Eindhoven University of Technology

MASTER

An optical amplifier using an erbium-doped fiber

Santbergen, M.T.M.C.

Award date:1992

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tLB EINDHOVEN UNIVERSITY OF TECHNOLOGYFACULTY OF ELECTRICAL ENGINEERING

TELECOMMUNICATIONS DIVISION EC

AN OPTICAL AMPLIFIERUSING AN ERBIUM-DOPED FIBER

by

M.T.M.C. Santbergen

Eindhoven, June 19th, 1992

Report of graduation studyperformed from September 1991 until June 1992Supervisor: prof.ir. G. D. Khoe ..:-Coaches: dr.ir. W.C. van Etten

ir. H.P .A. van den Boom

The faculty of Electrical Engineering of the Eindhoven University of Technologydoes not accept any responsibility regarding the contents of graduation reports.

Summary Ui

Summary

This graduation report is an account of my work for the Telecommunications Division of thefaculty of Electrical Engineering at the Eindhoven University of Technology. In this report theprinciple and the optimization of an Erbium-doped fiber amplifier (EDFA) is reviewed. Noise inErbium-doped fiber amplifiers is exarnined, and furthermore an account of the measurements ofabsorption and fluorescence spectra, amplification and noise figures is given. Finally a comparisonof optica! amplified IM/DD systems and coherent optica! systems is made.

Research in Erbium-doped amplifiers has indicated that pump wavelengths of 980 and 1480 omcan be used to arnplify signals with wavelengths of about 1550 om. An EDFA mainly consists ofan EDF, an optical coupier and a pump light source. Optimization of the EDFA involvesoptimizing gain, coupling efficiency and minimizing bending losses.

Noise in EDFAs consists of shot noise due to the signal and spontaneous emlSSlOn, signal­spontaneous beat noise and spontaneous-spontaneous beat noise. For high signal powers and highgain the signal-spontaneous beat noise dominates, and for lower signal powers the spontaneous­spontaneous beat noise dominates.

Comparison of optical amplified IMIDD systems with coherent detectioQ systems has shown,under ideal and equal conditions, a mere 3 dB SNR advantage of coherent systems over IMIDDsystems. This means that, through the introduction of EDFAs, the performance of IM/DD systemscan approach the performance of coherent systems. The choice between the two systems now onlyinvolves selectivity, channel spacing, costs and complexity of the receivers.

In preparation to the measurements, the 1480 om pump laser has been built in a housing. Thepump laser is therefore equipped with an adjustable current source and a temperature controller.The 1485/1555 om WDM coupier manufactured at the faculty is also built into that housing.Furthermore a Pascal program is written that transfers data from the Optical Spectrum Analyzer toa personal computer, where it can be written to disk or plotted on paper.

The measurements on the York EDF, using the 1480 om pump wavelength, have shown that thefiber didn't meet it's specifications. The optimum fiber length is much shorter than specified, theabsorption is higher and the amplification is lower than given by the manufacturer.

iv Contents

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 1

2. Optical amplifier using an Erbium-doped fiber 22.1. The principle of an Erbium-doped fiber amplifier 2

2.1.1. Three-Ievel laser 22.1.2. Laser threshold condition: population inversion 32.1.3. The Erbium atom doped in optical fiber 5

2.2. Components of the complete amplifier 82.3. Optimization of tbe amplifier 9

2.3.1. Working conditions of fiber amplifiers 92.3.2. Optimization of an Erbium-doped fiber 112.3.3. Optimum length of used York fiber 11

3. Noise in Erbium-doped fiber amplifiers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 123.1. The noise components 123.2. Noise figure 15

4. Comparison of optical amplified IMIDD systems and coherent optical systems4.1. The effects of ASE on a cascade of amplifiers4.2. Optica! amplified coherent detection systems4.3. Comparison of direct and coherent detection systems

. . . . . . .. 18182123

5. Measurements on tbe amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 245.1. Absorption spectrum 24

5.2. Fluorescence spectrum 265.3. Amplification 285.4. Noise figure 315.5. Conclusions 32

6. Conclusions and recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 33

References 34

List of abbreviations 35

List of constants and variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 36

Contents V

Appendix A: Specifications of the 1480 run pump laser. . . . . . . . . . . . . . . . . . . . . . . .. 38A.I About the laser 38

A.2 Charaeteristics 39A.3 Test data sheet CQF75-D #517 40

Appendix B: Data sheet of 148511555 nm WDM coupIer 41

Appendix C: Data of York DFI500 Erbium-doped fiber , 42

Appendix D: Power souree for the pump laserD. I Current souree0.2 Temperature controller

0.3 Power Supply

0.4 Housing of the pump laser

.............................. 4343485454

Appendix E: Listings PLOTFILE.PAS and IEEEIO.PAS 55

Introduction 1

1. Introduction

In optica! communication systems we can distinguish between two detection methods: directdetection and coherent detection. With direct detection (DD) the information carrying signa! isrecovered by detecting the power of the received optica! wave. Intensity modulation (IM) is afarm of amplitude modulation. It is not possible to recover FM or PM signaIs with directdetection (that is, if the optica! carrier is modulated), however if the received optica! wave ismixed with light from a loca! oscillator, an IF signa! is created, which contains both the phase andfrequency information of the transmitted signa!. This technique is called coherent detection.

The advantages of coherent systems over IMIDD systems are higher selectivity and moresensitivity. However, to realize such systems, the receiver has to be much more complicated. Thereceiver of an IM/DD system can be kept very simpIe, but for long distances more repeaters arenecessary.

We can amplify the optical signal by means of electronic regenerators or we can use optica!amplifiers. With electronic regenerators the optical signal is converted to an electrical signal,amplified electronica!ly and converted back to an optical signal for ftIrther transmission. Animportant drawback is the resulting electronic bottleneck. By using optical amplifiers this problemcan be solved. There are two types of optical amplifier:

• Semiconductor laser amplifier• Erbium-doped fiber amplifier (EDFA)

The semiconductor laser amplifier has the advantages of smaller size and lower power consump­tion. However, they are sensitive to polarization and have a large connection loss when connectedto transmission fibers. Therefore it seems that they are most suitable for use when combined withoptical integrated circuits and optoelectronic integrated circuits.

On the other hand, a fiber-type optical amplifier, like the EDFA, is directly connected to the

transmission fiber and connection loss is smal!. The power necessary for amplification is suppliedby pump light. Using this type of amplifier, many new appJications will become possible. The

EDFA has the merits of polarization insensitivity and low power level of pump light. Therefore, it

seems that EDFAs will be easily introduced into practical optica! transmission systems.

In this report the principle and the optimization of an Erbium-doped fiber amplifier will bediscussed (chapter 2). In chapter 3 noise in Erbium-doped fiber amplifiers will be discussed. Acomparison of optical arnplified fM/DD systems and coherent optica! systems will be made inchapter 4 and in chapter 5 absorption and fluorescence spectra, amplitïcation and noise figureswill be measured.

2 Optical amplifier using an Erbium-doped fiber

-2. Optical -amplifier- using-an Erbium::dopeçf fi~er_

In this chapter the optical amplifier will be discussed. First the principles of amplification using anErbium-doped fiber will be explained. Then the different components of the complete amplifier

wiJl be discussed and in the last section the optimization of an Erbium-doped fiber amplifier willbe examined.

2.1. The principle of 8n Erbium-doped fiber amplifier

The ampJification of light by means of an Erbium-doped fiber is based on the optical transitionsbetween different atomie levels. These levels have different energies and different populations. In

an Erbium-doped fiber amplifier (EDFA), optical signals can be amplified by means of a so-calledthree-Ievel laser system.

First the basic principles of the three-Ievel laser system wiJl be discussed [Wintjes].

2.1.1. Three-Ievellaser

Figure 2.1 shows both the relevant energy levels of a three-Ievel laser system and all relevanttransitions between these levels. Electrons are either in the El' the E2 or the E3 energy state. Nh

N2 and N3 are the number of electrons (population density) in the energy state Eh E2 and E3 ,

respectively. Photons can be absorbed or emitted by electrons that make transitions between two

states.

B

E 1 l

'-A 32

13 8 31 A 31 812 8 21

Figure 2.1 7hree-level transitions

A transition can either be spontaneous or stimulated, indicated by the Einstein's A and B coeffi­cients, respectively. In this simplified model Bif equals Bfi. A spontaneous transition occurs if an

electron spontaneously decays to a lower state.

Optical amplifier using an Erbium-doped fiber 3

A stimulated transition is induced by electromagnetic radiation in the appropriate frequency

region. The incident photons promote excitation of electrons to a higher level or de-excitation to alower level if the energy of the incident photons match the energy difference between the two

levels involved. Photons can therefore be absorbed or emitted. Stimulated ernission photons are inphase with the incident photons.

2.1.2. Laser threshold condition: population inversion

To understand which conditions have to be satisfied for light to be arnplified in this system, wemust consider the steady state situation of state Ez. A steady state implies that the number ofelectrons per unit of time arriving at an excited level (stimulated absorption rate) is equal to thenumber of electrons leaving the same level (stimulated emission ratel. We can write the following

rate equations [Wintjes]:

(2.1)

(2.2)

(2.3)

(2.4)

where Wp is the pump power density needed to excite electrons to the upper state (a signal with

photon-energy E)-EJ, W the signal power density (a signal with photon-energy E2-EI ) and NIOI the

total Erbium density.

The stimulated emission rate of photons with energy Ez-EI equals WB21N2 and the stimulatedabsorption rate of photons with energy E2-EI equals WBI~l' Since B21 = Blz, the populationthreshold for amplifying light (producing more photons than absorbing) is: N2 = NI' If Nz > NI

we speak of population inversion.

Combining equations (2.1) - (2.4) the population threshold (Nz/NI = 1), which determines theminimum power needed for amplification at a signal wavelength À = hcl(E2-EI ), can be stated as

follows:

(2.5)

4 Optical amplifier using an Erbium-doped fiber

The population threshold must- be minimized and therefore th~_ restrictions on a suitable three-Ievel

system are:

(2.6)

and1

't = -; the lifetime of state 2..411

(2.7)

(2.8)

State 2 must have a fairly long lifetime to lower the threshold and is therefore called a metastabIe

state. The decay from state 3 to state 2 however, is usually a very fast phonon-assisted non­

radiative process and therefore these levels are relatively close together. Of course one would Iike

the probability coefficient 8 13 to be as large as possible. If the pump rate Wp :: Wp B13N1 is high

enough to establish a population inversion, amplification can be accomplished.

Altogether four major conditions must be met to be able to establish amplification in a three-Ievel

system:

1: The existence of a metastabie state;

2: Fast decay from an upper state to this metastabie state;

3: High pump absorption probability;

4: High signal transition probability.

Only a few atoms do have three energy levels with characteristics that fit these requirements.

Erbium, doped in a host glass, is one of them.

Optical amplifier using an Erbium-doped fiber 5

2.1.3. The Erbium atom doped in optical fiber

So far the simplified three·level system. To have a good understanding of the Erbium-doped FiberAmplifier (EDFA) one has to consider the low-Iying energy levels of Erbium atoms doped in fiberglass as shown in figure 2.2 [Wintjes].

Energy ;\.

(eV) (nm)

4E-19 500

2H 11/2

ground state

metastablelevel

\asertranSit lons

15-161'm

_ non-radlatlve

de<:ays

/

qs 312

\ 9/2 ...4

I 912 "4

I 11/2"

4I 13/2

pump

absorpt Ion

I4I 15/2

,

1000

free-orblt

spln-orbit

spilt! Ing

crystal field spl,t!,ng

Figure 2.2 Energy levels ofErbium atoms doped in optica/fiber

Due to broadening of the energy levels (caused by Stark splitting, phonon broadening andtransition broadening) figure 2.2 shows transition bands instead of narrow transition lines.

