CN108287056B - System and method for evaluating coupling characteristics of optical fiber sensitive ring polarization mode - Google Patents
System and method for evaluating coupling characteristics of optical fiber sensitive ring polarization mode Download PDFInfo
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Abstract
The invention discloses an evaluation system and an evaluation method for the polarization mode coupling characteristic of an optical fiber sensing ring, which relate to the technical field of optical fiber sensing and comprise a pulse laser, a polarization controller, an optical fiber circulator, an optical fiber sensing ring, an online pulse polarization state receiver, a digital oscilloscope and a calculation module, wherein the pulse laser can emit continuous light and pulsed light, and the testing tail end of the optical fiber sensing ring is connected with a polarization analyzer; when the coupling characteristic is evaluated, firstly, a polarization analyzer is used for carrying out polarization eigenstate alignment, then pulsed light consistent with the polarization eigenstate is input, backscattered light is used for measuring the Stokes vector of reflected light of each point of the optical fiber sensitive ring, and finally, a three-point quaternion method and a birefringence vector projection method are used for calculating the polarization state electric field vector mode coupling coefficient and the extinction ratio of each point. The invention is suitable for high-precision measurement of high-quality optical fiber sensitive ring polarization mode coupling and the change of polarization maintaining optical fiber polarization mode coupling parameters caused by external factors, and realizes multi-parameter distributed sensing.
Description
Technical Field
The invention relates to the technical field of optical fiber sensing, in particular to an evaluation system and an evaluation method for coupling distribution characteristics of an optical fiber sensitive ring polarization mode.
Background
As a key inertial measurement technology, a Fiber Optic Gyroscope (FOG) is proposed for the first time in 1976, so that the FOG attracts high attention of many countries, and through rapid development of many years, the FOG becomes the mainstream choice in the field of inertial technology and is widely applied to navigation of airplanes and ships, armored cars, attitude control of tanks and pendulum trains, spacecraft stability and the like. The fiber-optic sensing ring is a core component of a fiber-optic gyroscope and is formed by winding a Polarization Maintaining Fiber (PMF) with good polarization maintaining performance, but the inherent polarization mode coupling caused by structural defects of the fiber and the induced polarization mode coupling caused by stress and main shaft misalignment in the winding process can cause remarkable polarization state fluctuation, so that null shift is caused. To improve the performance of the fiber optic gyroscope fundamentally, the polarization mode coupling coefficient of the fiber optic sensing ring needs to be measured more accurately, the positioning accuracy of the coupling point needs to be improved, and the polarization maintaining performance of the fiber optic sensing ring needs to be studied deeply.
Besides being applied to fiber optic gyroscopes, fiber optic sensing rings are also widely applied to fiber optic current sensors. Unlike the fiber optic sensing ring of a fiber optic gyroscope, it uses an elliptical polarization maintaining fiber whose intrinsic polarization states are two orthogonal ellipses.
At present, the method for polarization mode coupling detection of polarization-maintaining optical fibers is mainly an optical fiber white light interferometer, and the principle of the method is to determine a mode coupling point by compensating the optical path difference of an optical power coupling point; however, when this technique is used to measure polarization mode coupling, several problems arise: first, the broad spectrum light in the fiber-optic sensing ring can only resonate at a specific wavelength, and the polarization mode coupling is wavelength dependent; however, the light source of white light interference is a wide-spectrum light source, and the measured mode coupling intensity is the average mode coupling of the wide-spectrum light, so that the influence of the polarization mode coupling on the resonant wavelengths of different fiber resonators cannot be accurately described by using the average mode coupling of the wide-spectrum light measured by the white light interference; second, the polarization mode coupling coefficient can be positive or negative, because the polarization mode coupling is essentially caused by the misalignment of the birefringent axes (also called polarization main axes) of the two adjacent optical fibers, and the angle of the misalignment of the main axes can be positive or negative; the mode coupling coefficient measured by the white light interferometer is an intensity coupling coefficient, the values of which are positive and cannot reflect the coupling coefficient of a real polarization mode electric field vector; third, the use of a broad spectrum light source results in large polarization mode dispersion, the spatial resolution of which decreases with increasing measurement length; fourthly, the white light interferometer changes the optical path difference of two arms of the interferometer in a mechanical scanning mode, and the reflection mirror changes the polarization state of reflected light in the mechanical movement process, so that the measurement precision is influenced. Fifth, the white light interferometer uses a polarization splitting prism in the measurement process, so that only the polarization mode coupling of the line polarization state maintaining fiber can be detected, and the mode coupling between two orthogonal elliptical polarization states in the polarization elliptical fiber cannot be detected. Although fiber white light interferometer technology has reached a fairly high level for many years, it has been put to practical use and commercialized. However, for the optical fiber sensing ring with higher quality, the mode coupling coefficient is smaller, the measurement requirement is higher, and the white light interferometer is limited by the principle, and the space for further improving is limited. Therefore, it is an important requirement to search for a new method for further improving the detection sensitivity, detecting the mode coupling coefficient of the polarization mode electric field vector, and measuring the polarization mode coupling of the optical fiber sensitive loop made of the elliptical polarization state maintaining optical fiber.
