WO2021134747A1 - 双极性差分相位编码超高空间分辨率布里渊光时域反射仪 - Google Patents

双极性差分相位编码超高空间分辨率布里渊光时域反射仪 Download PDF

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WO2021134747A1
WO2021134747A1 PCT/CN2020/070085 CN2020070085W WO2021134747A1 WO 2021134747 A1 WO2021134747 A1 WO 2021134747A1 CN 2020070085 W CN2020070085 W CN 2020070085W WO 2021134747 A1 WO2021134747 A1 WO 2021134747A1
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instantaneous frequency
optical
signal
spatial resolution
time domain
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PCT/CN2020/070085
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French (fr)
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程凌浩
关柏鸥
马祥杰
朱金顶
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暨南大学
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Priority to US17/842,826 priority Critical patent/US11867540B2/en

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
    • G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
    • G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
    • G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
    • G01D5/353—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre
    • G01D5/35338—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using other arrangements than interferometer arrangements
    • G01D5/35354—Sensor working in reflection
    • G01D5/35358—Sensor working in reflection using backscattering to detect the measured quantity
    • G01D5/35364—Sensor working in reflection using backscattering to detect the measured quantity using inelastic backscattering to detect the measured quantity, e.g. using Brillouin or Raman backscattering

Definitions

  • the invention relates to the technical field of distributed optical fiber sensing, in particular to a bipolar differential phase encoding ultra-high spatial resolution Brillouin optical time domain reflectometer.
  • BOTDR Brillouin Optical Time Domain Reflectometer
  • the time distribution of the collected scattered light signal corresponds to the spatial distribution of the scattered light, which is the optical time domain reflectometry (OTDR).
  • OTDR optical time domain reflectometry
  • the Brillouin frequency shift caused by Brillouin scattering is related to the temperature and strain felt at the scattering position (generally a linear relationship). Therefore, by measuring the frequency shift of the Brillouin scattering signal, the temperature or strain at the scattering position can be reversed.
  • This technology that combines Brillouin scattering and OTDR is BOTDR.
  • the Brillouin scattering signal has poor coherence, so it has a certain line width.
  • the line width is about 30 MHz, and the spectrum of the scattered signal generally shows a Lorentz type.
  • the Brillouin frequency shift to be measured during sensing refers to the shift value of the center frequency (Brillouin frequency) of the Brillouin scattering spectrum.
  • the conventional technical method is to first measure the entire Brillouin scattering spectrum. Due to the influence of noise, the spectrum is generally not a smooth spectrum. Therefore, it is also necessary to perform Lorentz curve fitting on the obtained scattering spectrum to obtain an estimated spectrum with a smooth curve.
  • the frequency corresponding to the peak position of the smooth spectrum is sought to obtain the center frequency.
  • many different schemes have been proposed to measure the Brillouin frequency shift, basically all schemes are based on the measurement of the Brillouin scattering spectrum. In terms of specific technical approaches, it can be roughly divided into two types: frequency sweep-based schemes and digital spectrum analysis schemes based on Fast Fourier Transform (FFT).
  • the former uses a narrow-band filter to obtain the signal power in a narrow range near a certain frequency position in the Brillouin scattering spectrum, and obtains each frequency point of the Brillouin scattering spectrum by changing the center frequency (sweep frequency) of the filter.
  • the power to obtain the shape of the entire Brillouin scattering spectrum.
  • the latter performs FFT on the Brillouin scattering signal collected in a period of time through broadband reception, so as to obtain the Brillouin scattering spectrum.
  • the existing technology has the following shortcomings:
  • the Brillouin scattering signal must be squared to obtain intensity information.
  • the squaring operation makes all the measured values become positive, including the noise accompanying the signal. After the noise is squared, a constant positive noise power level is formed, which cannot be eliminated by multiple average superpositions, which limits the final measurement distance of these technologies.
  • the frequency spectrum cannot be continuously analyzed in frequency. There must be a frequency interval in the frequency analysis, which makes the resolution of the frequency spectrum. Restricted, and ultimately limited the accuracy of the measurement.
  • the embodiment of the present invention provides a bipolar differential phase encoding ultra-high spatial resolution Brillouin optical time domain reflectometer, which aims to solve the technical problems raised by the above-mentioned background art.
  • an embodiment of the present invention provides a bipolar differential phase encoding ultra-high spatial resolution Brillouin optical time domain reflectometer, including:
  • Narrow linewidth laser used to emit linearly polarized laser
  • the polarization maintaining coupler is used to divide the linearly polarized laser into two paths of light, wherein one path of light is transmitted to the Mach-Zehnder modulator for phase modulation, and the other path of light is transmitted to the coherent photoelectric receiver as a local oscillator light source;
  • the differential encoder is used to perform differential encoding on the original coding sequence; wherein the original coding sequence includes a first coding sequence and a second coding sequence, and the first coding sequence and the second coding sequence are two sets of mutual polarities. Is the opposite coding sequence;
  • the Mach-Zehnder modulator is used to phase-modulate the phase of the linearly polarized laser according to the differentially encoded encoding sequence, and output optical pulses;
  • optical circulator and a sensing optical fiber, the optical circulator is used to input the light pulse into the sensing optical fiber and receive the Brillouin scattered light returned by the sensing optical fiber;
  • the coherent photoelectric receiver is used to perform photoelectric mixing and coherent reception of the local oscillator light source and the Brillouin scattered light to obtain a microwave signal; wherein, the microwave signal includes Stokes signal and anti-Stork signal X signal;
  • the sideband separator is used to separate the Stokes signal and the anti-Stokes signal in the microwave signal, and send the Stokes signal and the anti-Stokes signal to Result measurement module;
  • the result measurement module is used to measure the first instantaneous frequency and the second instantaneous frequency according to the Stokes signal and the anti-Stokes signal, and to obtain the first instantaneous frequency and the second instantaneous frequency according to the first instantaneous frequency and the first instantaneous frequency. 2. Obtain the final measurement result from the instantaneous frequency.
