1. A high-stability dynamic phase demodulation compensation method based on polarization interference and DCM algorithm is characterized by comprising the following steps:

the method comprises the following steps that firstly, a phase demodulation system consisting of a Gaussian light source, an optical fiber circulator, an F-P sensor, an optical fiber beam splitter, a collimating mirror, a polarizer, a birefringent crystal, an analyzer and a photoelectric detector is constructed, light emitted by the Gaussian light source enters the F-P sensor through the optical fiber circulator, a dynamic signal modulates the cavity length of an F-P sensing element, and interference light returned after modulation is introduced into the optical fiber beam splitter;

the optical fiber beam splitter divides the interference light into three paths, the first path and the second path are demodulation interference signal paths, the third path is a reference compensation path, and the first path and the second path are connected to a collimating mirror to collimate the light beam and form a space light beam;

controlling the polarization directions of the polarizer and the analyzer to form an included angle of 45 degrees with the optical axis of the birefringent crystal, and simultaneously ensuring that the polarization directions of the polarizer and the analyzer are mutually vertical;

three spatial beams generate linear polarized light after passing through a polarizer, different optical path differences are introduced through birefringent crystals with different thicknesses, and low-coherence interference fringes are generated in the polarization direction of the spatial beams through a polarization analyzer; the demodulation interference circuits of the first path and the second path obtain low-coherence interference signals which are in an orthogonal relation with each other, and the reference compensation circuit of the third path obtains direct current quantity of the interference signals;

converting the direct current quantities of the two paths of low coherence interference signals and the one path of interference signal into analog signals through a photoelectric detector;

secondly, collecting analog signals and converting the analog signals into digital signals;

thirdly, processing the digital signal to complete normalization of the interference signal and compensation processing of the normalization result, demodulating the signal after compensation processing to obtain phase information of cavity length change, and the step specifically comprises the following processes:

after the direct current quantity is removed, the alternating current item of the interference signal is mapped to [ -1,1] by using the peak value of the signal peak to eliminate the alternating current coefficient, and the signal normalization is completed; the normalized signal expression is:

Q₁(t)＝cos{k₀[2l(t)-dΔn]}

Q₂(t)＝sin{k₀[2l(t)-dΔn]}

demodulating the collected signals to obtain phase information theta (t) of cavity length change, wherein the expression is as follows:

wherein Q is₁(t) is a function expression of the first path of signal after t time normalization, Q₂(t) is a function expression of the second path of signal after t time normalization, Q₁'(t)₁the t time is the differential, Q, of the first path of signal after normalization₂' (t) is the differential of the second signal after normalization at time t, k₀Is the center wavenumber, Z is a constant related to the initial phase value of the system, d is the thickness of the birefringent crystal, Δ n is the difference between the refractive indices of o and e of the birefringent crystal, and l (t) is the cavity length change at time t.

Background

Signals are carriers of information, and accurate sensing and accurate demodulation of signals play a crucial role in human life and military strategies. With the development of the optical fiber sensing technology, the optical fiber F-P sensor can be used for developing hands under extremely severe environment, and can be widely applied to the fields of dynamic detection of deep sea optical cables, aerospace, measurement of partial discharge and the like, and effective signal demodulation algorithms and systems become research hotspots.

At present, the demodulation method of the optical fiber F-P dynamic signal mainly takes an intensity demodulation method and a phase demodulation method as the leading parts. The intensity demodulation method has the advantages of high demodulation speed, simple structure and the like, but the static working point of the sensor is required to be in a linear working area, and the signal acquired by the sensor is distorted due to the cavity length drift. In the phase demodulation method, the DCM demodulation algorithm has formed a relatively complete theory at present, and is very suitable for measuring a large-range dynamic high-frequency signal. Compared with other phase demodulation technologies, the demodulation result obtained by the DCM demodulation algorithm in practical application is more easily interfered by environmental changes, including drift of the cavity length of the sensor, fluctuation of the output power of the light source, thermal interference and mechanical wave vibration in the environment, etc. In 2017, a DCM phase demodulation technology of a dual-wavelength short-cavity optical fiber interferometric acoustic sensor was proposed, however, the demodulation accuracy of the dual-wavelength system is easily affected by the wavelength drift of the light source. In 2019, etima et al propose a fiber Fabry-Perot interferometer based on birefringent crystals and a polarization technology, and utilize a DCM algorithm to demodulate orthogonal signals, so that measurement of large dynamic signals can be realized. Although the interferometer can adapt to the cavity length offset and the light source wavelength drift of the Fabry-Perot sensor, the interferometer has poor light intensity interference resistance. Just because DCM demodulation is very dependent on the stability of strength, this problem greatly limits the practical application of DCM demodulation technology in extreme environments; therefore, the method has profound significance in innovative technical research on how to improve the stability of the DCM demodulation system.

