A trace gas detection device based on optical interference compensation of photoacoustic thermoelastic spectrum

Abstract

The application discloses a kind of trace gas detection devices based on optical interference compensation of photo-induced thermoelastic spectrum, the device includes excitation semiconductor laser, laser collimation system, gas chamber of gas to be measured, focusing lens, quartz tuning fork, probe laser, optical fiber beam splitter, optical fiber circulator, single-mode optical fiber, photoelectric detector, signal generator, laser controller, adder, lock-in amplifier and computer.The application is based on the optical interference compensation technology of LITES, while playing the advantage that popper structure can effectively shield thermal noise, a light beam is irradiated on the bottom of quartz tuning fork, to form reference popper cavity, reference popper cavity and sensing popper cavity, which senses the vibration of quartz tuning fork, have the same structural parameters and environmental parameters, the difference between the intensity of the two reflected lights in reference popper cavity and sensing popper cavity is obtained, that is, the light intensity signal is not disturbed by the environment, to improve the long-term stability of popper demodulation method based on LITES technology.

Description

TECHNICAL FIELD

[0001] The present application relates to a trace gas detection device, in particular to a photo-thermal-elastic spectrum trace gas detection device based on optical interference compensation. BACKGROUND

[0002] In 2018, Professor Ma Yufei's research group from Harbin Institute of Technology discovered the photo-thermal-elastic effect of quartz tuning fork, i.e. the change of local heat on the surface of the tuning fork causes mechanical vibration, thus a new trace gas detection technology, i.e. photo-thermal-elastic spectrum (LITES for short) is proposed. The technology uses a quartz tuning fork as a light intensity detector. After gas absorption, laser irradiation is performed on the surface of the quartz tuning fork. The tuning fork absorbs laser energy to produce thermal-elastic deformation, which induces mechanical vibration of the tuning fork. The mechanical vibration is enhanced under the resonance characteristics of the quartz tuning fork. Due to the piezoelectric effect of the quartz material, the mechanical vibration generates an electric current signal. The obtained electric current signal is demodulated to obtain the gas concentration. However, in the traditional LITES system, the laser is directly incident on the surface of the tuning fork, thus thermal noise is generated. Moreover, the higher the optical power, the faster the noise increases, which presents an exponential growth trend, thus limiting the development of the system detection performance by increasing the optical power.

[0003] In order to reduce the thermal noise and further improve the sensitivity of the system, the vibration of the quartz tuning fork can be demodulated by the light intensity of the Pecher structure composed of the side wall of the quartz tuning fork, air and the end face of the optical fiber, i.e. the electric signal in the quartz tuning fork is not collected, but the optical signal in the Pecher structure is collected, which improves the signal-to-noise ratio of the system to a certain extent and improves the detection limit. However, the Pecher structure itself is very sensitive to environmental temperature and vibration, and the light intensity signal obtained by demodulation is easily affected by vibration, air flow and temperature change, thus the long-term stability of the system is difficult to guarantee. SUMMARY

[0004] In order to solve the problem of large thermal noise of the system caused by the electric signal collection of the vibration information of the quartz tuning fork in the traditional LITES technology, the present application provides a photo-thermal-elastic spectrum trace gas detection device based on optical interference compensation. The present application is based on the optical interference compensation technology of LITES. While taking advantage of the Pecher structure which can effectively shield the thermal noise, a light beam is irradiated on the bottom of the quartz tuning fork to form a reference Pecher cavity. The light intensity information obtained by the reference Pecher cavity is only related to the change of environmental refractive index and low-frequency vibration, and has the same structure parameters and environmental parameters as the sensing Pecher cavity which senses the vibration of the quartz tuning fork by irradiating on the tip of the quartz tuning fork. The difference between the two reflected light intensity values in the reference Pecher cavity and the sensing Pecher cavity can obtain the light intensity signal which is not disturbed by the environment to improve the long-term stability of the Pecher demodulation method based on the LITES technology.

