A distributed optical fiber gas detection device and method based on frequency sweeping technology

Abstract

The present invention discloses a distributed optical fiber gas detection device and method based on frequency sweeping technology, comprising a first laser, a first isolator, a first coupler, an acousto-optic modulator, a sensing optical fiber, a second erbium-doped optical fiber amplifier, a second laser, a second coupler, a polarization scrambler, a third coupler, a fourth coupler, a fifth coupler, a first photodetector, a second photodetector, and a pulse generator. Through the frequency sweeping technology, each detection point on the sensing optical fiber is scanned multiple times to achieve effective suppression of the dead zone. The detection pulse signal generates a backscattered Rayleigh scattering signal in the sensing optical fiber, and the concentration and composition information of the gas along the sensing optical fiber is obtained by detecting the phase information of the reflected Rayleigh scattering signal. The method of the present invention is simple, can realize the detection of gas concentration information with high precision, and has the same applicability to the detection of multiple gases.

Description

Technical Field

[0001] The present invention relates to a gas detection device and method, and in particular to a continuous distributed optical fiber gas detection device and method. Background Art

[0002] The rapid development of my country's economy has brought about serious safety problems. Frequent accidents of all kinds have not only caused personal injuries to relevant practitioners, but also caused huge economic losses to the country and produced adverse social impacts. These accidents basically involve rapid detection and real-time monitoring of hazardous gases and other related issues. In addition, environmental problems have become increasingly prominent in recent years. The greenhouse effect, acid rain, haze and other phenomena caused by air pollution have not only caused direct or indirect economic losses to the society, but also caused serious harm to people's health. Fiber optic sensing technology is a new type of sensing technology that uses optical fiber as a carrier to sense and transmit external measured signals. Compared with traditional electrical sensors, fiber optic sensors have many advantages such as anti-electromagnetic interference and radiation, light weight, insulation, multiple measurement objects, and strong multiplexing. Due to the various advantages of fiber optic sensing technology itself, it has received great attention once it appeared and has attracted the attention of researchers in various fields. Therefore, it is imperative to develop various fiber optic gas sensors with high sensitivity, fast response, and remote positioning detection, which has become the main research content in the field of sensing technology today. At present, the reported fiber optic gas sensors are mainly concentrated on point-type fiber optic gas sensors based on ordinary or photonic crystal fibers, which cannot realize long-distance online gas detection. Therefore, making full use of the advantages of photonic crystal fibers in gas detection and studying distributed fiber optic gas sensor technology has very important academic and application value. In 2014, Li Gang and other researchers proposed a distributed gas sensing system and its control method, with application number 201410708072.1. By controlling the multi-channel optical switch through the main control board, a laser was realized to control multiple channels for gas detection, which greatly reduced the system cost. In 2015, Zheng Guanghui and other researchers proposed a distributed fiber optic sensor, with application number 201510071655.2. The gas detection device and the host part including the laser source, demodulation device, and photoelectric detector were connected by optical fiber. The emitted laser and the reflected laser were transmitted between the above two through the optical fiber, which is suitable for long-distance site gas sensing detection. In 2015, Jin Wei and other researchers proposed a gas detection method and system based on the photothermal effect of hollow-core optical fiber, with application number 201510005210.4. The detection was carried out by using a dual laser scheme of pumping and detection. The method is simple and practical, and can achieve a very small spot area, greatly improving the optical power density, thereby enhancing the intensity of the photothermal signal. In 2018, Gao Jianjun and other researchers proposed a device and method for continuous distributed optical fiber gas detection, with application number 201810144950.X. They used femtosecond processing technology to make small holes on ordinary single-mode optical fiber as gas storage chambers, as the gas and light interaction area on the sensing optical fiber, and the gas absorbed the pump laser signal to produce modulation phenomenon. The back-Rayleigh scattering of the detection laser signal in the optical fiber was used to detect the phase information along the sensing optical fiber where the pump signal produced modulation phenomenon, and the gas concentration information along the sensing optical fiber was realized.Judging from these reports, these inventions are all point-type fiber optic gas sensors, which cannot be called distributed fiber optic gas sensors. Moreover, they cannot simultaneously measure multiple gas parameters along the optical fiber, making them difficult to implement industrially. Summary of the invention

[0003] The purpose of the present invention is to provide a distributed optical fiber gas detection device and method based on frequency sweeping technology, which can not only overcome the shortcomings and deficiencies of existing gas detection technology and realize optical fiber gas detection of distributed gas concentration, but also can achieve fast and high-precision measurement requirements and is easy to implement.

