Optical fiber sensing array with reciprocal structure, system based on optical fiber sensing array and demodulation method

Abstract

The invention belongs to the technical field of optical fiber sensing, and particularly discloses an optical fiber sensing array with a reciprocal structure, a system based on the optical fiber sensing array and a demodulation method. According to the invention, the first light path and the second light path are in the ith sensing channel, two beams of light in the same displacement sensing channel pass through the same delay optical fiber clockwise and anticlockwise respectively by using the transmission-type sensing probe, the phase information of the first light path comprises optical fiber light path and positive displacement information, and the phase information of the second light path comprises optical fiber light path and positive displacement information. The second optical path phase information comprises an optical fiber optical path and negative displacement information; after interference of the two paths of light, a phase item introduced by the time-delay optical fiber is completely eliminated, and then frequency noise and temperature noise introduced by the time-delay optical fiber are eliminated; a reference channel which does not contain a displacement sensitive structure and only contains an optical fiber reciprocity structure is arranged, phase information of the reference channel only contains front-end optical path noise, phase information of the optical fiber sensing array contains the front-end optical path noise and displacement information, and front-end common-mode noise is deducted through difference between the phase information of the optical fiber sensing array and the phase information of the reference channel.

Description

Technical Field

[0001] This application belongs to the field of fiber optic sensing technology, and more specifically, relates to a fiber optic sensing array with a reciprocal structure, a system based on the same, and a demodulation method. Background Art

[0002] Multichannel displacement measurement systems have been widely studied and applied in fields such as acoustic wave sensing arrays, hydrophone sensing arrays, and space inertial sensor measurements. Especially in the field of space inertial sensors, it is necessary to measure the low-frequency six-degree-of-freedom motion information of the test mass. The time-division multiplexing fiber optic heterodyne interference displacement sensing multiplexing system has the advantages of large multiplexing capacity and high measurement sensitivity, and has great potential in multichannel displacement measurement.

[0003] Pulsed light is generated in the light source part of the time-division multiplexing fiber optic heterodyne interference displacement sensing multiplexing system. By using delay optical fibers of different lengths in each sensing channel, the flight time of the pulsed light in each sensing channel is different, so that the interference signals of each sensing channel are separated in the time domain. By reading the signals in the corresponding time period, the information of each sensing channel can be obtained. In order to prevent signal overlap between sensing channels, the length difference of the delay optical fibers between adjacent sensing channels needs to reach the order of meters. However, the long delay optical fiber will introduce additional laser frequency noise and environmental noise, increasing the measurement noise. Currently, there are mainly two measurement noise deduction schemes, including a scheme of constructing a reference interferometer for common-mode noise suppression and a scheme of using the double-pulse differential method for common-mode noise suppression. However, these two schemes have the problem of poor fiber common-mode performance, resulting in poor measurement noise suppression effect. Summary of the Invention

[0004] Aiming at the deficiencies of the prior art, the purpose of this application is to provide a fiber optic sensing array with a reciprocal structure, a system based on the same, and a demodulation method, aiming to solve the problem of measurement noise introduced by long delay optical fibers in the existing time-division multiplexing fiber optic heterodyne interference displacement sensing multiplexing system.

[0005] The first aspect of this application relates to a transmissive sensing probe, comprising: a first polarization separation module, a polarization rotation module, and a second polarization separation module; The first polarization separation module is configured to receive vertically polarized light through a first fiber optic interface and directly transmit it to the polarization rotation module, receive horizontally polarized light through a second fiber optic interface, transmit it to the polarization rotation module after the transmission path is deflected; receive the output light of the polarization rotation module for polarization separation, so that the vertically polarized light is transmitted out through the first fiber optic interface, and the horizontally polarized light is output through the second fiber optic interface after the transmission path is deflected; The polarization rotation module is used to rotate the polarization state of the light output from the first polarization separation module clockwise by 90°, and then transmit it to the second polarization separation module. The polarization state of the light output from the second polarization separation module is rotated by 0°, and then transmitted to the first polarization separation module; The second polarization separation module is used to perform polarization separation on the light output from the polarization rotation module, so that the horizontally polarized light directly transmits through the probe head, and the vertically polarized light is reflected into the polarization rotation module; the horizontally polarized light directly transmitted is received and transmitted to the polarization rotation module.

[0006] Preferably, the first polarization separation module includes: a first polarization maintaining fiber collimator, a second polarization maintaining fiber collimator, and a birefringent crystal; The slow axes of the first polarization maintaining fiber collimator and the second polarization maintaining fiber collimator are placed perpendicular to each other; The vertically polarized light is incident from the first polarization maintaining fiber collimator, passes through the birefringent crystal without deviation, and directly transmits to the polarization rotation module; The horizontally polarized light is incident from the second polarization maintaining fiber collimator, and after the transmission path is deflected through the birefringent crystal, it is transmitted to the polarization rotation module; The birefringent crystal receives the light output from the polarization rotation module and performs polarization separation, so that the vertically polarized light exits from the first polarization maintaining fiber collimator, and the horizontally polarized light exits from the second polarization maintaining fiber collimator after the transmission path is deflected.

[0007] Preferably, the polarization rotation module includes a 45° Faraday rotator mirror and a half-wave plate; In the forward propagation process, the polarization state is rotated clockwise by 45° through the 45° Faraday rotator mirror, and then rotated clockwise by 45° through the half-wave plate, and transmitted to the second polarization separation module; In the reverse propagation process, the polarization state is rotated counterclockwise by 45° through the half-wave plate, and then rotated clockwise by 45° through the 45° Faraday rotator mirror, and transmitted to the first polarization separation module.

[0008] Preferably, the second polarization separation module is a polarization beam splitter prism, and one of the polarization end faces of the polarization beam splitter prism is coated with a reflective film, and the other polarization end face is not coated.

[0009] The second aspect of the present application relates to an optical fiber sensing array with a reciprocal structure, including N sensing modules; each sensing module includes two transmissive sensing probe heads as described in the first aspect and a delay optical fiber; The first transmissive sensing probe head and the second transmissive sensing probe head are symmetrically mirror-image arranged, and the horizontally polarized light directly transmitted out is facing each other; One end of the delay optical fiber is connected to the second optical fiber interface of the first transmissive sensing probe head, and the other end is connected to the second optical fiber interface of the second transmissive sensing probe head; The lengths of the delay optical fibers in different sensing modules are different, and the difference in the lengths of the delay optical fibers between adjacent sensing modules is determined by the width of the optical pulse received by the probe.

