Novel distributed birefringence measurement based on transient Brillouin grating in polarization-maintaining fibers and its application in sensing Yongkang Dong, Xiaoyi Bao*, Liang Chen, and Yuelan Lv Department of Physics, University of Ottawa, 150 Louis-Pasteur, Ottawa, ON, K1N 6N5, Canada ABSTRACT We propose a novel distributed birefringence measurement method based on transient Brillouin grating (TBG) in a polarization-maintaining (PM) fiber, which uses two short pump pulses to excite a TBG and a long probe pulse to map the transient Brillouin grating spectrum (TBGS) associated with the local birefringence. In experiment, the distributed birefringence of a 7-m Bow-tie PM fiber is measured, and the results show that it exhibits a periodical change with a period of ~ 3 m and variation amplitude of 1.6×10-6 along the entire fiber. In addition, based on the birefringence variation we also realize distributed temperature sensing using a PM photonic crystal fiber with a spatial resolution of 20 cm and a temperature measurement range of a few hundreds degrees Celsius with 2 ns pump pulses, and a temperature resolution of 0.07 °C with a 6 ns probe pulse. Keywords: Distributed birefringence; Distributed sensing; Polarization-maintaining fiber; Stimulated Brillouin scattering.
1. INTRODUCTION Brillouin scattering based distributed fiber sensor has been developed for two decades and shows capability of measuring temperature or strain for civil structural monitoring 1-3. In normal case, the spatial resolution is limited by the phonon lifetime (~10 ns) in optical fiber and is thus not better than 1 m. Light storage was recently described and experimentally demonstrated by stimulated Brillouin scattering in single-mode fiber 4. In this scheme, a transient Brillouin grating (TBG) was excited by two counter-propagating “data” pulse and “write” pulse, and the reflection of the following “read” pulse from the TBG was utilized to retrieve the data information. In our previous work, a frequency-shifted light storage was realized in a high birefringence polarization-maintaining (PM) fiber, and the optical frequency difference between the “write” and “read” pulses was proportional to the local birefringence 5. It has also been recently demonstrated that the birefringence is temperature- and strain-dependent, so the Brillouin grating spectrum associated with the birefringence can be used as a novel sensing mechanism for temperature or stain variation 6. More recently, a time-domain measurement of Brillouin grating spectrum in a Panda PM fiber with a spatial resolution of 2 m was demonstrated, where the Brillouin grating was excited continuously along the fiber by using one pump pulse and another CW pump 7. In this scheme, the spatial resolution is not better than 1 m due to the 10 ns phonon lifetime, and the temperature measurement range was a few tens degree Celsius limited by the detuning of Brillouin resonant frequency within natural Brillouin spectral width. Up to now, the reported measurement methods of birefringence in optical fiber can only get the average birefringence along the test fiber. In this paper, we first propose a distributed birefringence measurement method based on a local TBG in PM fibers and demonstrate its application in distributed temperature sensing. The length of the TBG (spatial resolution) is defined by two pump pulse width, and the position of TBG can be controlled by the time delay of the 2nd pump pulse, and the measured temperature range is determined by the convolution of two pump pulse spectrum, which is a few hundreds degrees Celsius for 2 ns pump pulse. In addition, polarization-maintaining photonic crystal fiber (PMPCF) could have a small core area, which provides a high power density compared with conventional PM fiber and thus could increase the signal intensity and signal-to-noise ratio.
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[email protected]; phone 1 613 562-5800 (6911); fax 1 613 562-5190 20th International Conference on Optical Fibre Sensors, edited by Julian Jones, Brian Culshaw, Wolfgang Ecke, José Miguel López-Higuera, Reinhardt Willsch, Proc. of SPIE Vol. 7503, 75037Y © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.848675 Proc. of SPIE Vol. 7503 75037Y-1
In this scheme 9, two short counter-propagating pump1 pulse and pump2 pulse are launched into the fast axis of the PM fiber to excite a TBG. The frequency of pump1 is larger than that of pump2 by a Brillouin frequency shift ν B of the PM fiber. Following pump2 pulse, a long probe pulse is launched into the slow axis, and energy from the probe pulse is partly transferred to the reflection pulse at the expense of TBG. A maximum reflection probe signal could be obtained when the frequency difference Δν between the pump2 and probe satisfies the following equation Δν = Δn ⋅ν n 9, where Δn is the local birefringence, n is the average refractive index and ν is the optical frequency of probe pulse. The spatial resolution is the length of the local TBG determined by the pump pulses, so that a high spatial resolution can be obtained by using short pump pulses. The measured TBGS is the convolution of the probe pulse spectrum and the intrinsic TBGS, therefore a long probe pulse is preferable to get a narrow TBGS. During this process, the probe pulse interacts with TBG instead of pump pulse, whereas this happens in a conventional BOTDA using single-mode fiber and causes the tradeoff between the spatial resolution and the Brillouin spectrum width, which provides an opportunity to obtain a high spatial resolution through short pump pulses and a narrow TBGS using a long probe pulse.
