development tunable laser based bragg grating …

Development of a Tunable Laser Based Bragg Grating Demodulation System

Laurie Chappell

A thesis submitted in w n f d t y with the requirements for the degree of Master of Applied Science

Graduate Department of Aerospace Engineering University of Toronto

O Copyright by Laune ChappeU 1998

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A novel Bragg grating demodulation system based on the use of a tunable gain-coupled

distributeci feedback (GC-DFB) semiconductor laser is developed and demonstrated. The

laser, donated by Norte1 Technologies, is packageci and characterizcd, including

wavelength tuning, power curve and temperature dependence characteristics. A current

driver is developed to precisely ramp the injection current of the tunable laser given

selactable user inputs. Severai data acquisition programs are written to facilitate the

different system modalities, and are combined into a final, all-encompassing product.

The s y s ~ is demonstrated to demodulate Bragg grating signals with a resolution of M.8

pe over a strain range of 6070 W. Demodulation of wavelength division multipiexed and

parallel multiplexed signals is demonstrateci, for both s e c and dynamic signals. A

hybrid serial / paraliel muitiplexed is demonstrated to simultaneously demodulate 12

separate Bragg grating signals. A tradeoff between system measurement range and

measurement bandwidth is discussed. An estirnate of the theoretical maximum number

of channels simultaneously adcimisable by the system is presented. Demodulation of

pultnided Bragg grating senson is presented. Suggestions for future systems are given.

Acknowledgements

h t and foremost, 1 would like to thank my supervisor, Dr. Raymond M. Measuns, for

the academic support thai enabled me to complete this thesis. I would also like to thank

Dr. Shangyuan Huang, Dr. Myo Ohn, and Trent Coroy. for always k ing there to answer

"just one more question." 1 would like to thank Gregory Fishbein for his pncision

craftmanship and Jake Unger for his invaluable knowledge of electronics. I would also

like to thank the other students in the FOSS Lab, past and present, for helping me out in

numerous different ways.

1 wodd üke to acknowledge ISIS Canada and Photonics Research Ontario (forrnerly

Ontario Laser and Lightwave Research Center) for their financial support. Without this

funding my thesis project could not have been completed

Finaliy, 1 would iike to thank Dr. ED. Chik and Dr. R Kriegler af Norte1 Technologies

for generously donating the GC-DFB laser that made this work possible.

LIST OF FIGURES ........................................................................................................................ VI

.................................................................................................. 1.1 Fiber Optic Sensors for Smart Structures 1 1.2 Development of Fiber Optic Sensors ........................................................................................................ 1 1.3 Thesis Objective ................................................................................................................................... 2 1.4 Thesis Organization ................................................................................................................................ 2

2 THEORETICAL ............................................................................................................................ 4

2.1 Fiber Bragg Gratings ................................................................................................................. ...,..... 4 2.1.1 The Bragg condition ......................................................................................................................... 5

............................................................................................................................. 2.1.2 Grating Rtsponse 6 21.21 Strain Sensitivity .................................. ..... ............................................................................................................. 7 2.1.2.2 Tan- Seasitivity ......................................................................................................................................... 9

2.1 -3 Grating Reflection Spectrurn ............................................................................................................. 9 ......................................................................................................................... 2.1.4 Grating Bandwidth 10

2.2 Multiplexing Techniques for Fibcr Bragg Graciag Scnsors ............ .... ..................................................... I l 2.2.1 Wavelength Division Multiplexing (WDM) ...................................................................................... 11 2.2.2 Time Division Multiplexing (TDM) ............................................................................................. 1 2 2.2.3 Parailel Mu1 tiplexing .............. .... ........................................................................................... 1 3 2.2.4 Hybrid Systcms ............................................................................................................................. 14

........................................................................... 2.3 Overview of Bragg Grating Demodulation Techniques 1 6 2.3.1 Broadband Source / Wavelength-Sclective Component Schemes ......................... ...... .............. 16

2.3.1.1 Passive Raîiometric Appmach ....................... ,... ....................................................................................... I 6 23.12 Waveleagth Division Coupla ............................................................................................................................ 1 7 23.1.3 ScaMing Fabry-Paot Fiter .. ....................................................................... ................................................. 1 8 23.1.4 Ac0wtd)pcic Tiurable Pilter ( A O ..................... ,, ......................................................................................... 19 23.15 fnt#fc~omcûic Detection Technique .......... ... ........... ...... ................................................................................ .. ...... 20 23.1.6 Quantum Wcii Eieccroabsorption Filtaing Detector .......................... .. .................................................................. 21

2.3.2 Nanowband TunabIt Source / Broadband Recciver Systems ......................... .... ......................... 2 2 .................................................................... 23.2.1 ninable Laser Basai ï)umduMon of Frbcr Bragg Graihg Sensors 22

3 SYSTEM COMPONeNTS ......................................................................................................... ...24 3.1 The Norte1 GC-DFB Laser .............. .. ................................................................................................. 24

...................................................................................................................... 3.1.1 Tùtory of Operation -24 ............. ............................. ................................. 3.1.1.1 Hïscory of the DFB taser ,, .- .................................................. 24 ............. 3.1.1.2Tbe Gain-Coupiai DFB Laser .................................................................................................................. 25

3.1.13 Theoieticai Analysis .... ............. - ...... " ............... . . . ....-...... - 26 ................................................ ..... ........................*. 3.1.1.4'Ihning ...... - ....,..-. .. ............ ....................... ................................................ .................................................. .. ....... 26

3.1.2 Packaging ...................................................................................................................................... 27 3.2 Curreat Supply .................................................................................................................................. 28 3.3 Bragg Gratings ............................... ....................................................................................................... 29 3.4 Data Acquisition / LabVIBW Program .................................................................................................... 30

3.4.1 Data Acquisition Boards ........................................................................................................... 3 0 ......................................................................................................................... 3.4.2 LabVIEW Program 30

3.5 Photodetcctors .... ......, ........ ... ....................................................................................................... ..33 3.6 1x4 SpUtter ....................... ....,....... 3 3 ...........................................................................................

4.1 Wavelength venus Injection Canent Characteristics ......................................................................... 3 5 4.1.1 DC Characteristics .......................................................................................................................... 35 4.1 -2 Rd-Time Measnremerits ................................................................................................................ 38 4.1.3 Polynomial CFuve Fit ..................................................................................................................... 42

4.2 Power vt. Current ............................................................................................................................... 43

4.3 Wavelength Dependence on Temperature ....................................... , 4.4 Linewidth ............................................................................................................................................. 45 4.5 Hysteresis ............................................................................................................................................. 47 4.6 Repeatabifity .................................................................................................................................... -47

........................................................................................ 5 SINGLE BRAGG GRATING SENSOR 50

5 . I Experimental Setup .............................................................................................................................. -50 ........................................................................................................................ 5.2 Rcsults ..................... ... -51

5.3 Tradeoff Betwetn Mcasurement Range and Measuremcnt Bandwidth ....................................................... 52

6 WAVELENGTH DIVISION MULTLPLEXING (WDM) .........e.e..e..e........................................... ..S3 6.1 Experimental Setup .................... ..., ............... .., .............................................................................. 53

6.1.1 Gratings ......................................................................................................................................... 54 6.2 Results ................................................................................................................................................. 55 6.3 Wavelength Consistcncy Check ............................................................................................................. 57

7 PARALLEL MULTIPLEXING ................... ............................................................................. -59

............................................................................................................................... 7.1 Expcrimental Setup 59 7.2 Rcsuits ................................................................................................................................................. 60 7.3 Theoretical Maximum Number of Channels ............................................................................................ 61

8 COMBINED WAVELENGTH DIVISION AND PARALLEL MULTIPLEXING ........................ A 4

8.1 Experimental Setup .......................... ,... ........................................................................................... 6 4 8.2 Resul ts ............................................................................................................................................. -65 8.3 Application: Smart Reinforcements .............................. ...,... ............................................................. 66

8.3.1 The Pultmsion Process ............................. ..,... ............................................................................... 67 8.3.2 Issues Rtgarding Pultrusion of Fiber Optic Scnsors .......................................................................... 68

8.3.21 Sensor f Host Bonding ........................................................................................................................................... 68 8.3.22 cmmctorization .................................................................................................................................................. 68

8.3.3 Pultnided Fiber Optic Sensors ......................................................................................................... 68 8.33.1 Scîup ........................W. .............. " ... - ..................................................................... "................ ........ 69 8332 RcsnIîs ................... ,. .........................--...... ,, ................................... ,, .................................................. - - ....... -70

9 CONCLUSIONS ..................m..........................................e.........e....m....e.........e................m............ e.72

9.1 Summary of Work Performed .......................... .... ................................................................................... 72 9.2 Suggestions for Further Reseatch and Development ................................. ,.. .................................. 7 3

List of Figures

Figure 1: Cross-section of0 Bragg g-g m g WIjform umplitudc. Urdur moduiatiùn a d PC* .................... -3 Figure 2: Schematic of Wavehgth Divnon Muüipkxîitg ........................................................................................ 11 Figure 3: Schemorif of T i DNhbn M&pl&nng .................................................................................................. 12 Figure 4: Schcmcrtic of ParaIIci MufipI-g ......................................................................................................... 13 Figure 5: Schemotic of C o & w T/WDM Q s t ~ m Proposcd by Berkoff et al. .......................................................... 14 Figure 6: Schematic of Combhd W , e h g t h Division Par&l MuItip&&ng ............ .... ...................... 15

.............. *.... Figure 7: Passive Ratiometrk Apprmch to Wme&ngth Sm Detecthn Proposed by Measutes a <iL .. 16 Figure 8: W~veJcngth Division C o ~ [ c r A p p r ~ c h Proposcd by Davis et . ai ...................... .. .................................. 17 Figure 9: ScMning Fiber Fabry-Perot system Proposcd by Keney et a l ........................................................... 1 8 Figure IO: Acourto-Optic TCUtLZble Fil&rApprwch Proposrd by Xu et al. .............................e................................. 19 Figure I l : Inte~eronaenic Appro~ch Propsed by Kersey et a l ............................................................................. 20

..................... Figure 12: Quantum Weil Electt&sorption Filtering Detecior Approcicih Proposcd by Coroy et aL .21 ................................................................................................................. Figure 13: apical Norte1 GC-DFB Lmer 26

Figwe 14: PÙwut Diagram ofNortel Ge-DFB Lmer ............................................................................................... 27 F i g m 15: Noml GC-DFB LaFL r aficr Packaging .................................................................................................... 28 F i g m 16: Photogruph of-r Di& D e e r rurd L U Tempcratwe Conîroller ................................... ..... 4 Figure 17: Schematîc FlowcM of UwEW Bragg G r d g DemOdIIIation Program .............. .... .................... 31 Figure 18: Screen Gtpture of LnbMEWProgram for 12-Channel Bragg Grcirug DemOdUlCItiOn System ............... 33 Figure 19: Phtograph of 1x4 Spl&er H o d g ....................................................................................................... 34 Figure 20: SchemcLtic of Wmekngth Cl&rprioti merimen&d Seîup .......................... .... ................................. 35 Figure 2 1: Photogmph of Laser Di& Autochar4~terùurion srstem ..................................................................... 36 Figure 22: Wavehgth vs . Injection Current Ckc~c:terLrrics for Norte1 GC-DFB Laser #I .................... .- .............. 37 Figure 23: W m h g t h us . Injection Current *~~terisitcs for Norte1 Laser #2 .................................................... 38 Figure 24: Schemaric of Michchon Imeferotneter .................................................................................................... 39

....................................................................... f i g ~ 25: O~CiIloscop Screen Gapture qf C~acicnrtic Frptges 39 Figure 26: P m r vs . Injecrion Curent for Norte1 GC-DFB h e r #I ..................................................................... 43 Figure 27: PM vs . Injcaion Cwcnt for Nonel GC-DFB Lrrrr .................................................................... 4 Fi- 28: DFB Luser Wmebgth Voriarlon with Tempetrrtur e. ............................................................................. 45 Figure 29: SchemQtic ofthe BBur*igh SA Phu Spectnun ha@r System ............................................................ 46 Figure 30: Norul GC-Dm k r e t Wmekn>glfi vs . Injection Cu- - Hysteresis &petimenî ................................ -47 Figwe 3 1: Rep/RtRhiüty o f Wovckngth versru C~u7cnt Scmu ...... ..................................... ................................. 48 Fi- 32: ExperhuW Setup Bmgg Gmthg Dunadukrtion wüli Rcference Photadetector .................................. 50 Fi- 33: Vruicrtion of Systun Respome with Applicd Smrfn .................................. .. 1 Figure 34: Schematic of Shgk-Ctuuurcl WDM @stem ...................................................................................... ...53 Figure 35: RcrpCciion Spectnm of WDM Bmgg Gmtlig String ............................ ... ..S .................................... Fi- 36: Bmgg G d g RcsponseEr, m 3-ClimYrcI WDM asîent ...... .. ........... .... ...........

