CN108731708B - Multi-channel low-coherence interference demodulation method capable of realizing arbitrary channel matching of sensor - Google Patents
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Abstract
The invention discloses a multi-channel low-coherence interference demodulation method capable of realizing any channel matching of a sensorp(ii) a Calculating a low-precision absolute phase estimate
Recovering the absolute phase of the low coherence interference fringe according to the fringe order n of the relative phase recovery interference fringe
Calculating the true envelope peak position K of the low coherence interference fringed(ii) a Secondly, selecting any sensing channel as a calibration channel; obtaining a real envelope peak position array corresponding to the scanning pressure array under the channel; establishing a corresponding relation between the low-coherence interference fringe envelope peak position of the F-P sensor and the external pressure; thirdly, calculating the scanning optical path difference OPD generated by the polarization low-coherence interferometer and the compensation quantity delta K of the envelope peak position among different sensing channels; fourthly, calculating the compensated envelope peak position KrUsing envelope peak position KrAnd demodulating the corresponding relation with the external pressure to obtain the measured pressure value of the corresponding sensing channel. The invention realizes the multichannel adaptive demodulation of the F-P sensor.
Description
Technical Field
The invention relates to the technical field of optical fiber sensing, in particular to a low-coherence interference demodulation method suitable for multi-channel application of a Fabry-Perot (F-P) sensor.
Background
Low coherence interferometry is an important measurement method in the field of optical interferometry, and is widely used in the fields of three-dimensional topography detection, optical coherence tomography, and fiber sensing, such as pressure, temperature, refractive index measurement, etc.
The low coherence interferometry and multichannel multiplexing sensing are combined, and the method has the advantages of simultaneous multipoint measurement, high sensing precision, low sensor expansion cost and the like. At present, wavelength division multiplexing, time division multiplexing and the like are common, wherein a wavelength division multiplexing method utilizes light sources with different spectrum center wavelengths as light sources of different sensors to further realize multiplexing of a plurality of sensors, but the multiplexing and demodulating modes of the method are complex, and the number of the sensors capable of being multiplexed is limited by the spectrum of the available light source. Compared with the time division multiplexing mode based on the multi-channel polarization low-coherence interferometer, the time division multiplexing mode based on the multi-channel polarization low-coherence interferometer uses one multi-channel optical fiber array to connect the multi-channel sensors, and the signals of the multi-channel sensors are sequentially demodulated in time by using the same demodulation optical path, so that the multiplexing mode is simple, and the number of the multiplexed sensors is large. However, due to the influences of factors such as non-vertical incidence of optical wedge beams, different positions of sensing channels, non-idealization of crystal processing, and inconsistent cavity length of batch-fabricated F-P sensors, calibration operation of the F-P sensor and a specific sensing channel is usually required to ensure demodulation accuracy and reliability of the fabry-perot (F-P) sensor. The calibration operation enables the F-P sensor to demodulate only on the corresponding calibration channel, but not be applied to other demodulation channels, which severely limits the application of the F-P sensor in multi-channel multiplexing sensing.
Disclosure of Invention
In order to overcome the technical problems in the prior art, the invention provides a multi-channel low-coherence interference demodulation method capable of realizing the matching of any channel of a sensor, which adopts a Fourier transform method to obtain the original enveloping peak position of a low-coherence interference signal with lower precision under any sensing channel, and then recovers the absolute phase of the selected monochromatic wavelength interference fringe by using the original enveloping peak position according to the phase distribution characteristic of the low-coherence interference fringe, thereby recovering the real enveloping peak position with high precision; according to the established position-optical path difference distribution model of the polarization low-coherence interferometer, the position compensation quantity of the corresponding sensing channel relative to the calibration channel is calculated by using the real envelope peak position, and the real envelope peak position under the sensing channel is compensated to the position under the calibration channel, so that the multi-channel application of the F-P sensor is realized.