[0005] The purpose of the present application is realized by the following technical solutions:

[0006] An optical interference compensation based photo-thermal spectrum trace gas detection device, comprising an excitation semiconductor laser, a laser collimation system, a gas chamber of a gas to be measured, a focusing lens, a quartz tuning fork, a probe laser, a fiber beam splitter, a fiber circulator, a single-mode optical fiber, a photodetector, a signal generator, a laser controller, an adder, a lock-in amplifier and a computer, wherein:

[0007] The low-frequency sawtooth wave generated by the signal generator and the high-frequency sine wave generated by the lock-in amplifier are sent to the adder, and the superimposed signal constitutes a modulation signal of the laser wavelength;

[0008] The modulation signal is sent to the laser controller, and the wavelength and power of the laser output by the excitation semiconductor laser are changed through the temperature module and the current module;

[0009] The laser beam output by the excitation semiconductor laser contains an absorption peak of the gas to be measured, and after collimation by the laser collimation system, it is incident into the gas chamber of the gas to be measured, and the target gas to be measured absorbs part of the laser energy;

[0010] The laser beam passes through the gas chamber of the gas to be measured, is focused to the root position of the prongs of the quartz tuning fork after the light spot of the focusing lens, and due to the photo-thermal effect, mechanical vibration is excited at the tip of the quartz tuning fork;

[0011] The laser beam output by the probe laser is divided into two beams by the fiber beam splitter, and after transmission through the fiber circulator and the single-mode optical fiber, it is respectively irradiated on the tip and the bottom of the prongs of the quartz tuning fork to form a sensing and a reference cavity;

[0012] The reflected light intensity signals of the sensing and the reference cavities are converted into electrical signals by the photodetector, and after differential processing, they are input into the lock-in amplifier for harmonic signal demodulation, and finally input into the computer for inversion of the gas concentration.

[0013] A method for optical interference compensation based photo-thermal spectrum trace gas detection using the above device, comprising the following steps:

[0014] Step one: adjust the optical path through the excitation semiconductor laser, the laser collimation system, the gas chamber of the gas to be measured, the focusing lens and the quartz tuning fork, so that the laser is focused at the best excitation position of the photo-thermal signal on the surface of the quartz tuning fork;

[0015] Step two: adjust the optical path through the probe laser, the fiber beam splitter, the fiber circulator, the single-mode optical fiber and the quartz tuning fork, so that the laser is divided into two beams and respectively irradiated on the tip and the bottom of the prongs of the quartz tuning fork to form a sensing and a reference cavity, and the cavity lengths of the sensing and the reference cavities are consistent;

[0016] Step three: the laser controller controls the output wavelength of the excitation semiconductor laser by adjusting the temperature and current, and finds the temperature and current corresponding to the detection gas absorption line;

[0017] Step four: the computer is used to control the phase-locked amplifier to scan the resonance frequency of the quartz tuning fork, and the 1 / n (n is the harmonic number) of the obtained resonance frequency is set as the modulation frequency of the excitation semiconductor laser, and then the modulation depth is scanned to obtain the optimal modulation depth;

[0018] Step five: the cavity length of the sensing and reference Popper cavities and the wavelength of the detection laser are adjusted to ensure that the wavelength of the detection laser is located at the working point of the free spectral range of the Popper cavity;

[0019] Step six: the reflected light intensity signals of the sensing and reference Popper cavities are obtained by the photoelectric detector respectively, and are sent into the phase-locked amplifier for differential calculation, and the harmonic signal is obtained by processing in the computer, and the concentration information of the target gas is obtained according to the harmonic signal.

[0020] Compared with the prior art, the present application has the following advantages:

[0021] 1. The present application introduces a reference Popper cavity, which improves the sensitivity of the single Popper demodulation system to the environmental temperature and noise, and the demodulation signal is easily affected by external factors such as vibration, airflow and temperature change, which is beneficial to improve the system detection performance and long-term stability.

[0022] 2. The detection device of the present application has the advantages of high sensitivity, low cost and small noise. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 Figure 1 is a structural diagram of the trace gas detection device based on optical interference compensation of the present application;

[0024] Figure 2 Figure 3 is a position relationship diagram of the excitation semiconductor laser beam, the quartz tuning fork and the single-mode optical fiber. DETAILED DESCRIPTION

[0025] The technical solutions of the present application will be further described below in conjunction with the drawings, but are not limited thereto, and any modification or equivalent replacement of the technical solutions of the present application without departing from the spirit and scope of the present application shall be covered in the protection scope of the present application.