................. F i , 37: Mana& OfEiqrcrimartrJ &tup for 4-CliamrcI ~amilèi Mulripbr,AQstun ..-................=.. 59 Figure 38: Srmic Tur w- R W ~ J ~ S for eCItonncI P M I MuM@trvA Systun ....................... .̂ .............. ..60 F i p z 39: DyMinic Test Experimcntal -for 2-ChmvrcI P&l M~itïpuIrrA *stem .................................... 61 Fi- 40: Epetiniartal SaupfDr vcuiprbk ~ t t m w w r ExperunCnr ............ ,., ........... ..... -........ 61 Ffgm 41: Varimion OfQstcm Rcsolrrtion willi A#CILUatiOn k e f .. .............- ...... ................... ...... Figure 42: ScAcmutk of 12 Chamte! Bmgg Grmhg D-n Systan .... ...... ...... ...................... Fgva 43: D ~ d S i @ a k j ' i v m I2-ChameI Bmgg Gmiag Systun .,., .... ........... .......... ......................... . Figure M: TIiC PUIiNIfOn PrOCCW- ....... .... .. - .... -. ............. ......... .... ......... .....- ...... ......................................O.... ..67 Fi- 4s: Pirotogrqph ofPuh&ù Tcndoru wiifr Embe&îèd Bmgg Gratfngs .......................... ., .......... ............. .... 69 Figwc 46= EkperinraJal Senrp for Pultnrdcd F i k t Optic Sensor Mc~pl(ramîs ..................................................... 70 Fi gvrt 47: estun Rwponrefbr Pultruded Fibcr Opdc Sau4r M - m ....................................................... 71 jr' 48: k g r a t c d O m S p h r . ..~.~....u.C~HI..~~~...~~~.~~~~.~..~~..~o~~~.~~~.~.........m....~............~~..~~..~~~~~..~~..o...~~..~~~.. -73 Fi- 49: Bookkm Ttchnob8y's Intcgmcd Op& Ttrrnsceher .............................................................................. 74

List of Tables

Table 1: Cotrelorion Between Laser Wavekngth Venus Cumnt Sc- ................................................................... 49 T& 2: Expected Reflccted Power from WDM Gmtings ..................................... .............................................. -54 Table 3: Results of Wmelength Consistency ErpcMunî ........................................................................................... 58

1 Introduction

1.1 Fiber Opfic Sensors for Smart Structures

The use of fiber optic sensors to m a u r e strain is becoming more and more popular as sensor performance and durability continues to improve. In many situations, fiber optic sensors can be placed where traditional foi1 strain gauges cannot, such as embedded in the matrix of a composite material or placed within the structure of a bridge. The emerging leader in the field of fiber optic sensor technology is the intracore fiber Bragg grating, which exhibits a shift in resonant wavelength due to changes in strain state or ambient temperature.

With the advent of fiber optic sensing technology, the concept of Fiber Optic Smn Structures sas conceived Il]. This involves outfitting a structure with monitoring equipment that allows the strain state of the structure to be known at all times, and in some cases, ailows the structure to react to changing conditions. This property is most desirable for large structures such as bridges, roads, pipelines, storage tanks, and airplanes because of their continual use and relatively high cost, which rnakes the expense associateci with the monitoring technology feasible.

The use of structural monitoring equipment makes it possible for fatigue to be monitored in an aircraft wing or imminent structurai failure of a bridge to be detected. Also, it rnakes possible repair stratepies that aiiow the replacement or refbrbishing of specifc stnicniral components to be d e d out only when necessary instead of as detennined by a generd maintenance schedule.

To support a large infrastructure of sensing elements, such as fiber Bragg gratings, a demodulation system is requireù that can not only accwatey and reliably monitor the sensing element, but ais0 handle multiplexing of multiple sensois in order to keep cost to a minimum.

1.2 Development of Fiber Optic Sensors

Various fiber optic sensor configurations have ben studied prior to fiber Bragg grathgs. including sensors that simply show a spot of light where damage or nbet bnakage has o c c d 121. polarimetric sensors 13.41, and many embodiments of interferometric sensors [5,6,7,8]. These types of sensors d e r h m various problems such as strain direction ambiguity and lack of multiplexing capability. The fiber Bragg grating

ovemornes these problems, and has thus becorne the dominant type of sensor in the field- Advantages of using a fiber Bragg grating over a traditional foil strain gauge include:

Easy integration into a wide variety of structures, including composite matends, with

little interference due to their smail size and cylindrical geornetry.

Sensors are Iightweight and relatively sturdy.

They are non-electricd and do not conduct elecaicity therefore they produce no

sparks.

They are immune to electromagnetic interference 0.

The cost of fiber optic technology is shrinking every day due to the explosion of its

use in the telecommunications industry-

They cm be readily multiplexed to form sensing networks.

They are not mechanicdiy cornplicated as the sensing conduit and the sensor itself is

one and the same (no wires hanging off of them)

AU of these advantages make the fiber Bragg grating sensor very attractive for many applications.

1.3 Thesis Objective

The objective of this thesis is to develop and demonstrate a new type of Bragg grating demodulation system based on the use of a tunabIe semiconductor laser. This type of system has many advaatages over current demodulation techniques due to the relatively large power budget afforded by a laser diode, because of the small size and therefore compactness of a semiconductor laser, and because this type of system readily addresses multiplexed sensors. Due to the requirements for hinable semiconductor lasers in the teIecomrnunications industry, their cost is quickly dropping as their performance continues to increase- This paves the way for a very powerfd Bragg m g demodulation system at a minimum cost.

This thesis is organized into 9 chaptets. The introduction, Chapter 1, discusses the importance of fiber Bragg gratings in the field of sensing, specifidy s& structures, and &O descn'bes the thesis objective and relevance of the work Cameil out.

Chapter 2 describes the characteristics of Bragg grating. inc1uding muitiplexhg and demodulation techniques. An analysis of the strain and temperature sensitivity of Bragg gratings is presented,

Chapter 3 outlines the various system components used to set up the tunable laser based Bragg grating demodulation facility.

Chapter 4 details the work camed out to package and characterize the Norte1 GC-DFB laser. Experimental resuits are presented, including wavelength-cmnt dependence, power-current dependence. temperature dependence, lincwidth, hysteresis, and wavelength-cumnt repeatzbiüty.

Chapter 5 discusses experimental results for a single Bragg grating sensor. including resolution measurements and bandwidth nonnalized strain resolution values. This chapter also discusses the system tradeoff between measurement range and measurement bandwidth.

Chapter 6 details the experimental results for staîic and dynamic measurements of wavelength division multiplexed sensors. This chapter also presents experimental results of a wavelength consistency check.

Chapter 7 outlines experimental resuits for a parallel rnultiplexed setup. including a theoretical estimate of the maxinium number of charnels that can be simultaneously monitored using the systea

Chapter 8 discusses experimental resdts from a hybnd wavelength and parallel multiplexed system, including a 12-channel system. This chapter also discusses measurements of Bragg gratings that had been puitruded in fiberglas tendons.

The conclusion, Chapter 9. gives a summary of work perfonned and suggestions for M e r research and development.

Appendices A, B, and C detail operation of and specifications for the laser diode cumnt driver, the photodetectors. and the 1x4 splitter.

Theoretical

Fiber Bragg grating sensors are taking the lead in the field of fiber optic sensing technology. Applications of Bragg grating fiber optic sensors as point sensors include static and dynamic loading of bearns [9], temperature measurements [IO], acoustic emission measurements [ 1 11, and composite cure monitoring [ 121.

2.1 Flber Bragg Gratings

A fiber Bragg grating is a periodic refractive index perturbation fonned in the core of an optical fiber through exposure to an intense W interference pattern. This interference pattern can be formed through the use of a split-beam interferometer [13] or a phase mask [14]. The phase mask technique is often preferred because it siflcantly d u c e s the W laser coherence and staûility requirements, however the interferometer technique is more flexible because the grating period can be adjusted by simply changing the angle between the interfering beams. With a phase mask, each separate wavelength quires a different mask, although some variation is possible by stretcbg the fiber within the fnnge pattern.

The modulation of the fiber wre index forms a structure which acts as a stopband Nter. A narrow band of light within the fiber is reflected by successive, coherent scattering induced by the index variations. The strongest interaction, or modecoupling takes place at the Bragg wavelength, &, given by

where ais the grating period; nN is the effective modal index of refraction of the fiber at

the Bragg wavelength, given by:

where n, is the wre or average index and anis the amplitude of index modulation. A îypical value of n, is beîween 1.45 and 1.5; typical values of &t range between 10-~ and IO-', and a t y p i d value of A is around 530 nm for a 1550 nm grating. Figun 1 depicts a Bragg grating having unifonn ampïitude, index modulation and penod

Bragg Graüng

Figure 1: Cross-section of a Bragg graîing having unifom amplirude, index modulation and penod .

Each reflection from a crest in the index perturbation is in phase with the next one at As. Any change in fiber properties, (such as strain, temperature or polarization) which varies the modal index or grating pitch will result in a shift in the Bragg wavelength [15,16].

2.1.1 The Bragg condition

When a guided fiber mode at the Bragg condition is incident upon a fiber Bragg grating, a certain percentage of the incident light will be scattered at each grating plane. In the nght circumstances, certain directions may be discovered where the wavelets created at each plane are in phase. If these directions correspond to a mode (guided, cladding, or free) of the fiber, then a resonant condition is satisfied and strong scattering will occur. In this situation, the forw ard wave momentum (proportional to i, , the forw ard-propagating modal wavevector) is reflected back by the grating rnomentum n 1171. Conservation of momentum dictates that

where t, is the modal wavevector of the fonvard-propagating wave

k, is the modal wavevector of the backward-propagating wave 2n

is the grating momentum = -, where A is the grating period A

As the photon frequencies are identical for the forward and backward propagating waves, 4 = 4 = &, where j3, is the wavelength of light that satisfies the Bragg condition. From equation 2.3 we obtain

which leads to the first order Bragg condition:

Thus Iight incident on a Bragg grating with period Aand effective modal index of refiaction na will be retlected back at the wavelength 4.

2.1.2 Grating Response

The Bragg resonance wavelength is altered by changes in applied sûain. ambient temperature and incident wavelength. The differentiai shift in Bragg wavelength resulting from a change in these parameters is given by:

The variation in r e b t i v e index due to a change in incident wavelength is negligible [18], which Ieaves

where is the change in grating length due to applied longitudinal strain and AT is the change in temperature

Equation (2.7) can be separated into two parts, the strain and the temperature sensitivity. and these parts can be anaiyzed separatedy.

The effects of an applied strain field on the value of the Bragg resonance wavelength are twofold Fim. strain variations shift the Bragg wavelength by expanding or cornpressing the grating, and thus changing the grating period, A. Second, the applied strain field causes a change in the e f f d v e index ttM ; this phenornenon is commonly known as the strain-optic effect The differential change in the Bragg wavelength resulting from an applied strain field is given by:

AL Substitution of e, = - into (2.8) gives L

Using the fact that

and

equation (2.9) can be rewritten as:

As outiined in [19], the strain-optic effect results in a change in the effective index of refkaction given by

where p, is the strain-optic tensor, and S) is the seain vector.

For longitudinal strain in the z-direction (dong the fiber axis), the strain vector is given

in the z-direction. where u is Poisson's ratio, and E, represents strain

And thus the shift in the Bragg wavelength for a aven axial strain is

where pN is the index-weighted strain-optic coefficient given by:

For a typical optical fiber, f i , = 0.1 13. fi , = 0.252, v =O. 16 [ml, and nQ = tt, = L48l8,

which givcs p, = 0.213. At 1550 nm, (2.17) thus predicts a wavelength strain

sensitivity of 1.2 ". The strain response is linear, with no evidence of hysteresis at w

2.1.2.2 Temperature Sensitivity

A change in temperature shifts the Bragg wavelength in two ways. Fit, thermal expansion (or contraction) changes the grating period A, and thus changes the Bragg wavelength. In addition, the effective index of refraction of the fiber, n, , is temperature dependent, and thus aiso affects the value of the Bragg wavelength [22].