Furthermore we can see that ao Erbium-doped fiber has several three-Ievel mechanisrns sincepumping cao be done fiom the ground state to one of the six possible upper states. The centralwavelengths of these six pump bands approximately are: 520, 540, 660, 800, 980 and 1480 om.

From all these upper states, Erbium ions cascade non-radiatively to the 411312 metastable statethrough multiphonon emission. Non-radiatively processes are inversely proportional to theexponential of the energy gap separating the two levels involved. The higher the phonon energy,the faster the non-radiative process. The energy gap between the 4113/2 and the 41 15/2 state is high

enough to reduce the decay to a large extent in comparison with the decay from higher pumplevels to the 41 13/2 state. This level is therefore metastabie. The pump transition probability depends

on which specific pump band is used.

6 Optical amplifier using an Erbium-doped fiber

- -All- levels wilt-be thermally- populated _acçQn;ling .10 Boltzman's statistics. This means that the

lower Stark components of both the 41 13/2 and the ground state will be higher populated than -thehigher Stark components. Thus absorption will be stronger at lower wavelengths (from lowercomponents of the ground state to higher components of the 411312 state) and emission will bestronger at higher wavelengths (from lower components of the 41 1312 state to higher components of

the ground state).

This Boltzman distribution thus results in different overall shapes of the absorption and emissionspectra of the metastable state and the ground state. The difference in wavelength between the

absorption and the emission band is called the Stokes shift. The Stokes shift permits the 1480 omband to be used as pump band. At 1480 om, unlike the other pump wavelengths, bath absorptionand emission occurs. As a consequence the population inversion will be somewhat lower than inthe case of, for instance, the 980 nm pump band.

Very important are the various loss processes. Apart from the already mentioned non-radiativedecay that can take place from any level, the most significant loss mechanism is excited stateabsorption (ESA). After excitation of an electron to an upper level, further excitation can occur 10

a still higher energy level through the absorption of anather pump photon or signal photon. This

can be seen in figure 2.3 in case of the 820 nm pump band.

45 3/2 t 800 nm_4_F.dg!...f/2~__-::;", .......I_Slgna I ESA

I.. -

4'11/? -~

4113/2

800 nmabsorpt Ion

4 115/2

Figure 2.3 800 nm signaJ ESA

The effect of ESA is detrimental because it depletes the metastabie level and makes inefficient useof pump energy. The electrons will ultimately return to the metastabIe state by non-radiative decay

but still a pump or signal photon will be lost to heat and therefore the overall pump efficiency wiJlbe reduced. ESA can take place fiom the final pump level (pump ESA) or the metastabie level

(signal ESA). Several higher states exist, resulting in unwanted ESA absorption bands.

OpticaI amplifier using an Erbium-doped fiber 7

Both known ESA bands and pump bands are shown in figure 2.4. As can be seen in this figure,

only the 980 nm and the 1480 nrn pump bands don't show any ESA. These bands are therefore infavor of the other pump bands, where efficient pumping is severely impeded. Within the pump

bands that suffer ESA, the pump wavelength must be adjusted to minimize ESA and maximizepump absorption at the same time.

10

• ESA

cQ 5-'Q.Lo(J"l

.D«

o~~....K.l.u..ul~ur&::W&Lu..L~.....L~.L.UL~......L:=~~m:L.u.o..ucuJ..uu..u..u.w.J

500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600

Wavelength (nm)

Figure 2.4 ESA and pump bands of an Erbium-doped fiber

8 Optical amplifier using an Erbium-doped fiber

2.2. Components of the co-m-plete amplifier - -

The three basic EDFA configurations are shown in figure 2.5 They are cJassified mainly by their

pump light propagation direction. The signa! light co-propagates with the pump light by forwardpumping and counter-propagates to the pump light by backward pumping.

Erblum-Ooped Fiber

Input 0----.,

Pump Light SOlTce

(Laser diode)

(a)

ErDlu",.- Doped FIler

Input~ ~'"'""-__-+

Opt lCallsola:<Y

Output

-+-__-0 Output

'nput o---j

Pump lig'1\ Source(Laser diode)

(c)

Optlcal

8and-Dass

Filter

Output

Figure 2.5 Basic EDFA eonjigurations (a) Forward pump type(b) Baekward pump type (e) Bidirectional pump type

The configurations shown in figure 2.5 cao be used as a power amplifier at the transmitter side inorder to increase output from a weak souree prior to transmission in a long systems fiber, as a

prearnplifier in order to amplify a weak signa! level to a level which is acceptable for the receiver

sensitivity, and as an in-line regenerative repeater.

An EDFA mainly consists of an Erbium-doped fiber, an optica! coupier and a pump light source.

In addition to these, an optica! isolator and/or an optica! band-pass filter is used to improve EDFA

performance.

Optical amplifier using an Erbium-doped fiber 9

Depending on which EDFA configuration is used, the optical isolator either prevents reflections toenter the EDFA from the output side or prevents the amplified signal and the pump signal fromreflecting back to the input of the amplifier (see figure 2.5). The amplified signal passes theoptical isolator with approximately 0.5 dB attenuation, but is attenuated 30-60 dB backwards.

The optical band-pass filter can be used to filter the ampJified spontaneous emission (ASE) inorder to minimize the amplifier saturation as weB as the noise due to ASE build up when more

than one amplifier is used. It can also be used to filter the unabsorbed pump light.

The fiber varies in length from a few meters to several hundreds of meters. Important parametersof the fiber are: the core diameter, the Erbium-radius to core-radius ratio, the erbium concentra­tion and the numerical aperture (NA) or the refractive index difference Á.

The performance of the EDFA is astrong function of the pump band used [Pedersen] and onlythose for which high-power semiconductor laser diodes are available are practical. This restriets

the choice to 980 and 1480 om, where relatively expensive lasers can be obtained and 800 nm forwhich inexpensive laser diodes are available. Unfortunately, pumping in the 800 nm band leads to

relatively poor performance in silica fiber because of the strong ESA as mentioned in the previous

paragraph.

For combining the signal and pump beam we use a wavelength-division-multiplexer (WDM)

coupier. A WDM coupier can be made of fused optica! fibers and can be used for one specificpump band only. It should have a minimum fiber-to-fiber insertion loss for the signal wavelength

and a maximum coupling ratio for the pump wavelength.

2.3. Optimization of the amplifier

In the previous paragraph several criteria that influence the performance of the amplifier (with

respect to the different amplifier components) were already mentioned. The most important factorhowever is the Erbium-doped fiber (EDF) itself. The EDF should be optimized in order to:

• optimize gain• optimize coupling efficiency• minimize bending losses

2.3.1. Working conditions of fiber amplifiers

Before going into the details of EDF optimization we will, with reference to figure 2.6 [Povisen],discuss some general properties of the fiber amplifier. Looking at figure 2.6 we can see that the

operation of the EDFA cao be divided into three regions. In region 1, at smal1 input power levels,

the gain is independent of the signal input power. In this region of operation the quantum

10 OpticaI amplifier using an Erbium-doped fiber

efficiency-of tbe pump power ~ yer'j p()Of, witl1__pu~p photons mainly being converted to uselessphotons of backward and forward ASE.

For higher signal input powers (region 2 of figure 2.6) tbe signal gain curve approaches tbedeclining curve of 'ideal amplification' (defined by unity quantum efficiency of pump photons).

..............

10 010 -2

- 8 0 L..o............~.........~"'""-..................................................-....J.......................................""--"-........J

10 -10

signa I Input power (mW)

Figure 2.6 Gain and ASE to signa/ ratio as presented in {Povlsenj

A number of advantages in operating tbe amplifier in region 2 can be Jisted: a pump efficiencyclose to unity, signal output power higher tban power of tbe ASE, i. e., spectral filtering of tbeoutput signal becomes less important tban in region 1, decreasing gain for increasing signal input

power, i.e., gain control of cascade coupled amplifiers will also be less important tban in region1, and finally tbe noise figure of tbe fiber amplifier exhibits its minimum in region 2.

The extreme signal input power case is covered by region 3 where tbe power of tbe signa! hasreached such magnitudes tbat it dominates tbe population inversion a10ng tbe fiber by establishinga counter-balance between stimulated emission and absorption. Operation of tbe fiber amplifier in

tbis region cannot be recommended since tbe gain is weak, witb tbe random emission andabsorption processes along tbe fiber merely acting as noise sourees on tbe signal.

The shape of tbe gain curve is similar for arbitrary fiber designs (Note tbat tbe numerical values

in figure 2.6 don't correspond with those of tbe fiber we've used). The location of tbe intersectionbetween region 1 and region 2 may vary witb fiber design. In an optimization of tbe fiber design

tbe task is to move tbis intersection to an input power level as smalI as possible or equivalently to

maximize tbe small signal gain.

Optical amplifier using an Erbium-doped fiber 11

2.3.2. Optimization of an Erbium-doped fiber

As mentioned earlier, gain, coupling efficiency and bending losses should be considered. In[PovJsen) is shown that the small signal gain decreases with increasing threshold pump power (thepump power for which the EDFA starts to amplify). This means that gain optimization is indepen­dent of the actual pump power and can be performed by minimizing the threshold pump power.

The threshold pump power depends on the design parameters of the fiber and the absorption andemission cross-sections of the Erbium ions. The threshold pump power decreases with increasing

NA, decreasing v and decreasing ad (NA = numerical aperture, v = normalized frequency, ad =Erbium-dopant radius to core-radius ratio). However since the fiber gain saturates for smalIthreshold pump powers [Povisen), there is no advantage in decreasing the threshold pump powerbelow the point of saturation (which depends on the pump power).

Minimum coupling losses require a mode field parameter (MFP) WO as close as possibJe to that ofa standard fiber [Etten]. In order to minimize the bending Josses, the v number of the EDF shouldbe maximized. Furthermore the bending losses decrease when .1 (or NA) increases and thebending radius is constant.

This shows that there is a tradeoff between maximum gain (minimum v) and minimum couplingand bending losses (maximum v).

2.3.3. Optimum length of used Vork fiber

Because several parameters of the purchased fiber weren't known or didn't correspond with thespecifications (see chapter 4), it wasn't possible to calculate the optimum fiber length. An othermethod to establish the optimum fiber length, is measuring the gain for several fiber lengths.These measurements were aJready performed by drs. M. Babeliowsky of PTf Research. Thesemeasurements indicated the optimum fiber Jength to be approximately 1.9 meters.

12 Noise in Erbium-doped fiber amplifiers

3. Noise in Erbium-doped fiber amplifiers _.

The output of the optical fiber amplifier is a combination of the amplified signal and broadbandarnplified spontaneous emission (ASE). There will be shot noise associated with both components.At the detector additional noise terms wiJl be introduced by mixing at the detector of the arnplifiedsignal and the spectral components of the ASE, to give signal-spontaneous beat noise andspontaneous-spontaneous beat noise.

3.1. The noise components

For simplicity , we assume an optical amplifier with unit coupling efficiency, uniform gain G, over

an optical bandwidth Bo' and an input power of Ps at frequency Ws centered in the optical passband

Bo '

The electrical field of the optical signal at the output of the amplifier may be written as [Olsson]

Bo/21lv

e(t) = E.expj«,).l+~,J) + L E.JP~expj«w" +27tköv)t+~k)t~(-BoI2&y)

(3.1)

where Es and cl>s are the amplitude and the phase of the optica! signal, respectively. Esp.k and cl>k arethe amplitude and a random phase for each component of spontaneous emission. ö" is the distance

between the spectrallines of the spontaneous emission.

The photodiode current is given by

i(t) oe e(t) 'e(t)" (3.2)

The spontaneous emission power in the optical bandwidth Bo (i.e. for frequencies ws·Y1.Bo S w ~

ws+ InBo) is given by [Yariv, Olsson]:

(3.3)

where JLsp = N1,I(N1,-NI ) is the atomic inversion factor of the transition. It accounts for the largervalue of N2, and hence larger spontaneous emission power, in atomie (amplifier) systems in which

NI 7Jl!. O. h is Planek's constant and " is the optical frequency.

