CN112461276A - System and method for reducing OFDR light source nonlinear phase influence - Google Patents
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- G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
- G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
- G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
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- G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
- G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
- G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
- G01D5/353—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre
- G01D5/35306—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using an interferometer arrangement
Abstract
The invention discloses a system and a method for reducing the nonlinear phase influence of an OFDR light source, belonging to the technical field of distributed optical fiber sensing. The invention solves the problems that the optical path is complex, the reference optical fiber has length limitation, external disturbance generates different noise items on the optical fibers at different disturbance positions, and the compensation of the nonlinear phase of the tunable light source is related to the length of the test optical fiber in the prior art. The optical path structure mainly comprises a tunable narrow linewidth frequency-sweeping laser, an optical fiber circulator, a polarization controller, an optical coupler, a polarization beam splitter, a photoelectric balance detector, a data acquisition card and a signal processing system.
Description
Technical Field
The invention belongs to the technical field of distributed optical fiber sensing, and particularly relates to a system and a method for reducing the nonlinear phase influence of an OFDR light source.
Background
The distributed optical fiber sensing technology is based on optical effects of light interference, Rayleigh scattering and the like in optical fibers, the optical fibers are used as sensors, and when light is modulated by loads such as vibration, strain, sound, temperature change, structural damage and the like loaded on a path in the optical fiber transmission process, optical signals distributed in different spaces along the path and changing along time also change correspondingly. Optical Frequency Domain Reflectometry (OFDR), an advanced optical reflectometry technique, has the advantages of high spatial resolution and large dynamic range compared to other sensing techniques.
In the OFDR technique, due to the objective existence of the nonlinear tuning effect of the tunable light source during linear tuning, beat frequency interference signals in the interferometer are not equal optical frequency intervals at the same time interval, so that the signals collected in the interferometer cannot be directly subjected to fourier transform. In the traditional method, the nonlinear effect of a light source is eliminated by utilizing a very typical system structure, namely two Mach-Zehnder interferometers are built, one is an auxiliary interferometer, a beat frequency signal of the auxiliary interferometer is generated by the interference of an optical signal of a local oscillator arm and an optical signal passing through a delay optical fiber in a test arm, and the change of phase information along with time can be obtained by monitoring the change of the light intensity of an interference signal; the other is a main interferometer, a beat signal of the main interferometer is generated by interference between an optical signal of a local oscillator arm and a backward rayleigh scattered optical signal passing through a delay optical fiber in a test arm, and because the rayleigh scattered optical signal contains phase information in the whole delay optical fiber, the phase information in the beat signal is superposition of phases in the whole delay optical fiber, and information of a nonlinear phase of a light source cannot be directly obtained from the superposition. Nonlinear phase information of the tunable light source is obtained by collecting interference signals of the auxiliary interferometer, the obtained nonlinear phase information and signals obtained by the main interferometer correspond to each other in time one by one, and the signals of the main interferometer are compensated by an algorithm.
The algorithm can realize equal optical frequency interval sampling, can effectively inhibit the nonlinear effect caused by a light source in a system, and effectively improve the quality of beat frequency interference signals. However, this system structure has a certain disadvantage while sampling the equal optical frequency, that is, the reference fiber and the test fiber are two different fibers, and different random phase noise caused by the external environment exists between the different fibers, and meanwhile, the nonlinear phase information of the light source obtained in the reference fiber is related to the distance, and is itself equivalent to a main interferometer signal with only one reflection point, and when the phase information obtained by the reference fiber is used to compensate the test fiber, the compensation effect is worse as the distance between the test fiber and the reference fiber increases, so that there is a certain error in the nonlinear compensation of the light source for the signal of the main interferometer by using the beat frequency interference signal of the auxiliary interferometer.
Disclosure of Invention
The invention provides a system and a method for reducing the nonlinear phase influence of an OFDR light source, which can improve the spatial resolution of the OFDR and simplify the light path structure.
The technical scheme adopted by the invention is as follows: a system for reducing the nonlinear phase influence of an OFDR light source comprises an OFDR light path system and a test optical fiber, wherein the OFDR light source system at least comprises an optical circulator and a 3dB optical coupler; one end of the test optical fiber is connected with the output port 2 of the optical circulator, and the other end of the test optical fiber is connected with the input port 2 of the 3dB optical coupler.
