Chaotic Raman distributed sensing device and method based on optical frequency domain reflection

Abstract

The invention relates to the field of distributed optical fiber sensing, in particular to a chaotic Raman distributed sensing device and method based on optical frequency domain reflection. Chaotic laser emitted by a chaotic light source in the device is sequentially subjected to frequency stepping modulation by an electro-optical modulator and amplified by an erbium-doped optical fiber amplifier and then is divided into two beams by an optical splitter, one beam serves as reference light and is received by a first photoelectric detector, and the other beam serves as detection light and enters a sensing optical fiber after passing through a wavelength division multiplexer. A Raman backward Stokes scattered light signal and a Raman backward anti-Stokes scattered light signal generated at each point along the sensing optical fiber are output by the wavelength division multiplexer and then are received by a second photoelectric detector; output signals of the first photoelectric detector and the second photoelectric detector are sent to a computer through a vector network analyzer, and temperature distribution along the sensing optical fiber is obtained through calculation and processing. According to the invention, effective sensing of distributed temperature in a long sensing distance can be realized at relatively low laser power.

Description

Technical Field

[0001] The invention relates to the field of distributed optical fiber sensing, and in particular to a chaotic Raman distributed sensing device and method based on optical frequency domain reflection. Background Art

[0002] Raman distributed fiber optic sensing technology uses optical fiber as a sensing element and transmission medium, and uses back Raman scattering information to detect temperature field changes at different locations along the sensing fiber. It has become an international key research field due to its advantages such as continuous temperature measurement, high safety, corrosion resistance, intrinsic safety, and anti-electromagnetic interference. It has great social needs and application prospects in the fields of polar scientific research, geothermal resource exploration and development, and large-scale infrastructure structural health monitoring.

[0003] Raman distributed fiber optic sensing technology can be divided into Raman optical time domain reflectometry technology and Raman optical frequency domain reflectometry technology. The light source used in Raman optical frequency domain reflectometry technology is usually a frequency-modulated continuous light source with a low peak power (milliwatt level). Compared with Raman optical time domain reflectometry technology that uses a high peak power (watt level) pulse light source, it not only makes the laser light source and other optical components less susceptible to damage, improves the reliability of the system, but also has cost advantages. In addition, Raman optical frequency domain reflectometry technology uses a continuous light source to perform narrow-band detection of optical reflection signals, which has the advantage of high spatial resolution (millimeter level).

[0004] However, in the traditional Raman optical frequency domain reflection distributed fiber optic sensing system, a continuous laser is used as the system light source. Since the backward Raman scattered light at different positions in the optical fiber has the same frequency but different phases, the scattered light will cancel each other out after reaching the photodetector. Therefore, as the modulation signal frequency increases, the backward Raman scattered light power decreases rapidly to the picowatt level, which greatly limits the maximum detection length of the system. That is, the effective scattered signal at a long sensing distance will be submerged in the noise, making the effective sensing distance far less than the theoretical calculated value. This makes it difficult to meet the long sensing distance requirements for temperature safety monitoring of major infrastructure projects such as tunnels, power cables, gas pipelines, and dams. For the above reasons, improving the effective sensing distance of the Raman optical frequency domain reflection distributed fiber optic sensing system while taking into account the spatial resolution performance is a key scientific issue and technical bottleneck for improving the performance of the system.

[0005] In summary, the existing Raman optical frequency domain reflection distributed optical fiber sensing system has a technical bottleneck in sensing distance that is difficult to break through, which greatly limits the application of Raman optical frequency domain reflection distributed optical fiber sensing technology. Therefore, it is necessary to improve the existing Raman optical frequency domain reflection distributed optical fiber sensing device and temperature demodulation method. Summary of the invention

[0006] In order to break through the technical bottleneck of traditional Raman optical frequency domain reflection distributed optical fiber sensing system, in which the effective scattered signal at a long sensing distance is submerged in the noise, resulting in a sensing distance far less than the theoretical calculated value, the present invention proposes a chaotic Raman distributed sensing device and method based on optical frequency domain reflection, so as to meet the requirements of Raman optical frequency domain distributed optical fiber sensing technology for long sensing distance and high spatial resolution.

