Differential-photothermal elastic spectroscopy trace gas detection device based on light intensity demodulation

Abstract

This invention discloses a differential-photothermoelastic spectroscopy trace gas detection device based on light intensity demodulation. The device includes an excitation semiconductor laser, a laser collimation system, a gas chamber, a focusing lens, a quartz tuning fork, a probe laser, an fiber optic beam splitter, a photodetector, a signal generator, a laser controller, a lock-in amplifier, and a computer. The quartz tuning fork resonates under the photothermoelastic effect. The probe laser, after being split by the fiber optic beam splitter, acts on both the inner and outer sides of the fork. The laser intensity transmitted through the quartz tuning fork is collected by a photodetector of the same type. When the quartz tuning fork vibrates, the laser intensities passing through the inner and outer sides of the fork generate signals with a phase difference of 180°. These two signals are sent to the lock-in amplifier for differential operation and demodulation. Ideally, after the differential operation, the signal will be amplified by two times, while the noise will cancel each other out. This invention has the advantages of noise immunity, high sensitivity, and low cost.

Description

Technical Field

[0001] This invention relates to a photothermal elastic spectroscopy trace gas detection device, specifically a differential-photothermal elastic spectroscopy trace gas detection device based on light intensity demodulation. Background Technology

[0002] Photothermoelastic spectroscopy based on quartz tuning forks is a highly sensitive trace gas detection technique. The laser wavelength corresponds to the absorption spectral lines of the gas being detected. In experiments, wavelengths with strong absorption lines in the near-infrared region that do not intersect with the absorption lines of gases present in high concentrations in the air, such as oxygen, water, and carbon dioxide, are typically selected. The laser wavelength is adjusted by a controller and periodically modulated by a high-frequency sinusoidal signal source. After collimation, the laser passes through the gas absorption region. The gas absorbs some of the laser energy, and the laser is then focused by a lens onto the root of the quartz tuning fork. The quartz absorbs the remaining laser energy, and under the photothermoelastic effect, it undergoes periodic thermoelastic deformation, generating mechanical vibrations at the same frequency as the sinusoidal modulation. When the sinusoidal modulation frequency equals the resonant frequency of the quartz tuning fork, the quartz tuning fork resonates, amplifying the mechanical vibrations. Due to the piezoelectric effect of the quartz tuning fork, these mechanical vibrations generate a current signal. Harmonic demodulation of this current signal reveals the gas concentration information.

[0003] In traditional photothermoelastic spectroscopy based on quartz tuning forks, laser light is absorbed by the gas and then irradiates the surface of the quartz tuning fork. While the laser induces a photothermoelastic signal, it also introduces thermal noise into the system. As the laser power increases, the thermal noise generated by the electron motion within the quartz tuning fork increases more dramatically, limiting the possibility of achieving a higher signal-to-noise ratio and improving system performance through increased laser power in photothermoelastic spectroscopy. Indirect demodulation methods, on the other hand, do not demodulate the electrical signal of the quartz tuning fork's vibration through its piezoelectric effect. Instead, they demodulate the optical signal by analyzing physical quantities such as the vibration, oscillation, and micro-displacement of the quartz tuning fork. Indirect demodulation methods help to avoid the thermal noise inherent in traditional photothermoelastic spectroscopy. Summary of the Invention

[0004] To address the issue of increasing laser power to achieve a higher signal-to-noise ratio and thus improve system detection performance in photothermal elastic spectroscopy, which is limited by thermal noise, this invention provides a differential-photothermal elastic spectroscopy trace gas detection device based on light intensity demodulation.

