Double-sideband FSI optical fiber measurement system and method based on temperature drift self-reference correction

Abstract

The invention discloses a double-sideband FSI optical fiber measurement system and a double-sideband FSI optical fiber measurement method based on temperature drift self-reference correction, which are used for correcting optical fiber short-time amplification measurement error and long-time optical path drift fluctuation caused by temperature drift. Comprising the following steps: 1) generating a double-sideband sweep frequency optical signal through an MZM; 2) establishing wavelength division multiplexing optical paths of the measurement interferometer and the self-reference interferometer; 3) performing frequency mixing interference on the multiplexing measurement light and the reference light, and acquiring an orthogonal interference signal; 4) resolving a corresponding phase difference of the two interferometers based on a time-shifting Ap-FFT algorithm; 5) constructing a double-sideband FSI dynamic measurement formula, and correcting a short-time temperature drift amplification error; 6, a long-time optical fiber optical path-temperature drift model is established, a temperature drift self-correction formula is derived, and long-time optical fiber temperature drift self-correction.According to the long-time optical fiber temperature drift self-correction method, the effect of complex active temperature control is achieved only by adding two optical fiber couplers, measurement limitation caused by temperature control limit is overcome, the optical fiber measurement precision and stability are remarkably improved, and meanwhile cost expenditure is reduced.

Description

Technical Field

[0001] This invention belongs to the field of optical precision measurement technology, and further relates to optical frequency scanning interferometry measurement methods. Specifically, it is a double-sideband optical frequency scanning interferometry (FSI) fiber optic measurement system and method based on temperature drift self-reference correction. It corrects the short-time amplification measurement error and long-term optical path drift fluctuation caused by temperature drift, and is used to provide a stable fiber length reference and high-precision fiber measurement. Background Technology

[0002] Fiber optic length measurement is a key technology in fiber optic communication, sensing, and industrial fields, and its accuracy directly affects network performance, fault location, and system reliability. With the increasing demand for high-precision fiber optic measurement in 5G networks, data centers, and aerospace, fiber optic length measurement technology under dynamic environments has become a research hotspot. Major challenges include additional optical path errors introduced by temperature drift, and real-time length changes caused by stretching, vibration, or bending during fiber length measurement. Among mainstream measurement technologies, optical time domain reflectometers (OTDRs) calculate distance by emitting light pulses and measuring the return time of backscattered light. Optical path fluctuations cause Doppler frequency shifts, which couple with the inherent frequency difference of the OTDR, producing periodic measurement ambiguity with errors reaching the millimeter level. Dual-comb interferometry (DCI) uses two sets of optical frequency combs for asynchronous sampling and eliminates Doppler errors through differential processing. However, it has high system complexity, requires two sets of mode-locked lasers, and is therefore costly, currently mainly used in laboratories and high-precision industrial scenarios. Double-sideband FSI generates double-sidebands through electro-optic modulation to cancel Doppler errors, enabling dynamic measurement of absolute distance. This invention applies the double-sideband FSI method to the field of fiber optic measurement, which can simultaneously balance accuracy, complexity, and cost.

[0003] While short-term temperature drift amplification error correction can be achieved using the double-sideband FSI method, temperature drift still leads to significant long-term optical path variability in fibers, with variability reaching millimeter levels in hundreds of meters of fiber. Methods for suppressing fiber temperature drift mainly include: one type is temperature drift error model compensation, which establishes a physical model of temperature drift and optical path variability for compensation and correction. This is limited by fiber parameters and the hysteresis between fiber temperature drift and ambient temperature; or it involves experimentally establishing a function curve of temperature and measurement error for compensation and correction, but this is not universally applicable due to differences between different types of measurement fibers. Another type is temperature control technology. Active temperature control systems adjust the temperature in real time using air conditioning or thermoelectric coolers; passive temperature control measures place the fiber optic equipment in a constant temperature chamber or sealed environment to reduce external temperature fluctuation interference. Fiber optic measurement accuracy depends on the temperature control effect, and there are temperature control limits due to measurement scenarios, system complexity, and cost. With the continuous development of fiber optic communication sensing and electro-optic modulation technology, increasingly higher requirements are placed on fiber optic measurement accuracy, stability, and cost control. There is an urgent need for a correction method that can further correct long-term optical path variability caused by temperature drift and break through the measurement accuracy limits. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies by proposing a double-sideband FSI fiber optic measurement system and method based on temperature drift self-reference correction. This system corrects short-time amplification measurement errors and long-term optical path drift errors caused by temperature drift. This invention innovatively proposes a fiber optic temperature drift self-correction method. It constructs a measurement interferometer and a self-reference interferometer based on the same measurement fiber, achieving temperature drift correction with only two additional fiber couplers. The system is simple, low-cost, and has great application potential. Combined with active temperature control, it can achieve extremely high measurement accuracy and stability.

[0005] The basic idea behind this invention is as follows: The physical mechanism of optical fiber path length variation with temperature drift was analyzed. The change in optical fiber path length is linearly related to the temperature change and the length of the optical path. However, the temperature drift measurement error coefficient is related to the physical properties of the measuring fiber. Different fibers have different temperature drift measurement error coefficients and sensed ambient temperature changes, making it impossible to use a reference interferometer to correct the measurement interferometer's values. This invention innovatively proposes using a wavelength division multiplexing (WDM) optical path design in two interferometers. In the measurement interferometer, light propagates back and forth in the measuring fiber, while in the reference interferometer, light propagates unidirectionally in the measuring fiber. The temperature drift measurement error coefficient and sensed ambient temperature change are necessarily the same for the same measuring fiber, differing only in the number of passes. The measuring fiber serves as both the measurement optical path and the reference optical path. Theoretically, this can perfectly correct the optical fiber path length fluctuations caused by temperature drift, always locking the continuous optical fiber path length measurement results to the measured values ​​at the initial temperature, achieving high-precision and stable measurement of the optical fiber path length. To implement this theoretical approach, only two fiber couplers were added to the traditional double-sideband FSI optical path. A wavelength division multiplexing optical path for the measurement interferometer and the self-reference interferometer was designed, and a shared solution and correction module ensured real-time synchronous sampling. An APFFT spectrum multiplexing algorithm was designed, and real-time synchronous solution was performed by the two interferometers. The temperature drift error formula and the self-correction formula were derived, thereby achieving temperature drift self-correction.