[0007] In order to solve the above technical problems, the technical solution adopted by the present invention is: a chaotic Raman distributed optical fiber sensing device based on optical frequency domain reflection, comprising: a chaotic light source, an optical isolator, an electro-optical modulator, an erbium-doped fiber amplifier, a light splitter, a first photodetector, a wavelength division multiplexer, a sensing optical fiber, a second photodetector, a vector network analyzer, and a computer;

[0008] The chaotic laser emitted by the chaotic light source passes through an optical isolator, an electro-optic modulator, and an erbium-doped fiber amplifier in sequence, and is then divided into two beams by a beam splitter, one of which is received by a first photodetector as a reference light, and the other is entered into a sensing optical fiber after passing through a wavelength division multiplexer as a detection light, and the Raman backward Stokes scattered light signal and the Raman backward anti-Stokes scattered light signal generated at each point along the sensing optical fiber are output by the wavelength division multiplexer and received by the second photodetector;

[0009] The electro-optic modulator is used to modulate the chaotic laser output by the chaotic light source under the drive of the vector network analyzer, so that it outputs a frequency-stepped sinusoidal chaotic laser. The first photodetector and the second photodetector are used to perform photoelectric conversion on the received optical signal and send it to the vector network analyzer. The vector network analyzer is used to collect the reference optical signal at each modulation frequency and the corresponding Raman back-Stokes scattered optical signal and the Raman back-anti-Stokes scattered optical signal. The converted electrical signals are sent to the computer, and the computer is used to calculate and process to obtain the temperature distribution along the sensing optical fiber.

[0010] The calculation formula for the temperature distribution along the sensing optical fiber obtained by computer calculation is:

[0011]

[0012] Where T is the sensing fiber temperature, Δv is the Raman frequency shift, h is the Planck constant, k is the Boltzmann constant, and h S (z) and h AS (z) respectively represent the time domain response functions corresponding to the Raman backscattered Stokes light signal and the Raman backscattered anti-Stokes light signal, Γ S , Γ ASare the capture coefficients of Raman backscattered Stokes signal and Raman backscattered anti-Stokes signal, α S With α AS They represent the power attenuation coefficients of Raman backscattered Stokes signal and Raman backanti-Stokes signal in the sensing optical fiber, respectively, and z represents the sensing distance.

[0013] The calculation formula of the time domain response function corresponding to the Raman backscattered Stokes light signal and the Raman backscattered anti-Stokes light signal is:

[0014] h S (z) = Re{IFFT(H S (ω m ))};

[0015] h AS (z) = Re{IFFT(H AS (ω m ))};

[0016] Where Re represents the real part operation, IFFT represents the inverse fast Fourier transform, and H S (ω m ) and H AS (ω m ) represent the frequency domain transfer functions of Raman backscattered Stokes signal and Raman backscattered anti-Stokes signal respectively, and their construction formula is:

[0017]

[0018] Among them, P MS (ω) and P MAS (ω) represents the maximum value of the intensity of the Raman backscattered Stokes signal and the Raman backscattered anti-Stokes signal collected within a period Q. and It represents the relative phase corresponding to the maximum intensity of the Raman back-Stokes scattered signal and the Raman back-anti-Stokes scattered signal.

[0019] The relative phase is calculated as:

[0020]

[0021] in, represents the phase of the reference light, and They represent the phases corresponding to the Raman backward Stokes scattered light signal and the Raman backward anti-Stokes scattered light signal respectively.

[0022] The chaotic light source comprises: a distributed feedback laser, a circulator, an optical coupler, a polarization controller, and a first attenuator; the distributed feedback laser is incident on the optical coupler via the circulator, the laser output from the first output end of the optical coupler is returned to the circulator after the polarization and intensity are adjusted in sequence by the polarization controller and the first attenuator, and then returned to the distributed feedback laser via the circulator, and its light field is disturbed so that it outputs chaotic laser, and the second output end of the optical coupler is used as the output end of the chaotic light source, and outputs the chaotic laser to the optical isolator.

[0023] The chaotic Raman distributed optical fiber sensing device based on optical frequency domain reflection further includes a second attenuator, which is arranged between the first photodetector and the spectrometer and is used to attenuate the light entering the first photodetector.

[0024] The second photodetector is an avalanche photodetector, which includes two measuring probes, which are respectively used to measure the electrical signals converted from the Raman backward Stokes scattered light signal and the Raman backward anti-Stokes scattered light signal.

[0025] In addition, the present invention also provides a sensing method of a chaotic Raman distributed optical fiber sensing device based on optical frequency domain reflection, comprising the following steps:

[0026] Steps: Control the vector network analyzer to output a frequency-stepped modulation signal;

[0027] Step: collecting the reference light detected by the first photodetector and the second photodetector, the Raman backscattered Stokes light signal and the Raman backanti-Stokes light signal converted into the electrical signal by a vector network analyzer;

[0028] Step 3: Calculate the relative phase of the Raman backscattered Stokes signal and the Raman backscattered anti-Stokes signal in each period, and construct the transmission function of the Raman backscattered Stokes signal and the Raman backscattered anti-Stokes signal in the frequency domain. The construction formula is:

[0029]

