Demodulation stability improving method and system based on primary ellipse fitting

Abstract

The invention discloses a demodulation stability improving method and system based on primary ellipse fitting, and belongs to the technical field of phase demodulation. According to the invention, based on a one-time ellipse fitting technology, an additional low-frequency sinusoidal signal is superposed in laser modulation, so that an interference signal generates a complete elliptical orbit, and an ellipse fitting algorithm is utilized to extract a direct current amount, an alternating current amplitude and a phase difference of fitting parameters in advance. A modulation depth value C is inquired and matched through fitting parameters in combination with a Bessel function table, accurate calibration of key parameters is achieved, then the phase difference of non-strict orthogonal signals is corrected, direct current is removed, and demodulation distortion caused by phase deviation is eliminated. Meanwhile, an intensity disturbance signal B containing light intensity fluctuation and circuit noise is calculated in real time based on the modulation depth value C and the fitting parameter, and is dynamically removed in the demodulation process, so that the intensity disturbance signal B is prevented from being superposed to a sensing signal, and the stability of a demodulation amplitude value is remarkably improved.

Description

Technical Field

[0001] The present invention relates to the technical field of phase demodulation, and particularly to a method and system for improving demodulation stability based on primary ellipse fitting. Background Art

[0002] Optical fiber sensors have advantages such as anti-electromagnetic interference, wide response frequency band, high sensitivity, and large dynamic range, and are widely used in military hydroacoustics, seismic wave detection, coal mine safety detection and other fields. Among various optical fiber sensors, the interferometric optical fiber sensor is based on the structure of an optical fiber interferometer. Through high-sensitivity coherent detection technology, the interference phase is changed by the action of the measured object on the optical fiber, and it has many advantages such as high sensitivity, multiple measured parameters, and low noise. The phase-generated carrier (PGC) modulation and demodulation technology is a relatively classic signal processing method for interferometric optical fiber sensors.

[0003] Real-time ellipse fitting includes external modulation ellipse fitting and internal modulation ellipse fitting. The external modulation ellipse fitting can be divided into two types: the ellipse fitting method based on an external signal and the ellipse fitting method based on environmental noise. The ellipse fitting method based on an external signal is relatively simple, but if a PZT or a large external signal is used to achieve ellipse closure, active devices will be introduced into the interferometer, which is not conducive to realizing a fully fiber-optic probe. Moreover, the large signal will also affect the stability of the system and reduce the system stability. The ellipse fitting method based on environmental noise does not require the application of an external signal or modulation, but there is a certain degree of uncertainty, and neither the accuracy nor the real-time performance can be guaranteed. The internal modulation ellipse fitting does not introduce active devices, but if a modulation signal is applied to the light source in real time, this modulation signal will be directly retained in the demodulation result. If a low-frequency signal is used for modulation, filtering at the demodulation end is difficult, and direct filtering will affect the test signal bandwidth; if a high-frequency signal outside the test bandwidth is used for modulation, signal aliasing may occur, affecting the dynamic range, and a higher sampling rate is required at the demodulation end, which has high requirements for hardware.

[0004] In summary, it is difficult to apply real-time ellipse fitting to the PGC demodulation scheme in practical engineering. A feasible scheme is the primary ellipse fitting scheme, that is, using primary ellipse fitting to pre-obtain orthogonal signal parameters for demodulation. However, in reality, these parameters are all unstable, especially the AC amplitude parameters of the orthogonal signals, including light intensity fluctuations and circuit disturbances. These intensity noise signals are reflected in the amplitude of the demodulation signal in the final signal demodulation, affecting the stability of the demodulation signal amplitude. In the PGC demodulation technology, compared with the PGC-DCM algorithm, although the PGC-Arctan algorithm can eliminate the influence of light intensity fluctuations, when the modulation depth value deviates, the demodulation result will have a significant distortion phenomenon, and there is a signal winding problem. When the amplitude of the signal to be measured exceeds π radians, signal jumps will occur, causing serious distortion of the signal. Summary of the Invention

[0005] In view of the above problems, a method and system for improving demodulation stability based on first-order ellipse fitting are provided. The present invention can suppress the interference of intensity noise signals based on first-order ellipse fitting. An additional low-frequency sine signal modulation is added to the laser modulation, and the orthogonal signal parameters are obtained in advance using the ellipse fitting algorithm. Using these parameters, the modulation depth value is calculated, the phase difference of the orthogonal signal is corrected to obtain a strictly orthogonal signal, and the intensity perturbation signal can be calculated by combining the orthogonal signal and the modulation depth value, and then removed. The technical solution of the invention can use the ellipse fitting algorithm in the measurement of small-amplitude signals, has the advantages of being easy to realize all-fiberization, low cost and low computational complexity, and moreover, removes the intensity perturbation and corrects the phase difference of the orthogonal signal, improving the stability and measurement accuracy of the demodulation system.