[0005] The objective of this invention is achieved through the following technical solution: A differential-photothermoelastic spectroscopy trace gas detection device based on light intensity demodulation includes an excitation semiconductor laser, a laser collimation system, a gas chamber, a focusing lens, a quartz tuning fork, a probe laser, an fiber optic beam splitter, a photodetector, a signal generator, a laser controller, a lock-in amplifier, and a computer. The device comprises: a low-frequency sawtooth wave generated by the signal generator and a high-frequency sine wave generated by the lock-in amplifier superimposed to form a modulation signal for the laser wavelength. This modulation signal is sent to the laser controller, which modulates the excitation semiconductor laser to output laser light. The laser light passes through the laser collimation system and enters the gas chamber. The target gas in the gas chamber absorbs part of the laser energy. After exiting the gas chamber, the laser light is focused onto the two prongs of the quartz tuning fork by the focusing lens. At the base of the finger, due to the modulation of the laser, the quartz tuning fork undergoes periodic elastic deformation under the photothermoelastic effect, causing the interdigitated fingers to oscillate. The fiber optic splitter divides the laser beam output from the probe laser into two equal parts and adjusts the relative position of the laser beam and the quartz tuning fork, so that the laser beam acts on the inner and outer sides of the interdigitated fingers of the quartz tuning fork respectively. After transmission, the probe laser on the inner and outer sides of the interdigitated fingers produces periodically changing light intensities. The light intensities transmitted through the inner and outer sides of the interdigitated fingers are detected by two photodetectors of the same type. Since the light intensity changes on the inner and outer sides of the interdigitated fingers are exactly opposite, the two are sent to the lock-in amplifier for differential operation and subsequent second harmonic demodulation. The harmonic data is input into the computer for processing to invert the gas concentration.

[0006] A method for detecting trace gases using differential-photothermal elastic spectroscopy based on light intensity demodulation using the above-mentioned device includes the following steps: Step 1: Adjust the optical path of the excitation semiconductor laser, laser collimation system, gas cell, focusing lens, and quartz tuning fork in sequence to ensure that the optical path is straight in both the horizontal and vertical directions; Step 2: The laser controller controls the output wavelength and optical power of the excited semiconductor laser by changing the temperature and current, thereby obtaining the temperature and current of the corresponding absorption line of the target gas and setting them correctly. Step 3: Use a computer to control the lock-in amplifier to scan the resonant frequency of the quartz tuning fork, and set half of the obtained resonant frequency as the frequency of the sine wave. Step 4: Adjust the position of the quartz tuning fork in three dimensions to ensure that the laser is incident at the base of the two fingers of the quartz tuning fork. At this time, the amplitude of the photothermal elastic signal is the largest. Step 5: Precisely adjust the relative position of the laser beam output by the detector laser and the quartz tuning fork to ensure that the intensity of the transmitted light detected by the photodetector changes periodically under the swing of the quartz tuning fork. Step Six: The signals detected by the two photodetectors are sent to the lock-in amplifier for differential operation. Then, the differential signal is demodulated using the second harmonic. The computer then uses the peak value of the second harmonic signal to deduce the concentration information of the target gas.

[0007] Compared with the prior art, the present invention has the following advantages: 1. This invention designs a differential-photothermoelastic spectroscopy trace gas detection device based on indirect demodulation of light intensity. A quartz tuning fork resonates under the photothermoelastic effect. The detection laser is split by an optical fiber and applied to the inner and outer sides of the fork. The laser intensity transmitted through the quartz tuning fork is collected by a photodetector of the same type. When the quartz tuning fork vibrates, the laser intensity passing through the inner and outer sides of the fork generates signals with a phase difference of 180°. These two signals are sent to a lock-in amplifier for differential operation and demodulation. Since the two differential signals are 180° phase, ideally, the signal will be amplified by two times after differential operation, while the noise will cancel each other out.

[0008] 2. The differential-photothermal elastic spectroscopy detection device based on indirect demodulation of light intensity of the present invention can improve the signal by up to two times while avoiding traditional thermal noise, thereby improving the signal-to-noise ratio of the photothermal elastic spectroscopy system.

