Modulation signal determination method for optimal modulation depth of reflective all-fiber current sensor

Abstract

The application discloses a method for determining a modulation signal of optimal modulation depth of a reflective all-fiber current sensor, and amplitude U of a modulation signal corresponding to the optimal modulation depth H is determined by adjusting amplitude of a modulation signal of a phase modulator PM When the peak-to-peak value of the light power received by the photodetector is maximum and the number of extreme points in one period is 3, the amplitude U of the modulation signal corresponding to the modulation depth H = π rad is obtained PMπ Thus, the electro-optic modulation coefficient k of the phase modulator is determined AP When the optimal modulation depth H o = 1.84 rad, the amplitude of the modulation signal corresponding to the optimal modulation depth is U PMo The method realizes online measurement of the half-wave voltage of the phase modulator, determines the modulation signal corresponding to the optimal modulation depth through the theoretically optimal modulation depth, does not introduce additional devices, and is simple and convenient to operate; and the application is not limited to the reflective all-fiber current sensor, and is suitable for any interference optical path structure which needs to measure the half-wave voltage.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of optical fiber current measurement, and particularly relates to a modulation signal determination method for optimal modulation depth of a reflective all-optical fiber current sensor. BACKGROUND

[0002] Compared with a traditional electromagnetic current transformer, the optical fiber current sensor has the advantages of small volume, light weight, large dynamic range, fast response speed, no magnetic saturation problem, safety and environmental protection, and can meet the measurement requirements of alternating current and direct current, and has been successfully applied to the fields of electrolytic aluminum industry, ultra-high voltage power network and pulse current measurement. At present, the optical fiber current sensor is mainly applied to a closed-loop data processing method, but the signal processing system design is complex and the cost is high. Therefore, a sine wave modulation technology is adopted, a correlation demodulation scheme is applied to realize data processing, which is a scheme that can greatly reduce the system cost and manufacturing difficulty, and has wide application requirements in places with low precision requirements.

[0003] Based on the optical fiber current sensing technology of the sine wave modulation, the modulation depth is determined by the amplitude and frequency of the phase modulator modulation signal, which directly affects the sensitivity of the system.

[0004] Usually, the scheme of measuring the half-wave voltage of the phase modulator is adopted, and then the modulation signal amplitude corresponding to the optimal modulation depth of the phase modulator is determined. At present, the main methods for measuring the half-wave voltage of the phase modulator are ① frequency doubling method. The direct current voltage and alternating current signal are loaded at the same time, the direct current voltage corresponding to the light intensity extreme value is adjusted, and the difference between the direct current voltages corresponding to the frequency doubling distortion of the alternating current signal is the half-wave voltage. This scheme requires simultaneous loading of alternating current and direct current signals, and the frequency doubling adjustment requires high requirements and is difficult to operate; ② the second is the spectrum analysis method. The half-wave voltage is obtained by comparing and measuring the odd and even components of the spectrum using a spectrum analyzer. This scheme is complex to measure, and the spectrum analyzer is limited by the resolution, and it is difficult to measure low frequency; ③ is the extreme value method. The direct current voltage loaded to the phase modulator is changed, and the extreme points of the output light intensity are determined by the optical interference structure. The half-wave voltage is the voltage difference corresponding to the adjacent maximum and minimum values. This scheme has a simple principle, but the operation is complicated and the measurement accuracy is low.

[0005] The Chinese invention patent "Half-wave voltage measuring device and method" (application number 202111173572.6) determines the half-wave voltage of the phase modulator by designing an interference optical path and using the periodic change of the output transmittance of the high-birefringence fiber ring mirror after the modulation phase modulator. This measurement process requires a spectrum analyzer, and the instrument equipment requirements for the measurement system are high, and the implementation process is complex.

[0006] The Chinese invention patent "Full-fiber current transformer open-loop demodulation and half-wave voltage tracking method" (application number 201911281442.7) adopts a square wave modulation scheme, and uses the time domain waveform characteristics corresponding to a special square wave modulation signal to realize the measurement of the half-wave voltage. This scheme is highly dependent on the modulation signal and cannot be applied to the case of sinusoidal modulation.

[0007] The Chinese invention patent "Full-fiber current transformer and half-wave voltage correction method thereof" (application number 201911120685.2) uses an additional light intensity difference method to realize the measurement of the half-wave voltage of the phase modulator. This scheme depends on the square wave modulation signal, and the additional light intensity difference calculation is easily affected by system noise. SUMMARY

[0008] The purpose of the present application is to overcome the shortcomings of the above-mentioned technology, and to provide a modulation signal determination method for the optimal modulation depth of a reflective full-fiber current sensor that can improve the sensitivity of the system.

