A voltage controlled oscillator based optical frequency domain reflectometry system and method

Abstract

This invention relates to an optical frequency domain reflection system and method based on a voltage-controlled oscillator (VCO). The system mainly includes an arbitrary function generator, a laser, a VCO, a single-sideband modulator, an auxiliary interferometer, a main interferometer, and a polarization beam splitter. The arbitrary function generator, laser, VCO, and VSB modulator work together to generate the sweeping light required for measurement. This sweeping light is split into two paths: one path enters the main interferometer as probe light, and the other path enters the auxiliary interferometer to obtain the instantaneous frequency of the probe light. Based on the instantaneous frequency, the signal from the main interferometer is resampled using a one-dimensional interpolation method. This invention uses a low-cost, small-sized VCO as the radio frequency signal generator to generate a sweeping signal with fast frequency modulation performance, and compensates for phase noise caused by the nonlinearity of the sweeping signal by designing the optical path of the auxiliary interferometer and using a linear interpolation algorithm.

Description

Technical Field

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

[0002] Currently, the Internet of Things (IoT) technology is developing rapidly and is widely used in various fields of production and daily life. To adapt to the wave of IoT technology development, various sensor technologies are constantly evolving. Due to its advantages such as high sensitivity, large dynamic range, good anti-electromagnetic interference performance, and strong stability, fiber optic sensors have become an important component of the IoT sensing layer.

[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 applied, including optical time domain reflector (OTDR) and optical frequency domain reflector (OFDR). As researchers continue to explore and optimize system performance, the spatial resolution of OTDR has reached its limit, making it difficult to achieve resolutions below the meter level. Currently, many industry solutions sacrifice system complexity and cost, hindering practical applications. However, OFDR technology, by emitting continuous light with a frequency that changes linearly over time and collecting the backscattered light signal in the fiber, processes the signal in the frequency domain, easily achieving sub-meter spatial resolution. By expanding the sweep frequency range of the light source, current OFDR technology can even detect fiber strain and temperature information at the micrometer scale. Based on the method of acquiring the sweep frequency light source, OFDR technology is divided into two types: direct modulation of the light source and external modulation of the electrical sweep frequency source. Externally modulated OFDR has the advantages of high measurement speed and long sensing distance. Currently, there are external modulation OFDR technologies based on high-order sideband modulation, four-wave mixing, and injection-locked technology with adjustable center wavelength. These technologies expand the sweep frequency range and achieve higher spatial resolution by utilizing various complex signal modulation techniques. However, they do not consider the practicality issues caused by increased system costs. The electrical sweep signals used in these technologies are generated by high-speed arbitrary waveform generators with large bandwidth. Although they can generate sweep signals with extremely high linearity, such devices are large, which is not conducive to system integration, and their high cost makes them unsuitable for practical applications.

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

[0005] To address the shortcomings or improvement needs of existing technologies, this invention focuses on the research of a dynamic OFDR distributed sensing system based on external modulation. It aims to reduce system costs by employing novel modulation devices, while simultaneously achieving a larger sweep range and higher sweep speed, thereby enabling rapid signal measurement with high spatial resolution. Based on this, this invention provides an optical frequency domain reflection system and method based on a voltage-controlled oscillator (VCO). A sawtooth wave output from a low-bandwidth arbitrary function generator (AFG) controls the VCO to generate a fast sweep signal, which is further supplemented by a single-sideband modulator to generate the sweep light. Furthermore, this invention compensates for phase noise caused by sweep nonlinearity by designing an auxiliary interferometer optical path and a linear interpolation algorithm. A polarization diversity receiving structure is also constructed at the signal receiver to reduce the impact of polarization fading. Ultimately, the proposed optical frequency domain reflection system and method based on a VCO achieves a 2.5 GHz sweep range, with a cost that is only a fraction of that of an arbitrary waveform generator with similar performance. The constructed OFDR sensing system based on external modulation achieved the detection of breakpoints on a 2600m optical fiber with a spatial resolution of up to 5cm, and also realized the detection of 10Hz vibration signals. This is of positive significance for realizing a low-cost and 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 (VCO), comprising an arbitrary function generator, a laser, a VCO, a single-sideband modulator, an auxiliary interferometer, a main interferometer, and a polarization beam splitter. The arbitrary function generator, laser, VCO, and single-sideband modulator work together to generate a sweeping beam required for measurement. This sweeping beam is split into two paths: one path enters the main interferometer as a probe beam, and the other path enters the auxiliary interferometer to acquire the instantaneous frequency of the probe beam. Based on the instantaneous frequency, the signal from the main interferometer is resampled using a one-dimensional interpolation method. The main interferometer includes a polarization controller, a circulator, and the optical fiber under test. The probe beam is split into two paths: one path enters the optical fiber under test through the circulator for detecting information along the fiber's path; the backscattered light returned from this path is output through the circulator and beats with the other probe beam that has passed through the polarization controller, generating a beat beam. This beat beam is split into two polarization states by the polarization beam splitter.

