Echo-wall mode microcavity-based opto-thermal photoacoustic spectrum trace gas detection device and method

Abstract

The application discloses a kind of based on echo wall mode microcavity's photo-induced thermoelastic spectrum trace gas detection device and method, the device includes signal generator No.1, lock-in amplifier, laser control unit, laser No.1, laser beam collimation unit, gas chamber, focusing lens, tuning fork, echo wall mode microcavity, coupling medium, laser No.2, signal generator No.2, photodetector, computer, laser No.1 output laser is incident to gas chamber in laser beam collimation unit, laser is focused to the fork finger root of tuning fork after being emitted;The laser output by laser No.2 is transmitted by coupling medium and is coupled into echo wall mode microcavity in the form of evanescent wave, echo wall mode microcavity occurs deformation, the resonance wavelength of echo wall mode microcavity in transmission spectrum moves, and the concentration of the gas to be measured in gas chamber is inverted according to the displacement of resonance wavelength.The application solves the problem that quartz tuning fork itself thermal noise is big and weak current signal is susceptible to electromagnetic interference.

Description

Technical Field

[0001] This invention relates to a photothermal elastic spectroscopy trace gas detection device and method, specifically to a photothermal elastic spectroscopy trace gas detection device and method based on a whispering-gallery mode microcavity. Background Technology

[0002] Photothermoelastic spectroscopy is a highly sensitive trace gas detection technique that offers advantages over other laser spectroscopy gas detection techniques, including high selectivity, high sensitivity, fast response speed, and non-contact measurement. This technology overcomes the limitations of traditional detection methods in complex environments such as corrosive gases and high-temperature combustion fields, and has already found applications in environmental monitoring, medical diagnostics, and aerospace.

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

[0004] Traditional demodulation methods directly utilize the electrical signal derived from the tuning fork to demodulate its vibration. However, because the laser irradiates the surface of the quartz tuning fork, the directly demodulated signal is affected by the thermal noise of the tuning fork itself. Thermal noise increases significantly with increasing laser power, severely limiting the improvement of the system's signal-to-noise ratio. Furthermore, the weak current signal generated by the tuning fork is susceptible to interference from external electromagnetic fields, requiring strict shielding.

[0005] An optical resonant cavity is an optical component that localizes and selects the frequency of light waves in both time and space. Its temporal confinement effect is characterized by the quality factor, and its spatial confinement effect by the mode volume. The whispering-gallery mode optical microcavity is one such example. Because electromagnetic waves undergo total internal reflection when propagating from a denser to a less dense medium, in a rotationally symmetric geometry, light propagating along the inner wall undergoes continuous total internal reflection, confining photons within the microcavity for an extended period, thus forming a whispering-gallery mode. Resonance occurs when the optical path length of the beam around the structure's boundary is an integer multiple of the wavelength. Whispering-gallery mode optical microcavities possess ultra-high quality factors (up to 10). 11It possesses superior characteristics such as extremely small mode volume and extremely narrow linewidth. Compared with other optical resonators such as Fabry-Perot cavities, it also has advantages such as simple structure, ease of fabrication, and ease of excitation and detection by optical fibers or waveguides. It has been proven that various crystalline and amorphous materials (such as lithium niobate, calcium fluoride, magnesium fluoride, silicon nitride, silicon dioxide, and silicon) can be used to fabricate whispering-gallery mode microcavities, and the microcavities have various structures such as spherical, ring, and disk shapes.

[0006] When the microcavity is subjected to pressure or vibration, its geometry undergoes slight deformation, leading to the resonance condition (2πn) of the whispering-gallery mode. eff R=mλ m , where n eff λ is the effective refractive index of the mode, m is the angular quantum number, and λ is the effective refractive index of the mode. m The amplitude or pressure magnitude can be deduced by detecting changes in the peak positions of the transmission spectrum (which is the resonant wavelength). In photothermoelastic spectroscopy, a quartz tuning fork vibrates mechanically when irradiated by a laser. This mechanical vibration acts as a sound source, generating a sound field. Therefore, combining photothermoelastic spectroscopy with a whispering-gallery mode microcavity (DMM) allows the MDM to measure the amplitude of the sound waves generated by the mechanical vibration of the tuning fork under laser irradiation. Since the amplitude is inversely proportional to the gas concentration, the amplitude of the tuning fork can be indirectly demodulated by the displacement of the peak positions of the transmission spectrum, thus inverting the concentration of the target gas. Summary of the Invention

