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Multiplexed sensor network employing birefringent-fibre WDMs
I November 1996
OPTICS COMMUNICATIONS ELSEVIER
Optics Communications 131 (1996) 290-294
Multiplexed sensor network employing birefringent-fibre WDMs H.D. Ford, R.P. Tatam Optical Sensors Group. Centrejbr Photonics and Optical Engineering, School of Mechanical Engineering, Cranfield University, Cranfield, Bedford, MK43 OAL. UK Received 3 January 1996; accepted 20 March 1990
Abstract A multiplexed sensor network incorporating wavelength-division demultiplexers (WDMs) constructed from birefringent fibre is presented. Experimental results from a hybrid fibre/bulk-optic version exhibit a worst case optical cross-talk, limited by the extinction ratio for the polarisation-splitting fibre component, of - 10 dB. Consideration of typical losses for an all-fibre version of the network suggests that, if polarisation-splitting couplers with a - 25 dB extinction ratio are employed, up to ten sensors can be multiplexed in this way, with cross-talk maintained below - 15 dB.
PACS: 42,8 I Keytw)~l,v: WDM: Birefrtngent fibre; Multiplexer; Fibre sensor
Wavelength.division multiplexing is used in optical fibre sensor systems to combine two or more signals at different wavelengths onto a single fibre for addressing a series of sensors at a remote location [ 1-3]. A new type of optical fibre wavelength-division multi/demultiplexer has been previously demonstrated by the authors, and is described elsewhere [4]. In brief, the optical fibre design exploits the periodic evolution with distance travelled of the state of polarisation (SOB of light propagating in a highly bi.'efringent (hi-bi) fibre. Any particular SOP is reproduced at intervals along the fibre axis [5] separated by the beat length L a, where L a - A / A n . Here A is the wavelength of the light in the fibre, and An the fibre birefringence. The beat length is thus a function of the wavelength of the light. It is
clear that an increase in wavelength results in a longer beat length, and that for constant birefringence A n, the relationship is linear. However, in any real fibre, An is also a function of wavelength, due to the effects of material and waveguide dispersion [6]. Thus when two optical signals having slightly different wavelengths are carried by a hi-bi optical fibre, the beat lengths for the two signals also differ slightly. It is assumed here that the signals, prior to entering the fibre, share a common SOP and therefore propagate initially along the fibre with closely matched polarisation states. However, as the distance from the fibre input face increases, the polarisation states become progressively more dephased. At a particular distance along the fibre, a situation arises in which one of the signals has completed an integral
0030.4018/96/$12.00 Copyright O 1996 Elsevier Science B.V. All rights reserved. PIi S0030-4018(96)00269-6
291
ll.D. Ford, R.P. Tamm / Optics" Communications 131 (1996) 290-294
number of polarisation cycles, while the other differ in phase by exactly one half of a polarisation cycle. Then, 2~'L
AI
ter, or a polarisation-splitting fibre coupler (PSC), having its eigenaxes aligned parallel with the polarisation directions of the two signals. The scheme described provides the basis for a polarisation-based wavelength demultiplexer. When it is required to separate signals at three or more equally-spaced wavelengths, demultiplexing can be achieved in stages, using a ladder-style network of concatenated fibre sections. The birefringence in highly birefringent fibre is temperature-dependent, hence the minimum crosstalk that can be maintained over an extended period is limited by sensitivity to ambient temperature fluctuations. The temperature sensitivity of the Eotec fibre used in this work, measured as the phase delay between axes, is 4.6 + 0.2 rad °C-I m - I , which implies that active temperature control is necessary in many practical situations. Choice of an appropriate fibre type can reduce the degree of temperature control required, since temperature sensitivity varies for fibres from different manufacturers. In general, geometrically birefringent fibre has a lower sensitivity than fibres incorporating stress lobes in the cladding. It has been suggested that application of polymeric coatings to fibres could reduce the temperature sensitivity [7], enabling a completely pas-
A2
where L is the position along the fibre, measured from the input face, & ha, is the fibre birefringence at free-space wavelength Ai, and m is an integer in the range 0, l, 2 . . . . . The shortest length of fibre for which this situation can be achieved is given by Eq. (1) with m = 0. Thus, a section of fibre cut to length L functions as a full-wave plate at wavelength AI, and as a half-wave plate at wavelength X2. For a half-wave plate, the phase retardation results in a sign change in the azimuth of the polarisation state, although the ellipticity remains unchanged. For a linear input polarisation state, having an azimuth of + 45 ° with respect to the axes of the fibre, both signals will emerge from the fibre with linear states of polarisation. However, the SOP of the signal at Xl has an azimuth of + 45° with respect to the fibre axes, while the SOP of the signal at X2 has an azimuth of - 4 5 °. These are independent, or orthogonal linear states. The two signals can therefore be spatially separated by a polarising beamsplit-
Diode laser I°..... i
PZTs
/\
[- ............... ] i Modulator 1 Oscilloscope
,
Isolator
\
QWP L , I(~
WDM B$
Peitier ----] !"~! controller J-"'Jt . .".~. .
