CN108731708A - The matched multichannel low coherence interference demodulation method in the arbitrary channel of sensor can be achieved - Google Patents

Abstract

The invention discloses a multi-channel low-coherence interference demodulation method capable of matching any channel of a sensor. The first step is to calculate and obtain the low-precision envelope peak position K _p through the Fourier transform method; calculate the low-precision absolute phase estimation value Recover the absolute phase of the low-coherence interference fringe by recovering the fringe order n of the interference fringe according to the relative phase Calculate the real envelope peak position K _d of the low-coherence interference fringes; the second step is to select any sensing channel as the calibration channel; obtain a real envelope peak position array corresponding to the scanning pressure array under this channel; establish F ‑P sensor low-coherence interference fringe envelope peak position and the corresponding relationship with the external pressure; the third step is to calculate the scanning optical path difference OPD generated by the polarization low-coherence interferometer and the envelope peak position compensation amount ΔK between different sensing channels ; The fourth step is to calculate the compensated envelope peak position K _r , and use the corresponding relationship between the envelope peak position K _r and the external pressure to demodulate the measured pressure value of the corresponding sensing channel. The invention realizes the multi-channel applicable demodulation of the F-P sensor.

Description

A multi-channel low-coherence interferometric demodulation method capable of matching any channel of the sensor

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 Fabry-Perot (F-P) sensors.

Background technique

Low-coherence interferometry is an important measurement method in the field of optical interference, and is widely used in the fields of three-dimensional shape detection, optical coherence tomography, and optical fiber sensing, such as pressure, temperature, and refractive index measurements.

The combination of low-coherence interferometry and multi-channel multiplexing sensing applications has the advantages of simultaneous measurement of multiple points, high sensing accuracy, and low cost of sensor expansion. At present, wavelength division multiplexing and time division multiplexing are relatively common. Among them, the wavelength division multiplexing method uses light sources with different spectral center wavelengths as light sources for different sensors, and then realizes the multiplexing of multiple sensors. However, the multiplexing method of this method The use and demodulation methods are complex, and the number of sensors that can be multiplexed is limited by the spectrum of the available light source. In comparison, the time-division multiplexing method based on a multi-channel polarization low-coherence interferometer uses a multi-channel optical fiber array to connect multiple sensors, and uses the same demodulation optical path to sequentially demodulate the signals of multiple sensors in time. The method is simple, and the number of reusable sensors is large. However, affected by factors such as non-vertical incidence of the wedge beam, different positions of the sensing channels, non-ideal crystal processing, and inconsistent cavity lengths of mass-produced F-P sensors, in order to ensure the accuracy of Fabry-Perot (F-P) sensor demodulation Accuracy and reliability usually require calibration of F-P sensors with a specific sensing channel. The calibration operation makes the F-P sensor demodulate only on the corresponding calibration channel, but cannot be applied to other demodulation channels, which severely limits the application of the F-P sensor in multi-channel multiplexing sensing.

Contents of the invention

In order to overcome the above-mentioned technical problems in the prior art, the present invention proposes a multi-channel low-coherence interference demodulation method that can realize any channel matching of sensors, and adopts the Fourier transform method to obtain low accuracy in any sensing channel. The original envelope peak position of the low-coherence interference signal, and then use this position to restore the absolute phase of the selected monochromatic wavelength interference fringe according to the phase distribution characteristics of the low-coherence interference fringe, and then restore the high-precision real envelope peak position; according to the established The position-optical path difference distribution model of the polarization low-coherence interferometer is used to calculate the position compensation amount of the corresponding sensing channel relative to the calibration channel by using the real envelope peak position, and compensate the real envelope peak position under the sensing channel To the calibration channel, so as to realize the multi-channel application of F-P sensor.