Noise in Erbium-doped fiber amplifiers 13

With E; ex GPz ' E;.A ex NJw (note that Esp,l isn't a function of k), M = Bj2ó/I and e."lh/l theresponsivity of the photodiode, equation (3.2) cao he written as

AI

+ 2JGPaN,,6v eT) :E cos(21tk6vt+~k-~.r)hv kc-AI (3.4)

JI M+ N(16v ell :E expj«<'>s+21tk6v)t+~I) • E exp-j«(,o).r+2'Rk6v)t+~k)

h~~ ~~

The three terms in (3.4) represent the signal, signal-spontaneous beat noise and spontaneous­

spontaneous beat noise, respectively. The power spectrum of is-sp(t) is uniform in the frequeney

interval 0 - Ih.Bo and has a density of:

eT'l)2 12JGP N 6v- '(2M+l)'-S (1 hv 2

1.B2 "

(3.5)

The third term in equation (3.4) can be rewritten as follows:

AI AI

isp_sp(t) = N,,6v e'l :E :E cos(21t(kJ)övt+~k-~J)hv l=-JI jc-M

The de term is obtained for k = j and there are 2M+ 1 such terms:

(3.6)

i dt =N av e'l (2M+l)sp " hv

(3.7)

If we organize the terms in (3.6) according to their frequencies, we can see that the terms with

same absolute frequency but of opposite sign add in phase. Therefore, the power spectrum of the

spontaneous-spontaneous beat noise extends from 0 to Bo with a triangular shape and a power

density near de of

(3.8)

14 Noise in Erbium-doped fiber amplifiers

power spectral denslty

53. (3)

(2)52 f------i--------i'

( 1)S1f--------c-----j------=::,'""'c--.:....--

o Be

B ---7wo

Figure 3. J Noise power spectral densities at the receiver(1) shot noise (2) s-sp beat noise (3) sp-sp beat noise

51, 52 and S3 represent the folIowing values:

2SI = 2 e Tl [GP + P ]

hv s sp

82 = 4( ~~rGPsNo

S3 = 2( eT) )2N2 Bhv 0 D

(3.9)

Note that we didn't take into account the random polarization of the spontanec)Us emission. 50percent of the spontaneous emission has a state of polarization that is orthogonal to the state ofpolarization of the signal. This would result in a reduction of the signal-spontaneous beat noiseand the spontaneous-spontaneous beat noise by a factor of 2.

Noise in Erbium-doped fiber amplifiers 15

3.2. Noise figure

Consider an optical in-Iine amplifier:

Gq

Figure 3.2 Optical amplifier with a power gain G

If we were to detect the signal at the input of the amplifier, the main noise contribution would, inan ideal case (a noiseless receiver), be that of the signal shot noise, sa that the signal-to-noisepower ratio (SNR) at the input to the amplifier is

( 'i..

( P6e" y , "Î'

SNR. = JIL = P6" l/ .-) (3.10)

'" P e" 2hvB J2e-6-B e, hv . e

J ""- j/ ',:/,_ /; Ij

where B, is the electrical bandwidth.

The noise power at the output is that of amplified signal shot noise, shot noise due to ASE, signal­spontaneous beat noise and spontaneous-spontaneous beat noise. The SNR at the output of theamplifier is thus [Olsson, Ramaswami]

(3.11)

The spontaneous-spontaneous beat noise is proportional to P~ and can be made negJigible to thesignal-spontaneous beat noise if the signal power P.(z) is not allowed to drop toa far and/or by

optical filtering. We can also neglect the shot noise due to ASE. For large gain G » 1, thesecond term in the denominator of (3.11) dominates and

(3.12)

16 Noise in Erbium-doped fiber amplifiers

11le noise figure NP,· oeing the ratio of the· input SNR to. the output valuez is tllus

(3.13)

which in an ideal three-level amplifier (NI = 0, #Lsp = 1) is equal to 2. The single high-gainoptical amplifier will thus degrade the SNR of the detected signal by a factor of 2 (3 dB).

If we consider the case where the amplifier is used as a preamplifier, the signal power usually isvery low and the third term in the denominator of (3.11) will dominate:

NF for P < .!hv (B -.!B )J:1 D:1 t

(3.14)

We can see that for very low signal powers the NF is inversely proportional to the signal power,

which means an increase of NF for decreasing signal powers. In figure 3.3 NF is shown as afunction of the signal power, for optical bandwidths Bo of 10 GHz, 1 THz and 100 THz,respectively.

20

~

.'" rI~ ,...

1-

m ~"Cl

<l!"-

10::lg'

<l!Cl]0Z

51r

J-70

B=100THZo

B= 10GHzo

, I ,~ , , :, , , , , , ,: , , I-60 -50 -40 -30 -20 -10 0

Signal power (dBm)

Figure 3.3 Noise figure as a junction of the signal power

In figure 3.3 the following values are assumed: gain G = 30 dB, 7/ = 1, #Lsp = 1, À = 1550 omand B. = 5 GHz. The figure shows that optical filtering improves the noise figure for low signalpowers considerably.

Noise in Erbium-doped fiber amplifiers 17

If more than one amplifier is used, noise will accumulate. We can rewrite (3.12) as:

(3.15)

For a cascade of n amplifiers the SNR at the output of the nth amplifier is

(3.16)

1 2hVB,(sm = ----p- 2J.Ll

tHII 6(3.17)

Using equation (3.13), the overall noise figure NFe is given by

NFe = 2f.ll +2J.L2

+ ... + 2f.l"G1 G1GZ,,·G"_1

(3.18)

:::: NP +NFz + '" +

NP,.1

Gel Gen

where Gei is the cumulative gain of the cascade up to the input of the ith amplifier and NFj the

noise figure of the ith amplifier. This formula differs from the Friis' formula, because the noise

figure of an EDFA is defined as the output signal-to-noise ratio compared to the shot noise

limitted SNR, which does not influence the ASE power.

'8 Comparison of optical amplified IM/DD systems and coherent optical systems

.4.. Comparison of. ()ptic:a.1 .amplified IM/DD systems andcoherent optical systems . . .. - ..

In very long fiber links, periodical amplification of the signal will be required. In this chapter we

will discuss the effects of ASE on a cascade of amplifiers and furthermore we will compareseveral system configurations.

4.1. The effects of ASE on 8 cascade of amplifiers

A generalization of expression (3.11) for the SNR of the detected signa} at an arbitrary point z

along the link is to write

SNR(z)(P.~~.~r

+ 4(e'l)2P (z)P (z) +hv " sp

(4.1)

where the last term in the denominator represents the mean-squared thermal noise of the receiver

(at point z) whose effective noise temperature is T,. R is the output impedance of the detector

including the receiver's input impedance. Equation (4.1) neglects the shot noise due to the ASE,

the spontaneous-spontaneous beat noise and intensity fluctuation noise of the souree laser. If the

signa} power P,(z) can be maintained above a certain level by repeated amplification, we can

neglect the receiver noise term. The SNR expression becomes:

SNR(z) (4.2)

where a 100 percent detector quantum efficiency (71 = 1) is assumed. Piz) is the signal power at

z, while P.,,(z) is the total ASE power at Z originating in all the preceding amplifiers (z' <z).

Let us consider the scenario of a long fiber with amplifiers employed serially at fixed and equal

intervaIs (Zo), as illustrated in figure 4.}.

Comparison of opticaI amplified IM/DD systems and coherent optical systems 19

PSP Cz)

P,;,o L ~,o ~,oL Ps,O ~,oL

G Jt1G Jt2 G

0 PsP,o Psp,oL 2Psp,o

+ ~

Zo

nPsP,O

Figure 4.1 A fiber link with periodie amplification

The signal power level Ps(z) at the fiber input and at the output of each amplifier is ps.D' The

signal is attenuated by a factor of L = exp(-azo) over the distance Zo between amplifiers and is

boosted back up by the gain G = V = exp(azo) by each amplifier to the initial level ps•D' The

spontaneous emission power Psp(z) is attenuated by a factor L between two neighboring amplifiers

and increases by an increment of Psp.D at the output of each amplifier. Using equation (3.3) and

assuming G » 1, we can write the SNR of the detected current at the output of the nth amplifier

as

SNR == PlI,o" 2[1 + 21l1ol..,,(exp(<<Zo) - l)]hvBe

where, because of the high signal and ASE levels, we neglected the thermal receiver noise.

(4.3)

Equation (4.3) suggests that the SNR at z can be improved by reducing Zo, i.e., by using smaller

intervals between amplifiers which entails reducing the gain G = exp(azo) of each amplifier. If we

take the limit of equation (4.3) as Zo -+ 0, the whole length of the fiber acts as a distributed

amplifier, with a gain constant g = ex just enough to maintain the signal at a constant value. The

SNR at z for a distributed amplifier is thus

SNR(z) (4.4)

We can compare the (ideaI) distributed amplifier with the discrete amplifier case of equation (4.3),

the case where only one preamplifier is used and the case where no amplifier is used.

20 Comparison of optical amplified IMIDD systems and coherent optical systems

Discreteamplifià-case:

p.,oSNR(z) =-------'~----­

2 [l + 2(z./Zo)~.1Jl(exp(uZo) - l)]hv B"

where we used n = z/Zo.

One preamplifier:

p.,oSNR(z) :; ----~~----

2[1 + 2~.1Jl(exp(uz) - l)]hvBt

where a gain G = exp(az) is assumed (compensation for fiber toss).

No amplifiers:

(P,.oexp( -uz)iSNR(z) = -------==-------

4kTRtB

.r ( he\l )22P"oexp(-uz)hvBt

+ -----=----=.

(4.5)

(4.6)

(4.7)

In a low-Ioss optical fiber, say with adB = 0.2 dB/km, the distanee between amplifiers that areplaced Q'l km apart would be 1/ln(10002) = 21.7 km.

80

60

,---, 40

rnD'--' 20rrZUJ 0

-20

-400 50 100

(dl

150 200 250 300

Fiber lengt h (km)

Figure 4.2 SNR's oJdirect detection systems

(a) continuous amplification (b) discrete amplifiers spaeed by Zo = Q'l

(e) one preamplifier (d) no amplification

Comparison of optical amplified IM/DD svstems and coherent opticaI svstems 21

The launched power ps•o is 5 mW, À = 1550 Dm, B. = 109 Hz, and adB = 0.2 dB/km. Curve (b)

is to be read only at multiples of z = a'l = 21.7 km, which are tbe output planes of tbe optica!

amplifiers. Curve (c) assumes a gain G = exp(az) , and curve (d) assumes detection with a

receiver witb T. = 725 K (F = 4 dB) and an input impedance of 1000 O.

Note that if, for example, we need to maintain a SNR exceeding 50 dB, we must use a fiber linkshorter tban 100 km if no amplifier is used, but if optica! amplifiers are used every Zo = a'l

(=21.7 km), fiber lengths in excess of 1000 km can be employed. The advantage of continuous

amplification compared to amplification every cll is seen to be less tban 2.5 dB so tbat the latter

may be taken as a practical optimum. We can also see tbat for short fiber link lengths (z < 40

km) amplification has no advantages.

4.2. Optical amplified coherent detection systems

The signal-to-noise ratio of a coherent detection scheme can be written as [Etten]

SNR(z) (4.8)

where Plo is tbe local oscillator power and Àb and Àd are tbe intensities of tbe Poisson processes of

the background radiation and the dark current.

If we neglect the noise due to tbe background radiation and the dark current and assume that11 = 1, tbe SNR for coherent systems without amplifiers cao be written as

hvB~ +

P,(Z)SNR(z) ::; --------

_1_ . 4JcT~B~(hV)22Pw R e

(4.9)

In practical situations, tbe local oscillator power Plo is much larger tban the signal power Ps(z),when the signa! has traversed a long length of fiber.

