CN115235367A - High-precision dual-frequency optical frequency domain reflectometer with large strain measurement range - Google Patents

Abstract

The invention discloses a high-precision dual-frequency optical frequency domain reflectometer with a large strain measurement range. , which is used to interfere the backscattered light emitted by the fiber to be tested with the sweep-frequency continuous light, and can adjust the polarization state of the backscattered light to obtain interference light; the photoelectric conversion module is used to convert the interference light into electrical Signal; acquisition and processing module, connected with the photoelectric conversion module, used to collect data, analyze and process the data. The dual-frequency optical frequency domain reflectometer uses the phase difference of the two frequency optical waves to measure the strain, which solves the problem that the maximum dynamic strain measurable by the single-frequency optical system is affected by the optical frequency when the repetition frequency of the sweep frequency and the vibration frequency of the applied strain remain unchanged. limit the problem and maintain high precision measurements.

Description

A high-precision dual-frequency optical frequency domain reflectometer with a large strain measurement range

technical field

The patent belongs to the field of optical fiber sensing, in particular to a high-precision dual-frequency optical frequency domain reflectometer with a large strain measurement range.

Background technique

Distributed optical fiber sensing technology has the advantages of anti-electromagnetic interference, high sensitivity, and easy implementation, and has been widely used in perimeter security, structural health monitoring, and seismic wave detection. When the fiber under test is disturbed by the external environment (such as dynamic strain), the length, core diameter and refractive index characteristics of the fiber will change, which will cause changes in the amplitude and phase of the Rayleigh scattered light in the fiber; The Rayleigh scattering signal is analyzed, and then the detection of the disturbance signal is realized. In the early sensing systems, the localization of perturbation events was only achieved based on the relative changes in the intensity of the Rayleigh scattering signal, and quantitative measurements could not be achieved. Further research shows that the amount of change in the phase of the Rayleigh scattering signal is linearly related to the magnitude of the strain applied to the fiber, so the magnitude of the dynamic strain can be quantitatively measured by demodulating the phase change of the probe light.

Among many detection methods, phase-sensitive optical frequency-domain reflectometry has received extensive attention due to its advantages of high resolution and high sensitivity. Optical frequency domain reflectometry uses frequency-modulated continuous wave as the detection light, and its spatial resolution depends on the frequency sweep range, which solves the problem of mutual restriction between spatial resolution and detection distance in the pulse detection method. The phase sensitive optical frequency domain reflectometer demodulates the phase spectrum of the Rayleigh scattering signal, performs phase difference before and after the strain event to obtain the magnitude of the phase change, and demodulates the dynamic strain applied to the fiber.

In the process of measuring dynamic strain with a phase-sensitive optical frequency-domain reflectometer, it is necessary to use an unwrapping algorithm to unwind the phase measurement values to make the phase continuous. However, the prerequisite for correct demodulation using the unwinding algorithm is that the absolute value of the phase change of adjacent measurement points cannot exceed π (π threshold condition), which limits the maximum range of measurable dynamic strain. Assuming that the applied dynamic strain is a single-frequency sinusoidal signal, according to the π threshold condition, the measurable dynamic strain of the system can be obtained as:

where f _p is the swept repetition frequency of the optical frequency domain reflectometer, and f _ε is the vibration frequency of the applied strain. In the current technology, the phase-sensitive optical frequency domain reflection system uses a single-frequency light with a wavelength of 1550nm (optical frequency ~ 193.5THz) for detection. It can be seen from the above formula that the frequency of the sweep repetition frequency and the applied strain vibration frequency remain unchanged. In the case of , the maximum measurable dynamic strain is limited by the optical frequency ν.

SUMMARY OF THE INVENTION

In order to overcome the problem that the maximum measurable dynamic strain is limited by the optical frequency ν when the frequency sweep repetition frequency and the applied strain vibration frequency remain unchanged, the present invention proposes a high-precision dual-frequency optical frequency domain reflectometer, which utilizes two The phase difference of the frequency light waves is used for strain measurement. This method is equivalent to forming a low-frequency carrier in the measurement system, which increases the strain measurement range; the measurement system uses the Fourier phase spectrum for demodulation, and guides the single-frequency light by means of the phase difference between the dual-frequency light. Phase unwinding enables high-precision measurements over a large dynamic strain range.

In order to achieve the above purpose, the present invention provides the following scheme: a high-precision dual-frequency optical frequency domain reflectometer with a large strain measurement range, comprising:

an optical modulation module, which is used to combine the continuous light emitted by the laser into a dual-frequency continuous light, and modulate the dual-frequency continuous light into a swept-frequency continuous light;

The optical interference module is used for interfering the backscattered light emitted by the fiber to be tested with the frequency sweeping continuous light, and can adjust the polarization state of the backscattered light to obtain the interference light;

a photoelectric conversion module for converting the interference light into an electrical signal;

The acquisition and processing module is connected with the photoelectric conversion module, and is used for analyzing and processing the electrical signal.