[0006] To achieve the above objectives, the technical solution of the present invention includes the following:

[0007] A double-sideband FSI fiber optic measurement system based on temperature drift self-reference correction includes: a double-sideband sweep frequency module, a temperature drift self-reference module, and a calculation and correction module; wherein the double-sideband sweep frequency module provides measurement light and reference light for the temperature drift self-reference module and the calculation and correction module, respectively; the temperature drift self-reference module constructs two Michelson interferometer optical paths around the measurement fiber, the optical paths passing through the measurement fiber twice constitute the measurement interferometer, and the optical paths passing through the measurement fiber once constitute the self-reference interferometer, which is used to correct the temperature drift optical path fluctuation of the same measurement fiber; the multiplexed measurement light carrying fiber length and temperature drift information is output to the calculation and correction module through wavelength division multiplexing;

[0008] The double-sideband sweep frequency module consists of a sweep frequency signal source, a driver amplifier, a narrow linewidth laser, a Mach-Zehnder intensity modulator (MZM), and a first fiber coupler. It is used to generate double-sideband sweep frequency light to offset the short-time amplification measurement error of the fiber.

[0009] The temperature drift self-reference module consists of an optical fiber circulator, a second optical fiber coupler, a third optical fiber coupler, a measurement optical fiber, and an optical fiber end face reflector. Based on the measurement optical fiber, it constructs the optical path of the measurement interferometer and the optical path of the self-reference interferometer to correct the influence of temperature drift on the optical fiber path length.

[0010] The calculation and correction module consists of a 90° optical mixer, two balanced detectors (BPDs), a data acquisition card, and an FPGA. It is used to perform optical digital coherent detection on the input multiplexed measurement light and the reference light, and to calculate the multi-frequency information in real time using the time-shifted Ap-FFT algorithm combined with wavelength division multiplexing to calculate the optical path length to be measured.

[0011] Furthermore, in the aforementioned double-sideband sweep frequency module, a narrow-linewidth laser generates a 1550nm fixed-wavelength optical carrier and outputs it to the MZM modulator; the sweep frequency signal source outputs a periodic sawtooth sweep frequency electrical signal, which is amplified by a driver amplifier and then output to the MZM modulator; the MZM modulator modulates the input sweep frequency electrical signal with the fixed-wavelength optical carrier, and the resulting double-sideband sweep frequency light is then output to the first fiber coupler, which splits the double-sideband sweep frequency light into two paths. One path is used as reference light and output to the 90° mixer in the solution correction module, and the other path is used as measurement light and output to the circulator of the temperature drift self-reference module.

[0012] Furthermore, the aforementioned temperature drift self-reference module includes a fiber optic circulator with three ports. The first port receives the measurement light output from the first fiber optic coupler in the double-sideband sweep frequency module. The second port of the fiber optic circulator transmits the measurement light to the measurement fiber, which is connected to the second fiber optic coupler. The measurement light is split into two paths according to a 5:5 beam split and enters two Michelson interferometer optical paths. In the measurement interferometer optical path, the fiber end face reflector reflects the measurement light back to the second fiber optic coupler, and then transmits it back to the second port of the fiber optic circulator via the measurement fiber. The measurement light is then transmitted to the third fiber optic coupler through the third port. In the self-reference interferometer optical path, the measurement light is directly transmitted from the second fiber optic coupler to the third fiber optic coupler. The third fiber optic coupler receives the two measurement lights and sends them to the 90° mixer in the solution correction module.

[0013] Furthermore, in the aforementioned calculation and correction module, the 90° optical mixer performs interference mixing on the received reference light and the multiplexed measurement light to generate four optical signals with phase differences of 0°, 90°, 180°, and 270°; the balanced detector is used for photoelectric conversion and DC elimination to generate two orthogonal electrical signals with phase differences of 0° and 90°; the acquisition card performs AD sampling to obtain the interference electrical signal; the FPGA integrates a time-shifted Ap-FFT algorithm to process the interference signal, synchronously calculate the phase difference in real time, correct fiber temperature drift, and obtain a stable fiber measurement length.

