Temperature sensor based on double core optical fibre

Ryszard Romaniuka, Jan Doroszb,

aWarsaw University of Technology, bBiałystok University of Technology,

ABSTRACT We report the theoretical and experimental study of a temperature sensor based on the changing of the refractive index with the temperature in double core optical fibre. Because of it the coupling coefficient between two cores depends on temperature. This sensor uses the double core fibre directly as a temperature sensor as well to a data transmission. The influence of variations of the temperature from 0oC to 135oC on the coupling coefficient (refractive index) is measured and we received the relation between the temperature and the signal in coupled core. This kind of sensor can be used for public safety, health and security systems.

Keywords: Optical fibre technology, optical fibre sensors, double-core optical fibres, temperature sensors

1. INTRODUCTION Thermal properties of optical fibres are utilized for construction of functional photonic devices nearly since the beginning of optical fibre technology [1,2]. Several mechanisms are used for changing the lightwave transmission like evanescent wave variations with temperature, fibre coupling, mode mixing, microbending, losses, phase shift, polarization changes, scattering and index variations [3,4]. In this respect two groups of mechanisms may be distinguished – passive (changing fibre parameter) and active (changing wave parameter), as well as two groups of devices amplitude and phase. Both optical fibre couplers and double core optical fibres are good candidates for photonic functional devices sensitive to temperature. Throughout this work they will be called thermally sensitive photonic functional devices rather than sensors because they can give rise not only to sensor but also make a number of thermally controlled couplers, phase shifters, fibre attenuators for interferometric and transmission purposes. We start this work with some general discussion of the properties of double core single mode and multimode optical fibres [5,8]. Then we follow to utilization of these properties to the construction of efficient, thermally sensitive, fibre optic photonic devices, based on doublecore optical filaments, for laboratory [6,7] as well as environmental and industrial [9] conditions.

2. THEORY OF DOUBLE-CORE SINGLEMODE FIBRE DEVICES The fundamental mode of a double core optical fibre HE11x will be totally coupled to the other core when the total phase shift ∆Φ satisfies the following condition ∆Φ=(2m+1)π. It is equivalent to the following equation, for the SA modes:

∫−

−− −=∆Φ1

1

11 ])()[( 11 dsVV AAp

ASpϖ , (1)

where VpAS is phase velocity of a given S.A. (symmetric-antisymmetric) mode.

The condition of full modal energy transfer is:

( )( ) ( )( ) πββε )12(]1111[ 2/12/11

1

11 +=−∆−−−∆−∫−

ndsk AAN

ASNcoreo

. (2)

The expression under the integral is simplified, for small values of ∆, to the following form ∆βNxx=βN

AS1-βNAA1. Let us

assume that HE11x mode is excited in one of the cores. This is equivalent to the excitation of that core with the following superposition of SA modes HE11x =AS1+AA1. At the output one should obtain again the HE11x mode, but from the other core. This time, it is equivalent to the following combination of AS modes HE11x =AS1 – AA1.

The coupling coefficient of the lightwave between the cores in a twin- core optical fibre depends on the fibre geometry and optical parameters, i.e. on the fibre material and technology. Here, we are interested in the design and manufacturing processes of an optical double core fibre of suitable coupling characteristics for particular applications. Choice of the proper parameters of the manufacturing process allows to produce fibres of strictly designed coupling

characteristics. The manufacturing parameters to be changed for optimal product characteristics are: glass compositions and pairing, kind of modification of the multicrucible process (like crucible and nozzle separation, diaphragming of inter-nozzle space, nozzle aperturing, etc.), details of the construction of individual crucibles, process temperature, speed of pulling. Fibre material has an essential influence on inter-core coupling.

