Optical frequency domain reflection system and method based on voltage-controlled oscillator

Abstract

The invention relates to an optical frequency domain reflection system and method based on a voltage-controlled oscillator. The system part mainly comprises an arbitrary function generator, a laser, a voltage-controlled oscillator, a single side band modulator, an auxiliary interferometer, a main interferometer and a polarization beam splitter. Wherein the arbitrary function generator, the laser, the voltage-controlled oscillator and the single sideband modulator are matched to generate sweep frequency light required by measurement; the sweep-frequency light is divided into two paths, one path enters the main interferometer to serve as probe light, the other path enters the auxiliary interferometer to be used for obtaining the instantaneous frequency of the probe light, and signals of the main interferometer are resampled through a one-dimensional interpolation method according to the instantaneous frequency. According to the invention, a voltage-controlled oscillator with low cost and small volume is used as a radio frequency signal generator to generate a sweep frequency signal with fast frequency modulation performance, and phase noise caused by sweep frequency nonlinearity is compensated by designing an auxiliary interferometer optical path and a linear interpolation algorithm.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber sensing technology, and in particular to an optical frequency domain reflection system and method based on a voltage-controlled oscillator. Background Art

[0002] At present, the Internet of Things technology is developing rapidly and is widely used in various fields of production and life. In order to adapt to the wave of development of Internet of Things technology, various sensor technologies are constantly developing. Due to its advantages such as high sensitivity, large dynamic range, good anti-electromagnetic interference performance and strong stability, optical fiber sensors have become an important part of the perception layer of the Internet of Things.

[0003] Distributed fiber optic sensing technology based on Rayleigh scattering has always been a research hotspot in the field of fiber optic sensing and has been widely used, including optical time domain reflector (OTDR) and optical frequency domain reflector (OFDR). With the continuous exploration and optimization of system performance by researchers, the spatial resolution of OTDR has reached its limit and is difficult to be lower than the meter level. At present, many solutions in the industry sacrifice system complexity and cost, which is not conducive to practical application. However, OFDR technology can easily achieve sub-meter spatial resolution by emitting continuous light whose frequency changes linearly with time, and after collecting the backscattered light signal in the optical fiber, it processes the signal in the frequency domain. By expanding the frequency sweep range of the light source, today's OFDR technology can even detect optical fiber strain and temperature information at the micrometer scale. Based on the way of obtaining the frequency sweep light source, OFDR technology is divided into two types: direct modulation of the light source and external modulation of the electrical frequency sweep source. Among them, the externally modulated OFDR has the advantages of fast measurement speed and long sensing distance. At present, there are external modulation OFDR technologies based on high-order sideband modulation, four-wave mixing technology and injection locking technology with adjustable center wavelength. They use various complex signal modulation technologies to expand the frequency sweep range and achieve higher spatial resolution, but they do not consider the practicality problems caused by the increase in system cost. The electrical frequency sweep signals used in the above technologies are all generated by high-speed arbitrary waveform generators with large bandwidth. Although they can generate highly linear frequency sweep signals, such devices are large in size, which is not conducive to system integration. At the same time, they are expensive and not conducive to practical applications.

[0004] In view of this, how to overcome the defects of the prior art and solve at least part of the above-mentioned technical problems is a difficult problem to be solved in this technical field. Summary of the invention

[0005] In view of the defects or improvement needs in the prior art, the present invention conducts research on a dynamic OFDR distributed sensing system based on external modulation, intending to reduce the system cost by adopting a new modulation device, and at the same time, achieve a larger sweep frequency range and a higher sweep frequency speed, thereby achieving fast signal measurement with higher spatial resolution. Based on this, the present invention provides an optical frequency domain reflection system and method based on a voltage-controlled oscillator, which generates a fast sweep frequency signal by controlling a voltage-controlled oscillator (VCO) output by a sawtooth wave of a low-bandwidth arbitrary function generator (AFG), and further generates a sweep frequency light in cooperation with a single-sideband modulator. In addition, the present invention also compensates for the phase noise caused by the nonlinearity of the sweep frequency by designing an auxiliary interferometer optical path and a linear interpolation algorithm, and also constructs a polarization diversity receiving structure at the signal receiving end to reduce the influence of polarization fading. Finally, the optical frequency domain reflection system and method based on a voltage-controlled oscillator proposed by the present invention achieves a 2.5GHz sweep frequency range, and compared with an arbitrary waveform generator with the same performance, the cost is only one-tenth of it. The external modulation OFDR sensing system built can detect breakpoints on 2600m optical fiber with a spatial resolution of up to 5cm, and can detect 10Hz vibration signals, which is of positive significance for realizing low-cost rapid measurement OFDR system.