Peltier ~element
tion ~
splitting coupler
Fig. I. Experimentalconfiguration for a hybrid two.wavelengthmultiplexed system, M: mirror, L: lens, QWP: quarter-waveplate, HWP: half-wave plate, PZT: piezoelectrictransducer,BS: beamsplittcr.
292
H.D. Ford. R.P. Tatam / Optics Communications 131 (1996)290-294
sive device to be constructed, but such fibres are not yet commercially available. A two-wavelength system is described here which demonstrates a possible signal-processing scheme for the birefringent demultiplexers. The experimental configuration used was the bulk-optic/fibre hybrid arrangement shown in Fig. I. A demultiplexer with good temperature stability, consisting therefore of only a short length of fibre, was required in order to minimise the cross-talk between the signals. The device used was constructed from Eotec FS-HB-3211 fibre. The operational wavelength of the PSC was 820 nm, and a Hitachi HL8311E diode laser, operating at a nominal wavelength of 830 nm, was thus chosen as an appropriate source. The maximum continuous tuning range of the diode laser was measured and found to be about 70 GHz, for demultiplexing of which about 5 m of fibre would be required. This length requires very careful temperature stabilisation for good longterm stability. It was found more satisfactory to set the temperature and injection current of the laser closely adjacent to a mode-hop position. A small modulation of the injection current about this point then caused a large and repeatable modulation of the optical frequency. For the particular laser used, setting the injection current to 90 mA and the temperature to between 29°C and 3 I°C produced a frequency shift of 1.5 nm (710 OHz) for a current modulation of under 2 mA. The centre wavelength under these conditions was 795 nm, which was much lower than the nominal wavelength. Only 0.505 m of fibre were required for demultiplexing of the 710 GHz frequency shift, with a concomitant temperature control requirement of 0.15°C. Tnis was readily achieved using a feedback circuit. The system was set up as in Fig. !, using the following method, The axes of the demultiplexer fibre were first located. This was achieved by launching broadband, linearly polarised light into the fibre, varying the input azimuth over a 90 ° range and monitoring the ratio of the maximum to minimum intensities observed as a polariser was rotated in the output light. Light was subsequently launched into one pola~"isation axis of the demultiplexer, and a half-wave plate used to match the fibre axes to the axes of the PSC. This half-wave plate was then rotated through 22.5 °. With the axes of all elements
in the system arranged in this way, a quarter-wave plate was positioned before the demultiplexer input, aligned at 45 ° to the polarisation axis of the input light so as to provide a circularly polarised input for the demultiplexer. The 50/50 beam splitter shown in the diagram was positioned to reflect half of any light returning towards the source onto a photodetector. A very low frequency (sub-hertz) square-wave modulation was now applied to the laser and the temperature of the Peltier element adjusted until each wavelength signal appeared on only one output arm of the PSC. The outputs were directed into two sensors, configured as bulk-optic Michelson intefferometers, to demonstrate the technique. One of the mirrors in each interferometer was mounted on a PZT cylinder, allowing a modulation signal to be imposed on either channel by application of a periodic voltage to the PZT cylinder, causing translation of the mirror along the axis of the interferometer. Light carrying this signal was then re-launched into the PSC from the output end, producing a response at the detector. The system was tested by modulating the PZT cylinders in the two sensing intefferometers with setrodyne voltages at different frequencies. For serrodyne modulation, linear response of the cylinders is obtained only at low frequencies. Values of 89 Hz and 179 Hz were selected, and the amplitude of the (o} 80O-.10
]]me Ires 0
10 i !