[0010] Preferably, the length of the delay optical fiber satisfies the following formula: , where is the refractive index of the optical fiber, is the difference in the lengths of two adjacent delay optical fibers, is the speed of light, is the pulse width.

[0011] The third aspect of the present application relates to an optical fiber sensing array with a reciprocal structure, including N sensing modules; each sensing module includes a transmissive sensing probe as described in the first aspect and two delay optical fibers with equal lengths; One end of the first delay optical fiber is connected to the first optical fiber interface of the transmissive sensing probe; one end of the second delay optical fiber is connected to the second optical fiber interface of the transmissive sensing probe.

[0012] The lengths of the delay optical fibers in different sensing modules are different, and the difference in the lengths of the delay optical fibers between adjacent sensing modules is determined by the width of the optical pulse received by the probe.

[0013] Preferably, the length of the delay optical fiber satisfies the following formula: , where is the refractive index of the optical fiber, , is the length of the first delay optical fiber of the th path, is the length of the second delay optical fiber of the th path, is the length of the first delay optical fiber of the th path, is the length of the second delay optical fiber of the th path.

[0014] The fourth aspect of the present application relates to an optical fiber displacement sensing multiplexing system, including a light source module, a first circulator, a second circulator, a first 1×(N + 1) optical fiber coupler, a second 1×(N + 1) optical fiber coupler, an optical fiber sensing array with a reciprocal structure as described in the second aspect, a reference optical fiber, and a signal detection and processing module; The light source module is used to provide two pulsed lights with a frequency difference; The input port of the first circulator is connected to the light source module, the reflection port is connected to the input port of the first 1×(N + 1) optical fiber coupler, and the output port is connected to the signal detection and processing module; The input port of the second circulator is connected to the light source module, the reflection port is connected to the input port of the second 1×(N + 1) optical fiber coupler, and the output port is connected to the signal detection and processing module; The first 1×(N + 1) fiber coupler, the input port is connected to the first circulator, and the output ports are respectively connected to the fiber optic sensing array and the reference fiber; The second 1×(N + 1) fiber coupler, the input port is connected to the second circulator, and the output ports are respectively connected to the fiber optic sensing array and the reference fiber; The reference fiber is used to output the light input by the first 1×(N + 1) fiber coupler clockwise from the second 1×(N + 1) fiber coupler, and output the light input by the second 1×(N + 1) fiber coupler counterclockwise from the first 1×(N + 1) fiber coupler; In the fiber optic sensing array, in each sensing module, the first fiber optic interface of the first transmissive sensing probe is connected to the output port of the first 1×(N + 1) fiber coupler, and the first fiber optic interface of the second transmissive sensing probe is connected to the output port of the second 1×(N + 1) fiber coupler. When the mirror to be measured is displaced, the first transmissive sensing probe will transmit the light carrying the positive displacement signal clockwise through the delay fiber, and transmit it to the second 1×(N + 1) fiber coupler through the second transmissive sensing probe; the second transmissive sensing probe will transmit the light carrying the negative displacement signal counterclockwise through the delay fiber, and transmit it to the first 1×(N + 1) fiber coupler through the first transmissive sensing probe. The mirror to be measured is located between the directly transmissive ends of the horizontal polarized light of the first transmissive sensing probe and the second transmissive sensing probe; The signal detection and processing module is used to receive and separate the sensing interference signals of each channel, then perform phase demodulation to obtain the phase of 1 reference fiber and the phases of N sensing channels, subtract the reference phase from the N sensing phases to obtain the phase change caused by the displacement of the mirror to be measured, and calculate the displacements of N mirrors to be measured according to the N phase changes; Or, It includes a light source module, a first circulator, a second circulator, a first 1×(N + 1) fiber coupler, a second 1×(N + 1) fiber coupler, a fiber optic sensing array with a reciprocal structure as described in the third aspect, a reference fiber, and a signal detection and processing module; The light source module is used to provide two pulsed lights with a frequency difference; The input port of the first circulator is connected to the light source module, the reflection port is connected to the input port of the first 1×(N + 1) fiber coupler, and the output port is connected to the signal detection and processing module; The input port of the second circulator is connected to the light source module, the reflection port is connected to the input port of the second 1×(N + 1) fiber coupler, and the output port is connected to the signal detection and processing module; The first 1×(N + 1) fiber coupler, the input port is connected to the first circulator, and the output ports are respectively connected to the fiber optic sensing array and the reference fiber; The second 1×(N + 1) fiber optic coupler, with its input port connected to the second circulator and its output ports respectively connected to the fiber optic sensing array and the reference optical fiber; The reference optical fiber is used to output the light input by the first 1×(N + 1) fiber optic coupler clockwise from the second 1×(N + 1) fiber optic coupler and output the light input by the second 1×(N + 1) fiber optic coupler counterclockwise from the first 1×(N + 1) fiber optic coupler; In the fiber optic sensing array, in each sensing module, the first delay optical fiber connects the output port of the first 1×(N + 1) fiber optic coupler and the first fiber optic interface of the transmissive sensing probe, and is used to transmit the optical pulse carrying displacement information clockwise through the first delay optical fiber and the second delay optical fiber, and then to the second 1×(N + 1) fiber optic coupler; the second delay optical fiber connects the output port of the second 1×(N + 1) fiber optic coupler and the second fiber optic interface of the transmissive sensing probe, and is used to transmit the optical pulse not carrying displacement information counterclockwise through the second delay optical fiber and the first delay optical fiber, and then to the first 1×(N + 1) fiber optic coupler. The mirror to be measured is located at the directly transmissive end of the horizontal polarized light of the transmissive sensing probe; The signal detection and processing module is used to receive and separate the sensing interference signals of each channel, and then perform phase demodulation to obtain 1 reference fiber optic phase and N sensing phases. Subtract the reference phase from the N sensing phases to obtain the phase change caused by the displacement of the mirror to be measured, and calculate the displacements of the N mirrors to be measured according to the N phase changes.