Disclosure of Invention

The invention aims to provide a high-stability dynamic phase demodulation compensation method based on polarization interference and a DCM algorithm, which realizes system demodulation with higher stability by compensating influences caused by light source fluctuation, sensor end face coupling efficiency change and the like.

The invention relates to a high-stability dynamic phase demodulation compensation method based on polarization interference and DCM algorithm, which specifically comprises the following steps:

firstly, constructing a phase demodulation system consisting of a Gaussian light source 1, an optical fiber circulator 2, an F-P sensor 3, an optical fiber beam splitter 4, a collimating mirror 5, a polarizer 6, a birefringent crystal 7, an analyzer 8 and a photoelectric detector 9, wherein light emitted by the Gaussian light source 1 enters the F-P sensor 3 through the optical fiber circulator 2, a dynamic signal modulates the cavity length of the F-P sensing element, and modulated and returned interference light is introduced into the optical fiber beam splitter 4;

the optical fiber beam splitter 4 divides the interference light into three paths, the first path and the second path are demodulation interference signal paths, the third path is a reference compensation path, and the first path and the second path are connected to a collimating mirror 5 to collimate the light beams and form space light beams;

the polarization directions of the polarizer 6 and the analyzer 8 are controlled to form an included angle of 45 degrees with the optical axis of the birefringent crystal, and the polarization directions of the polarizer and the analyzer are ensured to be mutually vertical;

three spatial beams generate linear polarization light after passing through a polarizer 6, different optical path differences are introduced through birefringent crystals 7 with different thicknesses, and low-coherence interference fringes are generated in the polarization direction of the spatial beams through an analyzer 8; the demodulation interference circuits of the first path and the second path obtain low-coherence interference signals which are in an orthogonal relation with each other, and the reference compensation circuit of the third path obtains direct current quantity of the interference signals;

the direct current quantities of the two paths of low coherence interference signals and the one path of interference signal are converted into analog signals through a photoelectric detector 9;

secondly, acquiring an analog signal by using a signal acquisition module 10, and converting the analog signal into a digital signal;

thirdly, processing the digital signal to complete normalization of the interference signal and compensation processing of the normalization result, demodulating the signal after compensation processing to obtain phase information of cavity length change, and the step specifically comprises the following processes:

mapping the interference signal alternating term to [ -1,1] by using the peak value of the signal peak, and completing signal normalization;

the collected signal is mapped into [ -1,1], and the signal expression after the direct current quantity and the alternating current coefficient are removed is as follows:

Q₁(t)＝cos{k₀[2l(t)-dΔn]}

Q₂(t)＝sin{k₀[2l(t)-dΔn]}

demodulating the collected signals to obtain phase information theta (t) of cavity length change, wherein the expression is as follows:

wherein Q is₁(t) is a function expression of the first path of signal after t time normalization, Q₂(t) is a function expression of the second path of signal after t time normalization, Q₁'(t)₁the t time is the differential, Q, of the first path of signal after normalization₂' (t) is the differential of the second signal after normalization at time t, k₀Is the center wavenumber, Z is a constant related to the initial phase value of the system, d is the thickness of the birefringent crystal, Δ n is the difference between the refractive indices of o and e of the birefringent crystal, and l (t) is the cavity length change at time t.

Compared with other optical fiber F-P dynamic phase demodulation algorithms, the method has the advantages that:

1) the method is suitable for eliminating the interference generated by external environment change and system fluctuation in an extreme application environment;

2) the stability of the optical fiber F-P dynamic phase demodulation method based on the DCM algorithm is improved;

3) the real-time compensation of Fabry-Perot (F-P) sensor cavity length demodulation can be realized.

Drawings

FIG. 1 is a schematic overall flow chart of the high-stability dynamic phase demodulation compensation method based on the polarization interference technique and DCM algorithm of the present invention;

FIG. 2 is a schematic diagram of an embodiment of a phase demodulation apparatus;

FIG. 3 is a diagram illustrating the phase demodulation results calculated by the system for compensating the fluctuation of the output power of the front light source from 50mW to 20 mW;

fig. 4 is a schematic diagram of the phase demodulation result calculated by the system after compensation of the output power of the light source from 50mW fluctuation to 20mW fluctuation.

Reference numerals:

1. the device comprises a Gaussian light source, 2, an optical fiber circulator, 3, an F-P sensor, 4, an optical fiber beam splitter, 5, a collimating mirror, 6, a polarizer, 7, a birefringent crystal, 8, an analyzer, 9, a photoelectric detector, 10, a signal acquisition module, 11 and a signal processing module.