  • the result measurement module specifically includes:
  • a first data collector configured to receive the Stokes signal and perform analog-to-digital conversion into a first digital signal
  • a first differential demodulator configured to perform differential demodulation on the first digital signal
  • a first correlator configured to perform correlation operations on the signals output by the first differential demodulator and the original coding sequence in sequence
  • a first superimposed averager configured to superimpose and average the correlation operation result output by the first correlator
  • a first measuring unit configured to obtain the first instantaneous frequency according to the superimposed average result output by the first superimposed averager
  • a second data collector configured to receive the anti-Stokes signal and perform analog-to-digital conversion into a second digital signal
  • a second differential demodulator configured to perform differential demodulation on the second digital signal
  • a second correlator configured to perform correlation operations on the signals output by the second differential demodulator and the original coding sequence in sequence
  • a second superimposed averager configured to superimpose and average the correlation calculation results output by the second correlator
  • a second measuring unit configured to obtain the second instantaneous frequency according to the superimposed average result output by the second superimposed averager
  • the third measurement unit is configured to obtain a final measurement result according to the first instantaneous frequency and the second instantaneous frequency.
  • the obtaining the final measurement result according to the first instantaneous frequency and the second instantaneous frequency is specifically:
  • the sensing optical fiber is a standard single-mode optical fiber or a plastic optical fiber.
  • the line width of the narrow linewidth laser is less than 100 kHz.
  • the Mach-Zehnder modulator adopts a lithium niobate wave conductive light Mach-Zehnder intensity modulator.
  • an optical pulse amplifier is further provided between the Mach-Zehnder modulator and the optical circulator, and the optical pulse amplifier is used to output the Mach-Zehnder modulator according to a preset first power value. Amplify the power of the light pulses, and input the amplified light pulses into the optical circulator.
  • an optical amplifier is further provided between the optical circulator and the coherent photoelectric receiver, and the optical amplifier is used to scatter the Brillouin emitted by the optical circulator according to a preset second power value.
  • the light performs power amplification, and the amplified Brillouin scattered light is input into the coherent photoelectric receiver.
  • the present invention has the following beneficial effects:
  • the embodiment of the present invention provides a bipolar differential phase encoding ultra-high spatial resolution Brillouin optical time domain reflectometer, including a narrow linewidth laser, a polarization maintaining coupler, a differential encoder, a Mach-Zehnder modulator, and an optical time domain reflectometer.
  • the implementation of the present invention can effectively improve the spatial resolution of the Brillouin scattering spectrum measurement, at the same time, the measurement accuracy can be effectively improved by increasing the number of superimposed averaging, and the measurement performance can be effectively improved by directly adopting the bipolar coding sequence.
  • FIG. 1 is a schematic diagram of the structure of a bipolar differential phase encoding ultra-high spatial resolution Brillouin optical time domain reflectometer provided by an embodiment of the present invention
  • FIG. 2 is another schematic diagram of the structure of a bipolar differential phase encoding ultra-high spatial resolution Brillouin optical time domain reflectometer provided by an embodiment of the present invention
  • FIG. 3 is a schematic flowchart of completing a measurement with a group of multiple different coding sequences according to an embodiment of the present invention
  • Figure 4 is a schematic structural diagram of a differential encoding provided by an embodiment of the present invention.
  • FIG. 5 is a schematic structural diagram of a de-differentiator provided by an embodiment of the present invention.
  • Fig. 6 is a schematic structural diagram of a correlator provided by an embodiment of the present invention.
  • an embodiment of the present invention provides a bipolar differential phase encoding ultra-high spatial resolution Brillouin optical time domain reflectometer, including:
  • Narrow linewidth laser used to emit linearly polarized laser
  • a polarization-maintaining coupler used to divide the linearly polarized laser into two paths of light, wherein one path of light is transmitted to the Mach-Zehnder modulator for phase modulation, and the other path of light is transmitted to the coherent photoelectric receiver as a local oscillator light source;
  • Differential encoder which is used to differentially encode the original coding sequence; wherein, the original coding sequence includes a first coding sequence and a second coding sequence, and the first coding sequence and the second coding sequence are two sets of polarity mutual Is the opposite coding sequence;
  • the Mach-Zehnder modulator is used to phase-modulate the phase of the linearly polarized laser according to the differentially encoded encoding sequence, and output optical pulses;
  • the optical pulse amplifier is used to amplify the power of the optical pulse output by the Mach-Zehnder modulator according to the preset first power value; (this optical pulse amplifier can be omitted in some applications);
  • optical circulator and a sensing optical fiber, the optical circulator is used to input the light pulse into the sensing optical fiber and receive the Brillouin scattered light returned by the sensing optical fiber;
  • the sensing optical fiber is a standard single-mode optical fiber or a plastic optical fiber.
