Heterodyne-enhanced photothermoelastic spectroscopy trace gas detection device and method

Abstract

The present invention discloses a heterodyne-enhanced photothermoelastic spectroscopy trace gas detection device and method. The device includes a first semiconductor laser, a second semiconductor laser, a first optical collimation system, a second optical collimation system, a gas chamber, a quartz tuning fork, an impedance amplifier, a laser control and data acquisition system, and a computer. The present invention improves the system signal-to-noise ratio from the perspective of increasing the signal, effectively increasing the measurement signal amplitude in two ways: on the one hand, the incidence of the dual light beam causes the quartz tuning fork to absorb higher power during damped oscillation, thereby increasing the piezoelectric signal; on the other hand, the new modulation signal can still participate in the generation of the beat frequency signal without changing its frequency, so that the beat frequency signal is significantly enhanced after superposition, thereby effectively improving the system signal-to-noise ratio. The present invention has the advantages of high sensitivity, fast response speed, and low cost.

Description

Technical Field

[0001] The present invention relates to a photothermoelastic spectroscopy gas detection device and method, and in particular to a heterodyne-enhanced dual-beam photothermoelastic spectroscopy gas detection device and method. Background Art

[0002] Photothermoelastic spectroscopy (PTES) is a trace gas detection technique based on the light absorption properties of a quartz tuning fork. Its excellent selectivity, high sensitivity, and contactless online measurement make it a promising technology for rapid gas detection. For gas sensors, the response time of the system is a key factor in determining their ability to rapidly detect gases. Therefore, optimizing the response mode of the PTS system and developing rapid signal demodulation methods are crucial for achieving rapid detection. Furthermore, to maximize the detection performance of PTS sensors, improving the system's signal-to-noise ratio and achieving a better detection limit are also key research areas.

[0003] Traditional photothermoelastic spectroscopy involves injecting a collimated, tunable laser into a gas chamber. Part of the laser's energy is absorbed by the gas being measured, and the remaining laser light strikes an external quartz tuning fork. The quartz tuning fork absorbs the remaining energy, causing it to undergo a transformation from light absorption to thermal expansion to elastic deformation. The laser energy striking the tuning fork undergoes periodic modulation, which in turn causes the elastic deformation of the tuning fork to also vary periodically, generating vibrations. When the laser's modulation frequency matches the tuning fork's resonant frequency, the tuning fork resonates and amplifies the vibrations, resulting in a correspondingly larger electrical signal generated by the tuning fork's piezoelectric effect. Demodulating this electrical signal allows the gas concentration to be retrieved.

[0004] Heterodyne quartz spectroscopy utilizes high-speed modulation of a tunable laser to induce a transient response from a quartz tuning fork, generating pulsed signals to be measured. In heterodyne quartz spectroscopy, the modulation frequency of the drive current is typically f = f0 ± Δf. Over a period of time, the vibrating quartz tuning fork undergoes damped oscillations due to a combination of extrinsic and intrinsic losses until the accumulated energy is depleted. In the absence of effective external excitation, the frequency of the damped oscillations of the quartz tuning fork is equal to its resonant frequency, meaning that the frequency of the piezoelectric signal generated by the quartz tuning fork is equal to its resonant frequency. If the piezoelectric signal generated by the quartz tuning fork is demodulated at a frequency that differs by a certain frequency difference Δf from the resonant frequency f0 of the quartz tuning fork, a beat frequency signal with a frequency of Δf is obtained. This beat frequency signal can be used to infer gas concentration information.

[0005] Existing heterodyne measurement is primarily used in quartz-enhanced photoacoustic spectroscopy (QEPAS). Its key feature is that it uses a control signal modulated with a frequency of f = f0 - Δf or f = f0 + Δf to generate a beat signal with the intrinsic response of the tuning fork. Gas concentration information is then inverted using the amplitude of the first harmonic of the beat frequency. According to the definition of the beat frequency, the generated beat signal amplitude ranges from |A1-A2| to A1+A2, where A1 and A2 represent the amplitudes of the two signals, respectively. Because the quartz tuning fork produces damped oscillations upon excitation, its amplitude decreases exponentially with time and is relatively low, resulting in a poor signal-to-noise ratio and limited detection capabilities. Summary of the Invention

[0006] To address the problem of small beat frequency signals and poor signal-to-noise ratios in conventional heterodyne trace gas sensors, which hinder improvements in the system's minimum detection limit, the present invention redesigns conventional heterodyne systems and proposes a heterodyne-enhanced photothermoelastic spectroscopy trace gas detection device and method. This device uses two laser beams modulated at frequencies f = f0 - Δf and f = f0 + Δf to detect the same target gas. These two laser beams are incident on either side of a quartz tuning fork, stimulating the fork to oscillate and generating an enhanced beat frequency signal. Gas concentration is then detected based on the enhanced beat frequency signal. This device offers the advantages of high sensitivity, fast response, and low cost.