[0004] Technical solution: To achieve the above purpose, the technical solution adopted by the present invention is:

[0005] A distributed optical fiber gas detection device based on frequency sweeping technology comprises a first laser, a first isolator, a first coupler, an acousto-optic modulator, a first erbium-doped optical fiber amplifier, a circulator, a sensing optical fiber, a second erbium-doped optical fiber amplifier, a second isolator, a second laser, a laser controller, a phase-locked amplifier, a second coupler, a polarization scrambler, a third coupler, a fourth coupler, a fifth coupler, a first photodetector, a second photodetector, a signal acquisition card, a signal processing and display unit, a pulse generator, a signal processing and display unit, wherein the first laser, the first isolator, the first coupler, the acousto-optic modulator, the first erbium-doped optical fiber amplifier, the circulator, the sensing optical fiber, the second erbium-doped optical fiber amplifier, the second isolator, the second laser, a laser controller, a phase-locked amplifier, a second coupler, a polarization scrambler, a third coupler, a fourth coupler, a fifth coupler, a first photodetector, a second photodetector, a signal acquisition card, a signal processing and display unit, a pulse generator, a signal processing and display unit, wherein the first laser, the first isolator, the first coupler, the acousto-optic modulator, the first erbium-doped optical fiber amplifier, the circulator, the sensing optical fiber, the second The optical fiber amplifier, the second isolator, the second laser, the laser controller, and the phase-locked amplifier are connected in sequence; the second coupler is respectively connected to the first coupler, the polarization scrambler, and the third coupler; the fourth coupler is respectively connected to the circulator, the third coupler, and the fifth coupler; the fifth coupler is respectively connected to the polarization scrambler and the first photodetector; the second photodetector is respectively connected to the third coupler and the signal acquisition card; the signal processing and display unit is respectively connected to the first laser and the signal acquisition card; the pulse generator is respectively connected to the acousto-optic modulator and the signal acquisition card; the signal acquisition card is respectively connected to the first photodetector, the phase-locked amplifier, and the signal processing and display unit.

[0006] The laser signal emitted by the first laser enters the first coupler through the first isolator. The first coupler divides the laser signal into two signals, namely the first laser signal and the second laser signal. The first laser signal enters the second coupler as a local oscillator signal. The second laser signal enters the acousto-optic modulator. The acousto-optic modulator modulates the second laser signal into a pulse signal, and the frequency of the signal is frequency-shifted and enters the first erbium-doped fiber amplifier. The pulse signal amplified by the first erbium-doped fiber amplifier enters the 1# port of the circulator, and is output from the 2# port of the circulator to enter the sensing optical fiber. In the sensing optical fiber, the pulse signal generates a backscattered Rayleigh signal, and the backscattered Rayleigh signal enters the circulator through the 2# port of the circulator. The back-scattered Rayleigh signal output from the 3# port of the circulator enters the fourth coupler, and the fourth coupler divides the back-scattered Rayleigh signal into two beams of signals, namely, the first beam of back-scattered Rayleigh signal and the second beam of back-scattered Rayleigh signal. The first beam of back-scattered Rayleigh signal enters the third coupler, and the second beam of back-scattered Rayleigh signal enters the fifth coupler; the second coupler divides the local oscillator signal into two beams of signals, namely, the first beam of local oscillator signal and the second beam of local oscillator signal. The first beam of local oscillator signal enters the third coupler, and the first beam of back-scattered Rayleigh signal and the first beam of local oscillator signal are coupled in the third coupler. The coupled signal output by the third coupler enters the first photodetector and is converted into an electrical signal, and the electrical signal is input into the signal acquisition card; The second local oscillator signal output from the second coupler enters the polarization scrambler, and the scrambled local oscillator signal enters the fifth coupler. The scrambled local oscillator signal and the second back-scattered Rayleigh signal are coupled in the fifth coupler. The signal output from the fifth coupler enters the second photodetector and is converted into an electrical signal, which is input into the signal acquisition card. One signal output from the signal acquisition card enters the phase-locked amplifier. The signal output from the phase-locked amplifier is connected to the laser controller. The output signal of the laser controller drives the second laser. The laser signal output from the second laser enters the second erbium-doped fiber amplifier through the second isolator. The signal amplified by the second erbium-doped fiber amplifier enters the sensing fiber. In the sensing fiber, the gas to be measured absorbs the signal from the first The signal output by the Erbium-doped fiber amplifier generates a phase modulation phenomenon, and the pulse signal input from the circulator's 2# port into the sensing optical fiber detects the phase information of the phase modulation phenomenon; the pulse electrical signal generated by the pulse generator is connected to the electrical signal input end of the acousto-optic modulator to drive the acousto-optic modulator to work, the synchronization signal output by the pulse generator and the signal processing and display unit is connected to the synchronization signal input end of the signal acquisition card to keep the signal acquisition card, the acousto-optic modulator and the first laser in a synchronous state, the signal processing and display unit is connected to the first laser to drive the first laser to perform frequency sweep control, and another signal output by the signal acquisition card is connected to the signal processing and display unit to obtain the gas concentration information along the sensing optical fiber.