[0007] To address the issues of high thermal noise from quartz tuning forks and susceptibility of weak current signals to electromagnetic interference in current photothermal elastic spectroscopy techniques, this invention provides a photothermal elastic spectroscopy trace gas detection device and method based on a whispering-gallery mode microcavity.

[0008] The objective of this invention is achieved through the following technical solution:

[0009] A trace gas detection device based on photothermal elastic spectroscopy using a whispering-gallery mode microcavity includes a signal generator (No. 1), a lock-in amplifier, a laser control unit, a laser (No. 1), a laser beam collimation unit, a gas chamber, a focusing lens, a tuning fork, a whispering-gallery mode microcavity, a coupling medium, a laser (No. 2), a signal generator (No. 2), a photodetector, and a computer. A low-frequency sawtooth wave generated by the signal generator (No. 1) and a high-frequency sine wave generated by the lock-in amplifier are superimposed to form a modulation signal for the laser wavelength. This modulation signal is sent to the laser control unit, and the modulated laser (No. 1) outputs laser light. The laser light is then incident on the gas chamber through the laser beam collimation unit. Inside, after the laser is emitted, it is focused by a focusing lens onto the base of the interdigitated finger of the tuning fork, causing it to undergo thermoelastic deformation and the interdigitated finger to vibrate mechanically. Signal generator 2 sends a modulation signal to laser 2, and the laser output from laser 2 is transmitted through 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 changes in light intensity 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 interdigitated finger, 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 based on the displacement of the resonant wavelength.

[0010] A method for detecting trace gases using photothermoelastic spectroscopy based on a whispering-gallery mode microcavity includes the following steps:

[0011] Step 1: Adjust the optical path of laser No. 1, laser beam collimation unit, gas 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 position of the base of the tuning fork finger.

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

[0013] Step 3: Use a computer-controlled lock-in amplifier to scan the resonant frequency of the tuning fork and set the obtained resonant 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 in five dimensions under a confocal microscope so that the distance between it and the whispering-gallery mode microcavity is less than 10 μm;

[0015] Step 5: Signal generator 2 sends a signal to laser 2, and the output laser is transmitted through 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 a photodetector. The vibration of the tuning fork causes the microcavity deformation of the whispering-gallery mode, resulting in a wavelength shift in the valley of the transmission spectrum.

[0017] Step 7: Change the concentration of the gas to be measured in the chamber from low to high with a fixed step size, and record the wavelength displacement corresponding to each gas concentration. Measure multiple times to 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] This invention utilizes the transmission spectrum variation of a microcavity in whispering-gallery mode to indirectly demodulate the vibration of a tuning fork, thereby obtaining information such as the concentration and type of the gas to be measured. The application of all-optical devices effectively avoids thermal noise and electromagnetic interference, reduces system noise, and improves the sensitivity and signal-to-noise ratio of the gas detection system. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of a photothermoelastic spectroscopy trace gas detection device based on a whispering-gallery mode microcavity.

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

[0022] Figure 3 Front view of the whispering-gallery mode microcavity;

[0023] Figure 4 This diagram shows the positional relationship between the interdigitated and whispering-gallery mode microcavities.

[0024] Figure 5 This is a curve showing the relationship between the valley displacement of the microcavity transmission spectrum and the acetylene gas concentration. Detailed Implementation

[0025] The technical solution of the present invention will be further described below with reference to the accompanying drawings, but it is not limited thereto. Any modifications or equivalent substitutions to the technical solution of the present invention that do not depart from the spirit and scope of the technical solution of the present invention should be covered within the protection scope of the present invention.