.eooLZ__l (b) 800"10
TimeIres 0
10
-e00
Fig. 2. Detector output from the two-wavelength multiplexed system, displayed on an oscilloscope, as the wavelength is switched to address (a) sensor ! at 89 Hz or (b) sensor 2 at 179 Hz.
293
H.D. Ford. R.P. Tatam / Optics Communications 131 (1996) 290-294 (al ? dBV rms
I
LogHag 10 dB Idiv
,, ,/t ,..nllPl -73J 0
i,,/!! I
A Itrl il I IIIJI.A /LLi LI^ I ¥ lYYt/IL iv
gl
200 FrequencyI Hz
(b) -/ dBV rmsl LogHag 10 dB Idiv
j 0
200 Frequency I Hz
Fig. 3. Detector output from the two-wavelength multiplexed system, displayed on a spectrum analyser, as the wavelength '" ~itched to address (a) sensor I at 89 Hz or (b) sensor 2 at 179 Hz.
modulation was set to drive the output of each interferometer over one fringe per modulation cycle. The amplified detector output was displayed both on a digital storage oscilloscope and on a spectrum analyser, as the low frequency square wave modulation was applied to the laser to address each sensing element in turn. Results are shown in Figs. 2 and 3. The oscilloscope traces (Fig. 2) clearly demonstrate the difference in frequency between the signals detected at the two wavelengths. From the spectrum analyser traces (Fig. 3), the cross-talk between channels can be determined. Optical cross-talk values are obtained by halving the electrical cross-talk, giving - 10 dB and - 14 dB for the two traces. The unequal values indicate a slight departure from the optimum operating point of the demultiplexer. The values are rather high, compared with theoretical predictions. This can be attributed principally to the poor extinction ratio of the PSC. The extinction ratio was measured to be only - 1 5 dB at 795 nm, which was 25 nm below the optimum operating wavelength of 820 nm. It should also be noted that the waveplates used in the multiplexed network were designed for use at 800 nm, whereas the laser centre wavelength was 795 nm. The SOP of the light was therefore slightly elliptical after passing through the waveplates. The cross-talk measured for the demultiplexer is consistent with these limitations.
Coupler
;K,.;k~.. WDM
X,,~.3.,. ~WDM ~
J
.
×.Xo.. ~.
WDM ..... WDM
__
_ _
-"
.×.
....... ~
....
-: .,,,.. s. i
!
' Source
WDM
'S, ' : ....
W D M ..... Xa,~.e...
-
~
X,...
"
Signal processing i
~q,;~l, 2.'"' ~llt' N
Fig. 4. Suggested all-fibre multiplexingconfiguration for a multi-wavelengthsystem. S t..... SN denote sensors used in reflection. WDM: wavelength-divisionmultiplexers, PSC: polarisation-splittingcoupler.
294
H.D. Ford. R.P. Tatam / Optics Communications 131 (1996)290-294
Although Fig. 1 shows a hybrid arrangement, all-fibre implementations are also possible. A multiwavelength all-fibre configuration is illustrated in Fig. 4. The number of sensors that can be combined in a multiplexed system is dependent on the power loss and cross-talk associated with each element in the system. In contrast to communications systems, optical fibre sensing systems generally require a relatively small number of channels [8]. In a wavelength-division multiplexed system, all the power in a particular wavelength band follows a specified route through the fibre, and the power loss for each channel is therefore simply the accumulated insertion loss of the system components for the route followed by light in that channel. Losses in polarisation-splitting couplers can be below 1.0 dB and in fused single-mode splices below 0.2 dB, which suggests a total insertion loss of no more than 12 dB for up to ten channels in the ladder topology of Fig. 4. The cross-talk of elements in this system is limited by the - 2 5 dB typical for a polarisation-splitting coupler. The total cross-talk thus exceeds - 2 0 dB for just three elements, and exceeds - 15 dB for ten sensors. In a WDM network, all the power in each wavelength channel is ideally transmitted to one particular sensor, the only cause of a reduction in power being losses in the optical components comprising the network. For a typical input power of 0 dBm, detector noise floor of - 7 0 dBm and sensor dynamic range of 20 dB, power losses up to a maximum of 50 dB per channel are acceptable [9], whereas cross-talk is usually required to be maintained below - 2 0 dB. The cross-talk therefore determines the maximum number of channels in this type of multiplexed system.
