CRDS multi-component trace gas measurement device and method combining optical feedback and PDH

Abstract

The present invention discloses a CRDS multi-component trace gas measurement device and method combining optical feedback and PDH phase locking, which relates to the field of trace gas measurement technology and includes a light source generating system for generating signal light, an optical resonant cavity for signal light to be injected and reciprocally reflected, an optical feedback system for stabilizing the frequency and power of the light source, a PDH phase locking system for frequency locking and noise suppression, and a signal processing system for monitoring the optical signal at the output end of the optical resonant cavity. The light source generating system provides signal light of two or more wavelengths, which is combined into a laser beam by a beam combiner and then injected into the optical resonant cavity and reciprocally reflected in the optical resonant cavity before being emitted to the optical feedback system. The present invention aims to achieve selective detection of multiple gas components while solving the problems of low sensitivity and insufficient measurement accuracy in the prior art. The device has high measurement accuracy and can be widely used.

Description

Technical Field

[0001] The present invention relates to the technical field of trace gas measurement, in particular to a CRDS multi-component trace gas measurement device and method combining optical feedback and PDH. Background Art

[0002] Trace gases (such as water vapor, carbon dioxide, and methane) are of great importance in environmental monitoring, climate research, and industrial process control. Traditional trace gas measurement devices are limited in their scope and suffer from low sensitivity and inadequate measurement accuracy. Cavity Ring-Down Spectroscopy (CRDS) technology, with its high sensitivity and precision, is an ideal choice for trace gas measurement. Summary of the Invention

[0003] To overcome the above-mentioned deficiencies in the prior art, the present invention provides a CRDS multi-component trace gas measurement device and method combining optical feedback and PDH, aiming to achieve selective detection of multiple gas components while solving the problems of low sensitivity and insufficient measurement accuracy in the prior art. The device has high measurement accuracy and can be widely used.

[0004] To achieve the above object, the present invention adopts the following technical solutions, including:

[0005] The CRDS multi-component trace gas measurement device combining optical feedback and PDH includes a light source generation system for generating signal light, an optical resonant cavity for signal light injection and reciprocating reflection, an optical feedback system for stabilizing the frequency and power of the light source, a PDH phase-locking system for frequency locking and noise suppression, and a signal processing system for monitoring the optical signal at the output end of the optical resonant cavity.

[0006] The light source generating system provides signal light of two or more wavelengths, which is combined into a laser beam by a beam combiner and then emitted into the optical resonant cavity and reflected back and forth in the optical resonant cavity before being emitted to the optical feedback system;

[0007] The optical feedback system includes a beam splitter, a polarizer, and a photodetector. The outgoing light of the optical resonant cavity is split by the beam splitter to separate the outgoing light of different wavelengths, and then passes through the polarizer to output the light to the photodetectors of the corresponding wavelengths. There are two or more photodetectors, each corresponding to the wavelength of the signal light.

[0008] The PDH phase-locked system includes two or more phase-locked amplifiers, each corresponding to the wavelength of the signal light. The phase-locked amplifiers are respectively connected to a photodetector and a light source generation system of the corresponding wavelength, and generate a PDH error signal based on the detection signal of the photodetector and a reference signal provided by the light source generation system. The laser frequency of the corresponding wavelength is adjusted in real time based on the PDH error signal to match the laser frequency with the resonant frequency of the optical resonant cavity.

[0009] The signal processing system includes a voltage comparator, an acquisition card and a processing unit; the detection signal of the photodetector is sent to the voltage comparator and the acquisition card respectively; the voltage comparator outputs a control signal to the processing unit according to the detection signal, and the processing unit controls the light source generation system according to the control signal; the acquisition card collects the detection signal in real time and sends it to the processing unit for calculation.

[0010] Preferably, the light source generation system includes a laser module, a laser interface and a mode field matching optical module; the laser module includes lasers of two or more wavelengths, electro-optical modulators (EOMs) corresponding to different wavelengths, and a beam combiner; the mode field matching optical module is used to control the beam waist position of the signal light to match the beam waist position of the resonant cavity, and includes an acousto-optic modulator (AOM), a collimator, an optical isolator, a focusing lens, and a reflector arranged in sequence along the light transmission direction;

[0011] The lasers output by lasers of different wavelengths are modulated by the EOM of the corresponding wavelength, combined into a laser beam by a beam combiner, and then pass through the AOM, collimator, optical isolator, and reflector in sequence to enter the optical resonant cavity.

[0012] Preferably, the optical feedback process of the optical feedback system is as follows:

[0013] Beam separation: Use a polarizer to separate a set proportion of light intensity from the outgoing light of the optical resonator as the feedback beam;

[0014] Polarization matching: Adjust the collimator and polarizer to make the polarization direction of the feedback beam consistent with the polarization direction of the laser output beam;

[0015] Feedback optimization: Adjust the polarization angle and incident angle of the feedback beam, and adjust the feedback light intensity to keep it within the set proportional range of the laser output intensity.

