A fiber perturbation compensation device and compensation method in coherent polarization state measurement

Abstract

The present invention relates to an optical fiber perturbation compensation device and compensation method in coherent polarization state measurement. After the radar echo signal is coupled into the polarization-maintaining optical fiber, it is divided into two coupled light beams propagating along the slow axis and the fast axis. Since the refractive indexes of the slow axis and the fast axis in the polarization-maintaining optical fiber are different, the external environment perturbation will cause the slow axis coupled light beam and the fast axis coupled light beam to add unequal phase changes. Based on the laser active coherent detection technology and the polarization radar technology, the present invention realizes optical fiber perturbation compensation through a reference optical path, eliminates the influence of different phase changes added to the slow axis coupled light beam and the fast axis coupled light beam after the echo light signal is coupled into the polarization-maintaining optical fiber, and adds the measurement of the echo polarization state on the basis of the measurement of the echo polarization component.

Description

Technical Field

[0001] The invention relates to the technical field of laser active coherent detection and polarization radar, and more specifically to an optical fiber perturbation compensation device and compensation method in coherent polarization state measurement. Technical Background

[0002] Laser radar uses the distance value of target pixels to estimate the three-dimensional shape of the target and identify the target. However, for targets with complex structures or camouflaged targets, there is ambiguity in determining the pixels of the target, which will lead to uncertainty in the recognition of the three-dimensional shape of the target. Using the polarization component of the reflected light to measure the target characteristics is a very promising method. Compared with the traditional polarization laser radar that uses direct detection to detect the Stokes parameters of the echo signal, the existing technology [Xu Qian, Sun Jianfeng, etc., Laser Active Coherent Balanced Detection Polarization Analyzer, CN106680831B[P]. 2019-02-26.] uses a full-space link and coherent balanced detection method to achieve high-sensitivity Stokes parameter measurement of the echo signal, and calculates its Mueller matrix through the change of the echo polarization state, analyzes the target reflected light intensity, polarization degree, depolarization and other characteristics, and obtains the characteristics of the target itself. Therefore, it is of great significance to accurately measure the polarization state of the echo signal.

[0003] The prior art [Xia Haiyun and Qiu Jiawei, et al., Single-detector all-fiber polarization laser radar based on time division multiplexing, CN106443709A[P].2017-02-22.] adopts an all-fiber link and uses a single detector to realize the simultaneous detection of horizontal polarization signals and vertical polarization signals in radar echoes. However, the phase disturbance caused by fiber perturbations is not considered. In practical applications, when measuring the polarization state of the echo signal, the fiber perturbation will cause the slow-axis coupled beam and the fast-axis coupled beam in the polarization-maintaining fiber to add unequal phase changes, making it difficult to measure the polarization state of the echo signal. It is usually necessary to improve the stability of the environment to reduce the impact of fiber perturbations, but this increases the difficulty of measurement; or simply consider that the fiber perturbations are "very small" and can be ignored, but because the slow-axis coupled beam and the fast-axis coupled beam add different phase changes, the measured polarization state of the echo signal is not the true polarization state of the echo signal. Summary of the invention

[0004] The present invention aims at the existing fiber polarization laser radar technology. Due to the external environmental perturbation causing the polarization-maintaining fiber slow-axis coupled beam and the fast-axis coupled beam to have additional unequal phase changes, it is difficult to measure the polarization state of the real echo signal. The present invention realizes fiber perturbation compensation through a reference optical path, eliminates the influence of environmental stability on the measurement, and adds the measurement of the echo polarization state on the basis of the measurement of the echo polarization component. The present invention realizes fiber perturbation compensation through a reference optical path, and in the laser active coherent detection and polarization radar technology, the polarization state of the echo signal can be measured without increasing the requirements for environmental stability.

[0005] The technical solution of the present invention includes the following steps:

[0006] 1. An optical fiber perturbation compensation device for coherent polarization state measurement, characterized in that it comprises a linear frequency modulated laser (1), a one-to-three polarization-maintaining optical fiber beam splitter (2), a polarization-maintaining optical fiber amplifier (3), a polarization-maintaining optical fiber circulator (4), a first optical fiber collimator (5), an adjustable optical fiber attenuator (6), a second optical fiber collimator (7), an optical reflector (8), a window plate (9), a polarization beam splitter (10), a one-to-two polarization-maintaining optical fiber beam splitter (11), a first 2×2 polarization-maintaining optical fiber coupler (12), a first balanced photodetector (13), a second 2×2 polarization-maintaining optical fiber coupler (14), a second balanced photodetector (15) and a data processor (16);

[0007] The chirp signal of the chirp laser output by the chirp laser (1) is divided into three optical signals with equal light intensities by a one-to-three polarization-maintaining optical fiber beam splitter (2). The first optical signal E A After the signal light is amplified by the polarization-maintaining optical fiber amplifier (3), it is input from the a port of the polarization-maintaining optical fiber circulator (4) and output from the b port, and the optical fiber output end face of the b port of the polarization-maintaining optical fiber circulator (4) is located on the front focal plane of the first optical fiber collimator 5; the signal light is collimated and output by the first optical fiber collimator (5) and becomes a spatial light beam, which is transmitted through the window plate 9 and irradiated onto the point target (17); the signal light is back-reflected by the point target (17) and reaches the first optical fiber collimator (5) through the window plate (9);

[0008] The second optical signal E B As reference light, it passes through an adjustable optical fiber attenuator (6), the optical fiber output end face of the adjustable optical fiber attenuator (6) is located on the front focal plane of the second optical fiber collimator (7), and the reference light is collimated and output by the second optical fiber collimator (7) to become a spatial light beam, which is reflected by an optical reflector (8) and a window plate (9) in sequence and then reaches the first optical fiber collimator (5);

