Method for improving precision and frequency band of white light interference phase demodulation system

Abstract

The invention discloses a method for improving the precision and the frequency band of a white light interferometric phase demodulation system, and belongs to the technical field of optical fiber sensing, and the method comprises the steps: theoretically analyzing the ratio of a demodulation result to a true value of the white light interferometric phase demodulation system, and taking the ratio as a compensation coefficient (CF); the CF is a quantity related to the frequency and the delay of a signal to be measured, and a group of cycle counts are generated to compensate the CF in a frequency spectrum demodulated by a white light interference phase demodulation system frequency by frequency; and recovering the compensated time domain signal from the compensated signal frequency spectrum by utilizing inverse Fourier transform. According to the invention, the working frequency band of the system is expanded while the white light interference distortion degree is reduced, so that the white light interference is more suitable for the sensing field.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber sensing technology, and in particular to a method for improving the accuracy and frequency band of a white light interferometric phase demodulation system. Background Art

[0002] Fiber optic sensing technology has been widely used in microseismic detection, deep-sea exploration and other fields due to its advantages such as high sensitivity and strong anti-electromagnetic interference ability. Narrow linewidth lasers are commonly used in fiber optic sensing demodulation systems. They have high stability and low requirements for interferometer arm length difference control. However, the production process of narrow linewidth lasers is complicated and costly. White light interferometry technology benefits from the use of broadband light sources instead of narrow linewidth lasers, and has achieved high sensitivity and high resolution. It has attracted the attention of many researchers.

[0003] The following problems exist in the white light interferometer phase demodulation system: Since a broadband light source is used instead of a high-cost narrow linewidth laser, the requirements for the interferometer structure are high, and crosstalk needs to be prevented while the arm length difference is controlled at the micron level. Therefore, the stable operation of white light interferometry requires the selection of a Segnac interferometer structure and the introduction of a delayed optical fiber to prevent crosstalk. In the white light interferometer phase demodulation system, due to the existence of delay, researchers regard the phase change introduced by the signal to be measured as the differential component of the signal to be measured, that is: ψ=φ(t)-φ(t-τ)=τφ tra '(t), and this equation is only valid when τ is a very small value. As τ and f increase, the distortion of the demodulation result will gradually increase. In order to control the distortion within 5%, the researchers have given certain restrictions on the operating frequency band of the system, that is, f<1 / 2πτ. The purpose of the present invention is to solve the distortion problem of the white light interferometric phase demodulation system and expand the system operating frequency band. Summary of the invention

[0004] The purpose of the present invention is to address the problem of distorted demodulation results and limited demodulation band caused by the resulting demodulation in a white light interferometric demodulation system, and to propose a method based on spectrum compensation and restoring the time domain signal through inverse Fourier transform, thereby establishing an improved white light interferometric phase demodulation system based on spectrum compensation.

[0005] The purpose of the present invention can be achieved by the following technical solutions: A method for improving the accuracy and frequency band of a white light interferometric phase demodulation system, comprising: Step 1: The ratio of the demodulation result of the white light interferometric phase demodulation system to the true value is theoretically analyzed and used as the compensation coefficient CF, the specific value of which is sinπfτ / πfτ, where f is the frequency of the signal to be measured and τ is the delay caused by the delayed optical fiber; Step 2: Obtain the value of the delay τ by calculation to accurately obtain the compensation coefficient at different frequencies; Step 3, calculating the compensation coefficients corresponding to the signals at different frequencies, and then compensating the compensation coefficients corresponding to the signals at different frequencies frequency by frequency in the spectrum obtained by the white light interferometric phase demodulation system to obtain the compensated signal spectrum; Step 4: Obtain the compensated time domain signal from the compensated signal spectrum using inverse Fourier transform to complete the compensation process.

[0006] As a further solution of the present invention, step one comprises the following steps: The phase information is demodulated using the ellipse fitting phase demodulation method. The two-way interference signal can be expressed by the following formula: ;Formula (1) Where a1 and a2 are the DC values ​​of the two interference signals, b1 and b2 are the fringe contrasts of the two interference signals, ψ is the phase change caused by the measured signal, and φ(t) represents the measured signal, where φ(t)=φ m sinωt, φ m and ω represent the amplitude and angular frequency of the measured signal, respectively, and β represents the phase difference caused by the 3×3 coupler; The two digital signals are demodulated digitally based on the ellipse fitting algorithm to obtain two phase differences ψ, and the approximation is: φ(t)-φ(t-τ)=τφ tra '(t).