The temperature-induced change in center wavelength of a fiber Bragg grating is given by

an, JA a - bM , where 5 is the thenn-optic coefficient, and = ail, Substitution of --

where a is the coefficient of thermal expansion of the fiber gives

which. upon substitution of the Bragg condition, gives

The value of 5 is approxunately 8.3 x 106 for a germania doped silica core; the value of a is 0.55 x 104 for s W [23]. This results in a temperature sensitivity of approximately 0.013 nmPC at 1550 m. This relation holds for temperatures up to 8S°C; above this temperature the sensitivQ increases and the response becornes moderately nonlinear W I *

2.1 8 Grating Ref lection Spectrum

Coupled-mode theory [25,26] can be used to mode1 the reflection pmperties of a fiber Bragg grating. As outlined in 1271, the mflectivity of a grating with constant modulation amplitude and pend is given by:

where R is the grating refiectivity

L is the grating length

B=- %c is the propagation constant A

n is the phase matched coupling coefficient 1

s = (a2 - ap2)T

At the resonant wavelength, there is no wavevector denining (Le. A S =O) , and thus equation (2.22) reduces to

From inspection of equations (2.22) and (2.23) it can be seen that ref'ectivity increases as i) the induced index change increases, or ii) the grating length increases.

2.1.4 Grating Bandwidth

A generai expression for the approximate spectral width of a fiber Bragg graîing is given by [28] to be:

Multiplexing is the pnictss of transmitting several messages or sigoals simultaneouly on the same circuit or chamei [29]. This is very importcmt in the field of fiber optic seasing, because it cuts dom on demodulation costs if many sensors can be monitored using one piece of equipment. The following sections describe the most cornmon methods of muitiplexing fiber Bragg gratings (FBGs).

2.2.1 Wavelength Division Multiplexing (WDM)

An experimental setup for this type of system, is shown in Figure 2. The pnnciple is that each FBG is written at a different center wavelength, so each grating can be identifieci by it's reflected wavelength. An operathg strain range for the sensor must be defined so that the wavelength spacing of the gratings on the fiber caa be detennined. For example, if each grating is going to be used to detect I5000 CLE, the gratings would need to be spaced approximately 12 nm apart so that no spectral overlap wodd occur-

Broadband LED

II SeriaJly Multipld Bragg G a n g Sensors

Advantages of this type of system inc1ude-.

Can address gratings louited spatidy very close îogetber Power budget is useci efficiently because sensors draw from different wavelength bands WeU-suitcd to FBGs because of theu w a v c 1 m ~ ~ c O d e d output

Limited by bandwidth of source a Requins gmîbgs at differeat wavelength - requires multiple phase masks a Grating fabrication cm be complex if gratings are very close together

If a wavelength demultiplexer is useci, availability and bandwidth could be a factor a Grating refiections could overlap and become indistinguishable if not judiciously

S P ~

2.2.2 Tirne Division Multiplexing (TDM)

This type of system, is shown in Figure 3. The principle of opedon involves pulsing an optical source to produce a pulse train. The FBGs are spaced at different distances dong the f i k r from the source such that a single pulse of appropriate duration produces a series of distinct reflection puises at the output. The duration of the hput pulse is detetmined the optical delay of the fiber comecting the FBGs. The gratin@ must be written with a low reflectivity because the source Light must pass thugh successive gratings at the same centex wavelength as it propagates, yet still have enough power to intemgate every sensor.

Advantages of this type of system include:

0 PBGs can aü be written at the same wavelength a Relatively n m w source bandwidth (12 am) nquirements

Disadvantages include:

More complex driver required to puise the source Difficult to manufacture sensors located spatially very close together Cross-talk between channels can lx an issue [30] If separation between gratngs is identical. ghost reflections can be created from sensors interfering with one another

2.2.3 Parallel Multiplexing

Parallel multiplexing, as s h o w in Figure 4, is conceptually the simplest method of intemgating multiple sensors with a single source. The principle of this type of multiplexing is that each grating is written in a separate opticai fiber, yet is addresseci by the same source. The source can address the array either through the use of an optical splitter, (if the power budget allows) or with some sort of switch.

Figure 4: Schentatic of Prarrlll Mdtiplèxing

PD 1 Ref. PD

This type of multiplexing is cspocially attractive for use with a tunable laser based system, where the laser power can be split many times before system resoIution suffers. Also, in the case of the tunable laser, separate demodulation systems for each channel are aot required

Tunabk tasecsourw

L k

O AI


AU the FBGs can be writîen at the same wavelength. so only one phase mask is =cl- If the fiber breaks or is discomected, only one sensor will be 10st

Disadvantages include:

Cost will not be reduced if each fiber requires a separate demodulation system If switching is mechmical. sensor interrogation will not be simultaneous. and failure is more likely than with a passive optical device.

2.2.4 Hybrid Systems

Hybrid systems are formed by the combination of two or more types of multiplexing. This includes the combination of thne and waveleagth division multiplexing 1311 as shown in Figure 5, and the combination of wavelength division and parailel multiplexing D2], as shown in Figure 6. Hybrid systems allow many more sensors to be mdtiplexed cornparad to systems that use only one multiplexing method because the maximum number of sensors for each multiplexing method cm be combined.

Figure 6: SclumOtic of Combined Wavekngth Division and Pmallel MulLi'plexing


The total number of charnels is the combination of the maximum of both mdtiplexing techniques used

May be more labour-intensive to set up

23 Overvfew of Bragg Grating Dernoduletion Tediniques

Several techniques have baen pmposed as to how to best demodulate transrnitted signal generated by a FBG. Each demodulation system falls

the reflected or into one of two

categories: those that are compriseci of a broadband source and a wavelength-selective component, or those that are compriseci of a tunable source and a broadband detector.

2.3.1 Broadband Source / WavelengthSelective Component Schemes

Passive Ratiometric Approach

The setup for this system as detailed in [33,34] is shown in the Figure 7. Light Born a broadband source is launched bto a 3 dB coupler, and subsequently intenogaies the fiber Bragg grating. The light raected h m the FBG is split into two components. Half of the teflected iight is passed through a wavelength &pendent filter, and the other half is passed directly to a non-Ntered photodetector to act as a -direct amplitude reference. These two signals are then d i v i a and the result is an amplitude function that is linear with wavelength. Th is type of system has demonstrated a strain resolution of 1 j~ over a range of 10.000 p [35].

* independent of source power fluctuations straightforward to implement

Disadvantages:

* sensitivity and alignment stability problems due to the use of bulle optic components 1361

* only one sensor c m be interrogated at a time (does not support WDM)

2.3.1.2 Wavelength Division Coupler

This system, as outlined in [37], is s h o w in Figure 8. The wavelength division coupler produces a monotonic change in coupling ratio for an input wavelength range of -50 nm. Thus, by monitoring the ratio of power From the two photodetectors, the reflected Bragg wavelength can be detennined. This type of system has demonstrated a strain resoIution of I3 p, over a strain range of -40,000 CIE [38].

insensitive to intensity variation effects large strainrange .

simple to implement

Disadvantages: - resolution not sufncient for some applications high polarizatio11 sensitivïty exhibitcd by fiber coupler (-1 96 RMS) [39)

2.3.1.3 Scanning Fabry-Perot Rlter

The setup for this type of demodulation technique, as discussed in [a], is shown in Figure 9. A broadband source is used to intemgatê one or more senaily multiplexed sensors. The reflection from the gratings 1s passeci t h u g h a Fabry-Perot filter, which passes a namwband wavelength component. The passband depends on the spacing between the mirrors on the device. A peizoelectric stack is used to hine the filter passband wavelength through the use of an electrical modulator. As the filter is tuned over the reflection signal from the gratings, the wavelengths cm be determined from the voltage applied to the flter when the tefiection signals are detected.

Tbis type of system has been demonstrated with a strain resolution of - 1 p~ over a 3.7 x lo4 range (iimited by FSR of the Wter 1411) and has recently been used to sequentiaily monitor an array of 60 fiber Bragg grating sensors r42.431.

Many gratings can be monitored on the same channe1 (WDM)

Cost may be an issue

2,3.1,4 Aoousto4ptic Tunable Fllter (AOTF)

This type of system, as described in [44], is shown in Figure 10. Light kom a broadband source is launched through the Bragg grating, and subsequently through the AOTE to a photodetector. The wavelength of light transmitted by the AOTF is a function of the RF frequency , as shown in the diagram.

Figure 10: Acousto-Op& Tknablè Fi&r Appto~ch Proposed by Xu et aL

The p ~ c i p l e of operation of the AOTF is outlined in [45,46]. Basically, an optical fiber is couplecl into a planar, optical and mrface acoustic waveguide, which is usually fabricated k m a lithium niobate crystrai. Lithium niobate exhibits both electrooptic and pi~zoelectric properties. The indcx of the crystal is perturbed by an m t i c wave. Mode ooupling wiii occur between the TM and TE waves when the acoustic wavelength A satisfies the phase-rnatching condition km = km = 2dA 147. This results in the polarization of the incident optical field being rotatad to the orthogonal state and being t d ü e d by the thin-fiirn polarize]~~. To track the instantaneous Bragg wavelength. a fecdback signal is employed to lock the mean optical wavelength of the filter to the instantaneous Bragg wavelength of the fiber m g . This system exhibits a strain resolution of 0.4 JE [48]. and a 3 x 1@ p range (iimited by the bandwidth of the source).

Exctiîent strain resolution Veryfastscanning

a C m track multiple gratllig lines by applying multipIe RF signals to the crystal behg used. Large strain range

Disadvantages:

Cost may be an issue

2.3.1.5 lnterferometrlc Detection Technique

This type of system, as detailed in [49], is shown in Figure I 1. Light from a broadband source passes through a coupler, and then intemgats the fiber Bragg grating. The reflection from the grating is passed through the arms of an unbalanced interferometer. Shifts in the Bragg wavelength are detected as phase shifts in the output of the interferometer, due to the inherent wavelength dependence of the phase of the unbdanced interferometer. The interferometer output can be modulated through control of the path imbalance. and through the use of a phase-reading technique. the grating wavelength can be detemined. Strain resolutions as small as -1 ne have been reported using this type of system [SOI.

Disadvantages:

Advantages:

Very good strain nsolution

Susceptible ta drift, therefore daes not perform weiï for quasi-static measurements 1s 11

2.3.1.6 Quantum Well Electroabsorption Filtering Detector

This system, as outlined in 1521, is shown in Figure 12. This type of system uses a semiconductor quantum well electroabsorption device as a optical filter. The principle involves tuning the absorption edge of the quantum well electroabsorption filtering àetector exhibiting the quantum confined Stark effect. A negative feedback approach is used to make the reverse bias voltage tune the spectral response of the device, and thus actively track the reflected wavelength of the FBG sensor. The reverse bias voltage is then calibrateci as a measure of the wavelength of the filtering detector. This system has demonstrated a straia resolution of f8 I~E, over a range of 3.4 x 104 p.^ [53].

ûptoektonics used have the potentiai to make a very small system System can be o p t i d y integrated ont0 the same mbstrate as other system components

Strain resolution not good enough for some appiidons EIectronics are iatricate Polarization sensitivity can be an issue

2-39 Narrowband Tunable Source / Broadband Recehrer Systems

2.3.2.1 Tunable Laser Based Demodulation of Fiber Bragg Grating Sensors

Rior to the work reported in this thesis, the number of published reports of systems using a Rinable laser source and broadband detector to demodulate fiber Bragg graîings has been very few.

In 1994. Ball et al. reported a system that used a mechanically tuned short cavity erbium doped f i k r laser to interrogate three wavelength division multiplexed gratings, with a strain resolution of 1.84 [54]. Another system was reported by Hjelme, et al. which used a DFB laser to interrogate fiber Bragg gratings [55.56], with a strain resolution of 1 p. A Ilimitation of both of these systems is that they suffcr from a very small dynamic range (400 pe). Another publication reportecl the use of a DBR laser to interrogate FBGs. however this system concentrated on exttacting distr-ibuted signals from the gratings 1573.

The system developed in this thesis uses a semiconductor gain-coupled distributed feedback (GC-DFB) laser to interrogate the FBG array. Light reflected fiom the grating array is collected by a photodetector array, and subsequently processeci by electronics or software. Correlation of the Bragg peak location to the location on the DFB injection current ramp allows the peak wavelength to be determined through the use of a calibration look-up table.