[0014] Furthermore, this invention also proposes a method for distance measurement using a double-sideband FSI fiber optic measurement system based on temperature drift self-reference correction, comprising the following steps:

[0015] (1) A swept frequency signal source generates a swept frequency electrical signal, which is amplified by a driver and then modulated with a carrier-suppressed double-sideband signal at a Mach-Zehnder intensity modulator (MZM) to generate a double-sideband swept frequency optical signal. The bias voltage is set at the minimum bias point;

[0016] (2) Double-sided band-sweep optical The light is fed into the first fiber coupler, which splits it into two paths. One path, with 90% of the split light used as a reference beam, is then fed into the first fiber coupler. The light is directly transmitted to a 90° mixer, and 10% of the other light is split off for measurement. Transmitted to the circulator;

[0017] (3) The temperature drift self-reference module constructs two Michelson interferometer optical paths: a measurement interferometer and a self-reference interferometer. In the measurement interferometer optical path, the measurement light passes through the measurement fiber twice, and in the self-reference interferometer optical path, the measurement light passes through the measurement fiber once. The measurement interferometer and the self-reference interferometer are wavelength divided and multiplexed through a third fiber coupler. The multiplexed measurement light is output to the solution and correction module. The steps are as follows:

[0018] (3.1) Measurement light output from the second port of the fiber optic circulator Ignoring the slight power loss and connector loss in fiber optic transmission, the second fiber coupler splits the measurement optical power transmitted through the measurement fiber into two parts: 50% of the power belongs to the measurement interferometer optical path, and 50% belongs to the self-reference interferometer optical path. Divided into Two parts are incident on the optical paths of the two interferometers;

[0019] (3.2) In the optical path of the measuring interferometer, The measurement light is struck by the fiber end-face reflector via the measurement fiber and the second fiber coupler. The fiber end-face reflector reflects the light back to the second fiber coupler, and then it is transmitted back to port 2 of the fiber circulator via the measurement fiber again. The measurement light is then transmitted to the third fiber coupler via port 3. The measurement interferometer returns the measurement light as... ;

[0020] (3.3) In the optical path of the self-reference interferometer, The measurement light is transmitted directly from the measuring fiber and the second fiber coupler to the third fiber coupler, and the measurement light returns from the reference interferometer as... ;

[0021] (3.4) Return to measurement light and The optical fibers are combined and multiplexed at the third fiber coupler and transmitted to the 90° mixer.

[0022] (4) The multiplexed measurement light and reference light interfere and beat at the 90° mixer. The interference light is detected by two balanced detectors (BPDs) to obtain orthogonal interference signals containing optical path measurement information. And collect data using a data acquisition card;

[0023] (5) The two orthogonal interference signals are sent into the FPGA, and the interference signals are processed using the Ap-FFT algorithm to identify and obtain the phase difference between the two interferometers. The steps are as follows:

[0024] (5.1) For complex signal data within one sweep frequency cycle, the spectrum and phase spectrum of the time-shifted data points are calculated using the time-shifted Ap-FFT algorithm;

[0025] (5.2) According to the spectrum distribution, the high-frequency components are the positive and negative interference frequencies of the measuring interferometer, and the low-frequency components are the positive and negative interference frequencies of the self-reference interferometer;

[0026] (5.3) Extract the phase corresponding to the interference frequencies of the measuring interferometer and the self-reference interferometer;

[0027] (5.4) Accumulate the phases of all time-shifted data points to obtain the upper and lower sideband phase differences between the measurement interferometer and the self-reference interferometer. and ;

[0028] (6) Construct a dynamic measurement formula for double-sideband FSI, substitute the phase difference between the upper and lower sidebands into the formula, correct the temperature drift amplification error, and calculate the real-time optical path of the measuring interferometer. Real-time optical path length of self-referenced interferometer ;

[0029] (7) Establish a long-time fiber optical path length-temperature drift model. Based on the fiber temperature drift self-correction principle, derive the temperature drift self-correction formula, lock the continuous fiber optical path length measurement results to the measurement values ​​under the initial conditions, and obtain the long-time stable fiber measurement values ​​based on temperature drift self-correction. .

[0030] Compared with the prior art, the present invention has the following advantages:

[0031] First, short-time temperature drift amplification error correction. Traditional FSI fiber measurement methods can accurately measure stable optical path lengths, but the frequency sweep measurement mechanism leads to short-time temperature drift amplification errors. Under the condition of a 1550nm center wavelength and an 8GHz bandwidth, according to the established temperature difference amplification error formula, the amplification factor is as high as 3850 times, meaning that a 1μm temperature drift optical path length change will result in a 3.85mm measurement error. This invention generates a frequency sweep double-sideband through electro-optic modulation, and the forward and reverse frequency sweeps cancel out the temperature drift amplification error term, correcting the short-time temperature drift amplification error of the traditional FSI fiber measurement method, and enabling high-precision real-time optical path length measurement of optical fibers.

[0032] Second, long-term fiber optic temperature drift self-correction. Temperature drift has a significant impact on the stability of long-term fiber optic measurements, even reaching the millimeter level. Traditional theoretical correction methods are limited by fiber parameters and the hysteresis between fiber temperature drift and ambient temperature, making them unsuitable for measurements of any fiber. The measurement accuracy of traditional active temperature control methods depends on the temperature control effect, and there are temperature control limits due to limitations in measurement scenarios, system complexity, and overhead. This invention establishes a long-term fiber optic path length-temperature drift model and innovatively proposes a temperature drift self-correction method, locking the continuous fiber optic path length measurement results to the measured values ​​under initial conditions. It achieves the effect of complex active temperature control by adding only two fiber couplers, and by combining it with active temperature control methods, it can overcome the measurement limitations caused by temperature control limits.

[0033] Third, it reduces sweep frequency nonlinearity errors. Traditional FSI fiber optic measurement methods treat the sweep speed across the entire sweep band as a fixed value, resulting in ideally linear frequency scanning. However, actual measurement systems experience short-term sweep frequency instability, and sweep frequency nonlinearity affects measurement accuracy. The method in this invention constructs a measurement interferometer and a self-reference interferometer. Simultaneously, it is affected by sweep frequency nonlinearity, which is proportional to the optical path length. During temperature drift error correction, the sweep frequency nonlinearity error is offset, reducing the system's sweep frequency nonlinearity error.

