Distributed optical fiber sensors have been extensively employed as an excellent method for high-precision monitoring in diverse fields, such as geophysical,1,2 structure safety,3
Advanced Photonics Nexus, Volume. 3, Issue 6, 066012(2024)
Brillouin optical correlation domain analysis based on a phase-chaos laser
The physical mechanism of gain motivation is the main theoretical bottleneck that restricts the signal-to-noise ratio (SNR) and results in a mono-merit implementation for the existing stimulated Brillouin scattering-based fiber sensors. A phase-chaos laser (PCL) is proposed and introduced in the Brillouin optical correlation domain analysis (BOCDA) scheme to promote the SNR and achieve a high-accuracy measurement. The PCL characteristics are presented, and a theoretical model of chaos gain accumulation and extraction is perfected. Then, the simulation results reveal that the SNR is improved by 5.56 dB, and the signal-to-background noise ratio (SBR) of the Brillouin gain spectrum (BGS) is promoted by 8.28 dB with a 100-km sensing distance. Further, the PCL is experimentally generated. In the proof-of-concept experiment, the accuracy of the Brillouin frequency shift is upgraded to ±0.64 MHz, and the SBR of BGS is improved by 10.77 dB. The PCL provides a new research direction for optical chaos, and the PCL-BOCDA showcases a promising future for optimal-merit-coupling sensing and its application.
1 Introduction
Distributed optical fiber sensors have been extensively employed as an excellent method for high-precision monitoring in diverse fields, such as geophysical,1,2 structure safety,3
Interestingly, in BOCDA schemes, the acoustic field of temporal-spatial independence is stimulated by the narrowband coherent optical fields, which could circumvent the phonon lifetime limitation and provide unique advantages in millimeter-level spatial resolution.15 The sinusoidal frequency-modulation BOCDA with the 40-cm resolution was first proposed in 2000,16 and that was improved to 1.6 mm in 2006.17 However, the feeble gain is easily overwhelmed by the accumulated noise along the fiber, or disturbed by the cross-talk signal of periodic correlation peaks (CPs), leading to a severe deterioration of the signal-to-noise ratio (SNR).18 Consequently, the sensing performance, encompassing sensing distance, spatial resolution, and measurement accuracy, is directly limited by the inferior SNR. Several enhanced BOCDA protocols have been inspired to improve the SNR. Methods for extracting pure Brillouin gain, such as intensity modulation,19 differential measurement,20,21 and phase-shift keying,22 have been employed to enhance the spatial resolution to 0.64 mm.15 The techniques aimed at suppressing accumulated noise, such as Golomb encoding,11,23 temporal gating,12,24,25 and gradient descent frequency shift estimation,26 have achieved a Brillouin frequency shift (BFS) error of <1 MHz. Concurrent interrogation of multiple CPs, employing techniques, such as time-domain data processing,12,24 hybrid aperiodic coding,27 and injection locking combined with distributed Raman amplification,28 has significantly promoted the sensing range to 72.65 km. However, the optimal merit is barely achieved in pairs. Exploring a multimeric coupling scheme reliant on a higher SNR shows a promising future.