[0009] The two spatial light beams are received by the first optical fiber collimator (5) and coupled into the polarization-maintaining optical fiber, propagated to the polarization beam splitter (10) through the polarization-maintaining optical fiber circulator (4), and split by the polarization beam splitter (10) into two beams of horizontal polarized light and vertical polarized light with orthogonal polarization directions, and the polarization directions of the two linear polarized lights are consistent with the slow axis direction of the optical fiber, and the two linear polarized lights respectively reach the first 2×2 polarization-maintaining optical fiber coupler (12) and the second 2×2 polarization-maintaining optical fiber coupler (14);

[0010] The third optical signal E C As the local oscillation light, it is split into two local oscillation light beams with equal light intensity by a one-to-two polarization-maintaining fiber beam splitter (11), and the polarization directions of the two local oscillation light beams are consistent with the direction of the slow axis of the optical fiber. The two local oscillation light beams arrive at a first 2×2 polarization-maintaining fiber coupler (12) and a second 2×2 polarization-maintaining fiber coupler (14) respectively;

[0011] After the horizontally polarized light and the vertically polarized light are mixed with the local oscillator light in the first 2×2 polarization-maintaining fiber coupler (12) and the second 2×2 polarization-maintaining fiber coupler (14), the first balanced photodetector (13) and the second balanced photodetector (15) convert the received optical mixing signals into electrical signals and transmit them to the data processor (16) for Fourier transformation. The signals are processed according to different frequency points, thereby eliminating the influence of different phase changes of the slow-axis coupled light beam and the fast-axis coupled light beam caused by external environmental perturbations, and realizing fiber perturbation compensation.

[0012] 2. A method for compensating optical fiber perturbations in coherent polarization state measurement, characterized in that it comprises the following steps:

[0013] S1. The output light field of the linear frequency modulated laser (1) is expressed as:

[0014]

[0015] in, represents the amplitude of the output beam of the linear frequency modulated laser (1), ω0 is the beam waist radius of the linear frequency modulated laser (1); is the Jones matrix of the linear frequency modulated laser (1) output polarized light transmitted in the polarization-maintaining optical fiber, the polarization direction of which is consistent with the slow axis direction of the optical fiber, f0 is the starting frequency of the linear frequency modulated laser (1), is the frequency modulation rate of the linear frequency modulated laser (1), is the initial phase of the linear frequency modulated laser (1);

[0016] S2. The chirp signal of the linear frequency modulated laser is divided into three optical signals of equal intensity by a one-to-three polarization-maintaining optical fiber beam splitter (2), one of which is used as a signal light path A, which is amplified by a polarization-maintaining optical fiber amplifier (3), enters the a port of the polarization-maintaining optical fiber circulator (4), and is output from the b port, and the optical fiber output end face is located on the front focal plane of the first optical fiber collimator (5); then, the signal light is collimated and output by the first optical fiber collimator (5) to become a spatial light beam, which is transmitted by the window plate (9) and irradiated onto the point target (17), and its light field expression is:

[0017]

[0018] Where ξ is the power gain of the polarization-maintaining fiber amplifier, For z t The spot radius at the point, For z t The radius of curvature of the wavefront at point is with z t The phase factor, z t is the distance between the point target (17) and the first optical fiber collimator (5), and satisfies z t >>F1, is the waist radius of the spatial light beam emitted after the signal light beam passes through the first optical fiber collimator (5), the waist position is located on the rear focal plane of the first optical fiber collimator (5), and F1 is the focal length of the first optical fiber collimator (5); and They represent the fiber coordinates (x f ,y f ) and the transformation matrix of the spatial coordinates (x, y) and the transmission Jones matrix of the window (9); n1, n s and n2 are the refractive indices of air, the slow axis of the polarization-maintaining optical fiber and the window plate (9), respectively; L1 is the optical fiber length between the linear frequency modulation laser 1 and the first optical fiber collimator (5); L w is the geometric length of the signal beam propagating in the window (9), and satisfies z t >>L w ; τ A1 The signal beam is output from a linear frequency modulated laser (1), and passes through a one-to-three polarization-maintaining fiber beam splitter (2), a polarization-maintaining fiber amplifier (3), a polarization-maintaining fiber circulator (4), a first fiber collimator (5), a fiber length L1, a window plate (9), and a space distance z in sequence. t Additional time delays;

[0019] S3. The point target (17) is back-reflected, undergoes far-field Fraunhofer diffraction, and the echo beam passes through the window (9) and reaches the first fiber collimator (5). The light field expression before the beam is:

[0020]

[0021] where ρ is the reflection coefficient of the point target (17), is the Jones matrix of the point target (17), τ A2 is the distance τ that the light beam travels through t and a time delay added to the window (9);

[0022] S4. The echo beam is received by the first optical fiber collimator (5) and coupled into the polarization-maintaining optical fiber. The optical field expression before propagating to the polarization beam splitter (10) is:

[0023]

[0024] Wherein, η is the coupling efficiency of the first optical fiber collimator (5), The Jones matrix of polarization-maintaining fiber, the slow axis is the light passing in the x direction, and the fast axis is the light passing in the y direction. and are the intrinsic phase constants of the slow axis beam and the fast axis beam of the polarization-maintaining fiber within the fiber distance L2, n f is the refractive index of the fast axis of the polarization-maintaining optical fiber, L2 is the length of the optical fiber between the first optical fiber collimator (5) and the polarization beam splitter (10), δ s and δ f They represent the additional phase changes of the slow-axis coupled beam and the fast-axis coupled beam caused by the external environment perturbation within the optical fiber distance of L2; is the spatial coordinate (x, y) and the fiber coordinate (x f ,y f )’s transformation matrix; τ A3 A time delay added to the light beam passing through the first optical fiber collimator (5), the polarization-maintaining optical fiber circulator (4) and the optical fiber length L2;