[0015] The fifth aspect of the present application relates to a demodulation method for the fiber optic displacement sensing multiplexing system as described in the fourth aspect, including: Measuring the front-end common mode noise in the reference fiber optic measurement optical path; Measuring the N displacements to be measured respectively through the fiber optic sensing array; Subtracting the front-end common mode noise by differentiating the phase information of the fiber optic sensing array and the phase information of the reference fiber optic.

[0016] Generally speaking, compared with the prior art through the above technical solutions conceived by the present application, the following beneficial effects are obtained: (1) The present application proposes a transmissive sensing probe, comprising: a first polarization separation module, a polarization rotation module, and a second polarization separation module. The vertically polarized light entering from the first optical fiber interface is directly transmitted to the polarization rotation module. After passing through the polarization rotation module, the polarization state of the input light rotates clockwise by 90° (the input light changes from vertically polarized light to horizontally polarized light) and is transmitted to the second polarization separation module. After polarization separation by the second polarization separation module, the horizontally polarized light is directly transmitted out of the probe, realizing the optical transmission from port 1 to port 2. The horizontally polarized light is reflected by the surface to be measured and enters the polarization rotation module from the second polarization separation module. After passing through the polarization rotation module, the polarization state rotates by 0° and is transmitted to the first polarization separation module. After polarization separation by the first polarization separation module, the horizontally polarized light is deflected and transmitted out through the second optical fiber interface, realizing the optical transmission from port 2 to port 3. The horizontally polarized light entering from the second optical fiber interface is deflected after passing through the birefringent crystal transmission path and then transmitted to the polarization rotation module. After passing through the polarization rotation module, the polarization state of the input light rotates clockwise by 90° (the input light changes from horizontally polarized light to vertically polarized light) and is transmitted to the second polarization separation module. Then it is directly reflected by the second polarization separation module. After reflection, the light passes through the polarization rotation module without changing its polarization state (vertically polarized light) and is transmitted out through the first optical fiber interface, realizing the optical transmission from port 3 to port 1.

[0017] (2) The present application proposes a fiber optic sensing array with a reciprocal structure, including a technical solution of a transmissive sensing probe and a technical solution of two transmissive sensing probes. In the i-th sensing channel of the sensing array, for the first optical path and the second optical path, using the transmissive sensing probe, two beams of light in the same displacement sensing channel pass through the same delay optical fiber clockwise and counterclockwise respectively. The phase information of the first optical path contains the optical path noise of the i-th optical fiber path and the positive displacement information, and the phase information of the second optical path contains the optical path noise of the i-th optical fiber path and the negative displacement information. After the two beams of light interfere, the phase terms introduced by the delay optical fiber can be completely eliminated, thereby eliminating the frequency noise and temperature noise introduced by the delay optical fiber and reducing the low-frequency noise of the multi-channel fiber optic sensing array.

[0018] (3) The present application proposes a fiber optic displacement sensing multiplexing system, which sets a reference channel without a displacement sensitive structure and only with a fiber optic reciprocal structure. The phase information of the reference channel only contains the front-end optical path noise, and the phase information of the fiber optic sensing array contains the front-end optical path noise and displacement information. By differentiating the phase information of the fiber optic sensing array and the phase information of the reference channel, the front-end common mode noise can be deducted, solving the problem of low low-frequency measurement sensitivity of the multi-channel fiber optic sensing array. Description of the Drawings

[0019] Figure 1 It is a schematic structural diagram of a transmissive sensing probe provided by an embodiment of the present application.

[0020] Figure 2It is a schematic structural diagram of a single-channel sensing module of an optical fiber sensing array with a reciprocal structure provided by an embodiment of the present application.

[0021] Figure 3 It is a schematic structural diagram of an optical fiber displacement sensing multiplexing system provided by an embodiment of the present application.

[0022] Figure 4 It is a time-domain diagram of undifferentiated phase data of each channel provided by an embodiment of the present application.

[0023] Figure 5 It is a time-domain diagram of phase data after differential between the 3-channel test path and the reference path provided by an embodiment of the present application.

[0024] Figure 6 It is an ASD spectrum of phase data after differential between the 3-channel test path and the reference path provided by an embodiment of the present application.

[0025] In all the drawings, the same reference numerals are used to represent the same elements or structures, where: 1 - Light source module; 2 - Light source; 3 - First coupler; 4 - First frequency shifter; 5 - Second frequency shifter; 6 - First pulse signal modulator; 7 - Second pulse signal modulator; 8 - First circulator; 9 - Second circulator; 10 - First 1×(N + 1) fiber coupler; 11 - Second 1×(N + 1) fiber coupler; 12 - Reference optical fiber; 13 - Optical fiber sensing array; 14 - Signal detection and processing module; 15 - Second coupler; 16 - Photoelectric detector; 17 - Data acquisition and processing system; 18 - First transmissive sensing probe; 19 - Second transmissive sensing probe; 20 - The i-th delay optical fiber; 21 - The i-th mirror to be measured; 22 - First polarization-maintaining fiber collimator; 23 - Second polarization-maintaining fiber collimator; 24 - Birefringent crystal; 25 - 45° Faraday rotator mirror; 26 - Half-wave plate; 27 - Polarizing beam splitter prism; 28 - Reflective film; 29 - First polarization separation module; 30 - Polarization rotation module; 31 - Second polarization separation module. Specific embodiments

[0026] In order to make the objectives, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0027] The embodiments of the present application will be described below with reference to the drawings in the embodiments of the present application.