In addition, the chaos BOCDA29 was proposed and experimentally demonstrated as another competitive long-range high-resolution configuration. The chaotic laser features broadband in the optical spectrum and random fluctuation in the power sequence.30 This utilization ensures the excitation of a sole narrow CP along the entire fiber, primarily avoiding high-rate external modulation and periodic CP interference. The bandwidth-enhanced chaotic laser has been applied to achieve a spatial resolution of 3.1 mm,31 and then the optimized time-gated scheme combined with differential denoising configuration has been proposed to increase the sensing range to 27.54 km.32 Similar to the above schemes, the best performances of chaos BOCDA are independently implemented, e.g., the sensing range being 19 m at a 3.1-mm spatial resolution, and the measurement accuracy being 2.98 MHz at 27.54-km sensing distance. The physical mechanism reveals that the SBS gain highly depends on the relative power variation of pump and probe waves. Specifically, chaos BOCDA suffers significant deterioration due to inherent power fluctuation. Simultaneously, the residual off-peak background noise is parasitically stimulated and consistently accumulated, which cannot be nearly inhibited completely. Therefore, in the original chaos BOCDA, the performances are enormously subject to the inferior SNR and are hardly to upgrade further.32
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In this paper, a novel high-correlated laser source, referred to as phase-chaos laser (PCL), is proposed and introduced in the BOCDA scheme to promote the SNR and achieve a high-accuracy measurement. The PCL and its characteristics are first presented, and the determinants of the SNR are comprehensively deduced, given the operating mechanism of chaos BOCDA. Then, the leading role of the chaos phase term in the temporal frequent excitation of the SBS acoustic field and the impact of the fluctuation and correlation characteristics of power on the pure gain extraction are investigated in the simulation. Further, the PCL is experimentally generated by an optical injection scheme, and its chaos properties are amply verified. Finally, the proposed PCL-BOCDA scheme markedly improves the accuracy of BFS and the signal-to-background noise ratio (SBR) of the Brillouin gain spectrum (BGS).
2 Methodology: Principle and Simulation
2.1 Operating Principle
The PCL, in definition, features a stochastic variation in phase and a constant in power. The PCL remains a Lorentz-shaped broadband in the optical spectrum. In contrast, the power fluctuation is approximately equivalent to that of the general laser, whose peak-to-peak value is approaching the noise floor in the time sequence, compared to the original chaotic laser.
The light field
Figure 1.The schematic diagram of the PCL-BOCDA principle. (a) The power sequence and phase sequence of the PCL. (b) Distribution map of beat spectra along the fiber. (c) Construction of the SBS gain along the fiber. (d) Gain accumulation of Stokes at a frequency of
In the BOCDA scheme, SBS interaction is stimulated by the highly correlated pump and probe waves, which are jointly modulated in phase or amplitude. Serving the PCL as the sensing signal, the beat interaction is operated between the counterpropagating pump and probe waves with the frequency detuning of
Further, the SBS acoustic fields are excited at the beat positions due to the electrostriction effect, and then the BGS at a particular position could be obtained by scanning the detuning frequency
The intensity of measured gain
The pure SBS gain
The first term of Eq. (7) on the right represents the influence of noise along the optical fiber. The second term of Eq. (7) on the right is the influence of light intensity fluctuation and the average number of times. Therefore, for the PCL system, the impact of
In BOCDA schemes, the acoustic field of temporal-spatial independence is stimulated by the narrowband coherent optical fields. However, tracing to the gain motivation mechanism, the feeble gain is easily overwhelmed by the accumulated noise along the fiber, or disturbed by the cross-talk signal of periodic CPs. According to Eq. (7), the SNR is severely deteriorated in principle. The sensing performance, including sensing distance, spatial resolution, and measurement accuracy, cannot be simultaneously improved due to the limitation of inferior SNR. Remarkably, the utilization of the PCL could achieve a higher measurement SNR by stimulating a purer gain and significantly suppressing the noise floor. In addition, this proposed model is suitable for clarifying the gain component and noise structure for the general BOCDA schemes. In the phase coding or sine frequency modulation schemes, the suborder CPs should be separate from the central CP, and the parasitic power fluctuation should be eliminated. For the amplified spontaneous emission source, the feeble gain is approximately submerged by the intensive amplitude fluctuation, resulting in a poor SNR.
2.2 Numerical Simulation
To determine the dominant role of the phase term in Brillouin gain excitation, the temporal–spatial distribution of the SBS acoustic field at
Figure 2.Simulation of temporal-spatial distribution of the acoustic field in different conditions. (a) Only phase term correlated. (b) Only amplitude term correlated.
The intensity of the acoustic field is further analyzed in both the phase chaos and original chaos schemes. As shown in Fig. 3(a), although the sharp acoustic field could be excited at the center CP in both schemes, the noise intensity of the suborder field is significantly reduced, with an SBR of 12.79 dB in the PCL-BOCDA,
Figure 3.Intensity distribution in the phase chaos and original chaos schemes. (a) Acoustic field. (b) Gain variation of the Stokes wave.