The optical time domain reflectometry is a commonly used technique for measuring the distribution parameters of an optical fiber, and the polarization state of rayleigh scattered light at a certain point in the optical fiber is consistent with incident light at the point, so that the change of the polarization state along the optical fiber can be detected by using the rayleigh scattered light. However, the optical time domain reflectometry technique has the following problems in specific applications: firstly, the pulse time for detecting the change of the polarization state must be less than the transmission time of the beat length of the optical fiber, and the beat length of the polarization-maintaining optical fiber is very small, usually about 2-3 mm, which requires the pulse to be in the ps or even fs magnitude; secondly, the optical time domain reflection technology generally used for detecting the polarization state adopts a linear polarization state with a fixed direction, which can not be aligned with the polarization main axis of the polarization-maintaining optical fiber, and particularly, no way is provided for ensuring that the polarization eigenstate of the optical fiber is maintained by aligning the elliptical polarization state; thirdly, the method for detecting the change of the polarization state can be divided into two methods, one is to use an analyzer to measure the optical work in a specific polarization direction, so that all stokes parameters of the polarization state cannot be detected; the other method is full polarization state detection, which is a technique for obtaining three Stokes parameters [ S1, S2, S3] distributed along the length of the optical fiber, namely complete polarization state information, but no on-line device suitable for full polarization state detection of high-speed pulses exists at present. Therefore, the existing optical time domain reflection technology based on full polarization state detection is only limited to a common single mode fiber, and is not used for polarization mode coupling performance measurement of a polarization maintaining fiber.
Disclosure of Invention
The invention aims to provide an optical fiber sensitive ring polarization mode coupling characteristic evaluation system which can measure the mode coupling performance of an optical fiber sensitive ring with higher quality, has higher detection sensitivity, can detect the coupling coefficient of a polarization mode electric field vector and can measure the polarization mode coupling of the optical fiber sensitive ring manufactured by an elliptical polarization state maintaining optical fiber, so as to solve the technical problems that the full polarization state can not be accurately detected and the polarization mode coupling performance of a polarization maintaining optical fiber can not be measured in the background technology.
In order to achieve the purpose, the invention adopts the following technical scheme:
an optical fiber sensitive ring polarization mode coupling characteristic evaluation system comprises a transmitting module, a receiving module and a calculating module,
the optical fiber ring tester is characterized in that the sending module comprises a pulse laser, a polarization controller and an optical fiber circulator which are sequentially connected, the output end of the pulse laser is connected with the input end of the polarization controller, the output end of the polarization controller is connected with the first end of the optical fiber circulator, and the second end of the optical fiber circulator is connected with the test starting end of the optical fiber sensitive ring to be tested.
The receiving module comprises an online pulse polarization state receiver and a digital oscilloscope, wherein the first input end of the online pulse polarization state receiver is connected with the third end of the optical fiber circulator, the output end of the online pulse polarization state receiver is connected with the input end of the digital oscilloscope, and the output end of the digital oscilloscope is connected with the calculating module.
And the testing tail end of the optical fiber sensitive ring to be tested is connected with a polarization analyzer.
Furthermore, the pulse laser comprises a narrow-band continuous optical laser, a lithium niobate modulator, a programmable pulse code generator, a modulator driver, an optical fiber amplifier and an optical filter; the narrow-band continuous optical laser is connected with a first input end of the lithium niobate modulator, an output end of the lithium niobate modulator is connected with an input end of an optical fiber amplifier, an output end of the optical fiber amplifier is connected with an input end of the optical filter, and an output end of the optical filter is connected with the polarization controller; the output end of the programmable pulse code generator is connected with the input end of the modulator driver, and the output end of the modulator driver is connected with the second input end of the lithium niobate modulator.
Further, the online pulse polarization state receiver comprises a polarization beam splitter, a coupler, a 90-degree optical mixer and 3 balanced optical detectors; the output end of the polarization beam splitter is connected with two couplers, the first output ends of the two couplers are respectively connected with two input ends of the 90-degree optical mixer, two output ends of the 90-degree optical mixer are respectively connected with a first balanced optical detector and a second balanced optical detector, the second output ends of the two couplers are connected with a third balanced optical detector, and the second output ends of the two couplers are respectively connected with two input ends of the third balanced optical detector; the input end of the polarization beam splitter is connected with the third end of the optical fiber circulator, and the output end of the first balanced light detector, the output end of the second balanced light detector and the output end of the third balanced light detector are connected with the digital oscilloscope.
Further, the on-line pulse polarization state receiver comprises a coupler, a 0-degree line analyzer, a 45-degree line analyzer, a right-handed circular analyzer and a balanced light detector; the three output ends of the coupler are respectively connected with the input ends of the 0-degree line analyzer, the 45-degree line analyzer and the right-handed circular analyzer, the output ends of the 0-degree line analyzer, the 45-degree line analyzer and the right-handed circular analyzer are respectively connected with a balanced light detector, the output ends of the three balanced light detectors are respectively connected with the digital oscilloscope, and the input end of the coupler is connected with the third end of the optical fiber circulator.
A method for evaluating the coupling characteristics of a sensitive ring polarization mode of an optical fiber by using the system as described above, comprising the following steps:
adjusting the polarization state of the optical signal input into the optical fiber sensing ring to be tested to be consistent with the polarization eigenstate of the optical fiber sensing ring to be tested, and inputting the optical signal consistent with the polarization eigenstate of the optical fiber sensing ring to be tested as a test optical signal;
collecting complete polarization state Stokes vectors S of the reflected light signals distributed along the longitudinal direction through the online pulse polarization state receiver and the digital oscilloscope;
and calculating the polarization mode coupling coefficient and the extinction ratio which are longitudinally distributed along the fiber sensitive ring to be detected according to the Stokes vector.
Further, the adjusting that the polarization state of the test optical signal input into the optical fiber sensing ring to be tested is consistent with the polarization eigenstate of the optical fiber sensing ring to be tested comprises adjusting the pulse laser to a continuous light output state, modulating the polarization state by the polarization controller, and observing the output polarization state of the optical fiber sensing ring to be tested by using a polarization analyzer, so that the input polarization state of the optical fiber sensing ring to be tested, which is modulated by the polarization controller, is consistent with the polarization eigenstate of the optical fiber sensing ring to be tested.
