High sensitivity rayleigh distributed temperature and strain measurement apparatus and method

Abstract

The application provides a high-sensitivity Rayleigh distributed temperature and strain measurement device and method, which comprises a first laser, a coupler, a pulsed light modulation light path, a laser frequency locking light path, a fiber to be measured and a detector; the light beam output by the first laser is divided into a first light beam and a second light beam through the coupler; the first light beam passes through the pulsed light modulation light path to form swept-frequency pulsed light of different frequencies; the second light beam passes through the laser frequency locking light path to form a compensation voltage fed back to the laser, so that the output frequency of the laser is locked; the pulsed light is injected from one end of the fiber to be measured, Rayleigh scattering occurs in the fiber to be measured to generate a back Rayleigh scattering signal; the detector collects the back Rayleigh scattering signal to obtain a back Rayleigh scattering spectrum, and the measured temperature and strain values are determined through the frequency shift value of the back Rayleigh scattering spectrum. The above-mentioned technology of the application can realize high-sensitivity absolute measurement of Rayleigh distributed temperature and strain.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of optical measurement, and particularly relates to a high-sensitivity Rayleigh distributed temperature and strain measurement device and method. BACKGROUND

[0002] With the birth and development of semiconductor lasers, the semiconductor lasers have been widely applied to high-resolution spectroscopy, optical communication, laser radar, frequency standard, precision measurement and other fields due to the advantages of easy tuning and modulation, small volume, light weight, wide wavelength coverage range and the like, and the role of the semiconductor lasers in aviation, national defense, medical care and the like is also more and more important. However, the noise and frequency drift of the free-running semiconductor laser in the working state can obviously exceed the requirements of high frequency stability and low noise in the above applications. Therefore, active frequency stabilization of the laser to improve the frequency stability has become a hot research direction today.

[0003] Compared with Brillouin and Raman sensing technology, the phase optical time domain reflectometer system based on Rayleigh scattering effect can achieve higher temperature and strain resolution. The change of the interference intensity of the Rayleigh backscattering signal is mainly subject to the change of the phase difference of the echo signal, and therefore, the schemes currently proposed to realize quantitative measurement of temperature and strain are mainly divided into two categories: a phase demodulation type phase optical time domain reflectometer scheme and a frequency compensation type phase optical time domain reflectometer scheme.

[0004] For the phase demodulation type phase optical time domain reflectometer system, the problems are the coherent fading phenomenon and the phase noise problem introduced by the coherent heterodyne detection. Both limit the sensing performance of the phase demodulation type phase optical time domain reflectometer system.

[0005] Compared with the phase demodulation type phase optical time domain reflectometer system, the frequency compensation type phase optical time domain reflectometer system does not have the above defects. The frequency compensation type phase optical time domain reflectometer system determines the change amount of temperature and strain by measuring the frequency shift amount between the demodulated backscattering Rayleigh spectrum and the reference spectrum, and then realizes quantitative measurement. However, when the frequency of the laser serving as the light source drifts, an additional frequency shift amount will be generated in the related spectrum, thereby causing a large error in the frequency shift estimation value and reducing the reliability and measurement accuracy of the system. SUMMARY

[0006] In view of the above problems, the application provides a high-sensitivity Rayleigh distributed temperature and strain measurement device and method to solve the above problems in the prior art.

[0007] According to an aspect of the present application, there is provided a high-sensitivity Rayleigh distributed temperature and strain measurement device based on laser frequency locking, comprising: a first laser, a coupler, a pulsed light modulation light path, a laser frequency locking light path, a fiber to be measured, and a detector group; the light beam output by the first laser is divided into a first light beam and a second light beam through the coupler; the first light beam passes through the pulsed light modulation light path to form swept-frequency pulsed light of different frequencies; the pulsed light is injected from one end of the fiber to be measured to generate a back Rayleigh scattering signal; the detector group collects the back Rayleigh scattering signal to obtain a back Rayleigh scattering spectrum, and the temperature and strain values to be measured are determined by the frequency shift value of the back Rayleigh scattering spectrum; the second light beam passes through the laser frequency locking light path to generate an error signal for locking the frequency of the first laser.

[0008] Further, the pulsed light modulation light path comprises a first electro-optic modulator, a first fiber loop, a second laser, a second electro-optic modulator, an arbitrary waveform generator, an erbium-doped fiber amplifier, and a second fiber loop; the first light beam and a microwave frequency agile signal output by the arbitrary waveform generator pass through the first electro-optic modulator to generate a double-sideband optical frequency agile signal; the double-sideband frequency agile optical signal is used as seed light, passes through the first fiber loop, and is injected into the second laser, the center frequency of the second laser is adjusted to coincide with the center frequency of the upper sideband optical frequency agile signal, and the upper sideband optical frequency agile signal is injection-locked; the upper sideband optical frequency agile signal after locking and the pulse signal generated by the arbitrary waveform generator pass through the second electro-optic modulator to generate a pulsed light signal; the pulsed light signal is amplified by the erbium-doped fiber amplifier, and then input from one end of the fiber to be measured through the second fiber loop, and the generated back Rayleigh scattering signal is received by the first detector.

