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Brillouin based distributed fibre sensor incorporating a mode-locked Brillouin fibre ring laser
1 July 1998
Optics Communications 152 Ž1998. 263–268
Brillouin based distributed fibre sensor incorporating a mode-locked Brillouin fibre ring laser V. Lecœuche, D.J. Webb, C.N. Pannell, D.A. Jackson Applied Optics Group, Physics Laboratory, UniÕersity of Kent, Canterbury CT2 7NR, UK Received 28 January 1998; accepted 25 March 1998
Abstract A pulsed Brillouin fibre ring laser has been developed and we describe its main features. The pump and the Brillouin laser are shown to form an excellent dual frequency source for distributed sensing. A first application for fire detection is demonstrated. q 1998 Elsevier Science B.V. All rights reserved.
For over 20 years, Brillouin Fibre Ring Lasers ŽBFRL. have been the subject of numerous studies, which essentially focused on the fundamental aspects of their dynamical behaviour w1–6x. However, only a few applications have thus far been found for these devices, presumably because of their high sensitivity to external perturbations. Another difficulty arises from the large range of experimentally observed behaviours, obtained at different working wavelengths, and with various cavity configurations, reinjection rates or lengths. Stable emission, periodic oscillations, pulsed regimes, quasiperiodic and chaotic motion have all been reported, often occurring alternately in a free running Brillouin laser. A tacitly adopted rule has been that CW emission can be obtained when the cavity Free Spectral Range ŽFSR. is large or of the order of the Brillouin gain linewidth, whereas pulsed regimes occur in the opposite case. These two regimes are obviously the ones of interest for a practical application, and efforts have been taken to stabilise these regimes. One possible solution is an active control either applied to the cavity length or the pump frequency w6,7x. This has allowed the use of a CW Brillouin laser as a gyroscope w8x and as a tunable frequency shifter w9,10x. An intracavity amplitude modulator can also be used to encourage mode-locking and reduce the fluctuations of the pulse intensity w11x. Stimulated Brillouin scattering has also been proven to be an efficient mechanism for implementing a distributed temperature and strain sensor w12x, especially when very
large sensing lengths are required w13x. Nevertheless, the need for two narrow linewidth laser sources, with a 12 GHz frequency difference controlled to about 1 MHz, is still a major factor limiting the development of a cost effective system. Several schemes have already been proposed in order to ‘‘synthesize’’ this dual frequency source from a single one. The approach taken in Ref. w14x involved a multipass fibre loop including a frequency shifter and an amplifier to overcome the cavity losses. Ultrafast electro-optic modulators have also been used to produce the frequency shift w15,16x. These solutions are still either complicated andror very costly. By contrast, a Brillouin generator would naturally have the required frequency shift w17x, and can be realised with the simplest standard telecommunication optical components. Recently, a global analysis of Brillouin laser dynamics has been undertaken, which elucidated the various regimes, and precisely defined the conditions of their occurrence in terms of a restricted number of parameters w18x. This work was used as a basis for the design of a mode-locked BFRL. We describe here the characteristics of this source, which appear to meet with the requirements of a Brillouin based distributed fibre sensor. We will show how it can be implemented in practice and demonstrate its efficient operation within an alarm system. The dynamic behaviour of a BFRL can be predicted as long as three parameters are known w18x, namely the number N of cavity modes under the gain curve Ždefined
0030-4018r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 3 0 - 4 0 1 8 Ž 9 8 . 0 0 1 8 7 - 4
264
V. Lecœuche et al.r Optics Communications 152 (1998) 263–268