[0025] S5. The coupled light beam is split into two linearly polarized lights with orthogonal polarization directions in the polarization beam splitter (10), and the polarization directions of the two linearly polarized lights are made consistent with the slow axis direction of the optical fiber by adjusting the two optical fibers at the output end of the polarization beam splitter (10). Subsequently, the light field expressions before reaching the first (second) 2×2 polarization-maintaining optical fiber coupler (12) ((14)) are respectively:

[0026]

[0027]

[0028] in, is the intrinsic phase constant added to the slow axis beam of the polarization-maintaining fiber within the fiber distance L3, L3 is the fiber length between the polarization beam splitter (10) (or the one-to-two polarization-maintaining fiber splitter (11)) and the first (second) 2×2 polarization-maintaining fiber coupler (12) ((14)), δ′ s is the additional phase change of the slow-axis coupled beam caused by the external environment perturbation within the fiber distance L3; represents the Jones matrix of the polarization beam splitter (10), It represents the conversion matrix of adjusting the fast axis direction of the polarization-maintaining fiber to the slow axis direction, τ A4 Adding a time delay to the light beam passing through the polarization beam splitter (10), the first (second) 2×2 polarization-maintaining fiber coupler (10) ((14)) and the fiber length L3; and E H (x f ,y f ; t) and E V (x f ,y f ; t) Simplified expression for time and space separation;

[0029] S6. The laser output optical signal passes through a one-to-three polarization-maintaining optical fiber beam splitter (2) and is divided into three optical signals of equal intensity, one of which is used as a reference optical path B; it passes through an adjustable optical fiber attenuator (6) and is collimated and output by a second optical fiber collimator (7), and its optical fiber output end face is located on the front focal plane of the second optical fiber collimator (7); then, the reference light becomes a spatial beam, and after being reflected by an optical reflector (8) and a window plate (9) in sequence, the propagation direction is changed, and the optical field expression before reaching the first optical fiber collimator (5) is:

[0030]

[0031] where ε is the power attenuation of the adjustable fiber attenuator (6), For z r The spot radius at the point, For z r The radius of curvature of the wavefront at point is with z r The phase factor, z r is the spatial distance between the reference light beam emitted from the second optical fiber collimator (7) and reflected by the optical reflector (8) and the window plate (9) to the first optical fiber collimator (5), is the waist radius of the spatial light beam emitted after the reference light beam passes through the second optical fiber collimator (7), the waist position is located on the rear focal plane of the second optical fiber collimator (7), and F′1 is the focal length of the second optical fiber collimator (7); and are respectively the reflection Jones matrices of the optical reflector (8) and the window plate (9); L′1 is the optical fiber length between the linear frequency modulated laser (1) and the second optical fiber collimator (7); τ B The light beam passes through a one-to-three polarization-maintaining optical fiber beam splitter (2), an adjustable optical fiber attenuator (6), a second optical fiber collimator (7), an optical fiber length L′1, and a spatial distance z r Additional time delays;

[0032] S7. The reference beam is also received by the first fiber collimator (5), and then coupled into the optical fiber. The optical field expression before propagating to the polarization beam splitter (10) is:

[0033]

[0034] Subsequently, the reference beam is also split into two linearly polarized beams with orthogonal polarization directions by the polarization beam splitter (10), and the polarization directions of the two linearly polarized beams are also adjusted to be consistent with the slow axis direction of the optical fiber. The light field expressions before reaching the first (second) 2×2 polarization-maintaining optical fiber coupler (12) ((14)) are respectively:

[0035]

[0036]

[0037] in, and is E′ H (x f ,y f ; t) and E′ V (x f ,y f ; t) A simplified expression for time and space separation.

[0038] S8. The laser output optical signal passes through the one-to-three polarization-maintaining fiber beam splitter (2) and is equally divided into three optical signals with equal light intensities, and the last one is used as the local oscillator optical path C. The optical field expression before propagating to the one-to-two polarization-maintaining fiber beam splitter (11) is:

[0039]

[0040] L″1 is the length of the optical fiber between the linear frequency modulated laser (1) and the one-to-two polarization-maintaining optical fiber beam splitter (11), τ C The time delay added to the light beam passing through the one-to-three polarization-maintaining fiber beam splitter (2) and the fiber length L″1.

[0041] Subsequently, the local oscillator beam is split into two identical linearly polarized beams through a one-to-two polarization-maintaining fiber beam splitter (11), and the polarization directions of the two beams are consistent with the direction of the slow axis of the optical fiber. The light field expressions before reaching the first (second) 2×2 polarization-maintaining fiber coupler (12) ((14)) are:

[0042]

[0043]

[0044] Among them, τ C1 The time delay added when the light beam passes through the one-to-two polarization-maintaining fiber beam splitter (11), the first (second) 2×2 polarization-maintaining fiber coupler (12) ((14)) and the fiber length L3, and E″ C1 (x f ,y f ; t) and E″ C2 (x f ,y f ; t) Simplified expression for time and space separation;

[0045] S9. The coupling ratio of the first (second) 2×2 polarization-maintaining fiber coupler (12) ((14)) is 1:1. The first balanced photodetector (13) converts the received optical mixing signal into an electrical signal, which is expressed as:

[0046]

[0047] The output electrical signal of the second balanced photodetector (15) is expressed as:

[0048]

[0049] S10. In the data processor (16), the I output by the first balanced photodetector (13) is H (t) Perform Fourier transform:

[0050]

[0051] Where T is the period of the FMCW signal, Among them, τ s =τ A1 -τ A2 -τ A3 -τ A4 , τ r =τ B -τ A3 -τ A4 , τ LO =τ C -τC1 ;

[0052] The I output by the second balanced photodetector (15) V (t) Perform Fourier transform:

[0053]

[0054] According to the different Fourier transform frequencies, Operation to achieve fiber perturbation compensation.