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

[0009] A sawtooth wave voltage is generated by an arbitrary function generator, which then controls the output frequency of the voltage-controlled oscillator to exhibit a sawtooth wave variation. This radio frequency signal is used as the driving signal for a single-sideband modulator. The light output from the laser is modulated by the radio frequency signal through the single-sideband modulator, and the output light is the frequency sweep light required for measurement.

[0010] In some implementations, the system also includes three detectors and an oscilloscope connected to the three detectors. One detector is positioned behind an auxiliary interferometer to receive the light after it has passed through the auxiliary interferometer. The other two detectors are positioned behind a polarization beamsplitter to receive the light in two polarization states after it has passed through the polarization beamsplitter.

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

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

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

[0014] Among them, f REF τ is the instantaneous frequency of the swept-frequency light source. REF To calculate the optical path difference between the two arms of the interferometer, the instantaneous frequency can be obtained using the Hilbert transform and arctangent as follows:

[0015]

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

[0017] After obtaining the instantaneous photon frequency at each sampling point, due to the arctangent function:

[0018]

[0019] Use the phase unwinding function to handle the discontinuities after phase unwinding.

[0020] In some implementations, the one-dimensional interpolation method includes cubic interpolation, successively interpolating with a cubic curve f(x) = a0 + a1x + a2x 2 +a3x 3 The minimum point approximation is used to find the minimum point of the function g(x), specifically:

[0021] Let \(x_1 \lt x_2\). In the search interval \([x_1, x_2]\), fit \(g(x)\) with \(f(x)\) such that:

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

[0023] Derive \(f(x)\) and set it equal to zero to obtain the expression of the minimum point of \(f(x)\) represented by \(a_0\), \(a_1\), \(a_2\) and \(a_3\) in \([x_1, x_2]\). Then solve for \(a_0\), \(a_1\), \(a_2\), \(a_3\), substitute them into the expression, and the approximate minimum point of \(g(x)\) can be obtained. 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.

[0024] In some embodiments, 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.

[0025] In some embodiments, 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.

[0026] In some embodiments, the probe light is divided into two paths by a 90:10 coupler. 90% of the probe light enters the待测光纤 (to-be-measured optical fiber) through the circulator for detecting the information along the to-be-measured optical fiber, and the returned backscattered light also outputs through the circulator and is beat with the 10% probe light passing through the polarization controller to generate the beat light.

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

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

[0029] Use the sawtooth wave output by a low-bandwidth arbitrary function generator to control the voltage-controlled oscillator to generate a fast frequency-sweeping signal, and further generate the swept-frequency light in cooperation with the single-sideband modulator;

[0030] Divide the swept-frequency light 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;

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

[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 in order to reduce the impact of polarization fading.

[0033] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0034] Extensive research has been conducted on improving the system performance of externally modulated optical frequency domain reflectometers. For example, fiber optic delay path technology is used to increase sensing distance; four-wave mixing technology and dynamic injection locking are employed to enhance sensing accuracy. However, these solutions all utilize costly and bulky high-speed arbitrary signal generators to generate swept-frequency signals, which hinders the practical application of this technology.