[0007] Through integration operation, we can get: Formula (2) where φ tra (t) represents the demodulation result before compensation. The ratio of the demodulation result of the differential algorithm to the true value is obtained through theoretical analysis and used as the compensation factor: Formula (3).

[0008] As a further solution of the present invention, step 2 comprises the following steps: Combining formula (2-3) we get: Formula (4) The value of the delay τ can be accurately obtained according to formula (4).

[0009] As a further solution of the present invention, in step three, the system generates a set of cycle counts as the values ​​of compensation coefficients corresponding to signals of different frequencies, and compensates them frequency by frequency in the signal spectrum obtained by the white light interferometric phase demodulation system; Formula (5) where φ com Represents the demodulation result of the compensated white light interferometric phase demodulation system.

[0010] A white light interferometric phase demodulation system with improved accuracy and frequency band, the system is used to execute the above-mentioned method for improving the accuracy and frequency band of the white light interferometric phase demodulation system; The demodulation system includes a spontaneous amplification light source, a 3×3 coupler, a 2×1 coupler, a second piezoelectric ceramic, and a Faraday rotation mirror. The two paths between the 3×3 coupler and the 2×1 coupler include a delay fiber and a first piezoelectric ceramic, respectively. The working principle is as follows: The optical signal emitted by the spontaneous emission light source is divided into two paths through a 3×3 coupler. The path of the first optical signal is: path 1-path 2-path 3-path 3-path 2-path 4, and the path of the second optical signal is path 4-path 2-path 3-path 3-path 2-path 1. The two optical signals interfere with each other at the 3×3 coupler. The piezoelectric ceramic 1 is used to apply modulation to ensure the normal operation of the ellipse fitting, and the piezoelectric ceramic 2 is used to apply the signal to be tested. The interfered optical signal is converted into an electrical signal after passing through a photoelectric detector, which is collected by a data acquisition card and then demodulated by the ellipse fitting phase demodulation technology. Path 1 is a path between the 3×3 coupler and the 2×1 coupler that includes a delay fiber; Path 4 is a path between the 3×3 coupler and the 2×1 coupler including piezoelectric ceramic 1; Path 2 is the path between the 2×1 coupler and the second piezoelectric ceramic; Path 3 is the path between piezoelectric ceramic 2 and the Faraday rotation mirror.

[0011] Beneficial effects of the present invention: The present invention expands the demodulation frequency band while solving the distortion problem, so that the working frequency band of the system is no longer limited by the distortion problem; the present invention solves the distortion problem of white light interference, making white light interference more suitable for the sensing field; and adopts a broadband light source instead of a narrow linewidth laser to reduce the production cost of the interferometer and simplify the optical path structure.

[0012] The present invention first theoretically analyzes the cause of the differential algorithm distortion, and calculates the ratio of the differential algorithm demodulation result to the theoretical true value, which is used as the compensation factor. Then the compensation factor is compensated frequency by frequency in the spectrum obtained by the differential algorithm, and then the time domain signal is obtained by inverse Fourier transform. After compensation, the distortion problem caused by the delay is eliminated, and the working frequency band of the system is also expanded. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The present invention will be further described below in conjunction with the accompanying drawings.

[0014] Figure 1It is a schematic block diagram of the structure of the white light interferometric phase demodulation system for improving the accuracy and frequency band of the present invention; Figure 2 It is a flow chart of a method for improving the accuracy and frequency band of a white light interferometric phase demodulation system according to the present invention; Figure 3 This is the CF compensation effect diagram; Figure 4 It is a comparison diagram of frequency domain signals before and after CF compensation; Figure 5 It is a comparison diagram of the time domain signals before and after CF compensation. DETAILED DESCRIPTION

[0015] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0016] like Figures 1 to 5 As shown, a method for improving the accuracy and frequency band of a white light interferometric phase demodulation system comprises: Step 1: Theoretically analyze the ratio of the demodulation result of the white light interferometric phase demodulation system to the true value and use it as the compensation coefficient (CF). The specific value is sinπfτ / πfτ, which is a quantity related to the frequency (f) and delay (τ) of the signal to be measured; Step 2: The compensation coefficient is a quantity related to the signal frequency (f) and the delay (τ). The value of the delay (τ) is obtained by calculation, thereby completing the accurate calculation of the compensation coefficient at different frequencies. Step 3, calculating the compensation coefficients corresponding to the signals at different frequencies, and then compensating the compensation coefficients corresponding to the signals at different frequencies frequency by frequency in the spectrum obtained by the white light interferometric phase demodulation system to obtain the compensated signal spectrum; Step 4: Obtain the compensated time domain signal from the compensated signal spectrum using inverse Fourier transform to complete the compensation process.