[0006] To achieve the above object, the technical solution adopted by the present invention is as follows.

[0007] A method for improving demodulation stability based on first-order ellipse fitting includes the following steps: Step 1: Apply a composite modulation signal including a high-frequency carrier signal and an additional low-frequency sine signal to the laser; Step 2: Obtain an interference signal through an interferometer, and obtain a non-strictly orthogonal signal after mixing and filtering; I x , I y ; Step 3: Perform ellipse fitting on I x , I y to extract the fitting parameters, namely the DC parameter, the AC amplitude parameter and the phase difference corresponding to the non-strictly orthogonal signal; ; Step 4: Calculate J 1( C ) / J 2( C ), and match the modulation depth value of the high-frequency carrier signal using a Bessel function table; C ; Step 5: Stop the additional low-frequency modulation of the additional low-frequency sine signal to the laser, remove the DC component of the non-strictly orthogonal signal, and correct the phase difference to obtain a strictly orthogonal signal V x , V y ; Step 6: Combine the modulation depth value C to calculate the intensity perturbation signal B , and for V x , Vy Perform normalization processing to obtain a normalized signal V 1、 V 2; Step 7: Perform PGC demodulation and high-pass filtering on the normalized signal, and output the sensing signal 。

[0008] Preferably, the high-frequency carrier signal in step 1 is much higher than the highest frequency of the sensing signal ; the amplitude of the additional low-frequency sine signal is greater than π / 2, and at the same time is much lower than the high-frequency carrier signal and avoids the corresponding frequency band of the sensing signal ; the additional low-frequency sine signal is 1 - 50 Hz.

[0009] Preferably, the interferometer in step 2 uses a Mach-Zehnder interferometer, a Michelson interferometer or a Sagnac interferometer.

[0010] Preferably, after the interference signal enters the multiplier, it is mixed with a first-harmonic carrier signal and a second-harmonic carrier signal respectively, and then a non-strictly orthogonal signal is obtained through low-pass filtering I x 、 I y 。

[0011] Preferably, the non-strictly orthogonal signals I x 、 I y are expressed as: where h and k are DC parameters, a and b are AC amplitude parameters, is the phase difference between the non-strictly orthogonal signals; includes an additional low-frequency modulation signal 、the sensing signal and the external environmental noise ; at the same time, the AC amplitude parameters satisfy the following relationship: where B is the intensity perturbation signal, C is the modulation depth value of the high-frequency carrier signal.

[0012] Preferably, the solution steps of the modulation depth value C in step 4 are as follows: Step S1: Calculate J 1( C ) / J2( C ): wherein, a and b are AC amplitude parameters, and is the phase difference between the non-strictly orthogonal signals; Step S2: Construct a Bessel function table that maps the modulation depth value J 1( C ) / J 2( C ) according to the formula in Step S1; C and J 1( C ) / J 2( C ). Step S3: Look up and match the corresponding modulation depth value J 1( C ) / J 2( C ) calculated from the subsequent AC amplitude parameters and phase differences. C .

[0013] Preferably, the strictly orthogonal signals V x , V y in Step 5 are expressed as:

[0014] Preferably, the steps for solving the normalized signals V 1, V 2 in Step 6 are as follows: Step S1: Calculate the intensity perturbation signal B : Step S2: Remove the modulation depth value V x , V y from the strictly orthogonal signals C and the intensity perturbation signal B to obtain the normalized signals V 1, V 2 after normalization:

[0015] Preferably, the PCG demodulation and solution steps for the sensing signal are as follows: Step S1: For the normalized signals V 1,V Derivation of 2 respectively gives and : Step S2: Perform cross - multiplication and subtraction operations on the and respectively to obtain V 3: Step S3: Integrate both sides of V 3 to obtain V 4: Step S4: Filter out the external environmental noise through high - pass filtering to obtain the sensing signal .

