Linearly configured BOCDA system using a differential measurement scheme Ji Ho Jeong,1 Kyu Hwang Chung,1,2 Sang Bae Lee,1 Kwang Yong Song,2 Je-Myung Jeong,3 and Kwanil Lee1,* 1
Center for Opto-Electronic Convergence Systems, Korea Institute of Science and Technology (KIST), Seoul 136791, South Korea 2 Dept. of Physics, Chung-Ang University, Seoul 156-756, South Korea 3 Dept. of Electrical and Computer Engineering, Hanyang University, Seoul 133-791, South Korea * [email protected]
Abstract: We experimentally demonstrate a linearly configured Brillouin optical correlation domain analysis (BOCDA) system enhanced by a differential measurement scheme. On-off control of the pump phase modulation with an intentional loss at the end of a fiber under test is applied for the acquisition of a Brillouin gain spectrum. This application leads to a four-fold enhancement of the spatial resolution and doubling of the measurement range in comparison with the former system under the same modulation parameters. ©2014 Optical Society of America OCIS codes: (060.2310) Fiber optics; (060.2370) Fiber optics sensors; (290.5900) Scattering, stimulated Brillouin; (120.5820) Scattering measurements.
References and links 1.
T. Horiguchi and M. Tateda, “BOTDA – nondestructive measurement of single-mode optical fiber attenuation characteristics using Brillouin interaction: theory,” J. Lightwave Technol. 7(8), 1170–1176 (1989). 2. X. Bao, D. J. Webb, and D. A. Jackson, “32-km distributed temperature sensor based on Brillouin loss in an optical fiber,” Opt. Lett. 18(18), 1561–1563 (1993). 3. M. Nikles, L. Thevenaz, and P. A. Robert, “Simple distributed fiber sensor based on Brillouin gain spectrum analysis,” Opt. Lett. 21(10), 758–760 (1996). 4. M. N. Alahbabi, Y. T. Cho, and T. P. Newson, “150-km-range distributed temperature sensor based on coherent detection of spontaneous Brillouin backscatter and in-line Raman amplification,” J. Opt. Soc. Am. B 22(6), 1321–1324 (2005). 5. K. Hotate and T. Hasegawa, “Measurement of Brillouin gain spectrum distribution along an optical fiber using a correlation-based technique - proposal, experiment and simulation,” IEICE Trans. Electron. E83-C, 405–412 (2000). 6. K. Y. Song, Z. He, and K. Hotate, “Distributed strain measurement with millimeter-order spatial resolution based on Brillouin optical correlation domain analysis,” Opt. Lett. 31(17), 2526–2528 (2006). 7. K. Y. Song and K. Hotate, “Distributed fiber strain sensor at 1 kHz sampling rate based on Brillouin optical correlation domain analysis,” IEEE Photon. Technol. Lett. 19(23), 1928–1930 (2007). 8. K. Y. Song and K. Hotate, “Brillouin optical correlation domain analysis in linear configuration,” IEEE Photon. Technol. Lett. 20(24), 2150–2152 (2008). 9. W. Zou, Z. He, and K. Hotate, “Single-end access correlation-domain distributed fiber-optic sensor based on stimulated Brillouin scattering,” J. Lightwave Technol. 28(18), 2736–2742 (2010). 10. J. H. Jeong, K. Lee, K. Y. Song, J.-M. Jeong, and S. B. Lee, “Differential measurement scheme for Brillouin Optical Correlation Domain Analysis,” Opt. Express 20(24), 27094–27101 (2012). 11. J. H. Jeong, K. Lee, K. Y. Song, J.-M. Jeong, and S. B. Lee, “Bidirectional measurement for Brillouin optical correlation domain analysis,” Opt. Express 20(10), 11091–11096 (2012).