The invention discloses a multi-channel low-coherence interference demodulation method capable of realizing any channel matching of a sensor, which comprises the following steps of:
firstly, calculating any low-coherence interference sensing signal under any demodulation channel by a Fourier transform method to obtain a low-precision envelope peak position Kp(ii) a Calculating a low-precision absolute phase estimate
Where q represents a selected Discrete Fourier Transform (DFT) number, corresponding to a certain monochromatic wavelength, here selected 61; n represents the total number of DFT discrete sequences, is equal to the total number of data points of a frame of low coherent interference signals, and has a value of 3000;
based on low-precision absolute phase estimates
And calculating the fringe order n of the recovered interference fringe by combining the DFT process:
wherein phi represents the relative phase obtained in the DFT process of the low coherent interference fringe, and the round () function returns the integer nearest to the parameter in the bracket;
recovering the absolute phase of the low coherence interference fringe according to the fringe order n and the relative phase phi of the interference fringe
Based on the recovered absolute phase
Calculating the true envelope peak position K of the low coherence interference fringed；
Selecting a calibration sensing channel s, scanning the external pressure of the F-P sensor through a pressure control system under the channel, collecting low-coherence interference fringe signals under each scanning pressure, recovering the real envelope peak position corresponding to each signal according to the mode of the first step, and obtaining a real envelope peak position array corresponding to the scanning pressure array; by the true envelope peak position KdThe array of (1) is a horizontal axis, the scanning pressure array is a vertical axis, 4-order polynomial fitting is carried out, and the corresponding relation between the low-coherence interference fringe envelope peak position of the F-P sensor and the external pressure is established;
and thirdly, calculating the scanning optical path difference OPD generated by the polarization low-coherence interferometer according to the envelope peak position of the sensing channel based on the system parameters of the polarization low-coherence interferometer:
OPD＝Fko(K,m)
wherein the function Fko() Representing the relation, wherein m represents the channel serial number of the sensing channel, and K represents the envelope peak position of the sensing channel;
calculating the position compensation quantity delta K of the envelope peak value among different sensing channels;
ΔK＝Fod(OPD,s,m)
and s represents the channel number of the calibration sensing channel.
Fourth, calculating the compensatedEnvelope peak position Kr；
Kr＝Kd+ΔK
Using envelope peak position K under calibration channelrAnd demodulating the corresponding relation with the external pressure to obtain the measured pressure value of the corresponding sensing channel.
Compared with the prior art, the invention has the beneficial effects and advantages that:
1. according to the invention, the multi-channel applicable demodulation of the F-P sensor is realized by an envelope peak position compensation mode according to the position-optical path difference distribution characteristics of the low-coherence interference system, and the channel adaptability of the F-P sensor is effectively improved;
2. the invention determines the interference level of the low coherent interference fringe by using the low-precision envelope peak position, greatly simplifies the interference level judgment process, recovers the real envelope peak position by comprehensively using an envelope peak method and a phase method, and has high demodulation precision;
3. the invention can use the F-P sensor on any sensing channel and realize high-precision demodulation only by calibrating the F-P sensor on the condition of one sensing channel without repeating the calibration process.
Drawings
FIG. 1 is a schematic diagram of a spatial scanning type low coherence interference fiber sensing atmospheric pressure demodulation device;
FIG. 2 shows a frame of interference signals of the F-P pressure sensor collected in the actual demodulation device under the pressure of 100kPa and the envelope thereof;
FIG. 3 is a schematic diagram of a process for recovering absolute phase in conjunction with the original envelope peak position and relative phase;
FIG. 4 is a graph of a relationship between an original envelope peak position, a true envelope peak position, and a scan calibration pressure during a pressure calibration process when the selected sensing channel 4 is a calibration channel;
FIG. 5 is a graph showing the relationship between the scanning optical path difference generated by the polarization low coherence interferometer as the abscissa and the position compensation between the other sensing channels and the calibration channel 4 as the ordinate;
FIG. 6 is a graph of the positions of the original envelope peaks of four sensing channels;
FIG. 7 is a graph of interference order estimation errors for four sensing channels;
FIG. 8 is a graph of the true envelope peak position recovered for four sensing channels;
FIG. 9 is a pressure demodulation error graph illustrating pressure demodulation performed after compensating the real envelope peak positions of four sensing channels to a calibration channel;
reference numerals: 1. the device comprises a broadband light source, a coupler, a 3, an F-P sensor, a 4, a multi-channel optical fiber array, a 5, a polarization low-coherence interferometer, a 6, a polarizer, a 7, a birefringent wedge, a 8, an analyzer, a 9, a linear array CCD, a 10 and a signal processing unit; 11. low coherence interference signal, 12, low coherence interference signal envelope; 13. relative phase 14, absolute phase estimation value 15, true absolute phase 16, channel one, channel 17, channel two, channel 18, channel three, channel 19 and channel four;
FIG. 10 is an overall flowchart of the multi-channel low coherence interference demodulation method capable of realizing any channel matching of the sensor according to the present invention.
