A device and method for optical thermal spectroscopy gas detection based on a tapered optical fiber

Abstract

The application belongs to the technical field of photo-thermal spectrum gas detection, and particularly relates to a photo-thermal spectrum gas detection device and method based on a tapered optical fiber, which comprises an optical fiber gas chamber, a pump laser, a first wavelength division multiplexer, a second wavelength division multiplexer, a circulator, a probe laser, a photoelectric detector, a low-pass filter, a PID feedback control module, a power divider, a processor and a lock-in amplifier. Compared with the existing photo-thermal spectrum gas detection method and device based on a hollow optical fiber, the application can accelerate the gas sensing reaction speed, and on the other hand, due to the fact that the thermal-optic effect and the thermal expansion coefficient of the optical fiber material are significantly greater than those of the gas, the photo-thermal spectrum signal intensity of the tapered optical fiber will be significantly greater than that of the hollow optical fiber.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of photo-thermal spectrum gas detection, and particularly relates to a photo-thermal spectrum gas detection device and method based on a tapered optical fiber. BACKGROUND

[0002] Photo-thermal spectrum is a high-sensitivity gas detection method. In the method, a pump-probe dual light source configuration is used. After pump light is absorbed by gas molecules to be detected, the temperature of the gas region is increased. When probe light passes through the region, the refractive index is changed due to the temperature, and the phase of the probe light is also changed, so that the gas component and concentration information can be detected. The photo-thermal spectrum method has the characteristics that the signal strength is related to the pump light power density. The higher the power density is, the stronger the photo-thermal signal is. Therefore, the current photo-thermal gas detection system usually uses spatial focusing or a hollow-core optical fiber to compress the pump light beam cross-sectional size, so as to improve the optical power density and enhance the photo-thermal signal strength. However, spatial focusing can only converge the light beam at one point, the light beam diverges fast, and the action distance is short. The hollow-core optical fiber is a special optical fiber with a central hole microstructure. The diameter of the central hole is only tens of microns. The central hole is filled with gas to be detected and conducts the light beam to propagate forward. The action distance is determined by the length of the optical fiber. Although the hollow-core optical fiber can improve the photo-thermal action distance to enhance the signal, the internal structure of the hollow-core optical fiber is long and narrow, the gas flow is limited, and the sensing reaction speed is very slow. Although the micro-channel on the side of the optical fiber can accelerate the gas exchange speed, or the gas flow is promoted by using a gas pump or other tools, these methods also increase the system complexity and device cost. SUMMARY

[0003] Based on the above problems, the application provides a photo-thermal spectrum gas detection method and device based on a tapered optical fiber, which can realize high pump light power density, long light-gas action distance, fast gas exchange speed, and fast sensing reaction speed. The technical scheme is as follows:

[0004] An optical thermal spectrum gas detection device based on a tapered fiber, comprising a fiber gas chamber, a first wavelength division multiplexer, a pump laser, a second wavelength division multiplexer, a low-pass filter, a photodetector, a power divider, a probe laser, a circulator, a lock-in amplifier, a processor and a PID feedback control module; the fiber gas chamber is used for placing a tapered fiber, and the gas chamber has an air inlet and an air outlet; one end of the fiber gas chamber is connected to the pump laser through the first wavelength division multiplexer, the other end of the fiber gas chamber is sequentially connected to the second wavelength division multiplexer and the circulator, and the circulator is connected to the probe laser; the first wavelength division multiplexer divides the probe light transmitted from the fiber gas chamber from the light path and transmits it to the photodetector; the photodetector converts the light intensity into an electrical signal, the electrical signal is divided into two paths by the power divider, the first path is connected to the low-pass filter, the PID feedback control module is connected to the control signal input end of the probe laser, and the wavelength of the probe laser is locked at a static operating point; the other path signal of the power divider is connected to the lock-in amplifier, the lock-in amplifier is connected to the processor, the processor outputs a modulation signal to the pump laser, the modulation frequency is f, and the lock-in amplifier demodulates the optical thermal harmonic signal from the power divider signal according to the modulation reference frequency from the processor; the harmonic signal is input to the processor and recorded.