[0030] Among them, P MS (ω) and P MAS (ω) represents the maximum value of the intensity of the Raman backscattered Stokes signal and the Raman backscattered anti-Stokes signal collected within a period Q. and represents the relative phase corresponding to the maximum intensity of the Raman backscattered Stokes signal and the Raman backscattered anti-Stokes signal; H S (ωm ) and H AS (ω m ) represent the frequency domain transfer functions of Raman backscattered Stokes signal and Raman backscattered anti-Stokes signal respectively;

[0031] Step 4: According to the transmission function in the frequency domain, the time domain response function of the Raman backscattered Stokes signal and the Raman backscattered anti-Stokes signal is calculated. According to the time domain response function, the temperature distribution along the sensing optical fiber is calculated. The calculation formula is:

[0032]

[0033] Where T is the sensing fiber temperature, Δv is the Raman frequency shift, h is the Planck constant, k is the Boltzmann constant, and h S (z) and h AS (z) respectively represent the time domain response functions corresponding to the Raman backscattered Stokes light signal and the Raman backscattered anti-Stokes light signal, Γ S , Γ AS are the capture coefficients of Raman backscattered Stokes signal and Raman backscattered anti-Stokes signal, α S With α AS They represent the power attenuation coefficients of Raman backscattered Stokes signal and Raman backanti-Stokes signal in the sensing optical fiber, respectively, and z represents the sensing distance.

[0034] In step 3, the calculation formula of the relative phase is:

[0035]

[0036] in, represents the phase of the reference light, and They represent the phases corresponding to the Raman backward Stokes scattered light signal and the Raman backward anti-Stokes scattered light signal respectively.

[0037] In step 4, the calculation formula of the time domain response function of the Raman backscattered Stokes signal and the Raman backscattered anti-Stokes signal is:

[0038] h S (z) = Re{IFFT(H S (ω m ))};

[0039] h AS (z) = Re{IFFT(H AS (ωm ))};

[0040] Wherein, Re represents real part operation, and IFFT represents inverse fast Fourier transform.

[0041] Compared with the existing Raman distributed optical fiber sensing system, the chaotic Raman distributed sensing device and method based on optical frequency domain reflection proposed in the present invention has the following advantages:

[0042] The present invention replaces the traditional continuous light with chaotic light, which has the characteristics of large amplitude noise. The broadband and wide spectrum characteristics of chaotic light greatly reduce the influence of noise on the Raman distributed optical fiber sensing system, thereby improving the signal-to-noise ratio of the system, that is, effective signals can still be detected at a long sensing distance, and due to the characteristics of large amplitude fluctuations in the timing of chaotic light, the stimulated threshold of the Raman spontaneous scattering signal can be greatly increased, thereby greatly increasing the intensity of the Raman scattering signal, thereby achieving effective distributed temperature sensing over a long sensing distance (100km) under the premise of taking into account the resolution at a lower laser power (about 50mW). BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 A schematic structural diagram of a high spatial resolution chaotic Raman distributed optical fiber sensing device based on optical frequency domain reflectometry technology provided by an embodiment of the present invention;

[0044] In the figure: 1: distributed feedback laser, 2: circulator, 3: optical coupler, 4: polarization controller, 5: first attenuator, 6: optical isolator, 7: electro-optic modulator, 8: erbium-doped fiber amplifier, 9: optical splitter, 10: second attenuator, 11: first photodetector, 12: wavelength division multiplexer, 13: sensing fiber, 14: second photodetector, 15: vector network analyzer, 16: computer. DETAILED DESCRIPTION

[0045] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all the embodiments; based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0046] like Figure 1As shown, an embodiment of the present invention provides a chaotic Raman distributed optical fiber sensing device based on optical frequency domain reflection, including: a chaotic light source, an optical isolator 6, an electro-optical modulator 7, an erbium-doped fiber amplifier 8, a splitter 9, a first photodetector 11, a wavelength division multiplexer 12, a sensing fiber 13, a second photodetector 14, a vector network analyzer 15, and a computer 16; the chaotic laser emitted by the chaotic light source passes through the optical isolator 6, the electro-optical modulator 7, and the erbium-doped fiber amplifier 8 in sequence, and is then divided into two beams by the splitter 9, one of which is received by the first photodetector 11 as a reference light, and the other is entered into the sensing fiber 13 after passing through the wavelength division multiplexer 12 as a detection light, and the Raman backward Stokes scattered light signal and the Raman backward anti-Stokes scattered light signal generated at each point along the sensing fiber 13 are output by the wavelength division multiplexer and received by the second photodetector 14.