Detailed Description
The technical solution of the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.
The multi-channel low-coherence interference demodulation method capable of realizing the matching of any channel of the sensor is combined with the measurement of the external atmospheric pressure. As shown in fig. 1, the spatial scanning type low coherence interference fiber sensing atmospheric pressure demodulation apparatus has the following operation procedures:
light emitted by a broadband light source (LED)1 reaches an F-P sensor 3 through a coupler 2, the F-P sensor 3 is a sensitive element for sensing external atmospheric pressure, two reflecting surfaces of an F-P cavity of the F-P sensor form a sensing interferometer, the distance between the two reflecting surfaces of the F-P cavity in the sensing interferometer is in linear relation with the atmospheric pressure, an optical signal modulated by the F-P sensor 3 is LED out from an outlet of the coupler 2, the optical signal is LED into a polarization low-coherence interferometer 5 through a multi-channel optical fiber array 4, the multi-channel optical fiber array 4 is in a single-row and one-word optical fiber structure, the polarization low-coherence interferometer consists of a polarizer 6, a birefringent wedge 7 and an analyzer 8, and due to the birefringent effect of the birefringent wedge 7, the optical signal forms a space low-coherence interference fringe through the birefringent wedge 7 and is received by a linear array CCD9, the signal processing unit 10 processes the interference fringe signal output from the line CCD 9. When the optical path difference caused by birefringent wedge 7 matches the optical path difference caused by F-P sensor 3, significant low coherence interference fringes are produced in the corresponding local area of linear array CCD 9.
The required atmospheric pressure is generated by a high-precision and high-stability pressure source, the pressure source can achieve the control precision of 0.02kPa, the effective pixel number of the linear array CCD is 3000, and each frame of data is formed by 3000 discrete data points. Each CCD pixel corresponds to an optical path difference generated by a fixed polarization low-coherence interference demodulator, and the optical path difference is represented by the position K of the CCD pixel. For the calibration channel 4, the optical path difference corresponding to the initial effective pixel is 31.302 μm, and the optical path difference between two adjacent CCD pixels is 0.0125 μm.
The space scanning type low-coherence interference optical fiber sensing atmospheric pressure demodulating device is utilized to sense atmospheric pressure through cavity length change, a polarization low-coherence interference demodulator composed of a polarizer, a birefringent wedge and an analyzer is used to demodulate an optical signal modulated by an F-P sensor, and space low-coherence interference fringes are formed in a local area with zero optical path difference. The invention discloses a multi-channel low-coherence interference demodulation method capable of realizing any channel matching of a sensor, which comprises the following specific implementation steps of:
firstly, solving the low-precision original envelope peak position of any low-coherence interference sensing signal under any demodulation channel, and recovering the high-precision real envelope peak position by combining phase characteristics;
1.1, as shown in fig. 2, the frame of low coherence interference signal output by the CCD when the external atmospheric pressure is 100kPa contains 3000 data points. Low-precision envelope peak position K calculated by Fourier transform methodpThe corresponding CCD pixel is 1602.