[0035] Therefore, this invention addresses this problem by proposing an optical frequency domain reflection system and method based on a voltage-controlled oscillator (VCO). It employs a low-cost, compact VCO as the radio frequency signal generator to produce a swept-frequency signal with fast frequency modulation performance. To compensate for the sensor resolution degradation caused by the VCO's swept-frequency nonlinearity, this invention uses a swept-frequency nonlinearity compensation scheme based on instantaneous frequency measurement and one-dimensional interpolation, and utilizes a single-sideband modulator and a polarization beam splitter to improve the signal-to-noise ratio of the received signal. Ultimately, a spatial resolution of 5 cm is achieved, and a 10 Hz vibration signal is successfully reproduced. This demonstrates excellent performance and represents a highly practical externally modulated optical frequency domain reflection system. Attached Figure Description

[0036] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments of the present invention will be briefly described below. Obviously, the drawings described below are merely some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without any creative effort.

[0037] Figure 1 This is a schematic diagram of an optical frequency domain reflection system based on a voltage-controlled oscillator provided in Embodiment 1 of the present invention;

[0038] Figure 2 A flowchart of an optical frequency domain reflection method based on a voltage-controlled oscillator provided in Embodiment 1 of the present invention;

[0039] Figure 3 This is a schematic diagram illustrating the principle of frequency sweep nonlinear compensation provided in Embodiment 2 of the present invention;

[0040] Figure 4 This is a schematic diagram of the measurement results provided in Embodiment 2 of the present invention. Detailed Implementation

[0041] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention. It should be noted that, unless otherwise specified, the various features in the embodiments of the present invention can be combined with each other, all within the protection scope of this application. Furthermore, although functional modules may be divided in the device schematic diagram, and a logical order may be shown in the flowchart, in some cases, the steps shown or described may be executed differently from the module division in the device or the order in the flowchart.

[0042] Unless otherwise defined, all technical and scientific terms used in this specification have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of the invention described below can be combined with each other as long as they do not conflict with each other.

[0043] Unless the context otherwise requires, throughout the specification and claims, the term "comprising" is interpreted as openly inclusive, meaning "including, but not limited to." In the description of the specification, terms such as "one embodiment," "some embodiments," "exemplary embodiment," "example," "specific example," or "some examples" are intended to indicate that a particular feature, structure, material, or characteristic associated with that embodiment or example is included in at least one embodiment or example of this disclosure. The illustrative representations of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics mentioned may be included in any suitable manner in any one or more embodiments or examples; that is, although they may be incorporated into embodiments or examples using the above terms for reasons such as order and position, it does not limit them to be incorporated in combination by a single embodiment or example.

[0044] Traditional OFDR systems based on external modulation employ high-speed arbitrary signal generators to produce highly linear frequency-sweeping signals; however, the equipment is expensive, resulting in high system costs. Voltage-controlled oscillators (VCOs) can output microwave signals of different frequencies by controlling the amplitude of the input DC voltage. Frequency-sweeping light can be obtained by rapidly sweeping the amplitude of the input voltage. However, due to the performance limitations of VCOs, the frequency of the output signal does not change completely linearly with the input voltage. This nonlinearity introduces significant phase noise into the frequency-sweeping light source, leading to a severe degradation of the spatial resolution of the constructed optical frequency domain reflection system. Furthermore, the phase noise of the electrical signal output by the VCO also restricts the system's spatial resolution. Eliminating the phase noise present in the frequency-sweeping light source is the key technical problem this invention aims to solve.