Fig. 1. Calculated, from the eigenequation and coupling equations, modal, dispersion and coupling characteristics of twin-core and double core optical fibres; a) Differential propagation constants of AS modes for adjacent and nonadjacent cores as functions of normalized frequency; b) Differential propagation constants of AS modes as function of fibre relative refraction for several normalized frequencies; c) Calculated intercore birefringence dispersion for five manufactured samples of double core optical fibres. The birefringence reaches zero for λo

dysp. This wavelength is a function of fibre geometry and material. It is also a strong function of temperature, on which thermal measurements of double core optical fibre base. Fibre data: 1- a=3µm, ∆=0,2%, core #2, a=9µm, ∆=0,15% ; 2 - a=3µm, ∆=0,3%, core #2, a=9µm, ∆=0,2% ; 3 - core #1, a=3µm, ∆=0,4%, core #2, a=9µm, ∆=0,2% ; 4 - core #1, a=3µm, ∆=0,4%, core #2, a=7,5µm, ∆=0,2% ; 5 - core #1, a=3µm, ∆=0,4%, core #2, a=6,5µm, ∆=0,2% ; d=10µm; d) Dispersion pf power transfer efficiency for the same samples of double core fibre; e) Dispersion characteristics of intercore contrast P12c in two samples of twincore fibre, fibre sample #1, core, a=3µm, ∆=0,4%, d= 3µm, V=2,2; – solid

line, fibre sample #2, core a=2µm, ∆=0,4%, d=4µm, V=1,5; – broken line; f) Dispersion characteristics of intercore contrast P12c in two samples of doublecore fibre; fibre sample #4 – solid line; fibre sample #5 – broken line.

When the cores in a double core fibre are not identical and initially not strongly coupled, then the propagation constants of the fundamental modes in each core HE11x are different and equal to β1 and β2. Direct power transfer between the fundamental modes is not possible, unless the modes are ideally fitted in phase. From the point of view of the material out of which the double core fibre is made and its dispersion properties, there is always a point on the dispersion characteristic for which ∆β=0. This condition is satisfied for a particular wavelength λ=λo

dysp. Fig 1. presents calculated dispersion curves for propagation constants βN(λ) and ∆β2(λ), differential propagation constants and coupled power for a few pulled and measured singlemode, doublecore optical fibres as functions of fibre refractive parameters, normalized frequency and wavelength. The curves were calculated for a few different fibres to compare the results and see the extent of design process. The dispersion curves for different fibres intersect in a few places. They reach zero value for different wavelengths λ in the range from 800nm to 1600nm. The value of λo

dysp at which zero phase shift is between fundamental modes of coupled cores in double core optical fiber increases with decreasing difference in core dimensions and with increasing height of the refractive index in a step-index fiber. The directional coupling coefficients C12 and C21 are not equal in double core optical fiber. The coupling may be considerably nonsymmetrical, stronger in one direction. Consequently, more power is propagated in one core. The measure of coupling symmetry and efficiency of power transfer, as a function of detuning from λo

dysp is relative coupling coefficient Cijr=Cij/Cji. The plots of function

power transfer efficiency times coupling coefficient C21r(λ)P12te(λ), relative to differential propagation constants curves

also presented in fig.1.

The calculated power transfer efficiency does not exceed 60% in the case of investigated double core optical fibres. Bigger efficiency is not possible for this fibre design. A few counteracting waveguide processes have influence on the shape of coupling efficiency characteristics. The coupling coefficients increase with wavelength for λ>λo

dysp. For bigger intercore separations the coupling decreases and is less sensitive to the difference in propagation constants in both coupled cores. The relative coupling coefficient Cij

r=Cij/Cji for maximum coupling wavelength λodysp depends solely on

the differential height in refractive levels in both cores. The power coupling optimisation process in double core optical fibers includes such parameters like: core separation d, absolute height of refractive profile and differential height ∆n, core diameters 2a, differential core diameters ∆aij. When one chooses the length of fibre equal to the beating length L≈LB, then the measured output signal is proportional to the inter-core contrast P12c. The dispersion functions of intercore contrast for measured samples of double core as well as twin-core fibres were presented in fig. 1. The spectral coupling characteristics may be narrowed by increasing the core separation. This, however, considerably increases the beating length. Spectral coupling characteristics have band pass properties and this unique feature of twin-core optical fibre may be used potentially for construction of tuneable filters.