1. Introduction Brillouin scattering based distributed fiber optic sensors for temperature and strain measurement have been widely studied as a tool for structural health monitoring, since they use the entire fiber as the sensing part, unlike the conventional sensors [1, 2]. There are several representative methods of the Brillouin sensors including Brillouin optical time domain analysis (BOTDA), Brillouin optical time domain reflectometry (BOTDR), and
#199368 - $15.00 USD (C) 2014 OSA
Received 14 Oct 2013; revised 17 Dec 2013; accepted 3 Jan 2014; published 15 Jan 2014 27 January 2014 | Vol. 22, No. 2 | DOI:10.1364/OE.22.001467 | OPTICS EXPRESS 1467
Brillouin optical correlation domain analysis (BOCDA) [3–5]. Pulse-based time-domain Brillouin sensors (BOTDR and BOTDA) share a common advantage of long measurement range up to 100 km: however, due to the nature of pulse-based operation, they generally show a limited spatial resolution (~1 m) and long measurement time (~several minutes). Meanwhile, the BOCDA system based on a synthesis of the optical coherence function can provide advantages of a higher spatial resolution (~mm order) and a higher sampling rate (~kHz) with random access of the sensing position in comparison with BOTDR and BOTDA. However, this comes at the cost of a limited measurement range (~several hundreds of meters) and complexity in the system configuration [5–7]. In the operation of BOCDA, a closed-loop configuration is commonly adopted for the counter-propagation of the pump and probe waves to induce stimulated Brillouin scattering (SBS), even though a single-end access to a fiber under test (FUT) is more desirable due to more flexibility in the deployment of the FUT. In 2008, a BOCDA system in a linear configuration was proposed by applying beat lock-in detection together with a narrowband optical filter [8]. However, the beat lock-in detection requires two intensity modulators and an additional function generator with rather complicated signal processing for the acquisition of the Brillouin gain spectrum (BGS). Recently, enlargement of the measurement range in the linearly-configured BOCDA was demonstrated by applying a polarization maintaining fiber (PMF) as FUT and two polarization beam splitters for the separation of the pump and the probe [9]. In 2010 a differential measurement scheme was applied to an ordinary BOCDA system where the BGS is acquired by on-off control of the phase modulation of the pump for the improvement of the spatial resolution [10]. In this paper, we newly apply the differential measurement scheme with an intentional loss at the end of FUT to the linearly-configured BOCDA system, resulting in a four-fold enhancement of the spatial resolution and double enlargement of the measurement range with a FUT of the conventional single-mode fiber (SMF) and a simpler configuration than those of previous works. The adoption of the differential measurement in our work additionally provides a new and important role to the linearly configured system that was overlooked in Ref.10: Conventional intensity-chop-based lock-in detection doesn’t work in the linear configuration due to large noise offset induced by the intensity variation of remaining pump wave even if suppressing the pump component with FBG. This problem is also described in Ref.8, where they apply the beat lock-in detection for solving this problem that requires an additional EOM and two function generators operated at different frequencies. Meanwhile, differential measurement scheme doesn’t suffer from the large noise offset since phase modulation doesn’t directly induce intensity variation of the pump. 2. Principle In ordinary BOCDA systems, strong SBS between the pump and the probe takes place at a correlation peak that periodically appears along the FUT by sinusoidal frequency modulation of a light source. The spatial resolution Δz and the measurement range L are determined by the following equations [5]: Δz =
Vg Δν B 2π f m Δf
L=
Vg 2 fm
(1)
(2)
where Vg is the group velocity of light, ΔνB is the Brillouin gain bandwidth, fm is the modulation frequency of the light source, and Δf is the amplitude of the modulation. For the acquisition of a local BGS, lock-in detection is applied where the pump wave is intensitychopped at the reference frequency (fL) of the lock-in amplifier.
#199368 - $15.00 USD (C) 2014 OSA
Received 14 Oct 2013; revised 17 Dec 2013; accepted 3 Jan 2014; published 15 Jan 2014 27 January 2014 | Vol. 22, No. 2 | DOI:10.1364/OE.22.001467 | OPTICS EXPRESS 1468
On the contrary, the pump wave is phase-modulated at a fixed frequency (Ω) in the differential measurement scheme, and the phase modulation is periodically turned on and off (at fL) to construct the BGS by the difference between them [10]. Figure 1 schematically shows the operation principle of the differential measurement.
0.0
0
Δf [MHz]
FUT
0
Δν [MHz]
Signal (Ω off)
Difference (by LIA)
1.0
−Ω +Ω
0.5
0.0
40 30
0
Local beat spectrum
Intensity
Brillouin gain
Σ
50
Δω [MHz]
Local BGS (Ω off)
1.0
60
0.5
0.0
0
Δf [MHz]
60
5
0
0
40
Output BGS 0
Δf [MHz]
Local BGS2 (Ω on)
10
Δf [MHz]
50
30
0
Signal amplitude
0.0
0.5
Signal amplitude
FUT
Intensity
Brillouin gain
Σ
0.5
Signal amplitude
1.0
1.0
Δν [MHz]
Local beat spectrum
Signal (Ω on)
Fig. 1. Operation principle of the differential measurement in the BOCDA: LIA, lock-in amplifier.