Further, the OFDR optical path system comprises a tunable narrow linewidth laser light source, a main interferometer and an auxiliary interferometer; the main interferometer comprises an optical coupler, an optical circulator, a polarization controller, a test optical fiber and a 3dB optical coupler, wherein an output port 2 of the optical coupler is connected with an input port 1 of the optical circulator, an output port 3 of the optical coupler is connected with the polarization controller, the output port 2 of the optical circulator is connected with the test optical fiber, the output port 3 of the optical circulator is connected with the input port 1 of the 3dB optical coupler, and an output end of the polarization controller is connected with the input port 2 of the 3dB optical coupler;
the auxiliary interferometer comprises an optical coupler, an optical circulator, a test optical fiber and a 3dB optical coupler, wherein an output port 3 of the optical coupler is connected with an input port 1 of the 3dB optical coupler, and the test optical fiber is connected with an input port 2 of the 3dB optical coupler.
Furthermore, the OFDR optical path system further comprises a polarization beam splitter, a photoelectric balance detector, a data acquisition card and a signal processing system, wherein an output port 3 of the 3dB optical coupler is connected with an input port 1 of the polarization beam splitter, an output port 4 of the 3dB optical coupler is connected with an input port 1 of the polarization beam splitter, an output port 2 of the polarization beam splitter is connected with an input port 1 of the photoelectric balance detector, and an output port 3 of the polarization beam splitter is connected with an input port 2 of the photoelectric balance detector; an output port 2 of the polarization beam splitter is connected with an input port 1 of the photoelectric balance detector, and an output port 3 of the polarization beam splitter is connected with the input port 2 of the photoelectric balance detector;
the output port 3 of the optical coupler is connected with the input port 1 of the photoelectric balance detector, the output port 4 of the optical coupler is connected with the input port 2 of the photoelectric balance detector, the output ends of the photoelectric balance detector, the photoelectric balance detector and the photoelectric balance detector are respectively connected with the data acquisition card, and the data acquisition card is connected with the signal processing system.
The invention also provides a method for reducing the nonlinear phase influence of the OFDR light source, which comprises the following steps: the method comprises the following steps:
step 1: after the tunable narrow-line laser emitted by the tunable narrow-line width laser light source is split by the optical coupler, one part of the tunable narrow-line laser enters the main interferometer, and the other part of the tunable narrow-line laser enters the auxiliary interferometer;
step 2: tunable narrow-line laser entering a main interferometer is split by an optical coupler, a part of light enters an input port 1 of an optical circulator and enters a test optical fiber from an output port 2 of the optical circulator, backward Rayleigh scattering light in the test optical fiber enters an output port 3 of the optical circulator from the port 2 to reach a 3dB optical coupler, the other part of light reaches the 3dB optical coupler after passing through a polarization controller and then is subjected to beat frequency interference with an optical signal from the optical circulator to obtain a beat frequency signal I1(t)；
After passing through the polarization beam splitter and the polarization beam splitter, the beat frequency interfered light is divided into P light and S light which are respectively detected by the balance detector and the balance detector;
and step 3: the signal light entering the test fiber enters the 3dB optical coupler through the test fiber,beat frequency interference is generated with the light from the output port 3 of the optical coupler, and the beat frequency signal output by the auxiliary interferometer is I2(t), the interference light is divided into two beams, and then the two beams are detected by a balance detector;
and 4, step 4: the balance detector converts the interference light signal into an electric signal and then transmits the electric signal to the data acquisition card, and the data acquisition card converts the analog signal into a digital signal through sampling and transmits the digital signal to the signal processing system for processing.
Further, the beat frequency signal I1The specific formula of (t) is:
wherein E is0Is the amplitude of the beat frequency optical signal; r (tau)z) Is the reflection coefficient of the fiber; f. of0Is the initial frequency of the light source; γ is the tunable rate of the light source; tau iszTesting the time delay of the optical fiber and the local oscillation arm optical fiber; wherein
Where c is the propagation speed of light in vacuum; n is the effective refractive index of the fiber; z is the length of the test fiber; phi is a1(t)-φ1(t-τz) Is a phase noise term caused by the nonlinearity of the light source in the main interferometer;
is the additional phase noise caused by the effect of external perturbations on the test fiber.
Further, the beat frequency signal I2The specific formula of (t) is:
wherein E is1Is the amplitude of the beat light signal of the auxiliary interferometer; phi is a2(t)-φ2(t-τz) Is the phase noise caused by the non-linearity of the light source itself in the auxiliary interferometerAn item;
is the additional phase noise caused by the influence of external disturbances on the auxiliary interferometer delay fiber.