[0028] In the first aspect, as Figure 1As shown in the figure, the present application relates to a transmissive sensing probe, comprising: a first polarization separation module 29, a polarization rotation module 30, and a second polarization separation module 31. The first polarization separation module 29 is configured to receive vertically polarized light through a first optical fiber interface and directly transmit it to the polarization rotation module 30, receive horizontally polarized light through a second optical fiber interface, and transmit it to the polarization rotation module 30 after the transmission path is deflected; receive the output light of the polarization rotation module 30 and perform polarization separation, so that the vertically polarized light is transmitted out through the first optical fiber interface, and the horizontally polarized light is output through the second optical fiber interface after the transmission path is deflected; the polarization rotation module 30 is configured to rotate the polarization state of the output light of the first polarization separation module 29 clockwise by 90°, then transmit it to the second polarization separation module 31, rotate the polarization state of the output light of the second polarization separation module 31 by 0°, and then transmit it to the first polarization separation module 29; the second polarization separation module 31 is configured to perform polarization separation on the output light of the polarization rotation module 30, so that the horizontally polarized light is directly transmitted out of the probe, and the vertically polarized light is reflected into the polarization rotation module 30; receive the horizontally polarized light and directly transmit it to the polarization rotation module 30.

[0029] As Figure 1 shown, port 1 receives incident light (light emitted by the light source), rotates the polarization state of the light clockwise by 90° and then transmits it to port 2; port 2 receives incident light (light reflected by the mirror) and directly transmits it to port 3; port 3 receives incident light (light emitted by the light source), rotates the polarization state of the light clockwise by 90° and transmits it to port 1.

[0030] As Figure 1 shown, port 1 receives incident light (light from port 3), transmits the light to a 1×(N + 1) fiber coupler; port 2 receives incident light (light from port 1), and exits to the mirror to be measured; port 3 receives incident light (light from port 2) and transmits it to the delay optical fiber.

[0031] Preferably, as Figure 1 shown, the first polarization separation module comprises: a first polarization maintaining fiber collimator 22, a second polarization maintaining fiber collimator 23, and a birefringent crystal 24; the slow axes of the first polarization maintaining fiber collimator 22 and the second polarization maintaining fiber collimator 23 are placed perpendicular to each other; the vertically polarized light is incident from the first polarization maintaining fiber collimator 22, passes through the birefringent crystal 24 without deviation, and is directly transmitted to the polarization rotation module 30; the horizontally polarized light is incident from the second polarization maintaining fiber collimator 23, and is transmitted to the polarization rotation module 30 after the transmission path is deflected through the birefringent crystal 24; the birefringent crystal 24 receives the output light of the polarization rotation module 30 and performs polarization separation, so that the vertically polarized light exits from the first polarization maintaining fiber collimator 22, and the horizontally polarized light exits from the second polarization maintaining fiber collimator 23 after the transmission path is deflected.

[0032] Preferably, as Figure 1As shown, the polarization rotation module includes a 45° Faraday rotation mirror 25 and a half-wave plate 26; during forward propagation, the polarization state is rotated 45° clockwise by the 45° Faraday rotation mirror 25, and then rotated 45° clockwise by the half-wave plate 26, and transmitted to the second polarization separation module 31; during reverse propagation, the polarization state is rotated 45° counterclockwise by the half-wave plate 26, and then rotated 45° clockwise by the 45° Faraday rotation mirror 25, and transmitted to the first polarization separation module 29.

[0033] Preferably, if Figure 1 As shown, the second polarization separation module 31 is a polarization beam splitter prism, one polarization end face of the polarization beam splitter prism is coated with a reflective film 28, and the other polarization end face is not coated. The reflectivity of the reflective film must be greater than 90%, and the material can be a metal reflective film (Au, Ag, Al, etc.) or a dielectric reflective film.

[0034] Second, as Figure 2 As shown, the present application relates to an optical fiber sensing array with a reciprocal structure, comprising N sensing modules; each sensing module comprises two transmission sensing probes as described in the first aspect and a delay optical fiber; the first transmission sensing probe 18 and the second transmission sensing probe 19 are symmetrically mirrored, and the ends of the horizontally polarized light directly transmitted are directly opposite, with a reflector 21 to be measured placed in the middle; one end of the delay optical fiber 20 is connected to the second optical fiber interface of the first transmission sensing probe, and the other end is connected to the second optical fiber interface of the second transmission sensing probe; the lengths of the delay optical fibers in different sensing modules are different, and the difference in the lengths of the delay optical fibers of adjacent sensing modules is determined by the width of the light pulses received by the probes.

[0035] Preferably, the delay optical fiber length satisfies the following formula: ,in, is the refractive index of the optical fiber, is the difference in length of two adjacent delayed optical fibers, is the speed of light, is the pulse width.

[0036] In a third aspect, the present application relates to a fiber optic sensing array with a reciprocal structure, comprising N sensing modules; each sensing module comprises a transmission sensing probe as described in the first aspect and two delay optical fibers of equal length; one end of the first delay optical fiber is connected to the first optical fiber interface of the transmission sensing probe, and one end of the second delay optical fiber is connected to the second optical fiber interface of the transmission sensing probe; the lengths of the delay optical fibers in different sensing modules are different, and the difference in the lengths of the delay optical fibers of adjacent sensing modules is determined by the width of the light pulse received by the probe.

[0037] Preferably, the delay optical fiber length satisfies the following formula: ,in, is the refractive index of the optical fiber, , is the length of the first extended optical fiber for the th path, is the length of the second extended optical fiber for the th path, is the length of the first extended optical fiber for the th path, is the length of the second extended optical fiber for the th path.