[0005] Preferably, Bragg gratings are machined at both ends of the tapered fiber to form a full-fiber resonant cavity with the tapered fiber as a cavity and the gratings as reflectors.

[0006] Preferably, the full-fiber resonant cavity comprises a left tapered transition zone and a right tapered transition zone, and a waist region between the two zones; a left grating is arranged in the left tapered transition zone, and a right grating is arranged in the right tapered transition zone.

[0007] Preferably, a long-period grating is machined at a central part of the tapered fiber, a center wavelength of a stop band of the long-period grating is set near a center of the probe light, left and right tapered transition zones are respectively arranged at two ends of the tapered fiber, a waist region is between the two zones, and the long-period grating is arranged in the waist region.

[0008] An optical thermal spectrum gas detection based on a tapered fiber, comprising the following steps:

[0009] S1. A gas sample to be measured is filled into a fiber gas chamber;

[0010] S2. A pump laser provides a light source, which enters the fiber gas chamber through a first wavelength division multiplexer; a second wavelength division multiplexer divides the pump light transmitted from the fiber gas chamber from the light path and leaves it unused;

[0011] S3. A light source output by a probe laser enters the fiber gas chamber through a circulator and a second wavelength division multiplexer, and a first wavelength division multiplexer divides the probe light transmitted from the fiber gas chamber from the light path and transmits it to a photodetector;

[0012] S4. The photoelectric detector converts the light intensity into an electrical signal, which is divided into two paths by a power divider, the first path is connected to a low-pass filter, and the control signal input end of the detection laser is connected to a PID feedback control module to lock the wavelength of the detection laser at a static working point;

[0013] S5. The processor outputs a modulation signal to the pump laser; the other path signal of the power divider is connected to a phase-locked amplifier, which demodulates the required optical thermal harmonic signal according to the reference frequency from the processor, and the harmonic signal is input to the processor for recording.

[0014] Preferably, the evanescent field generated by the pump light propagating in the tapered fiber is absorbed by the gas to be measured, and then the photo-thermal effect occurs. For a full-fiber resonant cavity with a tapered fiber as a cavity and a grating as a mirror, the wavelength of the detection light is set at the side of the interference peak. At this position, the intensity of the interferometer and the wavelength are linearly related. When the photo-thermal causes a phase change, the interference peak will move left and right, and the output intensity signal of the interferometer will change. Since the wavelength or intensity of the pump light is modulated at a frequency f, the output intensity signal of the interferometer is also modulated accordingly. The phase-locked amplifier is used to demodulate the modulation harmonic signals 1f, 2f, etc. The peak value of the harmonic signal is proportional to the concentration of the gas to be measured.

[0015] Preferably, the evanescent field generated by the pump light propagating in the tapered fiber is absorbed by the gas to be measured, and then the photo-thermal effect occurs. For a long-period grating processed in the waist region of the tapered fiber, the center wavelength of the stop band is set near the center of the detection light. The light intensity attenuation at the stop band appears a notch. The wavelength of the detection light is set at the side of the notch. At this position, the transmission intensity of the grating and the wavelength are linearly related. When the photo-thermal causes a phase change, the stop band position will move left and right, and the transmission intensity of the detection light through the grating will change. Since the wavelength or intensity of the pump light is modulated at a frequency f, the transmission intensity signal of the grating is also modulated accordingly. The phase-locked amplifier is used to demodulate the modulation harmonic signals 1f, 2f, etc. The peak value of the harmonic signal is proportional to the concentration of the gas to be measured.