When the lightwave is coupled initially only to one core, the normalized optical power in the second core is P(z)=sin2(Cz), where C – is intercore coupling coefficient, z-coupling length. Both possible coupling coefficients in the twin core fibre are equal C12=C21. The coupling coefficient in a twin core optical fibre is:

( ) ( )( )WK

aWdKVU

aC o

21

3

22/1 /2∆= , and the beating length is LB=π/C, (3)

where: U=ka(nr2-β2/k2)1/2, W=ka(β2/k2-np

2)1/2 - Bessel function arguments, V=kanco(2∆)1/2 – normalized frequency, ∆=(nco

2-ncl2)/2nco

2 – refractive index coefficient. The eigenequation was solved for several different sets of technological and material parameters. The results were presented in figures 1,2 and 3. The basic glass sets were: A - nco=1,516, ncl=1,510, B - nco=1,522, ncl =1511.

Twin core fibre, in comparison with double core fibre exhibits: C21=C12 and β1=β2 z=ZB. As a consequence, optical power can be transferred with 100% efficiency in an ideal fibre. The coupling efficiency does not show dispersive behaviour. Only the coupling coefficient is dispersive. All fibre non-idealities like non-cylindrical geometry, differences in core radiuses and refractive profiles, nonsymmetrical temperature distribution, stresses, non-linear effects, higher order effects, etc., change considerably the coupling, beat length and power transfer between cores. Fig. 1.c. shows calculated value of intercore contrast for twin-core optical fibre and fig.1.d. the same characteristic for double core fibre. Twin core fibre has nondispersive transfer which always equals to 100%.

Fig.2. Technological data of manufactured doublecore optical fibres; a) and b) Photographs of cross section (in uniform and interference fields) of sample twin core single mode optical fibres manufactured for the purpose of this work; a) Twin core fibre with W profile- refractive depletion around core; b) Twin core fibre Φ=125µm, 2a=10µm, d=4µm, NA=0,15; c) Equivalent refractive index profile of bent doublecore fibre with bending parallel to fibre plane and thermally changeable index of a region between the cores; d) Spectral transmission characteristics T(λ) of chosen manufactured samples of double core optical fibres for various core-cladding glass pairs ; e), f), g) Measured refractive index profiles (gradient, quasi-step and step-index) of individual cores in several samples of doublecore fibres; h), i), j), k) Measured ion profiles of manufactured samples of doublecore optical fibres made of lead and lead-barium glasses.

For the region of weak couplings in a twincore optical fibre, where the propagation constants are not coupled, the approximate dependency for beat length holds:

LB=(Cλo/n∆n) exp(C1V), C1=0,74+2,3(d/2a-1) (4)

For example, for: n1=1,5, ∆n=3%, d=2a, D/2a=1, V=2,4, λo=850nm, C≈8, C1=0,75, LB=0,9mm. Alternatively, for D/2a=1,5, V=2, C=25, C1=1,9, LB=2cm. Beat length is very sensitive to the core separation and external temperature.

Fig.3. Dependence of the twin core fibre coupling characteristics on the fibre material. The beating length as a function of intercore separation and wavelength LB(d) and LB(λ) for a few manufactured samples of double core optical fibres made of two glass sets; 1) Glass set A – nco=1,516, ncl=1,510; core radiuses and wavelengths: curve 1 - a=2,4µm, λ=0,85µm; curve 2 - a=2,4µm, λ=1,3µm; curve 3 - a=3,6µm, λ=0,85µm; curve 4 - a=3,6µm, λ=1,3µm; 2) Glass set B – nco=1,522, ncl=1,511; curve 1 - a=1,75µm, λ=0,85µm; curve 2 - a=1,75µm, λ=1,3µm; curve 3 - a=2,7µm, λ=0,85µm; curve 4 - a=2,7µm, λ=1,3µm; 3) and 4) Exemplary characteristics for different sets of core-cladding glasses; 5) Calculated LB(λ) for: a=2µm, curve 1 - d=5,5µm, curve 2 - d=11µm, curve 3 - d=16,5µm, curve 4 - 22µm; 6) Calculated LB(λ) for: a=2µm, curve 1 - d=6µm, curve 2 - d=10µm, curve 3 - d=15µm; 7) a=1,75µm, curve 1 - d=4µm, curve 2 - d=10µm, curve 3 - d=15µm; 8) a=1,5µm, curve 1 – d=6µm, curve 2 – d=10µm, curve 3 – d=15µm;