Further, the acquiring of the complete polarization stokes vector of the reflected light signal along the longitudinal distribution by the online pulse polarization state receiver and the digital oscilloscope includes coupling the test light signal through the optical fiber circulator, entering the optical fiber sensitive ring to be tested, forming a reflected light signal through backward rayleigh scattering, returning the reflected light signal to the second end of the optical fiber circulator, entering the optical fiber circulator, and entering the online pulse polarization state receiver through the third end of the optical fiber circulator.
Further, the calculating the polarization mode coupling coefficient and the extinction ratio longitudinally distributed along the fiber sensitive ring to be measured according to the stokes vector comprises calculating the change rate of the stokes vector S according to the complete polarization state stokes vector S
According to the formula
Calculating a birefringence vector
Unit vector
Representing the direction of the local principal axis of polarization, the magnitude | B | is the rate at which the polarization state rotates about the principal axis of polarization, i.e., the magnitude of birefringence at that principal axis; according to the formula
The out-of-mode coupling coefficient k is calculated.
Further, according to the Stokes vector relation of three adjacent detection points on the optical fiber sensing ring to be detected, a three-point quaternion method is adopted to calculate the birefringence vector B.
Further, in the above-mentioned case,according to the birefringence vector B on the Pongar sphere
And projecting the two axes to calculate the mode coupling coefficient k.
The principle of the invention for realizing the polarization mode coupling distributed measurement of the optical fiber sensing ring is as follows:
since the Stokes vector S rotates on the Ponga sphere, its radius is always 1, so its rate of change
Perpendicular to the current stokes vector, and thus has
Wherein
Its direction (unit vector)
) Is the direction of the local principal axis of polarization, and the magnitude | B | is the rate at which the polarization state rotates about the principal axis of polarization, i.e., the magnitude of birefringence at that principal axis. When considering the effect of polarization mode coupling on polarization state transmission, B should contain polarization mode coupling coefficient, and we use quaternion method to derive the transmission constant difference Δ β between B and two perpendicular polarization modes+-β-And their mode coupling coefficient k.
Neglecting the loss of the polarization maintaining fiber, the coupling between two orthogonal polarization states (including two orthogonal linear polarization states in a linear polarization maintaining fiber and two orthogonal elliptical polarization states in an elliptical polarization maintaining fiber) can be written as theoretically
When the formula (2) is expressed in the form of a Jones vector, it is
Expressing formula (3) in the form of a quaternion [32], as
Wherein the quaternion
(denoted by the English flower Edwardian Script ITC, the same applies hereinafter) is the Jones vector
The corresponding quaternion is then calculated using the corresponding quaternion,
is a Jones matrix
The corresponding quaternion, since the Jones matrix can be decomposed into
Wherein the average transmission constant of the fast and slow axes
Transmission constant difference Δ β ═ β+-β-The quaternion corresponding to the Jones matrix is obtained as
Thus, it is possible to provide
Wherein the content of the first and second substances,
is that
Hermitian transpose. Quaternion due to Stokes
Thus, it is possible to provide
Substituting (7) into (8) to obtain
Let Stokes quaternionWherein s is0Is a scalar part, S is a vector part, and is substituted into (9)
Comparing (11) with (1), it is possible to obtain
Therefore, only B pairs are measured
Two-axis projection is carried out, so that the transmission constant difference delta beta and the polarization mode coupling coefficient k of two orthogonal eigenstates can be obtained.
Based on the method of three-point quaternion, the B pairs can be calculated by only knowing the Stokes vectors of three adjacent points of a certain section of optical fiber
And (5) obtaining the transmission constant difference delta beta and the polarization mode coupling coefficient k of the small segment of the optical fiber through two-axis projection. In addition, the polarization state quaternion of adjacent three points A, B, C detected from the beginning of the fiber in the present system
Angle of rotation of the same as that detected at point A
The same angle of rotation between them, so S on the Poincare ballout(A)、Sout(B)、Sout(C) The variation relationship between S andA(A)、SA(B)、SA(C) the relationship between the changes is consistent. Therefore, in the present system, the stokes vectors S corresponding to the A, B, C three positions obtained by calculating the fiber start endout(A)、Sout(B)、Sout(C) And (3) describing A, B, C the polarization mode coupling between the adjacent three points according to the polarization mode coupling coefficient k calculated by the formula (12).
The invention has the beneficial effects that: the method is suitable for measuring high-quality optical fiber sensitive ring polarization mode coupling, overcomes the defects that the conventional white light interference technology is not high in the precision of measuring the optical fiber sensitive ring polarization mode coupling and is not suitable for maintaining the elliptical polarization state of the optical fiber, and can be used for measuring polarization-maintaining optical fiber polarization mode coupling caused by external factors (such as stress, bending, vibration and the like) to realize distributed measurement of various parameters.
Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings needed to be used in the description of the embodiments are briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without creative efforts.
Fig. 1 is a schematic diagram of a system for evaluating polarization mode coupling characteristics of a fiber-sensitive ring according to an embodiment of the present invention.
Fig. 2 is a block diagram of a pulse laser in the fiber-sensitive loop polarization mode coupling characteristic evaluation system according to the embodiment of the present invention.
Fig. 3 is a block diagram of an online pulse polarization state receiver in the fiber-sensitive loop polarization mode coupling characteristic evaluation system according to embodiment 1 of the present invention.
Fig. 4 is a block diagram of an online pulse polarization state receiver in the fiber-sensitive loop polarization mode coupling characteristic evaluation system according to embodiment 2 of the present invention.
FIG. 5 is a diagram of the distribution of Rayleigh scattered light power along the fiber sensor ring corresponding to the three components S1, S2, and S3 of the Stokes vector according to the embodiment of the present invention.
FIG. 6 is a graph of polarization mode coupling along the longitudinal dimension of a fiber optic sensor ring, as measured in both the forward and reverse directions of the fiber optic sensor ring, in accordance with an embodiment of the present invention.