[0006] The present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides an optical frequency domain reflection system based on a voltage-controlled oscillator, comprising an arbitrary function generator, a laser, a voltage-controlled oscillator, a single-sideband modulator, an auxiliary interferometer, a main interferometer and a polarization beam splitter; wherein the arbitrary function generator, the laser, the voltage-controlled oscillator and the single-sideband modulator cooperate to generate a swept-frequency light required for measurement; the swept-frequency light is divided into two paths, one path enters the main interferometer as a detection light, and the other path enters the auxiliary interferometer to obtain the instantaneous frequency of the detection light, and the signal of the main interferometer is resampled by a one-dimensional interpolation method according to the instantaneous frequency; the main interferometer comprises a polarization controller, a circulator and an optical fiber to be measured, the detection light is divided into two paths, one path enters the optical fiber to be measured through the circulator, and is used for detecting information along the optical fiber to be measured, and its returned backscattered light is output through the circulator, and beats with the other path of detection light passing through the polarization controller to generate beat-frequency light; the beat-frequency light is divided into two polarization states by the polarization beam splitter.

[0008] In some implementations, the arbitrary function generator, the laser, the voltage-controlled oscillator, and the single-sideband modulator cooperate to generate the swept frequency light required for measurement, specifically including:

[0009] The sawtooth wave voltage is generated by an arbitrary function generator, and then the voltage-controlled oscillator is controlled to output a radio frequency signal with a sawtooth wave variation in frequency, and the radio frequency signal is used as the driving signal of the single-sideband modulator; after the light output by the laser is modulated by the radio frequency signal through the single-sideband modulator, the output light is the swept frequency light required for measurement.

[0010] In some embodiments, three detectors and an oscilloscope connected to the three detectors are further included, wherein one detector is disposed behind the auxiliary interferometer to receive light after passing through the auxiliary interferometer; and the other two detectors are disposed behind the polarization beam splitter to respectively receive light of two polarization states after passing through the polarization beam splitter.

[0011] In some embodiments, acquiring the instantaneous frequency of the detection light specifically includes:

[0012] The light intensity I output after the auxiliary interferometer beat frequency interference REF It can be expressed as:

[0013] I REF (t)∝cos(2πf REF (t)τ REF )

[0014] Among them, f REF is the instantaneous frequency of the swept light source, τ REF is the optical path difference between the two arms of the auxiliary interferometer, and then the instantaneous frequency can be obtained by using Hilbert transform and inverse tangent as follows:

[0015]

[0016] I H (t) = H{I REF (t)}∝sin(2πf REF (t)τ REF )

[0017] After obtaining the instantaneous optical frequency of each sampling point, due to the inverse tangent function:

[0018]

[0019] Use the Phase Unwrapping function to handle discontinuities after phase unwrapping.

[0020] In some embodiments, the one-dimensional interpolation method includes a cubic interpolation method, which successively uses a cubic curve f(x)=a 0 +a 1 x+a 2 x 2 +a 3 x 3 The minimum point approximation seeks the minimum point of the function g(x), specifically:

[0021] Let x 1 <x 2 , in the search interval [x 1 ,x 2 ] fit g(x) with f(x) to satisfy:

[0022] g(x i )=f(x i ),g ′ (x i )=f ′ (x i ),(i=1,2)

[0023] Taking the derivative of f(x) and setting it equal to zero, we can find the value of f(x) in [x 1 ,x 2 ] is represented by a0, a1, a2 and a3, and then solved to get a 0 ,a 1 ,a 2 ,a 3 , substituting it into the expression, we can get the approximate minimum point of g(x). Iterate successively and stop when the absolute value of the derivative of f(x) at the approximate minimum point is less than the given error.

[0024] In some embodiments, the laser comprises a narrow linewidth laser with a linewidth of 1 kHz, and the arbitrary function generator generates a sawtooth wave voltage with a frequency of 200 Hz.

[0025] In some embodiments, the swept light is divided into two paths by a 99:1 coupler, 99% of the swept light enters the main interferometer as detection light, and 1% of the swept light enters the auxiliary interferometer to obtain the instantaneous frequency of the detection light.

[0026] In some embodiments, the detection light is divided into two paths by a 90:10 coupler, 90% of the detection light enters the optical fiber to be tested through a circulator, and is used to detect information along the optical fiber to be tested, and its returned backscattered light is output through the circulator and beats with the other 10% detection light that passes through the polarization controller to generate beat frequency light.

[0027] In some embodiments, the auxiliary interferometer includes a delay fiber and a normal fiber arranged in parallel.

[0028] On the other hand, the present invention provides an optical frequency domain reflectometry method based on a voltage-controlled oscillator, which is applied to the optical frequency domain reflectometry system based on a voltage-controlled oscillator as described in the first aspect, comprising:

[0029] The sawtooth wave output by a low-bandwidth arbitrary function generator is used to control a voltage-controlled oscillator to generate a fast frequency-sweep signal, which is then used in conjunction with a single-sideband modulator to further generate frequency-sweep light.

[0030] The frequency sweeping light is divided into two paths, one path enters the main interferometer as the detection light, and the other path enters the auxiliary interferometer to obtain the instantaneous frequency of the detection light;

[0031] The signal of the main interferometer is resampled by a one-dimensional interpolation method, thereby realizing equal-interval sampling of the beat frequency signal in the main interferometer at the optical frequency;

[0032] A polarization beam splitter is set at the signal receiving end to split the beat frequency light generated by the main interferometer into two polarization states to reduce the influence of polarization fading.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] A lot of research has been done on externally modulated optical frequency domain reflectometers to improve their system performance. For example, fiber delay path technology is used to improve the sensing distance; four-wave mixing technology and dynamic injection locking are used to improve the sensing accuracy. However, these solutions all use expensive and bulky high-speed arbitrary signal generators as devices to generate swept frequency signals, which is not conducive to the practical application of this technology.