[0007] Preferably, the first laser and the second laser are both lasers with tunable wavelength and power.

[0008] Preferably, the sensing optical fiber is a hollow core photonic crystal optical fiber.

[0009] Preferably, the sensing optical fiber is provided with small holes along the surface of the optical fiber as passages for gas to enter the hollow-core optical fiber.

[0010] Preferably, the diameter of the channel pore is about 1-10.0 μm.

[0011] Preferably, the first photodetector and the second photodetector are balanced detectors.

[0012] A detection method using a distributed optical fiber gas detection device based on a frequency sweeping technology, wherein a laser signal emitted by a first laser enters a first coupler through a first isolator, the first coupler divides the laser signal into two signals, namely, a first laser signal and a second laser signal, the first laser signal enters a second coupler as a local oscillator signal, the second laser signal enters an acousto-optic modulator, the acousto-optic modulator modulates the second laser signal into a pulse signal, and the frequency of the signal is frequency-shifted, and enters a first erbium-doped fiber amplifier, the pulse signal amplified by the first erbium-doped fiber amplifier enters port 1 of a circulator, and is output from port 2 of the circulator into a sensing optical fiber, in which the pulse signal generates a backscattered Rayleigh signal. The Rayleigh backscattered signal enters the circulator through the 2# port of the circulator, and the back Rayleigh backscattered signal output from the 3# port of the circulator enters the fourth coupler. The fourth coupler divides the back Rayleigh backscattered signal into two beams of signals, namely the first beam of back Rayleigh backscattered signal and the second beam of back Rayleigh backscattered signal. The first beam of back Rayleigh backscattered signal enters the third coupler, and the second beam of back Rayleigh backscattered signal enters the fifth coupler; the second coupler divides the local oscillator signal into two beams of signals, namely the first beam of local oscillator signal and the second beam of local oscillator signal. The first beam of local oscillator signal enters the third coupler, and the first beam of back Rayleigh backscattered signal and the first beam of local oscillator signal are coupled in the third coupler. The coupled signal output by the third coupler enters the first photodetector and is converted into an electrical signal , the electrical signal is input into the signal acquisition card; the second local oscillator signal output from the second coupler enters the polarization scrambler, and the scrambled local oscillator signal enters the fifth coupler, the scrambled local oscillator signal and the second back-scattered Rayleigh signal are coupled in the fifth coupler, and the signal output from the fifth coupler enters the second photodetector and is converted into an electrical signal, and the electrical signal is input into the signal acquisition card; one signal output from the signal acquisition card enters the phase-locked amplifier, and the signal output from the phase-locked amplifier is connected to the laser controller, and the output signal of the laser controller drives the second laser, and the laser signal output from the second laser enters the second erbium-doped fiber amplifier through the second isolator, and the signal amplified by the second erbium-doped fiber amplifier enters the sensing fiber, and in the sensing fiber, wait The measured gas absorbs the signal output from the second erbium-doped fiber amplifier to generate a phase modulation phenomenon, and the pulse signal input from the circulator's 2# port to the sensing optical fiber detects the phase information of the phase modulation phenomenon; the pulse electrical signal generated by the pulse generator is connected to the electrical signal input end of the acousto-optic modulator to drive the acousto-optic modulator to work, the synchronization signal output by the pulse generator and the signal processing and display unit is connected to the synchronization signal input end of the signal acquisition card to keep the signal acquisition card, the acousto-optic modulator and the first laser in a synchronized state, the signal processing and display unit is connected to the first laser to drive the first laser to perform frequency sweep control, and another signal output by the signal acquisition card is connected to the signal processing and display unit to obtain the gas concentration information along the sensing optical fiber.

[0013] Preferably: when displaying data, the relationship between concentration and phase size needs to be calibrated in advance. The curve correlation of the distributed optical fiber gas sensor obtained at different frequencies is expressed as:

[0014]

[0015] The minimum correlation of the curve is C = 0.99, which shows:

[0016]

[0017] Wherein, C is the curve correlation, W is the detection pulse width, and Δf is the frequency difference between the two curves.

[0018] Preferably, the frequency shift of the first laser and the second laser in 1 ms is less than or equal to 0.017M.