[0045] This invention utilizes a voltage-controlled oscillator (VCO) to replace the high-bandwidth, high-speed arbitrary signal generator used in traditional externally modulated optical frequency reflectometers, generating a fast frequency-sweeping signal with a large bandwidth, thereby producing a frequency-sweeping light source and significantly reducing system costs. Simultaneously, to compensate for the nonlinearity of the VCO output frequency-sweeping signal, an auxiliary interferometer system and a linear compensation algorithm were built and designed. Furthermore, a single-sideband modulator and a polarization diversity receiver are used to improve the signal-to-noise ratio of the received signal. Ultimately, centimeter-level spatial resolution fiber optic breakpoint monitoring and long-distance dynamic strain information measurement are achieved, representing a pioneering achievement for low-cost, high-performance optical frequency domain reflectometers.

[0046] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application. The invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0047] Example 1:

[0048] like Figure 1As shown, this embodiment of the invention provides an optical frequency domain reflection system based on a voltage-controlled oscillator (VCO). The system includes an arbitrary function generator, a laser, a VCO, a single-sideband modulator, an auxiliary interferometer, a main interferometer, and a polarization beam splitter. The arbitrary function generator, laser, VCO, and VSB modulator work together to generate a sweeping light required for measurement. This sweeping light is split into two paths: one path enters the main interferometer as probe light, and the other path enters the auxiliary interferometer to acquire the instantaneous frequency of the probe light. Based on the instantaneous frequency, the signal of the main interferometer is resampled using a one-dimensional interpolation method to achieve equally spaced sampling of the beat frequency signal in the main interferometer at the optical frequency, compensating for the nonlinearity of the sweeping signal output by the VCO. The auxiliary interferometer includes a delay fiber and a normal fiber connected in parallel. The main interferometer includes a polarization controller, a circulator, and the optical fiber under test. The probe light is split into two paths. One path enters the optical fiber under test through the circulator and is used to detect information along the optical fiber. The backscattered light returned by the circulator is output and beats with the other probe light that has passed through the polarization controller to generate beat-frequency light. The beat-frequency light is split into two polarization states by a polarization beam splitter. By merging the two polarization states using existing technology, the influence of polarization fading can be reduced. The specific polarization state merging is based on existing technology and will not be described in detail here.

[0049] Through the above-described configuration, this invention utilizes a voltage-controlled oscillator (VCO) to replace the high-bandwidth, high-speed arbitrary signal generator used in traditional externally modulated optical frequency reflectometers, generating a fast frequency-sweeping signal with a large bandwidth, thereby producing a frequency-sweeping light source and significantly reducing system costs. Simultaneously, to compensate for the nonlinearity of the VCO output frequency-sweeping signal, an auxiliary interferometer system and a linear compensation algorithm were built and designed. Furthermore, a single-sideband modulator and a polarization beam splitter are used to improve the signal-to-noise ratio of the received signal. Ultimately, centimeter-level spatial resolution fiber optic breakpoint monitoring and long-distance dynamic strain information measurement are achieved, representing a pioneering achievement for low-cost, high-performance optical frequency domain reflectometers.

[0050] In one embodiment, the arbitrary function generator, laser, voltage-controlled oscillator, and single-sideband modulator work together to generate the sweep light required for measurement, specifically including: generating a sawtooth wave voltage through the arbitrary function generator, thereby controlling the output frequency of the voltage-controlled oscillator to exhibit sawtooth wave variations, and using this radio frequency signal as the driving signal for the single-sideband modulator; the light output from the laser is modulated by the radio frequency signal through the single-sideband modulator, and the output light is the sweep light required for measurement.

[0051] In one embodiment, the system further includes three detectors and an oscilloscope connected to the three detectors. One detector is positioned behind an auxiliary interferometer to receive the light after it has passed through the auxiliary interferometer. The other two detectors are positioned behind a polarization beamsplitter to receive the light in two polarization states after it has passed through the polarization beamsplitter. In the auxiliary interferometer path, the light passes through a Mach-Zehnder interferometer and is then received by a detector. The output electrical signal is used to recover the instantaneous frequency of the light, and subsequently to compensate for frequency sweep nonlinearity.