[0016] Compared with the prior art, the application has the following advantages:

[0017] Compared with the existing photo-thermal spectrum gas detection method and device based on hollow core optical fiber, the application can speed up the gas sensing reaction speed. On the other hand, due to the thermo-optic effect and the thermal expansion coefficient of the optical fiber material being significantly greater than that of the gas, the photo-thermal spectrum signal intensity of the tapered fiber will be significantly greater than that of the hollow core optical fiber. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 Fig. (a) is a schematic diagram of a structure of a tapered fiber constituting a full-fiber resonant cavity; and Fig. (b) is a schematic diagram of a structure of a long-period grating processed on a tapered fiber.

[0019] Figure 2Fig. 1 is a schematic diagram of the transmission spectrum of F-P interference; Fig. 2 is a schematic diagram of the transmission spectrum of long period grating.

[0020] Figure 3 Fig. 3 is a schematic diagram of the system principle.

[0021] Figure 4 Fig. 4 is a light-thermal spectrum gas detection signal of the tapered fiber.

[0022] Figure 5 Fig. 5 is a comparison of gas detection reaction time.

[0023] In the figure: 1-left tapered transition zone, 2-waist zone, 3-right tapered transition zone, 4-left grating, 5-right grating, 6-long period grating; 7-first wavelength division multiplexer, 8-second wavelength division multiplexer, 9-circulator, 10-probe laser, 11-PID feedback control module, 12-low pass filter, 13-power divider, 14-optoelectronic detector, 15-phase-locked amplifier, 16-processor, 17-fiber gas chamber, 18-pump laser. DETAILED DESCRIPTION

[0024] In order to facilitate the understanding of the present application, the present application will be described more fully below with reference to the accompanying drawings. The embodiments of the present application are shown in the drawings. However, the present application can be realized in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present application more thorough and comprehensive.

[0025] The present application proposes a kind of light-thermal spectrum gas detection method and device based on tapered fiber, tapered fiber is a kind of special fiber, which is to make a section of ordinary optical fiber thin, become conical, processing method generally has flame fusion tapering method, carbon dioxide laser heating tapering method, optical fiber two ends are fixed on motor, middle section is heated and fused, motor moves outward and lengthens optical fiber, middle fusion part will be thin, until reach target size.The thinnest part in tapered fiber is called waist zone, diameter is close to or less than wavelength, part of optical fiber conduction light field will be exposed outside optical fiber, directly contact with external environment, it is called optical fiber evanescent field, the percentage of evanescent field energy in optical fiber guided mode is represented by Γ symbol.

[0026] The basic principle of tapered fiber light-thermal spectrum is that pump light produces absorption effect with external gas to be measured through evanescent field, and the light energy absorbed by the gas to be measured is released in the form of heat and conducted to the optical fiber to cause the optical fiber to heat up. At the same time, probe light also propagates in the tapered fiber, and due to the heating of the optical fiber, according to the thermal-optic effect and thermal expansion phenomenon existing in solid materials, the phase of the probe light will change, which is converted into the change of probe light intensity by phase detection device.

[0027] The first kind, Bragg grating is processed at both ends of the tapered fiber, the reflectivity of the probe light is greater than 99%, Fabry-Perot (F-P) interference is formed, and a full-fiber resonant cavity is constituted with the tapered fiber as the cavity and the grating as the mirror, as shown in Figure 1 (a), which comprises a left tapered transition zone 1, a waist zone 2, a right tapered transition zone 2, a left grating 4 and a right grating 5.

[0028] The transmission spectrum of F-P interference is shown in Figure 2 (a), the wavelength of the probe light is set at the side of the interference peak, and the intensity and wavelength of the interferometer are linearly related at this position. When the phase changes due to the light heat, the interference peak moves left and right, and the output intensity signal of the interferometer is obtained. Since the wavelength or intensity of the pump light is modulated at a frequency f, the output intensity signal of the interferometer is also modulated accordingly. The modulation harmonic signals 1f, 2f, etc. are demodulated using a lock-in amplifier, and the peak value of the harmonic signal is proportional to the concentration of the gas to be measured.