FIG. 7 is a diagram illustrating polarization state distribution on Pongall for positions A (a) and E (b) obtained through repeated measurements according to an embodiment of the present invention.
Wherein: 100-a sending module; 200-a receiving module; 300-a calculation module; 110-a pulsed laser; 120-a polarization controller; 130-fiber optic circulator; 400-optical fiber sensitive ring to be tested; 210-an on-line pulsed polarization state receiver; 220-digital oscilloscope; 500-polarization analyzer; 111-narrow band continuous light laser; 112-lithium niobate modulator; 113-programmable pulse code generator; 114-modulator driver; 115-fiber amplifier; 116-an optical filter; 211-a polarizing beam splitter; 212-a coupler; 213-90 degree optical mixer; 214-a first balanced light detector; 215-a second balanced light detector; 216-a third balanced photodetector; 211 a-coupler; 212a-0 degree line analyzer; 213a-45 degree line analyzer; 214 a-right-hand circular analyzer; 215 a-balanced light detector.
Detailed Description
Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like or similar reference numerals refer to the same or similar elements or elements having the same or similar function throughout. The embodiments described below by way of the drawings are illustrative only and are not to be construed as limiting the invention.
As used herein, the singular forms "a", "an", "the" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and/or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. It is to be understood that "connected" or "coupled" as used herein may include wirelessly connected or coupled, and that the term "and/or" as used herein includes any and all combinations of one or more of the associated listed items.
It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the prior art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
For the purpose of facilitating an understanding of the present invention, the present invention will be further explained by way of specific embodiments with reference to the accompanying drawings, which are not intended to limit the present invention.
Fig. 1 is a schematic diagram of an optical fiber sensitive loop polarization mode coupling characteristic evaluation system according to an embodiment of the present invention, fig. 2 is a block diagram of a pulse laser in the optical fiber sensitive loop polarization mode coupling characteristic evaluation system according to the embodiment of the present invention, fig. 3 is a block diagram of an online pulse polarization state receiver in the optical fiber sensitive loop polarization mode coupling characteristic evaluation system according to the embodiment 1 of the present invention, fig. 4 is a block diagram of an online pulse polarization state receiver in the optical fiber sensitive loop polarization mode coupling characteristic evaluation system according to the embodiment 2 of the present invention, fig. 5 is a longitudinal distribution diagram of rayleigh scattered light power along the optical fiber sensitive loop corresponding to three components S1, S2, and S3 of a Stokes vector according to the embodiment of the present invention, fig. 6 is a polarization mode coupling curve graph along the longitudinal distribution of the optical fiber sensitive loop obtained by forward and reverse measurements of the optical fiber sensitive loop according to the embodiment of the present invention, FIG. 7 is a diagram illustrating polarization state distribution on Pongall for positions A (a) and E (b) obtained through repeated measurements according to an embodiment of the present invention.
It should be understood by those skilled in the art that the drawings are merely schematic representations of embodiments and that the elements shown in the drawings are not necessarily required to practice the invention.
As shown in fig. 1 to 4, an embodiment of the present invention provides an optical fiber sensitive loop polarization mode coupling characteristic evaluation system, which includes a transmitting module 100, a receiving module 200, and a calculating module 300,
the sending module 100 includes a pulse laser 110, a polarization controller 120, and an optical fiber circulator 130, which are connected in sequence, wherein an output end of the pulse laser 110 is connected to an input end of the polarization controller 120, an output end of the polarization controller 120 is connected to a first end of the optical fiber circulator 130, and a second end of the optical fiber circulator 130 is connected to the test starting end of the optical fiber sensitive ring 400 to be tested;
the receiving module 200 includes an online pulse polarization state receiver 210 and a digital oscilloscope 220, a first input end of the online pulse polarization state receiver 210 is connected to the third end of the optical fiber circulator 130, an output end of the online pulse polarization state receiver 210 is connected to an input end of the digital oscilloscope 220, and an output end of the digital oscilloscope 220 is connected to the calculating module 300;
the test end of the optical fiber sensing ring 400 to be tested is connected with a polarization analyzer 500.
In a specific embodiment of the present invention, the pulse laser 110 includes a narrow-band continuous optical laser 111, a lithium niobate modulator 112, a programmable pulse code generator 113, a modulator driver 114, a fiber amplifier 115, and an optical filter 116; the narrow-band continuous optical laser 111 is connected to a first input end of the lithium niobate modulator 112, an output end of the lithium niobate modulator 112 is connected to an input end of an optical fiber amplifier 115, an output end of the optical fiber amplifier 115 is connected to an input end of the optical filter 116, and an output end of the optical filter 116 is connected to the polarization controller 120; the output end of the programmable pulse code generator 113 is connected to the input end of the modulator driver 114, and the output end of the modulator driver 114 is connected to the second input end of the lithium niobate modulator 112.
In one embodiment of the present invention, the online pulse polarization state receiver 210 comprises a polarization beam splitter 211, a coupler 212, a 90 ° optical mixer 213, 3 balanced photodetectors 214, 215, 216; the output end of the polarization beam splitter 211 is connected to two couplers 212, the first output ends of the two couplers 212 are respectively connected to two input ends of the 90 ° optical mixer 213, two output ends of the 90 ° optical mixer 213 are respectively connected to a first balanced photodetector 214 and a second balanced photodetector 215, the second output ends of the two couplers 212 are connected to a third balanced photodetector 216, and the second output ends of the two couplers 212 are respectively connected to two input ends of the third balanced photodetector 216; the input end of the polarization beam splitter 211 is connected to the third end of the fiber circulator 130, and the output ends of the first balanced photodetector 214, the second balanced photodetector 215, and the third balanced photodetector 216 are connected to the digital oscilloscope 220.
