| Phase-sensitive optical time domain reflectometry(φ-OTDR),a typical distributed optical fiber vibration sensor,uses the Rayleigh back-scattering light wave for vibration sensing.By demodulating the phase change of the Rayleigh back-scattering signal,vibration detection with a long distance can be achieved,so the technology has been widely concerned and studied in the academic world.However,with the development of large-scale infrastructure,communication and power networks,the vibration sensing performance of the coherent detection φ-OTDR system has been put forward higher requirements in engineering applications.Based on the traditional single probe pulse,pulse coding technology modulates a series of pulse sequences according to certain coding rules and injects them into the sensing fiber.At the receiving end,the corresponding data processing algorithm is used for encoding.The technology can break the constraints of spatial resolution,SNR,sensing distance,and other key parameters in the coherent detection φ-OTDR system.However,there are still some application limitations of the pulse coding technology in the coherent detection φ-OTDR system at present.Most of the coding techniques require multiple injections of coding sequences to meet the decoding requirements,which leads to great sacrifice in the frequency response range.At the same time,the coding gain in the coherent detection φ-OTDR system is affected by the pulsing mass,which is mainly reflected in the extinction ratio and pulse shape,but there is no relevant study.In addition,due to the transient effect in the erbium-doped fiber amplifier(EDFA),the coding pulse will have an obvious amplitude distortion after optical amplification,which greatly restricts the application of the coding system in the long-sensing distance.To solve the above problems,based on the in-depth study of the coherent detection φ-OTDR system,this paper proposes a method to improve the sensing performance of the system by using random number coding pulses,which can increase the SNR and sensing distance of the system while maintaining the original spatial resolution and frequency response range.Additionally,the corresponding coding gain optimization method and amplification distortion suppression method are proposed.The main research and results are as follows:(1)Aiming at the problem of frequency response range deterioration caused by multiple measurements in the coding pulse φ-OTDR,a random number coding method is proposed.The method only needs one sequence to complete the measurement,which means it can improve the system SNR while maintaining the original frequency response range.The coding pulse is modulated by physical random numbers and the encoding algorithm can be achieved by crosscorrelation operation between the system response signal and the random number coding pulse.The coding gain is analyzed theoretically according to the auto-correlation characteristics of the random number coding pulse.The experimental results show that when the single pulse width is 40 ns,the SNR of the demodulated vibration signal along a 2.2 km sensing fiber can be improved by 14.19 d B,and the original frequency response range is verified.In addition,compared with the traditional scheme with EDFA,the sensing distance of the system can be increased from less than 25 km to 42.338 km without any optical amplification,and the SNR of demodulated vibration signal can reach 21.5 d B.(2)Aiming at the feature that the SNR can be further improved due to the quality of pulse modulation in the φ-OTDR with random number coding,a coding gain optimization method based on synchronous cascade modulation(SCM)is proposed.By analyzing the effects of the extinction ratio and the rising edge of the probe pulse on the auto-correlation characteristics of the random number coding pulse,the influencing mechanisms of the pulse modulation quality on the coding gain and system SNR are explored.The theoretical simulation and experimental results show that the SCM scheme can improve the pulse extinction ratio and shorten the pulse rising edge,so as to further optimize the coding gain and improve the SNR of the system.The results show that the intensity SNR at the end of the 10.298 km sensing fiber is increased by8.01 d B,the phase demodulated signal obtained by SCM is more stable at 42.240 km sensing fiber in a static environment,and the SNR is increased by 7.82 d B in vibration environment.With the help of the SNR enhancement,the sensing distance of the system can be further extended,the sensing distance of the system can be extended to 62.440 km under a 40 ns pulse width,and the phase-demodulated SNR of the vibration signal is as high as 28.4 d B.(3)Aiming at the problem of the pulse amplification distortion caused by transient effect in the random number coding φ-OTDR,an amplification distortion suppression method based on quadrature phase shift keying(QPSK)is proposed.The basic principle of transient effect and amplification distortion is studied theoretically,and the influence law of amplification distortion is analyzed further.The phase modulation technique based on QPSK is used to replace the traditional amplitude modulation to generate random number coding pulses.The working principle of QPSK is described and the modulation process is designed based on an I/Q modulator.The auto-correlation characteristics of the QPSK modulated random number coding pulse are analyzed,and experimental results show that the coding gain can be further improved by 3 d B compared with the traditional scheme.In experiments,the QPSK modulated random number coding pulse can effectively suppress the amplitude distortion after optical amplification,the distortion coefficient decreases from 57.67% to 9.23%.Without distributed amplification,the QPSK modulated random number coding φ-OTDR system successfully achieves the vibration sensing along an 82.852 km fiber with a 40 ns pulse width,and the phasemodulated SNR of the vibration signal is as high as 21.99 dB. |
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