[0029] The second kind, a long-period grating is processed on the tapered fiber, and the center wavelength of the stop band is set near the center of the probe light, as shown in Figure 1 (b), which comprises a left tapered transition zone 1, a waist zone 2, a right tapered transition zone 2, and a long-period grating 6. The transmission spectrum of the long-period grating is shown in Figure 2 (b), and the light intensity attenuation at the stop band appears a notch. The wavelength of the probe light is set at the side of the notch, and the transmission intensity and wavelength of the grating are linearly related at this position. When the phase changes due to the light heat, the position of the stop band moves left and right, and the transmission intensity of the probe light through the grating changes. Since the wavelength or intensity of the pump light is modulated at a frequency f, the transmission light signal of the grating is also modulated accordingly. The modulation harmonic signals 1f, 2f, etc. are demodulated using a lock-in amplifier, and the peak value of the harmonic signal is proportional to the concentration of the gas to be measured.

[0030] Embodiment:

[0031] A single-mode optical fiber made of quartz glass with a cladding diameter of 125 microns is used to produce a tapered fiber with a waist zone length of 2 cm and a diameter of about 600 nm using a hydrogen-oxygen flame tapering machine. The evanescent field energy ratio Γ is measured to be 40%. A Bragg grating fiber is fused to each end of the tapered fiber, the grating reflection center wavelength is 1550 nm, the reflectivity is greater than 99%, and the bandwidth is 5 nm. Fabry-Perot (F-P) interference is formed in the middle of the grating, and a full-fiber resonant cavity is constituted with the tapered fiber as the cavity and the grating as the mirror, as shown in Figure 1 (a), which comprises a left tapered transition zone 1, a waist zone 2, a right tapered transition zone 2, a left grating 4 and a right grating 5. The transmission spectrum of F-P interference is shown in Figure 2As shown in (a), the wavelength of the probe light is set at the side of the interference peak, where the intensity of the interferometer is linearly related to the wavelength. When the photo-thermal effect causes a phase change, the interference peak will move left or right, and the output intensity of the interferometer will change.

[0032] A photo-thermal spectrum gas detection device based on a tapered fiber, Figure 3 As shown in the figure, the tapered fiber is encapsulated in a transparent fiber gas chamber 17, which has an inlet and an outlet for the gas to be tested. The right end of the fiber gas chamber 17 is connected to the pump laser 6 through the first wavelength division multiplexer 7, and the left end of the fiber gas chamber 17 is connected to the second wavelength division multiplexer 8 and the circulator 9 in turn, and the circulator is connected to the probe laser 10. The function of the circulator 9 is to prevent the probe light from being reflected by the fiber gas chamber 17 and entering the probe laser 10, which will cause damage to the device. The reflected light is separated from the optical path by the circulator 9 and is idle. The function of the second wavelength division multiplexer 8 is to separate the optical paths of the probe light and the pump light. The second wavelength division multiplexer 8 separates the pump light transmitted from the fiber gas chamber 17 from the optical path and idles it. The first wavelength division multiplexer 7 separates the probe light transmitted from the fiber gas chamber 17 from the optical path and transmits it to the photodetector 14. The photodetector 14 converts the light intensity into an electrical signal, which is divided into two paths by the power divider 13. The first path is connected to the low-pass filter 12 with a cutoff frequency less than 1 kHz, and the PID feedback control module 11 is connected to the control signal input end of the probe laser 10, which locks the wavelength of the probe laser 10 at the static operating point as shown in the figure. Figure 2 As shown in the figure, the right end of the fiber gas chamber 17 is connected to the pump laser 18, and the other path of the power divider 13 is connected to the phase-locked amplifier 15. The phase-locked amplifier 15 demodulates the photo-thermal harmonic signals 1f, 2f, etc. according to the modulation reference frequency from the processor 16, and the harmonic signals are input to the processor 16 for recording.