[0016] An optical fiber sensing system includes a modulation signal generator, a laser, a circulator, a Michelson interferometer, a photodetector, a data acquisition module, and a signal processor; the Michelson interferometer includes a coupler, a Faraday rotator mirror 1, and a Faraday rotator mirror 2; the modulation signal generator applies a composite modulation signal to the laser, the composite modulation signal includes a high - frequency carrier signal used as a carrier signal for PCG demodulation and an additional low - frequency sine signal used for phase modulation to achieve a first - order ellipse fitting, the laser outputs the modulated laser, which successively enters the circulator and the coupler and is split into sensing light and reference light, the sensing light and the reference light respectively enter the sensing arm and the reference arm of the Michelson interferometer, and after being reflected by the Faraday rotator mirror 1 and the Faraday rotator mirror 2, they are output to the coupler to form interference light, the interference light enters the photodetector through the circulator, is converted into an interference signal by the photodetector, collected by the data acquisition module and output to the signal processor, and the signal processor performs demodulation processing using the demodulation stability improvement method as defined in any one of claims 1 - 9, and finally outputs the sensing signal .

[0017] Due to the above - mentioned technical solutions, the present invention has the following beneficial effects.

[0018] (1) Based on the first - order ellipse fitting, the present invention adds an additional low - frequency sine signal modulation to the laser modulation, combines the ellipse fitting algorithm to pre - obtain the orthogonal signal parameters in advance, and calculates the modulation depth value using the obtained orthogonal signal parameters C , corrects the phase difference to obtain a strictly orthogonal signal, and the strictly orthogonal signal combines the modulation depth value C to calculate the intensity perturbation signal B, and then remove it. The technical solution of this invention can use the ellipse fitting algorithm when measuring small-amplitude signals, and has the advantages of being easy to realize all-fiberization, low cost, and low computational complexity. Moreover, it removes the intensity perturbation signal and corrects the phase difference of the orthogonal signal , improving the stability and measurement accuracy of the demodulation system.

[0019] (2) This invention realizes ellipse fitting through the internal modulation method (directly loading a low-frequency signal on the laser). By directly loading a low-frequency signal on the laser to replace traditional active devices such as external PZTs, there is no need to externally connect active devices such as PZTs. While avoiding system complexity, it realizes an all-fiberized architecture and significantly reduces the hardware cost. In addition, this invention calculates the ellipse fitting parameters once in real time and removes the intensity perturbation signals (light intensity fluctuations, circuit noise), eliminating their influence on the amplitude of the demodulated signal; corrects the phase difference of the orthogonal signal to be strictly orthogonal, avoiding demodulation distortion caused by phase deviation. At the same time, combined with the Bessel function table query mechanism, the real-time calculation load is reduced, making it possible to stably measure small-amplitude signals. Experiments show that this method effectively eliminates the double interference of intensity perturbation and phase deviation, and the amplitude stability of the demodulated signal is significantly improved from 12% of the conventional scheme to 2.46%. The dynamic range and signal-to-noise ratio are optimized synchronously, providing a demodulation solution with both high efficiency and reliability for high-precision sensing fields such as seismic wave detection and hydrophones.

[0020] (3) This invention subdivides the matching accuracy of the modulation depth value through the Bessel function table to ensure the reliability of parameter calibration and improve the dynamic range and measurement sensitivity; compared with the conventional single ellipse fitting scheme, the amplitude stability of the demodulated signal of this invention is significantly improved (optimized from 12.00% to 2.46%, and the experimental verification duration is 2 hours), and the Lissajous figure noise is lower.

[0021] (4) This invention can directly apply the ellipse fitting algorithm in the measurement of small-amplitude signals without relying on large-signal excitation, expanding the applicable scenarios. At the same time, the additional low-frequency modulation signal is outside the test bandwidth, avoiding aliasing with the high-frequency carrier signal; at the demodulation end, the sensing signal and the modulation residual noise are effectively separated through high-pass filtering to ensure signal integrity.