If we consider again tbe scenario of a long fiber with amplifiers employed serially at fixed and

equal intervals, as iIlustrated in figure 4.1, we cao write tbe SNR of tbe detected cureent at the

output of tbe nth amplifier as [Olsson]

22 Comparison of opticaI amplified IM/DD systems and coherent optical systems

(e" y2P1•oP",. hvrSNR" = ---------~..:........-~------

e" e" ~~~2eP -B + 4PID-n~ (G - l)eB +--liJ hv e hv sp e R

where the second term in the denominator stands for the local-spontaneous beat noise.

(4.10)

If we take 7J = I, G = exp(azo), n = z/~ and if we neglect the thermal noise, (4.10) can bewritten as

p.. oSNR(z) = --------..::.:.:'-----­

[1 + 2(~zo>~.(exp(ClZO) - l)]hvBe

For the case of a distributed amplifier the SNR becomes

P"oSNR(z) = -----"'~--[1 + 21-1.rpClZ]hv Be

(4.11)

(4.12)

80

60

40(IJU'--J 20CCZlf) 0

-20

-400 50 100

(a)

(b)

(c)

150 200 250 300

Fiber length(km)

Figure 4.3 SNR 's of coherent detection systems(a) distributed amplifier (b) discrete amplifiers (e) no amplification

The launched power ps•o is 5 rnW, the local oscillator power P/b is 5 mW, À = 1550 nm, B. =109 Hz, and ~cIB = 0.2 dB/km. Curve (b) is to be read only at multiples of z = a'l = 21.7 km,

which are the output planes of the optical amplifiers. Curve (c) assumes detection with a receiver

with T. = 725 K (NF = 4 dB) and an input impedance of 1000 D.

Comparison of optical amplified IM/OO systems and coherent optical systems 23

4.3. Comparison of direct and coherent detection systems

Note that equations (4.9), (4.11) and (4.12) differ from equations (4.7), (4.5) and (4.4),respectively, with a factor of 2 (3 dB). This means that , under ideal and equal conditions, theSNR of a coherent detection method is 3 dB Jarger than the SNR of a direct detection method.

However, in practice the SNR of an unamplified coherent detection system is 10-15 dB larger thatthe SNR of an unamplified IMIDD system (figure 4.4). With the use of optical amplifiers withIM/DD systems it is now possible to approach the SNR of a coherent detection system (there arereports of differences within 2 dB [Park]).

90 r--------------------,

80

---.J« 70'-----'

0::ZrJ)

60

-- ~ ..";:-'.

(a)

-- direct detect Ion- coherent detect Ion

(b)

(c)

(c) -~---------:~_-===:j

20015010050

50 L..--'-'-----'~~_____'___'_~~---'---'-~.........L___'____'>_~~~_____.L_~~_'_____"_'~-'--"

o

Fiber length (km)

Figure 4.4 Practical SNR 's ofboth direct and coherent detection systems(a) no amplification (b) distributed amplifier (c) discrete amplifiers

Coherent detection methods have advantages over IM/DD systems in terms of higher selectivity(channel selection by means of tuning the local oscillator laser) and smaller channel spacings.IM/DD systems are on the other hand cheaper to realize and are less complex.

24 Measurements on the amplifier

5 . Measurements on the -amplifier _

In preparation to the measurements the 1480 nm pump laser (datasheet in Appendix A) is built ina housing. The pump laser is tberefore equipped with an adjustable current source and atemperature controller (Appendix D). The 1485/1555 nm WDM coupler manufactured at thefaculty (datasheet in Appendix B) is also built in that housing. In order to be able to present the

measured data in a report, a Pascal program is written that transfers the data from the Optica!

Spectrum Analyzer (via an IEEE bus) to a personal computer, where it can be written to disk or

plotted on paper (Appendix E).

The measurements that were performed on the amplifier are the absorption and fluorescencespectra, the amplification and the noise figure.

5.1. Absorption spectrum

The absorption spectrum indicates which wavelengths can be used to pump the EDFA. However,one must keep in mind that several pump bands suffer from ESA (figure 2.4), which will degrade

the EDFA performance severely when using these bands. The absorption spectrum is measuredusing the setup in figure 5.1 :

Optlcal Spectrum

, 0 j~::~:.I"I~h?lPatch ...Cord .:

White Light Souree

iVIQCord

(a)

Erblum-Ooped Optlcal SpectrumFiber Ana Iyzer

I (() I Q lo.s.A.I"I~FiPateh. . ...Cord

(b)

Figure 5.1 Absorption spectrum measurement setup

The Optical Spectrum Analyzer we used is the ANDO AQ-631OB.

The spectrum measured with setup (a) is subtracted from the spectrum measured with setup (b),

resulting in the absorption spectrum in figure 5.2.

Measurements on the amplifier 25

25settings OptICai Spectrum Anallzer

Reference Level ·49d8mCenter Wavelength 9001 I100 rvn

20 5weep Wldth 100 rvnId>vResolutlon SM\Average Times 50

~

COU 15'--'cQ+-'Q~

0 10U1

..0<!:

5

0~~---'--'---l..-J..-'--'--"--,--'-'---'-'-~'--'--'c...L.J~---l..-J..--'---'-~-"--,--"--,--'-'---'-'--C-..J-~c...L.J-'--'---'--'---l..-J..--"--L..J

500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600

Wavelength (nm)

Figure 5.2 Absorption spectrum of1.9 m EDF

Additional measurements, performed with anoptical power meter, are:

• Coupling loss EDF to standard SMF forÀ = 1480 run: 1.7 dBÀ = 1538.6 nm: 1.7 dBÀ = 1551.8 nm: 1.7 dB

The values given in figures 5.2 and 5.3 include2 x 1.7 dB = 3.4 dB coupling losses.

20

m 152-cQ+--'Cl.(;

13 10«

~fefef\Ce Le....elCenter W8!velengtns...~WIOth~Iu~l()n

A...er-eoe Tmes

-<9 d8m1500 rrn2Orrnldl'1Q "'"50

The measurements show that the following pumpbands exist:

1600155015001450

o '-'--~~....L-..~~~_~~~-L...o~~-..J

1400

Wavelength(nm)

Figure 5.3 Absorption spectrum of 1.9 m EDFaround 1500 nm

• 515-545 nm 1

• 645-665 run 1

• 795-815 run 1

• 970-990 nm• 1480-1560 nm

1 These pump bands suffer from ESA (according to figure 2.4)

26 Measurements on the amplifier

5.2. Fluorescenét:fspe-ctrum

The fluorescence spectrum indicates which wavelengths can he amplified using the EDFA. The

spectrum is measured for both the co-propagating pump and the counter-propagating pump and asa function of the pump power coupled into the EDF (5.3, 10.6, 15.9, 21.2 and 28.1 mW,

respectively).

148D nm Erblum·Doped Optlcal SpectrumPUT1l) Laser Fiber Analyzer

I *' I f2xc~u~~re-----'-9~tch--+--1O~I--,--,2,---;,jO.SoA·ln••aCord Cord Cord f

(a)

1480 nm

Pump Laser

I ~LtJ P",hCord

"lhJl.O'!o.s.A.I~~ I I PatenCord

Optlcal Spectrum

WOMCoupier

Erblum·DopedFiber

Patch

Cord

Ana Iyzer(b)

Figure 5.4 F/uorescence spectrum measurement setup(a) co-propagating pump (b) counrer-propagating pump

Additional measurements, performed with an optical power meter, are:

• Attenuation WDM coupier for À = 1480 nm:À = 1538.6 nm:À = 1551.8 nm:

1.6 dB

2.1 dB1.6 dB

Figures 5.5 and 5.6 show that, for both the co-pTopagating and the counter-propagating pump,

signals with wavelengths of approximately 1536 nm and 1552 nm can be amplified. The

differences between the two spectra are due to the wavelength dependenee of the WDM coupleT

(Appendix B). When the spectra in figure 5.6 would be corrected fOT 0 dB attenuation of allwavelengths, the spectra would look similar to those in figure 5.5.

Measurements on the amplifier 27

Sêt tlr'qS (:PtlC.1 Spêctrum Analy'....

150

Ref~encl? levelCent.... W.velerqthSweepW,dthQ.esolut IonAverage TnT1ê!;

-40d~

1525 rrn20 rmldrv1rrn5

160015751550152515001475o1450

3' ~êl

C 100'-'...(l)

3: 281mW00- 212mW

<0 159mW\,!

50o-J l06mWG-O 53mW

Wavelength (nm)

Figure 5.5 F/uorescence spectra of 1.9 m EDFfor co-propagating pump

Settings OWcal Speel'''''' Analyzer

7S

Refererw:::e LevelCenter Wa ...~iengl hSWe€P WodlnResolu!rOnAverage TImes

-~J d&n1525 nrn20 nmld'V1 nm5

~ 50~ef

c'-'...(l)

:;;:00-

IO~ 25

+--'Q.0

o1450 1475 1500 1525 1550 1575 1600

Wavelength (nm)

Figure 5.6 F/uorescence spectra of 1.9 m EDF for counter-propagating pump

(28 Measurements on the amplifier

"-

5.3. Amplification

The amplification of the Erbium-doped fiber is measured as a function of the pump power and forboth the co-propagating and the counter-propagating configuration. The measurement setup isshown in figure 5.7.

1480 nm

Pump Laser

['tIQCord

~15386 I 15518 nm Cord

Signa: Laser (OrB)

WDMCoupier

Erblum-Ooped OptICai Spectrum

Fiber Analyzer

0 2,3({]) 4, 50 5lO.SoAolPatch PatchCord Cord

Ca)

1480 nm

Pump Laser

\~~6O.S.A.

Erbium· OopedFiber

[IlQ2,3~Cora Cord

15385/155; 8 nm

Signa l Laser (DFB)

WDM

CouPIer [21*1Cord

(b)

Patch

CordOptlcal Spectrum

Analyzer

Figure 5.7 Amplification measurement setup (a) co-propagating configuration(b) counter-propagating configuration

Additional rneasurements, performed with an optica! power meter, are:

• Signal power for À = 1538.6 nrn: 0.12 mW (-9.1 dBm)À = 1551.8 nrn: 66.0 IlW (-11.8 dBrn)

• Attenuation 1.9 m EDF, excluding 3.4 dB coupling losses:À = 1480 nm: 5.1 dB (2.7 dB/m)

À = 1538.6 nm: 29.6 dB (15.6 dB/m)À = 1551.8 nm: 16.9 dB (8.9 dB/m)

The signa! powers coupled into the EDF at 1538.6 and 1551.8 nrn are, in case of the co-propagat­ing setup 52.5 IlW (-12.8 dBm) and 30.9 JlW (-15.1 dBm), respectively, and in case of thecounter-propagating setup 83.2 JlW (-10.8 dBm) and 44.7 IlW (-13.5 dBm), respectively. In bothcases the pump signal is attenuated 1.6 + 1.7 = 3.3 dB before entering the EDF.