[0052] In one embodiment, the laser comprises a narrow-linewidth laser with a linewidth of 1 kHz. The arbitrary function generator generates a sawtooth wave voltage with a frequency of 200 Hz. The swept beam is split into two paths by a 99:1 coupler; 99% of the swept beam enters the main interferometer as probe beam, and 1% enters the auxiliary interferometer to obtain the instantaneous frequency of the probe beam. The probe beam is split into two paths by a 90:10 coupler; 90% of the probe beam enters the fiber under test through a circulator for detecting information along the fiber's path, and its returned backscattered light is output through a circulator and beats with the other 10% probe beam that has passed through a polarization controller to generate beat beam light.

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

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

[0055] Step 200: Split the swept light 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.

[0056] Step 300: The signal of the main interferometer is resampled by one-dimensional interpolation, thereby realizing the equal-interval sampling of the beat frequency signal in the optical frequency of the main interferometer.

[0057] Step 400: Set up a polarization beam splitter at the signal receiving end to split the beat frequency light generated by the main interferometer into two polarization states in order to reduce the impact of polarization fading.

[0058] In summary, this invention provides an optical frequency domain reflection system and method based on a voltage-controlled oscillator (VCO). It utilizes a low-cost, compact VCO as a radio frequency signal generator to produce a swept-frequency signal with fast frequency modulation capabilities. To compensate for the sensor resolution degradation caused by the VCO's swept-frequency nonlinearity, this invention employs a swept-frequency nonlinearity compensation scheme based on instantaneous frequency measurement and one-dimensional interpolation, and utilizes a single-sideband modulator and a polarization beam splitter to improve the signal-to-noise ratio of the received signal. Ultimately, a spatial resolution of 5 cm is achieved, and a 10 Hz vibration signal is successfully reproduced. This demonstrates excellent performance and represents a highly practical externally modulated optical frequency domain reflection system.

[0059] Example 2:

[0060] Based on Example 1, Example 2 will provide a more detailed explanation of the present invention through a specific example and principle.

[0061] OFDR (Optical Frequency Detection and Reflection) systems are distributed fiber optic sensing systems based on backscattering Rayleigh scattering. The basic principle is coherent detection based on Frequency Modulated Continuous Wave (FMCW) technology. In an OFDR system, two linearly chirped optical signals from coherent light sources at the same height interfere with each other, generating a beat frequency signal. Depending on the sweep frequency method, OFDRs can be divided into direct modulation by the light source and external modulation by the sweep source. This invention focuses on OFDRs with external modulation by the sweep source. The sweep light output from the modulator is split into a reference light and a measurement light by coupler A. The reference light is transmitted to coupler B through a reference path with a given optical distance, while the measurement light is transmitted to the fiber under test (FUT) through a measurement path. The FUT returns backscattered light. Coupler B then combines the reference light and the backscattered light to beat the signal, which is then detected by a photodiode. The detected signal is collected and analyzed by a data acquisition system (DAQ) to demodulate 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 under test, and Δν sweep This represents the frequency sweep range.

[0064] In OFDR systems based on external modulation, generating linear frequency modulated (LFM) signals with a large sweep range typically requires a high-bandwidth arbitrary signal generator, which significantly increases system cost. Therefore, this invention proposes using a voltage-controlled oscillator (VCO) as the source for generating LFM signals. A VCO is an oscillating circuit whose output frequency corresponds to its input control voltage; this correspondence is often nearly linear. By applying a linearly varying voltage signal to the VCO, a radio frequency signal whose frequency changes linearly with time can be obtained. Furthermore, VCOs offer advantages such as simple structure and low cost.