as the ratio of the Brillouin spontaneous gain linewidth to the cavity FSR., the cavity reinjection rate and the pump intensity. Periodic oscillations of the emitted light can be obtained for a certain range of values of N: small values lead to a monomode and stable emission whereas too large values may result in quasiperiodic oscillations. Whatever the value of N, stable emission can be obtained for sufficiently high pump power. The exact boundary values of the various domains depend on the cavity reinjection rate and are mapped in Ref. w18x. In addition to these main features, the detuning of the cavity modes with the pump or Brillouin gain peak frequencies may also have dramatic effects on the laser dynamics w4,6,19x. Fluctuations of the pump detuning leads to variations of the intracavity field amplitudes, which can then also affect the dynamic regime. Stokes detuning drifts mainly affect the monomode regime of oscillation by inducing mode-hops. Although both effects can be avoided with an active control of the cavity length, a free running laser may be more reliable and easy to use for long term operation in a practical system. The pump detuning problem can be avoided by inserting an intracavity isolator which cancels the cavity feedback. Furthermore, when N is large enough, a mode-locked regime becomes less sensitive to the Stokes detuning drifts, since active modes drifting away from the gain peak are replaced by those approaching it. Given the pump power available Žabout 50 mW. and the linewidth of the Brillouin spontaneous gain curve Ž35 MHz at 1.3 mm., we were then able to define the fibre length and the reinjection rate of the cavity for ideal lasing operation. In fact, many pairs of these two latter parameters are possible, but we found through experiment that the best pulses Žhigh amplitudershort duration. could be obtained for the lowest cavity feedback. The experimental
setup is shown in Fig. 1. The pump laser source is a 100 mW solid state CW Nd:YAG fibre pigtailed laser operating at 1319 nm. The BFRL consists of 600 m of standard telecommunication fibre Ž N s 100., the reinjection rate of the cavity is about 5%. The fibre has been wound by hand to minimise inhomogeneities of the strain, and the whole cavity is thermally stabilised in an oven, ensuring an homogeneous broadening of the Brillouin gain curve along the fibre, and thus a well defined lasing frequency. Two fibre couplers have been used in this cavity, DC1 is for the pump injection, DC2 extracts the backscattered Stokes wave, the aim being to improve the effective pumprStokes conversion. The polarisation controllers, PC1 and PC2, respectively allow the adjustment of the pump polarisation and the polarisation modes of the cavity. The optical isolator OI1 cancels the pump feedback. A few meters of the fibre is wound on a piezoelectric cylinder in order to permit a phase modulation to be applied to the cavity. The lasing threshold was reached with 33 mW of pump injected in the cavity. Just above the threshold, the laser emission was pulsed and the best pulses occurred for a pump power of about 50 mW, the maximum peak power at the output of the coupler DC2 then being 60 mW. When free running, peak power fluctuations were estimated to be 25% RMS over long term operation. By applying a phase modulation at twice the FSR frequency, we could reduce these fluctuations to 10% RMS. Fig. 2 shows a typical train of pulses with modulation applied. Pulse duration at halfwidth was 80 ns and their repetition time, equal to the transit time of the cavity, was 2.9 ms. The stability of the emission frequency can be estimated through the analysis of the pumprStokes beating spectrum. The pump frequency drift is estimated to be around 5 MHzrhour. The Stokes frequency should follow
Fig. 1. Experimental setup. PC: polarisation controller, DC: directional coupler Ž1: 55r45%, 2–5: 87r13%., D: detector, EOM: electro-optic modulator, OI: optical isolator, PZT: piezoelectric.
V. Lecœuche et al.r Optics Communications 152 (1998) 263–268
265
Fig. 2. Typical evolution of the Stokes intensity.
this slow drift and the resulting beating frequency should be roughly constant. As the laser emission is pulsed, the spectrum of the beating signal consists of a multi-peaked structure centred at a frequency of 12.8 GHz, the peaks being equally separated by one FSR. We assume that the highest peak frequency gives the pumprStokes frequency shift. The measurement of this latter quantity was automated and repeated over 4 hours, yielding the statistical distribution plotted in Fig. 3a. It reveals that the variance of the frequency shift is less than 0.8 MHz for long term
operation. Similar results were obtained for short term measurements, suggesting that the main fluctuations of the frequency occur on a timescale lower than one minute. The beating frequency can be tuned continuously by changing the temperature of the oven. The mean frequency is graphed as function of the temperature in Fig. 3b. This linear response allows a calibrated tuning over a few tens of MHz. However, it should be noted that the polarisation eigenstates of the cavity are also altered, so that any temperature change in excess of 58C requires a readjust-
Fig. 3. Ža. Statistical distribution of the pumprStokes beating frequency. Žb. Mean frequency shift as function of temperature.