[0019] Preferably, the sweep speeds of the first laser and the second laser are less than 17 M / S.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] The present invention uses femtosecond processing technology to make a small hole on a hollow-core photonic crystal fiber as a channel for gas to enter the hollow-core fiber. The gas absorbs the pump laser signal to generate a modulation phenomenon, and then uses the back-Rayleigh scattering of the detection laser signal in the optical fiber to detect the phase information along the sensing optical fiber where the pump signal generates the modulation phenomenon, so as to realize the gas concentration information along the sensing optical fiber. During the measurement process, the first laser is controlled by a computer to perform a slow frequency sweep to eliminate the influence of coherent fading noise on the distributed optical fiber gas detection system and reduce the blind area of ​​the distributed optical fiber gas detection measurement. The detection device has a simple system structure, high result accuracy, and good instrument stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic block diagram of the structure of the device of the present invention;

[0023] Figure 2 It is a schematic diagram of the structure of the sensing optical fiber of the present invention.

[0024] Figure 3 It is a frequency domain diagram of the detected gas position of the present invention.

[0025] Figure 4 It is a measurement curve diagram along the sensing optical fiber of the present invention. DETAILED DESCRIPTION

[0026] The present invention is further explained below in conjunction with the accompanying drawings and specific embodiments. It should be understood that these examples are only used to illustrate the present invention and are not used to limit the scope of the present invention. After reading the present invention, various equivalent forms of modifications to the present invention by those skilled in the art all fall within the scope defined by the claims attached to this application.

[0027] A distributed optical fiber gas detection device based on frequency sweeping technology, such as Figure 1 As shown, it includes a first laser 100, a first isolator 101, a first coupler 102, an acousto-optic modulator 103, a first erbium-doped fiber amplifier 104, a circulator 105, a sensing fiber 106, a second erbium-doped fiber amplifier 107, a second isolator 108, a second laser 109, a laser controller 110, a phase-locked amplifier 111, a second coupler 112, a polarization scrambler 113, a third coupler 114, a fourth coupler 115, a fifth coupler 116, a first photodetector 117, a second photodetector 118, a signal acquisition card 119, a signal processing and display unit 120, a pulse generator 121, and a signal processing and display unit 120, wherein the first laser 100, the first isolator 101, the first coupler 102, the acousto-optic modulator 103, the first erbium-doped fiber amplifier 104, the circulator 105, the sensing fiber 106, the second erbium-doped fiber amplifier 107, the second isolator 108, the second laser 109, a laser controller 110, a phase-locked amplifier 111, a second coupler 112, a polarization scrambler 113, a third coupler 114, a fourth coupler 115, a fifth coupler 116, a first photodetector 117, a second photodetector 118, a signal acquisition card 119, a signal processing and display unit 120, and a pulse generator 121. 7. The second isolator 108, the second laser 109, the laser controller 110, and the phase-locked amplifier 111 are connected in sequence. The second coupler 112 is connected to the first coupler 102, the polarization scrambler 113, and the third coupler 114 respectively. The fourth coupler 115 is connected to the circulator 105, the third coupler 114, and the fifth coupler 116 respectively. The fifth coupler 116 is connected to the polarization scrambler 113 and the first photodetector 117 respectively. The second photodetector 118 is connected to the third coupler 114 and the signal acquisition card 119 respectively. The signal processing and display unit 120 is connected to the first laser 100 and the signal acquisition card 119 respectively. The pulse generator 121 is connected to the acousto-optic modulator 103 and the signal acquisition card 119 respectively. The signal acquisition card 119 is connected to the first photodetector 117, the phase-locked amplifier 111, and the signal processing and display unit 120 respectively.