[0017] The present invention also discloses a white light interferometric phase demodulation system with improved accuracy and frequency band, the system is used to execute the above-mentioned method for improving the accuracy and frequency band of the white light interferometric phase demodulation system; Specifically, Figure 1 As shown, the demodulation system includes a spontaneous amplification light source, a 3×3 coupler, a 2×1 coupler, a second piezoelectric ceramic, and a Faraday rotation mirror. The two paths between the 3×3 coupler and the 2×1 coupler include a delay fiber and a first piezoelectric ceramic, respectively. The working principle is: The optical signal emitted by the spontaneous emission light source is divided into two paths through a 3×3 coupler. The path of the first optical signal is: path 1-path 2-path 3-path 3-path 2-path 4, and the path of the second optical signal is path 4-path 2-path 3-path 3-path 2-path 1. The two optical signals interfere at the 3×3 coupler. Piezoelectric ceramic 1 is used to apply modulation to ensure the normal operation of ellipse fitting, and piezoelectric ceramic 2 is used to apply the signal to be tested. The interfered optical signal is converted into an electrical signal after passing through a photoelectric detector and collected by a data acquisition card, and then demodulated by the ellipse fitting phase demodulation technology; Path 1 is a path between the 3×3 coupler and the 2×1 coupler that includes a delay fiber; Path 4 is a path between the 3×3 coupler and the 2×1 coupler including piezoelectric ceramic 1; Path 2 is the path between the 2×1 coupler and the second piezoelectric ceramic; Path 3 is the path between the second piezoelectric ceramic and the Faraday rotator mirror; Step one includes the following steps: The phase information is demodulated using the ellipse fitting phase demodulation method. The two-way interference signal can be expressed by the following formula: Formula (1); Where a1 and a2 are the DC values ​​of the two interference signals, b1 and b2 are the fringe contrasts of the two interference signals, ψ is the phase change caused by the measured signal, and φ(t) represents the measured signal, where φ(t)=φ m sinωt, φ m and ω represent the amplitude and angular frequency of the measured signal respectively, τ represents the delay caused by the delayed optical fiber, and β represents the phase difference caused by the 3×3 coupler.

[0018] The two digital signals are demodulated digitally based on the ellipse fitting algorithm to obtain two phase differences ψ, and the approximation is: φ(t)-φ(t-τ)=τφ tra '(t).

[0019] Therefore, through integration operation we can get: Formula (2) where φ tra (t) represents the demodulation result of the white light interferometric phase demodulation system, that is, the demodulation result obtained before compensation. Next, the ratio of the demodulation result of the differential algorithm to the true value is obtained through theoretical analysis and used as the compensation factor: Formula (3) Step 2 includes the following steps: Combining formula (2-3) we get: Formula (4) According to formula (4), the value of the delay τ can be accurately obtained, thereby accurately determining the value of the compensation coefficient CF to improve the accuracy of compensation.

[0020] In step three, the system generates a set of cycle counts as the values ​​of compensation coefficients corresponding to signals of different frequencies, and compensates them frequency by frequency in the signal spectrum obtained by the white light interferometric phase demodulation system.

[0021] Formula (5) where φ com The demodulation results of the white light interferometric phase demodulation system after compensation are shown in Figure 2. To verify the necessity and effectiveness of the compensation method, a set of calibration signals is generated and demodulated using the two demodulation methods before and after compensation. The demodulation results are compared. Figure 3 shown. Figure 3 (ab) represent the demodulation results when using 0.5 km, 3 km, 5 km, and 10 km delayed optical fibers, respectively. It can be seen that as the measured signal frequency and the delayed optical fiber length increase, the demodulation results before compensation gradually deviate from the calibration value, while the demodulation results after compensation are always close to the calibration value, thus verifying the necessity and accuracy of compensation.

[0022] Figure 4 The spectrum signals before and after compensation are shown.

[0023] Perform inverse Fourier transform on the compensated spectrum signal to obtain the time domain signal, which is compared with the time domain signal before compensation. Figure 5 shown.