[0022] (5) This invention solves the stability problems in traditional PGC demodulation caused by the non-ideality of orthogonal signals, deviation of modulation depth, and intensity perturbation through the ellipse fitting parameter correction and intensity noise signal suppression technology, and has the advantages of all-fiberization, high precision, and low computational complexity, and is applicable to fields such as seismic wave detection and hydrophones that have strict requirements for demodulation stability.

[0023] (6) Compared with the traditional PGC-DCM demodulation scheme and the conventional first-order ellipse fitting scheme, the present invention corrects the phase difference of the orthogonal signals, removes the influence of the intensity perturbation signal, and improves the stability and measurement accuracy of the demodulation system. In addition, compared with the real-time ellipse fitting scheme, the present invention does not require the real-time application of external modulation or internal modulation signals, and has the advantages of being easy to realize all-fiberization, low cost, and low computational complexity. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The following details the fabrication and application of the preferred embodiments of the present invention. It should be understood, however, that the present invention provides many applicable inventive concepts that can be embodied in various specific environments. The specific embodiments discussed are merely illustrative of the specific ways to make and use the present invention and do not limit the scope of the present invention. Those of ordinary skill in the art can obtain other drawings based on these drawings without creative efforts.

[0025] Figure 1 It is a flowchart of the method for improving the demodulation stability of the present invention.

[0026] Figure 2 It is a schematic structural diagram of the present invention.

[0027] Figure 3 It is a comparison diagram of Lissajous figures between the present invention and the conventional first-order ellipse fitting.

[0028] Figure 4 It is a comparison diagram of the demodulation amplitude stability between the present invention and the conventional first-order ellipse fitting.

[0029] Among them, 1 - modulation signal generator; 2 - laser; 3 - circulator; 4 - coupler; 5 - Faraday rotator mirror 1; 6 - Faraday rotator mirror 2; 7 - photodetector; 8 - data acquisition module; 9 - signal processor. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] The following details the fabrication and application of the preferred embodiments of the present invention. It should be understood, however, that the present invention provides many applicable inventive concepts that can be embodied in various specific environments. The specific embodiments discussed are merely illustrative of the specific ways to make and use the present invention and do not limit the scope of the present invention.

[0031] The present invention is based on the first-order ellipse fitting technology. By superimposing an additional low-frequency sine signal on the modulation of the laser 2, a complete elliptical trajectory is generated for the interference signal, and the direct current value, alternating current amplitude, and phase difference of the fitting parameters are pre-extracted using the ellipse fitting algorithm. The modulation depth value is queried and matched through the fitting parameters in combination with the Bessel function table. C, achieve precise calibration of key parameters, thereby correcting the phase difference of non-strictly orthogonal signals and removing the DC component to eliminate demodulation distortion caused by phase shift. At the same time, based on the modulation depth value C and the fitting parameters, calculate in real time the intensity perturbation signal containing light intensity fluctuations and circuit noise B , and dynamically remove it during the demodulation process to avoid its superposition on the sensing signal, significantly improving the stability of the demodulation amplitude.

[0032] The following will be further elaborated in conjunction with the attached Figures 1-4 drawings.

[0033] As Figure 1 shown, a method for improving demodulation stability based on first-order ellipse fitting includes the following steps: Step 1: Apply a composite modulation signal to the laser 2. The composite modulation signal includes a high-frequency carrier signal used as a carrier signal for PCG demodulation and an additional low-frequency sine signal used for phase modulation to achieve first-order ellipse fitting. The high-frequency carrier signal is much higher than the highest frequency of the sensing signal. In this embodiment, the high-frequency carrier signal is 100 kHz; the amplitude of the additional low-frequency sine signal is greater than π / 2, and at the same time, it is much lower than the high-frequency carrier signal and avoids the corresponding frequency band of the sensing signal to prevent the additional low-frequency sine signal from being superimposed on the sensing signal to be measured. In this embodiment, the additional low-frequency sine signal is 1 - 50 Hz.

[0034] Step 2: Obtain an interference signal through an interferometer, and obtain non-strictly orthogonal signals I x , I y after mixing and filtering, that is, the interference signal enters a multiplier and is mixed with a first-harmonic carrier signal and a second-harmonic carrier signal respectively, and then non-strictly orthogonal signals I x , I y are obtained through low-pass filtering; the interferometer uses a Mach-Zehnder interferometer, a Michelson interferometer or a Sagnac interferometer. In this embodiment, the interferometer uses a Michelson interferometer.