[0026] This invention provides a trace gas detection device based on photothermoelastic spectroscopy using a whispering-gallery mode microcavity, such as... Figure 1As shown, 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 signal generator 1 and the high-frequency sine wave generated by 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, and the modulated laser 4 outputs laser light. The laser light enters the gas chamber 6 through the laser beam collimation unit 5, and after exiting, it is focused through a lens. Mirror 7 focuses on the root of the interdigitated finger of the quartz tuning fork 8, causing it to undergo thermoelastic deformation and the interdigitated finger to vibrate mechanically. Signal generator 12 sends a modulation signal to laser 11, which is connected to tapered fiber 10. At its thinnest point, the tapered fiber 10 enters the whispering-gallery mode microcavity 9 as an evanescent wave. The transmission spectrum is detected and collected by photodetector 13, and the intensity change is transmitted to computer 14 for data processing. The whispering-gallery mode microcavity 9, close to the quartz tuning fork 8, undergoes slight deformation due to the interdigitated finger vibration, causing a shift in the resonant wavelength of the whispering-gallery mode microcavity 9 in the transmission spectrum. The concentration of the gas to be measured inside the gas chamber 6 can be retrieved based on the amount of wavelength shift. Specific implementation process:

[0027] Step 1: Adjust the optical path of laser 4, laser beam collimation unit 5, gas chamber 6, focusing lens 7, and quartz tuning fork 8 to ensure that the laser can be incident on the designated position of the device in sequence and finally irradiate the optimal position of the root of the quartz tuning fork 8.

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

[0029] Step 3: Use the software in computer 14 to control the lock-in amplifier 2 to scan the resonant frequency of the quartz tuning fork 8 and set the obtained resonant 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 in five dimensions 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: Signal generator 12 sends a signal with a scanning range of up to tens of GHz to laser 11. The output laser is transmitted by 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 the digital oscilloscope. The vibration of the quartz tuning fork 8 causes the deformation of the whispering-gallery mode microcavity 9, which leads to the wavelength shift of the transmission spectrum valley.

[0033] Step 7: Change the concentration of the gas to be measured in chamber 6 from low to high in fixed step sizes, and record the wavelength displacement corresponding to each gas concentration. Repeat the measurements to obtain the relationship curve between the displacement and the gas concentration. The relationship curve between the valley displacement of the microcavity transmission spectrum and the acetylene gas concentration is shown below. Figure 5 As shown, from Figure 5 It can be seen that there is a near-ideal linear relationship between the two, which can effectively invert gas concentration, proving the effectiveness of the method.

[0034] In this invention, laser 4 is a near-infrared band continuously tunable single longitudinal mode output distributed feedback semiconductor laser with a linewidth not greater than 1MHz, and laser 11 is a tunable continuous wave laser.

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

[0036] In this invention, in order to make the quartz tuning fork 8 vibrate more, the laser should be irradiated at the center of the root of the interdigital fingers. Under the same laser power, irradiating this position can make the tuning fork produce greater elastic deformation.

[0037] In this 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. Momentum mismatch makes it difficult for free-space light to couple into the whispering-gallery mode microcavity 9. Therefore, the gap between the tapered fiber 10 and the whispering-gallery mode microcavity 9 needs to be controlled within 100 nm to achieve near-field coupling. The placement of the two is as follows: Figure 4 As shown.

[0038] In this invention, such as Figures 2-4 As shown, the whispering-gallery mode microcavity 9 is a silica microbubble cavity, requiring high-purity quartz glass to ensure low optical loss and high thermal stability. The specific fabrication method is as follows: a quartz glass capillary is fixed on a support, and the middle of the capillary is locally heated (5-10W) by a carbon dioxide laser, causing it to expand locally into an elliptical microbubble. Subsequently, a secondary polishing process is performed by short-duration high-power laser ablation (15W) to reduce surface defects and improve the quality factor (reaching 10). 8 (Scale). After turning off the laser, allow the microbubbles to cool naturally to room temperature to avoid internal stress and cracks caused by sudden cooling.

[0039] In this invention, the modulation frequency of laser 11 is ≥10GHz.