In one embodiment of the present invention, the on-line pulsed polarization state receiver 210 includes a coupler 211a, a 0 ° line analyzer 212a, a 45 ° line analyzer 213a, a right-hand circular analyzer 214a, a balanced photodetector 215 a; three output ends of the coupler 211a are respectively connected to input ends of the 0 ° line analyzer 212a, the 45 ° line analyzer 213a, and the right-hand circular analyzer 214a, output ends of the 0 ° line analyzer 212a, the 45 ° line analyzer 213a, and the right-hand circular analyzer 214a are respectively connected to a balanced light detector 215a, output ends of the three balanced light detectors 215a are all connected to the digital oscilloscope 220, and an input end of the coupler 211a is connected to a third end of the optical fiber circulator 130.
As shown in fig. 5 to 7, an embodiment of the present invention further provides a method for evaluating a coupling characteristic of a polarization mode of a fiber ring by using the system described above, including the following steps:
adjusting the polarization state of the optical signal input into the optical fiber sensing ring 400 to be tested to be consistent with the polarization eigenstate of the optical fiber sensing ring 400 to be tested, and inputting the optical signal consistent with the polarization eigenstate of the optical fiber sensing ring 400 to be tested as a test optical signal;
collecting complete polarization state Stokes vectors S of the reflected light signals distributed along the longitudinal direction through the online pulse polarization state receiver 210 and the digital oscilloscope 220;
and calculating the polarization mode coupling coefficient and the extinction ratio longitudinally distributed along the fiber sensitive ring 400 to be tested according to the Stokes vector.
In the embodiment of the method, the adjusting that the polarization state of the test optical signal input to the optical fiber sensing ring 400 to be tested is consistent with the polarization eigenstate of the optical fiber sensing ring 400 to be tested includes adjusting the pulse laser 110 to a continuous light output state, modulating the polarization state by the polarization controller 120, and observing the output polarization state of the optical fiber sensing ring 400 to be tested by using the polarization analyzer 500, so that the input polarization state of the optical fiber sensing ring 400 to be tested, which is modulated by the polarization controller 120, is consistent with the polarization eigenstate of the optical fiber sensing ring 400 to be tested.
In the embodiment of the method according to the present invention, the acquiring, by the online pulse polarization state receiver 210 and the digital oscilloscope 220, the complete polarization state stokes vector of the reflected light signal distributed along the longitudinal direction includes coupling the test light signal through the optical fiber circulator 130, entering the optical fiber sensitive ring 400 to be tested, forming a reflected light signal through backward rayleigh scattering, returning the reflected light signal to the second end of the optical fiber circulator 130, entering the optical fiber circulator 130, and entering the online pulse polarization state receiver 210 through the third end of the optical fiber circulator 130.
In the embodiment of the method, the calculating the polarization mode coupling coefficient and the extinction ratio longitudinally distributed along the fiber sensitive ring 400 to be measured according to the stokes vector comprises calculating the change rate of the stokes vector S according to the complete polarization state stokes vector S
According to the formula
Calculating a birefringence vectorUnit vector
Representing the direction of the local principal axis of polarization, the magnitude | B | is the rate at which the polarization state rotates about the principal axis of polarization, i.e., the magnitude of birefringence at that principal axis; according to the formula
The out-of-mode coupling coefficient k is calculated.
In the embodiment of the method, the birefringence vector B is calculated by a three-point quaternion method according to the stokes vector relationship of three adjacent detection points on the optical fiber sensing ring 400 to be detected.
In the embodiment of the method, the birefringence vector B is applied to the Poincar sphere
And projecting the two axes to calculate the mode coupling coefficient k.
Example 1
The system for evaluating the coupling characteristics of the polarization mode of the fiber-sensitive ring in embodiment 1 of the present invention comprises a transmitting module 100, a receiving module 200, and a calculating module 300,
the sending module 100 includes a pulse laser 110, a polarization controller 120, and an optical fiber circulator 130, which are connected in sequence, wherein an output end of the pulse laser 110 is connected to an input end of the polarization controller 120, an output end of the polarization controller 120 is connected to a first end of the optical fiber circulator 130, and a second end of the optical fiber circulator 130 is connected to the test starting end of the optical fiber sensitive ring 400 to be tested;
the receiving module 200 includes an online pulse polarization state receiver 210 and a digital oscilloscope 220, a first input end of the online pulse polarization state receiver 210 is connected to the third end of the optical fiber circulator 130, an output end of the online pulse polarization state receiver 210 is connected to an input end of the digital oscilloscope 220, and an output end of the digital oscilloscope 220 is connected to the calculating module 300;
the test end of the optical fiber sensing ring 400 to be tested is connected with a polarization analyzer 500.
As shown in fig. 2, the pulse laser 110 according to embodiment 1 of the present invention includes a narrow-band continuous optical laser 111, a lithium niobate modulator 112, a programmable pulse code generator 113, a modulator driver 114, a fiber amplifier 115, and an optical filter 116; the narrow-band continuous optical laser 111 is connected to a first input end of the lithium niobate modulator 112, an output end of the lithium niobate modulator 112 is connected to an input end of an optical fiber amplifier 115, an output end of the optical fiber amplifier 115 is connected to an input end of the optical filter 116, and an output end of the optical filter 116 is connected to the polarization controller 120; the output end of the programmable pulse code generator 113 is connected to the input end of the modulator driver 114, and the output end of the modulator driver 114 is connected to the second input end of the lithium niobate modulator 112.