[0026] The present application provides a trace gas detection device based on optical interference compensation of the present application, as shown in Figure 1As shown, the device comprises an excitation semiconductor laser 1, a laser collimation system 2, a gas chamber 3 of the gas to be measured, a focusing lens 4, a quartz tuning fork 5, a probe laser 6, a fiber beam splitter 7, a fiber circulator 8, a single-mode optical fiber 9, a photodetector 10, a signal generator 11, a laser controller 12, an adder 13, a lock-in amplifier 14 and a computer 15, wherein:

[0027] The low-frequency sawtooth wave generated by the signal generator 11 and the high-frequency sine wave generated by the lock-in amplifier 14 are sent to the adder 13, and the superimposed signal constitutes the modulation signal of the wavelength of the excitation semiconductor laser 1;

[0028] The modulation signal is sent to the laser controller 12, and the wavelength of the output laser of the excitation semiconductor laser 1 is changed through the temperature module and the current module;

[0029] The laser beam containing the absorption peak of the gas to be measured output by the excitation semiconductor laser 1 is collimated by the laser collimation system 2 and then enters the gas chamber 3 of the gas to be measured, and the target gas absorbs part of the laser energy;

[0030] The outgoing laser of the gas chamber 3 is focused by the focusing lens 4 to the root position of the prongs of the quartz tuning fork 5, and the mechanical vibration of the tip of the quartz tuning fork 5 is excited due to the photo-thermal effect;

[0031] The outgoing laser beam of the probe laser 6 is split into two beams by the fiber beam splitter 7, and after transmission through the fiber circulator 8 and the single-mode optical fiber 9, it is respectively irradiated on the tip and the bottom of the prongs of the quartz tuning fork 5 to form a sensing and a reference cavity, and under the condition that the cavity length and the environment of the sensing and the reference cavities are the same, the reflected light intensity obtained by the sensing cavity contains signals and also carries environmental noise, while the reflected light intensity in the reference cavity only contains noise;

[0032] The reflected light intensity values of the sensing and the reference cavities are converted into electrical signals by the photodetector 10, and after differential operation to eliminate environmental noise, they are input into the lock-in amplifier 14 for harmonic signal collection and processing, and finally input into the computer 15 for subsequent processing to obtain the gas concentration.

[0033] A method for detecting trace gas based on optical interference compensation by using the above device, the specific implementation process is as follows:

[0034] Step one: adjust the light path through the excitation semiconductor laser 1, the laser collimation system 2, the gas chamber 3 of the gas to be measured, the focusing lens 4 and the quartz tuning fork 5 in turn, and ensure that the laser is focused at the best excitation position of the photo-thermal signal of the quartz tuning fork 5;

[0035] Step two: adjust the light path through the probe laser 6, fiber beam splitter 7, fiber loop 8, single-mode optical fiber 9, quartz tuning fork 5 in turn, so that the laser is divided into two beams, which irradiate the tip and bottom of the quartz tuning fork 5 to form a sensing and reference cavity, and ensure that the lengths of the two cavities are consistent.

[0036] Step three: the laser controller 12 controls the output wavelength of the excitation semiconductor laser 1 by changing the temperature and current, and finds the temperature and current corresponding to the detection gas absorption line;

[0037] Step four: use the computer 15 to control the phase-locked amplifier 14 to scan the resonance frequency of the quartz tuning fork 5, set the 1 / n (n is the harmonic number) of the obtained resonance frequency as the frequency of the sine wave, and then scan the modulation depth to obtain the optimal modulation depth and set it.

[0038] Step five: adjust the lengths of the sensing and reference cavities to the vicinity of the theoretical optimal value, and the specific value is determined according to the intensity of the reflected light, the wavelength coverage range of the probe laser, and the range of the free spectral region and other parameters; then adjust the wavelength of the probe laser 6 to ensure that the wavelength of the probe laser 6 is located at the working point of the free spectral region of the cavity.