[0065] In an OFDR system, to obtain the location information of the reflection point, the acquired signal needs to be transformed into the frequency domain using a Fast Fourier Transform (FFT). Then, based on the one-to-one correspondence between frequency and location, it is transformed into the distance domain to obtain the location of the reflection point on the fiber. If the laser performs a linear frequency sweep, the beat frequency signal is a single-cycle sinusoidal signal. However, the frequency sweep signal output by the voltage-controlled oscillator (VCO) is often not a strictly linear frequency sweep signal, meaning that the frequency intervals scanned by the light source within the same time interval are inconsistent. In this case, the signal sampling in the frequency domain is non-equal interval sampling, while FFT requires equal interval sampling in the frequency domain. If FFT is directly used to process the signal, it will cause bandwidth broadening and energy diffusion of the beat frequency signal, ultimately degrading the spatial resolution of the OFDR system and making it impossible to accurately locate the reflection point in the fiber. Therefore, in order to accurately locate the reflection point on the fiber, it is necessary to compensate for the nonlinear tuning effect of the laser caused by the VCO.

[0066] To address the nonlinear tuning effect of the swept-frequency light source, this invention employs a resampling method for compensation. The main idea is to construct an auxiliary interferometer and use it to demodulate the instantaneous frequency information of the voltage-controlled oscillator (VCO). Then, the signal from the main interferometer is resampled using a one-dimensional interpolation method, thereby achieving equal-interval sampling of the beat frequency signal in the optical frequency domain within the main interferometer. It should be noted that the instantaneous frequencies of the VCO, probe light, and swept-frequency light are all considered; the instantaneous frequency of the probe light is the same as that of the swept-frequency light, and their changes are identical to those of the VCO's instantaneous frequency.

[0067] In this embodiment, a system is built as follows: Figure 3 The Mach-Zehnder interferometer shown in (a) is used as an auxiliary interferometer. The specific process of frequency sweep nonlinear compensation is referred to in sequence. Figure 3 (a) Figure 3 (b) Figure 3 (c) Figure 3 As shown in (d) in the figure.

[0068] Based on the working principle of the Mach-Zehnder interferometer, the output light intensity I after beat frequency interference is... REF It can be expressed as equation (1):

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

[0070] Among them, f REF τ is the instantaneous frequency of the swept-frequency light source. REF Let the optical path difference between the two arms of the interferometer be denoted by Hilbert transform and arctangent, then the instantaneous frequency can be obtained using 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 photon frequency at each sampling point, due to the arctangent function:

[0074]

[0075] A phase unwinding function is needed to handle the discontinuities after phase unwinding.

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

[0077] This invention employs a one-dimensional interpolation method to resample the signal from the master interferometer, the principle of which is as follows: Figure 3 As shown in (d) in the figure. One-dimensional interpolation, as a commonly used method in data approximation, is relatively fast, has various forms, and can achieve different precision requirements, which meets the requirements of this invention for nonlinear tuning compensation. Its essence is to insert some unknown function values ​​into a number of known discrete function values ​​to estimate the approximate values ​​of some unknown data points within the interval. The basic idea of ​​the one-dimensional interpolation method is: assuming that the nodes are generated by unknown discrete functions g(x), that is, g(x) = ... j )=y j (j = 0, 1, 2, ..., n), construct a function f(x) to approximate g(x) such that f(x) passes through these n+1 nodes, i.e., satisfy f(x) = ... j )=y j (j=0,1,2,…,n), and then use f(x) to calculate the interpolation point x.i interpolation at, that is, y j = g(x j ). The 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), etc. 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. As a polynomial interpolation method, this method is a method of successively approximating the minimum point of the function g(x) with the minimum point of the cubic curve f(x) = a0 + a1x + a2x 2 + a3x 3 . The specific approach is as follows: Let x1 < x2, and in the search interval [x1, x2], 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] Take the derivative of 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. Then solve for a0, a1, a2, and a3 from the system of equations (4), substitute them into this expression, and the approximate minimum point of g(x) can be obtained. 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. Seeking the minimum point using the cubic interpolation method generally has a faster convergence speed and higher accuracy than using the quadratic interpolation method.