If the channel wavelength spacing is A A, the wavelength separation of the first WDM in the sys-
tern should also be A A, the wavelength separation of the next pair of WDMs 2 A A, that of the next set 4AA and so on until all wavelengths have been physically separated. It has been noted that the birefringence is wavelength dependent, implying inequality between the lengths of the WDMs in one level of the tree network. However, the effect is weak and, in practice, cross-talk of less than - 2 5 dB can be maintained for any particular WDM over a wavelength range of about +20 nm about its centre value. Thus for a total system bandwidth of a few nanometres, all WDMs in any given level can be equal in length, as stated above. The multiple wavelength channels required can be derived from individual diode lasers each running in a single longitudinal mode or, for better stability, from adjacent modes of one multimode diode laser. A wavelength-tunable single-mode diode laser can also be used to allow tuning to each sensor separately.
References [I] J.M. Senior and S.D. Cusworth, Int. J. Optoelectron. 6 (1991) 521. [2] A.D. Kersey, Electron. Lett. 27 (1991) 554. [3] F. Farahi, T.P. Newsox,, P. Akhavan Leilabady, J.D.C. Jones and D.A, Jackson, Int. J. Optoelectron. 3 (1988) 79. [4] H.D. Ford and R,P. Tatam, Optics Comm. 98 (1993) 151. [5] J,D.C, Jones and D.A. Jackson, Analytical Proceedings 22 (1985) 207. [6] D. Ologe, Appl. Optics II (1971) 2442. [7] J.M. Senior and S.D. Cuswortll, lEE Proceedings, 136 Pt. J (1989) 183. [8] Y. Kikuchi, R. Yamauchi, M. Akiyama, O. Fukuda and K. lnada, Proc. SPiE Int. Soc. Opt. Eng. 154 (1984) 395. [9] R. Kist, in: Optical Fiber Sensors: Systems and Applications, Vol. 2, eds. B. Cuishaw and J. Dakin (Artech House Inc., Norwood, Massachusetts, USA, 1989) pp. 525-53 I.
OPTICS COMMUNICATIONS ELSEVIER
Optics Communications 131 (1996) 290-294
Multiplexed sensor network employing birefringent-fibre WDMs H.D. Ford, R.P. Tatam Optical Sensors Group. Centrejbr Photonics and Optical Engineering, School of Mechanical Engineering, Cranfield University, Cranfield, Bedford, MK43 OAL. UK Received 3 January 1996; accepted 20 March 1990
Abstract A multiplexed sensor network incorporating wavelength-division demultiplexers (WDMs) constructed from birefringent fibre is presented. Experimental results from a hybrid fibre/bulk-optic version exhibit a worst case optical cross-talk, limited by the extinction ratio for the polarisation-splitting fibre component, of - 10 dB. Consideration of typical losses for an all-fibre version of the network suggests that, if polarisation-splitting couplers with a - 25 dB extinction ratio are employed, up to ten sensors can be multiplexed in this way, with cross-talk maintained below - 15 dB.