2. EXPERIMENT AND DISCUSSION The experimental setup of distributed sensing based on birefringence effect on TBG is shown in Fig. 1. Two narrow linewidth (3 kHz) fiber lasers operating at 1550 nm are used to provide the pump1 and pump2, respectively, and the frequency difference between is locked by a frequency counter. A tunable laser with a wavelength resolution of 0.1 pm is used as probe light. The electro-optic modulators EOM 1 is used to generate a 2 ns square-shaped pulse. Two high extinction ratio EOM2 and EOM3 are used to generate the square-shaped 2 ns pump2 pulse and probe pulse with the extinction ratio larger than 45 dB, which are then combined with a 3-dB coupler. The power of pump1 pulse, pump2 pulse and probe pulse in the PM-PCF are about 200 mW, 30 W and 30 W, respectively. The two pump pulses are launched into the fast axis and the delay between them is well controlled to excite the TBG at a specific location. Following the pump2 pulse with a time delay of 2 ns, the probe pulse is launched into the slow axis and its reflection by the local TBG is detected by a 1-GHz detector. A 7-m Bow-tie PM fiber and a 3-m PM-PCF are used as fibers under test. The Bow-tie PM fiber has a mode field diameter of 10.5 µm and a nominal birefringence of ~3.17×10-4. The measured ν B of the Bow-tie PM fiber at room temperature is 10.815 GHz and the Brillouin spectrum width is 61 MHz. The PM-PCF has a mode field diameter of 8 µm and a nominal birefringence of ~1.4×10-4. The measured ν B of the PM-PCF at room temperature is 11.131 GHz and the Brillouin spectrum width is 60 MHz.
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We first studied the dependence of TBGS on the probe pulse width. The probe pulse width is limited by the TBG lifetime of ~10 ns in optical fibers. Moreover, reflected light from the front part of the probe pulse could be depleted by the back part of the probe pulse, which decreases the signal intensity and is also a limitation to the probe pulse width.
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Figure 2 shows the measured TBGS using different probe pulse width with TBG excited at the location of 6.3m in Bowtie PM fiber. The measurement results show that the TBGS width decreases when increase the probe pulse width, and the two peaks are more distinct using 8 ns pulse width. (a)
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Fixing the frequency difference of pump1 and pump2 at 10.815 GHz, the TBGS of Bow-tie PM fiber is measured using a 8 ns probe pulse. Typical TBGSs are shown in Fig. 3, where most part of the fiber exhibit a four-peak spectrum shown as Fig. 3 (a) while two segments of 2.5 m~3.2m and 5.4 m~6.8 m have a two-peak spectrum shown as Fig. 3 (b). Note that in Fig. 3 (a) the frequency difference between peak a and peak b equals that between peak c and peak d with a value of ~160 MHz, and the frequency difference between peak a and peak c equals that between peak b and peak d with a value of ~280 MHz, which indicate that there are two modes existing in both fast and slow axes. Two-peak spectrum shown in Fig. 3 (b) gives that the frequency difference between peak a and peak b is also ~160MHz, which means that in this segment there are one mode in one axis and two modes in the other axis. The frequency difference of 160 MHz and 280 MHz are corresponding to the refractive index difference of 1.21×10-6 and 2.12×10-6, respectively. Because peak b is the maximum one and exists along the whole fiber, we chose peak b to calculate the birefringence of Bow-tie PM fiber, which is given in Fig. 3(c). The measured birefringence shows a periodical change with a period of ~ 3 m and variation amplitude of 1.6×10-6 along the 7-m fiber. As for the sensing application, PM-PCF is used as the sensing fiber with 1.5-m segment located from 0.9 m to 2.4 m in the oven. Fixing the frequency difference of pump1 and pump2 at 11.131 GHz, the TBGS is measured using a 6 ns probe pulse with the temperature at 15 °C, 35 °C, 55 °C and 75°C, respectively, and the results are provided in Fig. 4 (a). As the temperature increases, the excitation efficiency of the TBG decreases because of the increasing detuning of Brillouin frequency shift in the heated section. However, due to a few hundreds MHz bandwidth of 2 ns pump pulse, the TBG in the heated section could still be efficiently excited within a few hundreds degrees Celsius. Because the Brillouin gain coefficient increases with the temperature 8, the amplitude of the TBGS increases with the temperature as shown in Fig. 3(a). It is expected that the amplitude of the TBG has a maximum value at a high temperature and decreases beyond this point. The dependence of central frequency of TBGS on the temperature is plotted in Fig. 4 (b). The slope of the temperature sensitivity is determined by temperature-induced birefringence change and is measured to be -23.5 MHz/°C. The negative slope is in agreement with the result measured in a conventional PM fiber 6-8. The frequency uncertainty in Fig. 4 (a) is about 1.6 MHz, so that the temperature resolution could be 0.07 °C. Figure 4 (c) shows the reflection probe signal from different positions along the 3 m PM-PCF with the Δν fixed at 17.482 GHz. The sampling interval is 0.5 ns corresponding to a spatial step of 5 cm. The black squares show the results when the entire fiber is put in the room temperature. The amplitude variation at different positions reflects the fluctuation of the birefringence along the fiber. The red dots are the results when the heated section is put in the oven to keep 45 °C. The signals outside the oven are identical, while the birefringence change of the heated section is so large that no reflection probe signal could be observed. The signal within the transition region shows a spatial resolution of 20 cm, which is determined by 2 ns pump pulse.
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3. CONCLUSION In summary, we have proposed and demonstrated a time-domain distributed temperature fiber sensor based on birefringence effect on TBG by using two short pump pulses to excite a local TBG and a long probe pulse to measure the TBGS. The distributed birefringence of a 7-m Bow-tie PM fiber is measured, and a 20 cm spatial resolution and a temperature resolution of 0.07 °C using PM-PCF have been successfully realized. We believe that by increasing the peak power in the test fiber or using PM-PCF with smaller core area, shorter pump pulse (for example 1 ns) could be used to further improve the spatial resolution and detectable temperature range. In addition, a pulse train of pump1 could be used to perform multi-position measurement simultaneously to reduce the measurement time. The authors acknowledge the research supports from Natural Science and Engineering Research council of Canada and Canada Research Chair Program.
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