[0034] Fourth, the system is simple and low-cost. Traditional active temperature control methods rely on the effectiveness of temperature control for measurement accuracy. Cleanrooms and temperature control chambers limit the application scenarios and portability of the measurement system, and the cost of the temperature control system is high. Furthermore, the temperature control limits further restrict measurement stability. This invention achieves the effect of complex active temperature control by simply adding two fiber optic couplers to a double-sideband FSI measurement system, resulting in a simple and low-cost system. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the overall architecture of the system of the present invention;

[0036] Figure 2 This is a graph showing the measurement results before fiber optic temperature drift correction in an experimental part of this invention.

[0037] Figure 3This is a graph showing the measurement results after fiber optic temperature drift self-correction in part of the experiment in this embodiment of the invention.

[0038] Figure 4 This is a graph showing the measurement results before fiber optic temperature drift correction in Experiment 2 of this embodiment of the invention;

[0039] Figure 5 This is a graph showing the measurement results after fiber optic temperature drift self-correction in Experiment 2 of this embodiment of the invention.

[0040] Figure 6 This is a graph showing the measurement results of the fiber optic temperature drift before correction in Experiment 3 of this invention.

[0041] Figure 7 This is a graph showing the measurement results after fiber optic temperature drift self-correction in Experiment 3 of this embodiment of the invention. Detailed Implementation

[0042] The embodiments and effects of the present invention will be described in further detail below with reference to the accompanying drawings:

[0043] Example 1: Refer to Appendix Figure 1 This invention provides a double-sideband FSI fiber optic measurement system based on temperature drift self-reference correction, comprising: a double-sideband sweep frequency module, a temperature drift self-reference module, and a calculation and correction module; wherein the double-sideband sweep frequency module provides measurement light and reference light for the temperature drift self-reference module and the calculation and correction module, respectively; the temperature drift self-reference module constructs two Michelson interferometer optical paths around the measurement fiber, the optical paths passing through the measurement fiber twice constitute the measurement interferometer, and the optical paths passing through the measurement fiber once constitute the self-reference interferometer, used to correct the temperature drift optical path fluctuation of the same measurement fiber; the multiplexed measurement light carrying fiber length and temperature drift information is output to the calculation and correction module through wavelength division multiplexing;

[0044] The double-sideband sweep frequency module consists of a sweep frequency signal source, a driver amplifier, a narrow linewidth laser, a Mach-Zehnder intensity modulator (MZM), and a first fiber coupler. It is used to generate double-sideband sweep frequency light to offset the short-time amplification measurement error of the fiber.

[0045] The temperature drift self-reference module consists of an optical fiber circulator, a second optical fiber coupler, a third optical fiber coupler, a measurement optical fiber, and an optical fiber end face reflector. Based on the measurement optical fiber, it constructs the optical path of the measurement interferometer and the optical path of the self-reference interferometer to correct the influence of temperature drift on the optical fiber path length.

[0046] The calculation and correction module consists of a 90° optical mixer, two balanced detectors (BPDs), a data acquisition card, and an FPGA. It is used to perform optical digital coherent detection on the input multiplexed measurement light and the reference light, and to calculate the multi-frequency information in real time using the time-shifted Ap-FFT algorithm combined with wavelength division multiplexing to calculate the optical path length to be measured.

[0047] Furthermore, in the aforementioned double-sideband sweep frequency module, a narrow-linewidth laser generates a 1550nm fixed-wavelength optical carrier and outputs it to the MZM modulator; the sweep frequency signal source outputs a periodic sawtooth sweep frequency electrical signal, which is amplified by a driver amplifier and then output to the MZM modulator; the MZM modulator modulates the input sweep frequency electrical signal with the fixed-wavelength optical carrier, and the resulting double-sideband sweep frequency light is then output to the first fiber coupler, which splits the double-sideband sweep frequency light into two paths. One path is used as reference light and output to the 90° mixer in the solution correction module, and the other path is used as measurement light and output to the circulator of the temperature drift self-reference module.

[0048] Furthermore, the aforementioned temperature drift self-reference module includes a fiber optic circulator with three ports. The first port receives the measurement light output from the first fiber optic coupler in the double-sideband sweep frequency module. The second port of the fiber optic circulator transmits the measurement light to the measurement fiber, which is connected to the second fiber optic coupler. The measurement light is split into two paths according to a 5:5 beam split and enters two Michelson interferometer optical paths. In the measurement interferometer optical path, the fiber end face reflector reflects the measurement light back to the second fiber optic coupler, and then transmits it back to the second port of the fiber optic circulator via the measurement fiber. The measurement light is then transmitted to the third fiber optic coupler through the third port. In the self-reference interferometer optical path, the measurement light is directly transmitted from the second fiber optic coupler to the third fiber optic coupler. The third fiber optic coupler receives the two measurement lights and sends them to the 90° mixer in the solution correction module.

[0049] Furthermore, in the aforementioned calculation and correction module, the 90° optical mixer performs interference mixing on the received reference light and the multiplexed measurement light to generate four optical signals with phase differences of 0°, 90°, 180°, and 270°; the balanced detector is used for photoelectric conversion and DC elimination to generate two orthogonal electrical signals with phase differences of 0° and 90°; the acquisition card performs AD sampling to obtain the interference electrical signal; the FPGA integrates a time-shifted Ap-FFT algorithm to process the interference signal, synchronously calculate the phase difference in real time, correct fiber temperature drift, and obtain a stable fiber measurement length.