The switching of continuous light and pulsed light can be achieved by a pulsed laser 110 as shown in fig. 2. When continuous light is emitted, the continuous light is modulated by the polarization controller 120 to have a polarization state, enters the fiber sensing ring 400 to be tested through the second end of the fiber ring 130, enters the polarization analyzer 500 through the testing end of the fiber sensing ring 400 to be tested, observes the polarization state modulated by the polarization controller 120 through the polarization analyzer 500, and observes the output polarization state of the fiber sensing ring 400 to be tested by using the polarization analyzer 500, so that the polarization state of the input continuous light injected into the fiber sensing ring 400 to be tested after being modulated by the polarization controller 120 is consistent with the polarization eigenstate of the fiber sensing ring 400 to be tested. The pulse laser 110 is used for emitting pulse light, the pulse light signal is modulated by the adjusted polarization controller 120 and then enters the optical fiber sensitive ring 400 to be tested as a test signal from the second end of the optical fiber ring 130, a reflected light signal is formed after rayleigh scattering of the optical fiber sensitive ring 400 to be tested, the reflected light signal enters the receiving module 200 from the third end of the optical fiber ring 130, and the coupling characteristic evaluation is further completed.
As shown in fig. 3, the online pulse polarization state receiver 210 includes a polarization beam splitter 211, a coupler 212, a 90 ° optical mixer 213, 3 balanced photodetectors 214, 215, 216; the output end of the polarization beam splitter 211 is connected to two couplers 212, the first output ends of the two couplers 212 are respectively connected to two input ends of the 90 ° optical mixer 213, two output ends of the 90 ° optical mixer 213 are respectively connected to a first balanced photodetector 214 and a second balanced photodetector 215, the second output ends of the two couplers 212 are connected to a third balanced photodetector 216, and the second output ends of the two couplers 212 are respectively connected to two input ends of the third balanced photodetector 216; the input end of the polarization beam splitter 211 is connected to the third end of the fiber circulator 130, and the output ends of the first balanced photodetector 214, the second balanced photodetector 215, and the third balanced photodetector 216 are connected to the digital oscilloscope 220.
Example 2
The system for evaluating the coupling characteristics of the polarization mode of the fiber-sensitive ring in embodiment 2 of the present invention comprises a transmitting module 100, a receiving module 200, and a calculating module 300,
the sending module 100 includes a pulse laser 110, a polarization controller 120, and an optical fiber circulator 130, which are connected in sequence, wherein an output end of the pulse laser 110 is connected to an input end of the polarization controller 120, an output end of the polarization controller 120 is connected to a first end of the optical fiber circulator 130, and a second end of the optical fiber circulator 130 is connected to the test starting end of the optical fiber sensitive ring 400 to be tested;
the receiving module 200 includes an online pulse polarization state receiver 210 and a digital oscilloscope 220, a first input end of the online pulse polarization state receiver 210 is connected to the third end of the optical fiber circulator 130, an output end of the online pulse polarization state receiver 210 is connected to an input end of the digital oscilloscope 220, and an output end of the digital oscilloscope 220 is connected to the calculating module 300;
the test end of the optical fiber sensing ring 400 to be tested is connected with a polarization analyzer 500.
As shown in fig. 2, the pulse laser 110 according to embodiment 1 of the present invention includes a narrow-band continuous optical laser 111, a lithium niobate modulator 112, a programmable pulse code generator 113, a modulator driver 114, a fiber amplifier 115, and an optical filter 116; the narrow-band continuous optical laser 111 is connected to a first input end of the lithium niobate modulator 112, an output end of the lithium niobate modulator 112 is connected to an input end of an optical fiber amplifier 115, an output end of the optical fiber amplifier 115 is connected to an input end of the optical filter 116, and an output end of the optical filter 116 is connected to the polarization controller 120; the output end of the programmable pulse code generator 113 is connected to the input end of the modulator driver 114, and the output end of the modulator driver 114 is connected to the second input end of the lithium niobate modulator 112.
The switching of continuous light and pulsed light can be achieved by a pulsed laser 110 as shown in fig. 2. When continuous light is emitted, the continuous light is modulated by the polarization controller 120 to have a polarization state, enters the fiber sensing ring 400 to be tested through the second end of the fiber ring 130, enters the polarization analyzer 500 through the testing end of the fiber sensing ring 400 to be tested, observes the polarization state modulated by the polarization controller 120 through the polarization analyzer 500, and observes the output polarization state of the fiber sensing ring 400 to be tested by using the polarization analyzer 500, so that the polarization state of the input continuous light injected into the fiber sensing ring 400 to be tested after being modulated by the polarization controller 120 is consistent with the polarization eigenstate of the fiber sensing ring 400 to be tested. The pulse laser 110 is used for emitting pulse light, the pulse light signal is modulated by the adjusted polarization controller 120 and then enters the optical fiber sensitive ring 400 to be tested as a test signal from the second end of the optical fiber ring 130, a reflected light signal is formed after rayleigh scattering of the optical fiber sensitive ring 400 to be tested, the reflected light signal enters the receiving module 200 from the third end of the optical fiber ring 130, and the coupling characteristic evaluation is further completed.
As shown in fig. 4, the on-line pulsed polarization state receiver 210 according to embodiment 2 of the present invention includes a coupler 211a, a 0 ° line analyzer 212a, a 45 ° line analyzer 213a, a right-hand circular analyzer 214a, and a balanced photodetector 215 a; three output ends of the coupler 211a are respectively connected to input ends of the 0 ° line analyzer 212a, the 45 ° line analyzer 213a, and the right-hand circular analyzer 214a, output ends of the 0 ° line analyzer 212a, the 45 ° line analyzer 213a, and the right-hand circular analyzer 214a are respectively connected to a balanced light detector 215a, output ends of the three balanced light detectors 215a are all connected to the digital oscilloscope 220, and an input end of the coupler 211a is connected to a third end of the optical fiber circulator 130.
As shown in fig. 5, rayleigh scattered light powers corresponding to three components S1, S2 and S3 of the Stokes vector of the polarization state acquired by the digital oscilloscope 220 are distributed along the longitudinal direction of the fiber sensing loop. Complete polarization state information distributed along the longitudinal direction of the optical fiber sensitive ring can be obtained by using algorithm calculation.