[0028] The laser signal emitted by the first laser 100 enters the first coupler 102 through the first isolator 101. The first coupler 102 divides the laser signal into two signals, namely, the first laser signal and the second laser signal. The first laser signal enters the second coupler 112 as a local oscillator signal. The second laser signal enters the acousto-optic modulator 103. The acousto-optic modulator 103 modulates the second laser signal into a pulse signal, and the frequency of the signal is frequency-shifted and enters the first erbium-doped fiber amplifier 104. The pulse signal amplified by the first erbium-doped fiber amplifier 104 enters the 1# port of the circulator 105, and is output from the 2# port of the circulator 105 to enter the sensing fiber 106. In the sensing optical fiber 106, the pulse signal generates a back-Rayleigh scattered signal, and the back-Rayleigh scattered signal enters the circulator 105 through the 2# port of the circulator 105. The back-Rayleigh scattered signal output from the 3# port of the circulator 105 enters the fourth coupler 115. The fourth coupler 115 divides the back-Rayleigh scattered signal into two beams of signals, namely, a first beam of back-Rayleigh scattered signal and a second beam of back-Rayleigh scattered signal. The first beam of back-Rayleigh scattered signal enters the third coupler 114, and the second beam of back-Rayleigh scattered signal enters the fifth coupler 116. The second coupler 112 divides the local oscillator signal into two beams of signals, namely, a first beam of local oscillator signal and a second beam of local oscillator signal. The first beam of local oscillator signal enters the third coupler 114, and the second beam of back-Rayleigh scattered signal enters the fifth coupler 116. The first beam of Rayleigh backscattered signal and the first beam of local oscillator signal are coupled in the third coupler 114. The coupled signal output from the third coupler 114 enters the first photodetector 117 and is converted into an electrical signal, which is input into the signal acquisition card 119. The second beam of local oscillator signal output from the second coupler 112 enters the polarization scrambler 113. The polarized local oscillator signal enters the fifth coupler 116. The polarized local oscillator signal and the second beam of Rayleigh backscattered signal are coupled in the fifth coupler 116. The signal output from the fifth coupler 116 enters the second photodetector 118 and is converted into an electrical signal, which is input into the signal acquisition card 119. A signal outputted from the collector card 119 enters the phase-locked amplifier 111. The signal outputted from the phase-locked amplifier 111 is connected to the laser controller 110. The output signal of the laser controller 110 drives the second laser 109. The laser signal outputted from the second laser 109 enters the second erbium-doped fiber amplifier 107 through the second isolator 108. The signal amplified by the second erbium-doped fiber amplifier 107 enters the sensing fiber 106. In the sensing fiber 106, the gas to be measured absorbs the signal outputted from the second erbium-doped fiber amplifier 107, generating a phase modulation phenomenon. The pulse signal inputted from the 2# port of the circulator 105 into the sensing fiber 106 detects the phase information of the phase modulation phenomenon.The pulse electric signal generated by the pulse generator 121 is connected to the electric signal input end of the acousto-optic modulator 103 to drive the acousto-optic modulator 103 to work. The synchronization signal output by the pulse generator 121 and the signal processing and display unit 120 is connected to the synchronization signal input end of the signal acquisition card 119 to keep the signal acquisition card 119, the acousto-optic modulator 103 and the first laser 100 in a synchronous state. The signal processing and display unit 120 is connected to the first laser 100 to drive the first laser 100 to perform frequency sweep control. Another signal output by the signal acquisition card 119 is connected to the signal processing and display unit 120 to obtain the gas concentration information along the sensing optical fiber 106. ;

[0029] The first laser 100 and the second laser 109 are both lasers with tunable wavelength and power. The sensing optical fiber 106 is a hollow core photonic crystal optical fiber.

[0030] The sensing optical fiber uses femtosecond processing technology to make uniformly or non-uniformly distributed channel holes along the optical fiber surface as channels for gas to enter the hollow core optical fiber. The diameter of the channel holes is about 1-10.0 μm.

[0031] The first photodetector 117 and the second photodetector 118 are balanced detectors or other types of photodetectors.