[0080] Figure 1This is a structural diagram of the experimental system used in this invention. The light source is a narrow-linewidth laser with a linewidth of 1 kHz. A sawtooth wave voltage with a frequency of 200 Hz is generated by a low-bandwidth arbitrary function generator, which then controls the output frequency of the voltage-controlled oscillator to exhibit sawtooth wave variations. This RF signal is used as the driving signal for the single-sideband modulator. After the laser output light is modulated by the RF signal through the single-sideband modulator, the output light is the frequency sweep light required for measurement. The frequency variation law and range of the frequency sweep light are the same as those of the driving RF signal; in this experiment, it is set to 2.5 GHz. Subsequently, the frequency sweep light is split into two paths by a 99:1 coupler. 99% of the light enters the main interferometer as the probe light, and 1% of the light enters the auxiliary interferometer to obtain the instantaneous frequency of the probe light. The probe light is then split into two paths by a 90:10 coupler. 90% of the probe light enters the fiber under test through a circulator for probing along the fiber's path. The returned backscattered light is output through the circulator and beats with the remaining 10% of the probe light. Finally, to eliminate the effects of polarization fading, the beat-frequency light is split into two polarization states by a polarization beam splitter, and the two paths are received by two detectors. In the auxiliary interferometer path, the light passes through a Mach-Zehnder interferometer before being received by a detector. The output electrical signal is used to recover the instantaneous frequency of the light, which is then used to compensate for the frequency sweep nonlinearity.

[0081] Figure 4 The measurement results of the system are as follows, Figure 4 (a) in the figure represents the result of the breakpoint measurement. Figure 4 (b) shows the stress variation of the optical fiber from 2596m to 2612m over time. Figure 4 (c) in the figure illustrates the change of fiber stress at 2611m over time. To verify the performance of the voltage-controlled oscillator-based optical frequency domain reflection system proposed in this invention, we used ordinary single-mode fiber as the fiber under test and set its length to 2600m. Figure 4 Figure (a) shows the intensity distribution of the backscattered signal along the fiber after frequency sweep nonlinear compensation. The strong reflection peak at 2625m originates from the reflection at the fiber end. By measuring the full width at half maximum (FWHM) of the reflection peak, the spatial resolution of the system is found to be 5cm, which is close to the theoretical value. Subsequently, the fiber end is tightly wound onto a piezoelectric ceramic. Rapidly changing the driving voltage of the piezoelectric ceramic dynamically changes the diameter of the cylindrical ceramic, thereby changing the length of the attached 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 4Figure (c) shows the stress variation over time of the entire fiber end (2596m-2612m) and a single point (2611m). It can be seen that the sinusoidal vibration signal is well reproduced. Therefore, it can be seen that the optical frequency domain reflection system based on a voltage-controlled oscillator (VCO) of this invention not only has the advantage of low cost, but also achieves fiber break point measurement with high spatial resolution and vibration measurement with fast response speed, demonstrating good performance and application prospects.

[0082] In summary, extensive research has been conducted on improving the system performance of externally modulated optical frequency domain reflectometers. Examples include using fiber delay path technology to increase sensing distance, four-wave mixing technology, and dynamic injection locking to improve sensing accuracy. However, these solutions all employ costly and bulky high-speed arbitrary signal generators to generate sweep signals, which is detrimental to the practical application of this technology. Therefore, this invention addresses this problem by using a low-cost and compact voltage-controlled oscillator (VCO) as the RF signal generator to generate sweep signals with fast frequency modulation performance. To compensate for the sensor resolution degradation caused by the VCO's sweep frequency nonlinearity, this invention employs a sweep frequency nonlinearity compensation scheme based on instantaneous frequency measurement and one-dimensional interpolation, ultimately achieving a 5cm spatial resolution and successfully reproducing a 10Hz vibration signal. This demonstrates excellent performance and represents 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 all be listed. However, all optical frequency domain reflection technologies based on voltage-controlled oscillators used in the invention are within the scope of protection claimed by the present invention.

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

[0085] (1) Select appropriate sensing technologies as needed, including but not limited to OFDR, phi-OTDR, etc., and build a corresponding system based on the technology used. The aforementioned embodiment uses OFDR.

