Photo-thermoelastic spectrum trace gas detection device and method based on echo wall mode microcavity

Abstract

The invention discloses a light-induced thermoelastic spectrum trace gas detection device and method based on an echo wall mode microcavity. The device comprises a first signal generator, a lock-in amplifier, a laser device control unit, a first laser device, a laser beam collimation unit, a gas chamber, a focusing lens, a tuning fork, an echo wall mode microcavity, a coupling medium, a second laser device, a second signal generator, a photoelectric detector and a computer, the emergent laser is focused to the interdigital root of the tuning fork through the focusing lens; laser output by the second laser device is transmitted by the coupling medium and coupled into the echo wall mode microcavity in an evanescent wave form, the echo wall mode microcavity deforms, the resonant wavelength of the echo wall mode microcavity in the transmitted spectrum moves, and the concentration of the gas to be detected in the gas chamber is inverted according to the displacement of the resonant wavelength. The problems that a quartz tuning fork is large in thermal noise and weak current signals are easily interfered by an electromagnetic field are solved.

Description

Technical Field

[0001] The present invention relates to a photo-thermal elastic spectroscopy trace gas detection device and method, and particularly to a photo-thermal elastic spectroscopy trace gas detection device and method based on a whispering gallery mode microcavity. Background Art

[0002] Photo-thermal elastic spectroscopy technology is a high-sensitivity trace gas detection technology, which has advantages such as strong selectivity, high sensitivity, fast response speed and non-contact measurement compared with other laser spectroscopy gas detection technologies. This technology breaks through the application limitations of traditional detection methods in complex environments such as corrosive gases and high-temperature combustion fields, and has been applied in the fields of environmental monitoring, medical diagnosis, aerospace, etc.

[0003] Currently, the traditional demodulation method for trace gas detection based on photo-thermal elastic spectroscopy is the intensity demodulation method. The piezoelectric effect of a quartz tuning fork converts mechanical vibration into an alternating current signal, and the signal amplitude is positively correlated with the gas concentration. The lock-in amplifier uses twice the laser modulation frequency as the reference frequency to extract the corresponding second harmonic component in the signal, and establishes a linear relationship between the amplitude of the second harmonic signal and the gas concentration through a calibration experiment. During actual detection, the concentration value is calculated from the signal value based on this linear relationship.

[0004] In the traditional demodulation method, the vibration of the tuning fork is directly demodulated using the electrical signal derived from the tuning fork. However, since the laser irradiates the surface of the quartz tuning fork, the directly demodulated signal will be affected by the thermal noise of the tuning fork itself. The thermal noise will increase significantly with the increase of the laser power, seriously limiting the improvement of the system signal-to-noise ratio. In addition, the weak current signal generated by the tuning fork is vulnerable to external electromagnetic field interference and requires strict shielding.

[0005] An optical resonator refers to an optical component that plays a role in local enhancing light waves and frequency selection in time and space. Its limiting effect in time is characterized by the quality factor, and its limiting effect in space is characterized by the mode volume. The whispering gallery mode optical microcavity is one of them. Since total internal reflection occurs when electromagnetic waves propagate from an optically dense medium to an optically sparse medium, in a geometric structure with rotational symmetry, light rays will undergo continuous total internal reflection when propagating along the inner wall, localizing photons in the microcavity for a long time to form a whispering gallery mode. Resonance occurs when the optical path of the light beam around the structure boundary is an integer multiple of the wavelength. The whispering gallery mode optical microcavity has an ultra-high quality factor (up to 10 11) Superior characteristics such as extremely small mode volume and extremely narrow linewidth. Compared with other optical resonators such as Fabry-Perot resonators, it also has the advantages of simple structure, easy preparation, and being easily excited and detected by optical fibers or waveguides. It has been proven that whispering gallery mode microresonators can be fabricated using a variety of crystalline and amorphous materials (such as lithium niobate, calcium fluoride, magnesium fluoride, silicon nitride, silicon dioxide, and silicon, etc.), and the microcavities have various structures such as spherical, toroidal, and disk-shaped.