Further, the phase noise term phi caused by the self nonlinearity of the swept-frequency light source in the main interferometer system1(t)-φ1(t-τz) Equal to the phase noise term phi caused by the nonlinearity of the swept-source itself in the auxiliary interferometer2(t)-φ2(t-τz) Additional phase noise caused by the effect of external disturbances on the test fibre
Equal to the additional phase noise caused by the influence of external disturbance on the delay fiber of the auxiliary interferometer
Further, the beat frequency signal I obtained in the auxiliary interferometer is processed2(t) the established equation is further rewritten to establish an equation:
wherein, U0Is equivalent to I22E in (t)1 2，
Further, Hilbert transform is performed on equation (3), that is:
further, a formula for assisting the change rule of the phase delay of the beat signal output by the interferometer with time can be obtained according to the formula (2) and the formula (3):
furthermore, the phase output by the auxiliary interferometer corresponds to the phase change of the tunable optical signal generated at different positions in the test optical fiber in the main interferometer system one by one, and the phase change of the beat signal of the auxiliary interferometer system realizes linear output by compensating and nonlinear correcting the change of the phase along with time;
the phase change of the output signal of the main interferometer is in a linear relation with time or the position of the optical fiber, and the phase change is used for realizing frequency spectrum compression in a frequency domain and improving the spatial resolution of the system.
Furthermore, in the process of compensating and nonlinear correcting the change of the phase along with the time, an interpolation or a de-skew filtering algorithm is adopted.
Advantageous effects
1. In the invention, because the phase delay in the interferometer is in direct proportion to the length of the delay fiber, when the phase information obtained by the reference fiber in the auxiliary interferometer is used for compensating the test fiber in the main interferometer, the compensation effect is worse along with the increase of the distance between the test fiber and the reference fiber.
2. The invention eliminates the requirement on the length of the delay optical fiber in the traditional auxiliary interferometer and simplifies the optical path structure.
Drawings
FIG. 1 is a schematic diagram of the system framework of the present invention.
In the figure: 1a denotes a tunable narrow linewidth laser light source; 2a-3a each represent an optical coupler; 4a denotes an optical circulator; 5a denotes a polarization controller; 6a represents a test fiber; 7a denotes a 3dB optical coupler; 8a-9a each represent a polarizing beam splitter; 10 denotes a 3dB optical coupler; 11-13 each represent a photoelectric balanced detector; 14 denotes a data acquisition card; and 15, a signal processing system.
Detailed Description
For a better understanding of the present invention by those skilled in the art, the present invention will be described in further detail below with reference to the accompanying drawings and the following examples.
Example 1
Referring to fig. 1, the present embodiment provides a system for reducing the nonlinear phase effect of an OFDR optical source, further comprising a test optical fiber 6a, where the OFDR optical source system at least comprises an optical circulator 4a and a 3dB optical coupler 10; one end of the test optical fiber 6a is connected with the output port 2 of the optical circulator 4a, and the other end is connected with the input port 2 of the 3dB optical coupler 10. The OFDR optical path system comprises a tunable narrow linewidth laser light source 1a, an optical coupler 2a, a main interferometer and an auxiliary interferometer; the main interferometer comprises an optical coupler 3a, an optical circulator 4a, a polarization controller 5a, a test optical fiber 6a and a 3dB optical coupler 7a, wherein an output port 2 of the optical coupler 3a is connected with an input port 1 of the optical circulator 4a, an output port 3 of the optical coupler 3a is connected with the polarization controller 5a, an output port 2 of the optical circulator 4a is connected with the test optical fiber 6a, an output port 3 of the optical circulator 4a is connected with an input port 1 of the 3dB optical coupler 7a, and an output end of the polarization controller 5a is connected with an input port 2 of the 3dB optical coupler 7 a;
the auxiliary interferometer comprises an optical coupler 2a, an optical circulator 4a, a test optical fiber 6a and a 3dB optical coupler 10, wherein an output port 3 of the optical coupler 2a is connected with an input port 1 of the 3dB optical coupler 10, and the test optical fiber 6a is connected with an input port 2 of the 3dB optical coupler 10.