SBR is defined as the ratio of gain signal to background noise, where the noise level represents the level of the second largest peak that appears at
In addition, as the inset shows, the noise floor along the fiber presents a gentle variation in the PCL system rather than a drastic fluctuation. The gain intensity of the Stokes wave varying with the positions is further compared in Fig. 3(b). Obviously, in the PCL-BOCDA, the pure gain signal is sharply generated at the CP position, and there is negligible accumulated noise along the off-CP positions. Contrariwise, the effective gain is submerged by the noise structure in the original chaos scheme. According to Eq. (7), the SNR of the PCL-BOCDA is prominently increased to 8.05 dB, which is 5.56 dB higher than the original one.
When
Figure 4.The BGSs after single scanning and averaging 15 times in the (a1) original scheme and (b1) PCL-BOCDA scheme. The BGSs after averaging 15 times and their Lorentz-fitted curves in the (a2) original scheme and (b2) PCL-BOCDA scheme. The BGSs, after averaging 50 times, at different positions in the 100-km system at the (c) front and (d) end of the fiber.
The influence of the cumulative noise on the BGS measurement is further considered by increasing the sensing distance and the average times. The length of the fiber under test (FUT) with a BFS of 10.73 GHz is 100 km, and a 10 cm-long event section is set as a BFS of 10.64 GHz. The red curve of Fig. 4(c) demonstrates that when the central CP and the event region are located at the front of the FUT, the central BFS of the event region could be accurately distinguished with a neglected noise background, the SBR reaching 9.21 dB in the PCL-BOCDA scheme. Conversely, in the original system, the BGS performs bimodal and the SBR decreases to 2.07 dB, although the center BFS could be extracted. Figure 4(d) depicts that when the central CP and the event region are located at the end of the FUT, the central gain of the traditional system has been submerged by the accumulated noise along the fiber, presented as the subpeak being higher than the main peak. The SBR is severely deteriorated to
3 Experiment: Setup and Results
3.1 Experimental Setup
The experimental setup of the PCL-BOCDA is shown in Fig. 5. The PCL is generated by an optical open-loop injection configuration. The chaotic laser as the master laser (ML) is injected into the slave laser (SL) through the polarization controller (PC1) and variable optical attenuator (VOA). Utilizing optical injection, a strong semiconductor laser is controlled by a weak broadband chaotic laser. The center wavelength between the ML and the SL is adjusted to ensure the frequency detuning is within a certain range. The optical spectral shape of the output light of the SL is changed by adjusting the detuning frequency. The injection intensity is controlled by the VOA, and the PC1 can change the polarization-matching state of the two beams. The optical spectral width of the PCL is changed by adjusting the injection power and polarization-matching state. The PCL is split into two beams by an optical coupler, where the upper branch (90%) is used as the probe light and the lower branch (10%) is used as the pump light. The probe light is modulated in a suppress-carrier, double-sideband format by the electro-optic modulator (EOM), driven by the microwave signal generator (MWG), and the sideband frequency shift is approximately equal to that of the fiber BFS. The modulated probe light is transmitted through the programmable optical delay generator (PODG), amplified by the continuous-wave erbium-doped fiber amplifier (C-EDFA), and then launched into the end of the FUT through the polarization scrambler (PS) and the optical isolator (ISO). The pump light is modulated into the pulsed laser by the semiconductor optical amplifier (SOA) and then is launched into FUT via the pulse-EDFA (P-EDFA) and optical circulator (OC). The probe light is amplified by the SBS interaction along the FUT and filtered by the optical bandpass filter (BPF) with a bandwidth of 6 GHz. Finally, the filtered Stokes light is detected by a PD, sampled, and processed by a lock-in amplifier (LIA). Remarkably, the pulse modulation in this experiment is used as a trigging and chopping signal for lock-in detection, and the time-gated configuration is unadopted.