[0035] The non-strictly orthogonal signals I x , I y are expressed as: where h and k are DC parameters, a and b are AC amplitude parameters, is the phase difference between the non-strictly orthogonal signals; Including additional low frequency modulation signal , sensor signal and external environmental noise At the same time, the AC amplitude parameter satisfies the following relationship: in B is the intensity disturbance signal, C is the modulation depth value of the high frequency carrier signal.

[0036] Step 3: Right I x , I y Perform ellipse fitting to extract fitting parameters, namely DC parameters, AC amplitude parameters and phase difference corresponding to the non-strictly orthogonal signal .

[0037] Step 4: Calculate based on the fitting parameters J 1( C ) / J 2( C ), and use the Bessel function table to match the modulation depth value of the high frequency carrier signal C ; The modulation depth value C The steps to solve are as follows: Step S1: Calculation J 1( C ) / J 2( C ): in, a and b is the AC amplitude parameter, is the phase difference between the non-strictly orthogonal signals.

[0038] Step S2: According to step S1 J 1( C ) / J 2( C ) formula to construct the modulation depth value C and J 1( C ) / J 2( C ) values correspond to the mapped Bessel function table.

[0039] Step S3: The following AC amplitude parameters and phase difference are calculated. J 1( C ) / J 2( C ) value to find and match the corresponding modulation depth value C .

[0040] Step 5: Stop the additional low-frequency modulation of the additional low-frequency sine signal to the laser 2, remove the DC component of the non-strictly orthogonal signal, and correct the phase difference to obtain a strictly orthogonal signal V x 、 V y ; The strictly orthogonal signal V x 、 V y is expressed as:

[0041] Step 6: Combine the modulation depth value C to calculate the intensity perturbation signal B , and perform normalization processing on V x 、 V y to obtain the normalized signal V 1、 V 2; The solution steps of the normalized signals V 1、 V 2 are as follows: Step S1: Calculate the intensity perturbation signal B :

[0042] Step S2: Remove the modulation depth value V x 、 V y from the strictly orthogonal signals C and the intensity perturbation signal B to obtain the normalized signals V 1、 V 2 after normalization processing:

[0043] Step 7: Perform PGC demodulation and high-pass filtering on the normalized signals, and output the sensing signal . The PGC demodulation and solution steps of the sensing signal are as follows: Step S1: Differentiate the normalized signals V 1、 V 2 respectively to obtain 、 :

[0044] Step S2: For the 、 Perform cross - multiplication and subtraction operations separately to obtain V 3:

[0045] Step S3: Integrate both sides of V 3 to obtain V 4:

[0046] Step S4: Filter out the external environmental noise through high - pass filtering , and obtain the sensing signal .

[0047] As Figure 2 shown, an optical fiber sensing system includes a modulation signal generator 1, a laser 2, a circulator 3, a Michelson interferometer, a photodetector 7, a data acquisition module 8, and a signal processor 9; the Michelson interferometer includes a coupler 4, a Faraday rotator 5, and a Faraday rotator 6; the modulation signal generator 1 applies a composite modulation signal to the laser 2, the composite modulation signal includes a high - frequency carrier signal for PCG demodulation as a carrier signal and an additional low - frequency sine signal for phase modulation to achieve primary ellipse fitting, the laser 2 outputs the modulated laser, which successively enters the circulator 3 and the coupler 4 and is split into sensing light and reference light, the sensing light and the reference light respectively enter the sensing arm and the reference arm of the Michelson interferometer, and after being reflected by the Faraday rotator 5 and the Faraday rotator 6, they are output to the coupler 4 to form interference light, the interference light enters the photodetector 7 through the circulator 3, is converted into an interference signal by the photodetector 7, collected by the data acquisition module 8 and output to the signal processor 9, and the signal processor 9 performs demodulation processing using the above - mentioned demodulation stability improvement method, and finally outputs the sensing signal .

[0048] The following combines Figures 1-4 for further detailed elaboration.