A multi-channel low-coherence interference demodulation method capable of matching any channel of a sensor according to the present invention comprises the following steps:

The first step, for any low-coherence interference sensing signal under any demodulation channel, calculate the low-precision envelope peak position K _p by Fourier transform method; calculate the low-precision absolute phase estimation value

Among them, q represents the serial number of the selected discrete Fourier transform (DFT), which corresponds to a certain monochromatic wavelength, and 61 is selected here; N represents the total number of DFT discrete sequences, which is equal to the total number of data points of a frame of low-coherence interference signal, the value is 3000;

According to the low precision absolute phase estimate Combined with the DFT process, calculate the fringe order n of the restored interference fringe:

Among them, φ represents the relative phase obtained by the low-coherence interference fringe DFT process, and the round() function returns the nearest integer to the parameters in parentheses;

Recover the absolute phase of the low-coherence interference fringe according to the fringe order n and the relative phase φ of the interference fringe

According to the recovered absolute phase Calculate the real envelope peak position K _d of the low-coherence interference fringe;

The second step is to select the calibration sensing channel s, scan the external pressure of the FP sensor through the pressure control system under this channel, collect the low-coherence interference fringe signal under each scanning pressure, and restore each A real envelope peak position corresponding to a signal, to obtain a real envelope peak position array corresponding to the scanning pressure array; take the array of the real envelope peak position K _d as the horizontal axis, and the scanning pressure array as the vertical axis to perform a 4-degree polynomial simulation Combined, the corresponding relationship between the peak position of the low-coherence interference fringe envelope of the FP sensor and the external pressure is established;

The third step is to calculate the scanning optical path difference OPD generated by the polarization low coherence interferometer based on the system parameters of the polarization low coherence interferometer according to the peak position of the envelope of the sensing channel:

OPD＝F _ko (K,m)

Wherein, the function F _ko () characterizes the above relationship, m represents the channel number of the sensing channel, and K represents the peak position of the sensing channel envelope;

Calculate the envelope peak position compensation amount ΔK between different sensing channels;

ΔK＝F _od (OPD,s,m)

s represents the channel number of the calibration sensing channel.

The fourth step is to calculate the compensated envelope peak position K _r ;

K _r =K _d +ΔK

Using the corresponding relationship between the envelope peak position K _r under the calibration channel and the external pressure, the measured pressure value of the corresponding sensing channel is demodulated.

Compared with prior art, beneficial effect and advantage of the present invention are:

1. According to the position-optical path difference distribution characteristics of the low-coherence interference system, the present invention realizes the multi-channel applicable demodulation of the F-P sensor through the method of envelope peak position compensation, and effectively improves the channel adaptability of the F-P sensor;

2. The present invention uses the low-precision envelope peak position to determine the low-coherence interference fringe interference order, which greatly simplifies the interference order determination process, and comprehensively utilizes the envelope peak method and phase method to restore the real envelope peak position, and the demodulation accuracy high;

3. The present invention only needs to calibrate the F-P sensor under the condition of one sensing channel, without repeating the calibration process, it can use the F-P sensor on any sensing channel and realize high-precision demodulation.

Description of drawings

Figure 1 is a schematic diagram of a space-scanning low-coherence interference optical fiber sensing atmospheric pressure demodulation device;

Figure 2 is a frame of interference signal and its envelope of the F-P pressure sensor collected in the actual demodulation device under a pressure of 100kPa;

Fig. 3 is a schematic diagram of the process of recovering the absolute phase in combination with the original envelope peak position and the relative phase;

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 sensing channel 4 is selected as the calibration channel;

Fig. 5 is a relationship graph with the scanning optical path difference generated by the polarization low-coherence interferometer as the abscissa, and the position compensation amount between other sensing channels and the calibration channel 4 as the ordinate;

Figure 6 is a graph of the original envelope peak positions of the four sensing channels;

Fig. 7 is a curve diagram of the estimation error of the interference order of the four sensing channels;

Fig. 8 is a curve diagram of the real envelope peak position restored by four sensing channels;

Fig. 9 is a pressure demodulation error curve diagram of pressure demodulation after the real envelope peak positions of the four sensing channels are compensated to the calibration channel;