[0009] Further, the laser frequency locking light path comprises a phase modulator, a hydrogen chloride cell, a second detector, a radio frequency signal generator, a phase shifter, a mixer, a low-pass filter, and a proportional-integral-derivative controller; the second light beam and the radio frequency signal emitted by the radio frequency signal generator pass through the phase modulator to generate two modulation signals with equal size and opposite phase; the modulation signals pass through the hydrogen chloride cell to generate transmission spectrum signals, which are received and detected by the second detector; the signal received by the second detector and the radio frequency signal after phase shifting of the phase shifter pass through the mixer to mix, and then pass through the low-pass filter to obtain an error signal for frequency locking; the error signal is returned to the first laser through the proportional-integral-derivative controller for frequency locking.

[0010] According to another aspect of the present application, there is also provided a high-sensitivity Rayleigh distributed temperature and strain measurement method based on laser frequency locking, which is implemented by using the device as described above; the method comprises: a first light beam passes through a pulsed light modulation light path to form pulsed light, the pulsed light being pulsed light carrying a frequency agile microwave signal to be measured; a second light beam passes through a laser frequency locking light path to form a compensation voltage fed back to a laser, as an error signal for locking the frequency of the first laser to make the output frequency of the laser locked; the pulsed light is injected from one end of a fiber to be measured, Rayleigh scattering occurs in the fiber to be measured to generate a back Rayleigh scattering signal; the back Rayleigh scattering signal is collected to obtain a back Rayleigh scattering spectrum, and the temperature and strain values to be measured are determined by the frequency shift value of the back Rayleigh scattering spectrum.

[0011] Further, the first light beam and the second light beam are obtained by splitting the laser output by the same laser.

[0012] Further, the step of passing the first light beam through a pulsed light modulation light path to form pulsed light comprises: the first light beam and a microwave frequency agile signal output by an arbitrary waveform generator pass through a first electro-optical modulator to generate a double-sideband optical frequency agile signal; the double-sideband frequency agile optical signal is injected into the second laser as seed light through a first optical fiber circulator, the center frequency of the second laser is adjusted to coincide with the center frequency of the upper sideband optical frequency agile signal, and the upper sideband optical frequency agile signal is injection locked; the upper sideband optical frequency agile signal after locking and a pulse signal generated by the arbitrary waveform generator pass through the second electro-optical modulator to generate a pulsed light signal; the pulsed light signal is amplified by an erbium-doped fiber amplifier and then input from one end of the fiber to be measured through the first optical fiber circulator.

[0013] Further, the step of passing the second light beam through a laser frequency locking light path to form a compensation voltage fed back to a laser comprises: the second light beam and a radio frequency signal generated by the radio frequency signal generator pass through the phase modulator to generate two modulation signals with equal size and opposite phase; the modulation signals pass through the hydrogen chloride cell to generate a transmission spectrum signal; the transmission spectrum signal and the radio frequency signal after phase shifting by the phase shifter pass through the mixer to mix, and then pass through the low-pass filter to obtain an error signal for frequency locking; the error signal is returned to the first laser through the proportional-integral-derivative controller for frequency locking.

[0014] The high-sensitivity Rayleigh distributed temperature and strain measurement device and method of the present application suppress the frequency drift of the light source laser by frequency locking, thereby realizing high-sensitivity absolute measurement of Rayleigh distributed temperature and strain.

[0015] The high-sensitivity Rayleigh distributed temperature and strain measurement device and method of the present application realizes high-precision absolute measurement of Rayleigh scattering distributed temperature and strain. The laser frequency stabilization technology is combined with the swept frequency phase optical time domain reflectometer technology, the agile frequency microwave signal is loaded on the detection light emitted by the frequency-locked laser, and the pulse sequence modulation is performed on the detection light, so that the optical pulse sweeping is realized. The pulse light is input into the measured optical fiber to obtain the back Rayleigh scattering spectrum, and the temperature and strain change values are obtained according to the frequency shift values of the back Rayleigh scattering spectrum. Compared with the phase demodulation type phase optical time domain reflectometer system, there is no coherent fading phenomenon and the phase noise problem introduced by the coherent heterodyne detection, and compared with the traditional frequency compensation type phase optical time domain reflectometer system, the measurement precision is improved by suppressing the frequency drift of the laser serving as the light source. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 is a structural schematic diagram of a high-sensitivity Rayleigh distributed temperature and strain measurement device based on laser frequency locking according to an embodiment of the present application;

[0017] Figure 2 is a schematic diagram of a pulse light signal;

[0018] Figure 3 is a schematic diagram of a lock point in a hydrogen chloride gas cell absorption spectrum. DETAILED DESCRIPTION

[0019] In order for those skilled in the art to better understand the present application, exemplary embodiments or examples of the present application will be described below with reference to the accompanying drawings. Based on the embodiments in the present application, all other embodiments or examples obtained by those skilled in the art without creative labor shall fall within the scope of protection of the present application.