[0009] 3. The differential-photothermal elastic spectroscopy detection device based on indirect light intensity demodulation of the present invention has the advantages of noise immunity, high sensitivity and low cost. Attached Figure Description

[0010] Figure 1 This is a schematic diagram of a differential-photothermal elastic spectroscopy trace gas detection device based on indirect demodulation of light intensity. Figure 2 A schematic diagram illustrating the positions of the laser excitation and detection laser relative to the quartz tuning fork; Figure 3 This is a signal diagram of the differential-photothermoelastic spectrum based on indirect demodulation of light intensity. Detailed Implementation

[0011] 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.

[0012] This invention provides a trace gas detection device based on differential-photothermoelastic spectroscopy with indirect light intensity demodulation, such as... Figure 1As shown, the device includes an excitation semiconductor laser 1, a laser collimation system 2, a gas chamber 3, a focusing lens 4, a quartz tuning fork 5, a detection laser 6, a fiber optic splitter 7, a photodetector 8, a signal generator 9, a laser controller 10, a lock-in amplifier 11, and a computer 12. The low-frequency sawtooth wave generated by the signal generator 9 and the high-frequency sine wave generated by the lock-in amplifier 11 are superimposed to form a modulation signal for the laser wavelength. This signal is sent to the laser controller 10. The modulated excitation semiconductor laser 1 outputs laser light, which is then incident into the gas chamber 3 after passing through the laser collimation system 2. The target gas absorbs part of the laser energy. After the laser light exits from the gas chamber 3, it is focused by the focusing lens 4 onto the root of the fork of the quartz tuning fork 5. Due to the modulation of the laser light, the quartz tuning fork 5 undergoes periodic elastic deformation under the photothermoelastic effect, causing the fork to oscillate. Another detection laser 6 splits the laser beam into two equal parts via an optical fiber splitter 7, and adjusts the relative position of the beam with the quartz tuning fork 5, acting on the inner and outer sides of one side of the forked finger or the outer (inner) side of one side of the forked strand and the inner (outer) side of the forked strand on the other side. The light intensity transmitted through the inner and outer sides of the forked finger of the quartz tuning fork 5 is detected by two photodetectors 8 of the same type. Since the light intensity changes on the inner and outer sides are exactly opposite, both are sent to the lock-in amplifier 11 for differential operation and subsequent harmonic demodulation. The harmonic data is finally input into the computer 12 for processing to invert the gas concentration. Specific implementation process: Step 1: Adjust the optical path of the excitation semiconductor laser 1, laser collimation system 2, gas cell 3, focusing lens 4, and quartz tuning fork 5 in sequence to ensure that the optical path is straight in both the horizontal and vertical directions.

[0013] Step 2: The laser controller 10 controls the output wavelength and optical power of the excitation semiconductor laser 1 by changing the temperature and current, thereby obtaining the temperature and current of the corresponding target gas absorption line and setting them correctly.

[0014] Step 3: Using computer 12 to control lock-in amplifier 11, first scan the resonant frequency of quartz tuning fork 5, and set half of the obtained resonant frequency as the frequency of sine wave.

[0015] Step 4: Adjust the position of the quartz tuning fork 5 in three dimensions to ensure that the laser is incident at the base of the two interdigitated fingers of the quartz tuning fork 5. At this time, the amplitude of the photothermal elastic signal is maximized.

[0016] Step 5: Precisely adjust the relative position of the laser beam of the detection laser 6 and the quartz tuning fork 5 to ensure that the intensity of the transmitted light detected by the photodetector 8 changes periodically under the swing of the quartz tuning fork 5.

[0017] Step Six: The signals detected by the two photodetectors 8 are sent to the lock-in amplifier 11 for differential operation. Then, the differential signal is demodulated by second harmonics. The concentration information of the target gas can be inferred from the peak value of the second harmonic signal.

[0018] In this invention, the excitation semiconductor laser 1 is a near-infrared continuously tunable single longitudinal mode output distributed feedback semiconductor laser or a laser in other wavelength bands.