The differential measurement provides two prominent advantages over intensity-chopbased lock-in detection: One is enhancement of the spatial resolution, which originates from the effective removal of the noise structure as already demonstrated in a previous work [10]. The other point is that the system does not suffer from the noise coming from direct reflection of the pump, which generates a considerable amount of background noise in the lock-in detection based on the intensity-chop of the pump. This feature plays a key role in simplifying the structure of the currently proposed system in addition to the enhancement of the spatial resolution. Probe
IM1
50/50
(a) Pump
IM2
Probe
Signal
y
PBS
Pump
R ~ 100%
PBS y FUT (PMF)
x
(b)
FUT (SMF) L 2
length
Center for Opto-Electronic Convergence Systems, Korea Institute of Science and Technology (KIST), Seoul 136791, South Korea 2 Dept. of Physics, Chung-Ang University, Seoul 156-756, South Korea 3 Dept. of Electrical and Computer Engineering, Hanyang University, Seoul 133-791, South Korea * [email protected]
Abstract: We experimentally demonstrate a linearly configured Brillouin optical correlation domain analysis (BOCDA) system enhanced by a differential measurement scheme. On-off control of the pump phase modulation with an intentional loss at the end of a fiber under test is applied for the acquisition of a Brillouin gain spectrum. This application leads to a four-fold enhancement of the spatial resolution and doubling of the measurement range in comparison with the former system under the same modulation parameters. ©2014 Optical Society of America OCIS codes: (060.2310) Fiber optics; (060.2370) Fiber optics sensors; (290.5900) Scattering, stimulated Brillouin; (120.5820) Scattering measurements.
References and links 1.
T. Horiguchi and M. Tateda, “BOTDA – nondestructive measurement of single-mode optical fiber attenuation characteristics using Brillouin interaction: theory,” J. Lightwave Technol. 7(8), 1170–1176 (1989). 2. X. Bao, D. J. Webb, and D. A. Jackson, “32-km distributed temperature sensor based on Brillouin loss in an optical fiber,” Opt. Lett. 18(18), 1561–1563 (1993). 3. M. Nikles, L. Thevenaz, and P. A. Robert, “Simple distributed fiber sensor based on Brillouin gain spectrum analysis,” Opt. Lett. 21(10), 758–760 (1996). 4. M. N. Alahbabi, Y. T. Cho, and T. P. Newson, “150-km-range distributed temperature sensor based on coherent detection of spontaneous Brillouin backscatter and in-line Raman amplification,” J. Opt. Soc. Am. B 22(6), 1321–1324 (2005). 5. K. Hotate and T. Hasegawa, “Measurement of Brillouin gain spectrum distribution along an optical fiber using a correlation-based technique - proposal, experiment and simulation,” IEICE Trans. Electron. E83-C, 405–412 (2000). 6. K. Y. Song, Z. He, and K. Hotate, “Distributed strain measurement with millimeter-order spatial resolution based on Brillouin optical correlation domain analysis,” Opt. Lett. 31(17), 2526–2528 (2006). 7. K. Y. Song and K. Hotate, “Distributed fiber strain sensor at 1 kHz sampling rate based on Brillouin optical correlation domain analysis,” IEEE Photon. Technol. Lett. 19(23), 1928–1930 (2007). 8. K. Y. Song and K. Hotate, “Brillouin optical correlation domain analysis in linear configuration,” IEEE Photon. Technol. Lett. 20(24), 2150–2152 (2008). 9. W. Zou, Z. He, and K. Hotate, “Single-end access correlation-domain distributed fiber-optic sensor based on stimulated Brillouin scattering,” J. Lightwave Technol. 28(18), 2736–2742 (2010). 10. J. H. Jeong, K. Lee, K. Y. Song, J.-M. Jeong, and S. B. Lee, “Differential measurement scheme for Brillouin Optical Correlation Domain Analysis,” Opt. Express 20(24), 27094–27101 (2012). 11. J. H. Jeong, K. Lee, K. Y. Song, J.-M. Jeong, and S. B. Lee, “Bidirectional measurement for Brillouin optical correlation domain analysis,” Opt. Express 20(10), 11091–11096 (2012).