Measurements on the amplifier 29

-.......- co-gropalj;Jlt'r'lgPUflP --.-. counter~rOP6Q8tngp~

302520

Sq1Il W....lIr'vlh m' 8 rII'lSlQNIPc... 660"W(·"'&cl8'I)

5fltrlg5 C»taI5llec1rll!'l"rtllf2'J

Atr.en:::e l l ·2l dQI'le:-nt .. * ,...tfl t!I!l' 8..."S....,Wdth 02,....~"RnolwloOn 01 m........,,'"'" ~

15105

11

/

",ff

ff

1f

.;

Of-------...-L-+=-------------i

- 20 ...........~...........~............~--.J..~~ ...........~-'-'-~-'--"

o

-15

cQ -5

<1>Z

a.Ecoà; -10-Q

302520

~11~C»II(.t~'''"...,.~

~ferPl'a'lMI ·Ucl'8'r'lcent l'lQttl 'SlB 6 lWl

S....,WdU''1 OZ"""'d...A6olUtoon 01rrn~......~ T"," 5

1510

//

5

5,---------------,

01--------_------.....,

-5éD:sc -10Q~

co~

":::: -150-Ecoà; -20-Q"";-a;z -25

,-30 ,

-350

Pump power In EDF (mW) Pump power In EDF (mW)

Figure 5.8 Net-fiber amplification 1.9 m Erbium-dopedjiber

jor Às = 1538.6 nm (left) and Às = 1551.8 nm (right)

We have also measured the amplification for signal powers < -40 dBm. These results were evenworse than the results shown above and are therefore omitted. From figure 5.8 we can determine

the following figures (the numbers between brackets correspond with the numbers in figure 5.7):

Table 5.1 EDFA amplification measurement results

Ico-propagating pump counter-propagating pump

1538.6 nm 1551.8 nm 1538.6 nm 1551.8 run

Max. net-fiber amplif. ()-004) 1.8 dB 3.3 dB 2.2 dB 4.3 dB

Max. fiber-to-fiber amplif. (2~S) -1.6 dB -0.1 dB -1.2 dB 0.9 dB

Max. EDFA amplif. (I~) -3.7 dB -1.7 dB -3.3 dB -0.7 dB

Pump influence • 36.0 dB 23.9 dB 34.4 dB 24.4 dB

Threshold pump power 17 mW 12 mW 17 mW 9mW

• (Amplification with Ppwnp = max) - (Amplification with Ppwnp = 0)

30 Measurements on the amplifier

. Additional measurementLwerep~rt:.ormed,with a HP 81?54SM laser souree (1310/1550 nm), tomeasure the wavelength dependence of the amplification around i5'36 -rim.These rneasureinents·should indicate a similar shape of the amplification to that of the fluorescence spectrum and thus amaximum at 1536 nm. However, the measurements showed that the maximum amplificationoccurred at a wavelength of approximately 1538 nm. We also measured that the attenuation of theEDF has its maximum at 1535 nm (tabie 5.2).

Table 5.2 WaveJength dependenee of ampJification of 1.9 m EDF around 1536 nm

Peak level (dBm) EDF Fiber-to-fiberWavelength attenuation amplification

(om) after WDM after EDF after EDF (dB) (dB)coupier (Pp = 0) (Pp = max)

1531.7 -28.5 -59.9 -30.4 31.4 -1.9

1532.8 -25.1 -57.9 -25.6 32.8 -0.5

1533.9 -22.1 -61.2 -23.9 39.1 -1.8

1535.1 -22.8 -63.1 -22.3 40.3 0.5

1536.3 -19.4 -59.0 -18.5 39.6 0.9

1537.4 -19.5 -52.0 -17.3 32.5 2.2

1538.6 -23.0 -46.1 -20.2 23.1 2.8

1539.7 -20.6 -38.8 -19.3 18.2 1.3

1540.9 -17.8 -33.0 -15.4 15.2 2.4

Note that the different spectral lines of the laser all have different peak levels. For a moreaccurate measurement (canceling out the signal power dependence of the amplifier), the power ofthe laser souree should be adjusted for eaeh spectral lioe in order to measure with the same peaklevel for all spectrallines. However, measurement with a tunable DFB laser source is preferred.

Measurements on the amplifier 31

5.4. Noise figure

The noise figure is measured witb tbe setup shown in figure 5.9.

1480 nm

Pump Laser

15386 nm

5D MHz Modulated

Signal Laser (DFB)

I~ I f2Cord

~lJL~ Patch

Cord

WDM

Coupier

Erb lum- Doped Electrlca/Spectn.rnFiber Analyzer

0 , ({))I 0 1eoSoAo I

Patch Patch

Cord Cord

(a)

1538 6 nm

Erblum-Doped

Fiber

[jJ,~Q---+----'({))~,,4Cord Cord

50 MHz Modulated

Signa 1Laser (DFB)

(b)

WQM

Coupier

148D nm

Pump Laser

f21~1Cord

~le.S.A·1Patch

CordElectrlcal Spectrum

Analyzer

Figure 5.9 Noise figure measurement setup(a) co-propagating pump (b) counter-propagaring pump

The Electrical Spectrum Analyzer we used is the Hewlett Packard HP70000 System with theHP70810A Lightwave Section with optical input and the HP70206A System Graphics Display.

Tbe 1538.6 om signal laser was modulated with a 50 MHz rectangular wave (Ps = lOJAW). Thesignal level measured at 50 MHz, without tbe amplifier, was -55.8 dBmlHz. Tbe noise level ofthe signal was below the noise level of tbe Spectrum Analyzer (-71.3 dBmlHz). Therefore tbeSNR of the signal without amplification was measured to be ~ 15.5 dB. Tbe SNR of tbe signalwithout the amplifier was calculated, with equation (3.10), to be ~ 130 dB (shot-noise limit).

Tbe SNR of the signal after amplification with co-propagating pump (Pf'II'V' max.) was measured tobe 0.7 dB. Witb counter-propagating pump (Pf'II'V' max.) the SNR was measured to be 2.6 dB.

32 Measurements on the amplifier

This mêaJlS that the -noise figure NF- for the amplifie~ with c.o-propagatinRPump is ~~.8 dB ~

NF,o ~ 129.3 dB and with counter-propagating pump 12.9 dB ~ NFCOIWtr ~ 127.4 dB.

The SNRs after amplification are very low because the signal is attenuated and a lot of noise isintroduced by the fluorescence of the fiber and the shot noise of the pump laser. Optical filteringof the pump signal should therefore increase the SNRs.

5.5. Conclusions

The measurements have shown that the EDF didn't meet it's specifications (Appendix C). Apartfrom the low amplification and high absorption, the optimum fiber length is much shorter than isgiven by the manufacturer, which indicates that the Erbium concentration might be too high.The 1.7 dB coupJing losses, however, are due to the optimization of the EDF for coupJing withdispersion shifted tibers, which have a larger numerical aperture NA than standard single mode

fibers.

The measur~ments indicate that the bands around 530, 660, 800, 980 and 1485 run can be used aspump bands. Furthermore, signals with wavelengths of approximately 1536 and 1552 run can beamplified. The maximum net-fiber amplitication measured with our EDF is 4.3 dB. For thecomplete EDFA amplification this results in an "amplitication" of -0.7 dB. The high noise figureis a result of the attenuation of the signal and the addition of noise by the amplifier.

Judging the amplification and the noise figure, we can see that the counter-propagating setup isslightly in favor of the co-propagating setup. This might he a result of the decline in performanceof the EDFA for lower signaI powers. In case of the counter-propagating setup the pump powercoupled into the EDF is higher than in case of the co-propagating setup.

The measurements aIso show that higher pump powers will not result in a higher ampJificationbecause the amplifier is already in saturation for the maximum pump power (28 mW in EDF).The ampJification of this fiber may on the other hand be increased when a 980 run pump laser isused. As mentioned in chapter 2, at 980 nm only absorption occurs, which will enlarge thepopulation inversion and thus the arnpJification. The use of the 980 nm pump hand also makes iteasier to filter the pump signal from the output of the amplifier, what will reduce the noise at thedetector amd therefore improve the NF.

Furthermore I would Jike 10 point out that there are some variations in the measurement results ofthe absorption and ampJification of the EDF. These are due to different settings of the opticalspectrum analyzer (Resolution and Average Times) and due to the polarization dependence of the

O.S.A ..

Conclusions and recommendations 33

6. Conclusions and recommendations

Research in Erbium-doped amplifiers has indicated that pump wavelengths of 980 and 1480 omcan be used to amplify signaIs with wavelengths of about 1550 om. An EDFA mainly consists ofan EDF, an optical coupIer and a pump light source. An optical isolator and an optical filter can

be used to suppress reflections and reduce the noise generated by the EDFA, respectively.Optimization of the EDFA involves optimizing gain, coupling efficiency and minimizing bending

losses.

Noise in EDFAs consists of shot noise due to the signal and spontaneous emlSSlon, signal­spontaneous beat noise and spontaneous-spontaneous beat noise. For high signal powers and highgain the signal-spontaneous beat noise dominates, and for an ideal three-level amplifier the noisefigure is 3 dB. For lower signal powers the spontaneous-spontaneous beat noise dominates and thenoise figure is inversely proportional to the signal power. When amplifiers are used in cascadenoise will accumulate.

Comparison of optical amplified IM/DD systems with coherent detection systems has shown,

under ideal and equal conditions, a mere 3 dB SNR advantage of coherent systems over IM/DDsystems. This means that, through the introduction of EDFAs, the performance of IM/DD systems

can approach the performance of coherent systems. The choice between the two systems now onlyinvolves selectivity, channel spacing, costs and complexity of the receivers.

Measurements on the Vork EDF, using the 1480 nm pump wavelength, have shown that the fiberdidn't meet it's specifications. The optimum fiber length is much shorter than specified, theabsorption is higher and the amplification is lower than given by the manufacturer, which resultsin a high noise figure. Pumping of the fiber does, however, decrease the attenuation of the EDFwith approximately 30 dB.

A recommendation for further research is the use of a 980 om pump laser instead of the 1480 nm

pump laser. Because in this pump band only absorption occurs, this might result in a higher

population inversion and thus a higher amplification. The use of the 980 om pump band also

makes it easier to filter the pump signal from the output of the amplifier, what will reduce thenoise at the detector and therefore imprave the NF. The loss introduced by the WDM coupIer

could also be reduced. However, the best recommendation we can give is to purchase a newErbium-doped fiber, preferably one that is optimized for use with standard SMF.

34 References

References

[Wintjes]

[Pedersen]

[PovIsen]

[Etten]

[Olsson]

[Yariv]

Wintjes, RJ.W.

1he Erbium-doped Fiber Amplifier, Operation And ApplicationIVO-Report, Eindhoven: Instituut Vervolgopleiding, Technische UniversiteitEindhoven, 1991.

Pedersen, B. et al.

"Power Requirements for Erbium-Doped Fiber Amplifiers Pumped in the 800,980, and 1480 run Bands"

IEEE Phor. Technol. Lelt., Vol. 4, No. 1, pp. 46-49, 1992.

Povlsen, J.H. et al."Optimum Design of Erbium Fibre Amplifiers Pumped with Sourees Emitting at

1480 nm"

Electron. Lelt., Vol. 26, No. 17, pp. 1419-1421, 1990.

Etten, W. van and Plaats, J. van der

Fundamentals of Optical Fiber CommunicationsNew York: Prentice Hall International, 1991.

Olsson, N.A.

"Lightwave Systems With Optical Amplifiers"

J. Lighrwave Techno!., Vol. 7, No. 7, pp. 1071-1082,1989.

Yariv, Amnon

Optical Electronics, 4th ed.Philadelphia: Saunders College Publishing, 1991.

[Ramaswami] Ramaswami, R. and Humbiet, P.A.

"Amplifier Induced Crosstalk in Multichannel Optical Networks"J. Lighrwave Technol., Vol. 8, No. 12, pp. 1882-1896, 1990.

[Park] Park, Y.K. et al.

"Long Distanee Transmission with Erbium-Doped Fiber Amplifiers in Direct and

Coherent Detection Systems"

Invited paper ECOC-IOOC '91, pp. 86-96, 1991.

List of abbreviations

List of abbreviations 35

ASEDDE.S.A.EDF

EDFA

ESA

FMIMMFP

NANFO.S.A.PM

SMF

SNRWDM

Amplified spontaneous emissionDirect detection

Electrical spectrum analyzer

Erbium-doped fiber

Erbium-doped fiber amplifierExcited state absorption

Frequency modulationIntensity modulationMode field parameterNumerical apertureNoise figureOptical spectrum analyzerPhase modulationSingle mode fiber

SignaI-to-noise ratioWavelength division multiplexer

36 list of constants and variables

List of·constants and. variables

Constants:

cehk

Variables:

adA

B

B.Bo

eet)

El, E2 , E3

E.