  • the optical amplifier is used to amplify the power of the Brillouin scattered light emitted by the optical circulator according to the preset second power value; (this amplifier can be omitted in some applications);
  • the coherent photoelectric receiver is used to perform photoelectric mixing and coherent reception of the local oscillator light source and the Brillouin scattered light to obtain a microwave signal; wherein, the microwave signal includes Stokes signal and anti-Stork signal X signal;
  • the sideband separator is used to separate the Stokes signal and the anti-Stokes signal in the microwave signal, and send the Stokes signal and the anti-Stokes signal to Result measurement module;
  • the result measurement module is used to measure the first instantaneous frequency and the second instantaneous frequency according to the Stokes signal and the anti-Stokes signal, and to obtain the first instantaneous frequency and the second instantaneous frequency according to the first instantaneous frequency and the first instantaneous frequency. 2. Obtain the final measurement result from the instantaneous frequency.
  • the result measurement module specifically includes:
  • a first data collector configured to receive the Stokes signal and perform analog-to-digital conversion into a first digital signal
  • a first differential demodulator configured to perform differential demodulation on the first digital signal
  • a first correlator configured to perform correlation operations on the signals output by the first differential demodulator and the original coding sequence in sequence
  • a first superposition averager configured to superimpose and average the correlation operation results output by the first correlator
  • a first measuring unit configured to obtain the first instantaneous frequency according to the superimposed average result output by the first superimposed averager
  • a second data collector configured to receive the anti-Stokes signal and perform analog-to-digital conversion into a second digital signal
  • a second differential demodulator configured to perform differential demodulation on the second digital signal
  • a second correlator configured to perform correlation operations on the signals output by the second differential demodulator and the original coding sequence in sequence
  • a second superimposed averager configured to superimpose and average the correlation calculation results output by the second correlator
  • a second measuring unit configured to obtain the second instantaneous frequency according to the superimposed average result output by the second superimposed averager
  • the third measurement unit is configured to obtain a final measurement result according to the first instantaneous frequency and the second instantaneous frequency.
  • the obtaining the final measurement result according to the first instantaneous frequency and the second instantaneous frequency is specifically:
  • the line width of the narrow linewidth laser is less than 100 kHz. (Some applications can also use a laser with a wider line width, such as when the measuring distance is very short).
  • the Mach-Zehnder modulator adopts a lithium niobate wave conductive light Mach-Zehnder intensity modulator.
  • some applications can also use non-lithium niobate waveguides, such as silicon-based integrated photonic waveguides).
  • FIG. 2 is a block diagram of the bipolar differential phase encoding ultra-high spatial resolution Brillouin optical time domain reflectometer provided by the present invention, specifically:
  • the linearly polarized laser output from the narrow linewidth laser is divided into two paths by the polarization-maintaining coupler. One enters the Mach-Zehnder modulator for modulation, and the other enters the coherent photoelectric receiver as a local oscillator light source.
  • the Mach-Zehnder modulator is biased at the minimum power output point and modulates the differentially encoded code sequence to the phase of the laser.
  • Driving the Mach-Zehnder modulator is a bipolar signal.
  • the output of the Mach-Zehnder modulator is a series of light pulses. After these light pulses are amplified to a proper power level by the optical pulse amplifier, they pass through the optical circulator and enter the sensing fiber. When these light pulses propagate in the sensing fiber, Brillouin propagating backward is generated everywhere in the fiber. Scattered light. After passing through the optical circulator, the scattered light is amplified to a suitable power level by the optical amplifier, and then enters the coherent photoelectric receiver, and performs photoelectric mixing and coherent reception together with the local oscillator light output by the narrow linewidth laser.
  • the microwave signal includes Brillouin scattered Stokes signal and anti-Stokes signal.
  • the Stokes signal and the anti-Stokes signal are separated, and are converted into digital signals by the data collector 1 and the data collector 2 respectively.
  • the output of the data collector 1 As an example, its output first enters the differential demodulator 1 to demodulate the differential code. Then the output of the differential demodulator 1 is correlated with the original code sequence in turn, so that the scattered signal generated by each emitted light pulse will obtain a series of results after the correlation calculation. The first and second of these results in time represent the near and far in space where the scattered light is generated. Subsequently, the results obtained from multiple pulses at the same position are superimposed and averaged in the superposition averager 1 to improve the signal-to-noise ratio. The output result of the superposition averager 1 is used to obtain the instantaneous frequency 1. The instantaneous frequency 1 can be used as the measurement result 1. The instantaneous frequency 1, namely ⁇ 1 , is obtained by the following formula:
  • ⁇ 1 is the phase of the output result of the superposition averager 1
  • T is the sampling period of the data collector 1.
  • the instantaneous frequency 2 namely ⁇ 2
  • the measurement result 3 Take the superimposed average of the two results to get the measurement result 3, namely ⁇ 3 :
  • the spatial resolution ⁇ z of the measurement result is determined by the encoding rate used by the system:
  • c is the speed of light in vacuum and n 0 is the refractive index of the sensing fiber.
  • Narrow linewidth laser The linewidth of the laser should be narrow enough to ensure that there is no too much noise during coherent photoelectric reception. And the frequency stability of the laser should be sufficient to avoid interference with the measurement results. It is recommended to use a narrow linewidth laser with a linewidth below 100kHz.
  • the Mach-Zehnder modulator is a conventional lithium niobate wave conductive light Mach-Zehnder intensity modulator. When the modulator works, it needs to be DC biased at the lowest output power point, that is, when there is no external modulation signal, the output power of the modulator is the lowest.
  • the modulator is driven by a bipolar signal (both positive and negative level signals), so that the modulator converts the input continuous light according to the input code sequence to switch between two phases with a difference of ⁇ .