[0049] As Figure 2An optical fiber sensing system as shown includes a modulation signal generator 1, a laser 2, a circulator 3, a Michelson interferometer, a photodetector 7, a data acquisition module 8, and a signal processor 9; the interferometer uses a Mach-Zehnder interferometer, a Michelson interferometer, or a Sagnac interferometer. In this embodiment, the interferometer uses a Michelson interferometer. The Michelson interferometer includes a coupler 4, a Faraday rotator 1 5, and a Faraday rotator 2 6; the modulation signal generator 1 applies a composite modulation signal to the laser 2, and the composite modulation signal includes a high-frequency carrier signal for PCG demodulation as a carrier signal and an additional low-frequency sine signal for phase modulation to achieve a first-order ellipse fitting. The high-frequency carrier signal is much higher than the highest frequency of the sensing signal. In this embodiment, the high-frequency carrier signal uses 100 kHz; the amplitude of the additional low-frequency sine signal is greater than π / 2, much lower than the high-frequency carrier signal, and avoids the corresponding frequency band of the sensing signal to prevent the additional low-frequency sine signal from being superimposed on the sensing signal to be measured. In this embodiment, the additional low-frequency sine signal is 1 to 50 Hz.

[0050] The modulated laser output by the laser 2 enters the circulator 3 and the coupler 4 in sequence and is split into sensing light and reference light. The sensing light and the reference light enter the sensing arm and the reference arm of the Michelson interferometer respectively, and after being reflected by the Faraday rotator 1 5 and the Faraday rotator 2 6, they are output to the coupler 4 to form interference light. The interference light enters the photodetector through the circulator 3, is converted into an interference signal by the photodetector, and is collected by the data acquisition module 8 and output to the signal processor 9. The signal processor 9 uses Figure 1 The demodulation stability improvement method based on first-order ellipse fitting as described is used for demodulation processing, and finally the sensing signal is output .

[0051] The interference signal enters the multiplier and is mixed with the first-harmonic carrier signal and the second-harmonic carrier signal respectively, and then the non-strictly orthogonal signal is obtained through low-pass filtering I x , I y : where h and k are DC parameters, a and b are AC amplitude parameters, is the phase difference between the non-strictly orthogonal signals; includes an additional low-frequency modulation signal , the sensing signal and the external environmental noise .

[0052] In addition, the AC amplitude parameter satisfies the following relationship: where B is the intensity perturbation signal, C is the modulation depth value of the high-frequency carrier signal.

[0053] The non-strictly orthogonal signal can be pre-obtained with fitting parameters h , k , a , b , and values through ellipse fitting. Based on the fitting parameters, calculate J 1( C ) / J 2( C ), that is, the fitting parameters a , b , and C satisfy the following relationship:

[0054] According to J 1( C ) / J 2( C ) formula, construct the Bessel function table corresponding to the mapping of the modulation depth value C and J 1( C ) / J 2( C ) values. From the fitting parameters a , b、 of the subsequent ellipse fitting, the J 1( C ) / J 2( C ) ratio can be obtained. Substitute this ratio into the Bessel function table to find the closest C value, so as to obtain the modulation depth C value. The accuracy of obtaining the C value depends on the subdivision degree of the Bessel function table.

[0055] After obtaining the ellipse parameters, stop the additional low-frequency modulation of the laser 2, remove the DC component of the non-strictly orthogonal signal, and correct the phase difference to obtain the strictly orthogonal signal V x , V y ; The strictly orthogonal signal V x , V y is expressed as:

[0056] Combine the modulation depth value C Calculate the intensity perturbation signal B , the intensity perturbation signal can be obtained B :

[0057] For the strictly orthogonal signal V x , V y Perform the removal of the modulation depth value C and the intensity perturbation signal B , and obtain the normalized signal after normalization processing V 1,[[]]END]] V 2:

[0058] Derive the normalized signals V 1,[[]]END]] V 2 respectively, and obtain , :

[0059] Then perform cross multiplication and subtraction operations on the , respectively to obtain V 3:

[0060] Then integrate both sides of V 3 to obtain V 4:

[0061] Finally, filter out the external environmental noise through high-pass filtering , and obtain the sensing signal .

[0062] The conventional first-order ellipse fitting scheme directly normalizes the orthogonal signal by using the orthogonal signal parameters obtained by ellipse fitting in advance. It treats the orthogonal signal AC amplitude parameter as a fixed parameter, so it will be affected by the intensity perturbation signal during the demodulation process. In contrast, the present invention calculates the intensity perturbation in real time by using the orthogonal signal parameters of the first-order ellipse fitting scheme and removes it. The comparison diagram of the Lissajous figures formed by the normalized orthogonal signals of these two schemes is as shown in Figure 3 . It can be seen that due to the removal of the intensity perturbation signal, the Lissajous figure of the present invention has lower noise.