[0033] The photo-thermal detection system adopts pump-probe dual light source, the probe light source is 1550nm narrow linewidth laser, the pump light source is 1650nm tunable semiconductor laser, covering methane gas absorption line 1653.7nm. The tapered fiber is packaged in a fiber gas chamber 17, the fiber gas chamber 17 has a gas inlet and outlet, and can import and export the gas to be measured. The right end of the fiber gas chamber 17 is connected with the pump laser 18 through the first wavelength division multiplexer 7, the left end of the fiber gas chamber 17 is connected with the second wavelength division multiplexer 8 and the circulator 9 in turn, and the circulator is connected with the probe laser 10. The function of the circulator 9 is to prevent the probe light from being reflected by the fiber gas chamber 17 and entering the probe laser 10, causing the device to be damaged, and the reflected light is separated from the optical path by the circulator 9 and is idle. The function of the second wavelength division multiplexer 8 is to separate the optical path of the probe light and the pump light, and the transmitted pump light from the fiber gas chamber 17 is separated from the optical path and is idle; the function of the first wavelength division multiplexer 7 is to separate the optical path of the probe light and the pump light, and the transmitted probe light from the fiber gas chamber 17 is separated from the optical path and is transmitted to the photodetector 14. The photodetector 14 converts the light intensity into an electrical signal, the electrical signal is divided into two paths by the power divider 13, the first path is connected with the low-pass filter 12 with a cutoff frequency of 1khz, and the PID feedback control module 11 is connected with the control signal input end of the probe laser 10, so as to lock the wavelength of the probe laser 10 at the static operating point as shown in the figure. Figure 2 The processor 18 outputs a modulation signal to the pump laser 6, and the modulation frequency is 11khz. The other path signal of the power divider 13 is connected with the phase-locked amplifier 15, the phase-locked amplifier 15 demodulates the photo-thermal harmonic signal 1f according to the reference frequency from the processor 16, and the harmonic signal is input to the processor 16 for recording. In the experiment, 2%, 5%, 10% methane / nitrogen mixed gas is respectively introduced into the fiber gas chamber, and the photo-thermal 1f signal changes with the center wavelength of the pump laser as shown in the figure. Figure 4 The peak-to-peak value of the signal is linearly related to the concentration, Figure 4 The right side is the simulated 2f signal image. Then, the methane gas with a volume fraction of 2% and pure nitrogen gas are alternately filled into the sensing device, and the harmonic peak-to-peak value change trend is recorded. It is measured that the sensing device of the tapered fiber can reach a stable state in 1 minute, as a comparison experiment, the tapered fiber is replaced by a hollow core fiber, and other devices and experimental conditions are unchanged, and the sensing device of the hollow core fiber can reach a stable state in 5 minutes, so the tapered fiber has a better response speed than the hollow core fiber. Figure 5 The gas detection reaction time is compared.

[0034] In the description of the specification, the description of the terms "some embodiments", "other embodiments", "ideal embodiments", etc. means that the specific features, structures, materials or characteristics described in connection with the embodiments or examples are included in at least one embodiment or example of the present application. In the description, the illustrative description of the above terms does not necessarily mean the same embodiment or example.

[0035] Each of the technical features of the above-described embodiments can be combined arbitrarily. In order to make the description simple, all possible combinations of each technical feature in the above-described embodiments are not described, however, as long as the combinations of the technical features do not contradict, it should be considered that they are within the scope of the present specification.