[0035] Therefore, in order to solve this problem, the present invention proposes an optical frequency domain reflection system and method based on a voltage-controlled oscillator, which uses a low-cost and small-volume voltage-controlled oscillator as a radio frequency signal generator to generate a swept frequency signal with fast frequency modulation performance. In order to compensate for the problem of sensor resolution degradation caused by the nonlinearity of the voltage-controlled oscillator's swept frequency, the present invention adopts a swept frequency nonlinearity compensation scheme based on instantaneous frequency measurement and one-dimensional interpolation, and uses a single-sideband modulator and a polarization beam splitter to improve the signal-to-noise ratio of the received signal, ultimately achieving a 5cm spatial resolution and successfully restoring a 10Hz vibration signal, with good performance, and is a highly practical externally modulated optical frequency domain reflection system. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required to be used in the embodiments of the present invention. Obviously, the drawings described below are only some embodiments of the present invention, and for ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0037] Figure 1 A schematic structural diagram of an optical frequency domain reflectometry system based on a voltage-controlled oscillator provided in Example 1 of the present invention;

[0038] Figure 2 A flow chart of an optical frequency domain reflectometry method based on a voltage-controlled oscillator provided in Example 1 of the present invention;

[0039] Figure 3A schematic diagram of the principle of frequency sweep nonlinear compensation provided by Embodiment 2 of the present invention;

[0040] Figure 4 This is a schematic diagram of measurement results provided in Example 2 of the present invention. DETAILED DESCRIPTION

[0041] The present invention is described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, without departing from the concept of the present invention, several variations and improvements can also be made. These all belong to the protection scope of the present invention. It should be noted that, if there is no conflict, the various features in the embodiments of the present invention can be combined with each other, all within the protection scope of this application. In addition, although the functional module division may be performed in the device schematic diagram, and the logical order may be shown in the flow chart, in some cases, it may be different from the module division in the device, or the steps shown or described in the order in the flow chart may be performed.

[0042] Unless otherwise defined, all technical and scientific terms used in this specification have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used in this specification and in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. In addition, the technical features involved in each embodiment of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0043] Unless the context requires otherwise, throughout the specification and claims, the term "including" is to be interpreted as open inclusion, that is, "including, but not limited to". In the description of the specification, the terms "one embodiment", "some embodiments", "exemplary embodiments", "examples", "specific examples" or "some examples" and the like are intended to indicate that specific features, structures, materials or characteristics associated with the embodiment or example are included in at least one embodiment or example of the present disclosure. The schematic representation of the above terms does not necessarily refer to the same embodiment or example. In addition, the specific features, structures, materials or characteristics may be included in any one or more embodiments or examples in any appropriate manner, that is, although they may be carried in the embodiments or examples of the above terms due to reasons such as the order and position of appearance, it is not limited to that they can be carried in combination by one embodiment or example.

[0044] Traditional OFDR systems based on external modulation use high-speed arbitrary signal generators to generate highly linear swept-frequency models, but the equipment is expensive and the system cost is high. The voltage-controlled oscillator can output microwave signals of different frequencies by controlling the amplitude of the input DC voltage. By quickly sweeping the amplitude of the input voltage, a swept-frequency light can be obtained. However, due to the performance limitations of the voltage-controlled oscillator, the frequency of the output signal is not completely linear with the change of the input voltage, and the nonlinearity of the swept-frequency light source will introduce a large phase noise to the swept-frequency light source, which will lead to an extreme degradation of the spatial resolution of the constructed optical frequency domain reflection system. In addition, the inherent phase noise of the electrical signal output by the voltage-controlled oscillator also restricts the spatial resolution of the system. How to eliminate the phase noise existing in the swept-frequency light source is the key technical problem to be solved by the present invention.

[0045] The present invention uses a voltage-controlled oscillator to replace the high-speed arbitrary signal generator with large bandwidth used in the traditional externally modulated optical frequency reflectometer to generate a fast frequency-sweeping signal with a large bandwidth, and then generate a frequency-sweeping light source, which greatly reduces the system cost. At the same time, in order to compensate for the nonlinearity of the frequency-sweeping signal output by the voltage-controlled oscillator, an auxiliary interferometer system and a linear compensation algorithm are built and designed. In addition, a single-sideband modulator and a polarization diversity receiving device are used to improve the signal-to-noise ratio of the received signal. Finally, centimeter-level spatial resolution optical fiber breakpoint monitoring and long-distance dynamic strain information measurement are achieved, which is of pioneering significance for the realization of low-cost and high-performance optical frequency domain reflectometers.