[0039] Step six: the reflected light intensity signals of the sensing and reference cavities are obtained by the photodetector 10 and sent to the phase-locked amplifier 14 for differential calculation, and the harmonic signal is obtained by processing in the computer 15, and the concentration information of the target gas is obtained by inversion according to the harmonic signal.

[0040] In the present application, the excitation semiconductor laser 1 is a near-infrared continuous tunable single longitudinal mode output distributed feedback semiconductor laser or other wavelength tunable laser.

[0041] In the present application, the probe laser 6 is a broadband narrow linewidth tunable single longitudinal mode output semiconductor laser, the linewidth should be less than 50 MHz, the power stability should be less than 0.2 dB, the power should be greater than 5 mW, and the output wavelength should be in the communication wavelength band of the optical fiber device.

[0042] In the present application, the quartz tuning fork 5 is in a sealed gas chamber, and the gas pressure is between 50-500 Torr, and the specific gas pressure value is determined according to the relaxation time of the measured gas molecules.

[0043] In the present application, in order to make the quartz tuning fork 5 have a larger vibration amplitude, the laser irradiates the root position of the quartz tuning fork 5 (as shown in Figure 2 ), so that the tuning fork produces greater elastic deformation.

[0044] In the present application, the lengths of the sensing and reference cavities are the same, about 50-100 μm.

[0045] In the present application, the side light intensity reflectivity of the quartz tuning fork 5 should be >4%.

[0046] In the present application, the equivalent impedance value of the quartz tuning fork 5 should be less than 200 kΩ, and the quality factor should be greater than 10000, so as to eliminate electronic noise and increase signal value as much as possible.

[0047] In the present application, the power of the excitation semiconductor laser 1 should be >10 mW, and the output wavelength should be at the detection gas absorption peak position and should not overlap with the output wavelength of the probe laser 6.

[0048] In the present application, the insertion loss of the optical fiber beam splitter 7, the optical fiber circulator 8, and the single-mode optical fiber 9 should all be less than 0.25 dB in the optical fiber communication waveband, and the isolation of the optical fiber circulator 8 should be greater than 40 dB.

[0049] In the present application, the detection wavelength range of the photoelectric detector 10 should cover the output wavelength of the probe laser 6.

[0050] In the present application, wavelength modulation and harmonic demodulation technology is used to reduce the noise of the system, and the sine wave generated by the lock-in amplifier 14 modulates the output wavelength of the excitation semiconductor laser 1, and the modulation frequency is 1 / n of the resonance frequency of the quartz tuning fork 5, where n represents the harmonic number.

[0051] In the present application, the computer 15 is connected with the lock-in amplifier 14, and real-time control and signal acquisition processing are performed through software.

Claims

1. A photothermoelastic spectroscopy trace gas detection device based on optical interference compensation, characterized in that The device includes an excitation semiconductor laser, a laser collimation system, a gas chamber for a gas to be measured, a focusing lens, a quartz tuning fork, a detection laser, an optical fiber beam splitter, an optical fiber circulator, a single-mode optical fiber, a photodetector, a signal generator, a laser controller, an adder, a lock-in amplifier, and a computer, wherein: The low-frequency sawtooth wave generated by the signal generator and the high-frequency sine wave generated by the lock-in amplifier are fed into the adder, and the superimposed signals constitute a modulation signal of the laser wavelength; The modulation signal is sent to the laser controller, and the wavelength and power of the laser output by the semiconductor laser are changed through the temperature module and the current module; The laser beam output by the excitation semiconductor laser containing the absorption peak of the gas to be measured is collimated by the laser collimation system and then incident on the gas chamber to be measured, and the target gas to be measured absorbs part of the laser energy; The laser beam passes through the gas chamber to be measured and is focused by the focusing lens to the root of the quartz tuning fork finger. The photothermoelastic effect excites the tip of the quartz tuning fork to generate mechanical vibration. The laser beam output by the detection laser is divided into two beams by an optical fiber beam splitter, and after being transmitted through an optical fiber circulator and a single-mode optical fiber, it is respectively irradiated on the tip and bottom of the quartz tuning fork fingers to form a sensing Fabry-Perot cavity and a reference Fabry-Perot cavity. The cavity lengths of the sensing Fabry-Perot cavity and the reference Fabry-Perot cavity are the same, both of which are 50-100 μm. The reflected light intensity signals of the sensing Fabry-Perot cavity and the reference Fabry-Perot cavity are converted into electrical signals by a photoelectric detector, input into a lock-in amplifier for harmonic signal demodulation after differential processing, and finally input into a computer to invert the gas concentration.