[0055] The present invention has the following characteristics:

[0056] 1. The fiber perturbation compensation is realized by the reference optical path, which eliminates the influence of different phase changes of the slow-axis coupled beam and the fast-axis coupled beam caused by the external environment perturbation after the echo light signal is coupled into the polarization-maintaining fiber. On the basis of the measurement of the echo polarization component, the measurement of the echo polarization state is added.

[0057] 2. When measuring the echo polarization state, the signal beam, reference beam and local oscillator beam are the same light source, and no other measurement equipment is required. It has the advantages of system stability, compact structure and easy integration.

[0058] Technical effects of the present invention:

[0059] 1. The present invention adopts the method of fiber perturbation compensation to eliminate the influence of different phase changes of the slow-axis coupled beam and the fast-axis coupled beam caused by external environmental perturbations after the echo light signal is coupled into the polarization-maintaining fiber, and obtains the true polarization state of the echo signal. Even when the stability of the measurement environment is poor, the true polarization state of the echo signal can be obtained, which is an important method improvement for obtaining the polarization state information of the echo in polarization laser radar technology.

[0060] 2. The present invention uses the signal beam, reference beam and local oscillator beam as the same light source. When measuring the polarization state of the echo, there is no need to add other measuring equipment. The structure is compact and easy to integrate. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Figure 1 It is a structural diagram of the device of the present invention.

[0062] Figure 2 Schematic diagram of the end face of the polarization-maintaining optical fiber and the coordinate system built inside the optical fiber in the present invention. DETAILED DESCRIPTION

[0063] The present invention will be further described in detail below with reference to the accompanying drawings and examples, but the protection scope of the present invention shall not be limited thereto.

[0064] Figure 1It is a fiber perturbation compensation device in the coherent polarization state measurement of the present invention. As can be seen from the figure, the optical path diagram of the present invention includes a linear frequency modulation laser 1, a one-to-three polarization-maintaining fiber beam splitter 2; the signal light path A module includes a polarization-maintaining fiber amplifier 3, a polarization-maintaining fiber circulator 4, wherein a, b and c are respectively the three ports of the polarization-maintaining fiber circulator 4, and a first fiber collimator 5; the reference light path B module includes an adjustable fiber attenuator 6, a second fiber collimator 7, an optical reflector 8, and a window 9; the local oscillator light path C module; the processing module includes a polarization beam splitter 10, a one-to-two polarization-maintaining fiber beam splitter 11, a first 2×2 polarization-maintaining fiber coupler 12, a first balanced photodetector 13, a second 2×2 polarization-maintaining fiber coupler 14, a second balanced photodetector 15, and a data processor 16; and a point target 17. The solid line in the figure is the polarization-maintaining fiber optical path, and the dotted line is the spatial optical path.

[0065] Figure 2 is the coordinate system (x f ,y f ) schematic diagram. As can be seen from the figure, the x coordinate system constructed in the optical fiber in the present invention f The y axis of the constructed coordinate system coincides with the slow axis of the polarization-maintaining fiber. f The axis coincides with the fast axis of the polarization-maintaining fiber, and the beam propagation direction is the z-axis direction. The plane where the first fiber collimator 5 is located is the system platform transmitting / receiving surface, and its coordinate system is represented by (x, y). The coordinates of the point target 17 are set to (x T ,y T ).

[0066] The output light field expression of linear frequency modulated laser 1 is:

[0067]

[0068] in, represents the amplitude of the output light beam of the linear frequency modulated laser 1, and ω0 is the beam waist radius of the linear frequency modulated laser 1. is the Jones matrix of the linear frequency modulated laser 1 output polarized light transmitted in the polarization-maintaining fiber, and its polarization direction is consistent with the slow axis direction of the fiber. f0 is the starting frequency of the linear frequency modulated laser 1, is the frequency modulation rate of the linear frequency modulation laser 1, is the initial phase of the linear frequency modulated laser 1.

[0069] The chirp signal of the linear frequency modulated laser is divided into three light signals with equal light intensity through the one-to-three polarization-maintaining fiber beam splitter 2, one of which is used as the signal light path A. After the gain of the polarization-maintaining fiber amplifier 3, it enters the a port of the polarization-maintaining fiber circulator 4 and is output from the b port. The output end face of the fiber is located on the front focal plane of the first fiber collimator 5. Subsequently, the signal light is collimated and output by the first fiber collimator 5 and becomes a spatial beam, and its output beam waist radius is:

[0070]

[0071] Wherein, λ is the output wavelength of the linear frequency modulated laser 1, and F1 is the focal length of the first fiber collimator 5. The beam waist position is located on the back focal plane of the first fiber collimator 5.