1.2 enveloping the peak position K by low precision according to the phase distribution characteristicspCalculating a low-precision absolute phase estimate
Where q represents a selected Discrete Fourier Transform (DFT) number, corresponding to a certain monochromatic wavelength, here selected 61; n represents the total number of DFT discrete sequences, is equal to the total number of data points of a frame of low coherent interference signals, and has a value of 3000;
1.3 estimating the absolute phase value according to the low precision
Combining with DFT process to obtain fringe order n of relative phase recovery interference fringe:
wherein phi represents the relative phase obtained in the DFT process of the low coherent interference fringe, and the round () function returns the integer nearest to the parameter in the bracket;
1.4 recovering the absolute phase of the low coherence interference fringe according to the fringe order n and the relative phase phi of the relative phase recovery interference fringe
Fig. 3 shows a schematic diagram of the process of recovering the absolute phase by combining the original envelope peak position and the relative phase. The absolute phase recovery process includes: by low-precision original envelope peak position KpEstimated absolute phase estimate
And the recovered absolute phase
Only slight deviation exists between the interference orders, and the accurate interference order n can be recovered by combining the relative phase phi according to the property of phase distribution.
1.5 from the recovered absolute phase
Calculating the true envelope peak position K of the low coherence interference fringed；
Here the restored true envelope peak position Kd＝1605.84。
Secondly, selecting a calibration channel to calibrate the F-P sensor, and establishing a corresponding relation between the low-coherence interference fringe envelope peak position of the F-P sensor and the external pressure;
2.1, selecting a calibration sensing channel s, scanning the external pressure of the F-P sensor through a pressure control system under the channel, collecting low-coherence interference fringe signals under each scanning pressure, recovering the real envelope peak position corresponding to each signal according to a first step mode, and obtaining a real envelope peak position array corresponding to a scanning pressure array; FIG. 4 is a graph showing the relationship between the original envelope peak position, the real envelope peak position and the scanning calibration pressure during the pressure calibration process when the selected sensing channel 4 is the calibration channel, wherein the calibration pressure is increased by the corresponding original envelope peak position K in steps of 5kPa within the range of 100kPa and 200kPapAnd the recovered real envelope peak position Kd。
2.2, with true envelope peak position KdThe array of (1) is a horizontal axis, the scanning pressure array is a vertical axis, 4-order polynomial fitting is carried out, and the corresponding relation between the low-coherence interference fringe envelope peak position of the F-P sensor and the external pressure is established;
thirdly, establishing a relation between the scanning Optical Path Difference (OPD) generated by the polarization low-coherence interference demodulator and the position of the envelope peak value of the interference fringe through optical path calculation, and further deriving a relation between the scanning Optical Path Difference (OPD) and the compensation quantity of the envelope peak value positions of different sensing channels;
3.1, based on the system parameters of the polarization low coherence interferometer, calculating and analyzing the relationship between the envelope peak positions K of different sensing channels and the scanning optical path difference OPD generated by the polarization low coherence interferometer through an optical path:
OPD＝Fko(K,m)
wherein the function Fko() The above relationship is characterized, and m represents the channel number of the sensing channel.
3.2, based on the system parameters of the polarization low coherence interference demodulator, calculating and analyzing the relationship between the scanning optical path difference OPD generated by the polarization low coherence interference demodulator and the envelope peak position of different sensing channels through an optical path, and further deriving the relationship between the scanning optical path difference OPD generated by the polarization low coherence interference demodulator and the envelope peak position compensation quantity delta K between different sensing channels;
ΔK＝Fod(OPD,s,m)
wherein the function Fod() The relation is characterized, and s represents the channel serial number of the calibrated sensing channel. As shown in fig. 5, a graph of the relationship between the scanning optical path difference generated by the polarization low coherence interferometer as the abscissa and the position compensation amount between the other sensing channels and the calibration channel 4 as the ordinate is shown, and the simulation result of the position offset between the calibration channel 4 and the other sensing channels is obtained when the optical path difference increases from 30 μm to 70 μm at intervals of 0.1 μm;
fourthly, restoring the real envelope peak position and compensating the envelope peak position among channels, and demodulating the original envelope peak position K of any channelpAnd compensating to a calibration channel, and demodulating the pressure value by using the compensated envelope peak position.