[0019] In this invention, to improve the system signal-to-noise ratio, a high-power excitation semiconductor laser 1 can be used or the laser power can be amplified, and the laser power should be at least >10 mW.

[0020] In this invention, to improve the system signal, a gas chamber 3 with a long absorption range can be used, or prisms can be installed at both ends of the gas chamber to enhance absorption through multiple reflections. The gas absorption path should be at least >10 cm.

[0021] In this invention, the quartz tuning fork 5 is located in a sealed air chamber, and the air pressure in the sealed air chamber is between 50 and 760 Torr.

[0022] In this invention, to achieve a larger vibration amplitude in the quartz tuning fork 5, the laser emitted by the excitation semiconductor laser 1 should irradiate the root position of the prongs of the quartz tuning fork 5 (e.g., ...). Figure 2 As shown), so that the quartz tuning fork 5 can produce greater elastic deformation.

[0023] In this invention, a high-power probe laser 6 can be used to enhance the light intensity demodulation signal, or the laser power can be amplified. The laser power should be at least >10 mW.

[0024] In this invention, the probe laser 6 can be replaced with any type of tunable laser.

[0025] In this invention, the insertion loss of the fiber optic splitter 7 in the output band of the probe laser 6 should be <0.5 dB.

[0026] In this invention, in order to achieve a signal amplification of two times in subsequent differential operations, the specifications of the fiber optic spectrometer 7 should be 1×2 and the output power distribution should be 1:1.

[0027] In this invention, the relative position between the output end of the fiber optic splitter 7 and the quartz tuning fork 5 needs to be carefully adjusted so that the detection lasers on both the inner and outer sides of the fork of the quartz tuning fork 5 can generate periodically changing light intensity after transmission.

[0028] In this invention, wavelength modulation and second harmonic demodulation techniques are used to reduce system noise. The lock-in amplifier 11 and the signal generator 9 modulate the output wavelength of the excitation semiconductor laser 1. The modulation frequency should be equal to half the resonant frequency of the quartz tuning fork 5.

[0029] In this invention, computer 12 is connected to lock-in amplifier 11 and performs real-time control and signal acquisition and processing through software.

[0030] In the experiment, the photothermal elastic spectroscopy system measured the 2 when the laser beam acted alone on the inner and outer sides of a single prong of a quartz tuning fork. f The signals are 180° out of phase. When a laser beam simultaneously acts on both the inner and outer sides of a single fork of a quartz tuning fork, the 2... (The sentence is incomplete and requires more context to translate accurately.) f Signals such as Figure 3 As shown. By Figure 3 It can be seen that after differential calculation, the signal value is amplified to twice its original value; moreover, when the laser beam acts alone, the phases of the signals are 180° apart. Since the environmental noise from random fluctuations is independent of phase, the noise values ​​cancel each other out after differential calculation. Increasing the signal value and reducing the system noise improves the signal-to-noise ratio of the differential-photothermoelastic spectroscopy system based on light intensity demodulation, which is beneficial for improving the minimum detection limit of the system.

Claims

1. A differential-photothermal elastic spectroscopy trace gas detection device based on light intensity demodulation, characterized in that... The device includes an excitation semiconductor laser, a laser collimation system, a gas chamber, a focusing lens, a quartz tuning fork, a probe laser, a fiber optic splitter, a photodetector, a signal generator, a laser controller, a lock-in amplifier, and a computer. The signal generator generates a low-frequency sawtooth wave, and the lock-in amplifier generates a high-frequency sine wave, which are superimposed to form a modulation signal for the laser wavelength. This modulation signal is sent to the laser controller, which modulates the excitation semiconductor laser to output laser light. The laser light passes through the laser collimation system and enters the gas chamber. The target gas inside the gas chamber absorbs part of the laser energy. After exiting the gas chamber, the laser light is focused by the focusing lens onto the base of the two interdigitated fingers of the quartz tuning fork. Because the laser light is affected by… In the modulation process, the quartz tuning fork undergoes periodic elastic deformation under the photothermoelastic effect, causing the interdigitated fingers to oscillate. The fiber optic splitter divides the laser beam output from the probe laser into two equal parts and adjusts the relative positions of the laser beam and the quartz tuning fork, so that the laser beam acts on the inner and outer sides of the interdigitated fingers of the quartz tuning fork respectively. After transmission, the probe laser on the inner and outer sides of the interdigitated fingers produces periodically changing light intensities. The light intensities transmitted through the inner and outer sides of the interdigitated fingers are detected by two photodetectors of the same type. Since the light intensity changes on the inner and outer sides of the interdigitated fingers are exactly opposite, the two are sent to the lock-in amplifier for differential operation and subsequent second harmonic demodulation. The harmonic data is input into the computer for processing to invert the gas concentration.

