Fabry-Perot sensor demodulation dispersion compensation device and method based on birefringence interferometer

Abstract

The Fabry-Perot sensor demodulation dispersion compensation device and method based on birefringence interferometer belong to the field of optical fiber sensing; the device includes a broadband light source, a 2×2 coupler, a Fabry-Perot sensor, a computer, a high-speed data acquisition system, a linear array photosensitive device, an analyzer, a birefringent wedge 1, a birefringent wedge 2, a polarizer, a beam collimator and an optical fiber connector output end; the method uses a birefringent wedge 2 with a dispersion slope opposite to that of the birefringent wedge 1 to perform dispersion compensation, so that the fast axis and slow axis phase difference of the polarized light after passing through the birefringent wedge 1 and the birefringent wedge 2 do not change with the wavelength. The present invention has full hardware dispersion, does not consume computing resources, and has a good compensation effect; it can reduce or even eliminate the effects of dispersion on the signal, such as envelope broadening, signal strength reduction, envelope offset, phase offset and fringe period distortion; and has the characteristics of low sensitivity to the installation errors of the two birefringent wedges.

Description

Technical Field

[0001] The invention belongs to the field of optical fiber sensing, and in particular relates to a Fabry-Perot sensor demodulation dispersion compensation device and method based on a birefringence interferometer. Background Art

[0002] In order to evaluate the working status of aircraft engines with complex working conditions in real time, accurately and reliably, and to prevent major safety accidents, it is necessary to understand the air pressure field information of the engine inlet, compressor and combustion chamber in real time, which puts forward more stringent requirements for aerospace pressure sensors. The Fabry-Perot (Fabry-Perot)-based non-intrinsic optical fiber sensor has the advantages of anti-electromagnetic interference and high temperature resistance. Compared with traditional electrical sensors, it is more in line with the sensor requirements of aviation and aerospace engines with complex working conditions. At the same time, in view of the characteristics of high air pressure, high temperature, high requirements for demodulation accuracy and speed in the application of aerospace engines, the optical fiber Fabry-Perot sensor has a temperature coupling structure and the sensor diaphragm has the characteristics of large displacement (the maximum displacement is usually in the range of 10 to 20μm).

[0003] In recent years, with the maturity of fiber optic sensing technology, non-intrinsic Fabry-Perot fiber optic sensors have achieved rapid development. At present, there are relatively rich demodulation methods for this sensing technology, such as intensity demodulation, spectral demodulation, low-coherence interference demodulation, and phase carrier demodulation, all of which can be demodulated by the cavity length of the fiber Fabry-Perot sensor. However, since the above detection technical means are seriously inconsistent with the indicators required by the current application scenarios in terms of range, resolution, demodulation speed, and the combination of static and dynamic cavity length demodulation, it is necessary to study a demodulation technology that takes into account moderate range, fast speed, and the combination of dynamic and static cavity length demodulation.

[0004] In view of the above problems, a low-coherence demodulation method based on Fizeau interferometer has proposed a solution for the demodulation of fiber-optic gas Fabry-Perot sensors for aerospace. The two paths of light that form interference in the Fizeau interferometer are in the same path, and there are no moving parts such as piezoelectric ceramics, so the optical path structure is simple, easy to install and debug, and has good stability; using broadband light source demodulation, the absolute measurement of the sensor cavity length can be achieved, while taking into account the measurement capabilities of static and dynamic cavity lengths. Therefore, it is of great significance to study the low-coherence demodulation method based on Fizeau interferometer.

[0005] Considering that it is extremely difficult to process ordinary wedges, researchers use birefringent wedges instead of ordinary wedges for demodulation. However, birefringent wedges introduce wavelength-dependent dispersion, which in turn causes envelope broadening, signal intensity reduction, envelope shift, phase shift, and fringe period distortion of low-coherence interference fringes. At present, most methods compensate for dispersion from the perspective of signal processing, and the main problems are large computational complexity, time-consuming, and low precision. Summary of the invention

[0006] In order to overcome the problem of reduced demodulation accuracy caused by the dispersion caused by the use of birefringent wedges in the demodulation process of the existing fiber Fabry-Perot pressure sensor, the present invention proposes a hardware dispersion compensation method and device, namely a Fabry-Perot sensor demodulation dispersion compensation device and method based on a birefringence interferometer, which uses two crystal materials with positive and negative slope birefringence dispersion to compensate for the wavelength dependence of the birefringence optical path difference. The device and method have the advantages of good compensation effect, small amount of subsequent algorithm calculation, and high speed.