[0020] According to an embodiment of the present application, a high-sensitivity Rayleigh distributed temperature and strain measurement device based on laser frequency locking is provided, which comprises: a first laser, a coupler, a pulse light modulation light path, a laser frequency locking light path, a measured optical fiber, and a detector group; the light beam output by the first laser is divided into a first light beam and a second light beam through the coupler; the first light beam passes through the pulse light modulation light path to form swept frequency pulse light of different frequencies; the pulse light is injected from one end of the measured optical fiber to generate a back Rayleigh scattering signal; the detector group collects the back Rayleigh scattering signal to obtain a back Rayleigh scattering spectrum, and determines the measured temperature and strain values through the frequency shift values of the back Rayleigh scattering spectrum; the second light beam passes through the laser frequency locking light path to generate an error signal for locking the frequency of the first laser.

[0021] Next, the above-mentioned high-sensitivity Rayleigh distributed temperature and strain measurement device based on laser frequency locking will be described in conjunction with Figure 1 .

[0022] As Figure 1 shown, the device comprises a first laser 101, a coupler, a pulsed light modulation light path, a laser frequency locking light path, a fiber under test 110 and a detector group.

[0023] The detector group comprises a first detector 109 and a second detector 113.

[0024] The light beam output by the first laser 101 is split into a first light beam and a second light beam by the coupler (not shown in the figure, for example, it can be a beam splitter).

[0025] The first light beam passes through the pulsed light modulation light path to form swept-frequency pulsed light of different frequencies, so that the generated pulsed light is injected from one end of the fiber under test 110 to generate a backscattering Rayleigh signal.

[0026] The first detector 109 in the detector group collects the backscattering Rayleigh signal to obtain a backscattering Rayleigh spectrum, and the temperature and strain values to be measured are determined by the frequency shift value of the backscattering Rayleigh spectrum.

[0027] As an example, the pulsed light modulation light path comprises, for example, as Figure 1 shown, a first electro-optic modulator 102, a first fiber circulator 104, a second laser 103, a second electro-optic modulator 106, an arbitrary waveform generator 105, an erbium-doped fiber amplifier 107 and a second fiber circulator 108.

[0028] After the first light beam and the microwave frequency-changed signal output by the arbitrary waveform generator 105 pass through the first electro-optic modulator 102, a double-sideband optical frequency-changed signal is generated, which is injected into the second laser 103 as seed light through the first fiber circulator 104 to adjust the center frequency of the second laser 103, so that the center frequency of the second laser coincides with the center frequency of the upper sideband optical frequency-changed signal, thereby achieving injection locking of the upper sideband optical frequency-changed signal.

[0029] After the locked upper sideband optical frequency-changed signal and the pulse signal (which is an electrical signal) generated by the arbitrary waveform generator 105 pass through the second electro-optic modulator 106, a pulsed light signal (i.e. the pulsed light described above) is generated; after the pulsed light signal is amplified by the erbium-doped fiber amplifier, it is input from one end of the fiber under test through the second fiber circulator, and the generated backscattering Rayleigh signal is received by the first detector.

[0030] It should be noted that the above-mentioned double-sideband frequency-changed optical signal includes optical frequency-changed signals of the upper sideband and the lower sideband.

[0031] In addition, the second light beam passes through the laser frequency locking light path to generate an error signal for locking the frequency of the first laser 101.

[0032] As an example, the laser frequency locking optical path includes a phase modulator 111, a hydrogen chloride gas cell 112, a second detector 113, a radio frequency signal generator 114, a phase shifter 115, a frequency mixer 116, a low pass filter 117, and a proportional-integral-derivative controller 118.

[0033] As shown in Figure 1 The second light beam and the radio frequency signal emitted by the radio frequency signal generator 114 are modulated by the phase modulator 111 to generate a modulated signal, which is a modulated signal with two equal-size and opposite-phase sidebands.

[0034] The modulated signal passes through the hydrogen chloride gas cell 112 to generate a transmission spectrum signal, which is received and detected by the second detector 113.

[0035] The signal received by the second detector and the radio frequency signal after phase shifting by the phase shifter are mixed by the frequency mixer, and then pass through the low pass filter to obtain an error signal for frequency locking; the error signal is returned to the first laser through the proportional-integral-derivative controller for frequency locking.

