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Abstract
A pulse width modulation (PWM) Brillouin amplification has been proposed and demonstrated to improve the signal-to-noise ratio (SNR) and sensitivity of phase-sensitive optical time domain reflectometry (Ф-OTDR) especially for the far end of a sensing fiber. In the logarithmic unit, arbitrary gain distribution can be realized with the customizable PWM function. The gain distribution is adjustable by tuning the PWM parameters. To prove the concept, three typical gain distributions including up-ramp sawtooth, sine and triangle have been achieved with the corresponding driving functions. In experiments, a linear PWM pump light has been used to amplify the backscattering Rayleigh light. The signal at the leading end has been enhanced by about 11.5 dB. Meanwhile, 9 dB transmission attenuation (along 25 km SMF) has also been compensated excellently. To verify the effectiveness of attenuation compensation, two vibrations with a frequency of 100 Hz and 300 Hz have been recovered accurately at the trailing end. Besides, preamplifier and acoustic-optic modulator (AOM) was used to suppress the ASE noise and further improve the effective ER, respectively. With that, lower relative intensity noise (RIN) has been obtained in the proposed system compared to the conventional Brillouin amplification in Ф-OTDR. So the proposed PWM Brillouin amplification not only improves the SNR but also equalizes the sensitivity along whole sensing fiber. It avoids the complex calibration and suppresses the false alarm rate in field application. Foreseeably, this scheme is universal and can be adopted by other distributed fiber optic technique to enhance the system performance.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Over the past few decades, distributed fiber optical sensors (DOFS) have been intensively studied owing to the large-scale monitoring range, fully distributed manner, accurate localization and low cost per monitored point [1–4]. Among the numerous distributed sensing technologies, phase-sensitive optical time domain reflectometry (Ф-OTDR) has been demonstrated as a promising technique due to its overwhelming advantages of high sensitivity, broadband response and capability of multiple intrusions detection [5–8]. Recently, Ф-OTDR has been widely used in many fields such as perimeter intrusion monitoring, seismic wave mapping and pipe line protection, etc [9,10]. But the relatively low signal-to-noise ratio (SNR) restricts its reliability and it needs to be improved to reduce the Nuisance Alarm Rates. Besides, a degraded SNR exists at near and far end due to the inherent transmission attenuation α. Unfortunately, the degraded SNR reduces the sensitivity with the increase of the sensing range.
To improve SNR and extend the dynamic range, some proposals based on signal processing and light amplification have been put forward [11–19]. Although the method of signal processing can improve SNR without any hardware change, its SNR enhancement is determined by the system SNR inherently [11–14]. Therefore, the light amplification is more effective to improve the system performance compared to signal processing. For the light amplification in Ф-OTDR, several schemes have been demonstrated such as Raman, Brillouin and hybrid amplification [15–19]. Among them, Raman amplification improves probe/backscattering light effectively, but its practicability is still limited by high pump power, low efficiency and abundant loop structure [15–17]. Different from the Raman amplification, only a small pump power (much less than Raman pump) is required to achieve the same gain in Brillouin amplification [18]. In addition, Brillouin amplification has the characteristics of high gain, high efficiency and selective amplification such that it was regarded as a promising technique to improve system performance in many fields [20–23]. In DOFS (based on backscattering light detection), there are two types of Brillouin amplification including probe light amplification and backscattering light amplification. Most of the proposed schemes focused on the probe light amplification [18,19,23]. For them, optical frequency comb (OFC) pump and loop structure are necessary to maintain the pulse shape and amplify pulse, respectively. That greatly increases the system complexity and the cost for maintenance. Moreover, the gain fiber was acted as the sensing fiber simultaneously. The effect of stimulated Brillouin scattering (SBS) is susceptible to the external environment (temperature, strain and vibration) such that the gain fluctuates randomly. All of the drawbacks reduce their practicability and stability in practical applications. Compared to probe amplification, the backscattering light amplification has the advantages of simpler structure (OFC is not needed), single-end detection and stable gain [24,25]. Nevertheless, it just provides a constant gain to achieve a stable gain (the gain fiber does not act as the sensing fiber). Thus, the degraded SNR and the non-uniform sensitivity caused by the inherent transmission loss cannot be compensated. That makes the vibration/intrusion might not be exactly measured at far end.