[0007] Technical solution of the present invention

[0008] A Fabry-Perot sensor demodulation dispersion compensation device based on a birefringence interferometer comprises a broadband light source, a 2×2 coupler, a Fabry-Perot sensor, a computer, a high-speed data acquisition system, a linear array photosensitive device, an analyzer, a first birefringent wedge, a second birefringent wedge, a polarizer, a beam collimator and an optical fiber connector output end. The broadband light source and the 2×2 coupler, the 2×2 coupler and the Fabry-Perot sensor, the 2×2 coupler and the optical fiber connector output end are connected through optical fibers to form a path. The high-speed data acquisition system and the linear array photosensitive device, the computer and the high-speed data acquisition system are connected through cables to form a path. The linear array The photosensitive device, the polarizer, the first birefringent wedge, the second birefringent wedge, the polarizer, the beam collimator, and the optical fiber connector output end form an optical path, which is characterized in that the collimated light after the approximate point light source output light of the optical fiber connector output end passes through the beam collimator covers the polarizer, the first birefringent wedge, the second birefringent wedge, and the polarizer, and the coherent fringes formed are in the photosensitive surface of the linear array photosensitive device, the fast axis or the slow axis of the first birefringent wedge and the second birefringent wedge is at an angle of 45° with the polarization direction of the polarizer, the first birefringent wedge and the second birefringent wedge have opposite dispersion slopes, and the optical and geometric parameters need to satisfy α·d α +β·d β =0 and α·tan(γ α )+β·tan(γ β )=0, where α and β are the dispersion slopes of birefringent wedge 1 and birefringent wedge 2, i.e., the wavelength dependence coefficient of birefringence, d α and d β is the thickness of the smaller end of the wedge perpendicular to the optical axis of the birefringent wedge 1 and birefringent wedge 2 crystals, γ α and γ β is the wedge angle of birefringent wedge 1 and birefringent wedge 2 (29).

[0009] Among them, the collimated light at the output end of the optical fiber connector after passing through the beam collimator is a circular Gaussian light field or a linear collimated light beam.

[0010] Among them, based on the device, a Fabry-Perot sensor demodulation dispersion compensation method based on a birefringence interferometer can be used to achieve dispersion compensation, which is characterized in that the phase difference of the horizontal and vertical components of the polarized light generated by the polarizer changes after being transmitted through the fast axis and slow axis of the birefringent wedge one, and the dispersion slope of the birefringence of the birefringent wedge one material causes differences in the fast axis and slow axis phase differences of different wavelengths. A birefringent wedge two with an opposite dispersion slope to the birefringent wedge one is used for dispersion compensation, so that the fast axis and slow axis phase difference of the polarized light after passing through the birefringent wedge one and the birefringent wedge two does not change with the wavelength.

[0011] The advantages of the present invention are:

[0012] 1. The fast and slow light of the birefringent crystal are used to produce an optical path difference, which reduces the requirements for component processing accuracy compared to ordinary optical wedges.

[0013] 2. Use a birefringent wedge with an opposite birefringence dispersion slope to compensate for the dispersion produced by the wedge, reducing or even eliminating the effects of dispersion on the signal, such as envelope broadening, signal strength reduction, envelope shift, phase shift, and fringe period distortion.

[0014] 3. Using a fast and slow light interferometer consisting of two birefringent wedges, even if there is a small error in the relative position of the two wedges during installation, the impact on the interference fringes is less than the impact on the fringes caused by using a wedge without dispersion compensation function. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is a schematic diagram of a combination of a birefringent wedge 1 and a birefringent wedge 2 having a mutual compensation function;

[0016] In the figure: 11. Birefringent wedge 1, 12. Birefringent wedge 2, 13. Wedge angle γ of birefringent wedge 1 α , 14. Wedge angle γ of birefringent wedge 2 β , 15. The direction of the birefringent optical axis of birefringent wedge 1 and birefringent wedge 2.

[0017] Figure 2 It is a schematic diagram of a Fabry-Perot sensor demodulation dispersion compensation device based on a birefringence interferometer;

[0018] In the figure: 21. broadband light source, 22. 2×2 coupler, 23. Fabry-Perot sensor, 24. computer, 25. high-speed data acquisition system, 26. linear array photosensitive device, 27. polarizer, 28. birefringent wedge 1, 29. birefringent wedge 2, 210. polarizer, 211. beam collimator, 212. optical fiber connector output end. DETAILED DESCRIPTION

[0019] The specific implementation of the present invention is further described in detail below with reference to the accompanying drawings:

[0020] A Fabry-Perot sensor demodulation dispersion compensation device based on a birefringence interferometer comprises a broadband light source 21, a 2×2 coupler 22, a Fabry-Perot sensor 23, a computer 24, a high-speed data acquisition system 25, a linear array photosensitive device 26, an analyzer 27, a birefringent wedge 1 28, a birefringent wedge 29, a polarizer 210, a beam collimator 211 and an optical fiber connector output end 212. The broadband light source 21 and the 2×2 coupler 22, the 2×2 coupler 22 and the Fabry-Perot sensor 23, the 2×2 coupler 22 and the optical fiber connector output end 212 are connected through optical fibers to form a path, the high-speed data acquisition system 25 and the linear array photosensitive device 26, the computer 24 and the high-speed data acquisition system 25 are connected through cables to form a path. The linear array photosensitive device 26, the polarizer 27, the birefringent wedge 28, the birefringent wedge 29, the polarizer 210, the beam collimator 211, and the optical fiber connector output end 212 form an optical path, which is characterized in that the collimated light of the approximate point light source output end 212 of the optical fiber connector passes through the beam collimator 211 and covers the polarizer 27, the birefringent wedge 28, the birefringent wedge 29, and the polarizer 210, and the coherent fringes formed by the collimated light are in the photosensitive surface of the linear array photosensitive device 26, the fast axis or the slow axis of the birefringent wedge 28 and the birefringent wedge 29 is 45° with the polarization direction of the polarizer 210, the birefringent wedge 28 and the birefringent wedge 29 have opposite dispersion slopes, and the optical and geometric parameters need to satisfy α·d α +β·d β =0 and α·tan(γ α )+β·tan(γ β )=0, where α and β are the dispersion slopes of the birefringent wedge 1 28 and the birefringent wedge 29, i.e., the birefringence wavelength dependence coefficients, d α and d β is the thickness of the smaller end of the wedge perpendicular to the optical axis of the birefringent wedge 1 28 and the birefringent wedge 29, α and γ β is the wedge angle of birefringent wedge 1 28 and birefringent wedge 29.

[0021] The collimated light from the optical fiber connector output end 212 after passing through the beam collimator 211 is a circular Gaussian light field or a linear collimated light beam.

[0022] Among them, based on the device, a Fabry-Perot sensor demodulation dispersion compensation method based on a birefringence interferometer can be used to achieve dispersion compensation, which is characterized in that the phase difference of the horizontal and vertical components of the polarized light generated by the polarizer 210 changes after being transmitted through the fast axis and slow axis of the birefringent wedge 28, and the dispersion slope of the birefringence of the birefringent wedge 28 material causes differences in the fast axis and slow axis phase differences of different wavelengths. A birefringent wedge 29 with an opposite dispersion slope to the birefringent wedge 28 is used for dispersion compensation, so that the fast axis and slow axis phase difference of the polarized light after passing through the birefringent wedge 28 and the birefringent wedge 29 does not change with the wavelength.

[0023] The working process of the present invention is as follows:

[0024] The broadband light emitted by the broadband light source enters a port of the coupler and is guided to the optical fiber Fabry-Perot sensor. Under the influence of the outside world, the cavity length of the Fabry-Perot sensor changes, causing the property of the return light propagating in the optical fiber to change. The present invention uses the spectrum of the return light for demodulation. The optical signal of the Fabry-Perot sensor after spectrum modulation enters the demodulation device through the coupler. The demodulation device first collimates the modulated optical signal into a circular Gaussian light field or a linear collimated light beam. The collimated light beam enters the interference device composed of birefringent crystals to generate signal stripes. The signal stripes are detected by a linear array photosensitive device. The optical signal of the Fabry-Perot sensor is converted into an optical image. The optical image is collected by the data acquisition system, and the cavity length demodulation of the Fabry-Perot sensor is completed in the computer.

[0025] The principle of the present invention is as follows:

[0026] When using birefringent crystals, considering the influence of birefringence dispersion and the bandwidth of the light source, the refractive index difference between O light and E light is no longer a constant, but has a certain functional relationship with the wave number of the light source, which can be approximated as a linear relationship, expressed as n α (k) = n α +α(k-k0), where n α is the refractive index difference at the central wavelength of the light source, α is the slope of the birefringent wedge dispersion, k is the wave number of the light source, and k0 is the central wave number of the light source. It is precisely because of the introduction of this functional relationship that the interference fringes formed by the birefringent wedge have envelope broadening, signal intensity weakening, envelope shift, phase shift, and fringe period distortion compared to the interference fringes of the ordinary wedge, which leads to the problem of reduced subsequent demodulation accuracy.