PACS: 42,8 I Keytw)~l,v: WDM: Birefrtngent fibre; Multiplexer; Fibre sensor
Wavelength.division multiplexing is used in optical fibre sensor systems to combine two or more signals at different wavelengths onto a single fibre for addressing a series of sensors at a remote location [ 1-3]. A new type of optical fibre wavelength-division multi/demultiplexer has been previously demonstrated by the authors, and is described elsewhere [4]. In brief, the optical fibre design exploits the periodic evolution with distance travelled of the state of polarisation (SOB of light propagating in a highly bi.'efringent (hi-bi) fibre. Any particular SOP is reproduced at intervals along the fibre axis [5] separated by the beat length L a, where L a - A / A n . Here A is the wavelength of the light in the fibre, and An the fibre birefringence. The beat length is thus a function of the wavelength of the light. It is
clear that an increase in wavelength results in a longer beat length, and that for constant birefringence A n, the relationship is linear. However, in any real fibre, An is also a function of wavelength, due to the effects of material and waveguide dispersion [6]. Thus when two optical signals having slightly different wavelengths are carried by a hi-bi optical fibre, the beat lengths for the two signals also differ slightly. It is assumed here that the signals, prior to entering the fibre, share a common SOP and therefore propagate initially along the fibre with closely matched polarisation states. However, as the distance from the fibre input face increases, the polarisation states become progressively more dephased. At a particular distance along the fibre, a situation arises in which one of the signals has completed an integral
0030.4018/96/$12.00 Copyright O 1996 Elsevier Science B.V. All rights reserved. PIi S0030-4018(96)00269-6
291
ll.D. Ford, R.P. Tamm / Optics" Communications 131 (1996) 290-294
number of polarisation cycles, while the other differ in phase by exactly one half of a polarisation cycle. Then, 2~'L
AI
ter, or a polarisation-splitting fibre coupler (PSC), having its eigenaxes aligned parallel with the polarisation directions of the two signals. The scheme described provides the basis for a polarisation-based wavelength demultiplexer. When it is required to separate signals at three or more equally-spaced wavelengths, demultiplexing can be achieved in stages, using a ladder-style network of concatenated fibre sections. The birefringence in highly birefringent fibre is temperature-dependent, hence the minimum crosstalk that can be maintained over an extended period is limited by sensitivity to ambient temperature fluctuations. The temperature sensitivity of the Eotec fibre used in this work, measured as the phase delay between axes, is 4.6 + 0.2 rad °C-I m - I , which implies that active temperature control is necessary in many practical situations. Choice of an appropriate fibre type can reduce the degree of temperature control required, since temperature sensitivity varies for fibres from different manufacturers. In general, geometrically birefringent fibre has a lower sensitivity than fibres incorporating stress lobes in the cladding. It has been suggested that application of polymeric coatings to fibres could reduce the temperature sensitivity [7], enabling a completely pas-
A2
where L is the position along the fibre, measured from the input face, & ha, is the fibre birefringence at free-space wavelength Ai, and m is an integer in the range 0, l, 2 . . . . . The shortest length of fibre for which this situation can be achieved is given by Eq. (1) with m = 0. Thus, a section of fibre cut to length L functions as a full-wave plate at wavelength AI, and as a half-wave plate at wavelength X2. For a half-wave plate, the phase retardation results in a sign change in the azimuth of the polarisation state, although the ellipticity remains unchanged. For a linear input polarisation state, having an azimuth of + 45 ° with respect to the axes of the fibre, both signals will emerge from the fibre with linear states of polarisation. However, the SOP of the signal at Xl has an azimuth of + 45° with respect to the fibre axes, while the SOP of the signal at X2 has an azimuth of - 4 5 °. These are independent, or orthogonal linear states. The two signals can therefore be spatially separated by a polarising beamsplit-
Diode laser I°..... i
PZTs
/\
[- ............... ] i Modulator 1 Oscilloscope
,
Isolator
\
QWP L , I(~
WDM B$
Peitier ----] !"~! controller J-"'Jt . .".~. .
Peltier ~element
tion ~
splitting coupler
Fig. I. Experimentalconfiguration for a hybrid two.wavelengthmultiplexed system, M: mirror, L: lens, QWP: quarter-waveplate, HWP: half-wave plate, PZT: piezoelectrictransducer,BS: beamsplittcr.
