CN112097813A - 3X3 coupler photoelectric detection method and device based on optical frequency modulation - Google Patents
3X3 coupler photoelectric detection method and device based on optical frequency modulation Download PDFInfo
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Abstract
The invention belongs to the technical field of optical fiber sensing, in particular to a 3x3 coupler photoelectric detection method and a device based on optical frequency modulation, the method applies a sinusoidal signal modulation to the optical frequency of output laser of a single-frequency tunable laser through a signal generator, utilizes the phase difference between the output signals of the 3x3 coupler, combines any two output signals in three interference signals from a photoelectric detector in a least square fitting mode to obtain an elliptic curve, obtains an elliptic equation coefficient according to the fitted curve, solves the channel parameters of any two signals according to the corresponding relation between the parameters of the three channels and the coefficients in the fitted elliptic equation, can obtain the parameters of the three channels only by carrying out elliptic fitting on the three signals twice, can realize real-time adjustment, and successfully solves the problems of asymmetry and parameter degradation of the coupler, the stability and the reliability of system demodulation are improved, and therefore stable and accurate demodulation of external sensing signals is achieved.
Description
Technical Field
The invention belongs to the technical field of optical fiber sensing, and particularly relates to a 3x3 coupler photoelectric detection method and device based on optical frequency modulation.
Background
The optical fiber sensing system can adapt to various severe meteorological environments, does not need additional power supply, can realize long-distance transmission and the like, and is a research hotspot in the current sensing system industry. Compared with an intensity type sensing system, the interference type optical fiber sensing system has high sensitivity, becomes an important component of the research of the optical fiber sensing system, achieves the purpose of detection by utilizing the action of a measured object on the optical fiber to cause phase change when light passes through the optical fiber, is simple and easy to demodulate, and is widely applied to the fields of underwater acoustic detection, port base warning, oil exploration, seismic wave detection and the like.
The interference type optical fiber sensing system has the advantages of high sensitivity and resolution, flexible and various structure and wide application occasions. The interference type optical fiber sensing system has been paid attention to by military and civil research institutions at home and abroad, researchers of many institutions invest a large amount of manpower and material resources to research the interference type optical fiber sensing system, and the research on the signal detection technology of the interference type optical fiber sensing system has profound practical significance for promoting commercialization and practicability of the optical fiber sensing system.
Common interference type optical fiber sensing system signal detection techniques include a phase carrier modulation demodulation technique (PGC), a 3 × 3 coupler multiphase detection technique, and a heterodyne demodulation technique. Compared with heterodyne and PGC methods, the 3 multiplied by 3 coupler multiphase detection technology can enable the system to obtain larger working bandwidth and dynamic range, and the demodulation circuit is simple and easy to implement, thereby having obvious advantages.
However, the conventional demodulation method based on 3 × 3 fiber coupler requires that the splitting ratio of the coupler strictly satisfies 1:1:1 and the phase difference is equal to 120 °, in practice, a completely symmetric fiber coupler is difficult to prepare, generally speaking, an asymmetric 3 × 3 coupler, and the performance parameters of the fiber coupler are also degraded with time, which brings great difficulty to demodulation.
In recent years, some researchers have proposed solutions to the problems of asymmetry and parametric degradation of couplers. The invention patent of China (optical fiber laser hydrophone signal demodulation system) (patent number: ZL200910062834.4) obtains three-channel parameters by adding and subtracting maximum and minimum values, but the method has lower precision and more complex demodulation process. A paper (a new modulation technique for optical fiber sensors with [3 × 3] direct couplers, a bangting, a treble, an chengshen, a xiaoling, a grand dexing, a Chinese Optics Letters, 2008, and 01) proposes a novel demodulation algorithm for obtaining three channel parameters based on ellipse fitting of any two signals, which shows and verifies that the algorithm has a certain degree of polarization fading resistance, and simultaneously overcomes the problems of asymmetry of a coupler and inconsistency of conversion effects of a photodetector, but the algorithm adopts PZT to load large-signal modulation, introduces an active device, and is not beneficial to future large-scale application.
