A birefringent crystal demodulation system and method for fiber-optic fabry-perot sensor cavity length

Abstract

The present application relates to a fiber-optic F-P sensor cavity length demodulation system and method, in particular to a fiber-optic F-P sensor cavity length birefringent crystal demodulation system and method, which is used to solve the problem that in the traditional fiber-optic F-P sensor space scanning type low-coherence interference demodulation system, light passes through a fiber collimator and a cylindrical lens to form a linear exit light spot, and the light intensity is unevenly distributed in a Gaussian distribution, which leads to increased difficulty in demodulation, limits the demodulation range and precision of the demodulation system, and the aperture of the collimator is limited by the size of the cylindrical lens and the linear array CCD, so that the demodulation system is difficult to realize miniaturization, and the assembly and adjustment steps are many and difficult. The fiber-optic F-P sensor cavity length birefringent crystal demodulation system adopts an LED light source, which has a narrower bandwidth compared with an ASE broadband light source, and uses a birefringent crystal and a wire grid polarizer directly as a polarizing and detecting device for intensity regulation, which can significantly improve the interference fringe contrast and further obtain higher demodulation precision.

Description

TECHNICAL FIELD

[0001] The present application relates to a fiber-optic Fabry-Perot sensor cavity length demodulation system and method, in particular to a birefringent crystal demodulation system and method for fiber-optic Fabry-Perot sensor cavity length. BACKGROUND

[0002] The fiber-optic Fabry-Perot sensor is a kind of fiber-optic sensor with a Fabry-Perot cavity as a sensing head, which has the characteristics of small size, high temperature resistance, corrosion resistance, strong anti-electromagnetic interference ability, high measurement sensitivity, and is widely used in oil well logging, perimeter security, biological medicine, power monitoring and other fields. When the Fabry-Perot sensing head is affected by external physical quantities such as pressure, temperature, vibration, strain, etc., the cavity length of the Fabry-Perot cavity changes, therefore, the external physical quantity information to be measured can be obtained by demodulating the Fabry-Perot cavity length. The detection capability of the fiber-optic Fabry-Perot sensor is mainly limited by the structure of the Fabry-Perot sensing head and the rear-end demodulation method. It is relatively single and time-consuming to find a method to improve the sensitivity of the sensor from the aspects of Fabry-Perot sensing head material, processing technology, etc. The high-precision demodulation technology of the Fabry-Perot cavity length is the current mainstream development direction.

[0003] The low-coherence interference demodulation method matches the optical path difference of the return signal light of the Fabry-Perot sensing head, obtains the optical path difference matching position according to the position of the fringe peak, and then realizes the cavity length demodulation. According to the different matching optical path difference methods, it is mainly divided into two types of time scanning and space scanning. The time scanning system is generally based on a Michelson interferometer, which has a simple structure and a large measurement range, but uses mechanical scanning devices in the demodulation process, which reduces the measurement resolution and demodulation speed. The space scanning system uses a fixed spatial scanning device to match the optical path difference, avoids high-repetition mechanical movement, has the characteristics of compact structure, precision and speed, and is widely used.

[0004] The traditional fiber-optic Fabry-Perot sensor space scanning low-coherence interference demodulation system includes a broadband light source, a fiber coupler, a fiber-optic Fabry-Perot sensor, and a demodulation optical path. The demodulation optical path generally includes a fiber collimator, a cylindrical lens, an optical wedge, a linear array CCD, and a signal processing unit. The disadvantages include: (1) The cylindrical lens is used to achieve the purpose of converging light intensity, but at the same time, the light is compressed to form a linear exit light spot. The non-uniform Gaussian distribution of light intensity will increase the difficulty of demodulation, and limit the demodulation range and precision of the demodulation system; (2) There are many optical elements in the demodulation optical path, and the sizes of the optical elements are limited to each other. The size of the demodulation system is generally large, and the cost is also high; (3) The traditional space scanning low-coherence interference demodulation process involves beam collimation, and the precision requirement is high. Therefore, the optical and mechanical structure design needs to consider the fine adjustment mechanism of each optical element. The assembly and adjustment steps are complex and difficult, and the technical level of the assembly and adjustment personnel is also high. SUMMARY