A tunable GC-DFB semiconductor laser has many advantages over other types of tunable lasers. such as extemal cavity, DBR or Erbium doped fiber lasers, including:

very small size no mechanical moving parts tequired to tune (such as extemal grating or PZï stage) fast tuning speed (> 1 kHz for full waveiength range) acceptable tnning range (-10 nm) one input (injection current) controls the output wavelength tuning is continuous, with no rnodahops high output power (- 6 mW) narrow linewidth (-1 MHz) high staf,ility with respect to wavelength lower cost

As the pnce of tunable semiconductor lasers continues to drop, and their tuning range continues to increase, the tunable lasu based FBG demodulation system is quickly becoming the most attractive system available. The fact that this system is set up from widely available opticai components and that it does not nquire extensive electronics

keeps the system cost to a minimwn. Also, the large light budget afforded by the tunable laser aiiows many channeis to be addressed by the same source, through the use of various multiplexing techniques.

3 System Components

3.1 The N O M GGDFB Laser

Northem Telecom (Nortel) generously donated two Gain-Coupled Distributed Feedback (GC-DFB) Lasers to the FOSSLab for use in experimental research. The lasers have very impressive characteristics including a wide tuning range, a narrow linewidth, and relatively high output power. Details of the laser characterization are given in Chapter 4.

3.1.1 Theory of Operation

Semiconductor lasers that employ a conventional Fabry-Pemt-type caviw usually exhibit multilongitudinal-mode oscillation 1583. This results in two undesirable phenomena Fit, mode-hopping occurs, which is where power switching takes place between longinidinal modes. Secondly, power dropout can occur. Power dropout is when there is a transient decrutse of power in the main mode and increase of power in another submode. Thus, to avoid the unattractive side effects of rnultimode operation, single longitudinal mode operation is desirable.

Varions structures have been proposed that provide longiNdinal mode control, the most promising of which is the distributed feedback (Dm) laser. The DFB laser uses an optical grating that is incorporateci into the active region of the laser, and may be applied over the whole active length. When the periodicity of the grating is chosen correctly, the grating provides feedback via backw8Cd Bragg scattering h m the periodic perturbations of the cefiactive index andlor the gain of the laser medium itself. This 'distributed feedback' elbinates the need for conventional cavity mirrors. and selects a singIe longitudinal mode.

3.1.1.1 Hlstory of the DFB Laser

The concept of the DFB laser was introduced in 1971 by KogeInik and Shank [59], who fabricated the first DFB laser by using dyed gelath on a glass substrate. Nakamura et ai. reporteci the first semiconductor DFB laser oscillation in 1973 [60], which was achieved by optical pumping to a comgated GaAs surface. Subsequentiy. S c i h et al. reported the htst injection-type DFB laser, using a GaA1As/GaAs singlehetemjunction structure [61]. The nrst continuous-wave (cw) operation at room temperature was attained at almost the same time by Casey et al. 1621 and Nakamura et ai. 1631 in 1975. who both used a GaAiAs/GaAs sepamte-confinement hetemstnicture.

After the advent of cw operation in GaA1As/GaAs DFB lasers. the next challenge was to produce a DFB laser that would operate at the standard t e l t c o ~ ~ ~ m u n i d o n s wavelengths

of 1300 and 1550 m. Io 1979, Doi et al. reportai the first DFB laser to operate in the 1 micron wavelength range 1641. This laser was fabricated h m InGaAsPIInP, and operatecl at 1 150 nm, however only at low temperatuns Room temperature cw operation at 1500 nm was achieved in 198 1 by Utaka et. al. also using a InGaAsP/InP laser [65].

3.1 ,1.2 The Gain-Coupled DFB Laser

Historically, distributed feedback lasers have b e n realized by incorporating an index corrugation in the active region of the laser cavity which causes light feedback dong the cavity, and allows for single longitudinal-mode selaction 1661. This is called 'index- coupling".

Index-coupled DFB lasers have some inherent probIems, one king that a laser with this structure exhibits pairs of longitudinal modes having equal threshold gain, which typically leads to multimode operation [671. This mode degeneracy can be remedied through the use of an asymmetric facet coating (AR or HR-AR coating), however this approach cannot provide high single-mode yield because of random facet phases introduced by cleaving [68]. Another solution to this problem is to use a kl4 phase shift m g , which requires that the facet reflectivity be quite small (< 0.005), otherwise the yield falls off quickly. This approach, however, leads to spatial hole burning relatai unstable single-mode operation, and output power waste from the laser rear facet [69].

DFB lasers manufactureci with a gain-coupling mechanism have many advantages over index-coupled DFB lasers. A gaui-coupled DFB laser is nalized by having a gain comgation in the active region of the laser, which causes rdlections to occw due to the dependence of the index of refraction on the gain coefficient, niateci through the &amers-knig relations. Advantages of gaincoupled lasers include high single-mode yield less extemal reflection sensitivity, and reduced wavelength chirp [70]. Due to constraints in manuf'turing, it is difficult tu fabricate a purely gain-coupled (or loss- coupled) DFB laser, thus it is common to produce partly gain-coupleci lasers which improve laser performance without s d c i n g manufacturability and reliability.

Norte1 has developed a -1.55 p InGaAsP-TnP DFB laser which rises a str'ained layer multiquantum-well (MQW) active grating to nalue a mixed index and gain coupling [71]. The lasers are grown using a two-step metalorganic chernid vapor &position (MOCVD) process. A depiction of a typicai &vice s s ~ c h u e is shown in Figure 13.

Figure 13: IplpW Norccl GC-DFB h e t

Much work has been focused into the analysis of DFB laser oscillation. Kogelnik and Shank proposed a coupled-wave fonnalism for analyzing plane wave propagation in a periodic structure [72]. This analysis produced expressions for the DFB laser resonant wavelength and thnshold criteria for the modes of oscillation for both index and gain periodicities. This wupled-wave formdism was extended by Yariv 1731, who analyzed guideci-wave propagation in a ptriodically corrugated waveguide, which was more suitable for designhg heterostnictre lasers. Wang subsequently used the u loch- wave fomaiism to analyze wave propagation in periodic structures 1741. These two analyses were thenafter shown to be equivalent by Yariv and Gover (751.

Tuning

In standard Fabry-Perot lasers, any change in the refIactive index due to the local c&r density has no effect on the lashg frcquency. This is due to the fact that carrier density is clamped at the thnshold level, when averaged over the cavity, and since the leshg fnquency is determin& only by the averaged rehctive index, it should remsin constant The carrier density eff- aumot be elimirintPA in DIB lasers, because the lasing frrqutncy shift caused by the index change is nonlinear amund the Bragg wavelength, and also because cavity loss varies with carrier demity distribution. Thenfore, canier distribution created by the laser injection ament causes the lasing wavelength to shiff 1761. If the CLUTiet distribution is carefdiy controlled, smooth. conbinuous wavelength tuning can be achieved. Dl?B lasers have been demonstrated to be tuned over their range as quickly as 0.1 ns pq.

Alteniately, the DFB laser wavelength can bc tuned t h e d y , as changes in laser tcmpaatPre p&ce a change in the gratïng pcriod which in turn affecfs the output wavelength. However, this option is less atiractive than tuning using injection cumnt because the tuaing mte is very slow, and because relatively s d wavelength changes are offered by thermal variations.

The Norte1 GC-DFB laser can be temperature tuned by about lm 1 10 OC. Injection cumnt tuning scans the laser througb a continuou -9 am tuning range when the injection c m n t is ramped between 20 and 300 mk The system is currently set up to perfom this sweep in a time as short as 1 millisecond, aithough the lasex could probably be tuned at a faster rate. The laser tuning spted is luniteci by the injeaion cumnt transport mechanism, and the rate equations for the laser 1781.

3.1.2 Packaging

The laser diode itself is containeci in a 14-pin hemetically seded butterfly package. Also contained in the package is a thermister, a thermo-electric coder, an isolator, and a photodiode. A pin-out diagram of the laser diode is shown in Figure 14.

The laser was mounted on a )4 " alunlinun block measuring 3 inches by 4 inches to facilitate temperature control of the laser. A layer of t h e d paste was placed between the laser package and the heat sink to ensure good thermal couphg between the two objects.

The laser end the heat sink were then placed in a 4.68 inch by 3.68 inch by 2.06 inch Hammond mode1 1590C-BK box for a more permanent housing so that the laser diode wodd not be damageci h m everyday use- Ootput ports for the laser cumnt supply, the back facet photodiode. the thermister and the thermoelecffic cooler were mmted on the sides of the box. The appropriate pins of the laser diode were oripiaaiiy connecteci to the output ports using Pmduit MasCon CBlûûF26-8-D connectors, however this did not result in a robust connedion so the pins were soldcnd dVectly to the comector wires. A di& was soldemi across the positive and negative temiinals of the laser to ensure the laser could not be accidentally niined by incorrect biasing. The top view of this sthip is shown in Figure 15.

Figure 15: Nortel GC-DFB LaFer d e r Packagïng

The connector pigtailed to the laser diode at Nortel was not compatible with the standard connectors used in the FOSSLab, so it was removed and a new APC connector was fusion spliced on to the end of the pigtail.

3.2 Current Supply

A previous distributed sensing system set up in the FOSSLab tuned a DBR laser using a current supply that was ramped by voltage signals successively output from a D/A board. The ramp level increased in increments which were the equivalent of one bit, and therefore the smallest increase possible. This type of setup produces a "staircase" ramp, which lirnits the laser output wavelength resolution to the equivalent of 1 bit.

A new current supply was designed for use with the new GC-DFB lasers to produce a tmly linear, instead of stepwise linear, current ramp. Also, the new current supply was designed with the ability to Nne the laser through its full range in the span of one rnillisecond, which is considerably faster than is possible using signals generated from a data acquisition board. This allows the full potential of the laser to be realized.

The electronics for the current supply were designed and built by Jake Unger of UTIAS. The current supply can produce a given number of ramp cycles, set by the user, or it c m produce a continuous Stream of ramp cycles. The start and stop current values, as well as the ramp frequency and delay time between ramps are al1 set by the user. Outputs from the back of the driver include the connection to the laser diode itself, a gate signal that

can be used for data acquisition, and a voltage signal that is proportional to the cumnt ramp that cm be used to detemiine at what wavelength an event occumd.

Notes on the design and operation of the cumnt supply can be found in Appendix A. Figure 16 shows the laser diode driver with the IZX temperature controller.

Figwe Id: Photograph of Laser Diode Driver and !LX Temperature Controitèr

3.3 Bragg Oratings

The majority of gratings used for optical research in the FOSSLab are made at Photonics Research Ontario (fomerly Ontario Laser and Lightwave Research Center). Unfortunately phase masks required to make gratings at various locations within the range of the tunable laser were not available, so a -1542 nm phase mask was purchased fkom QPS of Montreai. This was used at the Bragg grating fabrication facility at the OLLRC to manufacture gratings in the appropriate wavelength window. Gratings that could not be made at PRO were purchased from Bragg Photonics.

3.4.1 Data Acquisition Boards

One data acquisition board was purchased for use with this project. This was a Gage model CSSIuPCI 5 megasample per second 12-bit PCI bus card with 2 channels. This was to be used in conjunction with an existing 2 charme1 Gage model CS512 5 megasample per second 12-bit ISA bus card. Unfominately the boards were taken over by another project and an altemate data acquisition card had to be used. This was the National Instruments AT-MI0 16XE-50 data acquisition carcl, which is an 8-channel, 16- bit, 20 kilosample per second c d Although the AT-MI0 16XE-50 card is not as fast as the car& purchased for use with the project, it has the advantages of higher resolution, and more data acquisition charnels. The speed of data acquisition is very important for this type of system; currently it is the limiting factor for the spesd of the whole system.

3.4.2 LabVlEW Program

Several LabVIEW pmgrams were h m n in support of the tunable laser based Bragg grating demodulation system. The flowchart shown in Figure 17 outihes the final 4- encompassing program. This flowchart is presented at a very high level, because program intricacies would be t w lengthy to detail.