[0027] The present invention uses a birefringent wedge with opposite birefringent dispersion slope to compensate for the problem of decreased demodulation accuracy caused by the above dispersion. Assuming that the refractive index difference at the central wavelength of the compensating wedge is n1 and the dispersion slope of the birefringent wedge is β, the refractive index difference caused by the wedge is n β (k) = n β+β(k-k0), combined with the functional relationship of the refractive index difference of the first birefringent crystal, the optical path difference formed by the overall wedge composed of these two birefringent crystals is s(k)=n α (k) d α +n β (k) d β =n α d α +n β d β +(α·d α +β·d β )(k-k0), where d α and d β It is the thickness of the smaller end of the wedge perpendicular to the optical axis of the two birefringent wedges. In order to make the optical path difference formed by the combined wedge unaffected by the wave number, α·d α +β·d β = 0, and the wedge angles γ of the two wedges α and γ β The relationship tan(γ1) / tan(γ2)=-β / α should be satisfied. At this time, according to the birefringence optical path difference of the two birefringent wedges and the cavity length of the Fabry-Perot sensor to be demodulated, the angle value, narrow side length, wide side length and other parameters of the two birefringent wedges can be designed, that is, s(k)=n α (k) d α +n β (k) d β Match the cavity length of the Fabry-Perot sensor to be demodulated.

[0028] The innovative features and technical effects of the present invention are:

[0029] The optical path difference between fast and slow light of a birefringent crystal is used, which reduces the requirements for component processing accuracy compared to ordinary optical wedges; a birefringent optical wedge with an opposite birefringence dispersion slope is used to compensate for the dispersion generated by the optical wedge, reducing or even eliminating the effects of dispersion on the signal, such as envelope broadening, signal intensity reduction, envelope offset, phase offset, and fringe periodic distortion; full hardware dispersion compensation measures are adopted, which does not consume computing resources and has a good compensation effect; a birefringent interferometer device composed of two birefringent optical wedges is used, so even if a small error occurs in the relative position of the two optical wedges during installation, the impact on the interference fringes is less than the impact on the fringes caused by using an optical wedge without a dispersion compensation function.

Claims

1. A Fabry-Perot sensor demodulation dispersion compensation device based on a birefringence interferometer, comprising a broadband light source (21), a 2×2 coupler (22), a Fabry-Perot sensor (23), a computer (24), a high-speed data acquisition system (25), a linear array photosensitive device (26), an analyzer (27), a birefringent wedge 1 (28), a birefringent wedge 2 (29), a polarizer (210), a beam collimator (211) and an optical fiber connector output end (212), wherein the broadband light source (21) is connected to the 2×2 coupler (22), the 2×2 coupler (22 ) and the Fabry-Perot sensor (23), the 2×2 coupler (22) and the optical fiber connector output end (212) are connected through optical fibers to form a path, the high-speed data acquisition system (25) and the linear array photosensitive device (26), the computer (24) and the high-speed data acquisition system (25) are connected through cables to form a path, the linear array photosensitive device (26), the polarizer (27), the birefringent wedge 1 (28), the birefringent wedge 2 (29), the polarizer (210), the beam collimator (211), and the optical fiber connector output end (212) form an optical path, characterized in that The collimated light of the approximate point light source outputted from the output end (212) of the optical fiber connector passes through the beam collimator (211), and covers the analyzer (27), the first birefringent light wedge (28), the second birefringent light wedge (29), and the polarizer (210). The coherent fringes formed therein are located within the photosensitive surface of the linear array photosensitive device (26). The fast axis or the slow axis of the first birefringent light wedge (28) and the second birefringent light wedge (29) are at an angle of 45° to the polarization direction of the polarizer (210). The first birefringent light wedge (28) and the second birefringent light wedge (29) have opposite dispersion slopes, and the optical and geometric parameters need to satisfy α·d α +β·d β =0 and α·tan(γ α )+β·tan(γ β )=0, where α and β are the dispersion slopes of the birefringent wedge 1 (28) and the birefringent wedge 2 (29), i.e., the birefringence wavelength dependence coefficients, d α and d β is the thickness of the smaller end of the wedge perpendicular to the crystal optical axis of the birefringent wedge 1 (28) and the birefringent wedge 2 (29), γ α and γ β is the wedge angle of birefringent wedge 1 (28) and birefringent wedge 2 (29).

2. The Fabry-Perot sensor demodulation dispersion compensation device based on a birefringence interferometer according to claim 1, characterized in that: The collimated light from the optical fiber connector output end (212) after passing through the beam collimator (211) is a circular Gaussian light field or a linear collimated light beam.

3. A dispersion compensation method for a Fabry-Perot sensor demodulation dispersion compensation device based on a birefringence interferometer as claimed in claim 1, characterized in that: The phase difference of the horizontal and vertical components of the polarized light generated by the polarizer (210) changes after being transmitted through the fast axis and slow axis of the birefringent wedge 1 (28). The dispersion slope of the birefringence of the birefringent wedge 1 (28) material causes the fast axis and slow axis phase differences of different wavelengths to differ. A birefringent wedge 2 (29) having a dispersion slope opposite to that of the birefringent wedge 1 (28) is used to perform dispersion compensation, so that the fast axis and slow axis phase differences of the polarized light after passing through the birefringent wedge 1 (28) and the birefringent wedge 2 (29) do not change with the wavelength.

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