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

[0041] In this invention, to ensure that the microcavity 9 in the whispering-gallery mode can be deformed by the vibration of the quartz tuning fork, the distance between the fork and the microcavity is not greater than 10 μm by observation and adjustment using a confocal microscope.

[0042] In this invention, a sine wave generated by lock-in amplifier 2 modulates the output wavelength of laser 4, and the modulation frequency is the same as the resonant frequency of the quartz tuning fork.

[0043] In this invention, the photodetector 13 is connected to the computer 14, and the software in the computer controls the signal demodulation unit and completes signal acquisition and data processing.

[0044] In this invention, the whispering-gallery mode microcavity 9 is not limited to... Figures 2-4 Microcavities, including common whispering-gallery mode optical microcavities such as microspheres, microbubbles, microdisks, and microrings, are all suitable for this invention.

[0045] In this invention, the coupling medium between the whispering-gallery mode microcavity 9 and the laser emitted from laser 2 11 is not limited to tapered fiber 10. Waveguides, prisms, or side-polished fibers can all replace tapered fiber 10 for coupling.

[0046] In this invention, the material of the tuning fork is not limited to quartz, but can be other materials with photothermoelastic effect.

[0047] In this invention, tunable semiconductor lasers of different wavelengths can be used depending on the absorption lines of the gas molecules to be measured.

Claims

1. A trace gas detection device based on photothermoelastic spectroscopy using a whispering-gallery mode microcavity, characterized in that... The device includes a signal generator (No. 1), a lock-in amplifier, a laser control unit, a laser (No. 1), a laser beam collimation unit, a gas chamber, a focusing lens, a tuning fork, a whispering-gallery mode microcavity, a coupling medium, a laser (No. 2), a signal generator (No. 2), a photodetector, and a computer. The low-frequency sawtooth wave generated by the signal generator (No. 1) 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, and the modulated laser (No. 1) outputs laser light. The laser light enters the gas chamber through the laser beam collimation unit and is then focused through a focusing lens. A mirror focuses on the base of the interdigitated fingers of the tuning fork, causing thermoelastic deformation and mechanical vibration of the interdigitated fingers. Signal generator #2 sends a modulation signal to laser #2, and the laser output from laser #2 is transmitted through 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 changes in light intensity are transmitted to a computer for data processing. The whispering-gallery mode microcavity close to the tuning fork undergoes slight deformation due to the vibration of the interdigitated fingers, 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 based on the displacement of the resonant wavelength.

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

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

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

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

6. The photothermoelastic spectroscopy trace gas detection device based on a whispering-gallery mode microcavity according to claim 1, characterized in that... The modulation frequency of the second laser is ≥10 GHz.

7. The photothermoelastic spectroscopy trace gas detection device based on a whispering-gallery mode microcavity according to claim 1, characterized in that... The sine wave generated by the lock-in 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 a whispering-gallery mode microcavity according to claim 1, characterized in that... The coupling medium is tapered fiber, waveguide, prism, or side-polished fiber.

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

10. A method for detecting trace gases using photothermoelastic spectroscopy based on a whispering-gallery mode microcavity using the apparatus described in any one of claims 1-9, characterized in that... The method includes the following steps: Step 1: Adjust the optical path of laser No. 1, laser beam collimation unit, gas 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 position of the base of the tuning fork finger. Step 2: Use the laser control unit to change the operating temperature and current of laser No. 1 so that the output wavelength of laser No. 1 matches the gas absorption line; Step 3: Use a computer-controlled lock-in amplifier to scan the resonant frequency of the tuning fork and set the obtained resonant 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 it and the whispering-gallery mode microcavity is less than 10 μm; Step 5: Signal generator 2 sends a signal to laser 2, and the output laser is transmitted through 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 microcavity deformation of the whispering-gallery mode, resulting in a wavelength shift in the valley of the transmission spectrum. Step 7: Change the concentration of the gas to be measured in the chamber from low to high with a fixed step size, and record the wavelength displacement corresponding to each gas concentration. Measure multiple times to obtain the relationship curve between the displacement and the gas concentration.

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