Channels fo Scan

Number of Samples Graph

Separate 1 ûatainto l

I Cornponents

1 DMdeout ' Power 1.: Graph '

Dependence 'M

Number of Fit Points i Perform Peak '

Peak Threshold '1 Detection

/'

1 I lnterpoluie : Peak Locations t

on Cunent Ramp

Convert +O Wavelenfi? 4 Change Peak - Location Values 'q Graptl

Temiserature 'i t0 W M e n g t h : 1

Save Data? Save Data t~Flle

Figure 17: Schematic Flouchart of LabVIEW Bragg Gra~ing DemoduCation Program

The program operates as foilows. Three software inputs are fed to the cornputer: which chamels to scan, scan rate and number of sarnples W. A gate signal frmn the laser diode current supply triggers the data acquisition to begin. Analog charnel 1 samples a signal generated by the laser diode current supply that is proportional to the current ramp supplied to the laser. Analog channel 2 samples the reference photodetector response. The rest of the channels are dedicated to sampling each channel of Bragg gratings. This data set is graphed on the screen.

The acquired data is then split into arrays representing the current ramp, the reference photodetector response, and the reflection spectrum from each channel. The reflection spectnim signals are divided by the reference photodetector signals to nullify any system power dependence. These results are also graphed.

Next, the reflection signals are fed to a peak detection aigorithm. The peak detection algorithm is a standard library function in the LabVIEW data andysis Library. Each reflection spectnim array is passed to the peak detection algorithm. which fits a quadcatic polynomid to a sequential set of data points and then determines the maximum value. The number of points used in the fit is specified by the user by setting the fit width variable. Software inputs tell the algorithm the desired peak threshold and peak width, so that random background noise signds will not be determineci to be peaks. A quick measurement technique tells the user the appmximate width of the Bragg peak so that an appropriate fit width can be determined.

The peak detection algorithm r e m s the locations of each peak in temis of sample number. These locations are then interpolateci on the current ramp data to find the value of the laser injection current at that sample location. This data set gives the current values associateci with the peak locations. If the user specifies thai the results should be converted to wavelength instead of king displayed in t e m of current, the wavelength is detemineci h m cuve fits supplied to the cornputer for each of the four calibrated laser temperatures. The laser temperature is input to the software by the user. The peak locations are then graphed vcrsus sample number. The user has the option to Save the data to a spreadsheet file. Figure 18 shows part of the program screen, including the raw data graph, and some of the anas where software-selatable inputs are entered.

Figure 18: Screen Capture of LdVIEW Program for 12-Channel Bragg Granig Demodulaiion System

3.5 Photodetectors

Three New Focus mode1 181 1 125 MHz, low-noise photodetectors were purchased for use with the tunable laser based system. Specifications of the photodetectors are shown in Appendix B.

A 1x4 splitter was purchased from EXB to be used for parallel multiplexing. The splitter was fabricated using the sarne technology as used to make typical 3 dB couplers, as opposed to an integrated optics technique. The splitter was packaged in a protective housing, with a coupler fusion spliced to each output ann to facilitate interrogation and monitoring of Bragg grating signals from each m. The specifications of the 1x4 splitter are shown in Appendix C. A picture of the 1x4 splitter housing, with side removed for viewing purposes, is shown in Figure 19.

Figure 19: Photogrprph of 1x4 Spliiîer Housing

Upon receipt of each of the Norte1 GC-DFB lasers, it was necessary to characteriz the laser to determine its attributes. Several experiments were carried out to meet this end.

4.1 WaveIength versus lnjectbn Current ChamcterIstics

The variation of wavebngth with injection current is somewhat non-linear for the GC- DFB laser, so it was necessary to calibrate the laser over the -10 nrn tuning range. This calibration was performed for DC (slow-varying) currents, as weii as for fast time-varying (rd-tirne) situations. There was no appreciable ciifference between the DC and real-the characteristics, as the maximum mal-tirne scaa rate was approximately 1 milliseconcl, which is sti I i slow on the time scale of the operation of the laser.

4.1.1 DC Characteristics

The apparatus shown in Figure 20 below was used to determine the DC wavelength vs. current characteristics nie experimental setup consisted of an HP model 70004A optical spectnim andyzer, an IIX lightwave model LDC-3722 laser diode controller, a cumnt supply designed for another projecf and a Packard Bell75 MHz Pentium cornputer with a National Instruments AT-MI0 16XE data acquisition board.

The LX laser diode controller was used to stabüize the temperature of the laser to a desired set point. A Lab- program was mitten to control the expriment The program's user-supplied software inputs included start current, end current, and current step. ûutputs from a data acquisition board were used to ramp the current supply. The output of the current supply was routed to the laser to provide the bias cumnt. A GPIB link was set up between the computer and the optical spectrum analyzer to allow the computer to control the OSA. At each current value on the injection current ramp, the computer would instmct the OSA to perfom a peak search, center the peak location on the OSA screen, and return the value of the peak wavelength to the computer. The computer would then write the c m n t value and the wavelength retunied from the OSA to a spreadsheet Ne. A photograph of the experimental setup is show in Figure 21.

Plots of laser wavelength versus current for Norte1 lasers #1 and #2 are shown in Figures 22 and 23.

Figure 23: W4yebng)r vs. Injecrion Crurent C ï w u c t d s i t c s for Nortei Laser #2

The graphs indicate that the wavelength-current relationship is very nearly linear, especially in the range between 100 and 275 mA. No appreciable drift has been obsemed in this relationship throughout the course of this work.

4.1 .2 Real-Time Measurements

To cietennine the wavelength versus current chmtenstics at operational speeds, an alternative form of measurement was necessary because the HP mode1 70004A optid spectmn analyzer is not capable of updating fast enough to colkt the required data. hstead, an interferometer was used to determine the wavelength vs. wnnt characteristics.

An all-nber Michelson interferometer was fabriCELted using a 50/50 fiber coupler witb mirrom fabricated on two of the fiber ends using a chernical technique. A depiction of the interfemmeter is shown below. -

Output 2-=cY The path Merence was set to be less than 2mm. which resulted in approximately 20 fringes appearing when the output of the DFF3 laser was passed through the interferorneter. An oscilloscope screen capture of the c m n t rarnp and the characteristic fiinges is shown in Figure 25.

Tek Run: l.0OMws Hl Res 71

x r ! !hW!

Wavefonn

Figure 25: OsciRoscope Screen Ciaplure of CIrrmrctenSlic Frhga

The intensity of the characteristic fringes is given by

where d =hi and ni is the path ciifference between the arms of the interfemmeter.

A givea scan wili result in a number of interference h g e s , as shown in the previous figure- For analysis of the fringes, an integer number of fkinges must be identifieci to work with. Thus an equal number of maxima and minima must be selected- Next the phase difference between the f i t and 1 s t mges must be deterhed. The phase at the f i t and last maxima is given by:

4mf q)i =- 4ml and #f =- Ai A I

w here:

Ai represents the wavelength at the fmt maximum A, represents the wavelength at the last maximum

The total phase difference is given by the difference between the two:

= Of -# i

and so the path difference of the interferometer c m be solved for:

where

A#, represents the total phase difference, which is equal to (# of mges - 1) X 2n

From the initiai scan of the interfemmeter, as show on the oscüloscope trace, we are left with a plot of intensity vernis tirne. Reat~anging the characteristic Michelson interferorneter interference equation gives

which can be used to solve for the value of 0 :

where I , is the ha-maximum of the interference fnnges and [ ( t h e ) is the varying intensity of the interferometer scan. Thus a plot of 9 versus time is generated fiom the plot of 1 versus time.

The phase total as a function of wavelength . denved in a manner analogous to the determination of the phase difference, can be written as

and in this manner we can solve for the wavelength as a function of the phase difference from the first maxima:

Equation 3.7 gave a relaiionship between running phase and tirne. This equaiion gives a relatiomhip between the total phase difference h m the first maxima and wavelength. In both cases, the definition of the phase is identical, so these two relationships can be comlated to give the relationship between wavelength and tirne. The ramp cumnt versus time is a known hct ion, so in hirn the relationship betwezn wavelength and cumnt can be detennined. F'mally, c w e fitting is used to detemine a mathematical relationship between wavelength and current.

In conclusion, cornparison of the slow and fast laser chafacterizations show no appreciable Merence.

4.1.3 Polynomial Curve Fit

The demodulation system determines the Bragg reflection peak location in terms of the injection current supplied to the laser. In order for the system to reNm re~dts in t e m of wavelength, a curve-fit or a look-up table must be supplied This is necessary because of the slight non-linearity of the wavelength-current relationship. Iiiitidy a look-up table was used, however that process took up too many system resources and slowed the cornputer down unacceptabiy. Subsquently curve fts were performed on the laser calibration curves s h o w in Figure 22. Sixth-order fits were performed using Microsoft Excel, which gave the foliowing results:

Laser Temperature: 20.22 OC:


Laser Temperature: 30.1 8 OC: n(T) = 6.770179E-141 - 5,194155E-111' + 1.4491 19E-081' - 2.000948E-061 + 2.370062E-041 + 5.78502 1E-032 + 1 .S369936+03, & = 9.988246E-01


where 1 is in units of mA, and h is in units of nm.

4.2 Power vs. Curr i f

The apparatus for the power vs. curnnt measurements was the same as that used for the wavelength vs. curent measurements outlined in Section 4.1.1, however the laser output was fed into the OSA power meter instead of the regular measurement photodetector. The LabVIEW GPIB software was modifiecl to operate the system in the power mode, and then the experiments were camied out. Plots of the power versus curent relationship for different temperatures are s h o w in Figures 26 and 27.

Figrtre 26: Power vs. Injection C-nt for Nottel CC-DFB Luser #I

-

Figure 27: Power vs. Injection Cument for Norte( GCIDFB Laser #S

Inspection of the graphs incikates that the power-current relationship is roughly parabolic concave d o m for both lasers, with the maximum power of - 5-6 mW occuning around 175 mA. Beyond 290 mA, the output power drops back to O. The m l e r the laser temperature, the higher the output power. Although this sounds attractive, the laser should not be moleci to too low a temperature or condensation could fom on the laser and destroy it.

4.3 Wavelength Dependence on Temperature

Figure 28 beiow shows the DFB laser wavelength dependence on tempe-, measured over a range of 10 to 35 OC, using a laser diode bias current of 50 rnA. This demonstrates the wavelength variation with temperature to be approximately 1 nrn 1 10 OC.

10 12 14 16 18 20 22 24 26 28 30 32 34 36

Tem perature (deg. C)

Figure 28: DFB Laer Wmekngth Vmiailon wüh Tempetuturc.

The linewidth of the laser could not be measuted accurately using the HP mode1 70004A optical spectnim analyzcr because it does not exhibit the required spectral resoIution. As an alternative, the Burleigh S A Plus Spectxum Analyzer System was set up to try and get a more accurate resuit. The SA PIUS System is a piezoeleztricaüy scaihed w n f d Fabry-Perot interfemmeter. The equipment was set up as shown in Figure 29, with an extra attachment made to the SA-900 Four-Axis Mount for fiber alignment.

rram-- -Rrip- M r 1 W ' ~

Figure 29: Schematic of the Burlngh SA PIUS Specinun A d y z e r System

The ramp generator provides a high voltage ramp for scanning the specinim aaalyzer through its full free spectral range. The detector picks up the resulting peaks. which can be displayed and compaied to the ramp on an oscilloscope. Given the fke spectral range of the system, the width of the signal can be compared to the width of the full range, and the value of the Iinewidth can be determined.

Unfortunately, the resolution of the interferorneter is limiteci by the fuiesse of the cavity. The spectral resolution of any Fabry-Perot interferorneter is FSWF, The maximum attainable finesse of this system was quoteci by the manufacturer to be approximately 300, which results in a spectral resolution of about 6.7 MHz. Ushg this technique, the linewidth of the laser was measured to be -7 MHz, which is not accurate because it has satnrated the resolution of the system.

After this atbmpt, it became clear that an alternaiive metbod of measuring the ünewidth was requirad such as a hetemdyne detection system. Unfortunately, the equipment nquir#l io perform such a meamernent was not available in the lab. The manufacturer of the laser was contacteci, who estimsrteri the linewidth of the laser to be "several hundred kHz" at a given wavelength, and "amund 2 MHz" when tmed at a rate of 1 m. This gives a coherence length of 12û m when tuned, and 1Oûû m when stationary, based on the assumption of a brentzian beam pronle p9]. It was decided that aj this time it was not n#xssary to acquire the extra equipment to more accurately &termine the laser linewidth, due to the fact that the cohexenœ Iength was so long that it would not be consequentid in the laboratory setup.