[0063] The above orthogonal signals are subjected to differential, cross - multiplication, and integration operations in PGC - DCM demodulation, and then the external environmental noise is filtered out by a high - pass filter. The required sensing signal can then be obtained. . The comparison chart of the amplitude stability of the sensing signals demodulated by the present invention and the conventional first - order ellipse fitting scheme in 2 - hour demodulation is as Figure 4 shown. Among them, the stability is obtained according to (maximum sensing signal amplitude - minimum sensing signal amplitude) / average sensing signal amplitude. From Figure 4 it can be seen that for a 2 - hour demodulation duration, the demodulation amplitude stability of the present invention is 2.46%, and its stability is far better than the 12.00% of the demodulation amplitude stability of the conventional first - order ellipse fitting scheme.

[0064] Compared with the traditional PGC - DCM demodulation scheme and the conventional first - order ellipse fitting scheme, the present invention corrects the phase difference of the orthogonal signals, removes the influence of intensity perturbation, and improves the stability and measurement accuracy of the demodulation system; compared with the real - time ellipse fitting scheme, the present invention does not require real - time application of external or internal modulation signals, and has the advantages of being easy to realize all - fiberization, low cost, and low computational complexity.

[0065] The present invention proposes a method for improving the stability of phase - generated carrier (PGC) demodulation based on first - order ellipse fitting, which suppresses the interference of intensity noise signals on the basis of first - order ellipse fitting. An additional low - frequency sine signal modulation is added to the modulation of the laser 2, and the orthogonal signal parameters are pre - obtained using the ellipse fitting algorithm. Using these parameters, the modulation depth value C is calculated, the phase difference of the orthogonal signals is corrected to obtain a strictly orthogonal signal. The orthogonal signal combined with the modulation depth value can C calculate the intensity perturbation signal B , and then remove it. The technical solution of the present invention can use the ellipse fitting algorithm in the measurement of small - amplitude signals, has the advantages of being easy to realize all - fiberization, low cost, and low computational complexity, and also removes the intensity perturbation signal and corrects the phase difference of the orthogonal signals, improving the stability and measurement accuracy of the demodulation system.

[0066] Although the specification has been described in detail, it should be understood that various changes, substitutions, and alterations can be made without departing from the spirit and scope of the present invention defined by the appended claims. In addition, the specific embodiments described do not limit the scope of the present invention. Those of ordinary skill in the art can easily understand based on the present invention that currently existing or future - to - be - developed processes, machines, manufactures, compositions of matter, means, methods, or steps can perform functions substantially the same as those of the embodiments of the present invention or obtain results substantially the same. Therefore, the appended claims are intended to include such processes, machines, manufactures, compositions of matter, means, methods, or steps within their scope.

Claims

1. A method for improving demodulation stability based on first-order ellipse fitting, characterized in that: It includes the following steps: Step 1: Apply a composite modulation signal including a high-frequency carrier signal and an additional low-frequency sine signal to the laser; Step 2: Obtain an interference signal through an interferometer, and obtain a non-strictly orthogonal signal after mixing and filtering I x , I y ; Step 3: For I x , I y perform elliptical fitting, and extract the fitting parameters, i.e., the DC parameter, AC amplitude parameter, and phase difference corresponding to the non-strictly orthogonal signal ; Step 4: Calculate based on the fitting parameters J 1( C ) / J 2( C ), and match the modulation depth value of the high-frequency carrier signal by using a Bessel function table C ; Step 5: Stop applying additional low-frequency modulation of the additional low-frequency sine signal to the laser, remove the DC component of the non-strictly orthogonal signal, and correct the phase difference to obtain a strictly orthogonal signal V x , V y ; Step 6: Combine the modulation depth value C Calculate the intensity perturbation signal B , and for V x , V y Perform normalization processing to obtain the normalized signal V 1,[[]] V 2; Step 7: Perform PGC demodulation and high-pass filtering on the normalized signal, and output the sensing signal .