[0038] In the fourth aspect, as Figure 3As shown, the present application relates to an optical fiber displacement sensing multiplexing system, which includes a light source module 1, a first circulator 8, a second circulator 9, a first 1×(N + 1) optical fiber coupler 10, a second 1×(N + 1) optical fiber coupler 11, an optical fiber sensing array 13 with a reciprocal structure as described in the second aspect, a reference optical fiber 12, and a signal detection and processing module 14; the light source module 1 is used to provide two pulsed lights with a frequency difference; the input port of the first circulator 8 is connected to the light source module, the reflection port is connected to the input port of the first 1×(N + 1) optical fiber coupler, and the output port is connected to the signal detection and processing module; the input port of the second circulator 9 is connected to the light source module, the reflection port is connected to the input port of the second 1×(N + 1) optical fiber coupler, and the output port is connected to the signal detection and processing module; the first 1×(N + 1) optical fiber coupler 10, the input port is connected to the first circulator, and the output ports are respectively connected to the optical fiber sensing array and the reference optical fiber; the second 1×(N + 1) optical fiber coupler 11, the input port is connected to the second circulator, and the output ports are respectively connected to the optical fiber sensing array and the reference optical fiber; the reference optical fiber 12 is used to output the light input from the first 1×(N + 1) optical fiber coupler clockwise from the second 1×(N + 1) optical fiber coupler, and output the light input from the second 1×(N + 1) optical fiber coupler counterclockwise from the first 1×(N + 1) optical fiber coupler; in each sensing module of the optical fiber sensing array 13, the first optical fiber interface of the first transmissive sensing probe is connected to the output port of the first 1×(N + 1) optical fiber coupler, and the first optical fiber interface of the second transmissive sensing probe is connected to the output port of the second 1×(N + 1) optical fiber coupler. When the mirror to be measured undergoes displacement, the first transmissive sensing probe will transmit the light carrying the positive displacement signal clockwise through the delay optical fiber, and transmit it to the second 1×(N + 1) optical fiber coupler through the second transmissive sensing probe; the second transmissive sensing probe will transmit the light carrying the negative displacement signal counterclockwise through the delay optical fiber, and transmit it to the first 1×(N + 1) optical fiber coupler through the first transmissive sensing probe. The mirror to be measured is located between the directly transmissive ends of the horizontal polarized lights of the first transmissive sensing probe and the second transmissive sensing probe; the signal detection and processing module 14 is used to receive and separate the sensing interference signals of each channel, then perform phase demodulation to obtain the phase of 1 reference optical fiber and the phases of N sensing channels, subtract the reference phase from the N sensing phases to obtain the phase change caused by the displacement of the mirror to be measured, and calculate the displacements of N mirrors to be measured according to the N phase changes; Or, It includes a light source module, a first circulator, a second circulator, a first 1×(N+1) fiber coupler, a second 1×(N+1) fiber coupler, a fiber optic sensing array with a reciprocal structure as described in the third aspect, a reference fiber, and a signal detection and processing module; the light source module is used to provide two pulsed lights with a frequency difference; the input port of the first circulator is connected to the light source module, the reflection port is connected to the input port of the first 1×(N+1) fiber coupler, and the output port is connected to the signal detection and processing module; the input port of the second circulator is connected to the light source module, the reflection port is connected to the input port of the second 1×(N+1) fiber coupler, and the output port is connected to the signal detection and processing module; the first 1×(N+1) fiber coupler, its input port is connected to the first circulator, and the output ports are respectively connected to the fiber optic sensing array and the reference fiber; the second 1×(N+1) fiber coupler, its input port is connected to the second circulator, and the output ports are respectively connected to the fiber optic sensing array and the reference fiber; the reference fiber is used to output the light input from the first 1×(N+1) fiber coupler clockwise from the second 1×(N+1) fiber coupler, and output the light input from the second 1×(N+1) fiber coupler counterclockwise from the first 1×(N+1) fiber coupler; in each sensing module of the fiber optic sensing array, the first delay fiber is connected to the output port of the first 1×(N+1) fiber coupler and the first fiber interface of the transmissive sensing probe, and is used to transmit the light pulse carrying displacement information clockwise through the first delay fiber and the second delay fiber, and then transmit it to the second 1×(N+1) fiber coupler; the second delay fiber is connected to the output port of the second 1×(N+1) fiber coupler and the second fiber interface of the transmissive sensing probe, and is used to transmit the light pulse not carrying displacement information counterclockwise through the second delay fiber and the first delay fiber, and then transmit it to the first 1×(N+1) fiber coupler, and the mirror to be measured is located at the directly transmissive end of the horizontal polarized light of the transmissive sensing probe; the signal detection and processing module is used to receive and separate the sensing interference signals of each channel, and then perform phase demodulation to obtain the phase of 1 reference fiber and the phases of N sensing channels, subtract the reference phase from the N sensing phases to obtain the phase change caused by the displacement of the mirror to be measured, and calculate the displacements of N mirrors to be measured according to the N phase changes.

[0039] The light source module 1 is used to provide two pulsed lights with a frequency difference as follows. As Figure 3 shown, it includes a light source 2, a first coupler 3, a first frequency shifter 4, a second frequency shifter 5, a first pulse signal modulator 6, and a second pulse signal modulator 7. Specifically, the single-frequency continuous laser emitted by the light source 2 is divided into a first optical path and a second optical path by the first coupler 3. The two optical paths respectively pass through the first frequency shifter 4 and the second frequency shifter 5 to generate and frequency modulations, and the frequency difference between the two is the heterodyne signal frequency. The lights after the two - path modulation respectively pass through the first pulse signal modulator 6 and the second pulse signal modulator 7 to modulate the continuous light into the first pulse light and the second pulse light, and then reach the sensing array through the first circulator 8 and the second circulator 9.

[0040] The first circulator 8 and the second circulator 9 are used to connect the light source module 1 and the sensing array. As Figure 3 shown, specifically, the a - ports of the first circulator 8 and the second circulator 9 are respectively connected to the first pulse light and the second pulse light emitted by the light source module; the lights incident on the a - ports of the first circulator and the second circulator are emitted from the b - ports and reach the sensing array; the lights reflected by the sensing array enter through the b - ports of the first circulator and the second circulator and reach the second coupler 15 through the c - ports for interference.

[0041] For the technical solution of two probes, the fiber optic sensing array is composed of N sensing modules. Specifically, the i - th sensing module is used to connect the i - th output port of the first 1×(N + 1) fiber optic coupler and the i - th output port of the second 1×(N + 1) fiber optic coupler.

[0042] The i - th sensing module includes a first transmissive sensing probe, a second transmissive sensing probe, and the i - th delay optical fiber. The first transmissive sensing probe is used to transmit the light input from its port 1 from port 2 to the A side of the reflecting surface, the reflected light is received by port 2, passes clockwise through the i - th delay optical fiber from port 3, and finally passes through the second transmissive sensing probe and is transmitted to the second 1×(N + 1) fiber optic coupler. The second transmissive sensing probe is used to transmit the light input from its port 1 from port 2 to the B side of the reflecting surface, the reflected light is received by port 2, passes counter - clockwise through the i - th delay optical fiber from port 3, and finally passes through the first transmissive sensing probe and is transmitted to the first 1×(N + 1) fiber optic coupler. The light emitted from port 2 of the first transmissive sensing probe 18 is perpendicularly incident on the A surface of the i - th mirror to be measured 21, and the light emitted from port 2 of the second transmissive sensing probe 19 is perpendicularly incident on the B surface of the i - th mirror 21. When the i - th mirror 21 undergoes displacement, the displacement information measured by the two transmissive sensing probes is equal in magnitude and opposite in direction.