The OFDR optical path system of the embodiment further includes a polarization beam splitter, a photoelectric balance detector, a data acquisition card 14 and a signal processing system 15, wherein an output port 3 of the 3dB optical coupler 7a is connected with an input port 1 of the polarization beam splitter 8a, an output port 4 of the 3dB optical coupler 7a is connected with an input port 1 of the polarization beam splitter 9a, an output port 2 of the polarization beam splitter 8a is connected with an input port 1 of the photoelectric balance detector 12, and an output port 3 of the polarization beam splitter 8 is connected with an input port 2 of the photoelectric balance detector 11; an output port 2 of the polarization beam splitter 9a is connected with an input port 1 of the photoelectric balance detector 11, and an output port 3 of the polarization beam splitter 9a is connected with an input port 2 of the photoelectric balance detector 12;
the output port 3 of the 3dB optical coupler 10 is connected with the input port 1 of the photoelectric balance detector 13, the output port 4 of the 3dB optical coupler 10 is connected with the input port 2 of the photoelectric balance detector 13, the output ends of the photoelectric balance detector 12, the photoelectric balance detector 11 and the photoelectric balance detector 12 are respectively connected with the data acquisition card 14, and the data acquisition card 14 is connected with the signal processing system 15.
The main interferometer and the auxiliary interferometer of the invention adopt the same test optical fiber, in the practical engineering application, no matter how long the optical fiber is used for testing, the lengths of the reference optical fiber and the test optical fiber can be ensured to be consistent at any time, and simultaneously, because the same optical fiber is used, the interference caused by the outside is also consistent, the invention eliminates the requirement on the length of the delay optical fiber in the traditional auxiliary interferometer and simplifies the optical path structure.
Example 2
Based on embodiment 1, the present invention further provides a method for reducing the nonlinear phase effect of the OFDR light source, which includes the following steps:
step 1: after the tunable narrow-line laser emitted by the tunable narrow-line width laser light source 1a is split by the optical coupler 2a, one part of the laser enters the main interferometer, and the other part of the laser enters the auxiliary interferometer;
step 2: tunable narrow-line laser entering a main interferometer is split by an optical coupler 3a, a part of light enters an input port 1 of an optical circulator 4a, the light exits from an output port 2 of the optical circulator 4a and enters a test optical fiber 6a, backward Rayleigh scattering light in the test optical fiber 6a enters an output port 3 of the optical circulator 4a from a port 2 and reaches a 3dB optical coupler 7a, the other part of light passes through a polarization controller 5a and reaches the 3dB optical coupler 7a, and beat frequency interference is generated between the backward Rayleigh scattering light and an optical signal exiting from the optical circulator 4a to obtain a beat frequency signal I1(t), the beat frequency-interfered light passes through the polarization beam splitter 8a and the polarization beam splitter 9aThen the light is divided into P light and S light which are respectively detected by a balance detector 12 and a balance detector 11;
beat frequency signal I1The specific formula of (t) is:
wherein E is0Is the amplitude of the beat frequency optical signal; r (tau)z) Is the reflection coefficient of the fiber; f. of0Is the initial frequency of the light source; γ is the tunable rate of the light source; tau iszTesting the time delay of the optical fiber and the local oscillation arm optical fiber; wherein
Where c is the propagation speed of light in vacuum; n is the effective refractive index of the fiber; z is the length of the test fiber; phi is a1(t)-φ1(t-τz) Is a phase noise term caused by the nonlinearity of the light source in the main interferometer;
is the additional phase noise caused by the effect of external perturbations on the test fiber.
And step 3: the signal light entering the test optical fiber 6a enters the 3dB optical coupler 10 through the test optical fiber 6a, and generates beat frequency interference with the light coming out from the output port 3 of the optical coupler 2a to obtain a beat frequency signal I2(t), the interference light is divided into two beams, and then detected by the balance detector 13;
beat frequency signal I2The specific formula of (t) is:
wherein E is1Is the amplitude of the beat light signal of the auxiliary interferometer; phi is a2(t)-φ2(t-τz) Is the phase noise term caused by the nonlinearity of the light source in the auxiliary interferometer;
is the additional phase noise caused by the influence of external disturbances on the auxiliary interferometer delay fiber.
And 4, step 4: the balance detector converts the interference light signal into an electrical signal and transmits the electrical signal to the data acquisition card 14, and the data acquisition card 14 converts the analog signal into a digital signal through sampling and transmits the digital signal to the signal processing system 15 for processing.