Preferably, the optical modulation module and the optical interference module are connected through a first optical coupler;

the optical interference module and the photoelectric conversion module are connected through a second optical coupler;

The first optical coupler and the second optical coupler are used for splitting light.

Preferably, the light modulation module includes a beam combining unit and a conversion unit;

The beam combining unit is used for combining continuous light into dual-frequency continuous light;

The conversion unit is used to modulate the dual-frequency continuous light into sweep-frequency continuous light.

Preferably, the beam combining unit includes a narrow linewidth laser and a first wavelength division multiplexer;

The narrow linewidth laser includes a first narrow linewidth laser and a second narrow linewidth laser;

the first narrow linewidth laser is used for emitting continuous light of the first optical frequency;

the second narrow linewidth laser is used to emit continuous light of the second optical frequency;

The first wavelength division multiplexer is used to combine the continuous light of the first optical frequency and the continuous light of the second optical frequency.

Preferably, the conversion unit includes an arbitrary waveform generator, a radio frequency amplifier, and a modulator;

The arbitrary waveform generator is used to send out a frequency sweep signal;

The radio frequency amplifier is connected to the arbitrary waveform generator, and is used for amplifying the frequency sweep signal;

The modulator, connected to the radio frequency amplifier, is used for modulating the dual-frequency continuous light into sweep-frequency continuous light.

Preferably, the optical interference module includes a first optical coupler, an optical amplifier, an optical fiber to be measured, an optical circulator, a polarization controller, and a second optical coupler;

The first optical coupler is used for splitting light waves, one of which is a detection path and one of which is a reference path;

the optical amplifier, connected to the first optical coupler, is used to increase the optical power of the incoming fiber;

The optical fiber to be tested is used to generate back Rayleigh scattered light;

The optical circulator is connected to the optical amplifier, the optical fiber to be measured, and the polarization controller, respectively, and is used to inject light waves into the optical fiber to be measured, to receive the backward Rayleigh scattered light generated by the optical fiber to be measured, and then to emit light. to the polarization controller;

The polarization controller is used to adjust the polarization state.

The second optical coupler is used to combine the light waves of the detection path and the reference path to interfere.

Preferably, the photoelectric conversion module includes a second wavelength division multiplexer, a third wavelength division multiplexer, a first photodetector, and a second photodetector;

the second wavelength division multiplexer and the third wavelength division multiplexer are connected to the second optical coupler for wavelength division multiplexing;

the first photodetector is connected to the second wavelength division multiplexer and the third wavelength division multiplexer respectively, and is used for receiving the beat frequency signal of the first optical frequency;

The second photodetector is respectively connected to the second wavelength division multiplexer and the third wavelength division multiplexer, and is used for receiving the beat frequency signal of the second optical frequency.

The present invention discloses the following technical effects:

1. The present invention is a phase sensitive optical frequency domain reflectometer, which can adjust the frequency sweep range to obtain high spatial resolution, and overcome the problem that spatial resolution and detection distance cannot be taken into account in the pulse detection method; and the frequency domain Fourier phase is adopted. For demodulation, it has the advantage of high sensitivity.

2. Using the dual-frequency measurement system proposed by the present invention, using the phase difference between the two frequencies for demodulation, compared with the traditional single-frequency optical detection method, the measurement range of dynamic strain is greatly improved. Using continuous light with frequencies of the first frequency and the second frequency for detection, the measurable dynamic strain of the dual-frequency measurement system is increased to the first frequency under the single-frequency measurement system divided by the frequency difference times (or under the single-frequency measurement system The second frequency divided by the frequency difference times).

3. In order to improve the measurement accuracy, the present invention uses the phase difference between the dual-frequency light to guide the phase of the single-frequency light to unwrap during phase demodulation, thereby improving the measurement accuracy of the dual-frequency optical system to the level of the single-frequency optical system. The high-precision dual-frequency optical frequency domain reflectometer proposed by the present invention provides an effective detection means for the application scenarios of large strain and high vibration frequency.

Description of drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings required in the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some of the present invention. In the embodiments, for those of ordinary skill in the art, other drawings can also be obtained according to these drawings without any creative effort.

1 is a schematic diagram of a system structure of a dual-frequency optical frequency domain reflectometer according to an embodiment of the present invention;

In the figure: 1 The first narrow linewidth laser, 2 The second narrow linewidth laser, 3 The first wavelength division multiplexer, 4 The modulator, 5 The arbitrary waveform generator, 6 The radio frequency amplifier, 7 The first optical coupler, 8 Optical amplifier, 9 optical circulator, 10 fiber to be tested, 11 polarization controller, 12 second optical coupler, 13 second wavelength division multiplexer, 14 third wavelength division multiplexer, 15 first photodetector, 16 second photodetector, 17 data acquisition and processor.