Hysteresis in the auiing of laser diodes has beea nported 1801. This refers to die fact that the injection cumnt versus wavelength relationship may depend on the tuning direction. For instance, if the laser was tuned from 30 mA to 3 0 mA, the resulting cume may not be the same as if the laser were tuned fiom 300 mA down to 30 m..

This characteristic was investigated using the sehip show in section 4.1.1, with a slightly modifieci software code which allowed the current to be ramped in the oppsite dinction to that previously reported. A jump was made to 290 mA, where the laser diode temperature was ailowed to stabilize at 25.14 OC, and then the current was subsequently ramped down to 30 mA. Figure 30 shows the results of the two mns, the top one king the nui fiom 30 to 290 mA, and the bottom one king the mn from 290 to 30 mA. The graph indicates that no appreciable hysteresis effects were observeci.

The tmiable laser characterization system, as outlined in section 4.1.1 uras set up to investigate the rcpeatability of the wavelength vs. cumnt ch~~8~tetistics of the Norte1 tunable GC-DFB laser. The temperature controiier was set to a temperature of 25.15 O C

M.Ol°C for this expriment. Seven separate scans were obtained, over the course of one &y. Figure 31 shows the d t s of the wavelength scans. The individual scans are not distinguishable because they are so similar.

In an effoa to cietennine the deviation in each data sequence. sixth order polynomial c w e fits were applied to each series. The value of the 1st numbet in each c w e fit, the constant, varied less than the resolution of the OSA. which is the instrument king used to determine the laser wavelength. This is the limiting factor in this experiment due to the fact that the OSA'S maximum remlution is only 0.08 nrn.

The correlation of data sets is defincd as the covariance divided by the product of the standard deviations of the data sets. Correlation is a measure of how alike the data sets are, with identical data sets having a correlation value of 1, and completely dissimilar data sets having a corrdation value of O. Table 1 shows the correlation values caiculated between each of the data sets. The values on the diagonal show perfect scores because a data set is. by definition, prfectly comlated to itself.

Table 1 shows very high comlation betwecn the &ta sets, only deviating Born a petfect score of 1 in the eighth decimal place. This gives a high confidence in the repeatability of the Iaser scans, even over the course of several hours.

In conclusion, the repeatability of the wavelength-current values has been show to be very g d An absolute measure of the variability of the scans, for example with respect to the 'average scan', is aot possible with this setup because the variation in wavelength between scans is beyond the resolution of the OSA.

Scan7

1 .O0000000

Scan5

1 .O0000000

Scan1 Scan 2 Scan 3

-6 Çcan2

1.00000000 0.99999993 0.99999991 0.99999992

Scanl 1 .O0000000 0.99999996 0.99999994

Scan 6 Scan 7

0.99999993 0.99999994

sau>3 1 Scan4

0.99999995 0.99999994

Scan 4 Scan 5

1 .O0000000 0.99999990 0.99999992

0.99999993 0.99999993

1.00000000 0.99999994-

0.99999993 0.99999992

0.99999994 0.99999994

0.99999996 0.99999995

1 .WûûOWû 0.99999996

Single Bragg Grating Sensor

The fmt measurements made with this tunable laser-based Bragg grating demodulation system were camed out on a single Bragg grating. The results are repotted in [81].

Figure 32 shows the experimental setup. A Bragg grating was mounted on an aluminum tapered cantilever beam, designed to ensure a constant strain proHe dong its iength. The grating was bonded to the beam using M-Bond AE-10 epoxy. The grating used in this experiment was a 1 cm long, 55% reflectivity, 0.2 nrn FWHM grating with a nominal Bragg wavelength of 154 1.5 nm.

The laser diode cumnt driver was used to repeattdly ramp the injection cumnt of the Norte1 gain-coupled distributed feedback (GC-DFB) laser between 20 and 300 mA, at a rate of 30 Hz. This caused the output wavelength of the laser to tune continuously between -1536 and -1544 MI, and the output power to vary between 0.5 and 6.25 mW. The output of the laser interrogated the fiber Bragg graîing &ter passing through an isolator and a 3dB coupler. The refiection from the grating was monitored using the refiection photodetector, and the laser power was monitored using the teference photodetector. The output of each photodetector, as well as a voltage signai proportional to the laser current ramp, was sampled at a rate of 10,000 samples per second using a 16- bit A D car& and subsequentiy the signals were processeci by the computer using the LabVIEPIr program outlined in Section 3.4.2. The ILX Lightwave mode1 LDC 3722B temperature controiier was used to stabilize the temperature of the laser to 20.32OC, M.01OC.

fiber Bragg

Peak variation due to laser power fluctuations and the laser injection-current output- power dependence was el imina. by dividing the refleztion photodetector respotise by the reference photodetector response. The peak of the Bragg grating reflection was

located using a peak detect aigorithm which employed a 4-point quadratc fit to interpolate the data The location of the peak was then comlated to the comsponding location on the laser current ramp, which, through the use of a calibration data look-up table, was correlateci to wavelength.

Figure 33 shows the system response for induced strains between -2200 and 2200 p. A conventional resistive strain gauge was mounted beside the Bragg grating to provide a strain calibration. As cm be seen, good linearity was observed throughout the measured region. The total strain range of the sysûm is limiteci by the tuning range of the laser, and for this setup was r n e a s d to be -6070 p. This could be broadened by the use of a more widely tunable laser.

For each data point, 50 consecutive measurements were taken. The mean of these values was used as data The variation in the measured values was consistently f0.00031 volts, which wmsponds to a molution of f0.8 p. Smpling at 30 Hz resulted in a bandwidth normalized strain resolution (BNSR) of 0.15 pe / ~ H z .

5.3 Tideoff Between Measumment Range and Mèasurement Bandwfdth

As the tunable laser based demodulation systern is nirrentiy set up, there exists a tradcoff between strain measunment range and measurement speed. For instance, in order to double the strain measurement range while maintainhg the same resolution, one must halve the meastuement speed, or bandwidth. Thus a bandwidth normalized srrain resolution of 0.025 j~ / dHz can be achieved by increasing the scan fiequency from 30 Hz to 1 kHz, while reduclng the strain range to only 180 p However, this tradeuîf can be eliminated by increasing the speed of data acquisition.

Waveiength Division Multiplexing (WDM)

The number of fiber Bragg gratings that can be multiplexed using a wavelength division multiplexing technique depends on the total tuning range of the laser and the s& range requirements for the sensors. The total stmin range for the tunable laser based demodulation system based on the use of the Nortcl GC-DFB laser was rneasured to be -6070 p&. This lends itself nicely to a string of 3 gratings, each having a 2000 p& range. Alternatively, a system could be set up with 6 gratings, each having a 1000 p range, or 2 gratings, each having a 3 0 range. In the end, the number of sensors used depends on the requirements of application k i n g monitored

At the time that these experiments were conducted, no strings of three gratings could be made with the gratings in the proper wavelength range, so 3 gratings had to be connected together to forrn the string. This presents certain complications, as discussed below. Subsequently, phase masks were bomowed that ailowed 3 gratings in the proper wavelength range to be d e n into the same optical fiber. This alleviates much of the complication involved in setting up a WDM system.

The use of the tunable laser in this configuration is very poweriùl because it allows multiple sensors to be intemgaîed without the addition of any equipment, which greatly reduces the per chamel cost.

6.1 Ekperimental Setup

A singlechamel, wavelength division multiplexing system was set up. as shown in Figure 34.

Figure 34: Sclumcrlic of SbagIc-C'hmeI WDM Systcm

Three gratings were used to cover the approximately 9 nm scanning range of the Norte1 GC-DFB laser. The center wavelengths of the gratings used were 1538.5, 1541.2 and 1544.3 nm. These gratings were well suited to be used in the - 1537 to - 1546 nm tuning range exhibited by the laser when operating at approximately 27.5 OC. The length of the rniddle grating was 1 cm (FWHM 0.15 nm), whereas the lengths of the other 2 gratings were 6 mm (E;WHM 0.2 nm).

The phase mask purchased for the fabrication of gratings for this laser was used to make the 1541.2 nm grating. The other gratings had to be ordered from Bragg Photonics of Montreal. Due to the fact that ali the gratings could not be fabricated at the same facility, they had to be connected together after fabrication to form a long string of gratings.

A critical issue that became apparent upon setup of the system is the order of the gratings in the fiber string. The grating closest to the laser experiences the fewest losses due to travel, connectors, etc., and thus gives a greatex reflection than those m e r down the line. Also, due to the fact that the laser output power is not constant over the tuning range, the graihg in the middle of the s p e c t m rdects a lot more iight than those at the edges because the incident power on the middle grating is so much higher than the incident power on the side gratings.

If these two effects are not taken into account, the photodetector rnight saturate for the middle grating reflection when the side grating reflections are stil l hardly discernible. Initially the middle grating had a refiectivity of approximately 90%, which led to photodetector saairation. A 1541.2 nm grating was subsequently manufactured with a -55% reflectivity to compensate for this.

Theoretically, the grating with the lowest expected reflected power should go fmt in the line because it wili experience the fewest round-trip losses. The grating with the highest expected nflected power shouid go 1st in the iine, because of higher expected round-trip losses. The follobhg table shows the reflectivities and laser current and power at the center wavelengths of each grating. From this the expected reflected power is caiculatod

Orating Rcflectivity Laser Current at & Laser Power at & Reflccted Power

Based on these calculations, the 1% 1.2 nm grating was placed first in the string, then the 1544.33 nm grating, and finaüy the 1538.5 nrn gratng. This gave a reflection spectnun with peaks that aiI met in the same photodetector range, as shown Figure 35.

1537 1538 1539 1540 1541 1542 1543 1544 1545 1546

Wave length (nm )

Figure 35: Refrccrion Spectnun of WDM Bragg Gtutîtag SnUig

The systern was initidy set up with connectors between each grating, and an U[B co~ectorized coupler linking the laser, the grating string and the photodetector. An isolator was placed between the laser output and the coupler to stop any backreflections h m interferhg with the laser.

The connectors linking the gratuigs resulted in high loss between gratings, such that the signal h m the third grating was not strong enough to be used. To cut-dom on the losses, the gratings were fusion spliced together. Another problem that arose resulted from the connectors on the EXB coupler. When comected with any connecter made in the FOSSLab, a Fabry-Perot cave was formed, creating unwanted noise on the grating signal. Even with the application of index-matchhg gel, the cavities were still present. This noise was not visible on the OSA or when slow scanning speeds were used, however the use of high-speed data acquisition boards (5 rnegasamples I second) aliowed this

noise to be viewed. The EXB connectors were subsequently removed and either replaced with a fusion spiice, a mechanical splice, or a connecter made in the FOSSLab.

As the laser was tuned over a scaming range of appmximately 9 nm, the reflected signal from the Bragg gratine was fed to the reflection photodetector. This signal. and a voltage signal from the current supply that correspondeci to the laser injection current ramp. were fed to the A/D card and monitored by the cornputer. Using the wavelength vs. current calibration for the laser would aUow the waveIength of the reflected signal, and subsequently the applied strain, to be determined*

A standard peak detection technique was used to monitor the three peaks reflected from the gratings. A 6 point quadratic curve fitting algorithm was applied to the signal to interpolate the data. More information of the signal processing is outlined in Section 3.4.2, which details the L a b M Program. Each grating had a strain range of approximately 2000 p ~ , due to the placement of the gratings and the nining range of the laser. Fewer channels or a wider laser tuning range would allow for a greater straïn range to be monitored. The scan range of 9 nm was covered in a scan period of 20 rns with a 1 rns &lay between scans, which resuits in a scan rate of approximately 48 Hz.

The longest wavelength grating was bonded to a cantilever beam which was placeci in a mechanical apparatus that allowed the beam to be dynamically strained at a rate of approximately 5 Hz, over a strain range of 2000 p. The center grating was bonded to a separate cantilever beam which had been tapered to ensure a constant strain distribution dong the length of the grating. This beam was lefi static. The third grating was not bondad, and was strained in a random fashion by hand. The response of the three gmtings venus t h e is shown in Figure 36.

1 O 15 20

Tlme (seconds)

Figure 36: Bragg G d g Response firom 3-Channel WDM System

It was deterrnined that the maximum dynamic signal that could be monitored was roughly 10 Hz. Above 10 Hz the dynamic signal could no longer be accurately represented. This is Limited purely by the speed of data acquisition.