[0036] In summary, the above-mentioned high-sensitivity Rayleigh distributed temperature and strain measurement device based on laser frequency locking includes a first laser, a coupler, a pulsed light modulation optical path, a laser frequency locking optical path, a to-be-measured optical fiber, and a detector. In the above-mentioned device, the light beam output by the first laser passes through the coupler to be divided into a first light beam and a second light beam; the first light beam passes through the pulsed light modulation optical path to form a swept-frequency pulsed light with different frequencies; the second light beam passes through the laser frequency locking optical path to form a compensation voltage fed back to the laser, so that the output frequency of the laser is locked; the pulsed light is injected from one end of the to-be-measured optical fiber, and Rayleigh scattering occurs in the to-be-measured optical fiber to generate a back Rayleigh scattering signal; the back Rayleigh scattering signal is collected by the detector to obtain a back Rayleigh scattering spectrum, and the to-be-measured temperature and strain values are determined by the frequency shift value of the back Rayleigh scattering spectrum.

[0037] In addition, the embodiment of the present application also provides a high-sensitivity Rayleigh distributed temperature and strain measurement method based on laser frequency locking, which is realized by using the above-mentioned device. The method comprises the following steps: the first light beam passes through the pulsed light modulation optical path to form a pulsed light, which is a pulsed light carrying a to-be-measured frequency agile microwave signal; the second light beam passes through the laser frequency locking optical path to form a compensation voltage fed back to the laser, which is an error signal for locking the frequency of the first laser to make the output frequency of the laser locked; the pulsed light is injected from one end of the to-be-measured optical fiber, and Rayleigh scattering occurs in the to-be-measured optical fiber to generate a back Rayleigh scattering signal; the back Rayleigh scattering signal is collected to obtain a back Rayleigh scattering spectrum, and the to-be-measured temperature and strain values are determined by the frequency shift value of the back Rayleigh scattering spectrum.

[0038] As an example, the first light beam and the second light beam are obtained by splitting the laser output by the same laser.

[0039] As an example, the step of forming the pulsed light by the first light beam passing through the pulsed light modulation optical path includes: the first light beam and the microwave frequency conversion signal output by the arbitrary waveform generator passing through the first electro-optical modulator to generate a double-sideband optical frequency conversion signal; the double-sideband frequency conversion light signal is injected into the second laser as seed light through the first fiber circulator, the center frequency of the second laser is adjusted to coincide with the center frequency of the upper sideband optical frequency conversion signal, and the upper sideband optical frequency conversion signal is injection locked; the locked upper sideband optical frequency conversion signal and the pulse signal generated by the arbitrary waveform generator pass through the second electro-optical modulator to generate a pulsed light signal; the pulsed light signal is amplified by the erbium-doped fiber amplifier and then input from one end of the to-be-measured optical fiber through the first fiber circulator.

[0040] As an example, the step of forming the compensation voltage feedback to the laser by the second light beam passing through the laser frequency locking optical path includes: the second light beam and the radio frequency signal generated by the radio frequency signal generator pass through the phase modulator to generate two modulation signals with equal size and opposite phase; the modulation signal passes through the hydrogen chloride cell to generate a transmission spectrum signal; the transmission spectrum signal and the phase-shifted radio frequency signal pass through the mixer after mixing and then pass through the low-pass filter to obtain the error signal for frequency locking; the error signal is returned to the first laser for frequency locking through the proportional-integral-derivative controller.

[0041] Preferred Example 1

[0042] Next, the preferred example is described in conjunction with Figure 1 The preferred example is described in conjunction with

[0043] As Figure 1 shown, the above-mentioned high-sensitivity Rayleigh distributed temperature and strain measurement device based on laser frequency locking is a Rayleigh scattering temperature and strain measurement device based on a stable light source. The device includes: a first laser 101, a coupler (not shown in the figure), a pulsed light modulation optical path, a laser frequency locking optical path, a to-be-measured optical fiber 110, and a detector (including a first detector 109 and a second detector 113).

[0044] The light beam output by the first laser 101 passes through the coupler and is divided into a first light beam and a second light beam.

[0045] The first light beam passes through the pulsed light modulation optical path to form a swept-frequency pulsed light of different frequencies.

[0046] The pulsed light is injected from one end of the to-be-measured optical fiber to generate a back Rayleigh scattering signal.

[0047] The first detector 109 collects the back Rayleigh scattering signal to obtain a back Rayleigh scattering spectrum, and determines the temperature and strain values to be measured according to the frequency shift value of the back Rayleigh scattering spectrum.

[0048] The second light beam passes through the laser frequency locking optical path to form a compensation voltage for the laser to lock the output frequency of the laser. In other words, the second light beam passes through the laser frequency locking optical path to generate an error signal for locking the frequency of the first laser.