[0005] The application aims at solving the problems in the conventional fiber-optic Fabry-Perot sensor space scanning type low-coherence interference demodulation system, i.e. the light forms a linear exit spot through a fiber collimator and a cylindrical lens, the light intensity is unevenly distributed in a Gaussian distribution, which leads to a difficulty in demodulation, limits the demodulation range and precision of the demodulation system, and the aperture of the collimator is limited by the size of the cylindrical lens and the linear array CCD, so the demodulation system is difficult to be miniaturized and has many steps of assembly and adjustment, and the application provides a fiber-optic Fabry-Perot sensor cavity length birefringent crystal demodulation system and method.

[0006] In order to solve the above problems in the prior art, the application provides the following technical solutions.

[0007] A fiber-optic Fabry-Perot sensor cavity length birefringent crystal demodulation system, which is characterized in that it comprises a demodulation light path and an LED light source, a fiber coupler and a fiber-optic Fabry-Perot sensor connected in sequence.

[0008] The light emitted by the LED light source enters the fiber-optic Fabry-Perot sensor through the fiber coupler, and the reflected signal light of the fiber-optic Fabry-Perot sensor enters the input end of the demodulation light path through the fiber coupler, and the demodulation light path comprises a birefringent crystal, a wire grid polarizer, a surface array CCD sensor, a signal conversion module and a signal processing unit arranged in sequence along the light path.

[0009] The birefringent crystal is used for forming two linearly polarized lights with an optical path difference and mutually orthogonal polarization states by the reflection signal light with different incident angles, and matching the optical path difference with the reflection signal light; the wire grid polarizer is used for projecting the two linearly polarized lights to generate interference fringes; the surface array CCD sensor is used for collecting the interference fringes and inputting the signal conversion module; the signal conversion module is used for converting the light signal of the interference fringes into an electric signal; and the signal processing unit is used for obtaining the Fabry-Perot cavity length of the fiber-optic Fabry-Perot sensor and the corresponding external pressure through the electric signal.

[0010] Further, the spectral range of the LED light source is 475-600 nm, the central wavelength is 520 nm, and the full width at half maximum is 30 nm; the transmission wave band of the birefringent crystal is 400-5000 nm; and the incident angle range of the wire grid polarizer is 0-45°, and the transmission wave band is 400-1200 nm.

[0011] Further, the fiber coupler adopts a 1x2 multimode fiber coupler, and the fiber type is 62.5 / 125 μm; the birefringent crystal adopts yttrium vanadate YVO4 material, and the size of the two end faces is 1x1 mm, and the thickness is 0.15 mm; and the wire grid polarizer adopts a nano aluminum wire+COP thin film base material, and the size is 5x5x0.19 mm.

[0012] Meanwhile, the application provides a birefringent crystal demodulation method for a fiber Fabry-Perot sensor cavity length, which is characterized by adopting the birefringent crystal demodulation system for the fiber Fabry-Perot sensor cavity length.

[0013] Step 1: light emitted by an LED light source reaches the fiber Fabry-Perot sensor through a fiber coupler, and after being modulated by the fiber Fabry-Perot sensor, two reflected signal lights with an optical path difference are generated;

[0014] Step 2: the two reflected signal lights enter a demodulation light path through the fiber coupler, the reflected signal lights with different incident angles are incident on the surface of the birefringent crystal to form two linearly polarized lights with an optical path difference and mutually orthogonal polarization states, the linearly polarized light and the reflected signal light are matched in terms of the optical path difference to generate low-coherence interference fringes, and then the interference fringes are projected by a wire grid polarizer to generate interference fringes; after the interference fringes are collected by a face array CCD sensor, the interference fringes are input into a signal conversion module, converted into interference fringe data, and input into a signal processing unit for interference fringe data processing, and finally the cavity length of the fiber Fabry-Perot sensor is demodulated.