[0009] To achieve the above-mentioned purpose, the modulation signal determination method for the optimal modulation depth of the reflective full-fiber current sensor designed by the present application is as follows:

[0010] Adjust the amplitude U of the modulation signal of the phase modulator PM When the peak-to-peak value of the optical power received by the photodetector is maximum and the number of extreme points in one period is 3, the amplitude U of the modulation signal corresponding to the modulation depth H = π rad is obtained PMπ , thereby determining the electro-optic modulation coefficient k of the phase modulator AP :

[0011]

[0012] When the optimal modulation depth H o = 1.84 rad, the amplitude of the modulation signal corresponding to the optimal modulation depth is U PMo :

[0013]

[0014] According to formula 13, the amplitude of the phase modulator modulation signal corresponding to the optimal modulation depth can be determined when the modulation is sinusoidal.

[0015] Further, the calculation process of the amplitude U PMπ of the modulation signal corresponding to the modulation depth H = π rad is as follows:

[0016] Observe whether the peak-to-peak value of the optical power received by the photodetector increases with the increase of the amplitude U PM of the modulation signal;

[0017] When U PM < πkAP / 2, -π<H sin(ω E ·t)<π, the peak-to-peak value of the optical power received by the photodetector is ΔP out :

[0018]

[0019] Therefore, when U PM <πk AP / 2, the peak-to-peak value of the optical power received by the photodetector is ΔP out gradually increases with the increase of U PM ;

[0020] When U PM ≥πk AP / 2, -H<H sin(ω E ·t)<H, the peak-to-peak value of the optical power received by the photodetector is ΔP out :

[0021] ΔP out =P in Equation 8

[0022] Therefore, when U PM ≥πk AP / 2, the peak-to-peak value of the optical power received by the photodetector is ΔP out remains unchanged with the increase of δ;

[0023] View the number of extreme points in one period;

[0024] Take the first-order derivative of the time of the optical power P out received by the photodetector,

[0025]

[0026] Then:

[0027]

[0028] Wherein, P in is the light power of the light source; θ is the rotation angle of the polarization plane of linearly polarized light, and θ=0 rad; ω E is the angular frequency; t is the time;

[0029] When U PM <πk AP / 2, -π<H sin(ω E ·t)<π, the extreme points of the optical power P out in one period are:

[0030]

[0031] Therefore, when U PM <πk AP / 2, the number of extreme points in a period is always 3, which does not change with the increase of U PM ;

[0032] When U PM ≥πk AP / 2, -H<H sin(ω E ·t)<H, the extreme points of the optical power P out received by the photodetector in a period are:

[0033]

[0034] In the formula, m is a positive integer, m0 is the minimum integer multiple of H as π; therefore, when U PM ≥πk AP / 2, the number of extreme points of the signal in a period gradually increases with the increase of U PM ;

[0035] That is, when the peak-to-peak value of the optical power received by the photodetector is the largest and the number of extreme points in a period is 3, the amplitude U PMπ of the modulation signal corresponding to the modulation depth H=π rad is obtained.

[0036] Further, the calculation process of the angular frequency ω E is as follows:

[0037] For the reflective all-fiber current sensor, the time τ C of the optical beam to pass through the phase modulator back and forth is calculated.

[0038]

[0039] In the formula, L is the optical fiber length between the phase modulator and the mirror, which can be accurately measured by using an optical time domain reflectometer; n is the refractive index of the optical fiber, and c is the speed of light in vacuum.

[0040] According to the formula 2, the angular frequency ω E of the modulation signal of the phase modulator is calculated.

[0041]

[0042] Further, the calculation process of the optical signal P out arriving at the photodetector is as follows:

[0043] When the modulation signal of the phase modulator is: u PM (t)=k AP ·δ·sin(ω E ·t)

[0044] wherein, kAP delta is the amplitude of the modulation signal; omega E is the angular frequency; k AP is the electro-optical modulation coefficient of the phase modulator, with the unit of V / rad; delta is the amplitude of the phase delay of the phase modulator, with the unit of rad;

[0045] The corresponding phase shift of the phase modulator is

[0046]

[0047] The phase shift angle generated by the light beam passing through the phase modulator twice is:

[0048]

[0049] Define the modulation depth H=2delta, then the optical signal P out reaching the photodetector is:

[0050]

[0051] P in is the optical power of the light source, theta is the rotation angle of the polarization plane of the linearly polarized light, at this time the amplitude U PM of the modulation signal of the corresponding phase modulator is:

[0052]

[0053] Compared with the prior art, the present application has the following advantages:

[0054] 1) The present application is based on the optical structure of the reflective all-fiber current sensor itself, realizes the measurement of the half-wave voltage of the phase modulator online, determines the modulation signal corresponding to the optimal modulation depth through the theoretically optimal modulation depth, does not introduce additional devices, and is simple and convenient to operate;