[0007] The purpose of the present invention is achieved through the following technical solutions:

[0008] A heterodyne-enhanced photothermoelastic spectroscopy trace gas detection device comprises a first semiconductor laser, a second semiconductor laser, a first optical collimation system, a second optical collimation system, a gas chamber, a quartz tuning fork, an impedance amplifier, a laser control and data acquisition system, and a computer, wherein:

[0009] The air chamber includes a first cavity and a second cavity, the first cavity and the second cavity are connected to each other, and the quartz tuning fork is located outside the air chamber and between the first cavity and the second cavity;

[0010] The laser control and data acquisition system modulates the output wavelengths of the first semiconductor laser and the second semiconductor laser so that the modulation frequency f1 of the laser output of the first semiconductor laser is equal to f0-Δf, and the modulation frequency f2 of the laser output of the second semiconductor laser is equal to f0+Δf, where f0 is the eigenfrequency of the quartz tuning fork, f is the beat frequency, and Δf is the difference between the eigenfrequency and the beat frequency;

[0011] The laser light output by the first semiconductor laser passes through a first optical collimation system and is incident on the first cavity. The laser light output by the second semiconductor laser passes through a second optical collimation system and is incident on the second cavity. After the gas to be measured absorbs part of its energy, the laser light output from the first cavity and the second cavity irradiates the quartz tuning fork from both sides.

[0012] The quartz tuning fork absorbs the energy of the laser and generates elastic deformation, and performs damped oscillation according to the eigenfrequency. According to the principle of generating a beat frequency signal, the piezoelectric signal generated by the quartz tuning fork and the two modulation signals output by the first semiconductor laser and the second semiconductor laser form a beat frequency current signal;

[0013] The current signal is transmitted to the impedance amplifier for amplification, and the laser control and signal acquisition system collects the amplified current signal and transmits it to the computer for demodulation and processing to invert the concentration of the detected gas.

[0014] A method for heterodyne-enhanced photothermoelastic spectroscopy trace gas detection using the above device comprises the following steps:

[0015] Step 1: Control the first semiconductor laser and the second semiconductor laser to output corresponding wavelengths and powers through a semiconductor laser controller;

[0016] Step 2: The laser control and data acquisition system scans the resonant frequency of the quartz tuning fork and modulates the corresponding laser by superimposing a low-frequency scanning signal with two high-frequency sinusoidal signals of specific frequencies f1 = f0 - Δf and f2 = f0 + Δf.

[0017] Step 3: The output beams of the first semiconductor laser and the second semiconductor laser are transformed into two collimated beams after passing through the first optical collimation system and the second optical collimation system respectively. The two beams are incident on the gas chamber containing the gas to be measured, and are partially absorbed by the gas before being emitted.

[0018] Step 4: Two laser beams are emitted from the gas chamber and illuminate the two sides of the quartz tuning fork respectively. After absorbing the energy of the laser, the quartz tuning fork produces elastic deformation and oscillates at a damped frequency according to its eigenfrequency until the absorbed energy decays to zero.

[0019] Step 5: The quartz tuning fork generates a current signal due to the piezoelectric effect, which is further amplified by the impedance amplifier and some noise is filtered out;

[0020] Step 6: The laser control and signal acquisition system collects the amplified current signal and processes it by a computer. The two demodulated signals and the amplified piezoelectric signal form an enhanced beat frequency signal, and the concentration of the detected gas is inverted according to its amplitude.

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] 1. The present invention improves the system signal-to-noise ratio from the perspective of signal amplification, effectively increasing the measurement signal amplitude in two ways: on the one hand, the incidence of the dual light beams causes the quartz tuning fork to absorb higher power during damped oscillation, thereby increasing the piezoelectric signal; on the other hand, the new modulation signal can still participate in the generation of the beat signal without changing its frequency, so that the beat signal is significantly enhanced after superposition, thereby effectively improving the system signal-to-noise ratio.