[0015] Compared with the prior art, the application has the following beneficial effects:

[0016] (1) The birefringent crystal demodulation system for the fiber Fabry-Perot sensor cavity length adopts an LED light source, and compared with an ASE broadband light source, the system has a relatively narrow bandwidth, and the birefringent crystal and the wire grid polarizer are directly used as a polarizing and detecting device for intensity adjustment, which can significantly improve the contrast of the interference fringes and further obtain a higher demodulation accuracy.

[0017] (2) The birefringent crystal demodulation system for the fiber Fabry-Perot sensor cavity length adopts a demodulation light path of the birefringent crystal combined with the wire grid polarizer, and has a simple structure and high stability; meanwhile, collimators and cylindrical lenses are not used, compared with a traditional demodulation light path, the system not only greatly reduces the volume and cost of the overall system, but also simplifies the adjustment difficulty of the optical and mechanical structure, improves the engineering portability and production efficiency.

[0018] (3) The birefringent crystal demodulation system for the fiber Fabry-Perot sensor cavity length adopts the birefringent crystal, and the optical path difference is matched through different incident angles, compared with a traditional spatial scanning type low-coherence interference demodulation system which matches the optical path difference of the fiber Fabry-Perot sensor head through the thickness of the optical wedge, the accuracy of the demodulation is greatly affected by the processing technology of the optical wedge, and the system greatly increases the adjustability of the demodulation light path while reducing the processing technology requirements. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 FIG. 1 is a structural schematic diagram of the birefringent crystal demodulation system for the fiber Fabry-Perot sensor cavity length;

[0020] Figure 2The optical demodulation schematic diagram of the birefringent crystal in the embodiment of the present application.

[0021] The reference signs are explained as follows: 1-LED light source; 2-optical fiber coupler; 3-optical fiber Fabry-Perot sensor; 4-birefringent crystal; 5-wire grid polarizer; 6-area array CCD sensor; 7-signal conversion module; 8-signal processing unit. DETAILED DESCRIPTION

[0022] The present application is further explained in conjunction with the accompanying drawings and exemplary embodiments.

[0023] Reference Figure 1 The birefringent crystal demodulation system of the optical fiber Fabry-Perot sensor cavity length comprises a demodulation light path, and LED light source 1, optical fiber coupler 2 and optical fiber Fabry-Perot sensor 3 arranged in sequence.

[0024] The output end of the optical fiber Fabry-Perot sensor 3 is connected with the input end of the demodulation light path through the optical fiber coupler 2, and the demodulation light path comprises birefringent crystal 4, wire grid polarizer 5, area array CCD sensor 6, signal conversion module 7 and signal processing unit 8 arranged in sequence along the light path.

[0025] The spectral range of the LED light source 1 is 475-600nm, the central wavelength is 520nm, and the full width at half maximum is 30nm.

[0026] The optical fiber coupler 2 is used for transmitting the light emitted by the LED light source 1 into the optical fiber Fabry-Perot sensor 3, and inputting the reflected signal light emitted from the optical fiber Fabry-Perot sensor 3 into the demodulation light path; the optical fiber coupler 2 adopts 1×2 multimode optical fiber coupler, and the optical fiber type is 62.5 / 125μm.

[0027] The optical fiber Fabry-Perot sensor 3 is used for converting the change of the external physical quantity to be measured into the change of the Fabry-Perot cavity length, the Fabry-Perot cavity is composed of the optical fiber end face and the diaphragm, the diaphragm is very sensitive to the external pressure, when the external pressure to be measured changes, the Fabry-Perot cavity length changes simultaneously, which causes the interference signal of the reflected signal light emitted by the optical fiber Fabry-Perot sensor 3 to change, the Fabry-Perot cavity length is demodulated through the interference signal, and then the external pressure to be measured is obtained.