  • the Mach-Zehnder modulator can also be replaced with other types of modulators, as long as it can achieve "converting the input continuous light according to the input code sequence into switching between two phases with a difference of ⁇ ".
  • the use of the Mach-Zehnder modulator described here is a widely used solution with better performance.
  • the data collector is the analog-to-digital converter, which converts analog signals into digital signals.
  • the signal processing after this module all work in the digital domain.
  • Two data collectors collect the Stokes Brillouin scattering signal and the anti-Stokes Brillouin scattering signal respectively.
  • Differential coding is the conventional encoder based on modulo-2 adder, as shown in Figure 4.
  • Differentiator is a conventional encoder based on delay multiplication, as shown in Figure 5.
  • the correlator is also a conventional structure, working in the state of stream processing, taking out N continuous data from the flowing data stream, multiplying the results with the N bits of the coding sequence and adding the results to output a new data stream. As shown in Figure 6.
  • Measurement results 1 and 2 represent the results of processing the Stokes and anti-Stokes Brillouin scattering signals respectively.
  • the accuracy of measurement results 1 and 2 is affected by the frequency stability of the narrow linewidth laser. The fluctuation of the laser frequency will interfere with the measurement results 1 and 2, causing measurement errors. When the laser frequency is very stable, both measurement results 1 and 2 can be used as the final measurement results.
  • Measurement result 3 is the average of measurement results 1 and 2, which eliminates the influence of laser frequency fluctuations and has better accuracy.
  • the system shown in Figure 1 involves more digital domain processing, including differential encoding, data acquisition, and all subsequent signal processing procedures.
  • the processing of these digital domains all need to work in the same clock domain, that is, the clocks they use are derived from the same clock source.
  • the present invention has the following beneficial effects:
  • the signal processing is simple, without complex algorithms such as spectrum analysis, spectrum fitting, peak finding, etc., only simple related operations and superimposed averaging operations are required, which can be highly parallelized and streamlined, and real-time signal processing can be achieved.
  • the spatial resolution is extremely high.