Reference signs: 1. broadband light source, 2. coupler, 3. F-P sensor, 4. multi-channel optical fiber array, 5. polarization low-coherence interferometer, 6. polarizer, 7. birefringent wedge, 8 , polarizer, 9, linear array CCD, 10, signal processing unit; 11, low coherence interference signal, 12, envelope of low coherence interference signal; 13, relative phase, 14, absolute phase estimate value, 15, real absolute phase , 16, channel one, 17, channel two, 18, channel three, 19, channel four;

FIG. 10 is an overall flow chart of the multi-channel low-coherence interference demodulation method capable of matching any channel of the sensor according to the present invention.

Detailed ways

The technical solutions of the present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

The multi-channel low-coherence interference demodulation method of the present invention, which can realize the matching of any channel of the sensor, is combined with the measurement of the external atmospheric pressure. As shown in Figure 1, it is a space-scanning low-coherence interference optical fiber sensing atmospheric pressure demodulation device. The working process is described as follows:

The light emitted by the broadband light source (LED) 1 reaches the F-P sensor 3 through the coupler 2. The F-P sensor 3 is a sensitive element that senses the external atmospheric pressure. The two reflective surfaces of the F-P cavity constitute a sensing interferometer. The distance between the two reflecting surfaces of the F-P cavity has a linear relationship with the atmospheric pressure. The optical signal modulated by the F-P sensor 3 is exported from the outlet of the coupler 2, and the optical signal is introduced into the polarization low-coherence interferometer through the multi-channel optical fiber array 4. 5. The multi-channel fiber array 4 is a single row inline lattice fiber structure. The polarization low coherence interferometric demodulator is composed of a polarizer 6, a birefringent wedge 7 and an analyzer 8. Due to the dual Due to the refraction effect, the optical signal passes through the birefringent wedge 7 to form spatial low-coherence interference fringes and is received by the linear array CCD9, and the signal processing unit 10 processes the interference fringe signals output by the linear array CCD9. When the optical path difference caused by the birefringent wedge 7 matches the optical path difference caused by the F-P sensor 3 , obvious low-coherence interference fringes will be generated in the corresponding local area of the linear array CCD 9 .

The required atmospheric pressure is generated by a high-precision and high-stable pressure source, which can reach a control accuracy of 0.02kPa. The effective pixel number of the linear array CCD is 3000 points, and each frame of data consists of 3000 discrete data points. Each CCD pixel corresponds to an optical path difference generated by a fixed polarization low-coherence interferometer, and the position K of the CCD pixel is used to represent the optical path difference. For 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.

Using the above-mentioned space-scanning low-coherence interference optical fiber sensing atmospheric pressure demodulation device, the atmospheric pressure is sensed through the change of the cavity length, and the polarization low-coherence interference demodulation device composed of a polarizer, a birefringent wedge and a polarizer is used. The optical signal modulated by the F-P sensor is demodulated to form spatial low-coherence interference fringes in the local area of zero optical path difference. The multi-channel low-coherence interference demodulation method that can realize the matching of any channel of the sensor in the present invention, the specific implementation steps are as follows:

The first step, for any low-coherence interference sensing signal under any demodulation channel, find its low-precision original envelope peak position, and combine the phase characteristics to recover the high-precision real envelope peak position;

1.1. As shown in Figure 2, a frame of low-coherence interference signal output by the CCD when the external atmospheric pressure is 100kPa, the frame of low-coherence interference signal contains 3000 data points. The CCD pixel corresponding to the low-precision envelope peak position K _p calculated by the Fourier transform method is 1602.