[0032] A detection method using a distributed optical fiber gas detection device based on a frequency sweeping technology, wherein a laser signal emitted by a first laser 100 (narrow linewidth external cavity tunable laser ECDL) enters a first coupler 102 (coupling ratio 80:20) via a first isolator 101, wherein the first laser is a narrow linewidth external cavity tunable laser ECDL, and the output detection signal wavelength is set to 1550.60nm, and the output power is set to 14dBm, and the first coupler 102 divides the laser signal into two signals, namely a first laser signal and a second laser signal, wherein the first laser signal (80%) enters a second coupler 112 as a local oscillator signal, and the second laser signal (20%) enters an acousto-optic modulator 103, wherein the acousto-optic modulator is a Gooch & House Go M040, the fixed frequency shift of the acousto-optic modulator is 200MHz, the period of the pulse light is T=3.3μs, the pulse width is w=100ns, the acousto-optic modulator 103 modulates the second laser signal into a pulse signal, and the frequency of the signal is frequency shifted, and enters the first erbium-doped fiber amplifier 104. The first erbium-doped fiber amplifier is KPS-BT2-C-30-PB-FA, and the output power range is 10-30dBm. The output power is set to 22dBm. The pulse signal amplified by the first erbium-doped fiber amplifier 104 enters the 1# port of the circulator 105, and is output from the 2# port of the circulator 105 to enter the sensing fiber 106. The structure of the sensing fiber is as follows: Figure 2As shown in the figure, the sensing fiber is a 1000m photonic crystal fiber. A circular hole is made on the sensing fiber at intervals of 50m using femtosecond processing technology as a channel hole. The diameter of the circular hole is 6.0μm. The sensing fiber is placed in a chamber with acetylene (C 2 H 2) gas environment, in the sensing optical fiber 106, the pulse signal generates a back Rayleigh scattered signal, the back Rayleigh scattered signal enters the circulator 105 through the 2# port of the circulator 105, and the back Rayleigh scattered signal output from the 3# port of the circulator 105 enters the fourth coupler 115, the coupling ratio of the fourth coupler 115 is 50:50, the fourth coupler 115 divides the back Rayleigh scattered signal into two beams of signals, namely the first beam of back Rayleigh scattered signal and the second beam of back Rayleigh scattered signal, the first beam of back Rayleigh scattered signal enters the third coupler 114 (coupling ratio 50:50), and the second beam of back Rayleigh scattered signal enters the fifth coupler 116 (coupling ratio 50:50); the second coupler 112 (coupling ratio 50:50) divides the local oscillator signal into two beams of signals, namely the first beam of local oscillator signal and the second beam of local oscillator signal, the first beam of local oscillator signal enters the third coupler 114, the first ... third coupler 114, the first beam of back Rayleigh scattered signal enters the fifth coupler 116 (coupling ratio 50:50), and the second beam of back Rayleigh scattered signal enters the fifth coupler 116 (coupling ratio 50:5 The Rayleigh scattered signal and the first beam of local oscillator signal are coupled in the third coupler 114. The coupled signal output by the third coupler 114 enters the first photodetector 117 and is converted into an electrical signal. The first photodetector is a balanced detector with a bandwidth of 350 MHz. The electrical signal is input into the signal acquisition card 119. The second beam of local oscillator signal output from the second coupler 112 enters the polarization scrambler 113. The scrambled local oscillator signal enters the fifth coupler 116. The scrambled local oscillator signal and the second beam of back Rayleigh scattered signal are coupled in the fifth coupler 116. The signal output from the fifth coupler 116 enters the second photodetector 118 and is converted into an electrical signal. The second photodetector is a balanced detector with a bandwidth of 350 MHz. The electrical signal is input into the signal acquisition card 119. One signal output from the signal acquisition card 119 enters the phase-locked amplifier 111. The phase-locked amplifier is SR865ALock-In Amplifier, the signal acquisition card is DAQPCIE 9081, and its sampling rate is 1.25GSa / s. The signal output by the phase-locked amplifier 111 is connected to the laser controller 110. The output signal of the laser controller 110 drives the second laser 109. The second laser is a DFB laser, and its central wavelength is 1527-1610nm, which is used as the output wavelength 1530.371nm, the output power is 0dBm, the laser signal output from the second laser 109 enters the second erbium-doped fiber amplifier 107 through the second isolator 108, the second erbium-doped fiber amplifier is a CEFA-C-BO-HP series C-band high-power continuous erbium-doped fiber amplifier, and the output power is set to 25dBm. The signal amplified by the second erbium-doped fiber amplifier 107 enters the sensing fiber 106. In the sensing fiber 106, the gas to be measured absorbs the signal output from the second erbium-doped fiber amplifier 107, generating a phase modulation phenomenon, and is input from the 2# port of the circulator 105 to the sensing fiber 106. The pulse signal detects the phase information of the phase modulation phenomenon; the pulse electrical signal generated by the pulse generator 121 is connected to the electrical signal input end of the acousto-optic modulator 103 to drive the acousto-optic modulator 103 to work, the pulse generator 121 and the synchronization signal output by the signal processing and display unit (computer) 120 are connected to the synchronization signal input end of the signal acquisition card 119 to keep the signal acquisition card 119, the acousto-optic modulator 103 and the first laser 100 in a synchronous state, the signal processing and display unit 120 is connected to the first laser 100, drives the first laser 100 to perform frequency sweep control, the pulse generator is Agilent 81110A, the frequency of the output signal is 0-330MHz, and the other signal output by the signal acquisition card 119 is connected to the signal processing and display unit 120 to obtain the gas concentration information along the sensing optical fiber 106. When the data is finally displayed, the relationship between the concentration and the phase size needs to be calibrated in advance. The curve correlation of the distributed optical fiber gas sensor obtained at different frequencies can be expressed as.