266
V. Lecœuche et al.r Optics Communications 152 (1998) 263–268
ment of the polarisation controllers. Otherwise, if the operating temperature is kept constant, the Brillouin source exhibited constant features for weeks without requiring any alignment. In a Brillouin based distributed temperature sensor, the repetition rate of the incoming pulses is limited to twice the transit time of light through the sensing fibre. The repetition rate of the Brillouin laser is therefore too high for sensing lengths in the kilometer range. This problem can be addressed by gating the pulses. This was carried out using a fibre pigtailed amplitude modulator in an arrangement depicted in Fig. 1. An electronic circuit was designed to generate a trigger signal once the signal from detector D1 reached a threshold value. The trigger enables the following pulse to go through the gate and then into the sensor. The circuit also permits the selection of pulses of the highest intensity through a fine adjustment of the threshold value, since neighbouring pulses tend to have similar intensities. After one pulse is detected, the circuit ignores the next pulses during a time delay fixed to 2 ms. Fig. 4 shows 10 selected pulses superimposed on the same graph, the trigger pulse was used to synchronize the data acquisition. The shape and amplitude of the pulses as well as the time delay with respect to the trigger pulse are highly reproducible. The method of Brillouin based distributed sensing is detailed in Ref. w13x. For the purposes of this demonstration, the system was configured as an alarm, designed so that an indication could be made whenever a region of the fibre reached a threshold temperature. About 6 mW of the remaining CW pump power is launched into one end of the sensor, Stokes pulses of 12 mW peak power are
launched into the other. The 15 km long sensing fibre is of the same kind as the one used to build the laser. Eight meters of fibre were wound on a metal cylinder, the temperature of which could be rapidly raised by blowing hot air through it. For this experiment, the oven temperature surrounding the Brillouin laser was set to an arbitrary value of 618C, which then defined the threshold temperature at which the sensor would respond. Data acquisition was synchronized to the incoming pulses and a time delay set to monitor the signal arising from the heated part of the fibre. In a fully engineered system, data should be recorded during a time of 2 nLrc following the pulse launching Ž n being the fibre index, L the sensing length and c the light velocity.. In this way, the whole fibre length could be monitored without penalty in acquisition time. Fig. 5a shows a simultaneous recording of the evolution of the detected signal and the temperature of the cylinder measured with a thermocouple. Each data point corresponds to 500 integrations of the signal, obtained every 2–3 s. During the first minute of this recording, the cylinder was held at room temperature. Then the heating was turned on; the signal gradually decreased with temperature, reaching a minimum at 618C. At this stage, the frequency shift between the pump and the Stokes waves exactly matched the resonance condition of a Brillouin interaction in the heated part of the fibre, resulting in a maximum depletion of the pump. Fig. 5b shows the evolution of the transmitted pump in this condition Žtime on the x-axis was replaced by the corresponding position on the sensing fibre.. From this trace, we can confirm that positional information can be determined with a resolution better than 10 m as expected from the Brillouin laser pulse width. As the temperature is
Fig. 4. Ten selected pulse profiles superimposed.
V. Lecœuche et al.r Optics Communications 152 (1998) 263–268
267
Fig. 5. Ža. Temperature Žcircles. and transmitted pump Žtriangles. evolutions while raising the temperature over the 8 m of the sensing fibre. Žb. Transmitted pump signal when the temperature of the sensing fibre equals the laser one.
further increased, the signal gradually returns to the initial value corresponding to an undepleted pump ŽFig. 5a.. The temperature resolution of a distributed sensor based on stimulated Brillouin scattering depends on the signal to noise ratio w20x, which in turn depends on the position along the fibre w13,21x. However, for an alarm application as described in this paper, the precise resolution is not critical since if there is a fire local to one region of the fibre, as the temperature rises it will at some stage be equal to the threshold temperature. It is not likely to be important if the alarm is raised when the temperature reaches precisely that value or 18C or even 58C before the threshold. We have realised and characterised a mode-locked BFRL. The main advantages of the source are the long term stability of the dynamic behaviour, and a well defined frequency shift which separates it from the pump source.
These features are of great interest for Brillouin based temperature sensing applications. The source was implemented in an alarm system which combines the advantages of fast response, auto-calibration and immunity to laser frequency drifts, without the requirements of high bandwidth electronics. The proposed sensor exhibits a spatial resolution better than 10 m over a 15 km sensing length. A fire condition was simulated by continuously raising the temperature of 8 m of the sensing fibre. If the temperature was not homogeneous or if smaller pieces of fibre were heated, signal variations would still occur but with less amplitude. Further tests should be performed to define the optimal Brillouin laser temperature and the threshold signal variation raising the alarm in order to optimise the sensitivity of the system. Alternatively, if the temperature along the sensing fibre is roughly constant, the same
268
V. Lecœuche et al.r Optics Communications 152 (1998) 263–268
arrangement could be used for strain detection. Actual distributed measurement of temperature Žor strain. could be achieved with acousto-optic modulators, allowing frequency tuning over tens of MHz, corresponding to a measurement range of tens of degrees.