In this work, we propose and demonstrate a novel pulse width modulation (PWM) Brillouin amplification for the backscattering Rayleigh light in Ф-OTDR. Both the theoretical analysis and experimental verification are implemented. Arbitrary gain distribution of the proposed method has been verified with three typical driving functions. To improve the system performance, the ASE noise was suppressed with pre-amplification and the relative intensity noise (RIN) enhancement of 9.4 dB has been achieved. Besides, the advantages of acoustic-optic modulator (AOM) over electrical optical modulator (EOM) has been discussed meticulously. About 2.2 dB RIN enhancement were obtained with AOM adopted pulse probe. By adopting linear PWM pump light, 9 dB attenuation along 25 km single mode fiber (SMF) was compensated well except for an offset gain of 11.5 dB. Two vibrations with frequency of 100 Hz and 300 Hz have been accurately measured at the trailing end of the test fiber.
2. Principle
In the traditional Brillouin amplification for the Rayleigh backscattering light, both the probe pulse and CW Brillouin pump are generated by electrical optical modulator (EOM) [24,25]. In this work, an acoustic-optic modulator (AOM) rather than EOM has been selected to generate the probe pulse. That can further improve the effective extinction ratio (ER) due to the frequency shift of AOM and the selectivity of Brillouin amplification. Besides, PWM pump has been proposed to compensate the inherent transmission loss except for providing a constant gain. Different from the conventional CW pump (providing a fixed gain), the PWM pump can provide a linear-growth gain as the time increasing in logarithmic unit. That compensates the inherent transmission loss wonderfully. Section 2.1 and 2.2 explain the advantage of AOM and the principle of PWM Brillouin amplification, respectively.
2.1 The effective extinction ratio improvement using AOM in Brillouin amplification
As proved in previous works, the ER of the probe pulse is crucial for Ф-OTDR and it has great effect on the system SNR and sensing range [26,27]. In conventional Brillouin amplification for the backscattering Rayleigh light, EOM was used to generate the probe pulse [24,25]. The pump light amplified signal and noise simultaneously. In this work, an AOM has been adopted to generate the probe pulse. Owing to the frequency shift of AOM and selectivity of Brillouin amplification, only signal can be amplified (the noise keeps unchanged). Therefore, the utilization of AOM can further improve the effective ER.
Figure 1 depicts the schematics of the probe pulse generated with EOM and AOM in Brillouin amplification. In backscattering Rayleigh light, the signal and noise are developed from the “ON state” and “OFF state”, respectively. For the EOM based probe pulse, same frequency (f0) exists at ON and OFF state, as illustrated in Fig. 1(a). So, the signal and noise have same frequencies. Both of them will be amplified with same gain in Brillouin amplification [24,25].
Fig. 1 The comparison of the probe pulse generated by EOM and AOM in Brillouin amplification. (a) The schematic of EOM and (b) AOM based probe pulse.
Different from the EOM based probe pulse, the light at ON and OFF state have different frequencies for the AOM based probe pulse, as shown in Fig. 1(b). The light frequencies are f0 + Δf and f0 at ON and OFF state, respectively. Δf is the frequency shift of AOM. Therefore, the signal and noise have different frequencies of f0 + Δf and f0. Benefited by adopting Brillouin amplification, only the signal with frequency of f0 + Δf (developed from “ON state”) can be amplified by the Brillouin pump. Thus, the effective ER can be partly improved in Brillouin amplification. Furthermore, the enhancement of the effective ER is determined by the Brillouin gain theoretically.
In Ф-OTDR incorporating with Brillouin amplification, the effective ER in the EOM based system is just determined by the ER of EOM. But for the AOM based system, it is codetermined by the AOM’s ER and the Brillouin gain. So, even if an EOM with same ER as AOM was adopted, the AOM adopted system can still achieve higher effective ER (such that higher SNR can be obtained).
2.2 The principle of PWM Brillouin amplification
Figure 2(a) and Fig. 2(b) show the schematics of the conventional and PWM Brillouin amplification, respectively. In conventional Brillouin amplification, a CW pump light with constant peak power was used to amplify the signal with a fixed Brillouin gain. As illustrated in Fig. 2(a), a single mode fiber (SMF) with length of L is acted as the gain medium. The signal and pump light are injected into SMF from two opposite directions [28,29]. Note that, the frequency of the pump light must be larger than the signal frequency. The frequency difference between the signal and pump light equals to the Brillouin frequency shift (BFS) of the gain fiber. In conventional Brillouin amplification, the power of the output signal (PSout) and the Brillouin gain (G) can be described as.
where PSout is the power of the output signal at position 0, PSin is the power of the input signal at position L, PP is the input pump power and it is a constant value in conventional Brillouin amplification, gB is the Brillouin gain coefficient of the SMF, Aeff is the nonlinear effective area of SMF and Leff is the effective fiber length (Leff = (1-e-αL)/α), α and L are the attenuation coefficient and fiber length, respectively. According to Eq. (1), a constant Brillouin gain G is obtained with the conventional method, as shown in Fig. 2(a). In Ф-OTDR, same gain is provided at near and far end, such that the inherent transmission loss cannot be compensated.Fig. 2 The comparison of the proposed method with the conventional method. (a) The schematic of the conventional CW pump and (b) the proposed PWM pump.