6.3 Wavelength Consistency Check

The final step of the LabVEW Bragg grating demodulation program is to convert cumnt values to wavelength. The LabVEW program allows the user to choose the program response to be either in current (mA) or wavelength (nm).

To convert the cumnt values to wavelength, the original laser wavelength calibration is used. Before taking any data, the temperature of operation of the laser must be specified by the user.. The temperature chosen has to be a temperature for which a calibration m e exists, however. The program then detects the peak locations in mA, and uses the appropriate calibration cuve to determine the grating response in uni& of wavelength.

In this program the power dependency of the wavclength response is taken into account. The laser power curve is monitorexi, and subsequently the reflection data is divided by the laser power curve. The peaks arc then detected h m the power-corrected data.

Values measured by the iunable-laser based system were measureâ against that of the OSA, and were found to be accurate, to within the resolution of the OSA. To chezk the

measured and their peak values converted to wavelength. Theoretidy, no matter what temperature the laser is operating at, the wavelength response of the grating shouid be the same. The results for the three gratings at the four different caiïbrated temperatures are shown in the table below.

Table 3: Resullr of Wavekngth Consistency EqerVnenf

Temperature (OC)

The difference in the values is well within the resolution of the OSA, indicating that if these values were measured using the OSA, no difference would be detected. The accuracy of the ILX temperature controller used to stabilize the laser diode temperature is f0.2 OC, giving a range of 0.4 OC. The temperature dependence of the laser was rneasure to be 1 nm per 10 OC. Thus a 1 degree temperature change would result in a 0.1 nm output wavelength change, and a 0.4 degree temperature change would result in a 0.04 nrn wavelength deviation. The values shown in the table are within this variation, and thus are acceptable. Also, the readings at different laser temperatures could not be made simultaneously, and thus temperature changes in the ambient lab temperature couid have slightly changed the values of the Bragg wavelength of the gratings.

In conclusion, this expriment shows that the tunable-laser based demodulation system is reasonably consistent over the range of calibrated ternperatures. Due to the accuracy of the IW( temperature controlIer, however, it is recommended that the laser tempera- be kept constant throughout the course of an experiment, because of the possible f0.2 OC emr. The stability of the controiler is very good - f0.01 O C , so once it is at a given temperature, it will remain true to that temperature.

Grating 1 1 Grathg 2 Grating 3

7 Parallel Multiplexing

The use of a tunable laser diode is powefil tool for demodulating paralle1 multiplexed fiber Bragg gratings because of the nlatively large amount of output power (-5mW) afforded by the laser. This ailows the light from the source to be split several times, allowing for many channels. This is very effective in cornparison with systems that use LEDs for sources, because in that case another source is required for every one or two channeis. Spreading the cost of the tunable laser diode over severai channels brings down the per channel cost dramatically.

7.1 Experimental Setup

For the parailel multiplexing system, the 1x4 splitter was used. A schematic of the experimental setup is shown in Figure 37. Four channels were set up by connecting a coupler to each arm of the splitter, and attaching a Bragg grating on one throughput arm of each coupler. The fmt channel also incorporateci the reference photodetector, used to divide out unwanted power dependencies resulting from laser power fluctuations, and the fact thai the laser power spectnim is not constant as the laser is scanneci through its tuning range. Four photodetectors were set up to capture the reflection spectmm from each of the Bragg grating channels. This data, dong with a voltage signal proportional to the current ramp, was routed to the cornputer to be evaluated by the LabVlEW program.

Figute 37: Scliematfc of pXrrerimentrJ Setq for 4c'kuuuf PQTQIlef Mulirjrkeà System

A static test was conducted for the fourchanne1 system. Figure 38 shows the peak Location versus the graph resuiting from the static test. This was conducted to ensure the resolution remainecl at the 0.8 p& value obtained in the single channel tests. Next a two- channel setup was used to show the dynamic response of the system. The resdts of this expriment are shown in Figure 39. For this experiment one grating was bonded to an aluminum beam and placed in an apparatus that dynamically strained the beam at a rate of - 1 Hz, through a strain range of - 2000 pz. The other grating was Ieft unbonded. The data acquisition card was set to sample at 1 0 . 0 sampIes per second, with the laser diode current driver rarnping the laser at a rate of - 48 Hz.

IO 15

Tlme (seconds)

Figute 39: Dyrramic Test Exprrimcntal Results fur 2-CIrmuut PmQlZeL MwCurcd Systcm

7.3 Theoretical Maximum Number of Channels

It is necessary to preüict an estimate of the theoretical maximum number of channels thaî coald be monitored by the parallei mdtiplexing scheme. To this end, one channel was monitored with a variable attenuator in place to simulate the effect of splitthg the laser power among several channels. The experimental setup is show0 in Figure 40.

fiber Bragg

A single Bragg grating was monitond, with a Wandel and Golte~118tlll Variable Optical Attenuator (mode1 numbtr OLA-15) intercepting the light fiam the laser. A LabViEW

peak detection program, as used in the other experiments, was used to detect the Bragg grating peak location.

Tnitially, the opticai attenuator was put in place and the location of the reflected peak was noted. The attenuation was increased until the program could no longer identify a Peak location. This occurred at an attenuation level of roughly 40 dB. When the attenuation was reciuced to 39.70 dB. a peak could be consistently identified. The attenuation was then gradually reduced. and resolution measurements were taken for severd attenuation levels.

For each aîtenuation level, 32 successive peak location rneasurements were taken. The f error was taken as half the peak to peak variation over the 32 measurements. Figure 41 shows the variation of resolution with attenuation level. The y-axis indicates the variation of the peak location of the static grating, converted into units of microstrain. The x-axis denotes the attenuation level, as indicated by the variable attenuator.

Although at an attenuation level of 39.7 dB a peak could be consistently identified, the variation over the 32 muimrements was 0.0026 V, which comsponds to a variation of approximately f7 pe. This value is quite high for most measmement requirernents. As shown on the graph, at an aîtenuation level of 30 dB the sharp dope flattens out, and the nsolution reaches a more acceptable level. At 30 dB. the measured peak value variation

was measured to be 0.00042 V, which corresponds to strain variation of f 1 p€- This value is more acceptable for most measunment systems.

As the attenuation was decreased, the value of the peak variation continued to drop. The lowest reading taken was at approximately 6 dB, and corresponded to a strain of f0.8 p. The attenuator could not accurately be set to values lower than this.

The theoretical attenuation levei where peak locations can be acceptably determineci lias been shown to be approximately 30 dB. This corresponds to the üght k i n g split 1000 tirnes, or a 1OOOchannel system. However, this does not include the losses associated with the 1xN splitter, nor the Iosses associated with long fiber leads or poor connections or splices. AIso, the reflectivity of the grating used in this setup was 83.03%. which is relatively high. Systems measuring gratings with a lower reflectiviv could not withstand 30 dB attenuation because the reflected signals would be too low to detect.

The worst case scenario wodd be to use a cascade of 2x2 splitters fusion spliced together to fom the 1xN splitter. Given that a typical insertion Ioss of a 3 dB coupler is 3.5 dB, and a good fusion splice loss is 0.03 dB, this would aUow 256 parallel multiplexed channels to be simuitaneously interrogated using a single laser source. Improvements could be made to this number, however, through the use of an integrated optic splitter.

Combined Wavelength Division and Parallel Multiplexlng

The ability to combine wavelength division and parailel multiplexing techniques ~ O W S many more sensors to be multiplexed than would be possible by just using one of the techniques by itself. This type of technique was demonstrated by Davis et al., 1821, to interrogate 60 separate fiber Bragg gratings. The drawback of that system, however, was that the sensors were set up in five strings of 12 gratiogs, and each string of gratings was individually addressed through the use of a switch. This was necessary because their demodulation technique, a scanning Fabry-Perot optical fdter, uses an LED as the Light source. The switching results in readuigs not king taken simuitaneously, and the speed of the system k ing Iimited by the speed of the switch.

The use of a tunable laser to carry out this type of multiplexing is very amactive because due to the large power budget of the laser, an optical splitter can be used instead of a switch. and hence ail the gratings can be addressed simultaneously.

8.7 Experimental Setup

The experimental setup is s h o w in Figure 42. The output fiom the laser was directeci through an isolator, and subsequently through a 1x4 splitter. The four channels of the splitter each had a 3 dB coupler fusion spliced to them, to facilitate the interrogation of the Bragg gratings. Every channel had a reflection photodetector to mmitor the response of the Bragg gratings, and the first channel incorporateci a reference photodetector to monitor the laser power. A string of three Bragg gratings was attached to each of the channels. Two of the channels had gratings with unsaained Bragg wavelengths of approximately 1537, 1541 and 1543 nm. 'ïhe other two channels had gratings with unstrained Bragg wavelengths of appmximately 1541, 1543 and 1546 m. The respollse fiom the 4 photodetectors, as well as the reference photodetector, were routed to an ND board, dong with a signal proportional to the cumnt ramp h m the laser diode driver. The laser was scanned £kom 50 to 305 mA at a rate of 10 Hz, with a 1 d s e c o n d delay. The laser temperature was stabilized at 30.0 OC, M.01 OC

Figure 43 shows peak wavelength versus time for the llchannel system. Six of the channels demonstrate dyoamic signals, and 6 channels show static signals. Both of the dynamic signals were produced by mouuting the fiber on a weighted cantilever beam and letting the beam oscillate. This explains the decay in amplinide of the dynamic signals with time. The beams useci, however, were of different length and f a b r i d h m different materials, which explains the different levels of strain induad. The static measUrexnent gratings were unbonda however the shortest wavelength grating on bath (1537 nm) had to be put in tension so that all the gratiags used in the expriment would faU within the laser wavelength operaîing range.

Time (seconds)

Figure 43: DemoduCrrfcd Sigmkfiom 12-Channel Bragg Gmting System

This system was subsequently used to mesure wavelength division multiplexed gratings that had k e n pultruded into glass fiber rods.

8.3 AppIimtion: Smsrt Reinforcements

Many wncrete structures that have been reinforcd with steel tendons are deteriorating kause the steel inside the structure corrodes when exposed to the elements. This is an enormous probiem when one considers the number of CO&, bridges and other structures that have been fabricated in this m e r . This issue has caused industry to look for innovative materials to replace steel in the fabrication of prestressing tendons; namely materials that produce lighter, stronger. and longer-lasting tendons. In arlnition, through hstnunentation of the tendons, a series of 'smart reinfonxments' can be pduced, whereby off-site personnel can remotely monitor and perhaps control the stresses and strahs encountered by the road, bridge, or structure at any üme.

Fiber minforceâ polymers (FRPs) are the most promising types of materials to be tested to replace steel tendons. These materiais consist of reinforcing fibers such as graphite, d d , or glass. embedded in a min such as epxy, polyester, or vhylester. These materials ail have a hi@ strength-&O-weight ratio, are corrosion mistant, and can be readily formed into tendons through the pultrnsioa process. Also, fiber optic sensors caa be embedded into the tendon while it is being manufactured, and thus the sensor instnimentation process is neither labour intensive nor timecollsuming.

8.3.1 The Pultrusion Process [83]

Pultnision is a continuous method of manufachinng reinforced plastic structures of unifom cross-section. The process involves 'pulling' resin and reinforcing fiber through a die to produce a continuous, high strength, composite material. Continuous strands, mats, and woven cloths can be combined with thermosetting resins and continuously forrned on-line.

The process is as follows. Dry fiben are pulled off reels (1) and submerged in a resin bath (2) , where they are thoroughly impregnated with resin. The wet fibers are then guided together and formed into a preliminary shape (3) by a preform tool. The wet, uncured bundle then enters the special pultrusion die (4).

The die is a precision mold, machined and ground to exacting tolerances. Within the die, heat causes the resin to crosslink, or cure. As the part exits the die, it is fully cured, and ready to be handled. The puller (5)- located downstream from the die, provides the force that keeps the process moving. M e r exiting the puller, the cured parts are cut to the desired length (6).

Figure 44: The P u h i o n Process

8.3.2 lesues Regarding Pultntsion of Rber Optic Sensors

Sensor / Host Bondlng

The interface between the puliruded optical fiber and host materiai is a very important factor on the performance of the fiber optic sensor at measwing internai strains. To effectively transfer strain from the resin matrix to an embedded fiber Bragg grating, a good bond between the optical fiber and the host material is necessary. In general, optical fibers are protected with coatings to increase their durability. The bond strength between the opticai fiber and the host material is thus dependent on i) the ability of the coating to survive the production conditions such as high temperature and ü) chernical compatibility with the resin rnatrix. Studies have shown that the standard acrylate coating on opticai fibers cannot withstand processing temperatures p a t e r than 85 OC [84], which is much lower than standard curing temperatures reached during the pultnision process. Polyimide coating, on the other han& can withstand temperatures up to 385 OC and has been shown to be chemically compatible with common resin matrices used [69], and hence creates a much better interface with host materials.