Disclosure of Invention
Aiming at the defects, the invention provides the 3x3 coupler photoelectric detection method and the device based on the optical frequency modulation with higher reliability and stability, compared with the traditional ellipse fitting algorithm based on the external large signal modulation, the method calibrates the three channel parameters by the optical frequency modulation method, avoids the influence of the external large signal on the system, can improve the stability and the reliability of the system demodulation to a great extent, and provides a basic support for the 3x3 demodulation system of the later large-scale array formation.
The idea of the invention is as follows: the invention provides a 3x3 coupler photoelectric detection method based on optical frequency modulation, which applies sinusoidal signal modulation to the output laser optical frequency of a single-frequency tunable laser through a signal generator, utilizes the phase difference between the output signals of the 3x3 coupler, forms Lissajous figures by any two paths of interference signals, constructs a mathematical model according to the characteristics of the Lissajous figures, and respectively draws the two paths of interference signals as horizontal and vertical coordinates on two coordinate axes which are vertical to each other, and the track of the mathematical model is an ellipse. The method has the advantages that an elliptic curve can be obtained by connecting any two output signals in three interference signals from the photoelectric detector in a least square fitting mode, an elliptic equation coefficient is obtained according to the fitting curve, channel parameters of any two signals can be solved according to the corresponding relation between the three channel parameters and each coefficient in the fitted elliptic equation, three channel parameters can be obtained only by carrying out elliptic fitting on the three signals twice, real-time adjustment and calibration can be realized, the problems of asymmetry and parameter degradation of a coupler are successfully solved, the stability and reliability of system demodulation are improved, and stable and accurate demodulation of external sensing signals is realized.
The technical scheme adopted by the invention is as follows: a3 x3 coupler photoelectric detection method based on optical frequency modulation is based on a photoelectric detection device which consists of a signal generator 1, a single-frequency tunable laser 2, an optical fiber circulator 3, a 3x3 coupler 4, a sensing arm 5, a reference arm 6, a first Faraday Rotator Mirror (FRM)7, a second Faraday Rotator Mirror (FRM)8, a first photoelectric detector 9, a second photoelectric detector 10, a third photoelectric detector 11, a data acquisition card 12 and a computer 13, and concretely comprises the following implementation steps:
the first step is as follows: the signal generator 1 applies an optical frequency modulation signal to the single-frequency tunable laser 2, so that laser output by the single-frequency tunable laser 2 is changed into frequency modulated laser;
the second step is that: the frequency-modulated laser enters the 3x3 coupler 4 through the optical fiber circulator 3;
the third step: the 3 × 3 coupler 4 divides input frequency-modulated laser into two beams which are respectively transmitted into a sensing arm 5 and a reference arm 6 of a michelson interferometer, and the michelson interferometer consists of the 3 × 3 coupler 4, the sensing arm 5, the reference arm 6, a first faraday rotator 7 and a second faraday rotator 8;
the fourth step: the light passing through the sensing arm 5 is reflected back into the 3 × 3 coupler 4 by the first faraday rotator mirror 7, and the light passing through the reference arm 6 is reflected back into the 3 × 3 coupler 4 by the second faraday rotator mirror 8;
the fifth step: two beams of light are converged by the 3 × 3 coupler 4 and then interfere with each other, and respectively enter the optical fiber circulator 3, the second photoelectric detector 10 and the third photoelectric detector 11, and the light passing through the optical fiber circulator 3 enters from the 3-2 port, is output from the 3-3 port and enters the first photoelectric detector 9;
and a sixth step: the first photodetector 9, the second photodetector 10, and the third photodetector 11 respectively send the acquired three-way interference signals to the computer 13 through three analog acquisition input ports CH1, CH2, and CH3 of the data acquisition card 12, where the three-way interference signals can be represented as follows:
wherein, mu1、μ2、μ3Respectively representing a first interference signal, a second interference signal and a third interference signal, A1、A2、A3Representing the DC-quantity parameters of the first interference signal, the second interference signal and the third interference signal, respectively, B1、B2、B3Respectively representing the amplitude parameters of the alternating terms of the first interference signal, the second interference signal and the third interference signal,
representing a signal to be detected;