Different from the conventional CW Brillouin amplification, the proposed PWM Brillouin amplification provides a variable pump power. Through adjusting the duty ratio of the pump pulse train, varying gain (with time increasing) can be achieved. After being amplified by the PWM Brillouin pump, the output signal and the Brillouin gain can be expressed by
According to Eq. (2), the Brillouin gain G is proportional to the pump power PP(t). With arbitrary pump power, similar gain distribution can be achieved. As shown in Fig. 2(b), a linear gain distribution (with time increasing) can be obtained with a linear-modulation pulse train. Except for an offset gain of G0, there is a linear growth of the Brillouin gain with time increasing. Through adopting the PWM Brillouin amplification, the attenuation of the backscattering Rayleigh light caused by transmission loss can be excellently compensated. That makes uniform SNR and sensitivity can be realized.
3. Experimental setup
A validation experiment has been implemented to prove the feasibility of the proposed PWM Brillouin amplification. Figure 3 shows the experimental setup and a narrow linewidth fiber laser (NLL) with linewidth of ~3 kHz has been acted as the light source. The center wavelength and output peak power of the generated continuous wave (CW) light are 1550.09 nm and 20 mw, respectively. The CW light was divided into two paths through an optical fiber coupler (50:50). In upper path, the CW light was modulated into optical pulse with width of 60 ns by an acoustic-optic modulator (AOM1, with frequency shift of 200 MHz), and then launched into 25 km sensing fiber via an optical circulator with repetition rate of 2 kHz. The CW light in lower path was used as pump light which amplifies the backscattered Rayleigh light. It was modulated by an electrical optical modulator (EOM) with frequency of fB (which equals to the equivalent BFS of the gain fiber), where the EOM worked at the carrier suppression mode (the transmission spectra before and after modulation are illustrated in Fig. 4(a)). After being amplified and filtered by an erbium doped fiber amplifier (EDFA) and a tunable optical filter consecutively, only the upper sideband (in frequency domain) was retained. The reserved upper sideband was then modulated into pulse train with tunable duty cycle through an AOM2 (whose frequency shift is 80 MHz). The pump pulse train amplified the backscattering Rayleigh light in a 5 km single mode fiber (SMF, which acted as the gain fiber and its Brillouin gain spectrum is shown in Fig. 4(b)). Lastly, the amplified light was converted into electrical signal at a photodetector (PD, Throlabs PDB425C) and sampled by an oscilloscope (OSC) with sample rate of 100 MS/s.
Fig. 3 Experimental setup. NLL: narrow linewidth laser; PC: polarization controller; AOM: acoustic-optic modulator; EDFA: erbium doped fiber amplifier; OBPF: optical bandpass filter; FUT: fiber under test; ISO: isolator; EOM: electrical optical modulator; PWM: pulse width modulation; SMF: single mode fiber; PD: photodetector; OSC: oscilloscope.
Fig. 4 (a) The transmission spectra before (ORG, blue solid line) and after (Pump, red solid line) carrier suppression modulation; (b) the Brillouin gain spectrum of the gain fiber.
4. Experimental results and discussion
Considering the intensive ASE noise in conventional Brillouin amplification [24], a preamplifier was used to amplify the backscattering Rayleigh light before Brillouin amplification. That suppresses the ASE noise effectively and all results are depicted in section 4.1. Besides, the advantages of AOM over EOM in Brillouin amplification have been verified. Compared with the EOM based system, higher effective ER can be achieved with the AOM based pulse, as shown in section 4.2. The proposed PWM Brillouin amplification has been validated and the results are illustrated in section 4.3. The proposed PWM Brillouin amplification compensates the inherent transmission loss except for providing an offset gain. That greatly improves the SNR at far end and measures the vibrations precisely.