4.1 original envelope Peak position K for an arbitrary sense channelpAccording to the first step, recovering the true envelope peak value position Kd；
4.2 OPD ═ F according to the relation in 3.1ko(K, m) Using KdThe demodulator corresponding to the position is calculated to scan the optical path difference OPD, and then the position is obtained according to 3.2And setting the offset delta K, and compensating the delta K to be under a calibration channel.
Kr＝Kd+ΔK
Wherein KrIs the compensated envelope peak position.
4.3, utilizing envelope peak position K under calibration channelrAnd the corresponding relation with the external pressure can demodulate the measured pressure value of the corresponding sensing channel, so that the high-precision demodulation of the F-P sensor under any channel is realized.
To more fully verify the feasibility of the method, taking a 4-channel polarization low coherence interferometer as an example, and taking channel 4 as a calibration channel, the experimental pressure monotonically increased from 100kPa to 200kPa at intervals of 0.5 kPa. The interference signal at each experimental pressure for each sensing channel is processed. Fig. 6, 7, and 8 are graphs of original envelope peaks, estimated interference level errors, and restored real envelope peak positions of four channels, respectively, and it can be seen that in the entire measurement range (100kPa-200kPa), the interference level estimation error is less than 0.15, which is less than the maximum effective estimation error of 0.5, the restored real envelope peak position linearity is very good, and no step error caused by interference level misjudgment occurs, and therefore the method of the present invention has strong reliability for restoring the real envelope peak position. FIG. 9 shows the pressure demodulation errors after the position compensation of each sensing channel, and it can be clearly seen that the demodulation errors of the method of the present invention can be maintained within 0.14kPa on any channel, so that the high-precision demodulation of the F-P sensor under multiple channels is realized.
The demodulation method of the present invention was verified experimentally, see fig. 4 to 9.
Claims (1)
1. A multi-channel low-coherence interference demodulation method capable of realizing any channel matching of a sensor is characterized by comprising the following steps:
firstly, calculating any low-precision envelope peak position K of any low-coherence interference sensing signal under any demodulation channel by a Fourier transform methodp(ii) a Calculating a low-precision absolute phase estimate
Wherein q represents a selected discrete fourier transform sequence number, corresponding to a certain monochromatic wavelength, here selected 61; n represents the total number of discrete sequences, which is equal to the total number of data points of a frame of low coherence interference signal, and the value is 3000;
based on low-precision absolute phase estimates
And calculating the fringe order n of the relative phase recovery interference fringes by combining a discrete Fourier transform process:
wherein phi represents the relative phase obtained in the DFT process of the low coherent interference fringe, and the round () function returns the integer nearest to the parameter in the bracket;
recovering the absolute phase of the low coherence interference fringe according to the fringe order n and the relative phase phi of the relative phase recovery interference fringe
Based on the recovered absolute phase
Calculating the true envelope peak position K of the low coherence interference fringed；
Secondly, scanning the external pressure of the F-P sensor through a pressure control system, collecting low-coherence interference fringe signals under each scanning pressure, recovering the real envelope peak position corresponding to each signal according to the mode of the first step, and obtaining a real envelope peak position array corresponding to the scanning pressure array; by the true envelope peak position KdThe array of (1) is a horizontal axis, the scanning pressure array is a vertical axis, 4-order polynomial fitting is carried out, and the corresponding relation between the low-coherence interference fringe envelope peak position of the F-P sensor and the external pressure is established;
and thirdly, calculating the scanning optical path difference OPD generated by the polarization low-coherence interferometer:
OPD＝Fko(K,m)
wherein the function Fko() Representing the relation, wherein m represents the channel serial number of the sensing channel, and K represents the position of the envelope peak of different sensing channels;
calculating the position compensation quantity delta K of the envelope peak value among different sensing channels;
ΔK＝Fod(OPD,s,m)
wherein s represents a channel serial number of the calibration sensing channel;
fourthly, calculating the compensated envelope peak position Kr；
Kr＝Kd+ΔK
Using envelope peak position K under calibration channelrAnd demodulating the corresponding relation with the external pressure to obtain the measured pressure value of the corresponding sensing channel.
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