  • the spatial resolution depends on the coding rate. In theory, it can achieve a spatial resolution of 2 cm or even better, which is currently unattainable by other technologies.
  • the coding rate is high, and the sensing system works in self-published Avalanche scattering, the nonlinear effect is greatly suppressed, and the transmission power can be greatly increased to improve the system performance.
  • the pulse transmission power of the prior art is greatly limited by the nonlinear effect, the transmission power is low, and the signal-to-noise ratio is low.
  • the frequency analysis is continuous, and in theory, any measurement accuracy can be achieved, as long as the number of superimposed averages is sufficient.
  • the measurement accuracy of the prior art is not only affected by the signal-to-noise ratio, but also limited by the spectral resolution.

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Abstract

本发明提供了一种双极性差分相位编码超高空间分辨率布里渊光时域反射仪，包括窄线宽激光器，保偏耦合器，差分编码器，Mach-Zehnder调制器，光脉冲放大器，光环形器，传感光纤，光放大器，相干光电接收机，边带分离器和结果测量模块。通过实施本发明能够有效提高布里渊散射谱测量的空间分辨率，同时通过增加叠加平均的次数能够有效提高测量精度，另外还通过直接采用双极性编码序列的方式从而有效提高测量的性能。

Description

双极性差分相位编码超高空间分辨率布里渊光时域反射仪 技术领域
本发明涉及分布式光纤传感技术领域，尤其是涉及一种双极性差分相位编码超高空间分辨率布里渊光时域反射仪。
背景技术
布里渊光时域反射仪(BOTDR)是一种分布式光纤传感技术。其基本原理是在光纤的一端发射一个宽度很窄(一般10～500ns)的激光脉冲。该激光脉冲沿着光纤传播的时候，会在光纤各处受到光纤中声子所导致的布里渊散射(Brillouin scattering)，从而在传播方向的反向上散射回一部分激光，并且该散射激光的频率相对于发射的光脉冲频率有一个频移。在标准单模光纤中，该频移约为10.8GHz，称为布里渊频移。光纤各处的散射光按照距离的远近，到达光纤发射端的时间也不同。因此，采集到的散射光信号在时间上的分布就对应了散射光在空间上的分布，这就是光时域反射技术(OTDR)。而布里渊散射所导致的布里渊频移，与散射位置处受感受到的温度和应变都有关系(一般是线性关系)。因此，通过测量布里渊散射信号的频移就可以反演出散射位置处的温度或者应变。这种将布里渊散射与OTDR结合的技术就是BOTDR。
布里渊散射信号的相干性较差，因此具有一定的线宽，在标准单模光纤中，该线宽有大约30MHz，而散射信号的频谱一般表现为洛伦兹型。由于布里渊散射谱具有一定宽度，因此传感时所要测量的布里渊频移是指布里渊散射谱中心频率(布里渊频率)的移动值。为了获得该中心频率，常规的技术手段是先测量获得整个布里渊散射谱。由于噪声的影响，该谱一般不是一个平滑的谱。因此还需要对得到的散射谱进行洛伦兹曲线拟合，以获得一个曲线平滑的估计谱。最后再寻求该平滑谱峰值位置处所对应的频率，从而求得中心频率。尽管目前已经提出了许多种不同方案来测量布里渊频移，但基本上所有的方案都是基于对布里渊散射谱的测量来实现的。具体的技术途径上，又可以大致分为基于扫频的方案和基于 快速傅里叶变换(FFT)的数字谱分析方案两类。前者通过一个窄带的滤波器来获取布里渊散射谱中某个频率位置附近很窄范围内的信号功率，通过改变滤波器的中心频率(扫频)来获取布里渊散射谱各个频点处的功率，从而获得整个布里渊散射谱的形状。后者通过宽带的接收，将一段时间内所采集到的布里渊散射信号做FFT，从而获得布里渊散射谱。
现有技术存在以下缺点：
1、基于测量布里渊散射谱的技术，都不可避免的要对布里渊散射信号进行强度检测，因为最终关心的是布里渊散射谱的强度信息。这就意味着在信号处理的某一个环节上，必须对布里渊散射信号进行平方运算来获取强度信息。平方运算使得所有的测量值全部变成了正值，包括伴随信号的噪声。噪声平方后形成了一个恒为正值的噪声功率水平，无法通过多次的平均叠加予以消除，限制了这些技术最终的测量距离。
2、对布里渊散射谱的测量，无论是采用扫频的方案还是采用FFT的方案，都不能对频谱进行频率上连续的分析，必然存在频率分析上的频率间隔，使得对频谱的分辨率受到限制，并最终限制了测量的精度。
3、由于需要依赖于强度检测，采用编码提高测量性能的方法受到了很大的限制。基本上所有可采用的编码序列都是双极性(1或者-1)的二进制序列，而不是单极性的(1或者0)。强度检测无法实现对-1的检测。因此，目前所提出的采用了编码的方案，基本上都需要做复杂的双极性到单极性编码的变换。并且单极性编码容易受到强度噪声和非线性效应的影响，性能上有很大制约。
发明内容
本发明实施例提供一种双极性差分相位编码超高空间分辨率布里渊光时域反射仪，旨在解决上述背景技术提出的技术问题。
为了解决上述技术问题，本发明实施例提供了一种双极性差分相位编码超高空间分辨率布里渊光时域反射仪，包括：
窄线宽激光器，用于发射线偏振激光；