4.1 The noise suppression with pre-amplification in Brillouin amplification
In conventional Brillouin amplification for the backscattering Rayleigh light, CW pump light amplified the backscattering Rayleigh light directly [24]. The intensive ASE noise induced by the weak backscattering Rayleigh light and strong pump degrades the system performance. To reduce the influence of ASE noise and measure the vibration precisely, many times average is required. Unfortunately, the average greatly decreases the frequency response. In this work, a preamplifier has been used to enhance the backscattering Rayleigh light before Brillouin amplification. To verify the effect of the pre-amplification in Brillouin amplification, contrast experiments (with and without pre-amplification) have been implemented. In experiments, CW pump light and a similar setup as Fig. 3 eliminating PWM module were adopted.
Figure 5(a) shows the original signals (blue lines) and the amplified signals (red lines, adopting direct Brillouin amplification). To observe the details clearly, a part of signals were illustrated in Fig. 5(b). Compared to the original signals, the amplified signals (without pre-amplification) fluctuate wildly. The intensive fluctuation seriously disrupts the signals and reduces the frequency response range. To quantify the fluctuation, standard deviation (σR) and relative intensity noise (RIN) of the traces have been calculated as the following expressions.
where σRj is standard deviation at position j, M is the total number of the traces, Ri,j is the value at position j of the i-th trace, is the average trace, SRIN is the average value of RIN. With the above calculation method, 15 traces were used to calculate SRIN. For the direct Brillouin amplification in Ф-OTDR, SRIN equals to 0.44.Fig. 5 Noise suppression with pre-amplification (PA) before Brillouin amplification (BA) for backscattering Rayleigh light. (a) The test results with direct Brillouin amplification (without pre-amplification) and (b) the corresponding details. (c) The test results with pre-amplification before Brillouin amplification and (d) the corresponding details.
In Brillouin amplification assisted by pre-amplification, the preamplifier improves the backscattering Rayleigh light. That greatly suppresses the ASE noise in Brillouin amplification. Amplified by the preamplifier and Brillouin pump consecutively, the overall signals and details are illustrated in Fig. 5(c) and Fig. 5(d) respectively. Obviously, the pre-amplification suppresses the noise and improves SNR greatly. With same calculation approach, the SRIN of the Brillouin amplification incorporating with pre-amplification has been calculated and it equals to 0.05. Compared to the direct Brillouin amplification (Fig. 5(a) and Fig. 5(b)), the Brillouin amplification with pre-amplification (Fig. 5(c) and Fig. 5(d)) improves the SRIN over 9.4 dB.
4.2 The effective extinction ratio improvement using AOM adopted pulse probe
According to the description in section 2.1, compared to the EOM based pulse, the AOM based pulse can get higher effective ER in Brillouin amplification. A verification experiment adopting CW pump light has been implemented. Here, a similar setup as Fig. 3 except for the PWM module was adopted. Considering there is no frequency shift for the EOM based pulse and its results can be easily deduced, we just implemented the demonstration experiments with AOM based pulse.
Firstly, we tested the performance of the AOM1, whose frequency shift is of 200 MHz. Figure 6(a) and 6(b) show the optical and electrical spectra, respectively. Here, the electrical spectrum was measured with coherent detection and the local light is from the laser source (without frequency shift). As depicted in Fig. 6(b), the frequency shift of 200 MHz and 400 MHz are generated by the fundamental frequency and second order harmonic, respectively. The ER at 200 MHz (green line) reaches up to 62 dB. More importantly, the frequency shift just appears at the ON state. That means the frequency of the leakage light (developed from “OFF state”) keeps unchanged. Assisted by the selectivity of the Brillouin amplification, the feature of frequency shift can get higher equivalent ER. That will further improve the system SNR.
Fig. 6 (a) The optical spectra and (b) the electrical spectrum when AOM turns on and off, respectively.
Three pump lights with different modulation frequencies were used to amplify the backscattering Rayleigh light and the test results are illustrated in Fig. 7. For simplicity, the pump lights with modulation frequency of 10670 MHz, 10770 MHz and 10870 MHz are named by pump1, pump2 and pump3, respectively. Here, the pump2 is acted as the reference, which cannot amplify the backscattering light generated at “ON state” and “OFF state” either. Figure 7(a) depicts the outputs of the PD at the condition of pulse off (“OFF state”). Due to the BFS of the gain fiber equals to 10670 MHz, as shown in Fig. 4(b), the leakage light is just amplified by the pump1 but remains unchanged when pump2 and pump3 were applied. That indicates the frequency of the leakage light keeps unchanged, which agrees well with the “OFF state” shown in Fig. 1(b) and Fig. 6(b).