[0072] After the signal light is transmitted through the window 9, it is irradiated onto the point target 17, and its light field expression is:

[0073]

[0074] Where ξ is the power gain of the polarization-maintaining fiber amplifier. For z t The spot radius at the point, For z t The radius of curvature of the wavefront at point is with z t The phase factor, z t is the distance between the point target 17 and the first optical fiber collimator 5, and satisfies z t >>F1. and They represent the fiber coordinates (x f ,y f ) and the transformation matrix of the spatial coordinates (x, y) and the transmission Jones matrix of window 9. n1, n s and n2 are the refractive indices of air, the slow axis of the polarization-maintaining fiber, and the window 9, respectively; L1 is the fiber length between the linear frequency-modulated laser 1 and the first fiber collimator 5; L w is the geometric length of the signal beam propagating in the window 9, and satisfies z t >>L w τ A1 The signal beam is output from the linear frequency modulation laser 1, and passes through the one-to-three polarization-maintaining fiber beam splitter 2, the polarization-maintaining fiber amplifier 3, the polarization-maintaining fiber circulator 4, the first fiber collimator 5, the fiber length L1, the window 9 and the space distance z in sequence. t Additional time delay.

[0075] The point target 17 is back-reflected, undergoes far-field Fraunhofer diffraction, and the return beam passes through the window plate 9. The light field expression before reaching the first fiber collimator 5 is:

[0076]

[0077] Where ρ is the reflection coefficient of the point target 17, is the Jones matrix of point target 17, τ A2 is the space distance z that the beam passes through t And window 9 additional time delay.

[0078] The echo beam is received by the first optical fiber collimator 5 and coupled into the polarization-maintaining optical fiber. The optical field expression before propagating to the polarization beam splitter 10 is:

[0079]

[0080] Wherein, η is the coupling efficiency of the first optical fiber collimator 5, The Jones matrix of polarization-maintaining fiber, the slow axis is the light passing in the x direction, and the fast axis is the light passing in the y direction. and are the intrinsic phase constants of the slow axis beam and the fast axis beam of the polarization-maintaining fiber within the fiber distance L2, n f is the refractive index of the fast axis of the polarization-maintaining optical fiber, L2 is the optical fiber length between the first optical fiber collimator 5 and the polarization beam splitter 10, δ s and δ f They respectively represent the additional phase changes of the slow-axis coupled beam and the fast-axis coupled beam caused by the external environment perturbation within the optical fiber distance L2. is the spatial coordinate (x, y) and the fiber coordinate (x f ,y f ) is the transformation matrix. A3 The time delay added to the light beam passing through the first fiber collimator 5, the polarization-maintaining fiber circulator 4 and the fiber length L2.

[0081] The coupled light beam is split into two linearly polarized lights with orthogonal polarization directions in the polarization beam splitter 10, and the polarization directions of the two linearly polarized lights are made consistent with the slow axis direction of the optical fiber by adjusting the two optical fibers at the output end of the polarization beam splitter 10. Subsequently, the light field expressions before reaching the first (second) 2×2 polarization-maintaining fiber coupler 12 (14) are respectively:

[0082]

[0083]

[0084] in, is the intrinsic phase constant of the slow-axis polarization-maintaining fiber beam added within the fiber distance L3, L3 is the fiber length between the polarization beam splitter 10 (or the one-to-two polarization-maintaining fiber splitter 11) and the first (second) 2×2 polarization-maintaining fiber coupler 12 (14), δ′ s It is the additional phase change of the slow-axis coupled beam caused by external environmental perturbations within the fiber distance L3. represents the Jones matrix of the polarization beam splitter 10, Represents the conversion matrix of adjusting the fast axis direction of the polarization-maintaining fiber to the slow axis direction. τ A4 The time delay added to the light beam passes through the polarization beam splitter 10, the first (second) 2×2 polarization-maintaining fiber coupler 12 (14) and the fiber length L3. and E H (x f ,y f ; t) and E V (x f ,y f ; t) A simplified expression for time and space separation.

[0085] The laser output optical signal is divided into three optical signals with equal light intensity through the one-to-three polarization-maintaining fiber beam splitter 2, one of which is used as the reference light path B. After passing through the adjustable fiber attenuator 6, it is collimated and output by the second fiber collimator 7, and its fiber output end face is located on the front focal plane of the second fiber collimator 7. Subsequently, the reference light becomes a spatial beam, and its output beam waist radius is:

[0086]

[0087] Wherein, F′1 is the focal length of the second optical fiber collimator 7 , and the beam waist position is located on the rear focal plane of the second optical fiber collimator 7 .

[0088] After the reference light is reflected by the optical reflector 8 and the window 9, the propagation direction is changed, and the light field before reaching the first fiber collimator 5 is expressed as:

[0089]

[0090] Where, ε is the power attenuation of the adjustable optical fiber attenuator 6, For z r The spot radius at the point, For z r The radius of curvature of the wavefront at point is with z r The phase factor, z rIt is the spatial distance length between the reference light beam emitted from the second optical fiber collimator 7 and reflected by the optical reflector 8 and the window plate 9 to the first optical fiber collimator 5 . and are the reflection Jones matrices of the optical reflector 8 and the window 9 respectively. L′1 is the fiber length between the linear frequency modulated laser 1 and the second fiber collimator 7, τ B The light beam passes through the one-to-three polarization-maintaining fiber beam splitter 2, the adjustable fiber attenuator 6, the second fiber collimator 7, the fiber length L′1 and the spatial distance z r Additional time delay.