[0043] For the technical solution of one probe, directly connect the port 1 of the probe to the first delay optical fiber, and then connect it to one of the output ports of the first 1×(N + 1) fiber optic coupler. Connect the port 3 of the probe to the second delay optical fiber, and then connect it to one of the output ports of the second 1×(N + 1) fiber optic coupler to realize the optical transmission from port 1 to port 2, from port 2 to port 3, and from port 3 to port 1.

[0044] The signal detection and processing module includes a second coupler 15, a photodetector 16, and a data acquisition and processing system 17.

[0045] The single-frequency continuous optical field signal emitted by the light source 2 is:

[0046] Wherein, is the amplitude of the emitted optical field, is the center frequency of the laser, is the initial phase. It is divided into a first optical field and a second optical field by the first coupler 3.

[0047] The optical field signals of the first optical field and the second optical field after passing through the frequency shifter and the pulse signal modulator are:

[0048]

[0049] Wherein, and are the amplitudes of the first optical field and the second optical field respectively, and are the modulation frequencies of the first frequency shifter and the second frequency shifter respectively, is the pulse signal frequency, are the phases of the first optical field and the second optical field respectively, is the square wave pulse function, specifically as follows:

[0050] Wherein, is the pulse width.

[0051] Due to the existence of the delay fiber, the pulse signals of each channel in the sensing array have the same flight time, and the pulse flight times between different sensing channels are different. Therefore, at the same moment, only the two optical signals of the same sensing channel will interfere, and the optical signals of other channels are in the off state. For convenience, the pulse signal modulation is ignored during the calculation.

[0052] The optical field signals passing through the reference fiber 12 clockwise and counterclockwise can be respectively expressed as:

[0053]

[0054] Wherein, and are the signal amplitudes, is the refractive index of the optical fiber, and are respectively the front optical path of the first optical field and the front optical path of the second optical field, is the length of the reference fiber 12, is the speed of light.

[0055] The optical field signals passing through the i-th sensing module clockwise and counterclockwise can be respectively expressed as:

[0056]

[0057] Wherein, and are the signal amplitudes, is the air refractive index, and are respectively the vertical distances from the ports 2 of the first transmissive sensing probe 18 and the second transmissive sensing probe 19 to the i-th mirror 21, is the displacement of the i-th mirror 21, is the length of the i-th delay optical fiber 20.

[0058] At the second coupler 15, the clockwise and counterclockwise optical fields of each sensing channel interfere, and then are received by the photodetector 16, and finally transmitted to the data acquisition and processing system 17 for phase demodulation.

[0059] Since , it can be ignored, and the phase expression of the interference signal of the reference channel is:

[0060] Wherein, is the central frequency of the laser, is the refractive index of the optical fiber, and are respectively the front optical path of the first optical field and the front optical path of the second optical field, is the speed of light.

[0061] The interference signal expression of the i-th sensing module is:

[0062] Wherein, is the air refractive index, and are respectively the vertical distances from the ports 2 of the first transmissive sensing probe 18 and the second transmissive sensing probe 19 to the i-th mirror 21, is the displacement of the i-th mirror 21.

[0063] The differential phase between the two is:

[0064] Wherein, is the distance difference between the ports 2 of the first transmissive sensing probe and the second transmissive sensing probe to the reflecting surface.

[0065] As can be seen from the above derivation, through the design of the transmissive sensing probe proposed in this application, the phase term introduced by the delay fiber can be completely eliminated, thereby reducing the additional low-frequency noise introduced by the delay fiber; by using the design of the reference interferometer, the common-mode noise introduced by the front-end optical path difference can be effectively deducted.

[0066] Fifthly, this application relates to a demodulation method for the fiber optic displacement sensing multiplexing system as described in the fourth aspect, including: Measuring the front-end common-mode noise in the reference fiber optic measurement optical path; Measuring N paths of displacements to be measured respectively through the fiber optic sensing array; Deducting the front-end common-mode noise by differentiating the phase information of the fiber optic sensing array and the phase information of the reference fiber.

[0067] Embodiment In this embodiment, a test device with 1 reference channel and 3 fiber optic sensing arrays is built. The output light frequency of the laser is 193.5 THz (corresponding to the center wavelength of the laser being 1550.12 nm), the frequency shift amount of the first frequency shifter is 80.01 MHz, the frequency shift amount of the second frequency shifter is 80 MHz, and the corresponding heterodyne frequency is 10 kHz; the modulation frequencies of the first pulse signal modulator and the second pulse signal modulator are both 1 MHz, the pulse width is 10 ns, and they have the same starting phase; the length of the reference fiber (regarded as the first delay fiber) is 2 m, and the lengths of the delay fibers of the 3 fiber optic sensing arrays are 10 m, 18 m, and 26 m respectively; the phase reading rate is 10 S / s, the type is polarization-maintaining fiber, and the reflective film is a metal reflective film (Au).

[0068] The time-domain diagrams of the phase data of each sensing channel without reference channel differentiation are as Figure 4 shown, and the phase fluctuation in 20 hours reaches ±5 rad; the time-domain diagrams of the phase data of each sensing channel after differentiation with the reference channel are as Figure 5 shown. After deducting through the reference channel differentiation, the phase fluctuation in 20 hours is reduced to ±0.04 rad, a reduction of two orders of magnitude, verifying the effectiveness of the reference channel differentiation deduction scheme for suppressing the front-end common-mode noise. In addition, according to Figure 5 the shown results, even if the length of the delay fiber is increased, the phase noise of each sensing channel does not increase significantly, indicating that the reciprocal structure proposed in this application has a good suppression effect on the additional noise introduced by the delay fiber.