保偏耦合器，用于将所述线偏振激光分为两路光，其中，将一路光传输到 Mach-Zehnder调制器进行相位调制，将另一路光作为本振光源传输到相干光电接收机；
差分编码器，用于将原始编码序列进行差分编码；其中，所述原始编码序列包括第一编码序列和第二编码序列，且所述第一编码序列和第二编码序列为两组极性互为相反的编码序列；
所述Mach-Zehnder调制器，用于根据经过差分编码的编码序列对所述线偏振激光的相位进行相位调制，并输出光脉冲；
光环形器和传感光纤，所述光环形器用于将所述光脉冲输入到所述传感光纤中，并接收所述传感光纤返回的布里渊散射光；
所述相干光电接收机，用于对所述本振光源以及所述布里渊散射光进行光电混频相干接收，得到微波信号；其中，所述微波信号包括斯托克斯信号与反斯托克斯信号；
边带分离器，用于将所述微波信号中的斯托克斯信号与反斯托克斯信号进行信号分离，并将所述斯托克斯信号和所述反斯托克斯信号发送到结果测量模块；
结果测量模块，用于分别根据所述斯托克斯信号和所述反斯托克斯信号测量得到第一瞬时频率及第二瞬时频率，并根据所述第一瞬时频率及所述第二瞬时频率求取最终测量结果。
进一步地，所述结果测量模块具体包括：
第一数据采集器，用于接收所述斯托克斯信号并进行模数转换为第一数字信号；
第一差分解调器，用于对所述第一数字信号进行差分解调；
第一相关器，用于将所述第一差分解调器输出的信号依次与所述原始编码序列进行相关运算；
第一叠加平均器，用于对所述第一相关器输出的相关运算结果进行叠加平均；
第一测量单元，用于根据所述第一叠加平均器输出的叠加平均结果求取所述第一瞬时频率；
第二数据采集器，用于接收所述反斯托克斯信号并进行模数转换为第二数字信号；
第二差分解调器，用于对所述第二数字信号进行差分解调；
第二相关器，用于将所述第二差分解调器输出的信号依次与所述原始编码序列进行相关运算；
第二叠加平均器，用于对所述第二相关器输出的相关运算结果进行叠加平均；
第二测量单元，用于根据所述第二叠加平均器输出的叠加平均结果求取所述第二瞬时频率；
第三测量单元，用于根据所述第一瞬时频率及所述第二瞬时频率求取最终测量结果。
进一步地，所述根据所述第一瞬时频率及所述第二瞬时频率求取最终测量结果，具体为：
计算所述第一瞬时频率及所述第二瞬时频率的平均值，得到所述最终测量结果。
进一步地，所述传感光纤为标准单模光纤或塑料光纤。
进一步地，所述窄线宽激光器的线宽小于100kHz。
进一步地，所述Mach-Zehnder调制器采用铌酸锂波导电光Mach-Zehnder强度调制器。
进一步地，在所述Mach-Zehnder调制器与所述光环形器之间还设置有光脉冲放大器，所述光脉冲放大器用于根据预设的第一功率值将所述Mach-Zehnder调制器输出的光脉冲进行功率放大，并将功率放大后的光脉冲输入到所述光环形器中。
进一步地，在所述光环形器与所述相干光电接收机之间还设置有光放大器，所述光放大器用于根据预设的第二功率值将所述光环形器发出的布里渊散射光进行功率放大，并将功率放大后的布里渊散射光输入到所述相干光电接收机中。
与现有技术相比，本发明具有如下有益效果：
本发明实施例提供了一种双极性差分相位编码超高空间分辨率布里渊光时域反射仪，包括窄线宽激光器，保偏耦合器，差分编码器，Mach-Zehnder调制器，光脉冲放大器，光环形器和传感光纤，光放大器，相干光电接收机，边带分离器，结果测量模块。通过实施本发明能够有效提高布里渊散射谱测量的空间分辨率，同时通过增加叠加平均的次数能够有效提高测量精度，另外还通过直接采用双极性编码序列的方式从而有效提高测量的性能。
附图说明
图1是本发明一实施例提供的双极性差分相位编码超高空间分辨率布里渊光时域反射仪的结构示意图；
图2是本发明一实施例提供的双极性差分相位编码超高空间分辨率布里渊光时域反射仪的另一结构示意图；
图3是本发明一实施例提供的用一组多个不同编码序列完成一次测量的流程示意图；
图4是本发明一实施例提供的差分编码的结构示意图；
图5是本发明一实施例提供的解差分器的结构示意图；
图6是本发明一实施例提供的相关器的结构示意图。
具体实施方式
下面将结合本发明实施例中的附图，对本发明实施例中的技术方案进行清楚、完整的描述，显然，所描述的实施例仅仅是本发明一部分实施例，而不是全部的实施例。基于本发明中的实施例，本领域普通技术人员在没有作出创造性劳动前提下所获得的所有其他实施例，都属于本发明保护的范围。
请参见图1，本发明实施例提供了一种双极性差分相位编码超高空间分辨率布里渊光时域反射仪，包括：
窄线宽激光器，用于发射线偏振激光；
保偏耦合器，用于将所述线偏振激光分为两路光，其中，将一路光传输到Mach-Zehnder调制器进行相位调制，将另一路光作为本振光源传输到相干光电接收机；
差分编码器，用于将原始编码序列进行差分编码；其中，所述原始编码序列包括第一编码序列和第二编码序列，且所述第一编码序列和第二编码序列为两组极性互为相反的编码序列；
所述Mach-Zehnder调制器，用于根据经过差分编码的编码序列对所述线偏振激光的相位进行相位调制，并输出光脉冲；
光脉冲放大器，用于根据预设的第一功率值将所述Mach-Zehnder调制器输出的光脉冲进行功率放大；(这个光脉冲放大器在某些应用场合可以省略不用)；
光环形器和传感光纤，所述光环形器用于将所述光脉冲输入到所述传感光纤中，并接收所述传感光纤返回的布里渊散射光；
在本发明实施例中，进一步地，所述传感光纤为标准单模光纤或塑料光纤。
光放大器，用于根据预设的第二功率值将所述光环形器发出的布里渊散射光进行功率放大；(这个放大器在某些应用场合可以省略不用)；
所述相干光电接收机，用于对所述本振光源以及所述布里渊散射光进行光电混频相干接收，得到微波信号；其中，所述微波信号包括斯托克斯信号与反斯托克斯信号；
边带分离器，用于将所述微波信号中的斯托克斯信号与反斯托克斯信号进行信号分离，并将所述斯托克斯信号和所述反斯托克斯信号发送到结果测量模块；
结果测量模块，用于分别根据所述斯托克斯信号和所述反斯托克斯信号测量得到第一瞬时频率及第二瞬时频率，并根据所述第一瞬时频率及所述第二瞬时频率求取最终测量结果。
在本发明实施例中，进一步地，所述结果测量模块具体包括：