References w1x K.O. Hill, B.S. Kawasaki, D.C. Johnson, Appl. Phys. Lett. 28 Ž1976. 608. w2x I. Bar-Joseph, A.A. Friesem, E. Lichtman, R.G. Waarts, J. Opt. Soc. Am. B 2 Ž1985. 1606. w3x E. Picholle, C. Montes, C. Leycuras, D. Legrand, J. Botineau, Phys. Rev. Lett. 66 Ž1991. 1454. w4x A.L. Gaeta, R.W. Boyd, Int. J. Nonlinear Physics 1 Ž1992. 581. w5x R.G. Harrison, P.M. Ripley, W. Lu, Phys. Rev. A 49 Ž1994. R24. w6x M. Dammig, G. Zinner, F. Mitschke, H. Weilling, Phys. Rev. A 48 Ž1993. 3301. w7x S.P. Smith, F. Zarinetchi, S. Ezekiel, Optics Lett. 16 Ž1991. 393. w8x R.K. Kadiwar, J.P. Giles, Electron. Lett. 25 Ž1989. 1729. w9x K. Kalli, D.O. Culverhouse, D.A. Jackson, Optics Lett. 16 Ž1991. 1538.
w10x O.S. Khan, R.P. Tatam, Optics Comm. 103 Ž1993. 161. w11x J. Botineau, C. Leycuras, C. Montes, E. Picholle, J. Opt. Soc. Am. B 6 Ž1989. 300. w12x T. Horiguchi, T. Kurashima, M. Tateda, IEEE Phot. Technol. Lett. 2 Ž1990. 352. w13x X. Bao, J. Dhliwayo, N. Heron, D.J. Webb, D.A. Jackson, J. Lightwave Technol. 13 Ž1995. 1340. w14x K. Shimizu, T. Horiguchi, Y. Koyamada, T. Kurashima, J. Lightwave Technol. 12 Ž1994. 730. w15x M. Nikles, L. Thevenaz, P.A. Robert, Optics Lett. 21 Ž1996. 758. w16x H. Izumita, T. Sato, M. Tateda, Y. Koyamada, IEEE Phot. Technol. Lett. 8 Ž1996. 1674. w17x N. Heron, X. Bao, D.J. Webb and D.A. Jackson, in: Distributed and Multiplexed Fibre Optic Sensors VI, A.D. Kersey, J.P. Dakin, Eds., Proc. SPIE 2838 Ž1996. 100. w18x V. Lecoeuche, B. Segard, J. Zemmouri, Optics Comm. 134 Ž1997. 547. w19x V. Lecoeuche, S. Randoux, B. Segard, J. Zemmouri, Phys. Rev. A 53 Ž1996. 2822. w20x C.N. Pannel, J. Dhliwayo, D.J. Webb, Meas. Science Technol. 9 Ž1998. 50. w21x T. Horiguchi, M. Tateda, J. Lightwave Technol. 7 Ž1989. 1170.
Optics Communications 152 Ž1998. 263–268
Brillouin based distributed fibre sensor incorporating a mode-locked Brillouin fibre ring laser V. Lecœuche, D.J. Webb, C.N. Pannell, D.A. Jackson Applied Optics Group, Physics Laboratory, UniÕersity of Kent, Canterbury CT2 7NR, UK Received 28 January 1998; accepted 25 March 1998
Abstract A pulsed Brillouin fibre ring laser has been developed and we describe its main features. The pump and the Brillouin laser are shown to form an excellent dual frequency source for distributed sensing. A first application for fire detection is demonstrated. q 1998 Elsevier Science B.V. All rights reserved.