Phase noise term phi caused by self nonlinearity of swept-frequency light source in main interferometer system1(t)-φ1(t-τz) Equal to the phase noise term phi caused by the nonlinearity of the swept-frequency light source in the auxiliary interferometer2(t)-φ2(t-τz) Additional phase noise caused by the effect of external disturbances on the test fibre
Equal to the additional phase noise caused by the influence of external disturbance on the delay fiber of the auxiliary interferometer
For beat frequency signal I obtained in auxiliary interferometer2(t) the established equation is further rewritten to establish an equation:
wherein, U0Is equivalent to I22E in (t)1 2，
The Hilbert transform is performed on the formula (3), wherein the formula is as follows:
the formula of the change rule of the phase delay of the beat signal output by the auxiliary interferometer along with time can be obtained according to the formula (3) and the formula (4):
the phase of the beat frequency signal output by the auxiliary interferometer corresponds to the phase change of the tunable optical signal generated at different positions in the test optical fiber in the main interferometer system one by one, and the linear output of the phase change of the beat frequency signal of the auxiliary interferometer system can be realized by adopting algorithms such as interpolation or deskew filtering and the like in the process of compensating and nonlinear correcting the change of the phase along with the time; the phase change of the output signal of the main interferometer is in a linear relation with time or the position of the optical fiber, so that the frequency spectrum compression is realized in a frequency domain, and the spatial resolution of the system is improved.
Although the present invention has been described with reference to a preferred embodiment, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.
Claims (10)
1. A system for reducing the nonlinear phase influence of an OFDR light source comprises an OFDR optical path system, and is characterized in that: the OFDR optical path system at least comprises an optical circulator (4a) and a 3dB optical coupler (10); one end of the test optical fiber (6a) is connected with the output port 2 of the optical circulator (4a), and the other end of the test optical fiber is connected with the input port 2 of the 3dB optical coupler (10).
2. The system of claim 1, wherein said system is further configured to reduce the effect of nonlinear phase of OFDR light source: the OFDR optical path system comprises a tunable narrow linewidth laser light source (1a), an optical coupler (2a), a main interferometer and an auxiliary interferometer;
the main interferometer comprises an optical coupler (3a), an optical circulator (4a), a polarization controller (5a), a test optical fiber (6a) and a 3dB optical coupler (7a), wherein an output port 2 of the optical coupler (3a) is connected with an input port 1 of the optical circulator (4a), an output port 3 of the optical coupler (3a) is connected with the polarization controller (5a), an output port 2 of the optical circulator (4a) is connected with the test optical fiber (6a), an output port 3 of the optical circulator (4a) is connected with an input port 1 of the 3dB optical coupler (7a), and an output end of the polarization controller (5a) is connected with an input port 2 of the 3dB optical coupler (7 a);
the auxiliary interferometer comprises an optical coupler (2a), an optical circulator (4a), a test optical fiber (6a) and a 3dB optical coupler (10), wherein an output port 3 of the optical coupler (2a) is connected with an input port 1 of the 3dB optical coupler (10), and the test optical fiber (6a) is connected with an input port 2 of the 3dB optical coupler (10).
3. The system of claim 1, wherein said system is further configured to reduce the effect of nonlinear phase of OFDR light source: the OFDR optical path system also comprises a polarization beam splitter, a photoelectric balance detector, a data acquisition card (14) and a signal processing system (15);
an output port 3 of the 3dB optical coupler (7a) is connected with an input port 1 of a polarization beam splitter (8a), an output port 4 of the 3dB optical coupler (7a) is connected with an input port 1 of a polarization beam splitter (9a), an output port 2 of the polarization beam splitter (8a) is connected with an input port 1 of a photoelectric balance detector (12), and an output port 3 of the polarization beam splitter (8) is connected with an input port 2 of a photoelectric balance detector (11); an output port 2 of the polarization beam splitter (9a) is connected with an input port 1 of a photoelectric balance detector (11), and an output port 3 of the polarization beam splitter (9a) is connected with an input port 2 of a photoelectric balance detector (12);
an output port 3 of the 3dB optical coupler (10) is connected with an input port 1 of the photoelectric balance detector (13), an output port 4 of the 3dB optical coupler (10) is connected with an input port 2 of the photoelectric balance detector (13), output ends of the photoelectric balance detector (12), the photoelectric balance detector (11) and the photoelectric balance detector (12) are respectively connected with a data acquisition card (14), and the data acquisition card (14) is connected with a signal processing system (15).