Esp,k

gG

Ge

Geiiet)i.(t).,k

lsp

ir-sp(t)

i~sp(t)

LMNI> N2, N3

NF

NFeNFco

NFcotlnl<r

NF;

NoNr-sp

N~sp

N/()/

Plo

Speed of light in vacuum = 2.99792 H)'l [mIs]Electron charge = 1.60210 10-19 [C]

Planck's constant = 6.62559 10-34 [Is]Boltzmann's constant = 1.38 10-23 [J/s]

Erbium-dopant radius to core-radius ratioEinstein's coefficient for spontaneous emission

Einstein's coefficient for stimulated emissionElectrical bandwidth

Optical bandwidthElectrical field of the optical signal

Energy statesAmplitude of the optical signal

Amplitude of the kth component of the spontaneous emissionGain constant

GainCumulative gain

Cumulative gain of the amplifier cascade up to the input of the ith amplifierPhotodiode current

Photodiode current due to signalDc-term of the photodiode current due to the spontaneous emission

Photodiode current due to signal-spontaneous beat noisePhotodiode current due to spontaneous-spontaneous beat noise

Fiber loss

2M+ 1 = Number of spectral Hnes of the ASE

Number of electrons in the energy state EI, E2, E), respectivelyNoise figure

Cumulative noise figureNoise figure for co-propagating configuration

Noise figure for counter-propagating configurationNoise figure of the ith amplifierSpontaneous emission noise power spectral density

Signal-spontaneous beat noise power spectral density

Spontaneous-spontaneous beat noise power spectral density

Total Erbium density

Local oscillator power

Ppumpp.p.,oPsp

RT.v

al!ll

ó"A

71

À

Àb

Àd

JL, JLspJI

T

cJ>1

cJ>s<Al

list of constants and variables 37

Pump power

SignaJ powerLaunched signaJ power

Spontaneous emission powerOutput impedanee of detector including the receiver's input impedaneeEffective noise temperatureNormaJized frequencyMode field parameterSigna! power density

Pump power densityDistance in fiber link

Distance between two amplifiersAttenuation factor

Attenuation factor in dBDistance between the spectra! components of spontaneous emission

Refractive index differenceCoupling efficiencyWavelengthIntensity of the Poisson process of the background radiation of a photodiodeIntensity of the Poisson process of the dark current of a photodiodeAtomie inversion factorOptical frequency

Lifetime of energy state

Random phase of the kth component of the spontaneous emissionPhase of optical signal

FrequencySignal frequency

1 2 3 4 5 {;

0 o 0

°10 0

,I

1000 Q1 014 13 12 R

BOTTOMVIEW

38 Appendix A: Specifications of the 1480 nm pump laser

.. -Appendix A: Specification$ .9fJ.he 1480 ':lm pump laser

A.1 About the laser

The high-output InGaAsP/InP laser diode has the double-channel, planar, buried-heterostructure(DC-PBH) and emits at a wavelength of 1480 nm. The wavelength is suited for pumping ofErbium~opedoptical fiber amplifiers. The laserdiode is mounted in a hermetically sealed metallicDIL14 package. The front facet of the laser is coupled to a 9/125 JLm single mode fiber pigtail.An InGaAs monitor diode is coupled to the rear facet for monitoring the optical output power. Inorder to achieve a case temperature independent performance, a therrnoelectric cooler and atemperature sensor are incorporated in the package.

In figure A.l the pinning of the 1480 nm pump laser is shown.

1 Cooler anode2 NC) NC4 NC5 L0 anode, therm Istor,

caseGND6 Ne7 PDcathodeB PO anode9 LOcathode

10 LOanode, thermistor.case GND

11 thermistor12 NC13 NC14 Cooler cathoae

Ne =Not Connected

Figure A.1 Pinning ofthe 1480 nm pump laser

Appendix A: Specifications of the 1480 nm pump laser 39

A.2 Characteristics

This data is provided by the manufacturer and is measured at Tcaae = 25°C, Ir = 500mA and~rmiolor = IOkO, unless otherwise specified.

Laser diodeRadiant output power from fiber

Threshold current

Forward voltage dropCentral wavelength

MonitordiodeDark reverse current

(Vmr = 10 V)Monitordiode currentCapacitance at V mr = 10 V

ThermistorResistance at TI == TIb.rmiator E 25°C

Imd

min. 30 mWtyp. 40 mWtyp. 50 mA

max. 100 mA

max. 2.5 Vmin. 1460 nmtyp. 1480 nm

max. 1500 nm

typ. 25 nAmax. 100 nA

min. 15 J.LAtyp. 10 pF

typ. 10 kO

Thermoelectric coolerCooler current at ITcaae - Tt I = 15°C

Cooler voltage at IT..... - TIl = 15°C

l cool max. 1.0 A

Vcool max. 2.1 V

» ".. 020 50 CA)

:.....CI)

40 :en ):>:r+ '01.5 'e. '0

CD

3: 'I» ~

r+ Co

2: .s 30 I» x'..... ·en ~(1) :::>

Ol 1.0 a. :::r<0 .....

Cl) en~ :::> '00 0 20 ,Cl) CD> ..... r+ 0..c ~(J)

.0 (').0.5 -' ClI

0 ...10 Ö·

'''T1 ~i" 11I

,U' 0-0 ' I0 ·0 ...:T0 100 200 300 400 500 0 100 200 300 400 500 (I)

=tt ....Current (mA) :U' ~

Current (mA) 00... 0: ..... ::l

Temp ( 'C) 0 25 40 3'0

60 Ith (mA) 25.9 25.7 254 c3

Ec (mW/A) 133 133 133 '0

iiï50 Rs (Q) 20 18 20 11I~

<[ V30 (V) 093 093 0.93:i 40 IJOmW (mA) 253 252 253.....

Wa.vel. (nm) 1494c(J)"- 30 Rntc (kQ) 100 9.9 98'--:::JU Ipelt (mA) -102 -410 -6790+;;:' 20 Ms (~A/mW) 1.0 1.0 1.0c0:2

10

PHILIPS OPTOELECTRONICS CENTRE0

0 10 20 30 40 50 (Copled by M Santbergen)

LJght output (mW)

»uuCD::J0.x'al

CIII...III

en:TCDCD...S..".coUI----UIUIUI::J

3~c~nocu~

CDJ~Q)

en:::TCDCD~

o.......a~00(J'1

--­...a(J'1(J'1(J'1

~

3~c3:(")oc:

'"C-CD...

»"'C"'CCD~

C._.><DJ

·49d9'rt1100 ...00.-.5""(J

~fer~rnLewt

Ctnt'" WI'l'tleorlQUrs....eo WcfthI«wlvliQnAvenQlt Trnn

1485+1555 nm output

1555 rm Ifl)ut

1485 rrn lrou!

5

Wavelength (nm)

OL.-.-....~.L......~__'__'_~L..l__o~_'_'_~....L..1.~ ............c..........~.L......~__'__'_~ ...........~.._...J

600 700 800 900 1000 1100 1200 1300 1400 1500 1500

10

15

20 r------------~---------___,

1485/1555 nm WDM Coup Ier

1555 nm Input

1485 nm Input

·~gd8T'l

1500,."}O tm/dlot,~

(J

·49d8'n1500 fTI'I20rmlarw5""10

1550 1575 1600

srtllngsODtlCfll SoectrtlT'l Ar>etyCE'f

RrtI",.ncelt'l91CtntOl" Wave~ngH·..SWlNIPWrdth~~tlon

....~a~T...-s

Rgfp.-nc:. un,l(l"nlet W.llW!'lpngths..ep WlO1hRloSOIUllOl'lA.frflQtT~

1475 1500 1525

1555 rm lrJ)ut

1425 1450

IQ

0~------~------~",_-----____,l...L----I

Wavelength (nm)

5

Wavelength (nm)

oL.......~-'---'-~~--'-~~-'--'-~___...J~~.o_L~~__'___...~~'_'_~__.J

1400

5

15 ,----------------------------'-------,

10

15

-5

20 ,-------------------------------,

-10

_ 15 '-'--~~........J.~~->-.L~~e-.L~~__'___'~~ .........~~L.._~........J.~~___..J

1400 1425 1450 1475 1500 1525 1550 1575 1500

'"a01cQ::JoU

42 Appendix C: Data of Vork OF' 500 Erbium-doped fiber

The York DF1500 amplifier fiber (serial number: ND712-o1) is designed for use at both 980 omand 1480 om pump-wavelengths. The mode field diameter (MFD) has been designed to match thatof dispersion-shifted telecommunications fiber.

Specifications:

Core composition:

Numerical aperture:Cut-off wavelength:Dopant-concentration:

Fiber geometry:

Cladding diameter:Cladding ovality:Core diameter:Concentricity:

Amplifier performance:

Maximum gain:

Gain coefficient:

Germano-Silicate Glass

0.2-0.22850-970 nm100-140 ppm

125.8 Ilm

1.3%

2.8 ILm

0.93 Ilm

approx. 25 dB1.75 dB/mW (achieved using 16 m of EDF, pumped at 1485 nmand with signal power < -42 dBm at 1535 nm)

Appendix 0: Power souree for the pump laser 43

Appendix 0: Power souree for the pump laser

0.1 Current souree

The principle of the current source is iIlustrated in figure 0.1:

Vref

P1

-vee

Figure D.l Principle of the current source

The variabie voltage V+ on the wiper of the potentiometer is placed over resistor Rl by means offeedback. This results in a current of (V + + Vcd/R1 through the laserdiode.

Because of the extreme sensitivity of the laserdiode to current spikes it is necessary to place acapacitor parallel to and an inductor in series with the laserdiode. To reduce the output-current ofthe opamp a transistor is added to the circuit (figure 0.2).

Vref

P1

-VCC'"

R5

-vce

c

Figure D.2 Current limitation ofthe current source and protection against spikes

44 Appendix 0: Power souree for the pump laser

Limitation of the current through the laserdiode can be realized by resistors R3, R4 and Rs anddiode D;. Take note that -V~e' -.<:: -Vee. -Thisis necessaryto- be able 10 realize à zero current­through the laserdiode.

Reference voltage VnJ is obtained by using a reference diode (figure 0.3). A maximum value for

voltage V+ (and thus the current (V+ +Ved/R I through the laserdiode) can be set by replacing PIin figure 0.2 with R7, PI and P2 in figure 0.3:

(D.1)

P2

P1 v+D2

L---r----J6

-vee

Figure D.3 Rejerence voltage and limitation of V+

As can be seen in appendix A the DILl4 package is equipped with a photodiode. This photodiode

can be used to monitor the optical output power of the laser. It is a1so necessary to monitor thelaser-current in order to establish any possible degeneration of the laser. The complete circuit is

iIlustrated in figure 0.4. Note that the numbering given in figure 0.4 differs from the numberingused in previous figures. In table 0.1 the component values for the 1480 nm laser are listed.

These components WIII not be placed on the PCB

-Q...:Yct>

Uc3u

äïenct>..

R7 RB R9 R1D

,2 1 3 Ii __I~~~~~~~_~=_=_~_~_~_O --l.-1--1c)q.76~_ U1 3=Vout I~

::::::: C1

-vcc

46 Appendix 0: Power souree for the pump laser

- Table D. J Listing of component values ofcurrentSOUfce

Components I1

ClC2C3C4CSC6C7CSDlD2D3D4LlLDMIM2PIP2P3P4RlR2R3R4R5R6R7R8R9RlORIlRI2TlTIUIU2

Values

tant. IOlLf22nF

tant. IJLFIJLF

tant. 3.3ILF22nF

tant. IIlFIOnFLED

KA3365VIN4007

Monitor diode220JLH

LaserdiodelOOJLA meterlOOJLA meter

20kOtOkOtOkO5kO

Ik2k433kIMIk

13k8k2130

3E3,3W3E3,3W

3k910k

BC414MJE3055LM790SLM741

Appendix D: Power souree for the pump laser 47

The design of the printed circuit board (PCB) is performed with the programs OrCAD and

Autotrax.