Detailed ways

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, but not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

In order to make the above objects, features and advantages of the present invention more clearly understood, the present invention will be described in further detail below with reference to the accompanying drawings and specific embodiments.

As shown in FIG. 1, the present invention provides a high-precision dual-frequency optical frequency domain reflectometer with a large strain measurement range, and its system structure is as follows: a first narrow linewidth laser 1 and a second narrow linewidth laser 2 emit frequency It is the continuous light of ν ₁ and ν ₂ , which is combined by the first wavelength division multiplexer 3, and the first wavelength division multiplexer 3 is connected to the optical input port of the modulator 4; the arbitrary waveform generator 5 is connected to the radio frequency amplifier 6, and the radio frequency The output end of the amplifier 6 is connected to the RF input end of the modulator 4, and the combined beam light is modulated into a frequency sweep continuous light. The optical output port of the modulator 4 is connected to the first optical coupler 7, and the first optical coupler 7 is divided into two channels, one of which is For the detection path, the optical amplifier 8 and the port a of the optical circulator 9 are connected in sequence, the port b of the optical circulator 9 is connected to the fiber to be tested 10, and the port c is connected to the polarization controller 11; the other output path of the first optical coupler 7 is used as a reference path. The detection path and the reference path are connected to the second optical coupler 12, and the two output ports of the second optical coupler 12 are respectively connected to the second wavelength division multiplexer 13 and the third wavelength division multiplexer 14, and the two wavelength division multiplexers The device separates the continuous light with frequencies of ν ₁ and ν ₂ , and then connects the first photodetector 15 and the second photodetector 16 respectively. After converting the optical signal into an electrical signal, it is connected to the data acquisition and processor 17.

Further optimizing the solution, the optical frequency ν ₁ of the first narrow linewidth laser and the optical frequency ν ₂ of the second narrow linewidth laser may differ from hundreds of GHz to several THz.

To further optimize the solution, the splitting ratio of the first optical coupler 7 is 90:10 or 80:20; the splitting ratio of the second optical coupler 12 is 50:50.

To further optimize the solution, the optical fiber 10 to be tested can be an ordinary single-mode optical fiber, a polarization-maintaining optical fiber, an FBG optical fiber, a Rayleigh scattering-enhanced optical fiber, and the like.

To further optimize the scheme, the demodulation method of the signal is as follows:

The first step is to transform the time domain beat signal collected in each frequency sweep cycle to the frequency domain for analysis. The size of the beat frequency reflects the position information of the fiber, and the Fourier phase at a certain frequency reflects the corresponding position. phase information at

和

The second step is to extract the Fourier phase information of the frequency domain signal in each frequency sweep period under the two single frequencies respectively, and obtain the strain caused by the two single frequencies at each moment through the difference between the distance axis and the slowly varying time axis. The winding values of the phase change are respectively

and

为：The third step is to find the phase difference between two single-frequency lights

for:

减小，因而可以测量更大范围的动态应变。利用频率为ν₁和ν₂的连续光进行探测时，双频测量系统的可测动态应变大小提升为单频ν₁测量系统下的ν₁/Δν倍(或单频ν₂测量系统下的ν₂/Δν倍)；In the formula, n is the refractive index of the fiber, κ is the strain coefficient of the fiber, L is the length of the strained region, ε is the strain size, and c is the speed of light in vacuum. where Δν=ν ₁ -ν ₂ is the frequency difference between the two light waves. It can be seen from the above formula that, compared with the single-frequency light, the phase change caused by the same strain in the dual-frequency measurement system is

reduced, so a wider range of dynamic strains can be measured. When the continuous light with frequencies of ν ₁ and ν ₂ is used for detection, the measurable dynamic strain of the dual-frequency measurement system is increased to ν ₁ /Δν times under the single-frequency ν ₁ measurement system (or under the single-frequency ν ₂ measurement system ν ₂ /Δν times);

引导单频光相位解缠绕。以频率ν₁下的单频光为例，由应变引起的单频光解缠绕后的相位变化

为缠绕相位

加2π的整数倍，即

解缠绕转化为求缠绕整数k₁的值；令比例因子M₁＝ν₁/Δν，根据式

求得整数k₁的值，进而解调得到

的值，保持了单频光测量下的精度；The fourth step, in order to improve the measurement accuracy under the dual-frequency system, use the phase difference between the dual-frequency light

Guided single-frequency optical phase unwinding. Taking the single-frequency light at frequency ν ₁ as an example, the phase change after unwinding of the single-frequency light due to strain

for winding phase

Add an integer multiple of 2π, i.e.