1.2. Calculate the low-precision absolute phase estimation value from the low-precision envelope peak position K _p according to the phase distribution characteristics

Among them, q represents the serial number of the selected discrete Fourier transform (DFT), which corresponds to a certain monochromatic wavelength, and 61 is selected here; N represents the total number of DFT discrete sequences, which is equal to the total number of data points of a frame of low-coherence interference signal, the value is 3000;

1.3. According to the low-precision absolute phase estimation value Combined with the DFT process, the fringe order n of the relative phase recovery interference fringe is obtained:

Among them, φ represents the relative phase obtained by the low-coherence interference fringe DFT process, and the round() function returns the nearest integer to the parameters in parentheses;

1.4. Recover the absolute phase of the low-coherence interference fringe according to the relative phase recovery of the fringe order n and the relative phase φ of the interference fringe

As shown in FIG. 3 , it is a schematic diagram of the process of recovering the absolute phase by combining the peak position of the original envelope and the relative phase. The absolute phase recovery process includes: the absolute phase estimate estimated by the low-precision raw envelope peak position _Kp Absolute phase with restored There is only a slight offset between them, and according to the nature of the phase distribution, the exact interference order n can be recovered by combining the relative phase φ.

1.5. According to the restored absolute phase Calculate the real envelope peak position K _d of the low-coherence interference fringe;

Here the recovered real envelope peak position K _d =1605.84.

The second step is to select the calibration channel to calibrate the F-P sensor, and establish the correspondence between the peak position of the low-coherence interference fringe envelope of the F-P sensor and the external pressure;

2.1. Select the calibration sensing channel s, scan the external pressure of the FP sensor through the pressure control system under this channel, collect the low-coherence interference fringe signal under each scanning pressure, and recover each signal according to the first step Corresponding to the real envelope peak position, obtain a real envelope peak position array corresponding to the scanning pressure array; as shown in Figure 4, the original envelope peak position of the pressure calibration process when the sensing channel 4 is selected as the calibration channel, The relationship curve between the true envelope peak position and the scanning calibration pressure, where the calibration pressure increases with a step size of 5kPa in the range of 100-200kPa, corresponding to the original envelope peak position K _p and the restored true envelope peak position K _d .

2.2. Take the array of the real envelope peak position K _d as the horizontal axis, and the scanning pressure array as the vertical axis to perform quadruple polynomial fitting, and establish the corresponding relationship between the envelope peak position of the low-coherence interference fringe of the FP sensor and the external pressure;

The third step is to establish the relationship between the scanning optical path difference (OPD) generated by the polarization low-coherence interferometer and the peak position of the interference fringe envelope through the calculation of the optical path, and further derive the scanning optical path difference (OPD) and different sensing channel packages. The relationship between the compensation amount of the network peak position;

3.1. Based on the system parameters of the polarization low-coherence interferometer, the relationship between the envelope peak position K of different sensing channels and the scanning optical path difference OPD generated by the polarization low-coherence interferometer is analyzed through optical path calculation:

OPD＝F _ko (K,m)

Wherein, the function F _ko () characterizes the above relationship, and m represents the channel number of the sensing channel.

3.2. Based on the system parameters of the polarization low-coherence interferometer, the relationship between the scanning optical path difference OPD generated by the polarization low-coherence interferometer and the peak position of the envelope of different sensing channels is analyzed through optical path calculation, and the polarization is further derived The relationship between the scanning optical path difference OPD generated by the low-coherence interferometer and the envelope peak position compensation ΔK between different sensing channels;

ΔK＝F _od (OPD,s,m)

Wherein, the function F _od () characterizes the above relationship, and s represents the channel number of the calibration sensing channel. As shown in Figure 5, take the scanning optical path difference produced by the polarization low-coherence interferometer as the abscissa, and take the position compensation amount between other sensing channels and the calibration channel 4 as the relationship curve of the ordinate, the optical path difference Simulation results of the position offset between calibration channel 4 and other sensing channels when increasing from 30 μm to 70 μm at 0.1 μm intervals;

The fourth step, the process of restoring the real envelope peak position and the compensation process of the envelope peak position between channels, compensates the original envelope peak position K _p of any demodulation channel to the calibration channel, and uses the compensated envelope peak position to carry out Pressure value demodulation.