Fig. 7 The effect of the pump frequency on the backscattering signals. (a) The outputs of the PD at the condition of pulse off; (b) the backscattering signals amplified by pump lights with modulation frequency of 10670, 10770 and 10870 MHz, respectively.
Meanwhile, the “ON state” has also been verified. Considering the BFS of the gain fiber and the frequency shift of AOM are 10670 MHz and 200 MHz, the effective pump light for the leakage light (noise) and signal are pump1 and pump3, respectively. Figure 7(b) shows the test results amplified by pump1 (blue lines), pump2 (red lines) and pump3 (green lines), respectively. To observe the signals clearly, only a part of traces are presented. Compared to the signals amplified by pump2 and pump3, the signals amplified by pump1 (blue lines) fluctuate widely. That means the leakage light (noise) in backscattering Rayleigh light is just amplified by pump1. Differently, the signals in backscattering light (developed from “ON state”) are just amplified by pump3 (green lines, before 24.9 km), as shown in Fig. 7(b). The above test results indicate the signal and noise in the backscattering Rayleigh light have different frequencies indeed. Besides, all test results are in good agreement with the theoretical analysis in section 2.1. To evaluate the performance exactly, the average RIN (SRIN) of the signals are calculated. When the backscattering Rayleigh light is amplified by pump1, pump2 and pump3, the corresponding SRIN equals to 0.13, 0.05 and 0.03, respectively. After being amplified by pump3, the SRIN enhancement is over 2.2 dB compared to the original signals.
In conventional Ф-OTDR, the system SNR and sensing range are mainly determined by the effective ER [26,27]. Compared to EOM (ER equals to 30 dB generally), AOM has much higher ER (over 50 dB) and it can get higher SNR and longer sensing range for the same probe power. Benefiting from the selectivity of Brillouin amplification and the frequency shift of AOM, the Ф-OTDR assisted by Brillouin amplification can further improve the effective ER and SNR. Even if EOM possesses same ER as AOM, the AOM based system can still obtain higher effective ER and SNR.
4.3 The test results adopting PWM Brillouin pump
In conventional Brillouin amplification, CW pump light was adopted. It just provides a constant gain but cannot compensate the inherent transmission loss. To compensate the loss, pump light with frequency sweeping was firstly considered [30,31]. However, the narrow bandwidth of the gain spectrum, bad linearity and ultra-fast sweeping make the linear gain (in logarithmic unit) hard to realize. Different from the CW and frequency-sweeping Brillouin amplification, this work proposes a PWM Brillouin amplification and linear gain distribution can be easily realized with great tunability.
According to the analysis in section 2.2, arbitrary gain distribution can be achieved if the corresponding modulation function was adopted. To verify its effectiveness, three typical functions including up-ramp sawtooth, triangle and sine functions were implemented and the test results are illustrated in Fig. 8. In the tests of the gain distribution, a weak CW light was amplified by the PWM pump light. Figures 8(a), 8(b) and 8(c) show the gain distributions both in linear and logarithmic units. In logarithmic unit, the gain distributions are similar to the driving functions. To observe the gain distributions clearly, the gain curves generated by three typical functions are depicted in Fig. 8(d). That can distinguish the difference of the triangle and sine function accurately. Besides, the gain distributions and the driving functions are exactly alike. That proves arbitrary gain distribution can be realized indeed.
Fig. 8 The gain distribution with different modulation functions of the PWM Brillouin amplification. (a), (b) and (c) are the gain distributions with modulation function of up-ramp sawtooth, triangle and sine, respectively; (d) the comparison of the gain distribution with different functions in logarithmic unit.
In Ф-OTDR, an inherent transmission loss exists. In logarithmic unit, the transmission loss grows linearly with the increase of sensing range (or transmission time). According the theoretical analysis and test results in Fig. 8, the transmission loss can be compensated perfectly with the driving function of up-ramp sawtooth (linear PWM function). To obtain a great gain curve, the exact parameters including modulation frequency, duty ratio and pulse period have been tested. In experiments, only one parameter was tuned but the others kept unchanged. For the discussion of the modulation frequency, pulse period of 1us and duty ratio from 10% to 90% were adopted. As shown in Fig. 9(a), the repetition rate of the gain curve is determined by the modulation frequency. After that, duty ratios with different range were tested and the results are shown in Fig. 9(b). Here, the modulation frequency and pulse period are 2 kHz and 1us, respectively. The growth rate (slope) of the gain curve increases with the range broadening. At last, different pulse periods were implemented and the results are depicted in Fig. 9(c). In this test, modulation frequency of 2 kHz and duty ratio from 10% to 90% were adopted. To observe the gain curves clearly, a part of Fig. 9(c) is shown in Fig. 9(d). The jitter of the gain curve increases as the pulse period broadening. It should be noted that the jitter is constant and periodic such that a stable gain is provided.