[0047] The electro-optic modulator 7 is used to modulate the chaotic laser output by the chaotic light source under the drive of the vector network analyzer 15, so that it outputs a frequency-stepped sinusoidal chaotic laser. The first photodetector 11 and the second photodetector 14 are used to perform photoelectric conversion on the received optical signal and send it to the vector network analyzer 15. The vector network analyzer 15 is used to collect the reference optical signal at each modulation frequency and the corresponding Raman back-Stokes scattered optical signal and the Raman back-anti-Stokes scattered optical signal converted into an electrical signal and send it to the computer 16. The computer 16 is used to calculate and process to obtain the temperature distribution along the sensing optical fiber.

[0048] Furthermore, in this embodiment, the second photodetector 14 is an avalanche photodetector, which includes two measuring probes, which are respectively used to measure the Raman backscattered Stokes light signal and the Raman backscattered anti-Stokes light signal.

[0049] Specifically, in this embodiment, the chaotic light source includes: a distributed feedback laser 1, a circulator 2, an optical coupler 3, a polarization controller 4, and a first attenuator 5; the distributed feedback laser 1 is incident on the optical coupler 3 through the circulator 2, and the laser output from the first output end of the optical coupler 3 returns to the circulator 2 after the polarization and intensity are adjusted in sequence by the polarization controller 4 and the first attenuator 5, and then returns to the distributed feedback laser 1 through the circulator 2, disturbing its light field so that it outputs chaotic laser, and the second output end of the optical coupler 3 serves as the output end of the chaotic light source, outputting the chaotic laser to the optical isolator 6.

[0050] The distributed feedback laser 1 is a continuous laser with a central wavelength of 1550 nm. The wavelengths of the generated Raman backscattered Stokes light signal and Raman backscattered anti-Stokes light signal are 1650 nm and 1450 nm respectively.

[0051] Specifically, in this embodiment, the a-port and c-port of the circulator 2 are located at the input end, and the b-port is located at the output end. The output end of the distributed feedback laser 1 is connected to the a-port of the circulator 2, the b-port of the circulator 2 is connected to the a-port of the optical coupler 3, the b-port of the optical coupler 3 is connected to the input end of the optical isolator 6, and the c-port thereof is connected to the input end of the polarization controller 4. In this embodiment, the splitting ratio of the two output ends of the optical coupler 3 is 50:50.

[0052] Specifically, in this embodiment, one output end of the optical splitter 9 is connected to the a port of the wavelength division multiplexer 12, the b port and the c port of the wavelength division multiplexer 12 are respectively connected to the two input ends of the second photodetector 14, and the d port of the wavelength division multiplexer 12 is connected to the sensing optical fiber 13, and the port wavelengths thereof are respectively: the wavelength of the a port is 1550nm, the wavelength of the b port is 1650nm, the wavelength of the c port is 1450nm, and the wavelength of the d port is 1650nm / 1450nm / 1550nm.

[0053] In addition, the chaotic light source in this embodiment may also be a chaotic laser in the prior art. The optical splitter 9 may be an optical fiber coupler, and its splitting ratio is reference light: detection light=1:99.

[0054] Furthermore, in this embodiment, a second attenuator 10 is also included. The second attenuator 10 is arranged between the first photodetector 11 and the beam splitter 9 and is used to attenuate the light entering the first photodetector 11 .

[0055] In this embodiment, the vector network analyzer 15 records the amplitude and phase values ​​of the Raman backward anti-Stokes signal, the Raman backward Stokes signal and the reference signal detected by the first photodetector 11 and the second photodetector 14 at each modulation frequency, and the temperature distribution information along the optical fiber can be obtained through data demodulation. The demodulation principle is as follows: first, a discrete transmission function is constructed, and then the transmission function is inversely transformed by fast Fourier transform to convert it to the time domain, and the temperature distribution information along the optical fiber can be obtained by using the ratio of the time domain response function of the Raman backward Stokes scattering signal and the Raman backward anti-Stokes scattering signal. The above demodulation process can be completed in the computer 16.

[0056] The distributed temperature detection and demodulation principle of the present invention is described in detail below.