1. Introduction Brillouin scattering based distributed fiber optic sensors for temperature and strain measurement have been widely studied as a tool for structural health monitoring, since they use the entire fiber as the sensing part, unlike the conventional sensors [1, 2]. There are several representative methods of the Brillouin sensors including Brillouin optical time domain analysis (BOTDA), Brillouin optical time domain reflectometry (BOTDR), and
#199368 - $15.00 USD (C) 2014 OSA
Received 14 Oct 2013; revised 17 Dec 2013; accepted 3 Jan 2014; published 15 Jan 2014 27 January 2014 | Vol. 22, No. 2 | DOI:10.1364/OE.22.001467 | OPTICS EXPRESS 1467
Brillouin optical correlation domain analysis (BOCDA) [3–5]. Pulse-based time-domain Brillouin sensors (BOTDR and BOTDA) share a common advantage of long measurement range up to 100 km: however, due to the nature of pulse-based operation, they generally show a limited spatial resolution (~1 m) and long measurement time (~several minutes). Meanwhile, the BOCDA system based on a synthesis of the optical coherence function can provide advantages of a higher spatial resolution (~mm order) and a higher sampling rate (~kHz) with random access of the sensing position in comparison with BOTDR and BOTDA. However, this comes at the cost of a limited measurement range (~several hundreds of meters) and complexity in the system configuration [5–7]. In the operation of BOCDA, a closed-loop configuration is commonly adopted for the counter-propagation of the pump and probe waves to induce stimulated Brillouin scattering (SBS), even though a single-end access to a fiber under test (FUT) is more desirable due to more flexibility in the deployment of the FUT. In 2008, a BOCDA system in a linear configuration was proposed by applying beat lock-in detection together with a narrowband optical filter [8]. However, the beat lock-in detection requires two intensity modulators and an additional function generator with rather complicated signal processing for the acquisition of the Brillouin gain spectrum (BGS). Recently, enlargement of the measurement range in the linearly-configured BOCDA was demonstrated by applying a polarization maintaining fiber (PMF) as FUT and two polarization beam splitters for the separation of the pump and the probe [9]. In 2010 a differential measurement scheme was applied to an ordinary BOCDA system where the BGS is acquired by on-off control of the phase modulation of the pump for the improvement of the spatial resolution [10]. In this paper, we newly apply the differential measurement scheme with an intentional loss at the end of FUT to the linearly-configured BOCDA system, resulting in a four-fold enhancement of the spatial resolution and double enlargement of the measurement range with a FUT of the conventional single-mode fiber (SMF) and a simpler configuration than those of previous works. The adoption of the differential measurement in our work additionally provides a new and important role to the linearly configured system that was overlooked in Ref.10: Conventional intensity-chop-based lock-in detection doesn’t work in the linear configuration due to large noise offset induced by the intensity variation of remaining pump wave even if suppressing the pump component with FBG. This problem is also described in Ref.8, where they apply the beat lock-in detection for solving this problem that requires an additional EOM and two function generators operated at different frequencies. Meanwhile, differential measurement scheme doesn’t suffer from the large noise offset since phase modulation doesn’t directly induce intensity variation of the pump. 2. Principle In ordinary BOCDA systems, strong SBS between the pump and the probe takes place at a correlation peak that periodically appears along the FUT by sinusoidal frequency modulation of a light source. The spatial resolution Δz and the measurement range L are determined by the following equations [5]: Δz =
Vg Δν B 2π f m Δf
L=
Vg 2 fm
(1)
(2)
where Vg is the group velocity of light, ΔνB is the Brillouin gain bandwidth, fm is the modulation frequency of the light source, and Δf is the amplitude of the modulation. For the acquisition of a local BGS, lock-in detection is applied where the pump wave is intensitychopped at the reference frequency (fL) of the lock-in amplifier.
#199368 - $15.00 USD (C) 2014 OSA
Received 14 Oct 2013; revised 17 Dec 2013; accepted 3 Jan 2014; published 15 Jan 2014 27 January 2014 | Vol. 22, No. 2 | DOI:10.1364/OE.22.001467 | OPTICS EXPRESS 1468
On the contrary, the pump wave is phase-modulated at a fixed frequency (Ω) in the differential measurement scheme, and the phase modulation is periodically turned on and off (at fL) to construct the BGS by the difference between them [10]. Figure 1 schematically shows the operation principle of the differential measurement.
0.0
0
Δf [MHz]
FUT
0
Δν [MHz]
Signal (Ω off)
Difference (by LIA)
1.0
−Ω +Ω
0.5
0.0
40 30
0
Local beat spectrum
Intensity
Brillouin gain
Σ
50
Δω [MHz]
Local BGS (Ω off)
1.0
60
0.5
0.0
0
Δf [MHz]
60
5
0
0
40
Output BGS 0
Δf [MHz]
Local BGS2 (Ω on)
10
Δf [MHz]
50
30
0
Signal amplitude
0.0
0.5
Signal amplitude
FUT
Intensity
Brillouin gain
Σ
0.5
Signal amplitude
1.0
1.0
Δν [MHz]
Local beat spectrum
Signal (Ω on)
Fig. 1. Operation principle of the differential measurement in the BOCDA: LIA, lock-in amplifier.
The differential measurement provides two prominent advantages over intensity-chopbased lock-in detection: One is enhancement of the spatial resolution, which originates from the effective removal of the noise structure as already demonstrated in a previous work [10]. The other point is that the system does not suffer from the noise coming from direct reflection of the pump, which generates a considerable amount of background noise in the lock-in detection based on the intensity-chop of the pump. This feature plays a key role in simplifying the structure of the currently proposed system in addition to the enhancement of the spatial resolution. Probe
IM1
50/50
(a) Pump
IM2
Probe
Signal
y
PBS
Pump
R ~ 100%
PBS y FUT (PMF)
x
(b)
FUT (SMF) L 2
length