[0049] The first laser 101 is a distributed feedback laser with a wavelength of λ0, for example.

[0050] The second light beam passes through the laser frequency locking optical path to generate a signal current for locking the frequency of the first laser.

[0051] As shown in Figure 1 The pulse light modulation optical path includes a first electro-optic modulator 102, a first fiber ring 104, a second electro-optic modulator 106, an arbitrary waveform generator 105, an erbium-doped fiber amplifier 107, and a second fiber ring 108, for example.

[0052] The first light beam and the microwave chirp signal output by the arbitrary waveform generator 105 pass through the first electro-optic modulator 102 to modulate and generate a double-sideband optical chirp signal. The double-sideband chirp optical signal, as a seed light, is injected into the second laser 103 through the first fiber ring 104, and the center frequency of the second laser 103 is adjusted to coincide with the center frequency of the upper sideband optical chirp signal, so that the upper sideband optical chirp signal is injection-locked. The locked upper sideband optical chirp signal and the pulse signal generated by the arbitrary waveform generator 105 pass through the second electro-optic modulator 106 to modulate and generate a pulse light signal; the pulse light signal is amplified by the erbium-doped fiber amplifier 107 and then input from one end of the optical fiber 110 (the end close to 108 shown in the figure) through the second fiber ring 108, and the back Rayleigh scattering signal generated thereby (again passing through 108) is received by the first detector 109.

[0053] In addition, the laser frequency locking optical path includes a phase modulator 111, a hydrogen chloride cell 112, a second detector 113, a radio frequency signal generator 114, a phase shifter 115, a mixer 116, a low-pass filter 117, and a proportional-integral-derivative controller 118.

[0054] As shown in Figure 1As shown, the second beam and the radio frequency signal emitted by the radio frequency signal generator 114 are modulated by the phase modulator 111 to generate a modulated signal with two sidebands of equal size but opposite phase. The modulated signal generates a transmission spectrum signal after passing through the hydrogen chloride gas cell 112, which is received and detected by the second detector 113. The signal received by the second detector 113 is mixed with the radio frequency signal after phase shifting by the phase shifter 115, and then passed through the low-pass filter 117 to obtain an error signal for frequency locking. The error signal is returned to the first laser for frequency locking by the proportional-integral-differential controller.

[0055] Preferred Example 2

[0056] Below, in conjunction with Figure 1 , Figure 2 A preferred example of a high-sensitivity Rayleigh distributed temperature and strain measurement device and method based on laser frequency locking is described.

[0057] like Figure 1 As shown, the high-sensitivity Rayleigh distributed temperature and strain measurement device based on laser frequency locking includes: a first laser, a coupler, a pulsed light modulation optical path, a laser frequency locking optical path, an optical fiber under test, and detectors (including the first and second detectors mentioned above).

[0058] The first beam passes through a pulsed light modulation optical path to form swept pulses of different frequencies.

[0059] The second beam passes through the laser frequency-locking optical path, which generates a compensation voltage that is fed back to the laser, thus locking the laser's output frequency.

[0060] A pulsed light is injected from one end of the fiber under test, where Rayleigh scattering occurs.

[0061] The detector collects the backscattered Rayleigh signal to obtain the backscattered Rayleigh spectrum, and the temperature and strain value to be measured are determined by the frequency shift value of the backscattered Rayleigh spectrum.

[0062] like Figure 2 As shown, the light output from the laser is modulated, and optical frequency agility technology is used to achieve rapid frequency switching of the modulated light. The duration of a single frequency is T. L (T L The light is then converted into a pulsed optical signal using a pulsed electrical signal, with the duration of each pulse corresponding to a specific frequency. This completes the frequency sweep of the pulsed light. After the pulse train enters the fiber under test, Rayleigh scattering signals at different frequencies are obtained. By obtaining the frequency variation of the Rayleigh backscattering spectrum, the measured temperature and strain value can be obtained. Here, n is the refractive index of the fiber, L is the fiber length, and c is the speed of light. Figure 2 In this context, N represents the number of pulses.

[0063] The first laser is a distributed feedback laser with a wavelength of λ0.

[0064] The pulse light modulation light path comprises a first electro-optic modulator, a first optical fiber circulator, a second electro-optic modulator, an arbitrary waveform generator, an erbium-doped fiber amplifier and a second optical fiber circulator.

[0065] The first light beam and the microwave chirp signal output by the arbitrary waveform generator are modulated by the first electro-optic modulator to generate a double-sideband optical chirp signal.

[0066] The double-sideband chirp optical signal is injected into the second laser as seed light through the first optical fiber circulator, and the temperature and current of the second laser are adjusted so that the center frequency of the second laser coincides with the center frequency of the upper sideband optical chirp signal, and the upper sideband optical chirp signal is injection-locked.