[0028] The birefringent crystal 4 is used for forming two beams of linearly polarized light with optical path difference and mutually orthogonal polarization states from the reflected signal light with different incident angles, and matching the optical path difference with the reflected signal light, so as to generate low-coherence interference fringes at the matching position. The demodulation principle of the birefringent crystal 4 is shown in Figure 2 , i1 and i3 are different incident angles of the reflected signal light on the surface of the birefringent crystal respectively; OPD i1 , OPD i3OPD1 and OPD3 are optical path differences of ordinary light and extraordinary light respectively after the reflected signal light with incident angles i1 and i3 passes through the birefringent crystal; OPD i2 OPD0 is the optical path difference of ordinary light and extraordinary light when the reflected signal light is perpendicularly incident on the surface of the birefringent crystal; i2 = |n o -n e |·d, wherein n o and n e are the refractive indexes of ordinary light and extraordinary light of the birefringent crystal respectively, and d is the thickness of the birefringent crystal. In the embodiment, the birefringent crystal 4 is made of yttrium vanadate YVO4 material, the transmission wavelength range is 400-5000 nm, the spectrum range of the LED light source 1 is included, the size of the two end faces of the birefringent crystal 4 is 1*1 mm, and the thickness is 0.15 mm.

[0029] The wire grid polarizer 5 is used for projecting interference of two mutually orthogonal linearly polarized lights of the reflected signal light. The wire grid polarizer 5 is made of nano-aluminum wire+COP thin film base material, the incident angle range is 0-45°, the transmission wavelength range is 400-1200 nm, the spectrum range of the LED light source 1 is included, and the size is 5*5*0.19 mm.

[0030] The area array CCD sensor 6 is used for synchronously receiving interference fringes generated after the reflected signal light from the fiber-optic F-P sensor 3 passes through the birefringent crystal 4 and the wire grid polarizer 5.

[0031] The signal conversion module 7 converts the optical signal of the interference fringes into an electrical signal by using a data acquisition card.

[0032] The signal processing unit 8 demodulates the interference fringes collected by the area array CCD sensor 6 by using a data acquisition card and a computer processing system, and obtains the F-P cavity length of the fiber-optic F-P sensor 3 and the corresponding external pressure.

[0033] The application discloses a birefringent crystal demodulation method for a fiber-optic F-P sensor cavity length.

[0034] Step 1: The light emitted by the LED light source 1 reaches the fiber-optic F-P sensor 3 through the fiber coupler 2, and two reflected signal lights with optical path differences are generated after being modulated by the fiber-optic F-P sensor;

[0035] Step 2, two beams of reflected signal light enter the demodulation light path through the optical fiber coupler 2, the reflected signal light of different incident angles is incident on the surface of the birefringent crystal 4, forming two beams of linearly polarized light with mutually orthogonal polarization states; due to the difference in refractive index and propagation distance, the two beams of linearly polarized light formed by different incident angles will produce different optical path differences after passing through the birefringent crystal 4, when the optical path difference of the linearly polarized light and the reflected signal light is matched, low-coherence interference fringes will be produced at the corresponding position;

[0036] Step 3, the two beams of linearly polarized light pass through the wire grid polarizer 5 to produce interference fringes;

[0037] Step 4, the interference fringes are collected by the area array CCD sensor 6, and then input into the signal conversion module 7, converted into interference fringe data input signal processing unit 8 for interference fringe data processing, and finally the cavity length of the fiber Fabry-Perot sensor is demodulated.

[0038] The above embodiments are only used to illustrate the technical solutions of the present application, but not to limit it. For ordinary skilled in the art, the specific technical solutions recorded in the foregoing embodiments can be modified, or some technical features can be replaced by equivalent ones, and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions protected by the present application.