Detailed Description

The technical solution of the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

Fig. 1 is a schematic overall flow chart of the high-stability dynamic phase demodulation compensation method based on the polarization interference technique and the DCM algorithm according to the present invention. The method comprises the following specific steps: the method comprises the following steps that firstly, a constructed phase demodulation system is utilized to obtain two paths of low-coherence interference signals and a direct current quantity analog signal of one path of interference signal; secondly, collecting analog signals and converting the analog signals into digital signals; and thirdly, carrying out data processing on the digital signal to complete normalization of the interference signal and compensation processing of a normalization result, and demodulating the acquired signal to obtain phase information of cavity length change.

Fig. 2 is a schematic diagram of an embodiment of a phase demodulating apparatus. In the device, light emitted by a Gaussian light source 1 enters an F-P sensor 3 through an optical fiber circulator 2, the cavity length of a Fabry-Perot sensing element is modulated by dynamic signals, multiple-beam interference is approximately equivalent to double-beam interference due to low reflectivity of the end face of an optical fiber, and the light is modulated and then returns to and is introduced into an optical fiber beam splitter 4. The optical fiber beam splitter 4 splits the interference light into three paths, the first path and the second path are demodulation interference signal paths, the third path is a reference compensation path, and each path is connected to the collimating mirror 5 to perform beam collimation and form a space beam. The polarization directions of the polarizer 6 and the analyzer 8 are controlled to form an included angle of 45 degrees with the optical axis of the birefringent crystal, and meanwhile, the polarization directions of the polarizer 6 and the analyzer 8 are ensured to be mutually vertical. Three spatial light beams generate linear polarization after passing through a polarizer 6, different optical path differences are introduced through three birefringent crystals 7 with different thicknesses, and finally low-coherence interference fringes are generated in the polarization direction of the three spatial light beams through a polarization analyzer 8, so that three demodulation interference signals are correspondingly obtained. Selecting the thickness of the birefringent crystal 7 in the two demodulation interference paths of the first path and the second path to be matched with twice of the cavity length of the sensor, ensuring that the optical path difference of the two paths of signals is pi/2, generating interference fringes near zero optical path difference, and obtaining two paths of orthogonal interference signals;

by utilizing a low coherence interference theory, selecting and using a light source with a Gaussian form, taking an F-P sensor as a sensing element, taking a polarizer, a birefringent crystal and an analyzer as a demodulation interferometer, generating interference fringes near zero optical path difference, and obtaining an interference signal light intensity expression as follows:

wherein, c (k) is a light source function expression with gaussian form, T (k, l) is a low coherent interference intensity function expression, and since the variation of the sensing interference term is small and can be ignored, and the negative first-order low coherent interference fringe cannot be received by the photodetector, the interference signal can be simplified into the form of direct current amount plus alternating current amount.

A is a direct current component of low coherent interference and is only related to a light source function expression and the thickness of the crystal; the alternating current component is approximately represented by first order low coherence interference fringes and is related to the expression of the light source function, the variation of the cavity length of the sensor and the thickness of the crystal. Wherein k is a wave number, k is 2 pi/lambda₀＝2π/λ₀Is the central wave number, k₁＝2π/λ₁，k₂＝2π/λ₂，λ₁Is the minimum wavelength of the light source spectrum, lambda₂Is the maximum wavelength of the light source spectrum, lambda₀Is the center wavelength of the light source,and delta n is the difference between the refractive indexes of o light and e light of the birefringent crystal, d is the thickness of the birefringent crystal, and l is the length of a working cavity of the sensor caused by an external signal.

And selecting the thickness of the birefringent crystal in the third reference compensation path to ensure that the optical path difference is out of the coherent range of the optical path difference introduced by the sensor, so as to obtain the direct current quantity of interference extraction interference signals, and the cavity length of the sensor is modulated by external dynamic signals.

The three paths of output interference signal intensity are expressed as follows:

f₁(t)＝A₁+B₁cos{k₀[2l(t)-dΔn]}

f₂(t)＝A₂+B₂sin{k₀[2l(t)-dΔn]}

f₃＝A₃

wherein A is₁、A₂、A₃Respectively, the direct current components of the first, second and third interference signals, B₁、B₂The amplitudes of the alternating current components of the first path of interference signal and the second path of interference signal are respectively.