Claims

1. A photothermal spectroscopic gas detection device based on tapered optical fiber, characterized in that, The system includes an optical fiber chamber, a first wavelength division multiplexer (WDM), a pump laser, a second WDM, a low-pass filter, a photodetector, a power divider, a probe laser, a circulator, a lock-in amplifier, a processor, and a PID feedback control module. The optical fiber chamber houses the tapered optical fiber and has an inlet and an outlet. One end of the optical fiber chamber is connected to the pump laser via the first WDM, and the other end is connected sequentially to the second WDM and the circulator, with the circulator connected to the probe laser. The first WDM splits the probe light transmitted from the optical fiber chamber into the optical path and transmits it to the light source. In the photodetector, the light intensity is converted into an electrical signal. This electrical signal is split into two paths by a power divider. The first path is connected to a low-pass filter, and a PID feedback control module is connected to the control signal input of the probe laser to lock the wavelength of the probe laser at the static operating point. The other path of the power divider is connected to a lock-in amplifier, which is connected to a processor. The processor outputs a modulation signal to the pump laser at a modulation frequency f. Based on the modulation reference frequency from the processor, the lock-in amplifier demodulates the photothermal harmonic signal from the power divider signal. The harmonic signal is then input to the processor and recorded. Bragg gratings are fabricated at both ends of the tapered optical fiber to form an all-fiber resonant cavity with the tapered optical fiber as the cavity and the gratings as mirrors. The all-fiber resonant cavity includes a left tapered transition region and a right tapered transition region, with a waist region in between. A left grating is located in the left tapered transition region, and a right grating is located in the right tapered transition region. The tapered optical fiber is fabricated with a long-period grating in the central part, and the stopband center wavelength is set near the center of the probe light. The tapered transition region is set at both ends of the tapered optical fiber, and a tapered transition region is set at the right end. The area between the two is the waist region, and the long-period grating is located in the waist region.

2. The detection method of the device according to claim 1, characterized in that, Includes the following steps: S1. Fill the optical fiber gas cell with the gas sample to be tested; S2. The pump laser provides the light source, which enters the fiber optic gas cell after passing through the first wavelength division multiplexer; the second wavelength division multiplexer separates the pump light transmitted from the fiber optic gas cell from the optical path and leaves it idle. S3. The light source output by the probe laser enters the fiber optic gas cell through the circulator and the second wavelength division multiplexer. The first wavelength division multiplexer separates the probe light transmitted from the fiber optic gas cell from the optical path and transmits it to the photodetector. S4. The photodetector converts light intensity into an electrical signal. The electrical signal is split into two paths by the power divider. The first path is connected to a low-pass filter. The PID feedback control module is connected to the control signal input terminal of the detection laser to lock the wavelength of the detection laser at the static operating point. S5. The processor outputs a modulation signal to the pump laser; the other signal from the power divider is connected to a lock-in amplifier. The lock-in amplifier demodulates the required photothermal harmonic signal based on the reference frequency from the processor, and the harmonic signal is input to the processor for recording.

3. The detection method according to claim 2, characterized in that, When the pump light propagates in the tapered fiber, the evanescent field generated is absorbed by the gas to be measured, resulting in a photothermal effect. For an all-fiber resonant cavity with the tapered fiber as the cavity and the grating as the reflector, the probe wavelength is set to the side of the interference peak. Here, the interferometer intensity and wavelength are linearly related. When the photothermal effect causes a phase change, the interference peak shifts left and right, and the interferometer outputs an intensity signal. Since the wavelength or intensity of the pump light is modulated, the modulation frequency f also modulates the interferometer output intensity signal. The modulated harmonic signals 1f and 2f are demodulated using a lock-in amplifier. The peak values ​​of the harmonic signals are proportional to the concentration of the gas to be measured.

4. The detection method according to claim 2, characterized in that, When the pump light propagates in the tapered fiber, the evanescent field generated is absorbed by the gas to be measured, resulting in a photothermal effect. For a long-period grating fabricated in the waist region of the tapered fiber, the stopband center wavelength is set near the center of the probe light. A dip appears at the light intensity attenuation in the stopband. The probe light wavelength is set on the side of the dip. Here, the grating transmitted light intensity and wavelength are linearly related. When photothermal effect causes a phase change, the stopband position will shift left and right, and the transmitted light intensity of the probe light through the grating will change. Since the wavelength or intensity of the pump light is modulated, the modulation frequency f also modulates the grating transmitted light signal accordingly. The modulation harmonic signals 1f and 2f are demodulated using a lock-in amplifier. The peak values ​​of the harmonic signals are proportional to the concentration of the gas to be measured.

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