the seventh step: performing ellipse fitting processing on a first interference signal and a second interference signal in the three-path signals to obtain a scatter diagram of the two-path interference signals, wherein the relationship between the two-path signals with fixed phase difference satisfies the following ellipse equation:
wherein
For the coefficients of an elliptic equation, to simplify the operation, the initial coefficient has been set to 1,
eighth step: the coefficient of the elliptic equation is directly obtained according to the fitted elliptic equation, and the direct current quantity of the two paths of signals can be obtained according to the coefficient of the elliptic equation obtained by calculation as shown in the formulas (0.5) and (0.6):
then, the amplitude of the alternating term of the two paths of signals is obtained as shown in the formulas (0.7) and (0.8):
solving to obtain a parameter A of the first and second channels1,A2,B1,B2；
The ninth step: performing the same ellipse fitting processing on the first interference signal and the third interference signal according to the eighth step to obtain a first channel parameter A and a third channel parameter A1,A3,B1,B3So far, three channel parameters A can be obtained by two times of ellipse fitting1,A2,A3,B1,B2,B3All the parameters are calculated, and then the calibration of the parameters is completed;
of course, the same ellipse fitting processing may also be performed on the second interference signal and the third interference signal according to the eighth step method to obtain the two-way and three-way channel parameters a2,A3,B2,B3；
The tenth step: after the parameters are calibrated, the signal generator 1 is removed, then normal demodulation is carried out, and after the external sensing signals are received, the signals are subjected to pretreatment of direct current reduction and alternating current reduction, as shown in the following three formulas (0.9), (0.10) and (0.11):
performing arc tangent operation on the preprocessed three-way data as follows:
the computer stipulates that the angles of the first quadrant and the second quadrant are 0-pi, and the angles of the third quadrant and the fourth quadrant are-pi-0, so that phase jump often occurs, the amplitude of the phase jump is 2-pi (winding of the phase), and therefore, phase unwrapping processing is performed on data after being subjected to arc tangent in the arc tangent operation, namely, a threshold value (usually set as pi) is set, jump is considered when the difference between two points before and after the data exceeds the threshold value, then + 2-pi or-2-pi operation is performed on the processed data, the phase is ensured not to jump, and the required external sensing signal can be obtained by performing low-frequency filtering on the processed data after the uncoiling processing.
Further, the waveform of the modulation signal loaded by the signal generator 1 is a sinusoidal signal, but is not limited to a sinusoidal signal, and other types of signals can also realize optical frequency modulation parameter calibration.
Further, the amplitude of the modulation signal applied by the signal generator 1 to the single-frequency tunable laser 2 should be large enough to fit the entire ellipse, otherwise the parameter calibration accuracy would be affected.
The present invention also provides a 3 × 3 coupler demodulation apparatus based on the optical frequency modulation method, including: the device comprises a signal generator 1, a single-frequency tunable laser 2, an optical fiber circulator 3, a 3x3 coupler 4, a sensing arm 5, a reference arm 6, a first Faraday Rotator Mirror (FRM)7, a second Faraday Rotator Mirror (FRM)8, a first photoelectric detector 9, a second photoelectric detector 10, a third photoelectric detector 11, a data acquisition card 12 and a computer 13; an output port 1-1 of the signal generator 1 is connected with a wavelength modulation input port 2-1 of a single-frequency tunable laser 2, an output port 2-2 of the single-frequency tunable laser 2 is connected with a first port 3-1 of an optical fiber circulator 3, a second port 3-2 of the optical fiber circulator 3 is connected with a first port 4-1 of a 3 × 3 coupler 4, a third port 3-3 of the optical fiber circulator 3 is connected with a first photoelectric detector 9, a second port 4-2 of the 3 × 3 coupler 4 is connected with a second photoelectric detector 10, a third port 4-3 is connected with a third photoelectric detector 11, a fourth port 4-4 of the 3 × 3 coupler 4 is connected with a first Faraday rotator 7 through a sensing optical fiber to form a sensing arm 5 of the Michelson interferometer, and a fifth port 4-5 is connected with a second Faraday rotator 8, a reference arm 6 of the michelson interferometer is formed, and the tail end of the optical fiber connected with the sixth port 4-6 of the 3 × 3 coupler 4 is disconnected for performing end face reflection elimination; the output ports of the first photodetector 9, the second photodetector 10 and the third photodetector 11 are respectively connected with three data acquisition input channels CH1, CH2 and CH3 of the data acquisition card 12, and the output port of the data acquisition card 12 is connected with the computer 13.