第一数据采集器，用于接收所述斯托克斯信号并进行模数转换为第一数字信号；
第一差分解调器，用于对所述第一数字信号进行差分解调；
第一相关器，用于将所述第一差分解调器输出的信号依次与所述原始编码序列进行相关运算；
第一叠加平均器，用于对所述第一相关器输出的相关运算结果进行叠加平 均；
第一测量单元，用于根据所述第一叠加平均器输出的叠加平均结果求取所述第一瞬时频率；
第二数据采集器，用于接收所述反斯托克斯信号并进行模数转换为第二数字信号；
第二差分解调器，用于对所述第二数字信号进行差分解调；
第二相关器，用于将所述第二差分解调器输出的信号依次与所述原始编码序列进行相关运算；
第二叠加平均器，用于对所述第二相关器输出的相关运算结果进行叠加平均；
第二测量单元，用于根据所述第二叠加平均器输出的叠加平均结果求取所述第二瞬时频率；
第三测量单元，用于根据所述第一瞬时频率及所述第二瞬时频率求取最终测量结果。
在本发明实施例中，进一步地，所述根据所述第一瞬时频率及所述第二瞬时频率求取最终测量结果，具体为：
计算所述第一瞬时频率及所述第二瞬时频率的平均值，得到所述最终测量结果。
在本发明实施例中，作为优选方案，所述窄线宽激光器的线宽小于100kHz。(某些应用也可使用线宽较宽的激光器，比如测量距离很短时)。
在本发明实施例中，作为优选方案，所述Mach-Zehnder调制器采用铌酸锂波导电光Mach-Zehnder强度调制器。(某些应用也可使用非铌酸锂波导，比如采用硅基集成光子波导)。
为了更好说明本发明技术方案，下面对本发明进行具体举例说明：
图2为本发明提供的双极性差分相位编码超高空间分辨率布里渊光时域反射仪的方案框图，具体地：
窄线宽激光器输出的线偏振激光被保偏耦合器分成两路光。一路进入 Mach-Zehnder调制器进行调制，另外一路作为本振光源进入相干光电接收机。
Mach-Zehnder调制器被偏置在最小功率输出点，并将经过差分编码的编码序列调制到激光的相位上。
驱动Mach-Zehnder调制器的是双极性信号。Mach-Zehnder调制器的输出是一系列的光脉冲。这些光脉冲在经过光脉冲放大器被放大到合适的功率水平后，经过光环形器而进入传感光纤，这些光脉冲在传感光纤中传播时，在光纤各处产生向后面传播的布里渊散射光。这些散射光在经过光环形器后被光放大器放大到合适的功率水平，然后进入相干光电接收机，与窄线宽激光器输出的本振光一起进行光电混频相干接收。
如果窄线宽激光器的输出波长是1550nm，且传感光纤是普通标准单模光纤，那么光电混频相干接收后将获得中心频率约为10.8GHz的微波信号。该微波信号中包含有布里渊散射的斯托克斯信号与反斯托克斯信号。
在该微波信号经过边带分离器后，斯托克斯信号与反斯托克斯信号会被分离开，分别被数据采集器1和数据采集器2模数转换为数字信号。
以数据采集器1的输出为例，其输出首先进入差分解调器1以解调差分编码。然后差分解调器1的输出依次与原始的编码序列进行相关运算，从而每一个发射的光脉冲所产生的散射信号都会在相关运算后得到一系列的结果。这些结果在时间上的先和后代表了散射光产生位置在空间上的近和远。随后，将相同位置处，多个脉冲所得到的结果在叠加平均器1中进行叠加平均，以提高信噪比。叠加平均器1的输出结果被用于求得瞬时频率1。该瞬时频率1可被作为测量结果1。瞬时频率1，即ω 1，由下式求得：
Figure PCTCN2020070085-appb-000001
其中，θ 1是叠加平均器1输出结果的相位，T是数据采集器1的采样周期。
根据同样的方法，由数据采集器2可以得到瞬时频率2，即ω 2，并可被作为测量结果2。取两个结果的叠加平均，即可得到测量结果3，即ω 3：
Figure PCTCN2020070085-appb-000002
而测量结果的空间分辨率Δz则决定于系统所采用的编码速率：
Figure PCTCN2020070085-appb-000003
其中c是真空中的光速，n 0是传感光纤的折射率。
需要说明的是，作为优选方案，以下为本发明方案部分模块的说明：
(1)窄线宽激光器：激光器的线宽要足够窄，以保证相干光电接收时不会产生太大的噪声。且激光器的频率稳定性要足够，以避免对测量结果形成干扰。推荐采用线宽在100kHz以下的窄线宽激光器。
(2)Mach-Zehnder调制器：Mach-Zehnder调制器是常规的铌酸锂波导电光Mach-Zehnder强度调制器。该调制器工作时需要被直流偏置在输出功率最低点，即在没有外加调制信号时，调制器的输出功率最低。驱动该调制器的是双极性信号(既有正电平也有负电平信号)，从而使得调制器将输入的连续光按照输入的编码序列变换为在相差为π的两个相位间切换。理论上，该Mach-Zehnder调制器也可被替换为其他类型的调制器，只要能够实现“将输入的连续光按照输入的编码序列变换为在相差为π的两个相位间切换”。但目前来说，采用这里所述的Mach-Zehnder调制器是广泛采用且性能较优的方案。
(3)相干光电接收机+边带分离器：经过边带分离器后，光纤产生的布里渊散射信号被分为斯托克斯散射信号和反斯托克斯散射信号，并分别输出。
(4)数据采集器：数据采集器即模数转换器，将模拟信号转换为数字信号。在此模块之后的信号处理，均工作在数字域。两个数据采集器分别采集斯托克斯布里渊散射信号和反斯托克斯布里渊散射信号。
(5)差分编码：
差分编码即常规的基于模2加法器的编码器，如图4所示。
(6)解差分器：
解差分器即常规的基于延迟相乘的编码器，如图5所示。
(7)相关器：
相关器也是常规的结构，工作在流处理的状态，从流动的数据流中取出N个连续的数据，与编码序列的N个比特按位相乘后将结果相加，输出新的数据流。 如图6所示。
(8)测量结果：
图2中有三个测量结果。测量结果1和2代表了对斯托克斯和反斯托克斯布里渊散射信号各自处理的结果。测量结果1和2的准确性受到窄线宽激光器频率稳定性的影响。激光器频率的波动会对测量结果1和2形成干扰，造成测量误差。在激光器频率很稳定时，测量结果1和2均可作为最终的测量结果。测量结果3是测量结果1和2的平均值，该结果消除了激光器频率的波动影响，具有更好的准确性。
(9)同步问题：