[0057] The distributed feedback laser 1 emits a stable continuous laser, which is incident on the circulator 2. The circulator 2 outputs the continuous light and enters the optical coupler 3 to be divided into two beams of light with a light intensity ratio of 50:50. One of the beams passes through the polarization controller 4 and the first attenuator 5, and then returns to the distributed feedback laser 1 through the circulator 2 to disturb the light field of the laser, thereby generating a chaotic laser. Here, the polarization controller 4 and the first attenuator 5 respectively control the polarization and power of the light to ensure the stability of the generated chaotic laser. The chaotic laser generated by this optical feedback method is then incident on the electro-optical modulator 7 through the circulator 2, the optical coupler 3 and the optical isolator 6. At the same time, the electro-optical modulator 7 is modulated by the sinusoidal modulation signal with a frequency step-by-step increase output from the output end of the vector network analyzer 15, and finally outputs a frequency-stepped sinusoidal chaotic optical signal at the output end of the electro-optical modulator 7. Here, the function of the optical isolator 6 is to prevent the chaotic laser from reflecting back and destroying the chaotic state of the chaotic laser. The sinusoidal chaotic signal is amplified by the erbium-doped fiber amplifier 8 and then divided into two beams of light with a light intensity ratio of 1:99 by the optical splitter 9. One beam with a light intensity ratio of 1% enters the second attenuator 10 as a reference light, and the signal after intensity attenuation is detected by the first photodetector 11 and converted into an electrical signal to be collected by the vector network analyzer 15. The light signal with a light intensity ratio of 99% is incident on the sensing optical fiber through the wavelength division multiplexer 12 as a detection light, and a 1650nm Raman backward Stokes scattered light signal and a 1450nm Raman backward anti-Stokes scattered light signal are generated at each point along the sensing optical fiber. The above Raman backscattered light signal is detected by the second photodetector 14 after passing through the wavelength division multiplexer 12 and converted into an electrical signal to be collected by the vector network analyzer 15. When the vector network analyzer 15 collects different modulation frequencies, the electrical signals detected by the first photodetector and the second photodetector are finally sent to the computer 16 for data processing, and the temperature distribution along the sensing optical fiber can be obtained.

[0058] In the traditional Raman OFDR distributed temperature sensing system, the sinusoidal intensity modulated laser power incident at the fiber incident end is:

[0059]

[0060] The power of the collected AC 1650nm Raman backscattered Stokes signal is expressed as:

[0061]

[0062] The power of the collected AC 1450nm Raman backscattered anti-Stokes signal is expressed as:

[0063]

[0064] where ωm represents the modulation frequency, t represents the transmission time, It represents the incident DC optical signal power at the incident end of the sensing optical fiber. represents the amplitude of the incident AC optical signal at the incident end of the sensing optical fiber, Γ S , Γ AS are the capture coefficients of Raman backscattered Stokes signal and Raman backscattered anti-Stokes signal, respectively, S , AS are the Bose-Einstein factors of the Raman backscattered Stokes signal and the Raman backscattered anti-Stokes signal, α0 and α S With α AS They represent the power attenuation coefficients of the incident light, Raman backscattered Stokes signal, and Raman backscattered anti-Stokes signal in the sensing fiber, respectively; z is the sensing distance, L is the fiber length, and n S 、n AS are the refractive indices of the Raman backscattered Stokes light signal and the Raman backscattered anti-Stokes light signal, respectively, and c is the speed of light.

[0065] Since DC light does not change with the modulation frequency, that is, its corresponding scattered light does not carry spatial distribution information, it is sufficient to analyze only the scattered light generated by AC light.

[0066] ρ S , AS The expression is as follows:

[0067]

[0068] Where Δv is the Raman frequency shift, h is the Planck constant, k is the Boltzmann constant, and T is the sensing fiber temperature.

[0069] It can be clearly seen from the expressions of equations (2)-(3) that the Raman back-Stokes scattered signal and the power of the Raman back-Stokes scattered signal are related to both the signal amplitude and the phase.

[0070] The Fourier transform formula is as follows:

[0071]

[0072] Where η is the frequency, F(η) is the frequency domain function, t is the time, f(t) is the space function, exp(-iηt) is the complex function, represents Fourier transform.

[0073] Combining equations (2)-(3) and (6), it can be seen that the Raman backscattered Stokes signal and the power expression of the Raman backscattered Stokes signal can be regarded as a Fourier transform from a spatial function to a frequency domain function.

[0074] When the modulation frequency is ω, the period of the sinusoidal modulation signal is Q (Q = 2π / ω), and the intensity of the reference signal detected by the first photodetector collected by the vector network analyzer is recorded as P R (ω,Q), the phase is recorded as is the initial phase, and the intensity of the 1650nm Raman backscattered Stokes signal detected by the second photodetector is recorded as P S (ω,Q), the phase is recorded as The intensity of the detected 1450nm Raman backscattered anti-Stokes signal is recorded as P AS (ω,Q), the phase is recorded as

[0075] The phase difference between the 1650nm Raman back-Stokes scattering signal and the reference signal is the phase change within the period Q, which is taken as the relative phase of the Raman back-Stokes scattering signal and is recorded as but:

[0076]

[0077] The phase difference between the 1450nm Raman backscattered anti-Stokes signal and the reference signal is the phase change within the period Q, which is taken as the relative phase of the Raman backscattered anti-Stokes signal and is recorded as but:

[0078]

[0079] The maximum value of the intensity of the 1650nm Raman back-Stokes scattering signal collected within a period Q is taken as the amplitude response of the 1650nm Raman back-Stokes scattering signal, denoted as P MS (ω), the relative phase value corresponding to the amplitude response is the phase response, which is recorded as Similarly, the maximum value of the intensity of the 1450nm Raman backscattering anti-Stokes signal collected within a period Q is taken as the amplitude response of the 1450nm Raman backscattering anti-Stokes signal, denoted as P MAS (ω), the relative phase value corresponding to the amplitude response is the phase response, which is recorded as

[0080] The vector network analyzer collects the intensity and phase information of the reference signal, 1650nm Raman back-Stokes scattering signal and 1450nm Raman back-anti-Stokes scattering signal detected by the photodetector and avalanche photodetector at different modulation frequencies, and sends them to the computer. After data processing, the amplitude response and phase response of the Raman back-Stokes scattering signal and the Raman back-anti-Stokes scattering signal at different modulation frequencies can be obtained.

[0081] Using different modulation frequencies ω m The amplitude response and phase response of the Raman back-Stokes scattered signal and the Raman back-anti-Stokes scattered signal under the condition can be used to construct the transmission function of the Raman back-Stokes scattered signal and the Raman back-anti-Stokes scattered signal in the frequency domain:

[0082]

[0083] Among them, H S (ω m ) and H AS (ω m ) represent the frequency domain transfer functions of Raman backscattered Stokes signal and Raman backscattered anti-Stokes signal respectively. MS (ω) and P MAS (ω) represents the maximum value of the intensity of the Raman backscattered Stokes signal and the Raman backscattered anti-Stokes signal collected within a period Q. and It represents the relative phase corresponding to the maximum intensity of the Raman back-Stokes scattered signal and the Raman back-anti-Stokes scattered signal.

[0084] Then, the frequency domain transfer function is subjected to inverse fast Fourier transform, and the real part is taken. Combining equations (2)-(3), the following corresponding time domain response function can be obtained, that is, the frequency domain information is converted into spatial information:

[0085] h S (z) = Re{IFFT(H S (ω m ))}=Γ S ρ S exp(-[(α0+α S )z]); (11)

[0086] h AS (z) = Re{IFFT(H AS (ω m ))}=Γ AS ρAS exp(-[(α0+α AS )z]); (12)

[0087] Among them, h S (z) and h AS (z) represents the time domain response functions corresponding to the Raman backscattered Stokes light signal and the Raman backanti-Stokes light signal, respectively, Re represents the real part operation, and IFFT represents the inverse fast Fourier transform.

[0088] Comparing the time domain response function, we can get:

[0089]

[0090] Therefore, the relationship between temperature T and sensing distance z is obtained:

[0091]

[0092] Where T is the sensing fiber temperature, Δv is the Raman frequency shift, h is the Planck constant, k is the Boltzmann constant, and h S (z) and h AS (z) respectively represent the time domain response functions corresponding to the Raman backscattered Stokes light signal and the Raman backscattered anti-Stokes light signal, Γ S , Γ AS are the capture coefficients of Raman backscattered Stokes signal and Raman backscattered anti-Stokes signal, α S With α AS They represent the power attenuation coefficients of Raman backscattered Stokes signal and Raman backanti-Stokes signal in the sensing optical fiber, respectively, and z represents the sensing distance.

[0093] Therefore, when different modulation frequencies are collected by the vector network analyzer, after the first photodetector and the second photodetector detect the obtained electrical signals, the frequency domain transmission functions of the Raman back-Stokes scattering signal and the Raman back-anti-Stokes scattering signal are constructed by equations (7) to (10), and then the corresponding time domain response functions are calculated by equations (11) to (12). Finally, the temperature at different positions of the entire sensing optical fiber can be demodulated by equation (14).

[0094] Embodiment 2

[0095] Embodiment 2 of the present invention provides a sensing method of a chaotic Raman distributed optical fiber sensing device based on optical frequency domain reflection, comprising the following steps:

[0096] Step 1: Control the vector network analyzer 15 to output a frequency-stepped modulation signal;

[0097] Step 2: The reference light detected by the first photodetector 11 and the second photodetector 14, the Raman backscattered light signal and the Raman backanti-Stokes scattered light signal are collected by the vector network analyzer 15 and converted into electrical signals;

[0098] Step 3: Calculate the relative phase of the Raman backscattered Stokes signal and the Raman backscattered anti-Stokes signal in each period, the calculation formula is the above formula (7) ~ (8), and construct the transmission function of the Raman backscattered Stokes signal and the Raman backscattered anti-Stokes signal in the frequency domain, the construction formula is the above formula (9) ~ (10);

[0099] Step 4: According to the transmission function in the frequency domain, the time domain response function of the Raman back-Stokes scattering signal and the Raman back-anti-Stokes scattering signal is calculated. The calculation formula is the above formula (11) to (12). According to the time domain response function, the temperature distribution along the sensing optical fiber is calculated. The calculation formula is the above formula (14).