Fig. 9 The effect of different parameters on the gain distribution in PWM Brillouin amplification. (a) The gain distribution with different modulation frequency (MF); (b) the gain distribution with different duty ratio; (c) the gain distribution with different pulse period and (d) the corresponding details of the gain curves.
In order to verify the effectiveness of the PWM Brillouin amplification, the demonstration experiments have been implemented according to the setup shown in Fig. 3. For comparison, the conventional CW Brillouin amplification has also been performed (the PWM module was eliminated). Figure 10(a) shows the backscattering traces with (red line) and without (blue line) CW Brillouin amplification, respectively. Compared to the original signal, a constant gain of ~12.2dB has been achieved along whole link. Except for the gain, the signal with CW Brillouin amplification has same attenuation as the original signal. That means the inherent transmission loss of ~9dB cannot be compensated with the CW Brillouin amplification. Therefore, the reduction of SNR is ~9dB at near and far end. Different from the CW Brillouin amplification, the PWM Brillouin amplification can provide a linear gain distribution, such that the transmission loss is compensated. To achieve stable gain distribution in Ф-OTDR, the modulation frequency of the PWM pump must equal to the repetition rate of the probe pulse (2 kHz). Besides, the pulse period of the PWM pump must be divisible by the period of the probe pulse (500 us). In experiments, 1us pulse period, 2 kHz modulation frequency and duty ratio from 10% to 90% were adopted. Figure 10(b) depicts the test results assisted by PWM Brillouin amplification. Note that, to verify the improvement along the whole link, the front of signal was adjusted almost the same in both methods through tuning the EDFA. The signals at the leading end have been improved about 11.5 dB in comparison with the original signals. Except for that, the signals at the trailing end have approximated intensity with the signals at the leading end. That means the inherent transmission loss of ~9 dB has been compensated excellently.
Fig. 10 The effect of the conventional CW Brillouin amplification and the proposed PWM Brillouin amplification on the backscattering Rayleigh signal. (a) The backscattering Rayleigh signals without and with CW Brillouin amplification (CW BA); (b) the backscattering Rayleigh signals without and with PWM Brillouin amplification (PWM BA).
In addition, vibration tests were implemented on the sensing fiber through a piezo-electric transducer (PZT). Extra 2 km SMF was spliced into the 25 km sensing fiber. A vibration with frequency of 300 Hz was applied at the position of ~25 km. Figure 11(a) shows the location and frequency of the measured vibration. Obviously, the test result is in good agreement with the applied vibration. After that, another vibration with frequency of 100 Hz was also applied at the same location. Figure 11(b) and Fig. 11(c) show the time domain signals and the corresponding frequency spectra, respectively. Moreover, same vibrations were also tested in the system assisted by conventional CW Brillouin amplification. Unfortunately, the vibrations cannot be measured. Note that, same gain as Fig. 10 was adopted. That means the PWM Brillouin amplification improves the SNR at the trailing end indeed.
Fig. 11 The vibration tests at 25km of the fiber link. (a) The location and frequency of the vibration; (b) and (c) the time domain signals and the corresponding frequency spectra of the vibrations.
5. Conclusion
We propose and experimentally demonstrate a novel Ф-OTDR incorporating with pulse width modulation (PWM) Brillouin amplification. Different from the conventional CW Brillouin amplification, the PWM Brillouin amplification compensates the inherent transmission loss except for providing an offset gain. Thus, a uniform sensitivity along fiber link and high SNR at the far end can be achieved.
In this work, a preamplifier is used to suppress the ASE noise in Brillouin amplification. Compared to the direct Brillouin amplification, the average relative intensity noise (SRIN) of 0.44 has been reduced to 0.05. Over 9.4 dB SRIN enhancement has been realized with the pre-amplification. Besides, an AOM rather than EOM has been adopted to generate the probe pulse. Benefiting from the selectivity of Brillouin amplification and the frequency shift of AOM, higher effective ER can be achieved. By combining AOM based pulse and Brillouin amplification, ~2.2 dB SRIN improvement has been achieved.