[0091] The reference beam is also received by the first fiber collimator 5, and then coupled into the optical fiber. The optical field expression before propagating to the polarization beam splitter 10 is:

[0092]

[0093] Subsequently, the reference beam is also split into two linearly polarized beams with orthogonal polarization directions by the polarization beam splitter 10, and the polarization directions of the two linearly polarized beams are also adjusted to be consistent with the slow axis direction of the optical fiber. The light field expressions before reaching the first (second) 2×2 polarization-maintaining fiber coupler 12 (14) are respectively:

[0094]

[0095]

[0096] in, and is E′ H (x f ,y f ; t) and E′ V (x f ,y f ; t) A simplified expression for time and space separation.

[0097] The laser output optical signal is divided into three optical signals with equal light intensity through the one-to-three polarization-maintaining fiber beam splitter 2, and the last one is used as the local oscillator optical path C. The optical field expression before propagating to the one-to-two polarization-maintaining fiber beam splitter 11 is:

[0098]

[0099] L″1 is the fiber length between the linear frequency modulated laser 1 and the one-to-two polarization-maintaining fiber beam splitter 11, τ C The time delay added when the light beam passes through the one-to-three polarization-maintaining fiber splitter 2 and the fiber length L″1.

[0100] Subsequently, the local oscillator beam is split into two identical linearly polarized beams by a one-to-two polarization-maintaining fiber beam splitter 11, and the polarization directions of the two beams are consistent with the slow axis direction of the fiber. The light field expressions before reaching the first (second) 2×2 polarization-maintaining fiber coupler 12 (14) are:

[0101]

[0102]

[0103] Among them, τ C1 The time delay added to the light beam passing through the one-to-two polarization-maintaining fiber splitter 11, the first (second) 2×2 polarization-maintaining fiber coupler 12 (14) and the fiber length L3. and E″ C1 (x f ,y f ; t) and E″ C2 (x F ,y F ; t) A simplified expression for time and space separation.

[0104] The coupling ratio of the first (second) 2×2 polarization-maintaining fiber coupler 12 (14) is 1:1. The first balanced photodetector 13 converts the received optical mixing signal into an electrical signal, which is expressed as:

[0105]

[0106] By adjusting the power attenuation ε of the adjustable optical fiber attenuator 6, the reference optical power is controlled to be much smaller than the local oscillator optical power, so that can be ignored, so:

[0107]

[0108] In the data processor 16, a Fourier transform is first performed:

[0109]

[0110] Where T is the period of the FMCW signal, Among them, τ s =τ A1 -τ A2 -τ A3 -τ A4 , τ r =τ B -τ A3 -τ A4 , τ LO =τ C -τ C1 .

[0111] Similarly, the output electrical signal of the second balanced photodetector 15 is expressed as:

[0112]

[0113] Its Fourier transform is:

[0114]

[0115]

[0116] Then, according to the different Fourier transform frequencies, the phase change of the slow axis coupled beam caused by the external environment perturbation within the fiber distance L3 is compensated: and Operation, also known as compensation Here, “*” indicates complex conjugate.

[0117] Finally, compensate for the additional phase change of the slow-axis coupled beam and the fast-axis coupled beam caused by the external environment perturbation within the fiber distance L2: Operation can eliminate

[0118] In the solution process, only is an unknown quantity that carries target information, and The transmission and reflection Jones matrices of other optical components can be calculated or measured. The fiber perturbation compensation is realized through the reference optical path to eliminate the influence of different phase changes of the slow-axis coupled beam and the fast-axis coupled beam caused by the external environment perturbation. Based on the measurement of the polarization component, the real polarization state of the echo signal is solved.

Claims

1. An optical fiber perturbation compensation device in coherent polarization state measurement, characterized in that: It comprises a linear frequency modulated laser (1), a one-to-three polarization-maintaining fiber beam splitter (2), a polarization-maintaining fiber amplifier (3), a polarization-maintaining fiber circulator (4), a first fiber collimator (5), an adjustable fiber attenuator (6), a second fiber collimator (7), an optical reflector (8), a window plate (9), a polarization beam splitter (10), a one-to-two polarization-maintaining fiber beam splitter (11), a first 2×2 polarization-maintaining fiber coupler (12), a first balanced photodetector (13), a second 2×2 polarization-maintaining fiber coupler (14), a second balanced photodetector (15) and a data processor (16); The chirp signal of the chirp laser output by the chirp laser (1) is divided into three optical signals with equal light intensities by a one-to-three polarization-maintaining optical fiber beam splitter (2). The first optical signal E A After the signal light is amplified by the polarization-maintaining optical fiber amplifier (3), it is input from the a port of the polarization-maintaining optical fiber circulator (4) and output from the b port, and the optical fiber output end face of the b port of the polarization-maintaining optical fiber circulator (4) is located on the front focal plane of the first optical fiber collimator 5; the signal light is collimated and output by the first optical fiber collimator (5) and becomes a spatial light beam, which is transmitted through the window plate 9 and irradiated onto the point target (17); the signal light is back-reflected by the point target (17) and reaches the first optical fiber collimator (5) through the window plate (9); The second optical signal E B As reference light, it passes through an adjustable optical fiber attenuator (6), the optical fiber output end face of the adjustable optical fiber attenuator (6) is located on the front focal plane of the second optical fiber collimator (7), and the reference light is collimated and output by the second optical fiber collimator (7) to become a spatial light beam, which is reflected by an optical reflector (8) and a window plate (9) in sequence and then reaches the first optical fiber collimator (5); The two spatial light beams are received by the first optical fiber collimator (5) and coupled into the polarization-maintaining optical fiber, propagated to the polarization beam splitter (10) through the polarization-maintaining optical fiber circulator (4), and split by the polarization beam splitter (10) into two beams of horizontal polarized light and vertical polarized light with orthogonal polarization directions, and the polarization directions of the two linear polarized lights are consistent with the slow axis direction of the optical fiber, and the two linear polarized lights respectively reach the first 2×2 polarization-maintaining optical fiber coupler (12) and the second 2×2 polarization-maintaining optical fiber coupler (14); The third optical signal E C As the local oscillation light, it is split into two local oscillation light beams with equal light intensity by a one-to-two polarization-maintaining fiber beam splitter (11), and the polarization directions of the two local oscillation light beams are consistent with the direction of the slow axis of the optical fiber. The two local oscillation light beams arrive at a first 2×2 polarization-maintaining fiber coupler (12) and a second 2×2 polarization-maintaining fiber coupler (14) respectively; After the horizontally polarized light and the vertically polarized light are mixed with the local oscillator light in the first 2×2 polarization-maintaining fiber coupler (12) and the second 2×2 polarization-maintaining fiber coupler (14), the first balanced photodetector (13) and the second balanced photodetector (15) convert the received optical mixing signals into electrical signals and transmit them to the data processor (16) for Fourier transformation. The signals are processed according to different frequency points, thereby eliminating the influence of different phase changes of the slow-axis coupled light beam and the fast-axis coupled light beam caused by external environmental perturbations, and realizing fiber perturbation compensation.