[0033]

[0034] Where W is the detection pulse width, and Δf is the frequency difference between the two curves. It can be seen from formula (1) that if the laser light source sweeps slowly enough so that the frequency drift is small enough, the measurement curve of the distributed optical fiber sensor can have a high correlation in a short time, and the ability to detect whether there is gas absorption along the sensing optical fiber is not affected. Since the distributed optical fiber sensor must use a light source with small frequency drift and narrow line width, this will introduce strong coherent fading noise when the distributed optical fiber sensor detects the sensing optical fiber. The coherent fading noise can be suppressed by sweeping the frequency. Therefore, the introduction of sweeping the frequency technology can ensure that the distributed optical fiber sensor can measure the phase change along the optical fiber without dead zone, and at the same time detect the loss event of the sensing optical fiber. In order to ensure the measurement performance of the distributed optical fiber sensor for gas detection along the sensing optical fiber, the minimum curve correlation is C = 0.99. It can be concluded from formula (1) that

[0035]

[0036] It can be seen from formula (2) that when the pulse width is determined, the maximum speed of the laser sweep will be determined. The frequency shift of the laser in 1ms is f≤0.0017 / W=0.017M. In the experiment, the pulse width is selected as 100ns, and the laser sweep speed should be less than 0.0017M / 0.0001S=17M / S. In order to effectively suppress the fading noise of the curve, the laser sweep rate is set to 10M / S. The frequency domain diagram of the phase change caused by the absorption of the gas to be measured on the sensing fiber is shown in the figure below. Figure 3 As shown in the figure, it can be seen that when the distributed optical fiber gas sensing system is swept at a speed of 10M / S, the measured signal frequency is 105.5Hz, the signal-to-noise ratio is 12dB, and the distributed optical fiber gas detection curves obtained under different accumulation times are as follows Figure 4 As shown, the coherent fading noise is well suppressed and some tiny loss events can also be observed.

[0037] During the measurement process, the first laser is controlled by a computer to perform a slow frequency sweep to eliminate the influence of coherent fading noise on the distributed optical fiber gas detection system and reduce the blind area of ​​the distributed optical fiber gas detection measurement.

[0038] The present invention uses hollow-core photonic crystal fiber as a sensing fiber, which is the area where the gas and light interact. In order to reduce the dead zone phenomenon caused by the coherent fading effect in the traditional distributed optical fiber gas detection device, each detection point on the sensing fiber is scanned multiple times through the frequency sweeping technology to achieve effective suppression of the dead zone. The signal emitted by the second laser is used as pump light to interact with the gas in the sensing fiber. After the gas absorbs the pump light, it produces a periodic modulation characteristic. The detection signal emitted by the first laser interacts with the periodically modulated gas, so that the phase of the detection signal changes. The detection pulse signal generates a backscattered Rayleigh scattering signal in the sensing fiber. By detecting the phase information of the reflected Rayleigh scattering signal, the concentration and composition information of the gas along the sensing fiber is obtained. The method of the present invention is simple, can realize the detection of gas concentration information with high precision, and has the same applicability to the detection of multiple gases.