[0046] In order to make the purpose, technical scheme and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not intended to limit the present application. The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0047] Embodiment 1:

[0048] like Figure 1As shown, an embodiment of the present invention provides an optical frequency domain reflectometry system based on a voltage-controlled oscillator, the system comprising an arbitrary function generator, a laser, a voltage-controlled oscillator, a single-sideband modulator, an auxiliary interferometer, a main interferometer and a polarization beam splitter; wherein the arbitrary function generator, the laser, the voltage-controlled oscillator and the single-sideband modulator cooperate to generate the swept frequency light required for measurement; the swept frequency light is divided into two paths, one path enters the main interferometer as a detection light, and the other path enters the auxiliary interferometer to obtain the instantaneous frequency of the detection light, and the signal of the main interferometer is resampled by a one-dimensional interpolation method according to the instantaneous frequency, so as to realize the equal-interval sampling of the beat frequency signal in the main interferometer at the optical frequency, and compensate for the nonlinearity of the swept frequency signal output by the voltage-controlled oscillator. The auxiliary interferometer comprises a delay fiber and a normal fiber arranged in parallel. The main interferometer includes a polarization controller, a circulator and an optical fiber to be tested. The detection light is divided into two paths. One path enters the optical fiber to be tested through the circulator and is used to detect information along the optical fiber to be tested. The returned backscattered light is output through the circulator and beats with the other detection light passing through the polarization controller to generate beat frequency light; the beat frequency light is divided into two polarization states by a polarization beam splitter; after the two polarization states are combined through the existing technology, the influence of polarization fading can be reduced. The specific polarization state combination is an existing technology and will not be repeated here.

[0049] Through the above-mentioned settings, the present invention uses a voltage-controlled oscillator to replace the high-speed arbitrary signal generator with large bandwidth used in the traditional externally modulated optical frequency reflectometer to generate a fast frequency-sweeping signal with a large bandwidth, and then generate a frequency-sweeping light source, which greatly reduces the system cost. At the same time, in order to compensate for the nonlinearity of the frequency-sweeping signal output by the voltage-controlled oscillator, an auxiliary interferometer system and a linear compensation algorithm are built and designed. In addition, a single-sideband modulator and a polarization beam splitter are used to improve the signal-to-noise ratio of the received signal. Finally, centimeter-level spatial resolution optical fiber breakpoint monitoring and long-distance dynamic strain information measurement are achieved, which is of pioneering significance for realizing low-cost and high-performance optical frequency domain reflectometers.

[0050] In one embodiment, the arbitrary function generator, laser, voltage-controlled oscillator and single-sideband modulator cooperate to generate the swept-frequency light required for measurement, specifically including: generating a sawtooth wave voltage through the arbitrary function generator, and then controlling the voltage-controlled oscillator to output a radio frequency signal with a sawtooth wave variation in frequency, and using the radio frequency signal as a driving signal for the single-sideband modulator; after the light output by the laser is modulated by the radio frequency signal through the single-sideband modulator, the output light is the swept-frequency light required for measurement.

[0051] In one embodiment, three detectors and an oscilloscope connected to the three detectors are also included, wherein one detector is arranged behind the auxiliary interferometer to receive the light after passing through the auxiliary interferometer; the other two detectors are arranged behind the polarization beam splitter to respectively receive the light of two polarization states after passing through the polarization beam splitter. In the auxiliary interferometer, the light passes through a Mach-Zehnder interferometer and is received by the detector, and the output electrical signal is used to recover the instantaneous frequency of the light, and then used to compensate for the nonlinearity of the frequency sweep.

[0052] In one embodiment, the laser includes a narrow linewidth laser with a linewidth of 1 kHz. The arbitrary function generator generates a sawtooth voltage with a frequency of 200 Hz. The swept light is divided into two paths by a 99:1 coupler, 99% of the swept light enters the main interferometer as detection light, and 1% of the swept light enters the auxiliary interferometer to obtain the instantaneous frequency of the detection light. The detection light is divided into two paths by a 90:10 coupler, 90% of the detection light enters the optical fiber to be tested through a circulator, and is used to detect information along the optical fiber to be tested. The backscattered light returned is output through the circulator, and beats with the other 10% detection light passing through the polarization controller to generate beat light.

[0053] refer to Figure 2 As shown, an embodiment of the present invention further provides an optical frequency domain reflection method based on a voltage-controlled oscillator. On the basis of the above-mentioned optical frequency domain reflection system based on a voltage-controlled oscillator, the method includes the following steps.

[0054] Step 100: Use a sawtooth wave output by a low-bandwidth arbitrary function generator to control a voltage-controlled oscillator to generate a fast frequency-sweep signal, and cooperate with a single-sideband modulator to further generate frequency-sweep light.

[0055] Step 200: Split the frequency-sweeping light into two paths, one path entering the main interferometer as detection light, and the other path entering the auxiliary interferometer to obtain the instantaneous frequency of the detection light.

[0056] Step 300: resample the signal of the main interferometer by a one-dimensional interpolation method, thereby achieving equal-interval sampling of the beat frequency signal in the main interferometer at the optical frequency.

[0057] Step 400: a polarization beam splitter is arranged at the signal receiving end to split the beat frequency light generated by the main interferometer into two polarization states to reduce the influence of polarization fading.