The beat length was investigated as a function of wavelength λ, core separation d and temperature T for the samples of manufactured double core optical fibres. Fig.3. presents the results of calculations. The function LogLB(d, λ) is linear for all combinations of arguments. The fibre parameters were chosen to maintain its singlemodedness. The beat length exhibits strong dependence on both d and λ. The calculated curves possess practical meaning, because indicate manufacturing conditions for fibers of particular coupling characteristics.

The measurement techniques of doublecore optical fibres require adaptation of optical power coupling methods, in order to excite individual cores of these fibres in a repeatable way as well as detect the output. The techniques should be compatible even with closely spaced cores. We used a method basing on using multicore fibre taper. The taper is manufactured form appropriate preform of bigger outside dimensions and the same core distribution. The taper is then cut at the cross-section compatible with measured doublecore fibre. The coupling end of taper is fitted to the measured fibre while the signal end is ended with individual fibres of standard dimensions.

Fig. 2. presents some of the characteristics of manufactured double core optical fibres used in the experiments. The basic ranges of parameters were: transparency 0,35µm – 1,45µm, attenuation 10-100dB/km, numerical aperture 0,12-0,5, fibre diameters from 35µm to 1mm, core diameters 2-25 and 50-65µm, core separation 3-25µm and adjacent, core radius ratio 1:1, 1:2 and other, sample lengths from 1 to 100m, jacket soft-hard.

3. THEORY OF DOUBLE-CORE MULTIMODE FIBRE DEVICES Thermally driven coupling between multimode cores, in double core optical fibre, depends on the evanescent field extensions in the cladding region between them. Relative power in the cladding is expressed by the dependence:

12/])/([/ 22222222 ++∆−= lnbnkaVknkaPP cotc β , (5)

where: nc(T)-core index, n(T)-cladding index, a-core radius, k=2π/λ, λ-wavelength, β-mode propagation constant, V=2k(2n∆n)1/2-normalized frequency, b=(β/k-n)/∆n-normalized propagation constant, l-mode radial index number (distinguished from azimuthal index number m).

To make the normalized power depend strongly on temperature one can assume that the cladding material depends much more on temperature than the core material, as is in the case of hybrid material fibres – holey ones impregnated with silicone, PMMA or organic low-loss, UV cured, glue. We assume standard dependence of involved refractive indexes on temperature: n(T)=no+α(T-20oC), where thermal dispersion of refractive index α= δn/δT [oC-1] for glasses is of the order of αg(lass)=+10-5/oC and αp(last)=-5*10-4 for such materials as PMMA, resins, silicone and UV/thermally curable highly transparent glues.

Multimode double core curved fibre exhibits extended temperature sensitivity. This fibre can be represented as straight one with linearly distorted refractive index profile. Depending on the plane of bending it affects more or less the intercore coupling. The number of modes supported by the bend fibre is expressed by:

]})2/3()/2)][(2/)2[(1{ 3/2kRnRaNN ostraightbend +∆+−= αα (6)

where: N – number of modes either in straight or in curved core, α – refractive index profile exponent (α=2 for parabolic profile, α=∞ for step index profile), R-bend radius. Multimode fibre with constant bend radius is equivalent to spatial high order modal filter. The losses due to curvature and temperature in a step-index fibre are:

])(/21log[10)(/1log[10][ 221 RTNAanRTadBL −=∆−= (7)

The parametric (scaling) factors for multimode doublecore optical fibre thermometer are core diameters, bending radius, refractive index difference between core and cladding (or numerical aperture), cores separation and the functional factor is the dependence of ∆(T) or NA(T).