The two orthogonal interference signals and the direct current of the interference signals pass through the photoelectric detector 9, and the optical signals are converted into analog electric signals. The signal acquisition module 10 is used to convert the analog electrical signal acquired by the photodetector 9 into a digital signal, and finally the acquired digital signal is transmitted to the signal processing module 11 for data processing. The treatment process comprises the following steps:

on the basis of realizing light power balance, the direct current quantity provided by the third reference compensation circuit can replace the direct current quantity of the two orthogonal signals, and the direct current quantity is subtracted from the two orthogonal signals respectively to obtain an interference signal alternating current term; and mapping the interference signal alternating term to [ -1,1] by using the peak value of the signal peak through a certain operation method to finish signal normalization. The signal expression after removing the direct current quantity and the alternating current coefficient is as follows:

Q₁(t)＝cos{k₀[2l(t)-dΔn]}

Q₂(t)＝sin{k₀[2l(t)-dΔn]}

finally, demodulating the acquired signals by using an improved DCM phase demodulation algorithm to obtain the cavity length phase change:

wherein Q is₁(t) is a function expression of the first path of signal after t time normalization, Q₂(t) is a function expression of the second path of signal after t time normalization, Q₁'(t)₁the t time is the differential, Q, of the first path of signal after normalization₂' (t) is the differential of the second signal after normalization at time t, k₀Is the center wavenumber, Z is a constant related to the initial phase value of the system, d is the thickness of the birefringent crystal, Δ n is the difference between the refractive indices of o and e of the birefringent crystal, and l (t) is the cavity length change at time t.

Through the processing, the interference of the fluctuation of the light source, the coupling efficiency of the sensor, the ambient temperature and other mechanical noises on the signal can be removed, the real-time compensation of the signal is realized, and the phase information theta (t) of the cavity length change of the F-P sensor is obtained.

This embodiment is merely an example, and is not intended to limit the specific structure of the demodulating apparatus. Those skilled in the art can substitute this embodiment with the same demodulation means as those of the related art.

An SLD light source is selected for experiments, the movement of the end face of the optical fiber is controlled through a high-precision nano displacement table, the change of the cavity length of the sensor is simulated, and experimental verification is carried out, specifically referring to fig. 2 and 3. It should be noted that, before the experiment, the optical path needs to be fixed, and each path uses the birefringent crystal with the same thickness to perform optical power compensation, so as to realize that the light intensity ratio of each path in the initial state is 1:1: 1. The change of the output power of the light source is changed from 50mW to 20mW, the end face of the optical fiber is controlled to move by the high-precision nanometer displacement table, and the coupling efficiency of the end face of the optical fiber is continuously changed due to the super-periodic vibration of the cavity length. The nano displacement table is set to move by sine wave with frequency of 20Hz and phase modulation larger than 2 pi, the target cavity length changes by 3 microns, but the actual cavity length changes by 1.99 microns due to the maximum power limitation of the controller. Under the experimental condition, the cavity length of the sensor is demodulated under the excitation of 1566nm of the central wavelength of the light source, 20mW of output power and 50mW of output power.

As shown in fig. 2, the phase demodulation results calculated for compensating the output power of the front light source from 50mW fluctuation to 20mW system are shown schematically. It can be seen that under the correct normalization of 50mW, the phase caused by the cavity length change of the sensor is correctly demodulated; when the optical power fluctuates to 20mW, the DCM demodulation waveform is distorted, the demodulation amplitude is wrong and a resonance phenomenon occurs.

As shown in fig. 3, the calculated phase demodulation result for compensating the output power of the light source from 50mW to 20mW system is shown. When the output power of the light source is 50mW, the phase change caused by the cavity length change of the sensor can be accurately demodulated, and when the output power of the light source fluctuates to 20mW, the phase waveform obtained by demodulation can still keep a true value, namely when the optical power fluctuates to 40% of the original value, the system can still correctly demodulate.

The embodiment can obtain the high-stability dynamic phase demodulation compensation method based on the polarization interference technology and the DCM algorithm, which can well eliminate the interference caused by light source fluctuation and the coupling of the optical fiber end face of the sensor, and the demodulation method has good stability.

Although the existing DCM algorithm solves the problem of the limitation of the working point of a sensor and can realize the measurement of a large-range high-frequency dynamic signal, the DCM is easily interfered by the light intensity generated by the change of an external environment and the fluctuation of a system, and the demodulation is very dependent on the stability of the intensity. The high-stability dynamic phase demodulation compensation method based on the polarization interference technology and the improved DCM algorithm extracts direct current quantity in real time from the angle of the system, independently extracts interference and system fluctuation introduced by the external environment, solves the influence caused by factors such as light source output power fluctuation, sensor coupling efficiency change and the like, and improves the stability of the system; compared with the proposed demodulation algorithm, the method has the advantages of simple operation, higher real-time performance, no low-pass and band-pass filter used in the compensation algorithm, and reduced limitation to the signal detection frequency range.