FIG. 6 is a polarization mode coupling curve calculated by Matlab algorithm from forward and reverse measurements of the fiber sensing ring, respectively, and distributed longitudinally along the fiber sensing ring; it can be seen that the two curves are substantially identical, with relatively large polarization mode coupling detected at A, B, C and D of the fiber sensor ring, and very little polarization mode coupling detected at E.
FIG. 7 is a graph of polarization state distributions on the Ponga sphere obtained by repeated measurements for two different locations, where (a) and (b) correspond to fiber sensor ring locations A and E, respectively, of FIG. 6. It can be seen that the polarization mode coupling at position a is large, resulting in the polarization state deviating from the polarization principal axis [1,0,0] distribution; and the polarization mode coupling at position B is small, and the polarization state is centrally distributed on the principal axes of polarization [1,0,0 ].
The principle of the invention for realizing the polarization mode coupling distributed measurement of the optical fiber sensing ring is as follows:
since the Stokes vector S rotates on the Ponga sphere, its radius is always 1, so its rate of change
Perpendicular to the current stokes vector, and thus has
WhereinIts direction (unit vector)
) Is the direction of the local principal axis of polarization, and the magnitude | B | is the rate at which the polarization state rotates about the principal axis of polarization, i.e., the magnitude of birefringence at that principal axis. When considering the effect of polarization mode coupling on polarization state transmission, B should contain polarization mode coupling coefficient, and we use quaternion method to derive the transmission constant difference Δ β between B and two perpendicular polarization modes+-β_And their mode coupling coefficient k.
Neglecting the loss of the polarization maintaining fiber, the coupling between two orthogonal polarization states (including two orthogonal linear polarization states in a linear polarization maintaining fiber and two orthogonal elliptical polarization states in an elliptical polarization maintaining fiber) can be written as theoretically
When the formula (2) is expressed in the form of a Jones vector, it is
Expressing formula (3) in the form of a quaternion [32], as
Wherein the quaternion
(denoted by the English flower Edwardian Script ITC, the same applies hereinafter) is the Jones vectorThe corresponding quaternion is then calculated using the corresponding quaternion,
is a Jones matrix
The corresponding quaternion, since the Jones matrix can be decomposed into
Wherein the average transmission constant of the fast and slow axes
Transmission constant difference Δ β ═ β+-β_The quaternion corresponding to the Jones matrix is obtained as
Thus, it is possible to provide
Wherein the content of the first and second substances,
is that
Hermitian transpose. Quaternion due to Stokes
Thus, it is possible to provide
Substituting (7) into (8) to obtain
Let Stokes quaternion
Wherein s is0Is a scalar part, S is a vector part, and is substituted into (9)
Comparing (11) with (1), it is possible to obtain
Thus, it is possible to provideAs long as B pairs are detected
Two-axis projection is carried out, so that the transmission constant difference delta beta and the polarization mode coupling coefficient k of two orthogonal eigenstates can be obtained.
Based on the method of three-point quaternion, the B pairs can be calculated by only knowing the Stokes vectors of three adjacent points of a certain section of optical fiberAnd (5) obtaining the transmission constant difference delta beta and the polarization mode coupling coefficient k of the small segment of the optical fiber through two-axis projection. In addition, the polarization state quaternion of adjacent three points A, B, C detected from the beginning of the fiber in the present system
Angle of rotation of the same as that detected at point A
The same angle of rotation between them, so S on the Poincare ballout(A)、Sout(B)、Sout(C) The variation relationship between S andA(A)、SA(B)、SA(C) the relationship between the changes is consistent. Therefore, in the present system, the stokes vectors S corresponding to the A, B, C three positions obtained by calculating the fiber start endout(A)、Sout(B)、Sout(C) And (3) according to the polarization mode coupling coefficient k calculated by the formula (12), the polarization mode coupling between the adjacent three points A, B and C can be described.
4000 times of repeated experiments show that the detection method has high repeatability, and the measurement spatial resolution reaches 1 meter. In addition, experiments show that the detection method can be used for measuring polarization mode coupling of the polarization maintaining optical fiber caused by external factors (such as stress, bending, vibration and the like) and realizing distributed measurement of various parameters.
In conclusion, the method can be used for measuring high-quality optical fiber sensitive ring polarization mode coupling, overcomes the defects that the conventional white light interference technology is not high in the precision of measuring the optical fiber sensitive ring polarization mode coupling and is not suitable for the elliptical polarization state maintaining optical fiber, and can be used for measuring polarization maintaining optical fiber polarization mode coupling caused by external factors (such as stress, bending, vibration and the like) and realizing distributed measurement of various parameters.
Those of ordinary skill in the art will understand that: the components in the device in the embodiment of the present invention may be distributed in the device in the embodiment according to the description of the embodiment, or may be correspondingly changed in one or more devices different from the embodiment. The components of the above embodiments may be combined into one component, or may be further divided into a plurality of sub-components.
The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are included in the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.