At last, the validation experiments of the PWM Brillouin amplification have been implemented. Driven by three typical functions including up-ramp sawtooth, sine and triangle, similar gain distributions have been achieved. That proves an arbitrary gain distribution can be obtained, which is consistent with the theoretical analysis. With a linear modulation (up-ramp sawtooth) PWM pulse train, ~11.5 dB signal enhancement has been realized. More importantly, ~9 dB transmission attenuation along the sensing fiber has been compensated simultaneously. In addition, two vibrations with different frequencies (100 Hz and 300 Hz) have been measured accurately. Foreseeably, the PWM Brillouin amplification also can be used in other distributed technologies.
Funding
International Science and Technology Cooperation Program of China (2014DFA11170); National Natural Science Foundation of China (NSFC) (No. 61475128, No. 61735015); Key Project of Sichuan Provincial Science and Technology Plan (2017GZ0091); Doctoral Innovation Fund Program of Southwest Jiaotong University (D-CX201718).
References and links
1. X. Bao and L. Chen, “Recent progress in distributed fiber optic sensors,” Sensors (Basel) 12(7), 8601–8639 (2012). [CrossRef] [PubMed]
2. M. Nikles, L. Thevenaz, and P. A. Robert, “Brillouin gain spectrum characterization in single-mode optical fibers,” J. Lightwave Technol. 15(10), 1842–1851 (1997). [CrossRef]
3. L. Thévenaz, “Next generation of optical fibre sensors: new concepts and perspectives,” Proc. SPIE 9157, 23rd International Conference on Optical Fibre Sensors, 9157AN (2014).
4. N. Guo, L. Wang, H. Wu, C. Jin, H. Y. Tam, and C. Lu, “Enhanced Coherent BOTDA System without Trace Averaging,” J. Lightwave Technol. 36(4), 871–878 (2018). [CrossRef]
5. Z. Qin, T. Zhu, L. Chen, and X. Bao, “High sensitivity distributed vibration sensor based on polarization-maintaining configurations of phase-OTDR,” IEEE Photonics Technol. Lett. 23(15), 1091–1093 (2011). [CrossRef]
6. Z. Wang, Z. Pan, Z. Fang, Q. Ye, B. Lu, H. Cai, and R. Qu, “Ultra-broadband phase-sensitive optical time-domain reflectometry with a temporally sequenced multi-frequency source,” Opt. Lett. 40(22), 5192–5195 (2015). [CrossRef] [PubMed]
7. H. He, L.-Y. Shao, B. Luo, Z. Li, X. Zou, Z. Zhang, W. Pan, and L. Yan, “Multiple vibrations measurement using phase-sensitive OTDR merged with Mach-Zehnder interferometer based on frequency division multiplexing,” Opt. Express 24(5), 4842–4855 (2016). [CrossRef] [PubMed]
8. Y. Lu, T. Zhu, L. Chen, and X. Bao, “Distributed vibration sensor based on coherent detection of phase-OTDR,” J. Lightwave Technol. 28(22), 3243–3249 (2010).
9. J. C. Juarez and H. F. Taylor, “Field test of a distributed fiber-optic intrusion sensor system for long perimeters,” Appl. Opt. 46(11), 1968–1971 (2007). [CrossRef] [PubMed]
10. H. Wu, Z. Wang, F. Peng, Z. Peng, X. Li, Y. Wu, and Y. Rao, “Field test of a fully distributed fiber optic intrusion detection system for long-distance security monitoring of national borderline,” In Proceedings of the 23rd OFS, OSA, Santander, 2014.
11. Z. Qin, L. Chen, and X. Bao, “Wavelet denoising method for improving detection performance of distributed vibration sensor,” IEEE Photonics Technol. Lett. 24(7), 542–544 (2012). [CrossRef]
12. T. Zhu, X. Xiao, Q. He, and D. Diao, “Enhancement of SNR and spatial resolution in φ-OTDR system by using two-dimensional edge detection method,” J. Lightwave Technol. 31(17), 2851–2856 (2013). [CrossRef]
13. M. A. Soto, J. A. Ramírez, and L. Thévenaz, “Intensifying the response of distributed optical fibre sensors using 2D and 3D image restoration,” Nat. Commun. 7, 10870 (2016). [CrossRef] [PubMed]
14. H. He, L.-Y. Shao, H. Li, B. Luo, X. Zou, Z. Zhang, W. Pan, and L. Yan, “SNR Enhancement in Phase-Sensitive OTDR with Adaptive 2-D Bilateral Filtering Algorithm,” IEEE Photonics J. 9(3), 1–10 (2017).