[0067] The injection-locked upper sideband optical chirp signal and the pulse signal generated by the arbitrary waveform generator are modulated by the second electro-optic modulator to generate a pulse light signal.

[0068] The pulse light signal is amplified by the erbium-doped fiber amplifier and then input from one end of the to-be-measured optical fiber through the first optical fiber circulator, and the back Rayleigh scattering signal generated is received by the first detector.

[0069] In the embodiment, the laser frequency locking light path comprises a phase modulator, a hydrogen chloride gas cell, a second detector, a radio frequency signal generator, a phase shifter, a mixer, a low-pass filter and a proportional-integral-derivative controller.

[0070] The second light beam and the radio frequency signal generated by the radio frequency signal generator are modulated by the phase modulator to generate two modulation signals with equal size and opposite phase.

[0071] The modulation signals pass through the hydrogen chloride gas cell to generate transmission spectrum signals, which are received and detected by the second detector.

[0072] The signal received by the detector and the radio frequency signal phase-shifted by the phase shifter are mixed by the mixer, and then pass through the low-pass filter to obtain an error signal for frequency locking.

[0073] The error signal is returned to the first laser through the proportional-integral-derivative controller for frequency locking.

[0074] As Figure 3As shown, the frequency of the laser is locked to the locking point on the right side of the spectrum line with a larger slope by using the different transmittance of the spectrum line of the hydrogen chloride gas cell with different laser frequencies. If the frequency of the laser deviates from the point, the light power transmitted through the gas cell changes, resulting in a change in the voltage output by the photodetector behind the gas cell, that is, an error signal is generated. In other words, the error signal is the deviation between the voltage value (which changes) output by the photodetector and the fixed voltage value at the locking point.

[0075] In this way, the photoelectric feedback system compares the changing voltage value generated by the frequency fluctuation with the fixed voltage value at the locking point, generates a compensation voltage fed back to the laser, and makes the output frequency of the laser re-locked to the locking point.

[0076] In the embodiment of the present application, the optical fiber to be measured is a single-mode optical fiber.

[0077] In the embodiment of the present application, the first and second electro-optical modulators work at the lowest operating point.

[0078] The temperature and strain measurement device of the embodiment of the present application is divided into upper and lower paths. The upper path generates a swept-frequency pulsed light, and the lower path generates an error signal for locking the frequency of the first laser. In the embodiment of the present application, the optical fiber to be measured is a single-mode optical fiber. The laser used has a linewidth of 1 MHz, and a narrow-linewidth distributed feedback semiconductor laser with a wavelength of 1550 nm is used as the first laser. A microwave frequency-agile signal is output by channel 1 of an arbitrary waveform generator and loaded onto the first electro-optical modulator to generate a double-sideband optical frequency-agile signal. The frequency range of the microwave frequency-agile signal is 10.6-11.1 GHz, and the step size is set to 5 MHz. The modulated double-sideband frequency-agile light signal is used as a seed light and injected into the second laser through the first circulator, and the injection locking of the upper sideband is achieved by adjusting the operating current and temperature of the second laser, thereby obtaining a power-stable and spectrum-pure upper sideband frequency-agile output. Then, the second electro-optical modulator is controlled by channel 2 of the arbitrary waveform generator to modulate the frequency-agile light signal into a pulsed light signal. The synchronization of channel 1 and channel 2 is adjusted to ensure that the pulsed light signal corresponds to the frequency components of the frequency-agile output one by one. The pulse width is 20 ns (the spatial resolution of the system is 2 m), the total number of pulses is 100, the pulse interval is 50 μs, and the maximum sensing distance of the system is 50 km. Then, the pulsed light is amplified to a power of 1 W by an erbium-doped fiber amplifier and then enters a 50 m long optical fiber to be measured, generates a backscattered Rayleigh signal, and then the signal is amplified by EDFA2, filtered by an adjustable filter to remove the spontaneous scattering noise, received by an optical-electric detector with a bandwidth of 120 MHz, and then the backscattered Rayleigh signal is collected by an oscilloscope with a sampling rate of 1 GSa / s.

[0079] The high-sensitivity Rayleigh distributed temperature and strain measurement method based on laser frequency locking comprises the following steps:

[0080] Step 1, the light beam output by the first laser is divided into a first light beam and a second light beam;

[0081] Step 2, the second light beam passes through a laser frequency locking light path to form a compensation voltage feedback to the laser, so that the output frequency of the laser is locked;

[0082] Step 3, the first light beam passes through a pulsed light modulation light path to form a swept frequency pulsed light of different frequencies;

[0083] Step 4, the pulsed light is injected from one end of the fiber to be measured, and Rayleigh scattering occurs in the fiber to be measured;

[0084] Step 5, the back Rayleigh scattering signal is collected to obtain the frequency change of the Rayleigh back scattering spectrum, and the temperature and strain values to be measured are determined.