For over 20 years, Brillouin Fibre Ring Lasers ŽBFRL. have been the subject of numerous studies, which essentially focused on the fundamental aspects of their dynamical behaviour w1–6x. However, only a few applications have thus far been found for these devices, presumably because of their high sensitivity to external perturbations. Another difficulty arises from the large range of experimentally observed behaviours, obtained at different working wavelengths, and with various cavity configurations, reinjection rates or lengths. Stable emission, periodic oscillations, pulsed regimes, quasiperiodic and chaotic motion have all been reported, often occurring alternately in a free running Brillouin laser. A tacitly adopted rule has been that CW emission can be obtained when the cavity Free Spectral Range ŽFSR. is large or of the order of the Brillouin gain linewidth, whereas pulsed regimes occur in the opposite case. These two regimes are obviously the ones of interest for a practical application, and efforts have been taken to stabilise these regimes. One possible solution is an active control either applied to the cavity length or the pump frequency w6,7x. This has allowed the use of a CW Brillouin laser as a gyroscope w8x and as a tunable frequency shifter w9,10x. An intracavity amplitude modulator can also be used to encourage mode-locking and reduce the fluctuations of the pulse intensity w11x. Stimulated Brillouin scattering has also been proven to be an efficient mechanism for implementing a distributed temperature and strain sensor w12x, especially when very
large sensing lengths are required w13x. Nevertheless, the need for two narrow linewidth laser sources, with a 12 GHz frequency difference controlled to about 1 MHz, is still a major factor limiting the development of a cost effective system. Several schemes have already been proposed in order to ‘‘synthesize’’ this dual frequency source from a single one. The approach taken in Ref. w14x involved a multipass fibre loop including a frequency shifter and an amplifier to overcome the cavity losses. Ultrafast electro-optic modulators have also been used to produce the frequency shift w15,16x. These solutions are still either complicated andror very costly. By contrast, a Brillouin generator would naturally have the required frequency shift w17x, and can be realised with the simplest standard telecommunication optical components. Recently, a global analysis of Brillouin laser dynamics has been undertaken, which elucidated the various regimes, and precisely defined the conditions of their occurrence in terms of a restricted number of parameters w18x. This work was used as a basis for the design of a mode-locked BFRL. We describe here the characteristics of this source, which appear to meet with the requirements of a Brillouin based distributed fibre sensor. We will show how it can be implemented in practice and demonstrate its efficient operation within an alarm system. The dynamic behaviour of a BFRL can be predicted as long as three parameters are known w18x, namely the number N of cavity modes under the gain curve Ždefined
0030-4018r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 3 0 - 4 0 1 8 Ž 9 8 . 0 0 1 8 7 - 4
264
V. Lecœuche et al.r Optics Communications 152 (1998) 263–268
as the ratio of the Brillouin spontaneous gain linewidth to the cavity FSR., the cavity reinjection rate and the pump intensity. Periodic oscillations of the emitted light can be obtained for a certain range of values of N: small values lead to a monomode and stable emission whereas too large values may result in quasiperiodic oscillations. Whatever the value of N, stable emission can be obtained for sufficiently high pump power. The exact boundary values of the various domains depend on the cavity reinjection rate and are mapped in Ref. w18x. In addition to these main features, the detuning of the cavity modes with the pump or Brillouin gain peak frequencies may also have dramatic effects on the laser dynamics w4,6,19x. Fluctuations of the pump detuning leads to variations of the intracavity field amplitudes, which can then also affect the dynamic regime. Stokes detuning drifts mainly affect the monomode regime of oscillation by inducing mode-hops. Although both effects can be avoided with an active control of the cavity length, a free running laser may be more reliable and easy to use for long term operation in a practical system. The pump detuning problem can be avoided by inserting an intracavity isolator which cancels the cavity feedback. Furthermore, when N is large enough, a mode-locked regime becomes less sensitive to the Stokes detuning drifts, since active modes drifting away from the gain peak are replaced by those approaching it. Given the pump power available Žabout 50 mW. and the linewidth of the Brillouin spontaneous gain curve Ž35 MHz at 1.3 mm., we were then able to define the fibre length and the reinjection rate of the cavity for ideal lasing operation. In fact, many pairs of these two latter parameters are possible, but we found through experiment that the best pulses Žhigh amplitudershort duration. could be obtained for the lowest cavity feedback. The experimental
setup is shown in Fig. 1. The pump laser source is a 100 mW solid state CW Nd:YAG fibre pigtailed laser operating at 1319 nm. The BFRL consists of 600 m of standard telecommunication fibre Ž N s 100., the reinjection rate of the cavity is about 5%. The fibre has been wound by hand to minimise inhomogeneities of the strain, and the whole cavity is thermally stabilised in an oven, ensuring an homogeneous broadening of the Brillouin gain curve along the fibre, and thus a well defined lasing frequency. Two fibre couplers have been used in this cavity, DC1 is for the pump injection, DC2 extracts the backscattered Stokes wave, the aim being to improve the effective pumprStokes conversion. The polarisation controllers, PC1 and PC2, respectively allow the adjustment of the pump polarisation and the polarisation modes of the cavity. The optical isolator OI1 cancels the pump feedback. A few meters of the fibre is wound on a piezoelectric cylinder in order to permit a phase modulation to be applied to the cavity. The lasing threshold was reached with 33 mW of pump injected in the cavity. Just above the threshold, the laser emission was pulsed and the best pulses occurred for a pump power of about 50 mW, the maximum peak power at the output of the coupler DC2 then being 60 mW. When free running, peak power fluctuations were estimated to be 25% RMS over long term operation. By applying a phase modulation at twice the FSR frequency, we could reduce these fluctuations to 10% RMS. Fig. 2 shows a typical train of pulses with modulation applied. Pulse duration at halfwidth was 80 ns and their repetition time, equal to the transit time of the cavity, was 2.9 ms. The stability of the emission frequency can be estimated through the analysis of the pumprStokes beating spectrum. The pump frequency drift is estimated to be around 5 MHzrhour. The Stokes frequency should follow
Fig. 1. Experimental setup. PC: polarisation controller, DC: directional coupler Ž1: 55r45%, 2–5: 87r13%., D: detector, EOM: electro-optic modulator, OI: optical isolator, PZT: piezoelectric.