The invention can achieve the following technical effects:
(1) compared with the traditional demodulation algorithm based on the symmetrical 3x3 coupler, the method has low performance requirement on the coupler, the traditional method requires three paths of couplers to be strictly symmetrical, and the splitting ratio meets 1:1:1 and the phase difference is 120 deg.. However, in practice, perfectly symmetrical 3x3 couplers are difficult to fabricate and performance parameters also degrade slowly over time. According to the method, parameters required by demodulation can be obtained by only carrying out ellipse fitting twice, and then real-time adjustment can be realized by only acquiring data in real time all the time and carrying out ellipse fitting on the data to obtain ellipse equation parameters, so that the signal detection precision is improved, and the stable detection of signals is realized;
(2) the method consists of a signal generator, a single-frequency tunable laser, an optical fiber circulator, three photoelectric detectors, two Faraday rotators, a computer and a data acquisition card, does not need to introduce an additional active device at a probe part, replaces large-amplitude modulation with an optical frequency modulation method, reduces the influence of the large-amplitude signal on the system, has simple structure, high reliability and high practicability, and provides a basic support for large-scale array formation of a 3 multiplied by 3 demodulation system;
drawings
FIG. 1 is a flow chart of a demodulation method provided by the present invention;
FIG. 2 is a diagram of an experimental setup provided by the present invention;
FIG. 3 is a waveform of three signals of the present invention during an experiment;
FIG. 4 is a graph of 200Hz optical frequency modulated sinusoidal signals provided to a single frequency tunable laser 2 in accordance with the present invention during an experiment;
FIG. 5 is a Lissajous diagram of the first and second paths of interference signals of the present invention during the experiment;
FIG. 6 is a Lissajous diagram of the first and third interference signals of the present invention during the experiment;
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
Aiming at the defects and requirements of the existing 3 × 3 coupler demodulation method and parameter calibration scheme, the invention provides a demodulation method of a 3 × 3 coupler based on optical frequency modulation, which comprises the steps of carrying out ellipse fitting on three interference signals subjected to optical frequency modulation to calibrate channel parameters as shown in fig. 1, then detecting external signals, eliminating direct current components and alternating current term amplitude components of the three interference signals, carrying out signal preprocessing, carrying out arc tangent and filtering processing on the preprocessed signals, and demodulating the needed external sensing signals.
As shown in fig. 2, the present invention also provides a 3 × 3 coupler demodulation apparatus based on optical frequency modulation, including: the device comprises a signal generator 1, a single-frequency tunable laser 2, an optical fiber circulator 3, a 3x3 coupler 4, a sensing arm 5, a reference arm 6, a Faraday Rotator Mirror (FRM)7, a Faraday Rotator Mirror (FRM)8, a first photoelectric detector 9, a second photoelectric detector 10, a third photoelectric detector 11, a data acquisition card 12 and a computer 13.
FIG. 3 is a time domain and frequency spectrum diagram of the three-way interference signal in the experiment, in which the signals within 0.373s-0.378s are cut off, and for the sake of easy distinction, V1 is represented by 'o', i.e. the first path of interference signal, 'x' represents V2, i.e. the second path of interference signal,
representing V3, a third interference signal, the spectrogram intercepts the frequency spectrum in the range of 0-1kHz, wherein the peak represents 200Hz and the frequency multiplication term of 200 Hz;
FIG. 4 is a loaded 1V, 200Hz optical frequency modulated sine wave signal;
FIG. 5 is a graph of the effect of fitting a signal ellipse for one and two channels, with measured data points represented by 'o' and the fitted ellipse represented by the line '-' from which the coefficients of the ellipse equation can be derived;
fig. 6 is an effect diagram of signal ellipse fitting of one channel and three channels, in which measured data points are represented by 'o' and fitted ellipses are represented by lines '-' to obtain an ellipse equation coefficient, and no two-channel and three-channel signal ellipse fitting is performed because three-channel parameters can be calculated by two-time ellipse fitting.