[0058] In summary, the embodiment of the present invention provides an optical frequency domain reflection system and method based on a voltage-controlled oscillator, which uses a low-cost, small-volume voltage-controlled oscillator as a radio frequency signal generator to generate a swept frequency signal with fast frequency modulation performance. In order to compensate for the problem of sensor resolution degradation caused by the nonlinearity of the voltage-controlled oscillator's swept frequency, the present invention adopts a swept frequency nonlinearity compensation scheme based on instantaneous frequency measurement and one-dimensional interpolation, and uses a single-sideband modulator and a polarization beam splitter to improve the signal-to-noise ratio of the received signal, ultimately achieving a 5cm spatial resolution and successfully restoring a 10Hz vibration signal, with good performance, and is a highly practical externally modulated optical frequency domain reflection system.

[0059] Embodiment 2:

[0060] Based on the first embodiment, the second embodiment describes the embodiment of the present invention in more detail through a specific example and principle.

[0061] The OFDR system is a distributed fiber optic sensing system based on backward Rayleigh scattering. The basic principle is based on the coherent detection technology of frequency modulated continuous wave (FMCW). In the OFDR system, interference occurs between two optical signals of linear frequency chirp from the same highly coherent light source to generate a beat signal. According to different frequency sweeping methods, it can be divided into direct modulation of the light source and external modulation of the frequency sweeping source. The present invention is directed to OFDR with external modulation of the frequency sweeping source, and the frequency sweeping light output from the modulator is divided into reference light and measurement light through coupler A. The reference light is transmitted to coupler B through a reference path with a given optical distance, and the measurement light is transmitted to the fiber under test (FUT) through the measurement path, and the FUT returns the backscattered light. After that, coupler B combines the reference light and the backscattered light to beat, and then detects the beat signal through a photodiode. The detected signal is collected and analyzed by a data acquisition system (DAQ), so as to demodulate and obtain the physical information detected on the FUT. The spatial resolution ΔZ of the OFDR system can be expressed as:

[0062]

[0063] c represents the speed of light, n represents the effective refractive index of the optical fiber to be tested, Δν sweep is the frequency sweep range.

[0064] In an OFDR system based on external modulation, in order to generate a linear frequency modulation signal with a large frequency sweep range, a large bandwidth arbitrary signal generator is usually required, which greatly increases the system cost. To this end, the present invention proposes to use a voltage-controlled oscillator (VCO) as a source of linear frequency modulation signals. VCO is an oscillation circuit in which the output frequency has a corresponding relationship with the input control voltage, and this corresponding relationship is often close to linear. By applying a linearly varying voltage signal to the VCO, a radio frequency signal whose frequency changes linearly with time can be obtained. And VCO has the advantages of simple structure and low cost.

[0065] In the OFDR system, in order to obtain the position information of the reflection point, the collected signal needs to be converted to the frequency domain through fast Fourier transform (FFT), and then converted to the distance domain according to the one-to-one correspondence between frequency and position, so as to obtain the position of the reflection point on the optical fiber. If the laser is linearly swept, the beat frequency signal is a single-cycle sine signal; however, the sweep frequency signal output by the voltage-controlled oscillator is often not a strictly linear sweep frequency signal, that is, the frequency intervals of the light source scanned in the same time interval are inconsistent. At this time, the sampling of the signal in the frequency domain is non-equally spaced sampling, and FFT requires the signal to be equally spaced in the frequency domain. At this time, if FFT is directly used to process the signal, the frequency band of the beat frequency signal will be widened and the energy will be diffused, which will eventually deteriorate the spatial resolution of the OFDR system and make it impossible to accurately locate the reflection point in the optical fiber. Therefore, in order to accurately locate the reflection point on the optical fiber, it is necessary to compensate for the nonlinear tuning effect of the laser caused by the voltage-controlled oscillator.

[0066] In view of the nonlinear tuning effect of the swept-frequency light source, the present invention adopts a resampling method to compensate for it. The main idea is to build an auxiliary interferometer and use it to demodulate the instantaneous frequency information of the voltage-controlled oscillator, and resample the signal of the main interferometer by a one-dimensional interpolation method, thereby realizing the equal-interval sampling of the beat frequency signal in the main interferometer at the optical frequency. It should be noted that for the instantaneous frequency of the voltage-controlled oscillator, the instantaneous frequency of the detection light, and the instantaneous frequency of the swept-frequency light, the instantaneous frequency of the detection light is the instantaneous frequency of the swept-frequency light, and the changes of the two are the same as the changes of the instantaneous frequency of the voltage-controlled oscillator.

[0067] Here, this embodiment builds a Figure 3 The Mach-Zehnder interferometer shown in (a) is used as an auxiliary interferometer. The specific process of performing frequency sweep nonlinear compensation is referred to in detail. Figure 3 (a) Figure 3 (b) Figure 3 (c) Figure 3 As shown in (d) in .