[0055] 2) The time-domain waveform analysis method is used to measure the half-wave voltage of the phase modulator in the present application, the approximate working range is determined according to the time-domain waveform characteristics, and the accurate measurement is realized according to the numerical calculation results, so that the measurement speed is fast and the precision is high;

[0056] 3) The present application is not limited to the reflective all-fiber current sensor, and is also applicable to any other interference optical path structure which needs to measure the half-wave voltage. BRIEF DESCRIPTION OF DRAWINGS

[0057] Figure 1 is a structural schematic diagram of the reflective all-fiber current sensor of the present application;

[0058] Figure 2 is the distribution characteristics of the optical time-domain signal corresponding to different amplitudes of the modulation signal. DETAILED DESCRIPTION

[0059] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0060] like Figure 1 The reflective all-fiber current sensor shown has a basic optical path structure including a light source, coupler, polarizer, phase modulator, and sensing unit. It receives optical power through a photodetector and displays the time-domain waveform of the optical power in real time via an oscilloscope or data acquisition card. By analyzing the waveform and performing moving average noise reduction calculations on the acquired data, the half-wave voltage of the phase modulator can be accurately measured, which is used to determine the modulation signal with the optimal modulation depth.

[0061] The method for determining the modulation signal for the optimal modulation depth of a reflective all-fiber current sensor is as follows:

[0062] For a reflective all-fiber current sensor, calculate the round-trip time τ of the beam passing through the phase modulator. C

[0063]

[0064] In the formula, L is the length of the optical fiber between the phase modulator and the reflector, which can be accurately measured using an optical time-domain reflectometer; n is the refractive index of the optical fiber; and c is the speed of light in a vacuum.

[0065] Calculate the angular frequency ω of the phase modulator signal using Formula 2. E

[0066]

[0067] When the modulation signal of the phase modulator is: u PM (t)=k AP ·δ·sin(ω E ·t)

[0068] Where, k AP δ is the amplitude of the modulating signal; ω E Angular frequency; k AP δ is the electro-optic modulation coefficient of the phase modulator, in V / rad; δ is the amplitude of the phase delay of the phase modulator, in rad.

[0069] The corresponding phase modulator phase shift is

[0070]

[0071] The phase shift angle generated by the beam passing back and forth through the phase modulator is:

[0072]

[0073] The modulation depth H = 2delta is defined, and the optical signal P reaching the photodetector is out :

[0074]

[0075] P in is the optical source power, theta is the rotation angle of the linearly polarized light polarization plane, and theta = 0 rad; omega E is the angular frequency; t is the time. At this time, the amplitude U of the corresponding phase modulator modulation signal is PM :

[0076]

[0077] As Figure 2 shown, the amplitude U of the phase modulator modulation signal is adjusted, the extreme point number in a cycle of the observed optical signal time domain waveform and the peak-to-peak value of the signal are observed, and the amplitude U of the modulation signal corresponding to the modulation depth H = pi rad is obtained PM . The specific method is divided into two steps: one is to observe the peak-to-peak value change of the signal, and the other is to observe the extreme point number in a cycle. In this process, the measured current is set to 0A, and a moving average denoising algorithm is used to reduce the influence of noise on the measurement result. PMπ

[0078] The specific process is: whether the peak-to-peak value of the optical power received by the photodetector increases with the increase of the amplitude U of the modulation signal PM ;

[0079] When U PM < pi k AP / 2, -pi < H sin (omega E · t) < pi, and the peak-to-peak value of the optical power received by the photodetector is delta P out :

[0080]

[0081] Therefore, when U PM < pi k AP / 2, the peak-to-peak value delta P out of the optical power received by the photodetector gradually increases with the increase of U PM ;

[0082] When U PM >= pi k AP / 2, -H < H sin (omega E · t) < H, and the peak-to-peak value of the optical power received by the photodetector is delta P out :

[0083] Delta P out = P in Equation 8​

[0084] Therefore, when U PM ≥ πk AP / 2, the peak-to-peak value ΔP out of the optical power received by the photodetector remains unchanged as δ increases;

[0085] Viewing the number of extreme points in one period;

[0086] Taking the first-order derivative of the time of the optical power P out received by the photodetector,

[0087]

[0088] Then:

[0089]

[0090] When U PM < πk AP / 2, -π < H sin(ω E · t) < π, the extreme points of the optical power P out in one period are 3:

[0091]

[0092] Therefore, when U PM < πk AP / 2, the number of extreme points in one period is always 3 and does not change as U PM increases;

[0093] When U PM ≥ πk AP / 2, -H < H sin(ω E · t) < H, the extreme points of the optical power P out received by the photodetector in one period are:

[0094]

[0095] In the formula, m is a positive integer, and m0 is the smallest integer multiple of π; therefore, when U PM ≥ πk AP / 2, the number of extreme points of the signal in one period gradually increases as U PM increases;

[0096] That is, when the peak-to-peak value of the optical power received by the photodetector is the largest and the number of extreme points in one period is 3, the electro-optical modulation coefficient k AP of the phase modulator is determined:

[0097]

[0098] When the optimal modulation depth H o = 1.84 rad, the modulation signal amplitude corresponding to the optimal modulation depth is U PMo :

[0099]

[0100] According to formula 13 and formula 2, the amplitude and frequency of the phase modulator modulation signal corresponding to the optimal modulation depth can be determined when the sine modulation.