The invention provides a 3x3 coupler photoelectric detection method and a device based on optical frequency modulation, the method comprises two steps of coefficient calibration and demodulation, the coefficient required by demodulation is calibrated by the optical frequency modulation in the early stage, the internal parameter calibration can be carried out without the help of external signals, the stability and the reliability are greatly improved, and the external sensing signals can be normally demodulated by an anti-tangential method of 3x3 demodulation after calibration.
In the embodiment of the invention, the method for modulating the optical frequency avoids the influence of a previously loaded large-amplitude signal on a system, and greatly improves the reliability and the practicability. Particularly, all parameters required by demodulation can be obtained through ellipse fitting, the method is accurate and fast, and stable detection of signals can be realized. A scatter diagram of the data is drawn in the experiment, ellipse fitting is carried out on the scatter diagram, whether the scatter diagram and the fitted ellipse are in line or not can be visually seen, and observation is facilitated.
Claims (6)
1. A3 x3 coupler photoelectric detection method based on optical frequency modulation is based on a photoelectric detection device consisting of a signal generator (1), a single-frequency tunable laser (2), an optical fiber circulator (3), a 3x3 coupler (4), a sensing arm (5), a reference arm (6), a first Faraday rotator mirror (7), a second Faraday rotator mirror (8), a first photoelectric detector (9), a second photoelectric detector (10), a third photoelectric detector (11), a data acquisition card (12) and a computer (13), and is characterized by comprising the following concrete implementation steps:
the first step is as follows: the signal generator (1) applies an optical frequency modulation signal to the single-frequency tunable laser (2) to change laser output by the single-frequency tunable laser (2) into frequency modulated laser;
the second step is that: the frequency-modulated laser enters a 3x3 coupler (4) through an optical fiber circulator (3);
the third step: the 3x3 coupler (4) divides input frequency-modulated laser into two beams which are respectively transmitted into a sensing arm (5) and a reference arm (6) of a Michelson interferometer, and the Michelson interferometer consists of the 3x3 coupler (4), the sensing arm (5), the reference arm (6), a first Faraday rotator mirror (7) and a second Faraday rotator mirror (8);
the fourth step: the light passing through the sensing arm (5) is reflected back to the 3x3 coupler (4) by the first faraday rotator mirror (7), and the light passing through the reference arm (6) is reflected back to the 3x3 coupler (4) by the second faraday rotator mirror (8);
the fifth step: two beams of light are converged by the 3 × 3 coupler (4) and then interfere with each other, and respectively enter the optical fiber circulator (3), the second photoelectric detector (10) and the third photoelectric detector (11), the light passing through the optical fiber circulator (3) enters from the second port (3-2) of the optical fiber circulator (3), is output from the third port (3-3) of the optical fiber circulator (3), and enters the first photoelectric detector (9);
and a sixth step: the first photodetector (9), the second photodetector (10) and the third photodetector (11) respectively send the collected three-way interference signals to the computer (13) through three analog collection input ports CH1, CH2 and CH3 of the data collection card (12), and the three-way interference signals can be respectively expressed in the following forms:
wherein, mu1、μ2、μ3Respectively representing a first interference signal, a second interference signal and a third interference signal, A1、A2、A3Representing the DC-quantity parameters of the first interference signal, the second interference signal and the third interference signal, respectively, B1、B2、B3Respectively representing the first interference signal and the second interference signalThe amplitude parameter of the alternating terms of the interference signal and the third interference signal,
representing a signal to be detected;
the seventh step: performing ellipse fitting processing on a first interference signal and a second interference signal in the three-path signals to obtain a scatter diagram of the two-path interference signals, wherein the relationship between the two-path signals with fixed phase difference satisfies the following ellipse equation:
wherein
For the coefficients of an elliptic equation, to simplify the operation, the initial coefficient has been set to 1,
eighth step: the coefficient of the elliptic equation is directly obtained according to the fitted elliptic equation, and the direct current quantity of the two paths of signals can be obtained according to the coefficient of the elliptic equation obtained by calculation as shown in the formulas (0.5) and (0.6):