[0086] (2) Building such Figure 1The illustrated system uses a 1550nm narrow-linewidth laser output. An arbitrary function generator controls a voltage-controlled oscillator to generate a linearly frequency-modulated (LFM) signal. This signal is then modulated by a single-sideband modulator to produce a sweep beam with a frequency that changes linearly with time. The LFM signal is then split into two paths by an optical coupler: the upper path enters an auxiliary interferometer, passes through a Mach-Zehnder interferometer, and is received by a photodetector. The detected signal is used to calculate the instantaneous photofrequency and compensate for the nonlinearity of the sweep beam. The lower path serves as the probe beam, which enters the main interferometer. This probe beam is split into two paths: one path passes through a circulator into the fiber under test for detection, and the returned backscattered information passes through the circulator again to beat the other path. The final beat signal is split into two paths by a polarization beamsplitter and received by two photodetectors to eliminate polarization fading effects.

[0087] (3) By resampling the time-domain signal of the main interferometer using the instantaneous photofrequency detected by the auxiliary interferometer, the spatial resolution degradation caused by the frequency sweep nonlinearity can be eliminated. The resampled signal can be processed according to the data processing scheme of the traditional OFDR technology: after Fourier transform, the intensity distribution of the backscattered signal along the fiber under test can be obtained. Then, by windowing and performing an inverse Fourier transform, and cross-correlation of the results of the two measurements, the stress at various points in the fiber under test can be obtained.

[0088] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; under the concept of the present invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of different aspects of the present invention as described above. For the sake of brevity, they are not provided in detail; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An optical frequency domain reflection 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; 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 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 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 and is used 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; 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; 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 whose frequency magnitude changes in a sawtooth wave, and this radio frequency signal is used as the drive 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; The one-dimensional interpolation method includes cubic interpolation, which successively uses a cubic curve f(x) = a0 + a1x + a2x. 2 +a3x 3 The minimum point approximation is used to find the minimum point of the function g(x), specifically: Let x1 < x2, and in the search interval [x1, x2], f(x) is used to fit g(x) 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, obtain the expression of the minimum point of f(x) in [x1, x2] represented by a0, a1, a2, and a3, then solve for a0, a1, a2, and a3, substitute them into the expression, and the approximate minimum point of g(x) can be obtained. 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.

2. The optical frequency domain reflection system based on a 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 and is used to receive the light after passing through the auxiliary interferometer; the other two detectors are arranged behind the polarization beam splitter and receive the light of the two polarization states after passing through the polarization beam splitter respectively.

3. The optical frequency domain reflection system based on a voltage-controlled oscillator according to claim 1, characterized in that, The specific process of obtaining the instantaneous frequency of the probe light includes: The light intensity I output after beat frequency interference by the auxiliary interferometer REF It can be represented as: I REF (t)∝cos(2πf REF (t)t REF ) Among them, f REF τ is the instantaneous frequency of the swept-frequency light source. REF To calculate the optical path difference between the two arms of the interferometer, the instantaneous frequency can be obtained using the Hilbert transform and arctangent 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.

4. The optical frequency domain reflection system based on a 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.

5. The optical frequency domain reflection system based on a voltage-controlled oscillator according to claim 4, 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 and is used for detecting the information along the fiber under test. 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.

6. The optical frequency domain reflection system based on a voltage-controlled oscillator according to any one of claims 1-5, characterized in that, The auxiliary interferometer includes a delay fiber and a normal fiber arranged in parallel.

7. A frequency-domain optical reflection method based on a voltage-controlled oscillator, using the frequency-domain optical reflection system based on a voltage-controlled oscillator as described in any one of claims 1-6, characterized in that, It includes: A sawtooth wave output from a low-bandwidth arbitrary function generator is used to control a voltage-controlled oscillator to generate a fast frequency sweep signal, which is then combined with a single-sideband modulator to further generate frequency sweep light. The frequency-sweeping light is split 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. The signal of the main interferometer is resampled by one-dimensional interpolation, thereby realizing equal-interval sampling of the beat frequency signal in the optical frequency in the main interferometer; 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 in order to reduce the impact of polarization fading.

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