Claims

1. A method for determining the modulation signal for the optimal modulation depth of a reflective all-fiber current sensor, characterized in that: The method for determining the optimal modulation depth corresponding to the modulation signal is as follows: Adjusting the amplitude U of the modulated signal by the phase modulator PM When the peak-to-peak optical power received by the photodetector is at its maximum and the number of extreme points in one cycle is 3, the amplitude U of the modulated signal corresponding to the modulation depth H = π rad is obtained. PMπ Thus, the electro-optic modulation coefficient k of the phase modulator is determined. AP : When the optimal modulation depth H o When the amplitude of the modulated signal is 1.84 rad, the amplitude of the modulated signal corresponding to the optimal modulation depth is U. PMo : Equation 13 can be used to determine the amplitude of the phase modulator signal corresponding to the optimal modulation depth during sinusoidal modulation.

2. The method for determining the modulation signal for the optimal modulation depth of the reflective all-fiber current sensor according to claim 1, characterized in that: The amplitude U of the modulated signal corresponding to the modulation depth H = π rad is obtained. PMπ The calculation process is as follows: Whether the peak-to-peak value of the optical power received by the photodetector changes with the amplitude U of the modulation signal. PM Increase and increase; When U PM <πk AP When / 2, -π <H sin(ω E ·t)<π, the peak-to-peak value of the optical power received by the photodetector is ΔP out : Therefore, when U PM <πk AP At / 2, the peak-to-peak optical power ΔP received by the photodetector is out With U PM The increase gradually increased; When U PM ≥ πk AP / 2, -H < H sin(ω E ·t) < H, and the peak-to-peak optical power received by the photodetector is ΔP out : ΔP out =P in Formula 8 Therefore, when U PM ≥πk AP At / 2, the peak-to-peak optical power ΔP received by the photodetector is out It remains constant as δ increases; View the number of extreme points within a period; The optical power P received by the photodetector out Find the first derivative over time. Among them, P in ω is the light power of the light source; θ is the rotation angle of the polarization plane of the linearly polarized light, and θ=0rad; E ω is the angular frequency; t is time; but: When U PM <πk AP When / 2, -π <H sin(ω E ·t)<π, optical power P in one period out The extreme points are: Therefore, when U PM <πk AP When the value is 2, the number of extreme points within one period is always 3, and does not change with U. PM Changes due to increases; When U PM ≥ πk AP / 2, -H < H sin(ω E ·t) < H, and the extreme points of the optical power P out received by the photodetector within one period are as follows: In the formula, m is a positive integer, and m0 is the smallest integer multiple of H being π; therefore, when U PM ≥πk AP When / 2, the number of extreme points of the signal within one period increases with U. PM The number gradually increased with the increase in [the number of people / entities]. That is, when the peak-to-peak optical power received by the photodetector is at its maximum and the number of extreme points in one cycle is 3, the amplitude U of the modulated signal corresponding to the modulation depth H = π rad is obtained. PMπ .

3. The method for determining the modulation signal for the optimal modulation depth of the reflective all-fiber current sensor according to claim 2, characterized in that: The angular frequency ω E The calculation process is as follows: For a reflective all-fiber current sensor, calculate the round-trip time τ of the beam passing through the phase modulator. C In the formula, L is the length of the optical fiber between the phase modulator and the reflector, which can be accurately measured using an optical time-domain reflectometer; n is the refractive index of the optical fiber; and c is the speed of light in a vacuum. Calculate the angular frequency ω of the phase modulator signal using Formula 2. E 4. The method for determining the modulation signal for the optimal modulation depth of the reflective all-fiber current sensor according to claim 3, characterized in that: The optical signal P that arrives at the photodetector out The calculation process is as follows: When the modulation signal of the phase modulator is: u PM (t)=k AP ·δ·sin(ω E ·t) Where, k AP δ is the amplitude of the modulating signal; ω E Angular frequency; k AP δ is the electro-optic modulation coefficient of the phase modulator, in V / rad; δ is the amplitude of the phase delay of the phase modulator, in rad. The corresponding phase modulator phase shift is The phase shift angle generated by the beam passing back and forth through the phase modulator is: Define the modulation depth H = 2δ, then the optical signal P reaching the photodetector out for: P in Let θ be the light source power, θ be the rotation angle of the linearly polarized light's polarization plane, and U be the amplitude of the corresponding phase modulator signal. PM for:

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