[0068] According to the working principle of Mach-Zehnder interferometer, the light intensity I output after the interferometer beat frequency interference REF It can be expressed as formula (1):

[0069] I REF (t)∝cos(2πf REF (t)τ REF )#(1)

[0070] Among them, f REF is the instantaneous frequency of the swept light source, τ REF is the optical path difference between the two arms of the interferometer, and then the instantaneous frequency can be obtained by using Hilbert transform and inverse tangent as shown in equations (2) and (3):

[0071]

[0072] I H (t) = H{I REF (t)}∝sin(2πf REF (t)τ REF )#(3)

[0073] After obtaining the instantaneous optical frequency of each sampling point, due to the inverse tangent function:

[0074]

[0075] The phase unwrapping function is needed to handle the discontinuity after phase unwrapping.

[0076] The instantaneous frequency obtained is then normalized and used as an independent variable to resample the signal detected by the main interferometer. In this way, the data obtained obeys an equally spaced distribution in the frequency domain, compensating for the nonlinearity of the frequency sweep and satisfying the conditions for performing FFT.

[0077] The present invention uses a one-dimensional interpolation method to resample the signal of the main interferometer. The principle is as follows: Figure 3 As shown in (d) in the figure. One-dimensional interpolation is a commonly used method in data approximation. It has fast operation and various forms. It can meet the requirements of different precisions and meet the requirements of the present invention for nonlinear tuning compensation. Its essence is to insert some unknown function values ​​into several known discrete function values ​​to estimate the approximate values ​​of some unknown data points in the interval. The basic idea of ​​the one-dimensional interpolation method is: assume that the node is generated by an unknown discrete function g(x), that is, g(x j )=y j (j=0,1,2,…,n), construct function f(x) to approximate g(x), so that f(x) passes through these n+1 nodes, that is, f(x) satisfies j )=y j (j=0,1,2,…,n), and then use f(x) to calculate the interpolation point xi The interpolation value at j =g(x j ). Commonly used one-dimensional interpolation methods mainly include nearest neighbor interpolation (Nearest), piecewise linear interpolation (Linear), cubic spline interpolation (Spline) and cubic polynomial interpolation (Cubic). Different interpolation methods have different usage characteristics. The present invention comprehensively considers the calculation error and calculation speed of the interpolation method, and finally selects the cubic interpolation method (cubic interpolation method). This method is a polynomial interpolation method, which is to successively use the cubic curve f(x)=a 0 +a 1 x+a 2 x 2 +a 3 x 3 The method of finding the minimum point of the function g(x) is as follows: Let x 1 <x 2 , in the search interval [x 1 ,x 2 ] fit g(x) with f(x) to satisfy:

[0078] g(x i )=f(x i ),g ′ (x i )=f ′ (x i ),(i=1,2)#(4)

[0079] Taking the derivative of f(x) and setting it equal to zero, we can find the value of f(x) in [x 1 ,x 2 ] is the minimum point expression represented by a0, a1, a2 and a3, and then the equation system (4) is solved to obtain a 0 ,a 1 ,a 2 ,a 3 , substituting it into this expression, we can get the approximate minimum point of g(x), and iterate successively. When the absolute value of the derivative value of f(x) at the approximate minimum point is less than a given error, the iteration stops. Using cubic interpolation to find the minimum point generally has a faster convergence speed and accuracy than using quadratic interpolation.

[0080] Figure 1This is a diagram of the structure of the experimental system used in the present invention. The light source uses a narrow linewidth laser with a linewidth of 1kHz. A sawtooth voltage with a frequency of 200Hz is generated by a low-bandwidth arbitrary function generator, and then the voltage-controlled oscillator is controlled to output a radio frequency signal with a sawtooth wave change in frequency, and the radio frequency signal is used as the driving signal of the single-sideband modulator. After the light output by the laser is modulated by the radio frequency signal through the single-sideband modulator, the output light is the swept frequency light required for measurement. The frequency change law and range of the swept frequency light are the same as those of the driving radio frequency signal. This experiment is set to 2.5GHz. The swept frequency light is then divided into two paths by a 99:1 coupler. 99% of the light enters the main interferometer as detection light, and 1% of the light enters the auxiliary interferometer to obtain the instantaneous frequency of the detection light. The detection light is then divided into two paths by a 90:10 coupler, of which 90% of the detection light enters the optical fiber to be tested through a circulator and is used to detect information along the optical fiber. The returned backscattered light is output through the circulator and beats with the 10% detection light. Finally, in order to eliminate the influence of polarization fading, the beat frequency light is divided into two polarization states by a polarization beam splitter, and the two paths of light are received by two detectors respectively. In the auxiliary interferometer path, the light passes through a Mach-Zehnder interferometer and is received by the detector. The output electrical signal is used to recover the instantaneous frequency of the light, and further used to compensate for the nonlinearity of the frequency sweep.