[0023] 2. The present invention increases the amplitude of the beat signal generated by adding an additional laser excitation method, thereby improving the signal-to-noise ratio and obtaining a better minimum detection limit. Using two laser beams with modulation frequencies of f = f0-Δf and f = f0+Δf, respectively, after passing through the same gas chamber, they are incident on the quartz tuning fork from both sides. The quartz tuning fork will still perform damped oscillation at its intrinsic frequency, but due to the addition of an incident laser beam, the signal amplitude will be significantly increased. At the same time, since the frequencies of the three signals are very close, beats will still occur. According to basic physical principles, the frequency of the beat is f, and the maximum amplitude of the beat signal is A1+A2+A3, where A1, A2 and A3 represent the signal amplitudes generated by the two modulation signals and the tuning fork, respectively. Compared with the maximum signal amplitude A1+A2 generated by ordinary heterodyne, through such a design, the detected beat signal has been significantly increased, so the signal-to-noise ratio of the photothermoelastic spectroscopy technology can be effectively improved, thereby improving the detection performance of the trace gas sensor. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a schematic diagram of the structure of a heterodyne-enhanced photothermoelastic spectroscopy trace gas detection device;

[0025] Figure 2 The laser incident process. DETAILED DESCRIPTION

[0026] The technical solution of the present invention is further described below with reference to the accompanying drawings, but is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention that does not depart from the spirit and scope of the technical solution of the present invention should be included in the scope of protection of the present invention.

[0027] The present invention provides a heterodyne enhanced photothermoelastic spectroscopy trace gas detection device, such as Figure 1As shown, the device includes a first semiconductor laser 1, a second semiconductor laser 2, a first optical collimation system 3, a second optical collimation system 4, a gas chamber 5, a quartz tuning fork 6, an impedance amplifier 7, a laser control and data acquisition system 8 and a computer 9. The two laser beams output by the first semiconductor laser 1 and the second semiconductor laser 2, which are modulated by modulation frequencies f1 = f0-Δf and f2 = f0+Δf respectively, respectively pass through the first optical collimation system 3 and the second optical collimation system 4 and are incident from both sides into the gas chamber 5. The laser beams are absorbed by the gas to be measured. After energy distribution, the emitted laser irradiates the quartz tuning fork 6 from both sides. Since there is a certain frequency difference Δf between the laser modulation frequencies f1 and f2 and the eigenfrequency f0 of the quartz tuning fork 6, the quartz tuning fork 6 undergoes damped oscillation after elastic deformation, with the frequency being its eigenfrequency. According to the principle of generating beat frequency signals, the piezoelectric signal generated by the quartz tuning fork 6 and the two modulation signals (for demodulation) can form a beat frequency current signal. This current signal is transmitted to the impedance amplifier 7 for amplification, and then transmitted to the laser control and data acquisition system 8 and the computer 9 for demodulation and processing. The specific implementation process is as follows:

[0028] Step 1: Control the first semiconductor laser 1 and the second semiconductor laser 2 to output corresponding wavelengths and powers through a semiconductor laser controller.

[0029] Step 2: The laser control and data acquisition system 8 scans the resonant frequency of the quartz tuning fork 6, and modulates the corresponding laser by superimposing a low-frequency scanning signal with two high-frequency sinusoidal signals of specific frequencies f1 = f0-Δf and f2 = f0+Δf.

[0030] Step 3: The output beams of the first semiconductor laser 1 and the second semiconductor laser 2 are transformed into two collimated beams after passing through the first optical collimation system 3 and the second optical collimation system 4 respectively. The two beams are incident on the gas chamber 5 containing the gas to be measured, and are partially absorbed by the gas before being emitted.

[0031] Step 4: After two laser beams are emitted from the gas chamber 5, they illuminate the two sides of the quartz tuning fork 6 respectively. The quartz tuning fork 6 will produce elastic deformation after absorbing the energy of the laser. Due to the particularity of the modulation frequency, it will eventually oscillate at a damped frequency according to the eigenfrequency until the absorbed energy decays to zero.

[0032] Step 5: The quartz tuning fork 6 generates a current signal due to the piezoelectric effect, which is further amplified by the impedance amplifier 7 and some noise is filtered out.

[0033] Step 6: The laser control and signal acquisition system 8 collects the amplified electrical signal and processes it through the computer 9. The two demodulated signals and the amplified piezoelectric signal form an enhanced beat signal, and the concentration of the detected gas is inverted according to its amplitude.