[0006] When the microcavity is subjected to pressure or vibration, its geometric structure will undergo a slight deformation, resulting in a change in the resonance condition of the whispering gallery mode (2πn eff R = mλ m , where n eff is the effective refractive index of the mode, m is the angular quantum number, and λ m is the resonance wavelength), and the magnitude of the amplitude or pressure can be deduced by detecting the change amount of the trough peak position in the transmission spectrum. In the photo-thermoelastic spectroscopy technique, when the quartz tuning fork is irradiated by a laser, mechanical vibrations are generated. This mechanical vibration is like a sound source and will generate a sound field. Therefore, by combining the photo-thermoelastic spectroscopy technique with the whispering gallery mode microcavity, the whispering gallery mode microcavity can be used to measure the acoustic wave amplitude generated by the mechanical vibration when the tuning fork is irradiated by the laser. Since the amplitude is inversely proportional to the gas concentration, the amplitude of the tuning fork can be indirectly demodulated by the displacement amount of the trough peak position in the transmission spectrum, and the concentration of the target gas can be inversely obtained. Summary of the Invention

[0007] In order to solve the problems of large thermal noise of the quartz tuning fork itself and the susceptibility of weak current signals to electromagnetic field interference in the current photo-thermoelastic spectroscopy technique, the present invention provides a photo-thermoelastic spectroscopy trace gas detection device and method based on a whispering gallery mode microcavity.

[0008] The object of the present invention is achieved by the following technical solutions:

[0009] An optothermal elastic spectroscopy trace gas detection device based on a whispering gallery mode microcavity, comprising a No. 1 signal generator, a lock-in amplifier, a laser control unit, a No. 1 laser, a laser beam collimation unit, a gas chamber, a focusing lens, a tuning fork, a whispering gallery mode microcavity, a coupling medium, a No. 2 laser, a No. 2 signal generator, a photodetector, and a computer. The low-frequency sawtooth wave generated by the No. 1 signal generator and the high-frequency sine wave generated by the lock-in amplifier are superimposed to form a modulation signal for the laser wavelength. The modulation signal is sent to the laser control unit. The modulated No. 1 laser outputs laser, which is incident into the gas chamber through the laser beam collimation unit. After the laser exits, it is focused by the focusing lens onto the root of the interdigital fingers of the tuning fork, causing it to undergo thermoelastic deformation and the interdigital fingers to vibrate mechanically. The No. 2 signal generator sends a modulation signal to the No. 2 laser. The laser output by the No. 2 laser is transmitted by the coupling medium and coupled into the whispering gallery mode microcavity in the form of an evanescent wave. The transmission spectrum is detected and collected by the photodetector, and the light intensity change is transmitted to the computer for data processing. The whispering gallery mode microcavity close to the tuning fork undergoes a small deformation under the influence of the tuning fork vibration, and the resonant wavelength of the whispering gallery mode microcavity in the transmission spectrum shifts. The concentration of the gas to be measured in the gas chamber is inverted according to the displacement of the resonant wavelength.

[0010] An optothermal elastic spectroscopy trace gas detection method based on a whispering gallery mode microcavity, comprising the following steps:

[0011] Step 1: Adjust the optical paths of the No. 1 laser, the laser beam collimation unit, the gas chamber, the focusing lens, and the tuning fork to ensure that the laser can be incident on the specified positions of the devices in sequence and finally irradiate the optimal action position at the root of the interdigital fingers of the tuning fork.

[0012] Step 2: Use the laser control unit to change the operating temperature and operating current of the No. 1 laser so that the output wavelength of the No. 1 laser coincides with the gas absorption line.

[0013] Step 3: Use the computer to control the lock-in amplifier to scan the resonance frequency of the tuning fork and set the obtained resonance frequency as the frequency of the high-frequency sine wave of the lock-in amplifier.

[0014] Step 4: Adjust the position of the tuning fork five-dimensionally under a confocal microscope so that the distance between it and the whispering gallery mode microcavity is less than 10 μm.

[0015] Step 5: The No. 2 signal generator sends a signal to the No. 2 laser, and the output laser is transmitted by the coupling medium and coupled into the whispering gallery mode microcavity in the form of an evanescent wave.

[0016] Step 6: The output light of the tapered fiber is detected by the photodetector, and the deformation of the whispering gallery mode microcavity caused by the tuning fork vibration results in the drift of the wavelength of the trough of the transmission spectrum.

[0017] Step 7: Change the concentration of the gas to be measured in the gas chamber from low to high at a fixed step size, and record the wavelength shift corresponding to each gas concentration. Through multiple measurements, obtain the relationship curve between the displacement and the gas concentration.