[0085] In the embodiment of the application, the step of the second light beam passing through the laser frequency locking light path to form a compensation voltage feedback to the laser in step 2 so that the output frequency of the laser is locked comprises: the second light beam and the radio frequency signal generated by the radio frequency signal generator pass through a phase modulator to produce two modulation signals with equal size and opposite phase sidebands; the modulation signals pass through a hydrogen chloride gas cell to produce transmission spectrum signals, which are received and detected by a second detector; the signal received by the detector and the phase-shifted radio frequency signal after the phase shifter pass through a mixer to mix, and then pass through a low-pass filter to obtain an error signal for frequency locking; the error signal is returned to the first laser through a proportional-integral-derivative controller for frequency locking.

[0086] In the embodiment of the application, the step of the first light beam passing through the pulsed light modulation light path to form pulsed light carrying the agile frequency signal in step 3 comprises: the first light beam and the microwave agile frequency signal output by the arbitrary waveform generator pass through a first electro-optic modulator to produce a double-sideband optical agile frequency signal; the double-sideband agile frequency optical signal is injected into the second laser as seed light through the first fiber ring, the center frequency of the second laser is adjusted to coincide with the center frequency of the upper sideband optical agile frequency signal, and the upper sideband optical agile frequency signal is injection locked; the locked upper sideband optical agile frequency signal and the pulse signal generated by the arbitrary waveform generator pass through a second electro-optic modulator to produce a pulsed light signal; the pulsed light signal is amplified by an erbium-doped fiber amplifier and then input from one end of the fiber to be measured through the first fiber ring.

[0087] After a disturbance is applied at a certain position of the fiber to be measured, the measurement is repeated again, and the Rayleigh scattering signal power I wd (v,z) is recorded. Wherein, v represents the incident light frequency, and z represents the position of the scattering point on the fiber to be measured. The Rayleigh scattering signals recorded twice are compared, and at the position where no disturbance occurs, I wd (v,z) is equal to I nd(ν,z) remains consistent, on the contrary, at the position where the disturbance occurs, the two will appear difference, at this time, I wd (ν+△ν,z) is the same as I nd (ν,z), because the change of frequency will make the phase difference before and after the disturbance remain unchanged, which also means that the change of temperature and strain will cause the distribution of the back Rayleigh scattering signal power in the laser frequency domain to shift. Wherein, I nd (ν,z) represents the measured Rayleigh backscattering signal power as a function of laser scanning frequency and fiber position. Considering these factors, the amount of change in laser frequency can be obtained by cross-correlation algorithm, and the relationship between the laser frequency compensation amount△ν and the temperature△T and strain△ε is:

[0088]

[0089]

[0090] In the formula:

[0091] v0-Initial frequency;

[0092] τ ik - The time difference required for light from the ith scattering point and the kth scattering point to be received by the detector from the beginning of the fiber incident; ik - The change amount of τ ik ;

[0093] ξ-thermal optical coefficient of silicon dioxide;

[0094] α T -thermal expansion coefficient of the optical fiber;

[0095] p e -effective strain optical coefficient;

[0096] K T -temperature coefficient;

[0097] K ε -strain coefficient.

[0098] The high-sensitivity Rayleigh distributed temperature and strain measurement device and method based on the frequency locking of the laser of the embodiment of the present application have the following advantages: the embodiment of the present application can determine the change amount of temperature and strain by demodulating the frequency shift amount (i.e. the frequency shift value described above) between the back Rayleigh scattering light measurement spectrum and the reference spectrum, and can realize absolute temperature and strain measurement through frequency locking and Rayleigh spectrum comparison, avoiding the phase noise problem introduced by the coherent fading phenomenon and coherent heterodyne detection.

[0099] The embodiment of the present application has high temperature and strain measurement sensitivity. The frequency drift of the light source laser is suppressed by frequency locking, which avoids the measurement error caused by the fact that the frequency drift of the backward Rayleigh scattering spectrum caused by the free operation of the laser cannot be distinguished from the frequency drift of the backward Rayleigh scattering spectrum caused by the change of the temperature or strain of the measured optical fiber. The measurement precision of mK and nano-strain can be achieved.

[0100] Although the present application has been described in connection with limited number of embodiments, those skilled in the art will appreciate that other embodiments can be envisaged within the scope of the present application as described hereinbefore. The disclosure of the present application is illustrative only and not restrictive of the present application, the scope of which is defined by the appended claims.