2. The demodulation stability improvement method based on primary ellipse fitting according to claim 1, characterized in that: The high-frequency carrier signal in the said step 1 is much higher than the highest frequency of the sensing signal ; the amplitude of the additional low-frequency sine signal is greater than π / 2, while much lower than the high-frequency carrier signal and avoiding the corresponding frequency band of the sensing signal ; the additional low-frequency sine signal is 1 to 50 Hz.

3. A method for improving demodulation stability based on primary ellipse fitting according to claim 1, characterized in that: The interferometer in Step 2 uses a Mach-Zehnder interferometer, a Michelson interferometer or a Sagnac interferometer.

4. A method for improving the demodulation stability based on primary ellipse fitting according to claim 1, characterized in that: After the interference signal enters the multiplier, it is mixed with the first harmonic carrier signal and the second harmonic carrier signal respectively, and then the non-strictly orthogonal signal is obtained through low-pass filtering. I x 、 I y 。 5. The demodulation stability improvement method based on primary ellipse fitting according to claim 4, characterized in that: The non-strictly orthogonal signal I x , I y is expressed as: Among them h and k are DC parameters, a and b are AC amplitude parameters, is the phase difference between the non-strictly orthogonal signals; includes an additional low-frequency modulation signal , a sensing signal and ambient noise ; meanwhile, the AC amplitude parameters satisfy the following relationship: Among them B is the intensity perturbation signal C is the modulation depth value of the high-frequency carrier signal 6. The demodulation stability improvement method based on primary ellipse fitting according to claim 1, characterized in that: The modulation depth value of step 4 C is solved as follows: Step S1: Calculate J 1( C ) / J 2( C ): Among them, a and b are AC amplitude parameters, is the phase difference between the non-strictly orthogonal signals; Step S2: According to J 1( C ) / J 2( C ) formula to construct the modulation depth value C and J 1( C ) / J 2( C ) value corresponding to the mapped Bessel function table; Step S3: Obtained through subsequent calculations of the AC amplitude parameter and the phase difference J 1( C ) / J 2( C ) value lookup and match the corresponding modulation depth value C .

7. The demodulation stability improvement method based on primary ellipse fitting according to claim 1, characterized in that: The strictly orthogonal signal in step 5 V x , V y is expressed as:

8. A method for improving demodulation stability based on primary ellipse fitting according to claim 1, characterized in that: The normalized signal in step 6 V 1. V The solution steps of 2 are as follows: Step S1: Calculate the intensity perturbation signal B :[[]] Step S2: Demodulation depth value V x and V y are removed from the strictly orthogonal signal, and intensity perturbation signal C is removed, to obtain a normalized signal B after normalization processing: V 1, V 2:

9. A method for improving the demodulation stability based on primary ellipse fitting as claimed in claim 5, wherein: The sensing signal The PCG demodulation and solution steps are as follows: Step S1: For the normalized signal V 1, V Deriving 1 and 2 respectively gives , : Step S2: For the , perform cross - multiplication and subtraction operations respectively to obtain V 3: Step S3: Integrate both sides of V 3 to obtain V 4: Step S4: Filter out the external environmental noise through high-pass filtering , and obtain the sensing signal .

10. An optical fiber sensing system, characterized in that: It includes a modulation signal generator, a laser, a circulator, a Michelson interferometer, a photodetector, a data acquisition module and a signal processor; the Michelson interferometer includes a coupler, a Faraday rotator mirror 1 and a Faraday rotator mirror 2; the modulation signal generator applies a composite modulation signal to the laser, the composite modulation signal includes a high-frequency carrier signal for PCG demodulation as a carrier signal and an additional low-frequency sine signal for phase modulation to achieve primary ellipse fitting, the modulated laser output by the laser sequentially enters the circulator and the coupler and is split into sensing light and reference light, the sensing light and the reference light respectively enter the sensing arm and the reference arm of the Michelson interferometer, and after being reflected by the Faraday rotator mirror 1 and the Faraday rotator mirror 2, they are output to the coupler to form interference light, the interference light enters the photodetector after passing through the circulator, is converted into an interference signal by the photodetector, collected by the data acquisition module and output to the signal processor, and the signal processor performs demodulation processing using the demodulation stability improvement method described in any one of claims 1-9, and finally outputs a sensing signal .

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