[0081] Figure 4 is the measurement result of the system, where Figure 4 (a) is the breakpoint measurement result. Figure 4 (b) shows the change of stress of the 2596m-2612m optical fiber over time. Figure 4 (c) in FIG. 2 is a schematic diagram of the change of the optical fiber stress at 2611 m with time. In order to verify the performance of the optical frequency domain reflectometry system based on the voltage-controlled oscillator proposed in the present invention, we use an ordinary single-mode optical fiber as the optical fiber to be tested, and set the length to 2600 m. Figure 4 (a) shows the intensity distribution of the backscattered signal along the optical fiber after the frequency sweep nonlinear compensation. The strong reflection peak at 2625m comes from the reflection at the end of the optical fiber. By measuring the half-height full width of the reflection peak, it can be obtained that the spatial resolution of the system is 5cm, which is close to the theoretical value. The end of the optical fiber is then tightly wrapped around the piezoelectric ceramic. Rapidly changing the driving voltage of the piezoelectric ceramic can dynamically change the diameter of the cylindrical ceramic, thereby changing the length of the attached optical fiber to apply dynamic strain. In this experiment, the driving voltage of the piezoelectric ceramic is set to a sinusoidal signal with a frequency of 10Hz. Figure 4 (b) and Figure 4(c) in the figure shows the stress variation over time of the entire section of optical fiber (2596m-2612m) and a single point (2611m) at the end of the optical fiber. It can be seen that the sinusoidal vibration signal is well restored. It can be seen that the optical frequency domain reflection system based on the voltage-controlled oscillator adopted in the present invention not only has the advantages of low cost, but also realizes the measurement of optical fiber breakpoints at a higher spatial resolution and the vibration measurement with a faster response speed, and has good performance and application prospects.

[0082] In summary, a lot of research work has been carried out on optical frequency domain reflectometers based on external modulation in order to improve their system performance. For example, the optical fiber delay path technology is used to improve its sensing distance, and the four-wave mixing technology and dynamic injection locking are used to improve its sensing accuracy. However, these solutions all use high-speed arbitrary signal generators with high cost and large volume as devices for generating swept frequency signals, which is not conducive to the practical application of this technology. Therefore, in order to solve this problem, the embodiment of the present invention proposes to use a low-cost and small-volume voltage-controlled oscillator as a radio frequency signal generator to generate a swept frequency signal with fast frequency modulation performance. In order to compensate for the problem of sensor resolution degradation caused by the nonlinearity of the voltage-controlled oscillator's swept frequency, the embodiment of the present invention adopts a swept frequency nonlinearity compensation scheme based on instantaneous frequency measurement and one-dimensional interpolation method, and finally achieves a 5cm spatial resolution and successfully restores a 10Hz vibration signal. It has good performance and is a highly practical externally modulated optical frequency domain reflectometer.

[0083] It should also be noted that there are many variations of the technical solution of the present invention, which cannot be listed one by one. However, any optical frequency domain reflection technology based on a voltage-controlled oscillator adopted in the invention is within the protection scope required by the present invention.

[0084] The embodiments of the present invention include but are not limited to the following:

[0085] (1) According to the needs, reasonably select appropriate sensing technology, including but not limited to OFDR, phi-OTDR, etc., and build a corresponding system according to the adopted technology. The above embodiment adopts OFDR.

[0086] (2) Build Figure 1The embodiment system shown. The laser outputs 1550nm narrow linewidth laser, and the voltage-controlled oscillator is controlled by an arbitrary function generator to generate a linear frequency modulation signal, and then the signal light is modulated by a single-sideband modulator to generate a swept frequency light whose frequency changes linearly with time, and then divided into two paths through an optical coupler: the upper path enters the auxiliary interferometer, passes through a Mach-Zehnder interferometer and is received by a photodetector, and the detected signal is used to calculate the instantaneous optical frequency, and then the nonlinearity of the swept frequency is compensated. The lower path enters the main interferometer as the detection light, and the detection light is divided into two paths. One path passes through the circulator to enter the optical fiber to be tested for detection, and the returned backscattered information passes through the circulator again to beat with the other path of light. The final beat signal is divided into two paths by the polarization beam splitter, and is received by two photodetectors respectively to eliminate the polarization fading effect.

[0087] (3) The problem of spatial resolution degradation caused by frequency scanning nonlinearity can be eliminated by resampling the time domain signal of the main interferometer using the instantaneous optical frequency detected by the auxiliary interferometer. The resampled signal can be processed according to the data processing scheme of traditional OFDR technology: after Fourier transform, the intensity distribution of the backscattered signal along the optical fiber to be tested can be obtained, and then the inverse Fourier transform is performed by windowing, and the results of the previous and subsequent measurements are cross-correlated to obtain the stress magnitude at each location of the optical fiber to be tested.

[0088] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them. Under the concept of the present invention, the technical features in the above embodiments or different embodiments may also be combined, the steps may be implemented in any order, and there are many other changes in different aspects of the present invention as described above, which are not provided in detail for the sake of simplicity. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An optical frequency domain reflectometry system based on a voltage-controlled oscillator, characterized in that: It includes an arbitrary function generator, a laser, a voltage-controlled oscillator, a single-sideband modulator, an auxiliary interferometer, a main interferometer, and a polarization beam splitter; among them, the arbitrary function generator, the laser, the voltage-controlled oscillator, and the single-sideband modulator cooperate to generate the swept-frequency light required for measurement; the swept-frequency light is divided into two paths, one path enters the main interferometer as the probe light, and the other path enters the auxiliary interferometer to obtain the instantaneous frequency of the probe light, and the signal of the main interferometer is resampled by the method of one-dimensional interpolation according to the instantaneous frequency; the main interferometer includes a polarization controller, a circulator, and a fiber under test, the probe light is divided into two paths, one path enters the fiber under test through the circulator for detecting the information along the fiber under test, and the returned backscattered light is output through the circulator again and is beat with the other path of the probe light passing through the polarization controller to generate a beat light; the beat light is divided into two polarization states by the polarization beam splitter.