[0069] The phase amplitude spectral density (ASD) spectra of each sensing channel after differentiation with the reference channel are as Figure 6 shown. The phase noise floors of the 3 sensing channels at 6 mHz are all lower than 0.016 , and they have good consistency. The displacement sensitivity of the system is Therefore, the equivalent displacement noise of the three sensing channels is better than .

[0070] It should be understood that expressions such as "including" and "may include" that can be used in this application indicate the existence of the disclosed functions, operations, or constituent elements, and do not limit one or more additional functions, operations, and constituent elements. In this application, terms such as "including" and / or "having" can be interpreted as indicating a specific characteristic, number, operation, constituent element, component, or a combination thereof, but cannot be interpreted as excluding the existence or possibility of addition of one or more other characteristics, numbers, operations, constituent elements, components, or a combination thereof.

[0071] In addition, in this application, the expression "and / or" includes any and all combinations of the associated listed words. For example, the expression "A and / or B" can include A, can include B, or can include both A and B.

[0072] In the description of the embodiments of this application, it should be noted that unless otherwise clearly specified and limited, the orientation terms mentioned in the embodiments of this application, such as "top", "bottom", "inside", "outside", "left", "right", etc., are only with reference to the direction of the accompanying drawings. Therefore, the orientation terms used are for better and clearer description and understanding of the embodiments of this application, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus cannot be understood as a limitation to the embodiments of this application.

[0073] In addition, in the embodiments of this application, mathematical concepts such as symmetry, equality, parallelism, and perpendicularity are mentioned. These limitations are all in view of the current technological level, rather than absolute strict definitions in the mathematical sense. A small deviation is allowed, and being approximately symmetric, approximately equal, approximately parallel, approximately perpendicular, etc. are all acceptable. For example, A is parallel to B means that A is parallel to B or approximately parallel to B, and the included angle between A and B can be between 0 degrees and 10 degrees. A is perpendicular to B means that A is perpendicular to B or approximately perpendicular to B, and the included angle between A and B can be between 80 degrees and 100 degrees.

[0074] The above is only the specific implementation manner of this application, but the protection scope of this application is not limited thereto. Any person skilled in the art within the technical scope disclosed in this application can easily think of changes or substitutions, which should all be covered by the protection scope of this application. Therefore, the protection scope of this application should be subject to the protection scope of the claims.

Claims

1. A transmissive sensing probe, characterized in that, Comprising: A first polarization separation module (29), a polarization rotation module (30), and a second polarization separation module (31); The first polarization separation module (29) is configured to receive vertically polarized light through a first optical fiber interface and directly transmit it to the polarization rotation module (30), receive horizontally polarized light through a second optical fiber interface, and transmit it to the polarization rotation module (30) after the transmission path is deflected; receive the output light of the polarization rotation module (30) for polarization separation, so that the vertically polarized light is transmitted out through the first optical fiber interface, and the horizontally polarized light is output through the second optical fiber interface after the transmission path is deflected; The polarization rotation module (30) is configured to rotate the polarization state of the output light of the first polarization separation module (29) clockwise by 90°, then transmit it to the second polarization separation module (31), rotate the polarization state of the output light of the second polarization separation module (31) by 0°, and then transmit it to the first polarization separation module (29); The second polarization separation module (31) is configured to perform polarization separation on the output light of the polarization rotation module (30), so that the horizontally polarized light is directly transmitted out of the probe head, and the vertically polarized light is reflected into the polarization rotation module (30); receive the horizontally polarized light and directly transmit it to the polarization rotation module (30).

2. The transmissive sensing probe according to claim 1, wherein, The first polarization separation module includes: a first polarization-maintaining fiber collimator (22), a second polarization-maintaining fiber collimator (23), and a birefringent crystal (24); The slow axes of the first polarization-maintaining fiber collimator (22) and the second polarization-maintaining fiber collimator (23) are placed perpendicular to each other; Vertically polarized light is incident from the first polarization-maintaining fiber collimator (22), passes through the birefringent crystal (24) without deviation, and is directly transmitted to the polarization rotation module (30); Horizontally polarized light is incident from the second polarization-maintaining fiber collimator (23), and is transmitted to the polarization rotation module (30) after the transmission path is deflected through the birefringent crystal (24); The birefringent crystal (24) receives the output light of the polarization rotation module (30) and performs polarization separation, so that the vertically polarized light exits from the first polarization-maintaining fiber collimator (22), and the horizontally polarized light exits from the second polarization-maintaining fiber collimator (23) after the transmission path is deflected.

3. The transmissive sensing probe according to claim 1, wherein The polarization rotation module includes a 45° Faraday rotator mirror (25) and a half-wave plate (26); During the forward propagation process, the polarization state is rotated clockwise by 45° through the 45° Faraday rotator mirror (25), and then rotated clockwise by 45° through the half-wave plate (26), and transmitted to the second polarization separation module (31); During the reverse propagation process, the polarization state is rotated counterclockwise by 45° through the half-wave plate (26), and then rotated clockwise by 45° through the 45° Faraday rotator mirror (25), and transmitted to the first polarization separation module (29).

4. The transmissive sensing probe according to claim 1, wherein The second polarization separation module (31) is a polarization beam splitter prism, and one of the polarization end faces of the polarization beam splitter prism is coated with a reflective film, and the other polarization end face is not coated.

5. An optical fiber sensing array with a reciprocal structure, characterized in that, Including N-channel sensing modules; each channel sensing module includes two transmissive sensing probe heads as described in any one of claims 1 to 4 and a delay optical fiber; The first transmissive sensing probe head and the second transmissive sensing probe head are symmetrically mirror-imaged, and the horizontally polarized light directly exits facing each other; One end of the delay optical fiber is connected to the second optical fiber interface of the first transmissive sensing probe, and the other end is connected to the second optical fiber interface of the second transmissive sensing probe; The lengths of the delay optical fibers in different sensing modules are different, and the length difference between the delay optical fibers of adjacent sensing modules is determined by the width of the optical pulse received by the probe.

6. The fiber optic sensing array according to claim 5, wherein The length of the delay optical fiber satisfies the following formula: , where is the refractive index of the optical fiber, is the difference in the lengths of adjacent delay optical fibers, is the speed of light, is the pulse width.