[0100] In summary, the present invention proposes a chaotic Raman distributed sensing device and method based on optical frequency domain reflection, which adopts chaotic light as the detection light source, and utilizes the large-amplitude noise-like characteristics of chaotic light. The broadband and wide-spectrum characteristics of chaotic light greatly reduce the influence of noise on the Raman distributed optical fiber sensing system, thereby improving the signal-to-noise ratio of the system. In addition, when constructing the transmission function of the Raman scattering signal, the chaotic light source allows the maximum intensity to be used as the amplitude response of the corresponding scattering signal during data processing. Therefore, the present invention improves the signal intensity, that is, effective signals can still be detected at a long sensing distance, and due to the large-amplitude fluctuation characteristics of the chaotic light sequence, the stimulated threshold of the Raman spontaneous scattering signal can be greatly increased, thereby greatly increasing the Raman scattering signal intensity, thereby realizing effective distributed temperature sensing over a long sensing distance at a relatively low laser power (about 50mW).

[0101] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A chaotic Raman distributed optical fiber sensing device based on optical frequency domain reflection, characterized in that: include: Chaotic light source, optical isolator (6), electro-optic modulator (7), erbium-doped fiber amplifier (8), optical splitter (9), first photodetector (11), wavelength division multiplexer (12), sensing fiber (13), second photodetector (14), vector network analyzer (15), computer (16); The chaotic laser light emitted by the chaotic light source passes through an optical isolator (6), an electro-optic modulator (7), and an erbium-doped fiber amplifier (8) in sequence, and is then divided into two beams by a beam splitter (9), one of which is received by a first photodetector (11) as a reference light, and the other is received by a sensing optical fiber (13) after passing through a wavelength division multiplexer (12) as a detection light, and the Raman backward Stokes scattered light signal and the Raman backward anti-Stokes scattered light signal generated at each point along the sensing optical fiber (13) are output by the wavelength division multiplexer and received by the second photodetector (14); The electro-optic modulator (7) is used to modulate the chaotic laser output by the chaotic light source under the drive of the vector network analyzer (15), so that it outputs frequency-stepped sinusoidal chaotic laser; the first photodetector (11) and the second photodetector (14) are used to perform photoelectric conversion on the received optical signal and send it to the vector network analyzer (15); the vector network analyzer (15) is used to collect the reference optical signal at each modulation frequency and the corresponding Raman back-Stokes scattered optical signal and the Raman back-anti-Stokes scattered optical signal converted into an electrical signal and send it to the computer (16); the computer (16) is used to calculate and process to obtain the temperature distribution along the sensing optical fiber.

2. The chaotic Raman distributed optical fiber sensing device based on optical frequency domain reflection according to claim 1 is characterized in that: The calculation formula for the temperature distribution along the sensing optical fiber obtained by computer calculation is: Where T is the sensing fiber temperature, Δv is the Raman frequency shift, h is the Planck constant, k is the Boltzmann constant, and h S (z) and h AS (z) respectively represent the time domain response functions corresponding to the Raman backscattered Stokes light signal and the Raman backscattered anti-Stokes light signal, Γ S , Γ AS are the capture coefficients of Raman backscattered Stokes signal and Raman backscattered anti-Stokes signal, respectively, S With α AS They represent the power attenuation coefficients of Raman backscattered Stokes signal and Raman backanti-Stokes signal in the sensing optical fiber, respectively, and z represents the sensing distance.

3. The chaotic Raman distributed optical fiber sensing device based on optical frequency domain reflection according to claim 2 is characterized in that: The calculation formula of the time domain response function corresponding to the Raman backscattered Stokes light signal and the Raman backscattered anti-Stokes light signal is: h S (z)=Re{IFFT(H S (ω m ))}; h AS (z)=Re{IFFT(H AS (ω m ))}; Where Re represents the real part operation, IFFT represents the inverse fast Fourier transform, and H S (ω m ) and H AS (ω m ) represent the frequency domain transfer functions of Raman backscattered Stokes signal and Raman backscattered anti-Stokes signal respectively, and their construction formula is: Among them, P MS (ω) and P MAS (ω) represents the maximum value of the intensity of the Raman backscattered Stokes signal and the Raman backscattered anti-Stokes signal collected within a period Q. and It represents the relative phase corresponding to the maximum intensity of the Raman backscattered Stokes signal and the Raman backscattered anti-Stokes signal.