Claims

1. A high-sensitivity Rayleigh distributed temperature and strain measurement device based on laser frequency locking, characterized in that, The device includes: The first laser, coupler, pulsed light modulation optical path, laser frequency-locked optical path, optical fiber under test, and detector group; The beam output from the first laser is split into a first beam and a second beam by the coupler; The first beam passes through the pulsed light modulation optical path to form swept pulses of different frequencies; The pulsed light is injected from one end of the optical fiber under test, generating a backscattered Rayleigh signal; The detector array collects the backscattered Rayleigh signal to obtain the backscattered Rayleigh spectrum, and determines the temperature and strain value to be measured by the frequency shift value of the backscattered Rayleigh spectrum. The second beam passes through the laser frequency-locking optical path to generate an error signal for locking the frequency of the first laser; The pulsed light modulation optical path includes a first electro-optic modulator, a first fiber optic circulator, a second laser, a second electro-optic modulator, an arbitrary waveform generator, an erbium-doped fiber amplifier, and a second fiber optic circulator. The first beam and the microwave frequency-agile signal output by the arbitrary waveform generator are modulated by the first electro-optic modulator to generate a double-sideband optical frequency-agile signal; The double-sideband optical frequency-agile signal is used as seed light and injected into the second laser through the first fiber optic circulator. The center frequency of the second laser is adjusted to coincide with the center frequency of the upper sideband optical frequency-agile signal to lock the upper sideband optical frequency-agile signal. The locked upper sideband optical agile frequency conversion signal and the pulse signal generated by the arbitrary waveform generator are modulated by the second electro-optic modulator to generate a pulse signal; The pulse signal is amplified by an erbium-doped fiber amplifier and then input from one end of the fiber under test through the second fiber circulator. The resulting backscattered Rayleigh signal is received by the first detector. The laser frequency-locked optical path includes a phase modulator, a hydrogen chloride gas cell, a second detector, a radio frequency signal generator, a phase shifter, a mixer, a low-pass filter, and a proportional-integral-derivative controller. The second beam and the radio frequency signal emitted by the radio frequency signal generator are modulated by the phase modulator to generate a modulated signal with two sidebands of equal size but opposite phase. The modulation signal generates a transmission spectrum signal after passing through the hydrogen chloride gas cell, which is received and detected by the second detector; The signal received by the second detector is mixed with the phase-shifted radio frequency signal of the phase shifter by the mixer, and then passed through the low-pass filter to obtain an error signal for frequency locking; The error signal is returned to the first laser via the proportional-integral-derivative controller for frequency locking.

2. A high-sensitivity Rayleigh distributed temperature and strain measurement method based on laser frequency locking, characterized in that, The method is implemented using the apparatus as described in claim 1; the method includes: The first beam passes through the pulse light modulation optical path to form pulse light, which is pulse light carrying a frequency-agile microwave signal of the frequency to be measured; The second beam passes through the laser frequency-locking optical path to form a compensation voltage fed back to the laser, which serves as an error signal for locking the frequency of the first laser so that the output frequency of the laser is locked. The pulsed light is injected from one end of the fiber under test, and Rayleigh scattering occurs in the fiber under test, generating a backscattered Rayleigh signal; The backscattering Rayleigh signal is acquired to obtain the backscattering Rayleigh spectrum, and the temperature and strain value to be measured are determined by the frequency shift value of the backscattering Rayleigh spectrum.

3. The method according to claim 2, characterized in that, The first beam and the second beam are obtained by splitting the laser beam output from the same laser.

4. The method according to claim 2 or 3, characterized in that, The steps of forming pulsed light by passing the first beam through a pulsed light modulation optical path include: The first beam and the microwave frequency-agile signal output by the arbitrary waveform generator are modulated by the first electro-optic modulator to generate a double-sideband optical frequency-agile signal; The double-sideband optical frequency-agile signal is used as seed light and injected into the second laser through the first fiber optic circulator. The center frequency of the second laser is adjusted to coincide with the center frequency of the upper sideband optical frequency-agile signal to lock the upper sideband optical frequency-agile signal. The locked upper sideband optical agile frequency conversion signal and the pulse signal generated by the arbitrary waveform generator are modulated by the second electro-optic modulator to generate a pulse signal; The pulse signal is amplified by an erbium-doped fiber amplifier and then input from one end of the fiber under test through the first fiber circulator.

5. The method according to claim 2 or 3, characterized in that, The steps for the second beam to pass through the laser frequency-locked optical path to form a compensation voltage fed back to the laser include: The second beam and the radio frequency signal emitted by the radio frequency signal generator are modulated by the phase modulator to generate two modulated signals with equal size and opposite phase sidebands; The modulation signal generates a transmission spectrum signal after passing through the hydrogen chloride gas cell. The transmission spectrum signal and the phase-shifted radio frequency signal are mixed by the mixer and then passed through the low-pass filter to obtain an error signal for frequency locking. The error signal is returned to the first laser via the proportional-integral-derivative controller for frequency locking.

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