[0018] Compared with the prior art, the present invention has the following advantages:

[0019] The present invention indirectly demodulates the vibration of the tuning fork by using the change of the transmission spectrum of the whispering gallery mode microcavity, so as to obtain information such as the concentration and type of the gas to be measured. The application of all-optical devices can effectively avoid thermal noise and electromagnetic interference, reduce the system noise, and improve the sensitivity and signal-to-noise ratio of the gas detection system. Description of the Drawings

[0020] Figure 1 is a schematic structural diagram of a photo-thermoelastic spectroscopy trace gas detection device based on a whispering gallery mode microcavity;

[0021] Figure 2 is a side view of the whispering gallery mode microcavity;

[0022] Figure 3 is a front view of the whispering gallery mode microcavity;

[0023] Figure 4 is a diagram showing the positional relationship between the interdigital electrodes and the whispering gallery mode microcavity;

[0024] Figure 5 is a relationship curve between the displacement of the trough of the microcavity transmission spectrum and the concentration of acetylene gas. Detailed Embodiment

[0025] The technical solution of the present invention will be further described below in conjunction with the drawings, but it is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention shall be covered by the protection scope of the present invention.

[0026] The present invention provides a photo-thermoelastic spectroscopy trace gas detection device based on a whispering gallery mode microcavity, as Figure 1As shown in the figure, the device includes a signal generator 1, a lock-in amplifier 2, a laser control unit 3, a laser 4, a laser beam collimation unit 5, a gas chamber 6, a focusing lens 7, a quartz tuning fork 8, a whispering gallery mode microcavity 9, a tapered fiber 10, a laser 11, a signal generator 12, a photodetector 13, and a computer 14. The low-frequency sawtooth wave generated by the signal generator 1 and the high-frequency sine wave generated by the lock-in amplifier 2 are superimposed to form a modulation signal for the laser wavelength. The modulation signal is sent to the laser control unit 3. The modulated laser 4 outputs laser light, which is incident into the gas chamber 6 through the laser beam collimation unit 5. After the laser light exits, it is focused by the focusing lens 7 onto the root of the interdigital fingers of the quartz tuning fork 8, causing thermoelastic deformation and mechanical vibration of the interdigital fingers. The signal generator 12 sends a modulation signal to the laser 11. The laser 11 is connected to the tapered fiber 10. The tapered fiber 10 enters the whispering gallery mode microcavity 9 in the form of an evanescent wave at the thinnest part. The transmission spectrum is detected and collected by the photodetector 13, and the change in light intensity is transmitted to the computer 14 for data processing. The whispering gallery mode microcavity 9 close to the quartz tuning fork 8 is slightly deformed under the influence of the vibration of the interdigital fingers, and the resonant wavelength of the whispering gallery mode microcavity 9 in the transmission spectrum shifts. According to the displacement of the resonant wavelength, the concentration of the gas to be measured in the gas chamber 6 can be inversely calculated. The specific implementation process is as follows:

[0027] Step 1: Adjust the optical paths of the laser 4, the laser beam collimation unit 5, the gas chamber 6, the focusing lens 7, and the quartz tuning fork 8 to ensure that the laser can be incident on the specified positions of the devices in sequence and finally irradiate the optimal action position at the root of the interdigital fingers of the quartz tuning fork 8.

[0028] Step 2: Use the laser control unit 3 to change the operating temperature and operating current of the laser 4 so that the output wavelength of the laser 4 coincides with the gas absorption line.

[0029] Step 3: Use the software in the computer 14 to control the lock-in amplifier 2 to scan the resonance frequency of the quartz tuning fork 8 and set the obtained resonance frequency as the frequency of the high-frequency sine wave of the lock-in amplifier 2.

[0030] Step 4: Adjust the position of the quartz tuning fork 8 five-dimensionally under a confocal microscope so that the distance between it and the whispering gallery mode microcavity 9 is less than 10 μm.

[0031] Step 5: The signal generator 12 sends a signal with a scanning range of up to more than a dozen GHz to the laser 11. The output laser is transmitted by the tapered fiber 10 and coupled into the whispering gallery mode microcavity 9 in the form of an evanescent wave.

[0032] Step 6: The output light of the tapered fiber 10 is detected by the photodetector 13 connected to a digital oscilloscope. The deformation of the whispering gallery mode microcavity 9 caused by the vibration of the quartz tuning fork 8 will cause the wavelength of the trough of the transmission spectrum to drift.

[0033] Step 7: Change the concentration of the gas to be measured in the gas chamber 6 from low to high at a fixed step size, and record the wavelength shift corresponding to each gas concentration. After multiple measurements, obtain the relationship curve between the displacement and the gas concentration. The relationship curve between the displacement of the trough of the microcavity transmission spectrum and the concentration of acetylene gas is as Figure 5 shown. From Figure 5 it can be seen that there is an almost ideal linear relationship between the two, which can effectively invert the gas concentration, proving the effectiveness of this method.