2. A method for compensating optical fiber perturbations in coherent polarization state measurement, characterized in that: The steps include: S1. The output light field of the linear frequency modulated laser (1) is expressed as: in, represents the amplitude of the output beam of the linear frequency modulated laser (1), ω0 is the beam waist radius of the linear frequency modulated laser (1); is the Jones matrix of the linear frequency modulated laser (1) output polarized light transmitted in the polarization-maintaining optical fiber, the polarization direction of which is consistent with the slow axis direction of the optical fiber, f0 is the starting frequency of the linear frequency modulated laser (1), is the frequency modulation rate of the linear frequency modulated laser (1), is the initial phase of the linear frequency modulated laser (1); S2. The chirp signal of the linear frequency modulated laser is divided into three optical signals of equal intensity by a one-to-three polarization-maintaining optical fiber beam splitter (2), one of which is used as a signal light path A, which is amplified by a polarization-maintaining optical fiber amplifier (3), enters the a port of the polarization-maintaining optical fiber circulator (4), and is output from the b port, and the optical fiber output end face is located on the front focal plane of the first optical fiber collimator (5); then, the signal light is collimated and output by the first optical fiber collimator (5) to become a spatial light beam, which is transmitted by the window plate (9) and irradiated onto the point target (17), and its light field expression is: Where ξ is the power gain of the polarization-maintaining fiber amplifier, For z t The spot radius at the point, For z t The radius of curvature of the wavefront at point is with Ω t The phase factor, z t is the distance between the point target (17) and the first optical fiber collimator (5), and satisfies z t >>F1, is the waist radius of the spatial light beam emitted after the signal light beam passes through the first optical fiber collimator (5), the waist position is located on the rear focal plane of the first optical fiber collimator (5), and F1 is the focal length of the first optical fiber collimator (5); and They represent the fiber coordinates (x f ,y f ) and the transformation matrix of the spatial coordinates (x, y) and the transmission Jones matrix of the window (9); n1, n s and n2 are the refractive indices of air, the slow axis of the polarization-maintaining optical fiber and the window plate (9), respectively; L1 is the optical fiber length between the linear frequency modulation laser 1 and the first optical fiber collimator (5); L w is the geometric length of the signal beam propagating in the window (9), and satisfies z t >>L w ; τ A1 The signal beam is output from a linear frequency modulated laser (1), and passes through a one-to-three polarization-maintaining fiber beam splitter (2), a polarization-maintaining fiber amplifier (3), a polarization-maintaining fiber circulator (4), a first fiber collimator (5), a fiber length L1, a window plate (9), and a space distance z in sequence. t Additional time delays; S3. The point target (17) is back-reflected, undergoes far-field Fraunhofer diffraction, and the echo beam passes through the window (9) and reaches the first fiber collimator (5). The light field expression before the beam is: where ρ is the reflection coefficient of the point target (17), is the Jones matrix of the point target (17), τ A2 is the space distance z that the beam passes through t and a time delay added to the window (9); S4. The echo beam is received by the first optical fiber collimator (5) and coupled into the polarization-maintaining optical fiber. The optical field expression before propagating to the polarization beam splitter (10) is: Wherein, η is the coupling efficiency of the first optical fiber collimator (5), The Jones matrix of polarization-maintaining fiber, the slow axis is the light passing in the x direction, and the fast axis is the light passing in the y direction. and are the intrinsic phase constants of the slow axis beam and the fast axis beam of the polarization-maintaining fiber within the fiber distance L2, n f is the refractive index of the fast axis of the polarization-maintaining optical fiber, L2 is the length of the optical fiber between the first optical fiber collimator (5) and the polarization beam splitter (10), and v s and δ f They represent the additional phase changes of the slow-axis coupled beam and the fast-axis coupled beam caused by the external environment perturbation within the optical fiber distance of L2; is the spatial coordinate (x, y) and the fiber coordinate (x f ,y f )’s transformation matrix; τ A3 A time delay added to the light beam passing through the first optical fiber collimator (5), the polarization-maintaining optical fiber circulator (4) and the optical fiber length L2; S5. The coupled light beam is split into two linearly polarized lights with orthogonal polarization directions in the polarization beam splitter (10), and the polarization directions of the two linearly polarized lights are made consistent with the slow axis direction of the optical fiber by adjusting the two optical fibers at the output end of the polarization beam splitter (10). Subsequently, the light field expressions before reaching the first 2×2 polarization-maintaining fiber coupler (12) and the second 2×2 polarization-maintaining fiber coupler (14) are respectively: in, is the inherent phase constant added to the slow axis light beam of the polarization-maintaining optical fiber within the optical fiber distance L3, L3 is the optical fiber length between the polarization beam splitter (10) or the one-to-two polarization-maintaining optical fiber splitter (11) and the first 2×2 polarization-maintaining optical fiber coupler (12) and the second 2×2 polarization-maintaining optical