4. LABORATORY EXPERIMENTS Two sets of laboratory sensors were manufactured. Singlemode doublecore fibre sensors based on two mechanisms: either thermal dispersion of λo

dysp wavelength or thermal dispersion of beating length (classical doublecore fibre) and thermal dispersion of differential core-cladding diffraction ∆n=nco-ncl (hybrid material fibre with organic impregnation). Multimode doublecore fibre sensors based on thermal dispersion of ∆n in holey fibers impregnated with highly transparent organic materials. Some hints concerning the preparation of both types of optical fibres were presented in fig.4.

Fig.5. shows block diagrams of practical laboratory set ups which were used for measuring, general optical, transmission as well as thermal properties of doublecore optical fibres. The multimode doublecore optical fibre was treated as an optical coupler and measured accordingly, while the singlemode doublecore fibre was treated as an intrinsic interferometer. Some initial models of multimode doublecore optical fibres were simulated, for set up calibration and signal levels assessment, by coupling two PCS fibres with removed plastic cladding and placed in thermally sensitive immersion environment (capillary micro-tube). The signal coupling was performed either with multi-beam optics or with doublecore fibre taper. The beating pattern of singlemode doublecore optical fibre and its dependence on temperature was observed and measured with the aid of an automated microscopic CCD system. Foundations of these observations were presented in fig.5. In particular, a location of the cores in the doublecore singlemode optical fibres were assessed by an interferometric method, using the back side of a pinhole diaphragm as an interference pattern observation screen. The sensitivity of this method was quite good, especially when the cores were placed in one plane in reference to the direction of the laser beam penetrating perpendicularly the doublecore optical fibre. Fig.6 presents photograph of laboratory hardware used for measurements, according to diagram in fig.5.

Fig.4. Construction details of glass and impregnated doublecore optical fibres.

Fig.5. Schematic block diagrams of laboratory set-ups used for measurements of general and thermal behaviour of doublecore optical fibres.

Fig.6. General view of laboratory set up for measuring of thermal properties of doublecore optical fibres.

Fig. 7. Measurements of relative thermal dispersion of refraction for different impregnation agents for doublecore optical fibres. Various refractive data apply for silica and silicone at T=20oC: n1=1,45; 1,47; 1,49; n2=1,40; 1,41; 1,44.

Fig.8. Measurements of beating phase shift in two singlemode doublecore thermal sensing fibres – glass and impregnated. DC stands for doublecore. The respective thermal sensitivities were approximately 0,05π/oC and 0,35π/oC.

Fig.9. Measurements of output signal from multimode doublecore optical fibre temperature sensor impregnated with highly transparent epoxy glue.

Fig.10. Measurements of output signal from multimode doublecore bent optical fibre temperature sensor. Bending perpendicular and parallel to the core plane. Bend radius around 6mm.

Fig.11. Comparison of thermal sensitivities between various kinds of optical fibres: elliptical, HB and doublecore – glass and impregnated.

Fig.7. presents the relative refraction data for organic materials used for gluing or/and impregnating the doublecore fibre sensor constructions. Some resins soften considerably above 135oC. The most resistant to high temperature was low-loss, transparent Teflon derivative. No visible softening changes were observed till 165oC for this material. The gluing and/or impregnation processes depended on the particular organic material. Some were UV, thermally or chemically curable. Some required solvents to decrease their viscosity prior to the impregnation process. Some resins and gums required multistage drying process and repeated impregnation. Quite considerable extent of ∆n values was obtained, exceeding 0,05 for T=100oC. The change of relative refraction was the biggest in the case of some kind of low density resin. The parameter ∂(∆n)/∂T was nearly 0,15 for ∆T=200oC. This gives refraction-temperature sensitivity for this fibre sensor construction of the order of 75*10-3. Low density resin was applicable only to 120oC. Transparent Teflon impregnation exhibited sensitivity of nearly order of magnitude lower but extended the range of measurements well above 150oC. The functional dependence for the refraction of one of silicone resins presented in fig.7. was n(T)=1,42-0,0005T. This dependence may be introduced in the relation on thermally dependent bent fibre losses (7), which leads to approximate relation α[dB]=10log{1- [1,45a/R[1,45-n(T)]. The bending losses decrease with temperature as the NA of the fibre increases and the intercore coupling decreases. Some sensitivity of this fibre sensor geometry was observed on the plane of bending in relation to the doublecore plane.