then, the amplitude of the alternating term of the two paths of signals is obtained as shown in the formulas (0.7) and (0.8):
solving to obtain a parameter A of the first and second channels1,A2,B1,B2；
The ninth step: performing the same ellipse fitting processing on the first interference signal and the third interference signal according to the eighth step to obtain a first channel parameter A and a third channel parameter A1,A3,B1,B3So far, three channel parameters A can be obtained by two times of ellipse fitting1,A2,A3,B1,B2,B3All the parameters are calculated, and then the calibration of the parameters is completed;
the tenth step: after the parameters are calibrated, the signal generator (1) is removed, then normal demodulation is carried out, and after the external sensing signals are received, the signals are subjected to pretreatment of direct current reduction and alternating current reduction, as shown in the following three formulas (0.9), (0.10) and (0.11):
performing arc tangent operation on the preprocessed three-way data as follows:
in the process of the arc tangent operation, the data after the arc tangent is subjected to phase unwrapping processing, namely a threshold value is set, jump is considered when the difference value between two points before and after the data exceeds the threshold value, then the data is processed by +2 pi or-2 pi operation, the phase is ensured not to jump, and the required external sensing signal can be obtained by carrying out low-frequency filtering on the data after the unwrapping processing.
2. A 3x3 coupler photodetection method based on optical frequency modulation according to claim 1, characterized in that: the waveform of the modulation signal loaded by the signal generator (1) is a sine signal.
3. A 3x3 coupler photodetection method based on optical frequency modulation according to claim 1, characterized in that: the amplitude of the modulation signal applied by the signal generator (1) to the single frequency tunable laser (2) should be large enough to fit the entire ellipse.
4. A 3x3 coupler photodetection method based on optical frequency modulation according to claim 1, characterized in that: in the ninth step, the second interference signal and the third interference signal may also be subjected to the same ellipse fitting processing according to the eighth step to obtain two-way and three-way channel parameters a2,A3,B2,B3。
5. A 3x3 coupler photodetection method based on optical frequency modulation according to claim 1, characterized in that: the threshold value at the time of performing the phase unwrapping process on the data after arc tangent in the tenth step is usually set to pi.
6. A 3x3 coupler demodulation apparatus based on the optical frequency modulation method according to any one of claims 1 to 3, characterized in that: the device comprises a signal generator (1), a single-frequency tunable laser (2), an optical fiber circulator (3), a 3 multiplied by 3 coupler (4), a sensing arm (5), a reference arm (6), a first Faraday rotator mirror (7), a second Faraday rotator mirror (8), a first photoelectric detector (9), a second photoelectric detector (10), a third photoelectric detector (11), a data acquisition card (12) and a computer (13); the output port (1-1) of the signal generator (1) is connected with the wavelength modulation input port (2-1) of the single-frequency tunable laser (2), the output port (2-2) of the single-frequency tunable laser (2) is connected with the first port (3-1) of the optical fiber circulator (3), the second port (3-2) of the optical fiber circulator (3) is connected with the first port (4-1) of the 3x3 coupler (4), the third port (3-3) of the optical fiber circulator (3) is connected with the first photoelectric detector (9), the second port (4-2) of the 3x3 coupler (4) is connected with the second photoelectric detector (10), the third port (4-3) is connected with the third photoelectric detector (11), the fourth port (4-4) of the 3x3 coupler (4) is connected with the first Faraday rotator mirror (7) through a sensing optical fiber, a sensing arm (5) of the Michelson interferometer is formed, a fifth port (4-5) is connected with a second Faraday rotator mirror (8) to form a reference arm (6) of the Michelson interferometer, and the tail end of an optical fiber connected with a sixth port (4-6) of the 3x3 coupler (4) is disconnected to be subjected to end face reflection elimination; the output ports of the first photoelectric detector (9), the second photoelectric detector (10) and the third photoelectric detector (11) are respectively connected with three data acquisition input channels CH1, CH2 and CH3 of a data acquisition card (12), and the output port of the data acquisition card (12) is connected with a computer (13).
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