[0034] In the present invention, the first semiconductor laser 1 and the second semiconductor laser 2 are both distributed feedback semiconductor lasers with near-infrared continuously tunable single longitudinal mode output. The two must have overlapping spectral ranges and ensure stable output performance under high-frequency modulation.

[0035] In the present invention, the near-infrared continuously tunable single longitudinal mode output distributed feedback semiconductor laser can be replaced by tunable lasers of other wavelength bands.

[0036] In the present invention, in order to ensure that the quartz tuning fork 6 can generate a sufficiently large damped oscillation amplitude, it is necessary to appropriately increase the laser incident power to ensure that the output power of the first semiconductor laser 1 and the second semiconductor laser 2 is greater than 15 mW.

[0037] In the present invention, the resonance frequency range of the quartz tuning fork 6 is 5 to 70 kHz, and it has a good quality factor (Q>8000).

[0038] In the present invention, the outgoing light beams of the first semiconductor laser 1 and the second semiconductor laser 2 after passing through the gas chamber 5 need to be incident from both sides of the quartz tuning fork 6. The incident position is the optimal position of the signal when the two are irradiated separately (usually the root of the fork leg or the root of the tuning fork arm), and the operating temperature range is -20 to 90°C.

[0039] In the present invention, the frequency of the high-frequency sinusoidal signal modulating the first semiconductor laser 1 and the second semiconductor laser 2 needs to differ from the resonant frequency of the quartz tuning fork 6 by ±Δf, and the range of Δf is 2 to 20 Hz. The specific value is optimized according to the size of the measurement signal.

[0040] In the present invention, the air chamber 5 includes a first cavity and a second cavity, the first cavity and the second cavity are connected to each other, and the quartz tuning fork 6 is located outside the air chamber 5 and between the first cavity and the second cavity. The air chamber 5 is sealed as a whole, and both the first cavity and the second cavity are provided with windows. The first cavity window and the second cavity window are coaxially arranged to form a laser channel. The first cavity window and the second cavity window must ensure that they have the same optical parameters (the specific laser incident process is as follows Figure 2 shown).

[0041] In the present invention, the gas chamber 5 can be replaced by two interconnected multi-optical path cells.

[0042] In the present invention, the air pressure range of the air chamber 5 is set to be between 10 mBar and 20 Bar, and the air flow rate is between 10 and 500 slm. The specific values ​​are determined according to the relaxation time and signal amplitude of the gas to be measured.

[0043] In the present invention, the quartz tuning fork 6 should have an equivalent impedance value less than 500 kΩ to reduce its own electronic noise as much as possible.

[0044] In the present invention, a wavelength modulation and first harmonic demodulation technique is adopted to process the received signal. The laser control and data acquisition system 8 modulates the output wavelengths of the first semiconductor laser 1 and the second semiconductor laser 2, controls and processes the data in real time through software, and feeds back and adjusts the modulation frequencies of the first semiconductor laser 1 and the second semiconductor laser 2 according to the f0 value of the quartz tuning fork calculated in real time.

[0045] In the present invention, the bandwidth B of the impedance amplifier 7 should be 1 Hz < B < 10 Hz to reduce the output noise brought by it.

[0046] In the present invention, the scanning frequency of the low-frequency signal for wavelength scanning needs to be greater than 50 cm -1 / s to ensure the generation of the beat signal. At the same time, the duration after scanning to the maximum current needs to be greater than 5 ms to ensure that the energy of the quartz tuning fork 6 is completely consumed.