Fig.8. shows the results of measurements of thermally induced beating phase shift in two singlemode doublecore thermal sensing fibres, one made of glass and second impregnated. The observed range of sensitivities changed an order of magnitude. Since the beating is very sensitive to the temperature, the possible measurement accuracy is high, better than ±0,1oC and depends on the quality of the optoelectronic detection and data processing system.

Fig.9. shows the results of measurements of thermally induced coupled optical power level change in a sample of multimode doublecore thermal sensing fibre. For various sensor constructions the relative signal level changed form

0,5dB to 6dB for ∆T=100oC. Several mechanisms are responsible for temperature sensitivity of doublecore multimode impregnated optical fibre.

If the sensing multimode fibre is bent, there is a considerable dependence of power losses in a single core on temperature due to strong ∂(∆n)/∂T relation, what was presented in fig.7. The difference between the characteristics with two orthogonal bending planes was presented in fig.10.

The second major mechanism of multimode doublecore fibre temperature sensitivity seems to be thermal dependence of mode mixing in a straight as well as bent fibre, which is also related to ∂(∆n)/∂T. The mean value of optical power coupling between respective m-modes in a doublecore fibre is:

4/12/131

4/112 )/1)(/]()/())(8[()( NmNmaknTTCmm −∆= π (8)

where: a-core radius in twincore fibre, n1- core refraction, N-total number of modes in multimode optical fibre, m-number of coupled modes, here ∆=1-n2/n1. This value, and its temperature dependence, can be calculated assuming the relation between m and N values. It is assumed for step index fibre that N=V2/2. The total mode coupling is a sum of couplings for all propagated modes. The m modal number can be treated as a continuous parameter, assuming equal powers carried by a large number of modes. The thermal dependence of coupling efficiency is then:

dzLTCNTN

])([sin)/1()( 2

0∫=η (9)

The measurement curve in fig. 9 can be normalized to the maximal values of the signal and approximated by the following function explicitly expressing the temperature T through the normalized signal value Sn from the sensor:

BSACT no += ln][ (10)

where A and B are sensor constants, different for particular sensor solutions. Sn is normalized to the range of (0, 1). For particular realized multimode doublecore sensor the values of these constants were A around 70 and B around 90. The error analysis showed the temperature measurement accuracy to be within ±3oC for multimode doublecore fibre sensor. The singlemode sensor accuracy is greater but depends rather more on the detection system than on the fibre itself. The measurements were done in both directions with increasing and decreasing temperature for several thermal cycles. No hysteresis was observed during the measurements of multimode sensor.

Fig 11. shows the summary of numerable measurements of thermal sensitivity of various kinds of specialty optical fibres, to compare the data between them and that of the doublecore fibres. It is interesting to note that double core optical fibre thermal sensors exhibit quite a range of sensitivities depending on sensor construction.

5. APPLICATIONS Some of the solutions of the investigated optical fibre thermometers were applied in real environment conditions. Some were placed in hybrid integrated optical probe for measurements of environmental water. The description of probe solution is included elsewhere [9]. Here we quote some water temperature measurements results with the aid of optical fibre thermometers. The measurements results were presented in fig.12.