7. An optical fiber sensing array with a reciprocal structure, characterized in that, It includes N sensing modules; each sensing module includes a transmissive sensing probe as described in any one of claims 1 to 4 and two delay optical fibers with equal lengths; One end of the first delay optical fiber is connected to the first optical fiber interface of the transmissive sensing probe, and one end of the second delay optical fiber is connected to the second optical fiber interface of the transmissive sensing probe; The lengths of the delay optical fibers in different sensing modules are different, and the length difference between the delay optical fibers of adjacent sensing modules is determined by the width of the optical pulse received by the probe.

8. The fiber optic sensing array according to claim 7, wherein The length of the delay optical fiber satisfies the following formula: , where is the refractive index of the optical fiber, , is the length of the first delay optical fiber of the th path, is the length of the second delay optical fiber of the th path, is the length of the first delay optical fiber of the th path, is the length of the second delay optical fiber of the th path.

9. An optical fiber displacement sensing multiplexing system, characterized in that, It includes a light source module, a first circulator, a second circulator, a first 1×(N + 1) optical fiber coupler, a second 1×(N + 1) optical fiber coupler, an optical fiber sensing array with a reciprocal structure as described in claim 5 or 6, a reference optical fiber, and a signal detection and processing module; The light source module is used to provide two pulsed lights with a frequency difference; The input port of the first circulator is connected to the light source module, the reflection port is connected to the input port of the first 1×(N + 1) optical fiber coupler, and the output port is connected to the signal detection and processing module; The input port of the second circulator is connected to the light source module, the reflection port is connected to the input port of the second 1×(N + 1) optical fiber coupler, and the output port is connected to the signal detection and processing module; The first 1×(N + 1) optical fiber coupler, the input port is connected to the first circulator, and the output ports are respectively connected to the optical fiber sensing array and the reference optical fiber; The second 1×(N + 1) optical fiber coupler, the input port is connected to the second circulator, and the output ports are respectively connected to the optical fiber sensing array and the reference optical fiber; The reference optical fiber is used to output the light input by the first 1×(N + 1) optical fiber coupler clockwise from the second 1×(N + 1) optical fiber coupler, and output the light input by the second 1×(N + 1) optical fiber coupler counterclockwise from the first 1×(N + 1) optical fiber coupler; Optical fiber sensing array, in each sensing module, the first optical fiber interface of the first transmissive sensing probe is connected to the output port of the first 1×(N + 1) optical fiber coupler, and the first optical fiber interface of the second transmissive sensing probe is connected to the output port of the second 1×(N + 1) optical fiber coupler. When the mirror to be measured undergoes displacement, the first transmissive sensing probe transmits the light carrying the positive displacement signal clockwise through the delay optical fiber and transmits it to the second 1×(N + 1) optical fiber coupler through the second transmissive sensing probe; the second transmissive sensing probe transmits the light carrying the negative displacement signal counterclockwise through the delay optical fiber and transmits it to the first 1×(N + 1) optical fiber coupler through the first transmissive sensing probe, and the mirror to be measured is located between the directly transmitted ends of the horizontal polarized lights of the first transmissive sensing probe and the second transmissive sensing probe; A signal detection and processing module, which is used to receive and separate the sensing interference signals of each channel, then perform phase demodulation to obtain the phase of 1 reference fiber and the phases of N sensing fibers, subtract the reference phase from the N sensing phases to obtain the phase change caused by the displacement of the mirror to be measured, and calculate the displacements of N mirrors to be measured according to the N phase changes. Or, It includes a light source module, a first circulator, a second circulator, a first 1×(N + 1) fiber coupler, a second 1×(N + 1) fiber coupler, a fiber optic sensing array with a reciprocal structure as claimed in claim 7 or 8, a reference fiber, and a signal detection and processing module; The light source module is used to provide two pulsed lights with a frequency difference. The input port of the first circulator is connected to the light source module, the reflection port is connected to the input port of the first 1×(N + 1) fiber coupler, and the output port is connected to the signal detection and processing module; The input port of the second circulator is connected to the light source module, the reflection port is connected to the input port of the second 1×(N + 1) fiber coupler, and the output port is connected to the signal detection and processing module; The first 1×(N + 1) fiber coupler, its input port is connected to the first circulator, and the output ports are respectively connected to the fiber optic sensing array and the reference fiber; The second 1×(N + 1) fiber coupler, its input port is connected to the second circulator, and the output ports are respectively connected to the fiber optic sensing array and the reference fiber; The reference fiber is used to output the light input by the first 1×(N + 1) fiber coupler clockwise from the second 1×(N + 1) fiber coupler, and output the light input by the second 1×(N + 1) fiber coupler counterclockwise from the first 1×(N + 1) fiber coupler; In the fiber optic sensing array, in each sensing module, the first delay fiber is connected to the output port of the first 1×(N + 1) fiber coupler and the first fiber interface of the transmissive sensing probe, and is used to transmit the optical pulse carrying displacement information clockwise through the first delay fiber and the second delay fiber, and then transmit it to the second 1×(N + 1) fiber coupler; the second delay fiber is connected to the output port of the second 1×(N + 1) fiber coupler and the second fiber interface of the transmissive sensing probe, and is used to transmit the optical pulse not carrying displacement information counterclockwise through the second delay fiber and the first delay fiber, and then transmit it to the first 1×(N + 1) fiber coupler, and the mirror to be measured is located at the directly transmissive end of the horizontal polarized light of the transmissive sensing probe; The signal detection and processing module is used to receive and separate the sensing interference signals of each channel, then perform phase demodulation to obtain the phase of 1 reference fiber and the phases of N sensing fibers, subtract the reference phase from the N sensing phases to obtain the phase change caused by the displacement of the mirror to be measured, and calculate the displacements of N mirrors to be measured according to the N phase changes.

10. A demodulation method for the fiber optic displacement sensing multiplexing system as described in claim 9, characterized in that, It includes: Measuring the front-end common-mode noise in the reference fiber measurement optical path; Measuring N displacements to be measured respectively through the fiber optic sensing array; Subtracting the front-end common-mode noise by differentiating the phase information of the fiber optic sensing array and the phase information of the reference fiber.

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