4. A method for reducing the nonlinear phase effect of OFDR light source, which is applied to the system for reducing the nonlinear phase effect of OFDR light source as claimed in any one of claims 1 to 3, and is characterized in that: the method comprises the following steps:
step 1: after the tunable narrow-line laser emitted by the tunable narrow-line width laser light source (1a) is split by the optical coupler (2a), one part of the laser enters the main interferometer, and the other part of the laser enters the auxiliary interferometer;
step 2: after tunable narrow-line laser entering a main interferometer is split by an optical coupler (3a), a part of light enters an input port 1 of an optical circulator (4a), and enters a test optical fiber (6a) from an output port 2 of the optical circulator (4a), backward Rayleigh scattering light in the test optical fiber (6a) enters an output port 3 of the optical circulator (4a) from a port 2 to reach a 3dB optical coupler (7a), and the other part of light reaches the 3dB optical coupler (7a) after passing through a polarization controller (5a), and then is subjected to beat frequency interference with an optical signal from the optical circulator (4a), so that a beat frequency signal I is obtained1(t)；
The light after beat frequency interference is divided into P light and S light after passing through a polarization beam splitter (8a) and a polarization beam splitter (9a), and the P light and the S light are respectively detected by a balance detector (11) and a balance detector (12);
and step 3: the signal light entering the test optical fiber (6a) enters a 3dB optical coupler (10) through the test optical fiber (6a) and generates beat frequency interference with the light coming out of an output port 3 of the optical coupler (2a) to obtain a beat frequency signal I2(t), the interference light is divided into two beams, and then the two beams are detected by a balance detector (13);
and 4, step 4: the balance detector converts the interference optical signal into an electric signal and then transmits the electric signal to the data acquisition card (14), and the data acquisition card (14) converts the analog signal into a digital signal through sampling and transmits the digital signal to the signal processing system (15) for processing.
5. The method of claim 4, wherein the method for reducing the nonlinear phase effect of the OFDR light source comprises: the beat frequency signal I1The specific formula of (t) is:
wherein E is0Is the amplitude of the beat frequency optical signal; r (tau)z) Is the reflection coefficient of the fiber; f. of0Is the initial frequency of the light source; γ is the tunable rate of the light source; tau iszTesting the time delay of the optical fiber and the local oscillation arm optical fiber; wherein
Where c is the propagation speed of light in vacuum; n is the effective refractive index of the fiber; z is the length of the test fiber; phi is a1(t)-φ1(t-τz) Is a phase noise term caused by the nonlinearity of the light source in the main interferometer;
is the additional phase noise caused by the effect of external perturbations on the test fiber.
6. The method of claim 4, wherein the method for reducing the nonlinear phase effect of the OFDR light source comprises: the beat frequency signal I2The specific formula of (t) is:
wherein E is1Is the amplitude of the beat light signal of the auxiliary interferometer; phi is a2(t)-φ2(t-τz) Is a light sourcePhase noise terms caused by self-nonlinearity in the auxiliary interferometer;
is the additional phase noise caused by the influence of external disturbances on the auxiliary interferometer delay fiber.
7. A method for reducing the nonlinear phase effect of OFDR light source according to claim 5 or 6, wherein: phase noise term phi caused by self nonlinearity of the swept-frequency light source in a main interferometer system1(t)-φ1(t-τz) Equal to the phase noise term phi caused by the nonlinearity of the swept-source itself in the auxiliary interferometer2(t)-φ2(t-τz)；
Additional phase noise caused by the effect of external disturbances on the test fiber
Equal to the additional phase noise caused by the influence of external disturbance on the delay fiber of the auxiliary interferometer
8. The method of claim 6, wherein said method comprises the steps of: for beat frequency signal I obtained in auxiliary interferometer2(t) the established equation is further rewritten to establish an equation:
wherein, U0Is equivalent to I22E in (t)1 2，
The Hilbert transform is performed on the formula (3), wherein the formula is as follows:
obtaining a formula of a change rule of the phase delay of the beat signal output by the auxiliary interferometer along with time according to a formula (3) and a formula (4):
9. the method of claim 8, wherein said method further comprises the step of: the phase of the beat frequency signal output by the auxiliary interferometer corresponds to the phase change of the tunable optical signal generated at different positions in the test optical fiber in the main interferometer one by one, and the phase change of the beat frequency signal of the auxiliary interferometer realizes linear output by compensating and nonlinear correcting the change of the phase along with time;
the phase change of the output signal of the main interferometer is in a linear relation with time or the position of the optical fiber, and the phase change is used for realizing frequency spectrum compression in a frequency domain and improving the spatial resolution of the system.
10. The method of claim 9, wherein the method of reducing the nonlinear phase effect of OFDR light source comprises: in the process of compensating and nonlinear correcting the change of the phase along with the time, an interpolation or deskew filtering algorithm is adopted.
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