[0050] Example 2: Refer to Appendix Figure 1 This embodiment provides a method for distance measurement using a double-sideband FSI fiber optic measurement system based on temperature drift self-reference correction, including the following steps:

[0051] Step 1) The sweep frequency signal source generates a sweep frequency electrical signal, which is amplified by the driver and then modulated with a carrier wave generated by a narrow linewidth laser at a Mach-Zehnder intensity modulator (MZM) with carrier suppression on a double-sideband, generating a double-sideband sweep frequency optical signal. The bias voltage is set at the minimum bias point;

[0052] In this embodiment, the meaningless initial phase is ignored, and the double-sideband swept light is... It is expressed as follows:

[0053] ,

[0054] in, For the upper band sweep frequency light, For the lower band sweep frequency light, For dual-side band-sweep optical amplitude, For the frequency of narrow linewidth lasers, The initial frequency of the radio frequency signal source. Let be the scan rate of frequency, and exp represent an exponential function with base e as the natural constant. To represent a complex number, Indicates time.

[0055] Step 2) Double-sided band-sweep optical The light is fed into the first fiber coupler, which splits it into two paths. One path, with 90% of the split light used as a reference beam, is then fed into the first fiber coupler. The light is directly transmitted to a 90° mixer, and 10% of the other light is split off for measurement. Transmitted to the circulator; the first fiber coupler transmits the double-sideband swept light. The spectrophotometer is:

[0056] ,

[0057] in, It is the amplitude value Reference light, It is the amplitude value The measurement light, the reference light and the measurement light satisfy the following conditions: Spectrophotometry.

[0058] Step 3) The temperature drift self-reference module constructs two Michelson interferometer optical paths: a measurement interferometer and a self-reference interferometer. In the measurement interferometer optical path, the measurement light passes through the measurement fiber twice, and in the self-reference interferometer optical path, the measurement light passes through the measurement fiber once. The measurement interferometer and the self-reference interferometer are wavelength divided and multiplexed through a third fiber coupler. The multiplexed measurement light is output to the solution and correction module. The steps are as follows:

[0059] (3a) Measurement light output from the second port of the fiber optic circulator Ignoring the slight power loss and connector loss in fiber optic transmission, the second fiber coupler splits the measurement optical power transmitted through the measurement fiber into two parts: 50% of the power belongs to the measurement interferometer optical path, and 50% belongs to the self-reference interferometer optical path. Divided into , Two parts of the light are incident on the optical paths of the two interferometers:

[0060] ,

[0061] in, It is the amplitude value The measurement interferometer incident measurement light, It is the amplitude value The incident measurement light of the self-reference interferometer satisfies the following conditions: Spectrophotometry.

[0062] (3b) In the optical path of the measuring interferometer, The measurement light is struck by the fiber end-face reflector via the measurement fiber and the second fiber coupler. The fiber end-face reflector reflects the light back to the second fiber coupler, and then it is transmitted back to port 2 of the fiber circulator via the measurement fiber again. The measurement light is then transmitted to the third fiber coupler via port 3. The measurement interferometer returns the measurement light as... ;

[0063] In traditional FSI fiber measurement, fiber temperature drift and strain directly cause optical path changes, and the system's sweep frequency nonlinearity is also equivalent to optical path changes, resulting in significant short-time amplification measurement errors. Optical path fluctuations cause the Doppler effect, manifested as a delay in the optical signal, and the reflected measurement light is superimposed with a Doppler frequency shift. It is expressed as follows:

[0064] ,

[0065] in, To measure the optical time delay corresponding to the relative optical path length in the interferometer, At the speed of light, To measure the initial relative optical path of the interferometer, for The change in optical path relative to time, measured by the interferometer returning the measured light. The expanded representation is as follows:

[0066] ,

[0067] (3c) In the optical path of the self-reference interferometer, The measurement light is transmitted directly from the measuring fiber and the second fiber coupler to the third fiber coupler, and the measurement light returns from the reference interferometer as... , means as follows:

[0068] ,

[0069] The optical delay corresponding to the relative optical path length in a self-referenced interferometer. The initial relative optical path of the self-reference interferometer, for The change in optical path relative to the time, from the reference interferometer back to the measurement light The expanded representation is as follows:

[0070] ,

[0071] (3d) Return to measurement light and The optical fibers are combined and multiplexed at the third fiber coupler and transmitted to the 90° mixer.

[0072] Step 4) The measurement light and the reference light are multiplexed and interfere with each other at the 90° mixer. The interference light is detected by two balanced detectors (BPDs) to obtain orthogonal interference signals containing optical path measurement information. The interference signal was acquired using a data acquisition card. , means as follows:

[0073] ,

[0074] in The DC component generated by interference, and To measure the interference signal generated by frequency sweeping between the interferometer's optical path and the reference optical path, and The optical path can be calculated using the phase information generated by frequency sweeping of the self-reference interferometer optical path and the reference optical path. and To measure the interference signal generated by the mutual interference between the interferometer's optical path and the self-reference interferometer's optical path, the amplitude of the interference signal after photoelectric conversion exhibits the following relationship. This facilitates subsequent phase extraction.