[0039] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A distributed optical fiber gas detection device based on frequency sweeping technology, characterized in that: The invention comprises a first laser (100), a first isolator (101), a first coupler (102), an acousto-optic modulator (103), a first erbium-doped fiber amplifier (104), a circulator (105), a sensing fiber (106), a second erbium-doped fiber amplifier (107), a second isolator (108), a second laser (109), a laser controller (110), a phase-locked amplifier (111), a second coupler (112), a polarization scrambler (113), a third coupler (114), a fourth coupler (115), a fifth coupler (116), a first photodetector (117), a second photodetector (118), a signal acquisition card (119), a signal processing and display unit (120), a pulse generator (121), and a signal processing and display unit (120), wherein: The first laser (100), the first isolator (101), the first coupler (102), the acousto-optic modulator (103), the first erbium-doped fiber amplifier (104), the circulator (105), the sensing fiber (106), the second erbium-doped fiber amplifier (107), the second isolator (108), the second laser (109), the laser controller (110), and the phase-locked amplifier (111) are connected in sequence; The second coupler (112) is connected to the first coupler (102), the polarization scrambler (113), and the third coupler (114) respectively; the fourth coupler (115) is connected to the circulator (105), the third coupler (114), and the fifth coupler (116) respectively; the fifth coupler (116) is connected to the polarization scrambler (113) and the first photodetector (117) respectively; the second photodetector (118) is connected to the third coupler (114) and the signal acquisition card (119) respectively; the signal processing and display unit (120) is connected to the first laser (100) and the signal acquisition card (119) respectively; the pulse generator (121) is connected to the acousto-optic modulator (103) and the signal acquisition card (119) respectively; the signal acquisition card (119) is connected to the first photodetector (117), the phase-locked amplifier (111), and the signal processing and display unit (120) respectively; The laser signal emitted by the first laser (100) enters the first coupler (102) through the first isolator (101), and the first coupler (102) divides the laser signal into two signals, namely a first laser signal and a second laser signal. The first laser signal enters the second coupler (112) as a local oscillator signal, and the second laser signal enters the acousto-optic modulator (103), and the acousto-optic modulator (103) modulates the second laser signal into a pulse signal, and the frequency of the signal is frequency-shifted, and enters the first erbium-doped fiber amplifier (104), and passes through the first The pulse signal amplified by the erbium-doped fiber amplifier (104) enters the 1# port of the circulator (105), is output from the 2# port of the circulator (105) and enters the sensing optical fiber (106), in which the pulse signal generates a back-scattered Rayleigh signal, which enters the circulator (105) through the 2# port of the circulator (105), and the back-scattered Rayleigh signal output from the 3# port of the circulator (105) enters the fourth coupler (115), which divides the back-scattered Rayleigh signal into two signal beams. The first beam of Rayleigh backscattered signal enters the third coupler (114), and the second beam of Rayleigh backscattered signal enters the fifth coupler (116); the second coupler (112) divides the local oscillator signal into two beams of signals, namely the first beam of local oscillator signal and the second beam of local oscillator signal, the first beam of local oscillator signal enters the third coupler (114), the first beam of Rayleigh backscattered signal and the first beam of local oscillator signal are coupled in the third coupler (114), and the third coupler (114) outputs the coupled local oscillator signal. The signal enters the first photodetector (117) and is converted into an electrical signal, which is input into a signal acquisition card (119); the second local oscillator signal output from the second coupler (112) enters the polarization scrambler (113), the polarized local oscillator signal enters the fifth coupler (116), the polarized local oscillator signal and the second backscattered Rayleigh signal are coupled in the fifth coupler (116), the signal output from the fifth coupler (116) enters the second photodetector (118) and is converted into an electrical signal, which is input into a signal acquisition card (119);A signal outputted from the signal acquisition card (119) enters a phase-locked amplifier (111), and a signal outputted from the phase-locked amplifier (111) is connected to a laser controller (110). The output signal of the laser controller (110) drives a second laser (109). The laser signal outputted from the second laser (109) enters a second erbium-doped fiber amplifier (107) through a second isolator (108). The signal amplified by the second erbium-doped fiber amplifier (107) enters a sensing fiber (106). In the sensing fiber (106), the gas to be measured absorbs the signal outputted from the second erbium-doped fiber amplifier (107), generating a phase modulation phenomenon. The pulse signal inputted from the 2# port of the circulator (105) into the sensing fiber (106) detects the phase modulation phenomenon. phase information; the pulse electric signal generated by the pulse generator (121) is connected to the electric signal input end of the acousto-optic modulator (103) to drive the acousto-optic modulator (103) to work; the synchronization signal output by the pulse generator (121) and the signal processing and display unit (120) is connected to the synchronization signal input end of the signal acquisition card (119) to keep the signal acquisition card (119), the acousto-optic modulator (103) and the first laser (100) in a synchronous state; the signal processing and display unit (120) is connected to the first laser (100) to drive the first laser (100) to perform frequency sweep control; another signal output by the signal acquisition card (119) is connected to the signal processing and display unit (120) to obtain gas concentration information along the sensing optical fiber (106); When displaying data, it is necessary to calibrate the relationship between concentration and phase size in advance. The curve correlation of the distributed optical fiber gas sensor obtained at different frequencies is expressed as: The minimum correlation of the curve is C = 0.99, which shows: Wherein, C is the curve correlation, W is the detection pulse width, and Δf is the frequency difference between the two curves.

2. According to claim 1, the distributed optical fiber gas detection device based on frequency sweeping technology is characterized in that: The first laser (100) and the second laser (109) are both lasers with tunable wavelength and power.

3. According to claim 2, the distributed optical fiber gas detection device based on frequency sweeping technology is characterized in that: The sensing optical fiber (106) is a hollow-core photonic crystal optical fiber.

4. The distributed optical fiber gas detection device based on frequency sweeping technology according to claim 3 is characterized in that: The sensing optical fiber (106) is provided with small holes along the surface of the optical fiber, which serve as passages for gas to enter the hollow-core optical fiber.

5. The distributed optical fiber gas detection device based on frequency sweeping technology according to claim 4, characterized in that: The channel pore diameter is about 1-10.0 μm.

6. The distributed optical fiber gas detection device based on frequency sweeping technology according to claim 5, characterized in that: The first photodetector (117) and the second photodetector (118) are balanced detectors.

7. The distributed optical fiber gas detection device based on frequency sweeping technology according to claim 6, characterized in that: The frequency shift of the first laser (100) and the second laser (109) in 1 ms is less than or equal to 0.017M.

8. The distributed optical fiber gas detection device based on frequency sweeping technology according to claim 7, characterized in that: The sweeping speed of the first laser (100) and the second laser (109) is less than 17 M / S.

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