2. The differential-photothermal elastic spectroscopy trace gas detection device based on light intensity demodulation according to claim 1, characterized in that... The excitation semiconductor laser is a distributed feedback semiconductor laser with near-infrared continuous tunable single longitudinal mode output.

3. The differential-photothermal elastic spectroscopy trace gas detection device based on light intensity demodulation according to claim 1 or 2, characterized in that... The laser power of the excitation semiconductor laser is at least >10 mW.

4. The differential-photothermal elastic spectroscopy trace gas detection device based on light intensity demodulation according to claim 1, characterized in that... In the gas chamber, the gas absorption path is at least >10 cm.

5. The differential-photothermal elastic spectroscopy trace gas detection device based on light intensity demodulation according to claim 1, characterized in that... The quartz tuning fork is located in a sealed air chamber with an air pressure between 50 and 760 Torr.

6. The differential-photothermal elastic spectroscopy trace gas detection device based on light intensity demodulation according to claim 1, characterized in that... The fiber optic splitter has an insertion loss of <0.5 dB within the output band of the probe laser.

7. The differential-photothermal elastic spectroscopy trace gas detection device based on light intensity demodulation according to claim 1 or 6, characterized in that... The laser power of the detection laser is at least >10 mW.

8. The differential-photothermal elastic spectroscopy trace gas detection device based on light intensity demodulation according to claim 1, characterized in that... The optical fiber splitter has a specification of 1×2 and an output power distribution of 1:

1.

9. The differential-photothermal elastic spectroscopy trace gas detection device based on light intensity demodulation according to claim 1, characterized in that... The lock-in amplifier and signal generator modulate the output wavelength of the excited semiconductor laser, and the modulation frequency is equal to half the resonant frequency of the quartz tuning fork.

10. A method for detecting trace gases using differential-photothermal elastic spectroscopy based on light intensity demodulation 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 the excitation semiconductor laser, laser collimation system, gas cell, focusing lens, and quartz tuning fork in sequence to ensure that the optical path is straight in both the horizontal and vertical directions; Step 2: The laser controller controls the output wavelength and optical power of the excited semiconductor laser by changing the temperature and current, thereby obtaining the temperature and current of the corresponding absorption line of the target gas and setting them correctly. Step 3: Use a computer to control the lock-in amplifier to scan the resonant frequency of the quartz tuning fork, and set half of the obtained resonant frequency as the frequency of the sine wave. Step 4: Adjust the position of the quartz tuning fork in three dimensions to ensure that the laser is incident at the base of the two fingers of the quartz tuning fork. At this time, the amplitude of the photothermal elastic signal is the largest. Step 5: Precisely adjust the relative position of the laser beam output by the detector laser and the quartz tuning fork to ensure that the intensity of the transmitted light detected by the photodetector changes periodically under the swing of the quartz tuning fork. Step Six: The signals detected by the two photodetectors are sent to the lock-in amplifier for differential operation. Then, the differential signal is demodulated using the second harmonic. The computer then uses the peak value of the second harmonic signal to deduce the concentration information of the target gas.

Images