2. The optical frequency domain reflectometry system based on voltage-controlled oscillator according to claim 1, characterized in that: The cooperation of the arbitrary function generator, the laser, the voltage-controlled oscillator, and the single-sideband modulator to generate the swept-frequency light required for measurement specifically includes: A sawtooth wave voltage is generated by the arbitrary function generator, and then the voltage-controlled oscillator is controlled to output a radio frequency signal with a sawtooth wave change in frequency magnitude, and this radio frequency signal is used as the driving signal of the single-sideband modulator; the light output by the laser is modulated by the radio frequency signal through the single-sideband modulator, and the output light is the swept-frequency light required for measurement.

3. The optical frequency domain reflectometry system based on voltage-controlled oscillator according to claim 1, characterized in that: It also includes three detectors and an oscilloscope connected to the three detectors. One detector is arranged behind the auxiliary interferometer for receiving the light after passing through the auxiliary interferometer; the other two detectors are arranged behind the polarization beam splitter for receiving the light of the two polarization states after passing through the polarization beam splitter respectively.

4. The optical frequency domain reflectometry system based on voltage-controlled oscillator according to claim 1, characterized in that: The obtaining of the instantaneous frequency of the probe light specifically includes: The light intensity I output after the auxiliary interferometer beat frequency interference REF It can be expressed as: I REF (t)∝cos(2πf REF (t)t REF ) Among them, f REF is the instantaneous frequency of the swept light source, τ REF is the optical path difference between the two arms of the auxiliary interferometer, and then the instantaneous frequency can be obtained by using Hilbert transform and inverse tangent as follows: I H (t)=H{I REF (t)}∝sin(2πf REF (t)τ REF ) After obtaining the instantaneous optical frequency of each sampling point, due to the arctangent function: Use the phase unwrapping function to process the discontinuity after phase unwrapping.

5. The optical frequency domain reflectometry system based on voltage-controlled oscillator according to claim 1, characterized in that: The one-dimensional interpolation method includes a cubic interpolation method, which successively uses a cubic curve f(x)=a0+a1x+a2x 2 +a3x 3 The minimum point approximation seeks the minimum point of the function g(x), specifically: Let x1 < x2, and fit g(x) with f(x) in the search interval [x1, x2] to satisfy: g(x i )=f(x i ),g ′ (x i )=f ′ (x i ),(i=1,2) Derive f(x) and set it equal to zero to obtain the expression of the minimum point of f(x) in [x1, x2] represented by a0, a1, a2, and a3, and then solve for a0, a1, a2, and a3. Substitute them into the expression to obtain the approximate minimum point of g(x). Iterate successively. When the absolute value of the derivative value of f(x) at the approximate minimum point is less than the given error, the iteration stops.

6. The optical frequency domain reflectometry system based on voltage-controlled oscillator according to claim 1, characterized in that: The laser includes a narrow-linewidth laser with a linewidth of 1 kHz, and the arbitrary function generator generates a sawtooth wave voltage with a frequency of 200 Hz.

7. The optical frequency domain reflectometry system based on voltage-controlled oscillator according to claim 1, characterized in that: The swept-frequency light is divided into two paths by a 99:1 coupler. 99% of the swept-frequency light enters the main interferometer as the probe light, and 1% of the swept-frequency light enters the auxiliary interferometer to obtain the instantaneous frequency of the probe light.

8. The optical frequency domain reflectometry system based on voltage-controlled oscillator according to claim 7, characterized in that: The probe light is divided into two paths by a 90:10 coupler. 90% of the probe light enters the fiber under test through the circulator for detecting the information along the fiber under test, and the returned backscattered light is output through the circulator again and is beat with the other 10% of the probe light passing through the polarization controller to generate a beat light.

9. The optical frequency domain reflectometry system based on a voltage-controlled oscillator according to any one of claims 1 to 8, characterized in that: The auxiliary interferometer includes a delay fiber and a normal fiber arranged in parallel.

10. An optical frequency domain reflectometry method based on a voltage-controlled oscillator, using the optical frequency domain reflectometry system based on a voltage-controlled oscillator as claimed in any one of claims 1 to 9, characterized in that: It includes: The sawtooth wave output by a low-bandwidth arbitrary function generator is used to control a voltage-controlled oscillator to generate a fast frequency-sweep signal, which is then used in conjunction with a single-sideband modulator to further generate frequency-sweep light. The frequency sweeping light is divided into two paths, one path enters the main interferometer as the detection light, and the other path enters the auxiliary interferometer to obtain the instantaneous frequency of the detection light; The signal of the main interferometer is resampled by a one-dimensional interpolation method, thereby realizing equal-interval sampling of the beat frequency signal in the main interferometer at the optical frequency; A polarization beam splitter is set at the signal receiving end to split the beat frequency light generated by the main interferometer into two polarization states to reduce the influence of polarization fading.

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