Claims

1. A heterodyne enhanced photothermoelastic spectroscopy trace gas detection device, characterized in that The device includes a first semiconductor laser, a second semiconductor laser, a first optical collimation system, a second optical collimation system, a gas chamber, a quartz tuning fork, an impedance amplifier, a laser control and data acquisition system, and a computer, wherein: The air chamber includes a first cavity and a second cavity, the first cavity and the second cavity are connected to each other, and the quartz tuning fork is located outside the air chamber and between the first cavity and the second cavity; The laser control and data acquisition system modulates the output wavelengths of the first semiconductor laser and the second semiconductor laser so that the modulation frequency of the laser output by the first semiconductor laser is , the modulation frequency of the second semiconductor laser output laser , is the eigenfrequency of the quartz tuning fork, is the difference between the eigenfrequency and the beat frequency; The laser light output by the first semiconductor laser passes through a first optical collimation system and is incident on the first cavity. The laser light output by the second semiconductor laser passes through a second optical collimation system and is incident on the second cavity. After the gas to be measured absorbs part of its energy, the laser light output from the first cavity and the second cavity irradiates the quartz tuning fork from both sides. The quartz tuning fork absorbs the energy of the laser and generates elastic deformation, and performs damped oscillation according to the eigenfrequency. According to the principle of generating a beat frequency signal, the piezoelectric signal generated by the quartz tuning fork and the two modulation signals output by the first semiconductor laser and the second semiconductor laser form a beat frequency current signal; The current signal is transmitted to the impedance amplifier for amplification, and the laser control and signal acquisition system collects the amplified current signal and transmits it to the computer for demodulation and processing to invert the concentration of the detected gas.

2. The heterodyne-enhanced photothermoelastic spectroscopy trace gas detection device according to claim 1, characterized in that The first semiconductor laser and the second semiconductor laser are both distributed feedback semiconductor lasers with near-infrared continuously tunable single longitudinal mode output, and the output power is greater than 15 mW.

3. The heterodyne-enhanced photothermoelastic spectroscopy trace gas detection device according to claim 1, characterized in that The resonant frequency range of the quartz tuning fork is 5-70 kHz, the quality factor Q>8000, and the quartz tuning fork has an equivalent impedance value less than 500 kΩ.

4. The heterodyne-enhanced photothermoelastic spectroscopy trace gas detection device according to claim 1, characterized in that The emitted laser is incident from both sides of the quartz tuning fork, with the incident position being the root of the fork leg or the root of the tuning fork arm. The operating temperature range is -20~90 ℃.

5. The heterodyne-enhanced photothermoelastic spectroscopy trace gas detection device according to claim 1, characterized in that described The range is 2~20 Hz.

6. The heterodyne-enhanced photothermoelastic spectroscopy trace gas detection device according to claim 1, characterized in that The first cavity and the second cavity are both provided with windows, and the first cavity window and the second cavity window are coaxially arranged to form a laser channel.

7. The heterodyne-enhanced photothermoelastic spectroscopy trace gas detection device according to claim 1, characterized in that The air pressure range of the air chamber is set to between 10 mBar and 20 Bar, and the air flow rate is between 10 and 500 slm.

8. The heterodyne-enhanced photothermoelastic spectroscopy trace gas detection device according to claim 1, characterized in that The bandwidth B of the impedance amplifier satisfies the following conditions: 1 Hz <B<10 Hz。 9. A method for detecting trace gases using heterodyne-enhanced photothermoelastic spectroscopy using the device according to any one of claims 1 to 8, characterized in that The method comprises the following steps: Step 1: Control the first semiconductor laser and the second semiconductor laser to output corresponding wavelengths and powers through a semiconductor laser controller; Step 2: The laser control and data acquisition system scans the resonant frequency of the quartz tuning fork and uses a low-frequency scanning signal and two specific frequencies and The high-frequency sinusoidal signals are superimposed and modulated to correspond to the laser; Step 3: The output beams of the first semiconductor laser and the second semiconductor laser are transformed into two collimated beams after passing through the first optical collimation system and the second optical collimation system respectively. The two beams are incident on the gas chamber containing the gas to be measured, and are partially absorbed by the gas before being emitted. Step 4: Two laser beams are emitted from the gas chamber and illuminate the two sides of the quartz tuning fork respectively. After absorbing the energy of the laser, the quartz tuning fork produces elastic deformation and oscillates at a damped frequency according to its eigenfrequency until the absorbed energy decays to zero. Step 5: The quartz tuning fork generates a current signal due to the piezoelectric effect, which is further amplified by the impedance amplifier and some noise is filtered out; Step 6: The laser control and signal acquisition system collects the amplified current signal and processes it by the computer. The two demodulated signals and the amplified piezoelectric signal form an enhanced beat frequency signal, and the concentration of the detected gas is inverted according to its amplitude.

10. The heterodyne-enhanced photothermoelastic spectroscopy trace gas detection method according to claim 9, characterized in that The scanning frequency of the scanning signal is greater than 50 cm -1 / s to ensure the generation of the beat frequency signal; at the same time, the time required to scan to the maximum current is greater than 5 ms to ensure that the energy of the quartz tuning fork is completely consumed.

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