[0024] The above contents are merely examples and explanations of the present invention. Those skilled in the art may make various modifications or additions to the specific embodiments described or replace them in a similar manner. As long as they do not deviate from the invention or exceed the scope defined by the claims, they shall all fall within the protection scope of the present invention.

Claims

1. A method for improving the accuracy and frequency band of a white light interferometric phase demodulation system, characterized in that: include: Step 1: The ratio of the demodulation result of the white light interferometric phase demodulation system to the true value is theoretically analyzed and used as the compensation coefficient CF, the specific value of which is sinπfτ / πfτ, where f is the frequency of the signal to be measured and τ is the delay caused by the delayed optical fiber; Step 2: Obtain the value of the delay τ by calculation to accurately obtain the compensation coefficient at different frequencies; Step 3, calculating the compensation coefficients corresponding to the signals at different frequencies, and then compensating the compensation coefficients corresponding to the signals at different frequencies frequency by frequency in the spectrum obtained by the white light interferometric phase demodulation system to obtain the compensated signal spectrum; Step 4: Obtain the compensated time domain signal from the compensated signal spectrum using inverse Fourier transform to complete the compensation process.

2. A method for improving the accuracy and frequency band of a white light interferometric phase demodulation system according to claim 1, characterized in that: Step one includes the following steps: The phase information is demodulated using the ellipse fitting phase demodulation method. The two-way interference signal can be expressed by the following formula: ;Formula (1) Where a1 and a2 are the DC values ​​of the two interference signals, b1 and b2 are the fringe contrasts of the two interference signals, ψ is the phase change caused by the measured signal, and φ(t) represents the measured signal, where φ(t)=φ m sinωt, φ m and ω represent the amplitude and angular frequency of the measured signal, respectively, and β represents the phase difference caused by the 3×3 coupler; The two digital signals are demodulated digitally based on the ellipse fitting algorithm to obtain the phase signal ψ, and the approximation is: φ(t)-φ(t-τ)=τφ tra '(t) Through integration operation, we can get: Formula (2) where φ tra (t) represents the demodulation result obtained by the demodulation system before compensation. The ratio of the demodulation result of the differential algorithm to the true value is obtained through theoretical analysis and used as the compensation factor: Formula (3).

3. A method for improving the accuracy and frequency band of a white light interferometric phase demodulation system according to claim 2, characterized in that: Step 2 includes the following steps: Combining formula (2-3) we get: Formula (4) The value of the delay τ can be accurately obtained according to formula (4).

4. The method for improving the accuracy and frequency band of a white light interferometric phase demodulation system according to claim 3, characterized in that: In step three, the system generates a set of cycle counts as the values ​​of compensation coefficients corresponding to signals of different frequencies, and compensates them frequency by frequency in the signal spectrum obtained by the white light interferometric phase demodulation system; Formula (5) where φ com Represents the demodulation result of the compensated white light interferometric phase demodulation system.

5. A white light interferometric phase demodulation system with improved accuracy and frequency band, the system being used to execute a method for improving the accuracy and frequency band of a white light interferometric phase demodulation system as described in any one of claims 1 to 4; The demodulation system includes a spontaneous amplification light source, a 3×3 coupler, a 2×1 coupler, a second piezoelectric ceramic, and a Faraday rotation mirror. The two paths between the 3×3 coupler and the 2×1 coupler include a delay fiber and a first piezoelectric ceramic, respectively. The working principle is as follows: The optical signal emitted by the spontaneous emission light source is divided into two paths through a 3×3 coupler. The path of the first optical signal is: path 1-path 2-path 3-path 3-path 2-path 4, and the path of the second optical signal is path 4-path 2-path 3-path 3-path 2-path 1. The two optical signals interfere with each other at the 3×3 coupler. The piezoelectric ceramic 1 is used to apply modulation to ensure the normal operation of the ellipse fitting, and the piezoelectric ceramic 2 is used to apply the signal to be tested. The interfered optical signal is converted into an electrical signal after passing through a photoelectric detector, which is collected by a data acquisition card and then demodulated by the ellipse fitting phase demodulation technology. Path 1 is a path between the 3×3 coupler and the 2×1 coupler that includes a delay fiber; Path 4 is a path between the 3×3 coupler and the 2×1 coupler including piezoelectric ceramic 1; Path 2 is the path between the 2×1 coupler and the second piezoelectric ceramic; Path 3 is the path between piezoelectric ceramic 2 and the Faraday rotation mirror.

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