292
H.D. Ford. R.P. Tatam / Optics Communications 131 (1996)290-294
sive device to be constructed, but such fibres are not yet commercially available. A two-wavelength system is described here which demonstrates a possible signal-processing scheme for the birefringent demultiplexers. The experimental configuration used was the bulk-optic/fibre hybrid arrangement shown in Fig. I. A demultiplexer with good temperature stability, consisting therefore of only a short length of fibre, was required in order to minimise the cross-talk between the signals. The device used was constructed from Eotec FS-HB-3211 fibre. The operational wavelength of the PSC was 820 nm, and a Hitachi HL8311E diode laser, operating at a nominal wavelength of 830 nm, was thus chosen as an appropriate source. The maximum continuous tuning range of the diode laser was measured and found to be about 70 GHz, for demultiplexing of which about 5 m of fibre would be required. This length requires very careful temperature stabilisation for good longterm stability. It was found more satisfactory to set the temperature and injection current of the laser closely adjacent to a mode-hop position. A small modulation of the injection current about this point then caused a large and repeatable modulation of the optical frequency. For the particular laser used, setting the injection current to 90 mA and the temperature to between 29°C and 3 I°C produced a frequency shift of 1.5 nm (710 OHz) for a current modulation of under 2 mA. The centre wavelength under these conditions was 795 nm, which was much lower than the nominal wavelength. Only 0.505 m of fibre were required for demultiplexing of the 710 GHz frequency shift, with a concomitant temperature control requirement of 0.15°C. Tnis was readily achieved using a feedback circuit. The system was set up as in Fig. !, using the following method, The axes of the demultiplexer fibre were first located. This was achieved by launching broadband, linearly polarised light into the fibre, varying the input azimuth over a 90 ° range and monitoring the ratio of the maximum to minimum intensities observed as a polariser was rotated in the output light. Light was subsequently launched into one pola~"isation axis of the demultiplexer, and a half-wave plate used to match the fibre axes to the axes of the PSC. This half-wave plate was then rotated through 22.5 °. With the axes of all elements
in the system arranged in this way, a quarter-wave plate was positioned before the demultiplexer input, aligned at 45 ° to the polarisation axis of the input light so as to provide a circularly polarised input for the demultiplexer. The 50/50 beam splitter shown in the diagram was positioned to reflect half of any light returning towards the source onto a photodetector. A very low frequency (sub-hertz) square-wave modulation was now applied to the laser and the temperature of the Peltier element adjusted until each wavelength signal appeared on only one output arm of the PSC. The outputs were directed into two sensors, configured as bulk-optic Michelson intefferometers, to demonstrate the technique. One of the mirrors in each interferometer was mounted on a PZT cylinder, allowing a modulation signal to be imposed on either channel by application of a periodic voltage to the PZT cylinder, causing translation of the mirror along the axis of the interferometer. Light carrying this signal was then re-launched into the PSC from the output end, producing a response at the detector. The system was tested by modulating the PZT cylinders in the two sensing intefferometers with setrodyne voltages at different frequencies. For serrodyne modulation, linear response of the cylinders is obtained only at low frequencies. Values of 89 Hz and 179 Hz were selected, and the amplitude of the (o} 80O-.10
]]me Ires 0
10 i !
.eooLZ__l (b) 800"10
TimeIres 0
10
-e00
Fig. 2. Detector output from the two-wavelength multiplexed system, displayed on an oscilloscope, as the wavelength is switched to address (a) sensor ! at 89 Hz or (b) sensor 2 at 179 Hz.
293
H.D. Ford. R.P. Tatam / Optics Communications 131 (1996) 290-294 (al ? dBV rms
I
LogHag 10 dB Idiv
,, ,/t ,..nllPl -73J 0
i,,/!! I
A Itrl il I IIIJI.A /LLi LI^ I ¥ lYYt/IL iv
gl
200 FrequencyI Hz
(b) -/ dBV rmsl LogHag 10 dB Idiv
j 0
200 Frequency I Hz
Fig. 3. Detector output from the two-wavelength multiplexed system, displayed on a spectrum analyser, as the wavelength '" ~itched to address (a) sensor I at 89 Hz or (b) sensor 2 at 179 Hz.
modulation was set to drive the output of each interferometer over one fringe per modulation cycle. The amplified detector output was displayed both on a digital storage oscilloscope and on a spectrum analyser, as the low frequency square wave modulation was applied to the laser to address each sensing element in turn. Results are shown in Figs. 2 and 3. The oscilloscope traces (Fig. 2) clearly demonstrate the difference in frequency between the signals detected at the two wavelengths. From the spectrum analyser traces (Fig. 3), the cross-talk between channels can be determined. Optical cross-talk values are obtained by halving the electrical cross-talk, giving - 10 dB and - 14 dB for the two traces. The unequal values indicate a slight departure from the optimum operating point of the demultiplexer. The values are rather high, compared with theoretical predictions. This can be attributed principally to the poor extinction ratio of the PSC. The extinction ratio was measured to be only - 1 5 dB at 795 nm, which was 25 nm below the optimum operating wavelength of 820 nm. It should also be noted that the waveplates used in the multiplexed network were designed for use at 800 nm, whereas the laser centre wavelength was 795 nm. The SOP of the light was therefore slightly elliptical after passing through the waveplates. The cross-talk measured for the demultiplexer is consistent with these limitations.