Figure 5.Experimental setup of the proposed PCL-BOCDA. Chaos-LD, chaos laser diode; DFB-LD, distributed feedback-laser diode; PC, polarization controller; VOA, variable attenuator; OC, optical circulator; EOM, electro-optic modulator; MWG, microwave signal generator; PODG, programmable optical delay generator; C-EDFA, continuous-wave erbium-doped fiber amplifier; PS, polarization scrambler; ISO, isolator; SOA, semiconductor optical amplifier; P-EDFA, pulse erbium-doped fiber amplifier; BPF, optical bandpass filter; PD, photodetector; LIA, lock-in amplifier.
3.2 Measurement Results
In this work, the PCL is experimentally generated. Figure 6 shows the typical properties of the original chaos and the proposed PCL. Both optical spectra present a wideband distribution and the
Figure 6.Characteristics of the (1) original chaos and (2) PCL. (a) Optical spectrum. (b) Phase sequence. (c) Power sequence. (d) Probability density distribution of the power. (e) Autocorrelation curve of power sequence.
Serving these two lasers as the sensing source, the measured BGSs are illustrated in Fig. 7(a). In the lock-in detection system, each BGS was obtained via a single measurement, where MWG was scanned from 10.68 to 10.88 GHz with a 1 MHz step. It only consumes 0.2 s for a single measurement, which is only determined by the sweeping speed of MWG and the sampling rate of LIA, with a sensitivity of
Figure 7.Measured (a) BGSs and (b) BFS errors in the 1 km-long FUT in different schemes.
Finally, the BGSs of the relatively long-range systems are measured to conduct the suppression of background noise in the PCL-BOCDA. Figure 8(a) shows the structure of the FUT, which consists of 1400-m SMF1 with a BFS of 10.748 GHz and a 10-m SMF2 with a BFS of 10.805 GHz. Figures 8(b) and 8(c) describe the measured BGSs at the front and end of the FUT under different schemes. When the central CP is located at the SMF1, the BGS of the original chaos scheme shows a slightly bimodal structure with a gain fluctuation, and the SBR of that is decreased to 4.36 dB. The deterioration of the original chaos BGS is aggregated, as the central CP is located at the SMF2, where a powerful subpeak induced by the secondary beat and off-peak substrate results in the SBR value is as low as 2.20 dB. Significantly, whether the central CP is placed at the front or tail of FUT, the BGSs of the phase-chaos scheme present a nearly coincident Lorenz shape, whose SBRs are promoted to 12.60 and 12.97 dB, respectively. Therefore, this proof-of-concept implementation of the PCL-BOCDA scheme presents a higher SNR and enormous development potential in high-performance sensing.
Figure 8.(a) Structure of the FUT. The measured BGSs at the (b) front and (c) tail of the FUT in different schemes.
4 Discussion
In BOCDA schemes, the acoustic field of temporal-spatial independence is stimulated by the narrowband coherent optical fields, which could circumvent the phonon lifetime limitation and provide unique advantages in high spatial resolution. However, the feeble gain is easily overwhelmed by the accumulated noise along the fiber or disturbed by the cross-talk signal of periodic CPs.
For all BOCDA schemes, two indices are mainly proposed to estimate the sensing performance.
Remarkably, the SBR of BGS is directly related to the system SNR. The SBR improvement is one of the manifestations of the higher SNR and could be constantly degraded with further improvement of the sensing performance.
A brief overview of the SBR or SNR of BOCDA is summarized in Table 1. The highest SBR of 12.97 dB has been experimentally demonstrated in this paper, the accuracy of BFS being prior. With the sensing range increasing, the SBR of the traditional scheme severely deteriorates to 0.36 dB, although the extra method of optimal temporal gating is applied. In contrast, a higher SBR of 5.35 dB with a range of 100 km is theoretically achieved, and an enormous potential for long-reach high-resolution application is presented in the PCL scheme. Remarkably, although the differential measurement and Golomb coding are employed to reduce the noise floor and promote the SBRs in sine-FM and phase modulation schemes, respectively, the PCL-BOCDA system could reduce the power fluctuation of the light source and not be limited by the bandwidth of the modulation device with the same or even higher SBR.