Unwinding is transformed into the value of the winding integer k ₁ ; let the scaling factor M ₁ =ν ₁ /Δν, according to the formula

Find the value of the integer k ₁ , and then demodulate to get

The value of , maintains the accuracy under single-frequency optical measurement;

The fifth step is to obtain the strain size ε according to the linear relationship between the phase change and the strain. The strain size ε obtained by demodulation at each moment corresponds to the slow time axis, and the time-varying strain curve caused by vibration is obtained. .

The above-mentioned embodiments are only to describe the preferred modes of the present invention, but not to limit the scope of the present invention. Without departing from the design spirit of the present invention, those of ordinary skill in the art can make various modifications to the technical solutions of the present invention. Variations and improvements should fall within the protection scope determined by the claims of the present invention.

Claims (7)

1. a high-precision dual-frequency optical frequency domain reflectometer with a large strain measurement range, is characterized in that, comprising: an optical modulation module, which is used to combine the continuous light emitted by the laser into a dual-frequency continuous light, and modulate the dual-frequency continuous light into a swept-frequency continuous light; The optical interference module is used for interfering the backscattered light emitted by the fiber to be tested with the frequency sweeping continuous light, and can adjust the polarization state of the backscattered light to obtain the interference light; a photoelectric conversion module for converting the interference light into an electrical signal; The acquisition and processing module is connected with the photoelectric conversion module, and is used for analyzing and processing the electrical signal. 2. the high-precision dual-frequency optical frequency domain reflectometer of large strain measurement range according to claim 1, is characterized in that, the light modulation module and the light interference module are connected through a first optical coupler; The optical interference module and the photoelectric conversion module are connected through a second optical coupler. 3. The high-precision dual-frequency optical frequency domain reflectometer of large strain measurement range according to claim 1, is characterized in that, The light modulation module includes a beam combining unit and a conversion unit; The beam combining unit is used for combining continuous light into dual-frequency continuous light; The conversion unit is used to modulate the dual-frequency continuous light into sweep-frequency continuous light. 4. The high-precision dual-frequency optical frequency-domain reflectometer of large strain measurement range according to claim 3, characterized in that, The beam combining unit includes a narrow linewidth laser and a first wavelength division multiplexer; The narrow linewidth laser includes a first narrow linewidth laser and a second narrow linewidth laser; the first narrow linewidth laser is used for emitting continuous light of the first optical frequency; the second narrow linewidth laser is used to emit continuous light of the second optical frequency; The first wavelength division multiplexer is used to combine the continuous light of the first optical frequency and the continuous light of the second optical frequency. 5. The high-precision dual-frequency optical frequency-domain reflectometer with a large strain measurement range according to claim 3, characterized in that, The conversion unit includes an arbitrary waveform generator, a radio frequency amplifier, and a modulator; The arbitrary waveform generator is used to send out a frequency sweep signal; The radio frequency amplifier is connected to the arbitrary waveform generator, and is used for amplifying the frequency sweep signal; The modulator, connected to the radio frequency amplifier, is used for modulating the dual-frequency continuous light into sweep-frequency continuous light. 6. The high-precision dual-frequency optical frequency domain reflectometer with a large strain measurement range according to claim 1, characterized in that, The optical interference module includes a first optical coupler, an optical amplifier, an optical fiber to be measured, an optical circulator, a polarization controller, and a second optical coupler; The first optical coupler is used for splitting light waves, one of which is a detection path and one of which is a reference path; the optical amplifier, connected to the first optical coupler, is used to increase the optical power of the incoming fiber; The optical fiber to be tested is used to generate back Rayleigh scattered light; The optical circulator is connected to the optical amplifier, the optical fiber to be measured, and the polarization controller, respectively, and is used to inject light waves into the optical fiber to be measured, to receive the backward Rayleigh scattered light generated by the optical fiber to be measured, and then to emit light. to the polarization controller; The polarization controller is used to adjust the polarization state; The second optical coupler is used to combine the light waves of the detection path and the reference path to interfere. 7. The high-precision dual-frequency optical frequency domain reflectometer of large strain measurement range according to claim 1, wherein, The photoelectric conversion module includes a second wavelength division multiplexer, a third wavelength division multiplexer, a first photodetector, and a second photodetector; the second wavelength division multiplexer and the third wavelength division multiplexer are connected to the second optical coupler for wavelength division multiplexing; the first photodetector is connected to the second wavelength division multiplexer and the third wavelength division multiplexer respectively, and is used for receiving the beat frequency signal of the first optical frequency; The second photodetector is respectively connected to the second wavelength division multiplexer and the third wavelength division multiplexer, and is used for receiving the beat frequency signal of the second optical frequency.

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