Coupler
;K,.;k~.. WDM
X,,~.3.,. ~WDM ~
J
.
×.Xo.. ~.
WDM ..... WDM
__
_ _
-"
.×.
....... ~
....
-: .,,,.. s. i
!
' Source
WDM
'S, ' : ....
W D M ..... Xa,~.e...
-
~
X,...
"
Signal processing i
~q,;~l, 2.'"' ~llt' N
Fig. 4. Suggested all-fibre multiplexingconfiguration for a multi-wavelengthsystem. S t..... SN denote sensors used in reflection. WDM: wavelength-divisionmultiplexers, PSC: polarisation-splittingcoupler.
294
H.D. Ford. R.P. Tatam / Optics Communications 131 (1996)290-294
Although Fig. 1 shows a hybrid arrangement, all-fibre implementations are also possible. A multiwavelength all-fibre configuration is illustrated in Fig. 4. The number of sensors that can be combined in a multiplexed system is dependent on the power loss and cross-talk associated with each element in the system. In contrast to communications systems, optical fibre sensing systems generally require a relatively small number of channels [8]. In a wavelength-division multiplexed system, all the power in a particular wavelength band follows a specified route through the fibre, and the power loss for each channel is therefore simply the accumulated insertion loss of the system components for the route followed by light in that channel. Losses in polarisation-splitting couplers can be below 1.0 dB and in fused single-mode splices below 0.2 dB, which suggests a total insertion loss of no more than 12 dB for up to ten channels in the ladder topology of Fig. 4. The cross-talk of elements in this system is limited by the - 2 5 dB typical for a polarisation-splitting coupler. The total cross-talk thus exceeds - 2 0 dB for just three elements, and exceeds - 15 dB for ten sensors. In a WDM network, all the power in each wavelength channel is ideally transmitted to one particular sensor, the only cause of a reduction in power being losses in the optical components comprising the network. For a typical input power of 0 dBm, detector noise floor of - 7 0 dBm and sensor dynamic range of 20 dB, power losses up to a maximum of 50 dB per channel are acceptable [9], whereas cross-talk is usually required to be maintained below - 2 0 dB. The cross-talk therefore determines the maximum number of channels in this type of multiplexed system.
If the channel wavelength spacing is A A, the wavelength separation of the first WDM in the sys-
tern should also be A A, the wavelength separation of the next pair of WDMs 2 A A, that of the next set 4AA and so on until all wavelengths have been physically separated. It has been noted that the birefringence is wavelength dependent, implying inequality between the lengths of the WDMs in one level of the tree network. However, the effect is weak and, in practice, cross-talk of less than - 2 5 dB can be maintained for any particular WDM over a wavelength range of about +20 nm about its centre value. Thus for a total system bandwidth of a few nanometres, all WDMs in any given level can be equal in length, as stated above. The multiple wavelength channels required can be derived from individual diode lasers each running in a single longitudinal mode or, for better stability, from adjacent modes of one multimode diode laser. A wavelength-tunable single-mode diode laser can also be used to allow tuning to each sensor separately.
References [I] J.M. Senior and S.D. Cusworth, Int. J. Optoelectron. 6 (1991) 521. [2] A.D. Kersey, Electron. Lett. 27 (1991) 554. [3] F. Farahi, T.P. Newsox,, P. Akhavan Leilabady, J.D.C. Jones and D.A, Jackson, Int. J. Optoelectron. 3 (1988) 79. [4] H.D. Ford and R,P. Tatam, Optics Comm. 98 (1993) 151. [5] J,D.C, Jones and D.A. Jackson, Analytical Proceedings 22 (1985) 207. [6] D. Ologe, Appl. Optics II (1971) 2442. [7] J.M. Senior and S.D. Cuswortll, lEE Proceedings, 136 Pt. J (1989) 183. [8] Y. Kikuchi, R. Yamauchi, M. Akiyama, O. Fukuda and K. lnada, Proc. SPiE Int. Soc. Opt. Eng. 154 (1984) 395. [9] R. Kist, in: Optical Fiber Sensors: Systems and Applications, Vol. 2, eds. B. Cuishaw and J. Dakin (Artech House Inc., Norwood, Massachusetts, USA, 1989) pp. 525-53 I.