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The SNR or SBR of the proposed PCL protocols has been promoted, although some indices should be further enhanced compared to the state-of-art methods. In addition, PCL optimizes the optical spectral shape of chaotic light, which can further provide a positive improvement in the measurement of spatial resolution. PCL BOCDA performs a significant competitiveness in multimerit coupling achievement and is expected to highlight the future avenues after revising the intrinsic drawbacks of the traditional chaos.
5 Conclusion
In summary, we propose and demonstrate a novel BOCDA scheme based on a PCL. It is understood that the SNR of SBS-based sensors is principally limited by the gain stimulation mechanism, which severely restricts the development of the SBS-based sensors. The PCL is proposed and subsequently utilized to deduce the SNR impact factors of the chaos BOCDA. Then, the SNR of the PCL-based scheme is increased by 5.56 dB, and the SBR of BGS is improved by 8.28 dB at an FUT of 100 km, which significantly promotes the theoretical optimal sensing performance. Further, the PCL is experimentally generated by the optical injection scheme for the first time. In the proof-of-concept PCL-BOCDA, the measurement accuracy of BFS is preliminarily improved from
The generation and utilization of the PCL provide a new research direction for optical chaos, and an advanced exploration would be further conducted including spectral characteristics, relevant dimensions, and applications in optical coherence tomography. In addition, the SNR model, aiming at the impact factors and improvable avenues, offers some experience and instructions for the general BOCDA schemes. The PCL-BOCDA provides a competitively novel solution for high-SNR measurement, and the multimeric coupling sensing promises to be achieved by combining broadband enhancement, differential measurement, and temporal gating in the future.
Lintao Niu received her BS degree in 2021 in photoelectric information science and technology from the College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan, China, where she is currently working toward the PhD in optics engineering. Her current research interests include chaotic laser and optical fiber sensing.
Yahui Wang is an assistant research fellow at Taiyuan University of Technology, Taiyuan, China. He received his PhD in optics engineering from Taiyuan University of Technology, Taiyuan, China, in 2021. His research interests include chaotic laser and its Brillouin distributed fiber sensing.
Jing Chen received her BS degree in applied physics from the College of Physics and Optoelectronic Engineering, Taiyuan University of Technology, Taiyuan, China, in 2022, where she is currently working toward a master’s degree in optics engineering. Her current research interests include chaotic laser and optical fiber sensing.
Haochen Huang received his BS degree in optoelectronic information science and Engineering from the College of Physics and Optoelectronic Engineering, Taiyuan University of Technology, Taiyuan, China, in 2022, where he is currently working toward the master’s degree in optics engineering. His current research interests include chaotic laser and optical fiber sensing.
Lijun Qiao received her PhD in microelectronics and solid electronics from the Institute of Semiconductors, Chinese Academy of Sciences, Beijing, China, in 2017. She is currently an associate professor at the College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan, China. Her research interests include nonlinear dynamics of laser diodes and photonic integrated broadband chaotic semiconductor lasers.
Mingjiang Zhang is a professor and PhD supervisor at Taiyuan University of Technology, Taiyuan, China. He received his PhD in optics engineering from Tianjin University in 2011. He was a visiting scholar at the University of Ottawa, Canada, in 2016. His current research interests include photonic integrated chaotic lasers and distributed optical fiber sensing. He also serves as a reviewer for IEEE, OSA, and Elsevier journals.
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Lintao Niu, Yahui Wang, Jing Chen, Haochen Huang, Lijun Qiao, Mingjiang Zhang, "Brillouin optical correlation domain analysis based on a phase-chaos laser," Adv. Photon. Nexus 3, 066012 (2024)
Category: Research Articles
Received: May. 30, 2024
Accepted: Oct. 23, 2024
Published Online: Nov. 19, 2024
The Author Email: Wang Yahui (wangyahui@tyut.edu.cn), Zhang Mingjiang (zhangmingjiang@tyut.edu.cn)