[0075] Step 5) Input the two orthogonal interference signals into the FPGA, process the interference signals using the Ap-FFT algorithm, identify and obtain the phase difference between the two interferometers. The steps are as follows:

[0076] (5.1) For complex signal data within one sweep frequency cycle, the spectrum and phase spectrum of the time-shifted data points are calculated using the time-shifted Ap-FFT algorithm;

[0077] (5.2) According to the spectrum distribution, the high-frequency components are the positive and negative interference frequencies of the measuring interferometer, and the low-frequency components are the positive and negative interference frequencies of the self-reference interferometer;

[0078] (5.3) Extract the phase corresponding to the interference frequencies of the measuring interferometer and the self-reference interferometer;

[0079] (5.4) Accumulate the phases of all time-shifted data points to obtain the upper and lower sideband phase differences between the measurement interferometer and the self-reference interferometer. and ;

[0080] According to the expression of the interference signal Construction of the interception time Phase difference between upper and lower sidebands of internal measurement interferometer Phase difference between the upper and lower sidebands of the self-referenced interferometer :

[0081] ,

[0082] Step 6) Short-time temperature drift amplification error correction: Construct a dynamic measurement formula for double-sideband FSI, substitute the phase difference between the upper and lower sidebands into the formula, correct the temperature drift amplification error, and calculate the real-time optical path of the measuring interferometer. Real-time optical path length of self-referenced interferometer ;

[0083] The traditional FSI measurement formula for single-frequency sweep is:

[0084] ,

[0085] in, For the initial relative optical path, Time period The relative optical path change caused by temperature drift For the sweep frequency phase difference, To account for temperature drift amplification error, under the condition of a center wavelength of 1550nm and a bandwidth of 8GHz, the amplification factor is... A temperature drift of up to 3850 times, or 1μm, can result in a measurement error of 3.85mm.

[0086] The temperature drift amplification error term is canceled out by forward and reverse frequency sweeping, and a dynamic measurement formula for double-sideband FSI is constructed to account for the phase difference between the upper and lower sidebands. and Substituting the values ​​into the formula and correcting for temperature drift amplification errors, the real-time optical path length of the measuring interferometer can be calculated. Real-time optical path length of self-referenced interferometer :

[0087] ,

[0088] Step 7) Long-term fiber temperature drift self-correction: Establish a long-term fiber optical path length-temperature drift model. Based on the fiber temperature drift self-correction principle, derive the temperature drift self-correction formula. Lock the continuous fiber optical path length measurement results to the measurement values ​​under the initial conditions. Obtain stable long-term fiber measurement values ​​based on temperature drift self-correction. ;

[0089] Within one frequency sweep cycle, short-term temperature drift amplification error correction was achieved, enabling accurate measurement of the optical path length. However, under the influence of long-term fiber drift, the optical path length remains unstable and requires correction.

[0090] The relationship between the change in the distance light travels in any medium and temperature drift can be expressed as follows:

[0091] ,

[0092] in, For the distance light travels, This represents the change in distance. For the temperature of the medium, The change in temperature The refractive index of the medium, Thermo-optic coefficient, Let be the coefficient of thermal expansion. The relationship between the change in optical path length and temperature drift can be simplified as follows:

[0093] ,

[0094] Among them, the definition The optical fiber temperature drift coefficient is given by the thermo-optical coefficient of 6.7 × 10⁻⁶. -6 / ℃, coefficient of thermal expansion 5.5×10 -7 The fiber optic temperature drift coefficient is calculated to be 7.25 μm / (m·℃), meaning that a 1℃ temperature drift in 1m of fiber optic path will result in a 7.25μm measurement path drift. The long-term fiber optic path-temperature drift model for the measurement interferometer and the self-reference interferometer is established as follows:

[0095] ,

[0096] in, The initial measurement time, For continuous measurement time, To measure the temperature change of the interferometer, This represents the temperature change of the self-reference interferometer. To measure the temperature drift coefficient of the optical fiber used in the interferometer, This is the temperature drift coefficient of the optical fiber used in the self-reference interferometer.

[0097] Since both the measuring interferometer and the self-reference interferometer use the same measuring fiber, differing only in the number of passes, they satisfy the following within the same measurement time. , The relationship between temperature drift and self-reference correction formula is as follows:

[0098] ,

[0099] Based on the temperature drift self-reference correction formula, the fiber optic measurement system, on the basis of short-term temperature drift amplification error correction, always locks the continuous fiber optical path measurement results to the measured values ​​under the initial conditions (temperature), thus eliminating fiber optical path fluctuations caused by temperature drift and realizing long-term fiber temperature drift self-correction.

[0100] The effectiveness of this invention can be further illustrated by the following experimental results.

[0101] I. Experimental Conditions

[0102] In this experiment, the laser wavelength was 1550nm, the initial frequency of the sweep signal source was 8GHz, the sweep period was 10ms, the sweep bandwidth was 8GHz, the sampling frequency was set to 20MHz, and the sampling time was 0.5s.

[0103] II. Experimental Content and Results

[0104] Experiment 1: Comparative Experiment on Temperature Drift Correction under Active Temperature Control. Under the experimental conditions described above, active temperature control was activated inside the cleanroom, with an air conditioning temperature control accuracy of ±0.1℃. Based on the relationship between optical path length variation and temperature drift, the theoretical measurement error caused by temperature drift over a 150m fiber optic path was calculated to be ±108.75μm, and the theoretical measurement error caused by temperature drift over a 300m fiber optic path was calculated to be ±217.50μm. Figure 2 As shown, the measurement interferometer measures unidirectional transmission in the fiber optic cable with a range of 157.18 μm and a standard deviation of 42.68 μm; the measurement interferometer measures bidirectional round-trip transmission in the fiber optic cable with a range of 309.71 μm and a standard deviation of 80.60 μm. According to the measurement error graph, the error curves of the two interferometers are positively correlated, and the fluctuation of the error curves is related to the active temperature control adjustment of the air conditioner. Figure 3 As shown, the measurement error correction curves of the two interferometers are in excellent agreement. After further self-reference correction calculations, the measurement fiber in the self-reference interferometer has a unidirectional transmission range of 26.70 μm and a standard deviation of 7.62 μm; the measurement fiber in the measurement interferometer has a bidirectional round-trip transmission range of 53.23 μm and a standard deviation of 15.20 μm. The proposed temperature drift self-correction method has a stability that is 6 times higher than that of the active temperature control method.