0 J1fm mlO 0iIlIl... 0:: n .... 0L

~g=! QJgo :lI'" ::YI

15 r / dQ C» ..Jl'" "t: ro1cm fT -, ---- f\ +, lÏ

/Ij % E~ ~ ao :J; +>o n +>c "~ cr al'" ro 0 coc: SJCl ;) coI nIU '!! UCl 0Cl C 0-t: /Ij<I( z::'"n 0.....

0.-~

L~_ __ef 0.- co0 - 0 I

~l-C/.)

Figure D.5 PCB-design of current source

P3

OLD L1

-e::J;~ O=~ DJ

J=-O" ~O:O=O D.OND

~+D+ 0tu

l P2 Ct

O·t.8~t- ~u Cl

-c::J- +DD~~~ M+l\J+

5

*~a uu

l~

Figure D.6 Placemenr of the components

::YId-'

LQJ>oa.aI-

couQ..z:.o~coI~

l­C/.)

48 Appendix 0: Power souree for the pump laser

-.. - -0.2 Temperature controller._,

The temperature is measured with an NTC resistor which is used in a Wheatstone bridge.

Vref

R1

R T

6V

Figure D. 7 WheatslOne bridge with me resistor

The resistance of the NTC resistor as a function of the temperature can be written as

/(0.2)

where T is the absolute temperature in K and A and B are constants. For the Siemens Kt9 NTCresistor, B equals 3340 K and with RT = tOkO at a temperature of 25°C; this means thatA = 0.136 O. The voltage AV as a function of the temperature reads:

(0.3)

RJ will he chosen in such a way that the relation between ~V and T is as linear as possible at atemperature To:

(0.4)

With (0.2), (0.3) and (0.4) RJ can he determined:

(D.S)

Appendix 0: Power souree tor the pump laser 49

We require the bridge to he in balance for T = To. This puts conditions on the values of Rz andR'j:

AV(TJ =0

Rz = Rl

~ Rr

For example: Rz = Rl = 7kO and R) = RT = 10kO.

(0.6)

(0.7)

Temperature To can be adjusted by adding a potentiometer P in series of resistor R) in figure D.7.

Minimum temperature: TIIIÏD

Maxim T 3340um temperature: max = ----

m(0.~6)3440

With R3 = 8.2kO and P = 5kO: l70e < 1'0 < 30oe.

(D.8)

(D.9)

To keep the temperature constant we use feedback: the temperature signal À V is carried back tothe Peltier element by means of negative feedback. Voltage VR can be used to vary the tempera­ture, but since this is not necessary for our purposes VR equaIs 0 V (figure 0.8).

R5

II

K2BVR

A

-L

lp

Figure D.8 Feedback circuit

Kl presents a differential amplifier (figure D.9) and Kz is the final stage which converts thevoltage on point A linearly to the Peltier current lp and in this manner acts as a current source(figure D.10).

(2P

I )V = K 'A V 0; - +1 'A VB 1 ~

and (0.10)

50 Appendix 0: Power souree for the pump laser

+

6V

P7 R7

P6

P1 >-_.L---o Vs

R6

R7R7

Figure D.9 Differential amplifier

+ 15V

PEL TIER

RB

Ii

l-1SV

Figure D.JO Current souree for Peltier element

The complete circuit of the temperature controller is depicted in figure 0.11 and the component

values for the pump laser are Iisted in table 0.2. Note that the numbering given in figure D.ll

differs from the numbering used in previous figures.

Wheatstone brIdge wrth NTC

+15V

differentlal amplifier f,nalstage

+ 15V .15V

T2

R17R16R15

-15Y-15V

R12

These components wlll not be placed on the PCB

r------,, ,I ,I II ,L J

R10

R 11R9

RB

R7

1=16

P2

R20-15V

R2

D2 R3 R4

R18

oetect,on of extreme temperature

.15V

R1

j----------III

: qFELTIER qIIIL _

.------ --I, ,: 01 '":: ~:!.._---- --!

U'I....

52 Appendix 0: Power souree for the pump laser

Table D.2 -Listingof component va(~fs oftemperature controller

Components 11

DlD2D3D4

NTCPELTIER

PIP2P3P4RlR2R3R4R5R6R7R8R9

RIORIlRl2Rl3Rl4Rl5Rl6R17R18Rl9R20Tl1'2UIU2

Values

LEDKA3365V

tN4148LED

ThermistorPeltier element

SkOtOkO20kOtokO

lk52k46k86k88k24k74k73k33k33k33k310k

Ik10k5k6240

3E3,3W10k42klk5

BC414MJE3055LM4741LM747

Appendix 0: Power souree for the pump laser 53

The design of the printed circuit board for the temperature controller is also performed with theprograms OrCAD and Autotrax.

+0:;>o

o

I- 0 I- 0~

~ 0 .c Ij ~

Q.I

~f'~ :JI

~ Öl ./d

+> "C ,.., -':J +> 0:J % Er .r Nlil ::i 0Ol' +>lil C1- I +>Q lTQ r 0r 0 f:Q

.si_ :l

11 n f:QI: 't!Q 0 U" c

ë1\1 CL,... l-n..... z:.

a:; 0

~ 0U

~CL

tl :LWI-

Figure D.l2 PCB-design of temperature controller

0 '1 ~ tl

-e:::J- ~~ O~'"(I~ ca IIn

~.---~---'~~~~D)­21lJ-;:::::]-

--e:J!.~

gU~;9/J ~~

-c::J- -;:::::]-~ll~f""---j~110 JiN

D*~Ol~

Figure D.l3 Placement of componems

:Yld-"

s...QJ>oQ.oI-

f:QUCLl­r.oUCl..:LWI-

54 Appendix 0: Power souree for the pump laser

- . -.

0.3 Power Supply

The power supply provides the voltages -]2/15 (adjustab]e), 0 and + 12/15 V and a maximum

current of 2.5 A. The module can be used with the 19 inch cases.

0.4 Housing of the pump laser

The circuits, together with the laser, are built into a module that can be used with the 19 inchcases. The 1485/1555 run WDM coupIer is also built in a ]9 inch module. The fronts of themodules are shown in figure 0.14:

stroom aan

x5mA mW

optisch uit

EB EBtemperatuur

UIT EB AAN EB EB

t(8regelaar te

EC aan hoog

I

EBstroom door laser

1480 nm pomp laser

optisch vermogen

Figure D.14 Fronts of the modules

WDM-eoupler

1555nmin

G1485nmin

G1555+1485 uit

GG

tC8EC

Appendix E: Listings PlOTFILE.PAS and IEEEIO.PAS 55

Appendix E: Listings PLOTFILE.PAS and IEEEIO.PAS

PLOTFILE.PAS

This program enables the user to transfer data from the Optical Spectrum Analyzer (O.S.A.), viathe IEEE bus, to a personal computer, write it to disk and/or to plot the data that is visible on theO.S.A.-screen. The O.S.A. and the plotter shauld be connected ta the computer with a GP-IBcabie. The adresses of the O.S.A. and the plotter should be set to [30] and [05], respectively.When started, the program will prompt the user for input when necessary.

PROGRAM PLOTFILE:

USES IEEEIO,CRT:

VAR Marker,Response,YssKey,Value,SensitivityTemp,Temp1,Error,Stap,x,y,sCoordX,CoordY,Reflev,Swpwd,Hokjes,Ctrwl,Resoln,Avr,YsReceiveResul tInit,AccAB,NegFout,stoppenlblFileStringOutFile

String;Cher:Integer;

Real;Array[O •• 4072J Of Char;Array[1 ..581l Of Real;Booleen;Stri ng [50J;String[20J;Text:

Procedure Number(Var Result:Real);Var Strng : String;

Error : Integer;Begin

Resul t := 0;Error := 0:Strng := ":Temp := Tetrp+1;While (Receive[TempJ <> ',') And (Temp < 4073) 00Begin

Strng:=Strng+Receive[TempJ;Temp:=Tetrp+1;

End:Val(Strng,Result,Error):If (Error <> 0) And (NegFout = False) ThenBegin

IJri teln:Wri teln:Writeln('Fouten in de Data·overdracht.'):Writeln;Writeln('A. Stoppen.'):Writeln('B. Doorgaan, waarbij het programma de foute data aanpast. Het resultaat zal'):Writeln(' dan wat de foute data betreft niet geheel juist zijn.'):Writeln;Writeln('Geef keuze [A/BJ .');Key := Upcase(Readkey):While (Key <> 'A') And (Key <> '8') 00 Key := Upcase(Readkey);If Key = 'B' Then NegFout:=True

Else RunErrorjEnd'If (AccAB) And (Error <> 0) Then Result := 4;

End; {procedure Number}

Procedure Plot;Begin

Writeln('Geef de titel van de te plotten grafiek aub. Maximaal 50 tekens.'):Readln(lbl) :Writeln;Writeln('Het plotten duurt ongeveer 3 minuten.'):

56 Appendix E: Listings PLOTFllE.PAS and IEEEIO.PAS

--(Begin met plottenl _.Wri teln(leeeOut, 'OUTPUT 05; 1NvSlOSP1PAPUO, OPD9367, 0,9367,1047,0, 1041,0,OPUO,

349P09367,349PU9367, ,698POO,698');WritelnCleeeOut,'OUTPUT 05;PU837,1413P0837,7326,9367,7326,9367, 1413,837,1413SP2');

Tenp1 := 0;Tenp := 1413;Stap := RoundC5913 / hokjes);While CTl!q) < 7325 - Stap) 00Begin

Tenp := Tl!q) + Stap;If (ABSCTl!q) - 5848) > 50) or (AccAB = True) ThenBegin

lf CTemp1 =0) ThenBegin

Writeln(leeeOut,'OUTPUT 05;PU837,' ,Temp,'PD9367,' ,Temp);Temp1 := 1;

EndElseBegin

Writeln(leeeOut,'OUTPUT 05;PU9367,' ,Temp,'PD837,' ,Temp);Temp1 := 0;

End;End;

End; <whi le}Tl!q) := 1690;Whi Ie (Temp < 7661) DoBegin

Writeln(leeeOut,'OUTPUT OS;PU' ,Temp,' ,1413PD' ,Temp,' ,7326');Temp := Temp + 853;Writeln(leeeOut,'OUTPUT OS;PU' ,Temp,' , 7326PD' ,Temp,', 1413');Temp := Temp + 853;

End;Writeln(leeeOut,'OUTPUT 05;PU8514,1413PD8S14,7326');If CAccAB = False) ThenBegin

Writeln(leeeOut,'OUTPUT OS;LT2SPZPU9367,5849PD837,5849LTPU887,5900L01SI.3,.3SLCSOSSLBREF' ,CHR(3»;WritelnCleeeOut,'OUTPUT 05;PUO,5900LB' ,Reflev:2:0,chr(3»;Writeln(leeeOut,'OUTPUT OS;PUO,S700LB dB' ,chr(3»;

End·If (ReceiveC69] = '.') Then <output in dB}Begin

CoordX := 822.293;Writeln(leeeOut,'OUTPUT 05;SP3PU837,1413');For Temp := 1 To 581 DoBegin

CoordX := CoordX + 14.707;CoordY := 5849 - (Reflev-ResultCTemp]) * 739.125 / Ys:If (CoordY < 1413)Then Writeln(leeeOut, 'OUTPUT 05;PA' ,Round(CoordX),', 1413PU')Else If (CoordY > 7326)

Then If (Result[Temp] =0) Then Writeln(leeeOut,'OUTPUT 05;PAPU' ,Round(CoordX),' ,7328')Else Writeln(leeeOut,'OUTPUT 05;PA' ,Round(CoordX),' ,7328PU')

Else Writeln(leeeOut,'OUTPUT 05;PAPD',Round(CoordX),' ,',Round(CoordY»;End;

EndElse {output lineair}Begin

CoordX := 822.293;WritelnCleeeOut,'OUTPUT 05iSP3PU837,1413'):For Temp := 1 To 581 DoBegin

CoordX := CoordX + 14.707;If (AccAB = False)Then CoordY := 1413 + Result[Templ * 4436Else If Ys=O.2 Then CoordY := 1413 + ResultCTempJ / Ys * 985.5