[0034] In the present invention, the No. 1 laser 4 is a distributed feedback semiconductor laser with a continuously tunable single longitudinal mode output in the near-infrared band, and its linewidth should not be greater than 1 MHz. The No. 2 laser 11 is a tunable continuous wave laser.

[0035] In the present invention, the quartz tuning fork 8 is obtained by peeling off the external cylindrical metal protective shell of a commercial tuning fork type quartz crystal oscillator, and the characteristic frequency of its symmetric vibration within the interdigital surface does not exceed 35 kHz.

[0036] In the present invention, to make the quartz tuning fork 8 have a larger vibration amplitude, the laser should be irradiated at the center position of the interdigital root. At the same laser power, irradiating at this position can cause a greater elastic deformation of the tuning fork.

[0037] In the present invention, since the whispering gallery mode is based on the principle of total internal reflection, the traveling wave vector in the microcavity is greater than the free space wave vector, and the momentum mismatch makes it difficult for free space light to couple into the whispering gallery mode microcavity 9. Therefore, it is necessary to control the gap between the tapered fiber 10 and the whispering gallery mode microcavity 9 within 100 nm to achieve near-field coupling. Their placement positions are as Figure 4 shown.

[0038] In the present invention, as Figures 2 to 4 shown, the whispering gallery mode microcavity 9 is a silica microbubble cavity, and high-purity quartz glass needs to be selected to ensure low optical loss and high thermal stability. The specific preparation method is as follows: Fix the quartz glass capillary on a bracket, and locally heat (5 - 10 W) the middle part of the capillary by a carbon dioxide laser to make it locally expand into an elliptical microbubble. Subsequently, it is also necessary to perform secondary polishing by burning with a high-power (15 W) laser for a short time to reduce the surface defects of the microbubble and improve the quality factor (reaching the order of 10 8 magnitude). After turning off the laser, the microbubble should be allowed to cool naturally to room temperature to avoid internal stress and cracks caused by rapid cooling.

[0039] In the present invention, the modulation frequency of the No. 2 laser 11 ≥ 10 GHz.

[0040] In the present invention, the quartz tuning fork 8, the whispering gallery mode microcavity 9, and the tapered fiber 10 are placed in a sealed clean container to reduce external interference and contamination.

[0041] In the present invention, to ensure that the whispering gallery mode microcavity 9 can be deformed under the action of the vibrating quartz tuning fork, the distance between the interdigital electrodes and the microcavity is observed and adjusted by a confocal microscope to be no greater than 10 μm.

[0042] In the present invention, the sine wave generated by the lock-in amplifier 2 modulates the output wavelength of the No. 1 laser 4, and the modulation frequency is the same as the resonance frequency of the quartz tuning fork.

[0043] In the present invention, the photodetector 13 is connected to the computer 14, and the software in the computer is used to control the signal demodulation unit and complete signal acquisition and data processing.

[0044] In the present invention, the whispering gallery mode microcavity 9 is not limited to Figures 2 to 4 the microbubble cavity therein. Common whispering gallery mode optical microcavities such as microsphere cavities, microbubble cavities, microdisk cavities, and microring cavities are all applicable to the present invention.

[0045] In the present invention, the coupling medium between the whispering gallery mode microcavity 9 and the laser beam emitted from the No. 2 laser 11 is not limited to the tapered fiber 10, and waveguides, prisms, or side-polished fibers can all replace the tapered fiber 10 for coupling.

[0046] In the present invention, the material of the tuning fork is not limited to quartz and can be other materials with the photo-thermoelastic effect.

[0047] In the present invention, tunable semiconductor lasers with different wavelength bands can be used according to the different absorption lines of the gas molecules to be measured.

Claims

1. A photothermoelastic spectroscopy trace gas detection device based on a whispering gallery mode microcavity, characterized in that The device comprises a No. 1 signal generator, a phase-locked amplifier, a laser control unit, a No. 1 laser, a laser beam collimation unit, an air chamber, a focusing lens, a tuning fork, a whispering gallery mode microcavity, a coupling medium, a No. 2 laser, a No. 2 signal generator, a photodetector, and a computer. The low-frequency sawtooth wave generated by the No. 1 signal generator and the high-frequency sine wave generated by the phase-locked amplifier are superimposed to form a modulation signal of the laser wavelength. The modulation signal is sent to the laser control unit, and the modulated No. 1 laser outputs laser light. The laser light is incident into the air chamber through the laser beam collimation unit, and the laser light is emitted through the focusing lens. The mirror is focused to the base of the tuning fork's fingers, causing them to undergo thermoelastic deformation and mechanical vibration. Signal generator No. 2 sends a modulation signal to laser No.