4. The chaotic Raman distributed optical fiber sensing device based on optical frequency domain reflection according to claim 3 is characterized in that: The relative phase is calculated as: in, represents the phase of the reference light, and They represent the phases corresponding to the Raman backward Stokes scattered light signal and the Raman backward anti-Stokes scattered light signal respectively.

5. The chaotic Raman distributed optical fiber sensing device based on optical frequency domain reflection according to claim 1 is characterized in that: The chaotic light source comprises: a distributed feedback laser (1), a circulator (2), an optical coupler (3), a polarization controller (4), and a first attenuator (5); the distributed feedback laser (1) is incident on the optical coupler (3) via the circulator (2); the laser output from the first output end of the optical coupler (3) is adjusted in polarization and intensity by the polarization controller (4) and the first attenuator (5) in sequence and then returns to the circulator (2); and returns to the distributed feedback laser (1) via the circulator (2), disturbing its light field so that it outputs chaotic laser; the second output end of the optical coupler (3) is used as the output end of the chaotic light source, outputting the chaotic laser to the optical isolator (6).

6. The chaotic Raman distributed optical fiber sensing device based on optical frequency domain reflection according to claim 1, characterized in that: The invention also comprises a second attenuator (10), which is arranged between the first photodetector (11) and the optical splitter (9) and is used for attenuating the light entering the first photodetector (11).

7. The chaotic Raman distributed optical fiber sensing device based on optical frequency domain reflection according to claim 1 is characterized in that: The second photodetector (14) is an avalanche photodetector, which comprises two measuring probes, which are respectively used to measure the electrical signals converted from the Raman backscattered Stokes light signal and the Raman backscattered anti-Stokes light signal.

8. The sensing method of a chaotic Raman distributed optical fiber sensing device based on optical frequency domain reflection according to claim 1 is characterized in that: The following steps are involved: Step 1: Control the vector network analyzer (15) to output a frequency-stepped modulation signal; Step 2: collecting the reference light detected by the first photodetector (11) and the second photodetector (14), the Raman backscattered light signal and the Raman backanti-Stokes scattered light signal converted into the electrical signal by means of a vector network analyzer (15); Step 3: Calculate the relative phase of the Raman backscattered Stokes signal and the Raman backscattered anti-Stokes signal in each period, and construct the transmission function of the Raman backscattered Stokes signal and the Raman backscattered anti-Stokes signal in the frequency domain. The construction formula is: Among them, P MS (ω) and P MAS (ω) represents the maximum value of the intensity of the Raman backscattered Stokes signal and the Raman backscattered anti-Stokes signal collected within a period Q. and represents the relative phase corresponding to the maximum intensity of the Raman backscattered Stokes signal and the Raman backscattered anti-Stokes signal; H S (ω m ) and H AS (ω m ) represent the frequency domain transfer functions of Raman backscattered Stokes signal and Raman backscattered anti-Stokes signal respectively; Step 4: According to the transmission function in the frequency domain, the time domain response function of the Raman backscattered Stokes signal and the Raman backscattered anti-Stokes signal is calculated. According to the time domain response function, the temperature distribution along the sensing optical fiber is calculated. The calculation formula is: Where T is the sensing fiber temperature, Δv is the Raman frequency shift, h is the Planck constant, k is the Boltzmann constant, and h S (z) and h AS (z) respectively represent the time domain response functions corresponding to the Raman backscattered Stokes light signal and the Raman backscattered anti-Stokes light signal, Γ S , Γ AS are the capture coefficients of Raman backscattered Stokes signal and Raman backscattered anti-Stokes signal, respectively, S With α AS They represent the power attenuation coefficients of Raman backscattered Stokes signal and Raman backanti-Stokes signal in the sensing optical fiber, respectively, and z represents the sensing distance.

9. The sensing method of the chaotic Raman distributed optical fiber sensing device based on optical frequency domain reflection according to claim 8 is characterized in that: In step 3, the calculation formula of the relative phase is: in, represents the phase of the reference light, and They represent the phases corresponding to the Raman backward Stokes scattered light signal and the Raman backward anti-Stokes scattered light signal respectively.

10. The sensing method of the chaotic Raman distributed optical fiber sensing device based on optical frequency domain reflection according to claim 8, characterized in that: In step 4, the calculation formula of the time domain response function of the Raman backscattered Stokes signal and the Raman backscattered anti-Stokes signal is: h S (z)=Re{IFFT(H S (ω m ))}; h AS (z)=Re{IFFT(H AS (ω m ))}; Wherein, Re represents real part operation, and IFFT represents inverse fast Fourier transform.

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