[0105] Experiment 2: Temperature Drift Correction Comparison Experiment without Active Temperature Control (Heating Up). Under the experimental conditions described above, the cleanroom was set at an active temperature of 20.6℃. After the air conditioning was turned off and the environment was allowed to naturally heat up to 22.5℃, the temperature change was +1.9℃. Based on the relationship between optical path length change and temperature drift, the theoretical measurement error caused by temperature drift in a 150m fiber optic path was calculated to be +2066.25μm, and the theoretical measurement error caused by temperature drift in a 300m fiber optic path was calculated to be +4132.50μm. Figure 4As shown, the measurement of unidirectional transmission in the fiber optic cable using the self-reference interferometer has a range of 1959.11 μm and a standard deviation of 616.37 μm; the measurement of bidirectional round-trip transmission in the fiber optic cable using the measurement interferometer has a range of 3917.88 μm and a standard deviation of 1224.37 μm. According to the measurement error graph, the error curves of the two interferometers are positively correlated. The error curves initially remain stable and then gradually increase, which is related to the fiber optic cable's slow sensing of environmental temperature drift, exhibiting a certain time lag. Figure 5 As shown, the measurement error correction curves of the two interferometers are in excellent agreement. After further self-reference correction calculations, the measurement fiber in the self-reference interferometer has a unidirectional transmission range of 146.54 μm and a standard deviation of 22.81 μm; while the measurement fiber in the measurement interferometer has a bidirectional round-trip transmission range of 292.19 μm and a standard deviation of 45.47 μm. The stability of the proposed temperature drift self-correction method is improved by 13 times.

[0106] Experiment 3: Temperature Drift Correction Comparison Experiment without Active Temperature Control (Cooling). Under the experimental conditions described above, the cleanroom was set at an active temperature of 25.05℃. The air conditioning was then turned off, and the environment was allowed to cool naturally to 24.06℃, a temperature change of -1℃. Based on the relationship between optical path length change and temperature drift, the theoretical measurement error caused by temperature drift in a 150m fiber optic path was calculated to be -1087.50μm, and the theoretical measurement error caused by temperature drift in a 300m fiber optic path was calculated to be -2175μm. Figure 6 As shown, the measurement interferometer for unidirectional transmission of fiber optic cable in the self-reference interferometer has a range of 1069.23 μm and a standard deviation of 345.71 μm; the measurement interferometer for bidirectional round-trip transmission of fiber optic cable in the measurement interferometer has a range of 2092.22 μm and a standard deviation of 693.90 μm. According to the measurement error graph, the error curves of the two interferometers are positively correlated. The error curves initially remain stable and then gradually decrease, which is related to the fiber optic cable's slow sensing of environmental temperature drift, exhibiting a certain time lag. Figure 7 As shown, the measurement error correction curves of the two interferometers are in excellent agreement. After further self-reference correction calculations, the measurement fiber in the self-reference interferometer has a unidirectional transmission range of 58.12 μm and a standard deviation of 12.94 μm; while the measurement fiber in the measurement interferometer has a bidirectional round-trip transmission range of 115.88 μm and a standard deviation of 25.81 μm. The stability of the proposed temperature drift self-correction method is improved by 18 times.

[0107] Based on three sets of comparative experiments on temperature drift correction under different conditions, temperature drift has a significant impact on the stability of long-duration fiber optic measurements, even reaching the millimeter level. Traditional theoretical correction methods are limited by fiber parameters and the hysteresis between fiber temperature drift and ambient temperature, making them unsuitable for measurements on any fiber. The measurement accuracy of traditional active temperature control methods depends on the temperature control effect, and there are temperature control limits due to limitations in measurement scenarios, system complexity, and overhead. The proposed temperature drift self-correction method achieves the effect of complex active temperature control by adding only two fiber couplers. Combining the active temperature control method and the proposed temperature drift self-correction method, it can achieve extremely high measurement accuracy and stability, controlling the measurement range of 300m fiber optical path to below 50μm and the standard deviation to below 20μm.

[0108] The above experiments demonstrate the correctness and effectiveness of the method proposed in this invention.

[0109] The parts of this invention not described in detail are common knowledge to those skilled in the art.

[0110] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Obviously, those skilled in the art, after understanding the content and principle of the present invention, may make various modifications and changes in form and detail without departing from the principle and structure of the present invention. However, these modifications and changes based on the concept of the present invention are still within the scope of protection of the claims of the present invention.

Claims

1. A dual-band FSI optical fiber measurement system based on temperature drift self-referencing correction, characterized in that, The application relates to a fiber-optic temperature drift self-referencing and correction system. The application relates to a fiber-optic temperature drift self-referencing and correction system. The application relates to a fiber-optic temperature drift self-referencing and correction system. The application relates to a fiber-optic temperature drift self-referencing and correction system. The application relates to a fiber-optic temperature drift self-referencing and correction system.

2. The system of claim 1, wherein: The application relates to a fiber-optic temperature drift self-referencing and correction system. The application relates to a fiber-optic temperature drift self-referencing and correction system.

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The system of claim 1, wherein: The solving correction module, wherein the 90° optical frequency mixer interferes and mixes the received reference light and the multiplexed measurement light to generate four light signals with phase differences of 0°, 90°, 180° and 270°; the balanced detector is used for photoelectric conversion and direct current elimination to generate two orthogonal electrical signals with phase differences of 0° and 90°; the acquisition card is used for AD sampling to obtain interference electrical signals; the FPGA integrated with a time-shifted Ap-FFT algorithm is used for processing the interference signals, synchronously and real-timely solving the phase differences, correcting the fiber temperature drift and obtaining a stable fiber measurement length.