Claims

1. A birefringent crystal demodulation system for a fiber optic Fabry-Perot sensor cavity, characterized in that: It includes a demodulation optical path, and an LED light source (1), an optical fiber coupler (2), and an optical fiber Fabry-Perot sensor (3) connected in sequence; The light emitted by the LED light source (1) enters the fiber optic Fabry-Perot sensor (3) through the fiber optic coupler (2). The reflected signal light of the fiber optic Fabry-Perot sensor (3) enters the input end of the demodulation optical path through the fiber optic coupler (2). The demodulation optical path includes a birefringent crystal (4), a wire grid polarizer (5), and an area array CCD sensor (6) arranged sequentially along the optical path. The output end of the area array CCD sensor (6) is connected to the signal conversion module (7) and the signal processing unit (8) in sequence. The birefringent crystal (4) has two end faces of 1×1mm and a thickness of 0.15mm. It is used to form two linearly polarized beams with optical path difference and mutually orthogonal polarization states by incident reflected signal light at different incident angles, and to match the optical path difference with the reflected signal light. The linear grid polarizer (5) is used to project the intensity of the two linearly polarized beams to generate interference fringes. The area array CCD sensor (6) is used to collect the interference fringes and input them into the signal conversion module (7). The signal conversion module (7) is used to convert the optical signal of the interference fringes into an electrical signal. The signal processing unit (8) is used to obtain the Fabry-Perot cavity length of the fiber optic Fabry-Perot sensor (3) and the corresponding external pressure through the electrical signal.

2. The birefringent crystal demodulation system for a fiber optic Fabry-Perot sensor cavity length according to claim 1, characterized in that: The LED light source (1) has a spectral range of 475-600nm, a center wavelength of 520nm, and a full width at half maximum (FWHM) of 30nm; the birefringent crystal (4) has a transmission band of 400-5000nm; and the linear grating polarizer (5) has an incident angle range of 0-45° and a transmission band of 400-1200nm.

3. The birefringent crystal demodulation system for a fiber optic Fabry-Perot sensor cavity length according to claim 2, characterized in that: The fiber coupler (2) is a 1×2 multimode fiber coupler with a fiber type of 62.5 / 125μm; the birefringent crystal (4) is made of yttrium vanadate (YVO4); the wire grid polarizer (5) is made of nano-aluminum wire + COP thin film substrate material with a size of 5×5×0.19mm.

4. A method for demodulating the cavity length of a fiber optic Fabry-Perot sensor using a birefringent crystal, characterized in that, The birefringent crystal demodulation system using the fiber optic Fabry-Perot sensor cavity length described in claim 1 includes the following steps: Step 1: The light emitted by the LED light source (1) reaches the fiber optic Fabry-Perot sensor (3) through the fiber optic coupler (2). After being modulated by the Fabry-Perot sensor, two beams of reflected signal light with optical path difference are generated. Step 2: Two reflected signal beams enter the demodulation optical path through the fiber coupler (2). The reflected signal beams with different incident angles are incident on the surface of a birefringent crystal (4) with a size of 1×1mm and a thickness of 0.15mm at both ends, forming two linearly polarized beams with optical path difference and mutually orthogonal polarization states. The linearly polarized beams and the reflected signal beams are matched by optical path difference to generate low-coherence interference fringes. The interference fringes are then generated by intensity projection through a wire grid polarizer (5). The interference fringes are collected by a CCD sensor (6) and input into a signal conversion module (7) to convert them into interference fringes data. The interference fringes data are then input into a signal processing unit (8) for interference fringes data processing, and finally the cavity length of the fiber Fabry-Perot sensor is demodulated.

5. The method for demodulating the cavity length of a fiber optic Fabry-Perot sensor using a birefringent crystal according to claim 4, characterized in that: The LED light source (1) has a spectral range of 475-600nm, a center wavelength of 520nm, and a full width at half maximum (FWHM) of 30nm; the birefringent crystal (4) has a transmission band of 400-5000nm; and the linear grating polarizer (5) has an incident angle range of 0-45° and a transmission band of 400-1200nm.

6. The method for demodulating the cavity length of a fiber optic Fabry-Perot sensor using a birefringent crystal according to claim 5, characterized in that: The fiber coupler (2) is a 1×2 multimode fiber coupler with a fiber type of 62.5 / 125μm; the birefringent crystal (4) is made of yttrium vanadate (YVO4) material; the wire grid polarizer (5) is made of nano-aluminum wire + COP thin film substrate material with a size of 5×5×0.19mm.

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