2. The laser output by laser No. 2 is transmitted by a coupling medium and coupled into the whispering gallery mode microcavity in the form of an evanescent wave. The transmission spectrum is detected and collected by a photodetector, and the light intensity changes are transmitted to a computer for data processing. The whispering gallery mode microcavity close to the tuning fork undergoes a slight deformation due to the vibration of the fingers, and the resonant wavelength of the whispering gallery mode microcavity in the transmission spectrum moves. The concentration of the gas to be measured in the gas chamber is inverted based on the displacement of the resonant wavelength.

2. The photothermoelastic spectroscopy trace gas detection device based on whispering gallery mode microcavity according to claim 1 is characterized in that The laser No. 1 is a near-infrared band continuously tunable single longitudinal mode output distributed feedback semiconductor laser with a line width of no more than 1 MHz, and the laser No. 2 11 is an adjustable continuous wave laser.

3. The photothermoelastic spectroscopy trace gas detection device based on whispering gallery mode microcavity according to claim 1 is characterized in that The characteristic frequency of the in-plane symmetrical vibration of the tuning fork fingers does not exceed 35 kHz.

4. The photothermoelastic spectroscopy trace gas detection device based on whispering gallery mode microcavity according to claim 1 is characterized in that The gap between the coupling medium and the whispering gallery mode microcavity is controlled within 100 nm.

5. The photothermoelastic spectroscopy trace gas detection device based on whispering gallery mode microcavity according to claim 1 is characterized in that The whispering gallery mode microcavity is a microsphere cavity, a microbubble cavity, a microdisk cavity or a microring cavity.

6. The photothermoelastic spectroscopy trace gas detection device based on whispering gallery mode microcavity according to claim 1 is characterized in that The modulation frequency of the No. 2 laser is ≥10 GHz.

7. The photothermoelastic spectroscopy trace gas detection device based on whispering gallery mode microcavity according to claim 1 is characterized in that The sine wave generated by the phase-locked amplifier modulates the output wavelength of laser No. 1, and the modulation frequency is the same as the resonance frequency of the tuning fork.

8. The photothermoelastic spectroscopy trace gas detection device based on whispering gallery mode microcavity according to claim 1 is characterized in that The coupling medium is a tapered optical fiber, a waveguide, a prism or a side-polished optical fiber.

9. The photothermoelastic spectroscopy trace gas detection device based on whispering gallery mode microcavity according to claim 1 is characterized in that The tuning fork is made of a material having a photothermoelastic effect.

10. A method for detecting trace gases by photothermoelastic spectroscopy based on whispering gallery mode microcavity, characterized in that The method comprises the following steps: Step 1: Adjust the optical path of laser No. 1, laser beam collimation unit, air chamber, focusing lens, and tuning fork to ensure that the laser can be incident on the designated position of the device in sequence and finally irradiate the optimal working position of the root of the tuning fork finger; Step 2: Use the laser control unit to change the operating temperature and operating current of laser No. 1 so that the output wavelength of laser No. 1 coincides with the gas absorption line; Step 3: Use a computer to control the lock-in amplifier to scan the resonance frequency of the tuning fork and set the obtained resonance frequency as the frequency of the high-frequency sine wave of the lock-in amplifier; Step 4: Adjust the position of the tuning fork in five dimensions under a confocal microscope so that the distance between the tuning fork and the whispering gallery mode microcavity is less than 10 μm; Step 5: Signal generator No. 2 sends a signal to laser No. 2, and the output laser is transmitted by the coupling medium and coupled into the whispering gallery mode microcavity in the form of an evanescent wave; Step 6: The output light of the tapered fiber is detected by a photodetector. The vibration of the tuning fork causes the deformation of the whispering gallery mode microcavity, resulting in the wavelength drift of the transmission spectrum trough. Step 7: Change the concentration of the gas to be measured in the gas chamber from low to high with a fixed step length, and record the wavelength displacement corresponding to each gas concentration. Measure multiple times to obtain a relationship curve between the displacement and the gas concentration.

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