5. A measurement method based on the system of claim 1, characterized by, The method comprises the following steps: (1) The frequency-sweep signal source generates a frequency-sweep electrical signal, which is power-amplified by a driver and subjected to carrier-suppressed double-sideband modulation with an optical carrier generated by a narrow-linewidth laser at a Mach-Zehnder intensity modulator (MZM) to generate double-sideband frequency-sweep light , the bias voltage is set at the minimum bias point; (2) Double sideband swept light into a first fiber coupler, through which it is split into two paths, one path splitting 90% as reference light directly transmitted to a 90° mixer, the other path splitting 10% as measurement light transmitted to a circulator; (3) The temperature drift self-reference module constructs two Michelson interferometer optical paths, measures the interferometer and the self-reference interferometer; the measurement light in the measurement interferometer optical path passes through the measurement fiber twice, the measurement light in the self-reference interferometer optical path passes through the measurement fiber once, the measurement interferometer and the self-reference interferometer are wavelength division multiplexed through the third fiber coupler, the multiplexed measurement light is output to the solving correction module, and the implementation steps are as follows: (3.1) The second port of the fiber circulator outputs the measurement light Without considering the weak fiber transmission power loss and joint loss, the second fiber coupler divides the measurement light power transmitted through the measurement fiber into two parts, 50% of which belongs to the measurement interferometer optical path, and 50% of which belongs to the self-reference interferometer optical path, and the measurement light is divided into , two parts and incident on the two interferometer optical paths; (3.2) measuring the optical path of the interferometer, The measured fiber and the second fiber coupler hit the fiber end face reflector, which reflects it back to the second fiber coupler, and is transmitted back to the port of the fiber circulator 2 again through the measured fiber, and the measurement light is transmitted to the third fiber coupler through the 3 port, and the return measurement light of the interferometer is ; (3.3) from the reference interferometer optical path, The measured optical fiber and the second optical fiber coupler are directly transmitted to the third optical fiber coupler, and the reference interferometer returns the measurement light as ; (3.4) return measurement light and The light is combined at a third fiber coupler and transmitted to a 90° hybrid mixer. (4) The measurement light and the reference light are interfered and beat at a 90° mixer, the interference light is detected by two balanced photodetectors (BPDs) to obtain the quadrature interference signal containing the optical path measurement information and collected by a data acquisition card; (5) The two orthogonal interference signals are sent into the FPGA, the Ap-FFT algorithm is used to process the interference signals, the phase differences corresponding to the two interferometers are identified and obtained, and the implementation steps are as follows: (5.1) The frequency spectrum and the phase spectrum of the time-shifted data points are calculated through the time-shifted Ap-FFT algorithm for the complex signal data in one sweep cycle; (5.2) According to the frequency spectrum distribution, the high-frequency components are the positive and negative interference frequencies of the measurement interferometer, and the low-frequency components are the positive and negative interference frequencies of the self-reference interferometer; (5.3) The phases corresponding to the interference frequencies of the measurement interferometer and the self-reference interferometer are extracted; (5.4) Accumulate the phases of all time-shifted data points to obtain the upper and lower sideband phase differences of the measurement interferometer and the self-referenced interferometer and ; (6) Constructing the double sideband FSI dynamic measurement formula, the phase difference of upper and lower sidebands is brought into the formula, the temperature drift amplification error is corrected, and the real-time optical path of the measurement interferometer and the real-time optical path of the self-reference interferometer are solved. (7) The long-time fiber optical path temperature drift model is established, the temperature drift self-correction formula is derived according to the fiber temperature drift self-correction principle, the continuous fiber optical path measurement result is locked to the measurement value under the initial condition, and the long-time fiber stable measurement value is obtained based on the temperature drift self-correction .

6. The method of claim 5, wherein: The double sideband swept light of step (1) is represented as follows ， wherein, is a double sideband swept light is an amplitude of the double sideband swept light, is a frequency of the narrow linewidth laser, is an initial frequency of the radio frequency signal source, is a scan rate of the frequency; exp denotes an exponential function with the natural constant e as base, denotes a complex number, denotes time; the first fiber coupler splits the double sideband swept light : ， wherein, is a reference light having an amplitude , is a measurement light having an amplitude , and a ratio of the amplitude of the reference light to the amplitude of the measurement light satisfies a splitting ratio.

7. The method of claim 5, wherein: the measurement light in the measurement interferometer optical path and the measurement light in the self-referencing interferometer optical path is represented as follows: ， ， wherein c is the speed of light, is the initial relative optical path of the measurement interferometer, is the initial relative optical path of the reference interferometer, is the change in relative optical path at time t, is the initial relative optical path of the measurement interferometer, is the initial relative optical path of the reference interferometer, is the change in relative optical path at time t.

8. The method of claim 5, wherein: Measuring the upper and lower sideband phase difference of the interferometer and self-referenced interferometer of step (5.4) and is represented as follows: ， wherein, is the time taken for the swept segment.

9. The method of claim 5, wherein: The real-time optical path of the interferometer is measured in step (6) and the real-time optical path of the self-referencing interferometer is represented as follows: 。 10. The method of claim 9, wherein: The long-time fiber optical path-temperature drift model of the measurement interferometer and the self-reference interferometer in step (7) is: ， wherein, is the initial measurement time, is the continuous measurement time, is the measured interferometer temperature change amount, is the reference interferometer temperature change amount, is the fiber temperature drift coefficient used by the measurement interferometer, is the fiber temperature drift coefficient used by the reference interferometer; and the temperature drift self-reference correction formula is: 。

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