4.1. For the original envelope peak position K _p of any sensing channel, recover its true envelope peak position K _d according to the first step;

4.2. According to the relational expression OPD=F _ko (K, m) in 3.1, use K _d to calculate the demodulator scanning optical path difference OPD corresponding to this position, then calculate the position offset ΔK according to 3.2, and compensate ΔK Go to the calibration channel.

K _r =K _d +ΔK

Where K _r is the position of the envelope peak after compensation.

4.3. Using the corresponding relationship between the envelope peak position K _r and the external pressure under the calibration channel, the measured pressure value of the corresponding sensing channel can be demodulated, and high-precision demodulation of the FP sensor under any channel can be realized.

In order to more comprehensively verify the feasibility of this method, a 4-channel polarization low-coherence interferometer is taken as an example, with channel 4 as the calibration channel, and the experimental pressure increases monotonously from 100kPa to 200kPa at intervals of 0.5kPa. The interference signal at each experimental pressure for each sensing channel is processed. Figure 6, Figure 7, and Figure 8 are graphs of the original envelope peak value, the estimated interference order error, and the restored real envelope peak position of the four channels respectively. It can be seen that within the entire measurement range (100kPa-200kPa), the interference level The secondary estimation error is less than 0.15, which is less than the maximum effective estimation error of 0.5, and the linearity of the restored real envelope peak position is very good, and there is no step error caused by the misjudgment of the interference order. The network peak position has strong reliability. Figure 9 shows the pressure demodulation error after position compensation for each sensing channel. It can be clearly seen that the demodulation error of the method of the present invention can be kept within 0.14kPa on any channel, realizing the F-P sensor under multi-channel High-precision demodulation.

The demodulation method of the present invention has been verified through experiments, see FIG. 4 to FIG. 9 .

Claims (1)

1. A multi-channel low-coherence interference demodulation method that can realize sensor arbitrary channel matching, is characterized in that, the method comprises the following steps: The first step, for any low-coherence interference sensing signal under any demodulation channel, calculate the low-precision envelope peak position K _p by Fourier transform method; calculate the low-precision absolute phase estimation value Among them, q represents the serial number of the selected discrete Fourier transform (DFT), which corresponds to a certain monochromatic wavelength, and 61 is selected here; N represents the total number of DFT discrete sequences, which is equal to the total number of data points of a frame of low-coherence interference signal, the value is 3000; According to the low precision absolute phase estimate Combined with the DFT process, calculate the fringe order n of the restored interference fringe: Among them, φ represents the relative phase obtained by the low-coherence interference fringe DFT process, and the round() function returns the nearest integer to the parameters in parentheses; Recover the absolute phase of the low-coherence interference fringe according to the fringe order n and the relative phase φ of the interference fringe According to the recovered absolute phase Find the true envelope peak position K _d of the low-coherence interference fringes: The second step is to select the calibration sensing channel s, scan the external pressure of the FP sensor through the pressure control system under this channel, collect the low-coherence interference fringe signal under each scanning pressure, and restore each A real envelope peak position corresponding to a signal, to obtain a real envelope peak position array corresponding to the scanning pressure array; take the array of the real envelope peak position K _d as the horizontal axis, and the scanning pressure array as the vertical axis to perform a 4-degree polynomial simulation Combined, the corresponding relationship between the peak position of the low-coherence interference fringe envelope of the FP sensor and the external pressure is established; The third step is to calculate the scanning optical path difference OPD generated by the polarization low coherence interferometer based on the system parameters of the polarization low coherence interferometer according to the peak position of the envelope of the sensing channel: OPD＝F _ko (K,m) Wherein, the function F _ko () characterizes the above relationship, m represents the channel number of the sensing channel, and K represents the peak position of the sensing channel envelope; Calculate the envelope peak position compensation amount ΔK between different sensing channels: ΔK＝F _od (OPD,s,m) s represents the channel number of the calibration sensing channel. The fourth step is to calculate the compensated envelope peak position K _r : K _r =K _d +ΔK Using the corresponding relationship between the envelope peak position K _r under the calibration channel and the external pressure, the measured pressure value of the corresponding sensing channel is demodulated.