15. Y. Kong, Y. Liu, Y. Shi, F. Ansari, and T. Taylor, “Research on the 𝜙-OTDR fiber sensor sensitive for all of the distance,” Opt. Commun. 407, 148–152 (2018). [CrossRef]
16. H. F. Martins, S. Martin-Lopez, P. Corredera, M. L. Filograno, O. Frazão, and M. Gonzalez-Herraez, “Phase-sensitive optical time domain reflectometer assisted by first-order Raman amplification for distributed vibration sensing over >100 km,” J. Lightwave Technol. 32(8), 1510–1518 (2014). [CrossRef]
17. H. F. Martins, H. F. Martins, S. Martin-Lopez, P. Corredera, J. D. Ania-Castañon, O. Frazão, and M. Gonzalez-Herraez, “Distributed vibration sensing over 125 km with enhanced SNR using phi-OTDR over a URFL cavity,” J. Lightwave Technol. 33(12), 2628–2632 (2015). [CrossRef]
18. Z. N. Wang, J. Li, M. Q. Fan, L. Zhang, F. Peng, H. Wu, J. J. Zeng, Y. Zhou, and Y. J. Rao, “Phase-sensitive optical time-domain reflectometry with Brillouin amplification,” Opt. Lett. 39(15), 4313–4316 (2014). [CrossRef] [PubMed]
19. Z. N. Wang, J. J. Zeng, J. Li, M. Q. Fan, H. Wu, F. Peng, L. Zhang, Y. Zhou, and Y. J. Rao, “Ultra-long phase-sensitive OTDR with hybrid distributed amplification,” Opt. Lett. 39(20), 5866–5869 (2014). [CrossRef] [PubMed]
20. X. S. Yao, “Brillouin selective sideband amplification of microwave photonic signals,” IEEE Photonics Technol. Lett. 10(1), 138–140 (1998). [CrossRef]
21. O. Terra, G. Grosche, and H. Schnatz, “Brillouin amplification in phase coherent transfer of optical frequencies over 480 km fiber,” Opt. Express 18(15), 16102–16111 (2010). [CrossRef] [PubMed]
22. C. G. Atkins, D. Cotter, D. W. Smith, and R. Wyatt, “Application of Brillouin amplification in coherent optical transmission,” Electron. Lett. 22(10), 556–558 (1986). [CrossRef]
23. H. Q. Chang, X. H. Jia, X. L. Ji, C. Xu, L. Ao, H. Wu, Z. N. Wang, and W. L. Zhang, “DBA-based BOTDA using optical comb pump and pulse coding with a single laser,” IEEE Photonics Technol. Lett. 28(10), 1142–1145 (2016). [CrossRef]
24. D. Mermelstein, E. Shacham, M. Biton, and S. Sternklar, “Brillouin amplification and processing of the Rayleigh scattered signal,” Opt. Lett. 40(14), 3340–3343 (2015). [CrossRef] [PubMed]
25. E. Liokumovitch, D. Gotliv, and S. Sternklar, “Detuned Brillouin amplification of OTDR signals with an enhanced signal-to-noise ratio,” Opt. Lett. 42(24), 5166–5169 (2017). [CrossRef] [PubMed]
26. M. Ren, D. P. Zhou, L. Chen, and X. Bao, “Influence of finite extinction ratio on performance of phase-sensitive optical time-domain reflectometry,” Opt. Express 24(12), 13325–13333 (2016). [CrossRef] [PubMed]
27. C. Baker, B. Vanus, M. Wuilpart, L. Chen, and X. Bao, “Enhancement of optical pulse extinction-ratio using the nonlinear Kerr effect for phase-OTDR,” Opt. Express 24(17), 19424–19434 (2016). [CrossRef] [PubMed]
28. G. P. Agrawal, Nonlinear Fiber Optics (Academic press, 2007).
29. L. Thévenaz, S. F. Mafang, and J. Lin, “Effect of pulse depletion in a Brillouin optical time-domain analysis system,” Opt. Express 21(12), 14017–14035 (2013). [CrossRef] [PubMed]
30. D. Zhou, Y. Dong, B. Wang, C. Pang, D. Ba, H. Zhang, Z. Lu, H. Li, and X. Bao, “Single-shot BOTDA based on an optical chirp chain probe wave for distributed ultra-fast measurement,” Light Sci. Appl. 7(1), 32 (2018). [CrossRef]
31. H. Zheng, J. Zhang, T. Zhu, G. Yin, Y. Bai, D. Qu, X. Huang, and F. Qiu, “Polarization independent fast BOTDA based on pump frequency modulation and cyclic coding,” Opt. Express 26(14), 18270–18278 (2018). [CrossRef] [PubMed]