With a good bond, it has b e n shown that embedded optical fibers do not have a signifiant effect on the tensile properties of pultruded fiber reinforced polymers, but they do slightly detenorate the shear strength, by about 7% for glass FRP rod with one embedded optical fiber, and by about 1% for carbon FRP rod with one embedded optical fiber 1851.

8-3.2,2 Connectorization

Ixiherent in the nature of the pultnision process is the fact that as the product is formed, it is cut into specific-length rods. Unfortunately, if an optical fiber is embe-dded into the rod at the t h e of manufachire, the end of the fiber will be cut blunt against the end of the rod, and no lead WU remsin that cao easily be co~ectorked.

One solution to this problem involves polishing the end of the pultruded FRP rod and connectoriwng the polished end, after the rod is complete. Another solution involves the use of an opticai 'smart cap' 1861. In this embodiment, an end cap is placed over the polished end surface of an embedded pultnided tendon. The 'smart cap' then aligns a fiôer with the puitruded optical fiber, using alignment optics and three-dimensiond piezo- eleztric positioners.

8.3.3 Pultruded Fiber Optic Sensors

ISIS Theme 3.4 researchers from DalTech University (fomerly Technical University of Nova Swtia) have done considerable work in the field of pultrusion. Working with Paui

Mulvihill of the UTIAS FOSSLab, they have produced pultruded FRP tendons that contain strings of serially multiplexed fiber Bragg grating sensors. The sensors are embedded in the tendon during fabrication, without any special protection, and without recoating the gratings. Two pultmded glas reinforced vinyl ester rods (volume fiaction of E-glass fibers 62%) were sent from DalTech to the FOSSLab for testing. These tendons each contained a pultmded optical fiber with three gratings wrinen in it, at wavelengths of approximately 1541, 1543 and 1546 nm. One tendon had the string of gratings located near the surface; the other tendon had the string of gratings placed near the centerline. Figure 45 shows the two pultruded rods set up in a cantilever fashion in the FOSSLab. The three dark marks on each tendon represent an approximate Bragg grating location within the tendon.

Figure 45: Photograph of Puunrded Tendons with Embedded Bragg Gratings

8.3.3.1 Experimental Setup

The pultmded gratings, when slightly in compression, fell into the wavelength range of the tunable laser. A combination serial / paralle! demodulation system was set up using two chmnels of a 1x4 splitter unit. The setup is shown in Figure 46. Two parallel

channels were set up, and the rods, each containing three gratings, were hooked up to the two channeIs. Thus, in total, a 6-channel system was establishecl. This could have a i l y been a twelve-channel system simply by hooking up two more Igrating strings to the remaining two channels of the 1 x4 splitter.

Figure 46: ErperimerrLal Setup for PuCiruded Fiber Qtic Sensor Measurernents

8.3.3.2 Results

The tendons were supported in a cantilever fashion, and each was strained by weighting the end of the rod. One rod was oscillating, and the other was left stationary. The measurernents were taken at a rate of 10 Hz. Figure 47 shows the reflected peak wavelength vs. tirne for the six gratings. The three gratings with the sinusoicial peak patterns were those located on the oscilMing rod. The gratùig with the highest amplitude oscillations was the grating closest to the root of the beam; the next highest oscillations were produced at the rnid-section of the beam; the smdest oscillations were the response of the grating closest to the end of the beam.

Figure 47: Sysiem Response for PuCtnrded Fiber Optic Sensor Measurements

This work shows the fmt demodulation of pultruded Bragg fiber optic sensors using a tunable-laser based system. This system is important for making measutements of the pdtxuded gratings because other systerns, such as the optical spectmm analyzer or the FLS 3000, cannot make real-time measurements of wavelength division multiplexed gratings. These results bnng the FOSSLab one step doser to fulfilling HL5 goals outlined in Theme 2 - Integrated Fiber Optic Sensing, section T2.2 - Material Integration and Applications of Fiber Optic Structural Sensing.

A novel Bragg grating demodulation system based on the use of a GC-DFB tunable semiconductor laser has been developed. This type of system is very powemil because of the wide range of multiplexing techniques that it can support, and will become more and more widespread as the price of tunable serniconductor lasers continues to decrease and their performance continues to increase.

The objective of this thesis was to develop a tunable laser based Bragg grating demodulation system. The fmt step undertaken to complete this objective was to package the two GC-DFB lasers generously donated to the FOSSLab by Norte1 Technologies of Ottawa Subsequent to packaging, the lasers were characterized for wavelength-current dependence (including both real-tirne and static meaurements), power-current dependence, wavelength-temperature dependence, linewidth and hysteresis characteristics. in order to drive the laser, a laser diode current driver was developed and produced in conjunction with Jake Unger. This allowed the user to select desired current ramp characterisitics and ramp the injection current of the laser.

Severai items were then specified and purchased for use with this project. These included a Gage CS5 1WCI 5 megasample per second data acquisition board, 3 New Focus mode1 18 11 high-speed, low-noise photodetectors, an EXB 1x4 splitter module, various EXB 50/50 couplers, a - 1541 nrn QPS phase mask, and other Bragg gratings that could not be fabricated in- house.

Subsequently, several LabVIEFC' pro- were written to perform the data acquisition and analysis. DBerent programs were written depending on the type of multiplexing desired In the end, these separate pro- were incorporateci into one aii-encompassing program that can demodulate a i l demonstrated forms of multiplexed fiber Bragg grating sensors.

The fmt fuil-system tests were performed on a single Bragg graîing sensor. The resolution of the system was measured to be M.8 CLE, with a bandwidth normalized strain resolution of 0.15 p 1 d ~ z . System response was demonstrated over a 4400 y range. The total range of the system was measured to be roughly 6070 p. The tradeoff between measurement range and measurement bandwidth was discussed.

Next the demodulation of wavelength division multiplexed sensors was demonstrated, including static and dynamic measurements of a 3-channcl system. Using a 3channel system, each grating can travel over a strain range of 2000 p.& without overlapping another grating. Possible complications in setting up a WDM system are discussed, and the consistency of the wavelength determination portion of the system is verified. The

upper M t of dynamic signals that can be measured using this system was detennined to be roughly 10 Hz, Limiteci by the speed of data acquisition.

Subsequently, a parailel multiplexed system was set up and demonstrated. Static measurements of a khanne1 system are presented, as are dynamic measurements of a 2- channel system. A theoretical anaiysis of the maximum number of channels that can be simultaneously addressecl by the system is presented, showing that 256 channels could be in terrogated.

Finally, a hybnd WDM and paralle1 multiplexed system was demonstrated. Static and dynamic measurements of a l2thannel system were taken. This system was then used to demonstrate the demodulation of 2 strings of 3 gratings that had been pultmded into two separate fiberglass rods.

9.2 Suggestions for Further Research and Development

Design of a high-speed demodulation unit is currently being undertaken by Thierry Cherpillod. This system wili use dedicated electronics to acquire the Bragg grating reflection spectnim and iden@ the peak locations. This will replace the data acquisition board and cornputer currently used to demodulate the Bragg grating signals. The use of the high-speed electronics will dramatically incnase the speed of the entire system which is cmnt ly Lirnited by the speed of data acquisition.

Another possible Unprovernent to the system would be to replace a lot of the bulk components with integrated optics. With sophisticated techniques now available for manufacture of integrated optics, companies such as Bookham Technology are producing user-specific integrated optical circuits. The use of integrated optics would dramatidy decrease the size of the system, and cut down on losses induced from hision splices, couplers and connectors. Integrated optical components commercially available are shown in Figures 48 and 49. PIacing the spfitter from Figure 48 in the path of the laser in Figure 49 and duplicating the rest of the setup would reproduce the demodulation system reporteci in this thesis, without the use of bullc components.

Figute 48: Integrracd Optic Sp&ter

Figure 49: Bookhm Techology 's Integrated Optic Trcursceiver

10. References

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Laser Diode Driver

Front Panel Controls:

START:

STOP:

FREO:

CYCLES:

DELAY:

MODE SW:

TRIG: - LEn IND:

Dial indicates starting current in mA. Range is 10-300 mA.

Dia1 indicates stop current in mA. Range is 10-300 mA.

Dial indicates ramp rate in Hz. Range is 10-999 Hz. Note: A ramp rate of 500 Hz would translate into a ramp time of 2 milliseconds.

Dia1 indicates number of ramps produces in the burst mode. Range is 1- 100 ramps. Note: 00 dia1 reading is 1 0 ramps

DiaI indicates time in milliseconds between end and begiming of successive rarnps in continuous or burst mode. Range is 1 - 1000 miiiiseconds. Note: 000 diai reading is 1Oûû miiliseconds

Selects continuous ramps or 1 -10 ramps depending on cycle switch setîing .

Switch initiates ramps in both burst or continuous modes.

LED 'ON' indicates the unit is producing ramps. Note: Delay tirnes between ramps of more than 200 milliseconds cause the LED to flash.

Rear Panel Si-:

Iout: - Laser Diode Drive Current Ramp output. Range 10-300 mA.

REF RAMP: Reference voltage ramp for the output c m n t source. Scale Factor is 0.01V / mA Range is 0.1-3V equaling 10-300 rnA. Output z = lm

1 MON: Indicator of output drive current. Scale factor = 0.0 1 V / mA Output Z = lm

A 1 out: Reference voltage ramp with the start point in mA normalized to zero. Scale factor = 0.0 1 V / rnA Output z = lm

A 1 D.C.: D.C. voltage representing the difference between start and stop signds. Scale factor is 0.02 V / rnA Range 10-300 mA. Output z = lm

GATE:

GATE:

Rectangular waveform 0-5V active high dwing ramp on times. Output Z = 100 $2

Inverted gate signal (active low) Output Z = 100 R

OPERATION:

1 ) Comect power cord of unit to 1 15 VAC *** Make sure laser diode TEC is hooked up and m e d on *** 2) C o ~ e c t laser diode to Iout 3) T m on power switch 4) Select start and stop currents, ramp rate, delay, and cycles if in burst mode 5 ) Press TRIG to initiate ramping 6) To inhibit ramping when in continuous mode, simply toggle mode switch to burst 7 ) To rwume ramping in continuous mode, toggle mode switch back to CONT and

push TRIG

NOTES:

1) Laser diode should not be connecteci or disconnected to unit during ramping. ALWAYS INHIBJT RAMPING FIRST

2) Start and Stop currents, Ramp Rate and Delay may be altered during ramping. However, extreme nonlinearity will be observed during the settling time.

3) It is recommendcd that ramping be stopped before shutting down power.

Model1811 Specif ications

Cuupiing: DC or AC

Bmdwidth (3 dB): DC-125 MHz (typ. DC) 25 kHz-125 MHz (m. AC)

WavelengthRange: 800-1800nm

Photodiode Material: InGaAs PIN

Photodiode Size: 300-pn diameter (FS) 100-pm diameter (FC, ST)

Power Requirements: I 15 V DC; 250 mA

Risetirne: 3 ns (typ)

'Djpical Current Gain: AC-Coupled Version 40 V/mA (AC)

1V/mA (DC) DC-Coupled Version 4ûV/mA

Input Noise Current 2 plV'I~z @ 10 MHz (typ) 20 gvdh @ 100 M H z (typ)

Output Current 40 mA (max into 50 a) mut Power: 53 pW (max @ 1.3 p) (Linear ûperation)

input Power (CW) : 7 mW (max w/o damage)

Connectors Input: FC, ST, or direct RF Output SMA DC bias monitor: SMB (ACaupled units only)

DATE: 2/20/97

JOB NO. : C-373 A

PO# : 961 223

PART NO B1-14SXX-15NE30-EO

DATA SHEET

1 M /= T: OUTPUT

IMAGE EVALUATION TEST TARGET (QA-3)

APPLlED - IMAGE. lnc = 1653 East Main Strr.et - -* - - Rochester, NY 14639 USA -- ,== Phone: 71 61482-0300 -- -- - - Fax: 71 61288-5989

O 1993. Appiied Image. Inc. All Rlghis Reserveâ

development tunable laser based bragg grating …

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