V. Lecœuche et al.r Optics Communications 152 (1998) 263–268
265
Fig. 2. Typical evolution of the Stokes intensity.
this slow drift and the resulting beating frequency should be roughly constant. As the laser emission is pulsed, the spectrum of the beating signal consists of a multi-peaked structure centred at a frequency of 12.8 GHz, the peaks being equally separated by one FSR. We assume that the highest peak frequency gives the pumprStokes frequency shift. The measurement of this latter quantity was automated and repeated over 4 hours, yielding the statistical distribution plotted in Fig. 3a. It reveals that the variance of the frequency shift is less than 0.8 MHz for long term
operation. Similar results were obtained for short term measurements, suggesting that the main fluctuations of the frequency occur on a timescale lower than one minute. The beating frequency can be tuned continuously by changing the temperature of the oven. The mean frequency is graphed as function of the temperature in Fig. 3b. This linear response allows a calibrated tuning over a few tens of MHz. However, it should be noted that the polarisation eigenstates of the cavity are also altered, so that any temperature change in excess of 58C requires a readjust-
Fig. 3. Ža. Statistical distribution of the pumprStokes beating frequency. Žb. Mean frequency shift as function of temperature.
266
V. Lecœuche et al.r Optics Communications 152 (1998) 263–268
ment of the polarisation controllers. Otherwise, if the operating temperature is kept constant, the Brillouin source exhibited constant features for weeks without requiring any alignment. In a Brillouin based distributed temperature sensor, the repetition rate of the incoming pulses is limited to twice the transit time of light through the sensing fibre. The repetition rate of the Brillouin laser is therefore too high for sensing lengths in the kilometer range. This problem can be addressed by gating the pulses. This was carried out using a fibre pigtailed amplitude modulator in an arrangement depicted in Fig. 1. An electronic circuit was designed to generate a trigger signal once the signal from detector D1 reached a threshold value. The trigger enables the following pulse to go through the gate and then into the sensor. The circuit also permits the selection of pulses of the highest intensity through a fine adjustment of the threshold value, since neighbouring pulses tend to have similar intensities. After one pulse is detected, the circuit ignores the next pulses during a time delay fixed to 2 ms. Fig. 4 shows 10 selected pulses superimposed on the same graph, the trigger pulse was used to synchronize the data acquisition. The shape and amplitude of the pulses as well as the time delay with respect to the trigger pulse are highly reproducible. The method of Brillouin based distributed sensing is detailed in Ref. w13x. For the purposes of this demonstration, the system was configured as an alarm, designed so that an indication could be made whenever a region of the fibre reached a threshold temperature. About 6 mW of the remaining CW pump power is launched into one end of the sensor, Stokes pulses of 12 mW peak power are
launched into the other. The 15 km long sensing fibre is of the same kind as the one used to build the laser. Eight meters of fibre were wound on a metal cylinder, the temperature of which could be rapidly raised by blowing hot air through it. For this experiment, the oven temperature surrounding the Brillouin laser was set to an arbitrary value of 618C, which then defined the threshold temperature at which the sensor would respond. Data acquisition was synchronized to the incoming pulses and a time delay set to monitor the signal arising from the heated part of the fibre. In a fully engineered system, data should be recorded during a time of 2 nLrc following the pulse launching Ž n being the fibre index, L the sensing length and c the light velocity.. In this way, the whole fibre length could be monitored without penalty in acquisition time. Fig. 5a shows a simultaneous recording of the evolution of the detected signal and the temperature of the cylinder measured with a thermocouple. Each data point corresponds to 500 integrations of the signal, obtained every 2–3 s. During the first minute of this recording, the cylinder was held at room temperature. Then the heating was turned on; the signal gradually decreased with temperature, reaching a minimum at 618C. At this stage, the frequency shift between the pump and the Stokes waves exactly matched the resonance condition of a Brillouin interaction in the heated part of the fibre, resulting in a maximum depletion of the pump. Fig. 5b shows the evolution of the transmitted pump in this condition Žtime on the x-axis was replaced by the corresponding position on the sensing fibre.. From this trace, we can confirm that positional information can be determined with a resolution better than 10 m as expected from the Brillouin laser pulse width. As the temperature is
Fig. 4. Ten selected pulse profiles superimposed.