Claims (8)
1. An optical fiber sensitive ring polarization mode coupling characteristic evaluation system comprises a sending module (100), a receiving module (200) and a calculating module (300), and is characterized in that,
the sending module (100) comprises a pulse laser (110), a polarization controller (120) and an optical fiber circulator (130) which are connected in sequence, wherein the output end of the pulse laser (110) is connected with the input end of the polarization controller (120), the output end of the polarization controller (120) is connected with the first end of the optical fiber circulator (130), and the second end of the optical fiber circulator (130) is connected with the test starting end of the optical fiber sensitive ring (400) to be tested; the testing tail end of the optical fiber sensitive ring (400) to be tested is connected with a polarization analyzer (500);
the pulse laser (110) comprises a narrow-band continuous optical laser (111), a lithium niobate modulator (112), a programmable pulse code generator (113), a modulator driver (114), a fiber amplifier (115) and an optical filter (116); the narrow-band continuous optical laser (111) is connected with a first input end of the lithium niobate modulator (112), an output end of the lithium niobate modulator (112) is connected with an input end of an optical fiber amplifier (115), an output end of the optical fiber amplifier (115) is connected with an input end of the optical filter (116), and an output end of the optical filter (116) is connected with the polarization controller (120); the output end of the programmable pulse code generator (113) is connected with the input end of the modulator driver (114), and the output end of the modulator driver (114) is connected with the second input end of the lithium niobate modulator (112);
the receiving module (200) comprises an online pulse polarization state receiver (210) and a digital oscilloscope (220), wherein a first input end of the online pulse polarization state receiver (210) is connected with a third end of the optical fiber circulator (130), an output end of the online pulse polarization state receiver (210) is connected with an input end of the digital oscilloscope (220), and an output end of the digital oscilloscope (220) is connected with the calculating module (300).
2. The fiber-sensitive ring polarization mode coupling characteristic evaluation system according to claim 1, wherein the online pulse polarization state receiver (210) comprises a polarization beam splitter (211), a coupler (212), a 90 ° optical mixer (213), 3 balanced photodetectors (214, 215, 216); the output end of the polarization beam splitter (211) is connected with two couplers (212), the first output ends of the two couplers (212) are respectively connected with two input ends of the 90-degree optical mixer (213), two output ends of the 90-degree optical mixer (213) are respectively connected with a first balanced optical detector (214) and a second balanced optical detector (215), the second output ends of the two couplers (212) are connected with a third balanced optical detector (216), and the second output ends of the two couplers (212) are respectively connected with two input ends of the third balanced optical detector (216); the input end of the polarization beam splitter (211) is connected with the third end of the optical fiber circulator (130), and the output ends of the first balanced light detector (214), the second balanced light detector (215) and the third balanced light detector (216) are connected with the digital oscilloscope (220).
3. The fiber-sensitive ring polarization mode coupling characteristic evaluation system according to claim 2, wherein the on-line pulse polarization state receiver (210) comprises a coupler (211a), a 0 ° line analyzer (212a), a 45 ° line analyzer (213a), a right-hand circular analyzer (214a), a balanced photodetector (215 a); the three output ends of the coupler (211a) are respectively connected with the input ends of the 0-degree line analyzer (212a), the 45-degree line analyzer (213a) and the right-handed circular analyzer (214a), the output ends of the 0-degree line analyzer (212a), the 45-degree line analyzer (213a) and the right-handed circular analyzer (214a) are respectively connected with a balanced light detector (215a), the output ends of the three balanced light detectors (215a) are respectively connected with the digital oscilloscope (220), and the input end of the coupler (211a) is connected with the third end of the optical fiber circulator (130).
4. A method for evaluating the coupling characteristics of a sensitive ring polarization mode of an optical fiber using the system according to any of claims 1-3, comprising the steps of:
adjusting the polarization state of the optical signal input into the optical fiber sensitive ring (400) to be tested to be consistent with the polarization eigenstate of the optical fiber sensitive ring (400) to be tested, and inputting the optical signal consistent with the polarization eigenstate of the optical fiber sensitive ring (400) to be tested as a test optical signal; adjusting the pulse laser (110) to a continuous light output state, modulating the polarization state by the polarization controller (120), and observing the output polarization state of the optical fiber sensitive ring (400) to be detected by using the polarization analyzer (500), so that the input polarization state of the optical fiber sensitive ring (400) to be detected, which is modulated by the polarization controller (120), is consistent with the polarization eigenstate of the optical fiber sensitive ring (400) to be detected;
a pulse laser (110) is used for emitting pulse light, the pulse light signal is modulated by an adjusted polarization controller (120) and then enters a fiber sensitive ring (400) to be tested as a test signal from the second end of the fiber ring (130), and a reflected light signal is formed after Rayleigh scattering of the fiber sensitive ring (400) to be tested;
collecting complete polarization state Stokes vectors S of the reflected light signals distributed along the longitudinal direction through the online pulse polarization state receiver (210) and the digital oscilloscope (220);
and calculating the polarization mode coupling coefficient and the extinction ratio which are longitudinally distributed along the fiber sensitive ring (400) to be tested according to the Stokes vector.
5. The method of claim 4, wherein the acquiring the full polarization Stokes vectors of the reflected light signals along the longitudinal direction by the online pulse polarization receiver (210) and the digital oscilloscope (220) comprises: and coupling the test optical signal through the optical fiber circulator (130), then entering the optical fiber sensitive ring (400) to be tested, forming a reflected optical signal through backward Rayleigh scattering, returning the reflected optical signal to the second end of the optical fiber circulator (130), entering the optical fiber circulator (130), and entering the online pulse polarization state receiver (210) through the third end of the optical fiber circulator (130).
6. The method according to claim 4, wherein the calculating the polarization mode coupling coefficient and the extinction ratio distributed along the longitudinal direction of the fiber sensitive ring (400) to be tested according to the Stokes vector comprises:
calculating the change rate of the Stokes vector S according to the complete polarization state Stokes vector S
According to the formula
Calculating a birefringence vector
Unit vector
Representing the direction of the local principal axis of polarization, the magnitude | B | is the rate at which the polarization state rotates about the principal axis of polarization, i.e., the magnitude of birefringence at that principal axis;
according to the formula
The out-of-mode coupling coefficient k is calculated.
7. The method according to claim 6, characterized in that the birefringence vector B is calculated by a three-point quaternion method according to the Stokes vector relationship of three adjacent detection points on the fiber-sensitive ring (400) to be tested.
8. The method of claim 7, wherein the birefringence vector B is applied to the Poincar sphere
And projecting the two axes to calculate the mode coupling coefficient k.
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