fiber coupler (14), δ s ′ is the additional phase change of the slow-axis coupled beam caused by the external environment perturbation within the fiber distance L3; represents the Jones matrix of the polarization beam splitter (10), It represents the conversion matrix of adjusting the fast axis direction of the polarization-maintaining fiber to the slow axis direction, τ A4 Adding a time delay to the light beam passing through the polarization beam splitter (10), the first 2×2 polarization-maintaining fiber coupler (12), the second 2×2 polarization-maintaining fiber coupler (14), and the fiber length L3; and E H (x f ,y f ; t) and E V (x f ,y f ; t) Simplified expression for time and space separation; S6. The laser output optical signal passes through a one-to-three polarization-maintaining optical fiber beam splitter (2) and is divided into three optical signals of equal intensity, one of which is used as a reference optical path B; it passes through an adjustable optical fiber attenuator (6) and is collimated and output by a second optical fiber collimator (7), and its optical fiber output end face is located on the front focal plane of the second optical fiber collimator (7); then, the reference light becomes a spatial beam, and after being reflected by an optical reflector (8) and a window plate (9) in sequence, the propagation direction is changed, and the optical field expression before reaching the first optical fiber collimator (5) is: where ε is the power attenuation of the adjustable fiber attenuator (6), For z r The spot radius at the point, For z r The radius of curvature of the wavefront at point is with z r The phase factor, z r is the spatial distance between the reference light beam emitted from the second optical fiber collimator (7) and reflected by the optical reflector (8) and the window plate (9) to the first optical fiber collimator (5), F1 is the waist radius of the spatial beam emitted after the reference beam passes through the second optical fiber collimator (7), and the waist position is located on the rear focal plane of the second optical fiber collimator (7). ′ is the focal length of the second optical fiber collimator (7); and are the reflection Jones matrices of the optical reflector (8) and the window (9), respectively; L ′ 1 is the length of the optical fiber between the linear frequency modulated laser (1) and the second optical fiber collimator (7), τ B The light beam passes through a one-to-three polarization-maintaining optical fiber beam splitter (2), an adjustable optical fiber attenuator (6), a second optical fiber collimator (7), an optical fiber length L ′ 1 and the spatial distance z r Additional time delays; S7. The reference beam is also received by the first fiber collimator (5), and then coupled into the optical fiber. The optical field expression before propagating to the polarization beam splitter (10) is: Subsequently, the reference beam is also split into two linearly polarized beams with orthogonal polarization directions through the polarization beam splitter (10), and the polarization directions of the two linearly polarized beams are also adjusted to be consistent with the slow axis direction of the optical fiber; the optical field expressions before reaching the first 2×2 polarization-maintaining optical fiber coupler (12) and the second 2×2 polarization-maintaining optical fiber coupler (14) are respectively: in, and is E′ H (x f ,y f ; t) and E ′ V (x f ,y f ; t) Simplified expression for time and space separation; S8. The laser output optical signal passes through the one-to-three polarization-maintaining fiber beam splitter (2) and is divided into three optical signals with equal light intensities, and the last one is used as the local oscillator optical path C; the optical field expression before propagating to the one-to-two polarization-maintaining fiber beam splitter (11) is: L″1 is the length of the optical fiber between the linear frequency modulated laser (1) and the one-to-two polarization-maintaining optical fiber beam splitter (11), τ C The time delay added to the light beam passing through the one-to-three polarization-maintaining optical fiber beam splitter (2) and the optical fiber length L″1; Subsequently, the local oscillator beam is split into two identical linearly polarized beams through a one-to-two polarization-maintaining fiber beam splitter (11), and the polarization directions of the two beams are consistent with the direction of the slow axis of the optical fiber. The light field expressions before reaching the first 2×2 polarization-maintaining fiber coupler (12) and the second 2×2 polarization-maintaining fiber coupler (14) are respectively: Among them, τ C1 The time delay added to the light beam passing through the one-to-two polarization-maintaining fiber beam splitter (11), the first 2×2 polarization-maintaining fiber coupler (12), the second 2×2 polarization-maintaining fiber coupler (14) and the fiber length L3, and E″ C1 (x f ,y f ; t) and E″ C2 (x f ,y f ; t) Simplified expression for time and space separation; S9. The coupling ratios of the first 2×2 polarization-maintaining fiber coupler (12) and the second 2×2 polarization-maintaining fiber coupler (14) are both 1:1, and the first balanced photodetector (13) converts the received optical mixing signal into an electrical signal, which is expressed as: The output electrical signal of the second balanced photodetector (15) is expressed as: S10. In the data processor (16), the I output by the first balanced photodetector (13) is H (t) Perform Fourier transform: Where T is the period of the FMCW signal, Among them, τ s =τ A1 -τ A2 -τ A3 -τ A4 , τ r =τ B -τ A3 -τ A4 , τ LO =τ C -τ C1 ; The I output by the second balanced photodetector (15) i (t) Perform Fourier transform: According to the different Fourier transform frequencies, Operation to achieve fiber perturbation compensation.

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