Else CoordY := 1413 + Result[Tenp] / Ys * 739.125;If (CoordY = 1413)Then Writeln(leeeOut,'OUTPUT 05 PAPU' ,RoundCCoordX),', 1413')Else lf CCoordY > 7326) Then Wr teln(leeeOut,'OUTPUT 05:PAPU' ,Round(CoordX),' ,7328')

Else Wr teln(leeeOut,'OUTPUT 05;PAPD' , Round(CoordX),, ,',Round(CoordY»;End;

End;

Appendix E: Listings PLOTFILE.PAS and IEEEIO.PAS 57

Writeln(leeeOut,'OUTPUT 05;SP1PU100,7S0SI.3,.SLBSWP , ,S~WO:5:1,'nm/div' ,CHR(3»;writeln(leeeOut,'OUTPUT 05;PR100,-52PRPOO,349');Writeln(leeeOut,'OUTPUT 05;PRPU100,-299lBYS ' ,Ys:5:1,Yss,CHR(3»;Writeln(JeeeOut,'OUTPUT OS;PR100,-52PRPDO,349');writeln(IeeeOut,'OUTPUT 05;PRPU100,-299LBRES ' ,Resoln:4:1,'nm',chr(3»;Writeln(leeeOut,'OUTPUT OS;PR100,-S2PRPOO,349');Writeln(JeeeOut,'OUTPUT OS;PRPU100,-299LBAVR ' ,Avr:3:0,chr(3»;Writeln(JeeeOut,'OUTPUT 05;PR100,-52PRPOO,349');Writeln(IeeeOut,'OUTPUT 05;PRPU100,-299LB' ,Receive[72],chr(3»;Writeln(leeeout,'OUTPUT 05;PAPU100,1150LB' ,Ctrwl-5*Swpwd:6:1,'nm',' ':16,

Ctrwl:6:1,'nm',' ':16,Ctrwl+5*Swpwd:6:1,'nm' ,chr(3»;Writeln(JeeeOut,'OUTPUT OS;PAPU100,401LBWMKR' ,chr(3»;Writeln(JeeeOut,'OUTPUT 05;PAPU100,52LBLMKR' ,chr(3»;Writeln(IeeeOut,'OUTPUT OS;PAPUO,7400LBSPECTRUM ' ,Lbl,chr(3»;Writeln(leeeOut,'OUTPUT OS;PUSP');

End; {Procedure Plot}

, ,Result[x+1]:5:2);

, ,Reflev:5:2,' dam');',Ctrwl:S:1,' nm');, ,Swpwd:3:1,' nm/div');',Resoln:3:1,' mi'>;, ,Avr:3:0);',Ys:3:1,' ',Yss);

Procedure WriteToDisk;Begin

Writeln('Geef de naam van de te bewaren file (max 20 karakters).');Readln(Filestring);AssignCOutFile,Filestring);Rewrite(OutFile);Writeln;Writeln('Header toevoegen aan data-file? (JIN)');Writeln;Key:=Upcase(ReadKey);While (Key <> 'J') And (Key <> 'N') 00 Key := UpcaseCReadKey);If CKey = 'J') ThenBegin

WritelnCOutFile,'Reference LevelWriteln(OutFile,'Center WavelengthWriteln(OutFile,'Sweep WidthWritelnCOutFile,'ResolutionWriteln(OutFile,'Average TimesWriteln(OutFile,'Y'Scale

End;For x := 0 To 580 DoBegin

Writeln(OutFile,(Ctrwl'5*Swpwd)+(10*x/580*Swpwd):9:4,'End-Clo;e(OutFile);

End; {Procedure WriteToDisk}

Aansluitingen kontroleren. Loopt er een GP-JB kabel van de computer naar');de plotter en een GP-IB kabel van de computer naar de AQ-6310B Optical');Spectrum Analyzer?');Staat de plotter aan en zit er A4 papier in?');Zitten de goede pennen in de plotter:');

Pen 1: Zwart');Pen 2: Groen');Pen 3: Rood.');

Analyzer [adres 30] en van de plotter [adres 05]');

Begin {Main program}Stoppen := False;Init := False;While Not(Stoppen) DoBegin

While Not(lnit) DoBegin

ClrScr;Writeln('Plot programma voor Optical Spectrum Analyzer');Writeln('Ook geschikt voor schrijven van data naar disk');Writeln;Writeln;Writeln('VOORBEREJOING:');Writeln;Writeln('1.Writeln('Wri teln('Writeln('2.Writeln('3.Wri teln('Writeln('Writeln('Writeln('4. Zijn de adressen van deWriteln(' goed ingesteld?');Writeln;Writeln;Writeln('Oruk op J indien gereed.');Wri teln;Key:=Upcase(Readkey);While (Key <> 'J') Do Key:=Upcase(Readkey);Init:=True;

End;

58 Appendix E: Listings PLOTFILE.PAS and IEEEIO.PAS

-ctrScr·· .Writel~('Druk op <J> indien data op OSA relevant-fs,·);Writeln;Key:=Upcase(Reedkey);While (Key <> 'J') Do Key:=Upcase(Readkey);

NegFout := False;Writeln(leeeOut,'TIME OUT 26');Writeln(leeeout,'OUTPUT 30;DDATA');Writeln('De data wordt van de Spettrum Analyzer naar de computer gestuurd.');Writeln(leeeout,'ENTER 30 #4073 BUFFER',

Seg(Receive[O]),':' ,Ofs(Receive[O]»;Writeln;Writeln('Data zichtbaar maken op scherm? (JIN).');Key := Upcase(Readkey);While (ley <> 'J') And (Key <> 'N') Do Key := Upcase(Readkey);If (ley = 'J') ThenBegin

For Temp1 := 0 Ta 6 Do Write(Receive[Temp1]);Writeln;For Temp := 1 Ta 580 DoBegin

Write('RESULT=',Temp,' TEMP=' ,Temp*7,' .• ',Temp*7+6,' ');For Temp1:=0 To 6 Do Write(Receive[Temp*7+Temp1]);Writeln;If Temp mod 20=0 Then Key := Readkey;

End;Write('RESULT=581 TEMP=4067..4072 ');For Temp1:=4067 To 4072 Do Write(Receive[Temp1);

End·Writeln;Writeln('Even rekenen •.•. ');

AccAB:=Receive[8]='.';Temp := 0;Temp1 := 1;While (Receive[temp) <> ',') Do Temp:=T~1;

Whi le (Tetrp < 4073) DoBegin

Number(Result[Temp1]);Temp1 := Temp1 + 1;

End;

Writeln(leeeOut,'OUTPUT 30;STATE');Writeln(leeeOut,'ENTER 30 #77 BUFFER' ,Seg(Receive[O),':' ,Ofs(Receive[O]»;Temp := 50;Number(Reflev) ;Temp := 12;Number(Ctrwl);Temp := 25;NI.iN:ler ( Swpwd) ;Temp := 38;Nl.iN:ler(Resoln);Temp := 57;Nl.iN:ler(Avr);

CASE -ROUND(Reflev) MOO 10 Of0,3,6 Hokjes:= 6.67;1,4,8 Hokjes:= 10.6;2,5,9 : Hokjes := 8.43;7 : Hokjes := 5.32;

End·If (Receive[69] = '.') ThenBegin

Tel'I'P :=66;Nl.iN:ler(Ys);Hokjes := 8;Yss := 'dB/div';

End

{Yscale in dB}

Appendix E: Listings PLOTFILE.PAS and IEEEIO.PAS 59

Else If (AccAB : True)Then {lineair geval met AccA/B Of AccB/A}Begin

Yss := '/div ';Ys := 0.2;Hokjes := 6;for Temp := 1 To 581 Do If (Result[Temp] > 1.2) ThenBegin

Ys := 0.5;Hokjes := 8;Temp := 581;

End;EndElse {andere lineaire gevallen}Begin

If (-Round(Reflev) div 28 = 0) Then Yss := '~/div'

Else If (-Round(Reflev+28) div 30 = 0) Then Yss := 'nnVdiv'Else Yss := 'pmldiv';Ys := Reflev;If (-Ys >= 30) Then Ys := Ys + 30;If (-Ys >= 30) Then Ys := Ys + 30;CASE -Round(Ys) Of

28,29,0 Ys := 200;1.2.3 Ys := 100;4.5.6.7 Ys := 50;8.9.10 Ys := 20;11,12.13 Ys:=10;14,15.16.17 Ys:= 5;18,19.20 Ys := 2;21,22.23 Ys·-l·24.25.26.27 YS;: 0:5;

End; {case}End; {Else}

Writeln;Writeln('Plotten Of Bewaren op disk? (P/B)');I.Iriteln;Key:=Upcase(Readkey);While (Key <> 'P') And (Key <> 'B') Do Key := Upcase(Readkey);]f (Key : 'P') Then Plot;If (Key = 'B') Then WriteToOisk;Writeln;Writeln('Stoppen? [JIN]');Key:=Upcase(Readkey);While (Key <> 'J') And (Key <> 'N') Do Key := Upcase(ReadKey);If (Key = 'J') Then Stoppen := True;

End;END.

Register definition for low-level callsText files for IEEE communicetion

60 Appendix E: listings PlOTFllE.PAS and IEEEIO.PAS

IEEEIO.PAS

This Pascal unit is used by the program PLOTFILE.PAS.

Unit leeelO;

( Support file for IEEE 488 epplicetions.

Contains:Var Regs

leeeout, leeeln

Procedure locnIOCTlReadRawModeleeeCClqllete

>Interface

Uses Dos;

Sends locn BREAK cOIlIIIlInd to Personal488Reeds loens from Personal488Sets fi les into "raw" (bill8ry) modeCloses files, shuts down.

VerRegsleeeout, leeeln

Registers;Text;

Procedure IOCTL;Procedure IOCTLRead(Ver Command:String);Procedure RewMode(Var AFile:Text);Procedure IeeeCClqllete;

IIJ1)lementat ; on

Procedure IOCTL;( This is the equivalent of the BASIC statment IOCTL#',"BREAK"

es described in Chapter 6 of the Personal488 manual.>Const Break : String = 'BREAK';Var

IeeeFileHandle : integer absolute IeeeOut;Begin

With Regs 00 BeginAX := S4403; (MS-OOS function code)BX := IeeeFileHandle;CX := length(Break);OS := Seg(Break);OX := Ofs(Break).'

End;MsOos(Regs)

End;

Procedure IOCTlRead(Var Command:String);{ This is the equivalent of the BASIC function IOCTLS as described

in Chapter 6 of the Persoll8l488 manual. >Var

IeeeFileHandle: integer absolute IeeeOut;Begin

With Regs Do BeginAX := S4402; (MS-OOS function code)BX := IeeeFileHandle;CX := 255; (Length)OS := Seg(Command);OX := Ofs(Command)+';MsDos( Regs);Command[Ol := Char(Regs.AX) (Received length>

End'End; ,

Appendix E: listings PlOTFllE.PAS and IEEEIO.PAS 61

Procedure RawMode(Var AFile:Text);( This sets the file into "rBW" Or binary mode. This greatly

improves the efficiency of communication.)Var

FileHandle : integer absolute AFile;Begin

With Regs Do BeginAX := 14400: (MS-DOS Get Device Data function)BX := FileHandle;

End:MsD05 (Regs):Wlth Reg5 Do Begin

AX := 14401:BX := FileHandle;DH := 0; {DH must be O}DL := DL Or $20 (Set Binary Mode bit)

End:MsOos(Regs)

End;

Procedure IeeeComplete:Begin

Close(leeeDut);CloseCl eeel n)

End:

BeginAssign(leeeDut,'leee~ut'); Rewrite(leeeDut);Assign(leeeln,'leeeln'); Reset(leeeln);

RawMode(leeeDut);RawMode(leeeln);

lOCH:Writeln(leeeout,'RESET');Writeln(JeeeDut,'FILL ERROR')

End.