2. The optical interference compensation-based photothermoelastic spectroscopy trace gas detection device according to claim 1 is characterized in that The excitation semiconductor laser is a distributed feedback semiconductor laser with a near-infrared continuously tunable single longitudinal mode output or a tunable laser of other wavelength bands, with a power greater than 10 mW, an output wavelength at the detection gas absorption peak position and no overlap with the output wavelength of the detection laser, and a modulation frequency of 1 / n of the resonance frequency of the quartz tuning fork, where n represents the harmonic order.

3. The optical interference compensation-based photothermoelastic spectroscopy trace gas detection device according to claim 1 is characterized in that The detection laser is a broadband, narrow-linewidth, tunable single-longitudinal-mode output semiconductor laser with a linewidth less than 50 MHz, a power stability less than 0.2 dB, a power greater than 5 mW, and an output wavelength in the optical fiber device communication band.

4. The optical interference compensation-based photothermoelastic spectroscopy trace gas detection device according to claim 1 is characterized in that The quartz tuning fork is placed in a sealed air chamber, and the air pressure of the air chamber is between 50 and 500 Torr.

5. The photothermoelastic spectroscopy trace gas detection device based on optical interference compensation according to claim 1 or 4, characterized in that The quartz tuning fork has a side light intensity reflectivity greater than 4%, an equivalent impedance value less than 200 kΩ, and a quality factor greater than 10,000.

6. The optical interference compensation-based photothermoelastic spectroscopy trace gas detection device according to claim 1, characterized in that The insertion losses of the optical fiber splitter, optical fiber circulator, and single-mode optical fiber are all less than 0.25 dB in the optical fiber communication band, and the isolation of the optical fiber circulator is greater than 40 dB.

7. The optical interference compensation-based photothermoelastic spectroscopy trace gas detection device according to claim 1 is characterized in that The detection wavelength range of the photodetector covers the output wavelength of the detection laser.

8. A method for detecting trace gases by photothermoelastic spectroscopy based on optical interference compensation using the device according to any one of claims 1 to 7, characterized in that The method comprises the following steps: Step 1: Adjust the optical path through the excitation semiconductor laser, laser alignment system, gas chamber to be measured, focusing lens, and quartz tuning fork to ensure that the laser is focused on the optimal excitation position of the photothermoelastic signal on the surface of the quartz tuning fork; Step 2: Adjust the optical path through the detection laser, fiber beam splitter, fiber circulator, single-mode fiber, and quartz tuning fork so that the laser is split into two beams, which are respectively irradiated on the tip and bottom of the quartz tuning fork fingers to form the sensing Fabry-Perot cavity and the reference Fabry-Perot cavity, and ensure that the cavity lengths of the sensing Fabry-Perot cavity and the reference Fabry-Perot cavity are consistent; Step 3: The laser controller controls the output wavelength of the excitation semiconductor laser by adjusting the temperature and current to find the temperature and current corresponding to the detection gas absorption line; Step 4: Use a computer to control the lock-in amplifier to scan the resonance frequency of the quartz tuning fork. Set 1 / n of the obtained resonance frequency as the modulation frequency of the excitation semiconductor laser, where n is the harmonic order. Then scan the modulation depth to obtain the optimal modulation depth. Step 5: Adjust the cavity lengths of the sensing Fabry-Perot cavity and the reference Fabry-Perot cavity and the wavelength of the detection laser to ensure that the wavelength of the detection laser is located at the operating point of the free spectrum region of the Fabry-Perot cavity; Step 6: The reflected light intensity signals of the sensing Fabry-Perot cavity and the reference Fabry-Perot cavity are obtained through photoelectric detectors, and sent to a phase-locked amplifier for differential calculation. The harmonic signals are processed in a computer and the concentration information of the target gas is inverted based on the harmonic signals.

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