图1中所展示的系统涉及到较多的数字域处理，包括差分编码、数据采集器以及之后的所有信号处理流程。这些数字域的处理均需要工作在同一个时钟域，即他们所使用的时钟均衍生于同一个时钟源。
请参见图3，在实际测量中，一次完整的测量，需要发射一组多个不同的编码序列。事实上，对每一个发射的二进制编码序列A，还另外需要发射一个二进制编码序列-A，即将A中的1变成0，将A中的0变成1。采用这样的方法，可以消除系统中的噪声对测量结果的干扰。当发射的编码序列是包含序列A和序列B在内的互补序列时，那么还需要另外发射一对互补序列-A和-B，也是同样的将A和B中的1变成0，将A和B中的0变成1。以发射互补序列为例，在一次测量中，发射的这些不同的编码序列所得到的接收信号之间的信号处理流程如图2所示，同样的也是以数据采集器1的输出为例。
因此，在一次测量中，需要将所发射的同组内多个不同的编码序列所得到的相关器输出都进行叠加平均，然后再进行瞬时频率的计算获得测量结果。
与现有技术相比，本发明具有如下有益效果：
(1)信号处理简单，无需频谱分析、频谱拟合、寻峰等复杂的算法，只需要简单的相关操作和叠加平均操作，可以高度的并行化、流水线化，可以做到实时信号处理。
(2)空间分辨率极高。空间分辨率决定于编码速率。理论上可以达到2厘米甚至更优的空间分辨率，是目前其他技术无法达到的。
(3)编码速率高，传感系统工作于自发布里渊散射，非线性效应受到很大抑制，可以大幅提高发射功率从而改善系统性能。而现有技术的脉冲发射功率受到非线性效应很大的限制，发射功率低，信噪比较低。
(4)频率分析是连续的，理论上可以达到任意的测量精度，只要叠加平均的次数足够多。而现有技术的测量精度除了受信噪比影响外，还受限于频谱分辨率。
以上所述是本发明的优选实施方式，应当指出，对于本技术领域的普通技术人员来说，在不脱离本发明原理的前提下，还可以做出若干改进和润饰，这些改进和润饰也视为本发明的保护范围。

Claims (8)

  1. 一种双极性差分相位编码超高空间分辨率布里渊光时域反射仪，其特征在于，包括：
    窄线宽激光器，用于发射线偏振激光；
    保偏耦合器，用于将所述线偏振激光分为两路光，其中，将一路光传输到Mach-Zehnder调制器进行相位调制，将另一路光作为本振光源传输到相干光电接收机；
    差分编码器，用于将原始编码序列进行差分编码；其中，所述原始编码序列包括第一编码序列和第二编码序列，且所述第一编码序列和第二编码序列为两组极性互为相反的编码序列；
    所述Mach-Zehnder调制器，用于根据经过差分编码的编码序列对所述线偏振激光的相位进行相位调制，并输出光脉冲；
    光环形器和传感光纤，所述光环形器用于将所述光脉冲输入到所述传感光纤中，并接收所述传感光纤返回的布里渊散射光；
    所述相干光电接收机，用于对所述本振光源以及所述布里渊散射光进行光电混频相干接收，得到微波信号；其中，所述微波信号包括斯托克斯信号与反斯托克斯信号；
    边带分离器，用于将所述微波信号中的斯托克斯信号与反斯托克斯信号进行信号分离，并将所述斯托克斯信号和所述反斯托克斯信号发送到结果测量模块；
    结果测量模块，用于分别根据所述斯托克斯信号和所述反斯托克斯信号测量得到第一瞬时频率及第二瞬时频率，并根据所述第一瞬时频率及所述第二瞬时频率求取最终测量结果。
  2. 根据权利要求1所述的双极性差分相位编码超高空间分辨率布里渊光时域反射仪，其特征在于，所述结果测量模块具体包括：
    第一数据采集器，用于接收所述斯托克斯信号并进行模数转换为第一数字信号；
    第一差分解调器，用于对所述第一数字信号进行差分解调；
    第一相关器，用于将所述第一差分解调器输出的信号依次与所述原始编码序列进行相关运算；
    第一叠加平均器，用于对所述第一相关器输出的相关运算结果进行叠加平均；
    第一测量单元，用于根据所述第一叠加平均器输出的叠加平均结果求取所述第一瞬时频率；
    第二数据采集器，用于接收所述反斯托克斯信号并进行模数转换为第二数字信号；
    第二差分解调器，用于对所述第二数字信号进行差分解调；
    第二相关器，用于将所述第二差分解调器输出的信号依次与所述原始编码序列进行相关运算；
    第二叠加平均器，用于对所述第二相关器输出的相关运算结果进行叠加平均；
    第二测量单元，用于根据所述第二叠加平均器输出的叠加平均结果求取所述第二瞬时频率；
    第三测量单元，用于根据所述第一瞬时频率及所述第二瞬时频率求取最终测量结果。
  3. 根据权利要求1或2所述的双极性差分相位编码超高空间分辨率布里渊光时域反射仪，其特征在于，所述根据所述第一瞬时频率及所述第二瞬时频率求取最终测量结果，具体为：
    计算所述第一瞬时频率及所述第二瞬时频率的平均值，得到所述最终测量结果。
  4. 根据权利要求1所述的双极性差分相位编码超高空间分辨率布里渊光时域反射仪，其特征在于，所述传感光纤为标准单模光纤或塑料光纤。
  5. 根据权利要求1所述的双极性差分相位编码超高空间分辨率布里渊光时域反射仪，其特征在于，所述窄线宽激光器的线宽小于100kHz。
  6. 根据权利要求1所述的双极性差分相位编码超高空间分辨率布里渊光时域反射仪，其特征在于，所述Mach-Zehnder调制器采用铌酸锂波导电光Mach-Zehnder强度调制器。
  7. 根据权利要求1所述的双极性差分相位编码超高空间分辨率布里渊光时域反射仪，其特征在于，在所述Mach-Zehnder调制器与所述光环形器之间还设置有光脉冲放大器，所述光脉冲放大器用于根据预设的第一功率值将所述Mach-Zehnder调制器输出的光脉冲进行功率放大，并将功率放大后的光脉冲输入到所述光环形器中。
  8. 根据权利要求1所述的双极性差分相位编码超高空间分辨率布里渊光时域反射仪，其特征在于，在所述光环形器与所述相干光电接收机之间还设置有光放大器，所述光放大器用于根据预设的第二功率值将所述光环形器发出的布里渊散射光进行功率放大，并将功率放大后的布里渊散射光输入到所述相干光电接收机中。
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