6. CONCLUSIONS Doublecore optical fibres exhibit considerable temperature sensitivity. This sensitivity is composed of the following factors. The temperature changes the length of optical fibre and its refractive index profile. These properties are dependent on material characteristics and are the same in single-core and doublecore fibres. To increase the thermal sensitivity of an optical fibre a transparent organic material is applied, having much stronger temperature dependence of its refraction than the glass. Among these substances there are transparent, low-loss resins, glues, lacquers, gums and silicones as well as Teflon derivatives. Specific to the doublecore fibre is the dependence of intercore optical power transfer and intercore signal interference (beating) on the temperature. The coupling between the cores depends on the core separation and particularly on the refractive index of the region between the cores. If the refraction of this region is strongly temperature dependent, then one can obtain sensitive temperature sensor. The multimode doublecore fiber

sensor relies on dependence of modal coupling on temperature. The singlemode doublecore fiber sensor relies either on dependence of interference pattern on temperature or the spectral band pass intercore coupling characteristics on temperature. With bent doublecore fibre, the temperature sensitivity depends on the location of plane of bending relative to the plane of cores. The effect of loss dependence on temperature is added here. If both planes agree the sensitivity is maximized because the power loss in internal core (which is more bent) is catched by the outside core (which is less bent). The differential bending is at the maximum. If these planes are perpendicular the intercore coupling does not change, because both cores suffer equal bending.

Fig.12. Environmental measurements with the aid of optical fibre thermometers; a) FOT ver.3.0, Water Inlet Czerniakowska, June; b) FOT ver.2.0, Sewage Cleaning Factory Pruszków, July;

We have shown the feasibility of manufacturing of singlemode as well as multimode doublecore optical fibre temperature sensors. The sensor detection circuit is quite simple and cheap. The sensors are susceptible to mass manufacturing, and construction of measurement multi-sensor networks.

7. ACKNOWLEDGEMENTS The work was partially supported by the following grants: “Photonics Engineering” Research Program of Warsaw University of Technology under the topic “Doublecore Optical Fibres” [8], Environmental Fiberoptic Monitoring System [9] and statute own research works of Białystok University of Technology no S/WE/2/98 and [6,7].

8. REFERENCES 1. P.Heinzmann, R.Hofstetter, Temperature dependence of PCS fiber characteristics, Proc of SPIE, vol. 584, Cannes

Optoelectronics Symposium, 1985, p. 123; 2. P. Lecoy, et al., New fiber optic distributed temperature sensor, Proc of SPIE, vol. 718, Fibre optic Sensors II,

1987, p.131; 3. L.Rui-fu, et al., A fiber optic temperature sensor based on bend losses; Proc of. SPIE, vol. 1572, Optical Fibre

Sensors in China, 1991, p. 180; 4. J.Pluciński, Optical fibre pseudo-distributed transmissive temperature sensor, VIIth Conf. Optical Fibres and Their

Applications, Krasnobród 1999, p. 351; 5. R.Romaniuk, J.Dorosz, Multicore single-mode soft-glass optical fibers, Optica Applicata, vol. 24, No 1-2, 1999, p.

15-50; 6. K.Biryłko, Temperature dependence on core coupling in doublecore optical fibre, M.Sc. Thesis, Białystok

University of Technology, 2000, (J.Dorosz, tutor); 7. R.Szczuka, Influence of coupling length on the power transfer in double core optical fibre; M.Sc. Thesis, Białystok

University of Technology, 2000, (J.Dorosz, tutor); 8. R.Romaniuk (edit), Doublecore optical fibres, Photonics Engineering Research Program Technical Reports, WUT,

Warsaw 1998-2000; http://www.nms.ise.pw.edu.pl ; Photonics Information Centre; 9. R.Romaniuk (edit), Integrated, multi-parameter, fiber optic probe for measurements surface environmental water,

Engineering Research Program Technical Reports, WUT, Warsaw 1995-2000; http://www.nms.ise.pw.edu.pl ; Photonics Information Centre;

Published in Proc SPIE, vol. 4887; www.spie.org