V. Lecœuche et al.r Optics Communications 152 (1998) 263–268
267
Fig. 5. Ža. Temperature Žcircles. and transmitted pump Žtriangles. evolutions while raising the temperature over the 8 m of the sensing fibre. Žb. Transmitted pump signal when the temperature of the sensing fibre equals the laser one.
further increased, the signal gradually returns to the initial value corresponding to an undepleted pump ŽFig. 5a.. The temperature resolution of a distributed sensor based on stimulated Brillouin scattering depends on the signal to noise ratio w20x, which in turn depends on the position along the fibre w13,21x. However, for an alarm application as described in this paper, the precise resolution is not critical since if there is a fire local to one region of the fibre, as the temperature rises it will at some stage be equal to the threshold temperature. It is not likely to be important if the alarm is raised when the temperature reaches precisely that value or 18C or even 58C before the threshold. We have realised and characterised a mode-locked BFRL. The main advantages of the source are the long term stability of the dynamic behaviour, and a well defined frequency shift which separates it from the pump source.
These features are of great interest for Brillouin based temperature sensing applications. The source was implemented in an alarm system which combines the advantages of fast response, auto-calibration and immunity to laser frequency drifts, without the requirements of high bandwidth electronics. The proposed sensor exhibits a spatial resolution better than 10 m over a 15 km sensing length. A fire condition was simulated by continuously raising the temperature of 8 m of the sensing fibre. If the temperature was not homogeneous or if smaller pieces of fibre were heated, signal variations would still occur but with less amplitude. Further tests should be performed to define the optimal Brillouin laser temperature and the threshold signal variation raising the alarm in order to optimise the sensitivity of the system. Alternatively, if the temperature along the sensing fibre is roughly constant, the same
268
V. Lecœuche et al.r Optics Communications 152 (1998) 263–268
arrangement could be used for strain detection. Actual distributed measurement of temperature Žor strain. could be achieved with acousto-optic modulators, allowing frequency tuning over tens of MHz, corresponding to a measurement range of tens of degrees.
References w1x K.O. Hill, B.S. Kawasaki, D.C. Johnson, Appl. Phys. Lett. 28 Ž1976. 608. w2x I. Bar-Joseph, A.A. Friesem, E. Lichtman, R.G. Waarts, J. Opt. Soc. Am. B 2 Ž1985. 1606. w3x E. Picholle, C. Montes, C. Leycuras, D. Legrand, J. Botineau, Phys. Rev. Lett. 66 Ž1991. 1454. w4x A.L. Gaeta, R.W. Boyd, Int. J. Nonlinear Physics 1 Ž1992. 581. w5x R.G. Harrison, P.M. Ripley, W. Lu, Phys. Rev. A 49 Ž1994. R24. w6x M. Dammig, G. Zinner, F. Mitschke, H. Weilling, Phys. Rev. A 48 Ž1993. 3301. w7x S.P. Smith, F. Zarinetchi, S. Ezekiel, Optics Lett. 16 Ž1991. 393. w8x R.K. Kadiwar, J.P. Giles, Electron. Lett. 25 Ž1989. 1729. w9x K. Kalli, D.O. Culverhouse, D.A. Jackson, Optics Lett. 16 Ž1991. 1538.
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