[0016] Preferably, the locking process of the PDH phase-locked system is as follows:

[0017] Two or more wavelengths of laser light are introduced into the RF modulation signal through the EOM respectively, and the modulation depth is adjusted so that the modulation amplitude of each wavelength of laser light meets the PDH phase-locking requirement;

[0018] After passing through the beam splitter and polarizer, the outgoing light from the optical resonant cavity is introduced into the photodetector of the corresponding wavelength. The detection signal of the photodetector is input into the phase-locked amplifier of the corresponding wavelength, demodulated using the RF reference signal to generate a PDH error signal. Finally, the PDH error signal is input into the frequency control module of the laser of the corresponding wavelength to adjust the frequency of the laser in real time to match the resonant frequency of the optical resonant cavity.

[0019] Preferably, in the signal processing system, the voltage comparator compares the voltage of the detection signal. If the voltage exceeds the set threshold voltage, a control signal is output to the processing unit. The processing unit triggers the AOM according to the control signal to shut down the optical path, thereby obtaining a ring-down signal.

[0020] Preferably, the optical resonant cavity is provided with an air inlet and an air outlet respectively; a filter and a ball valve are sequentially provided on the air inlet connecting pipe, and the flow rate of the gas to be measured is controlled by the ball valve after the gas to be measured passes through the filter; a pressure gauge and a vacuum pump are sequentially provided on the air outlet connecting pipe;

[0021] The optical resonant cavity comprises a cavity, a high-reflection mirror, a window lens, and a high-reflection mirror adjustment frame;

[0022] Both ends of the cavity are equipped with high-reflection mirror adjustment frames; a high-reflection mirror is fixed on the inner side of the high-reflection mirror adjustment frame via an O-ring, and the two high-reflection mirrors in the two high-reflection mirror adjustment frames at both ends of the cavity are exactly the same in size and shape, and both high-reflection mirrors are provided with small holes for laser injection / exit, and the two high-reflection mirrors serve as the front cavity mirror and the rear cavity mirror of the optical resonant cavity respectively; a window lens is fixed on the outer side of the high-reflection mirror adjustment frame via an O-ring, serving as a cavity sealing cover to isolate the outside air; the high-reflection mirror is coated with high-reflection films and anti-reflection films of two or more wavelength bands corresponding to the laser wavelength; the window lens is coated with anti-reflection films of two or more wavelength bands corresponding to the laser wavelength.

[0023] Preferably, a piezoelectric ceramic, PTZ, is installed on the rear cavity mirror of the optical resonant cavity for dynamically adjusting the cavity length so that the optical resonant cavity maintains synchronous resonance with the laser.

[0024] Preferably, the device includes signal lights of three wavelengths, with center wavelengths of 1392 nm, 1605 nm, and 1654 nm respectively.

[0025] Preferably, the device further comprises a temperature control module and a pressure control module for controlling the temperature and pressure in the optical resonant cavity.

[0026] The present invention also provides a CRDS multi-component trace gas measurement method combining optical feedback and PDH, which is applicable to the above-mentioned CRDS multi-component trace gas measurement device combining optical feedback and PDH, and the method comprises the following steps:

[0027] S1, by measuring the ring-down time when there is a gas to be measured in the cavity and when there is no gas to be measured, the gas absorption coefficient α(v) is obtained; τ(v) represents the ring-down time when there is a gas to be measured in the cavity, and τ0 represents the ring-down time when there is no gas to be measured; the absorption coefficient Wherein, v represents the absorption wavelength of the gas to be measured;

[0028] S2, using the Beer-Lambert law, the relationship between the absorption coefficient α(v) and the gas concentration C and the absorption cross section σ(v) is: α(v) = C·σ(v);

[0029] S3, the calculation formula of gas concentration C is obtained:

[0030] S4, measure the ring-down time τ at different wavelengths λi when there is no gas to be measured λi , calculate the absorption coefficient α of the gas to be measured for each wavelength λi λi , establish the equations between the absorption coefficient and gas concentration:

[0031]

[0032] in, is the jth gas to be measured at wavelength λ i The absorption cross section at j is the concentration of the jth gas.

[0033] The advantages of the present invention are:

[0034] (1) The present invention realizes the selective detection of multiple gas components with high measurement accuracy and sensitivity, and can be widely used.

[0035] (2) Optical feedback technology and PDH phase-locked frequency stabilization technology further improve the measurement accuracy and sensitivity of CRDS technology by stabilizing the frequency of the laser. Among them, optical feedback technology adjusts the frequency of the laser to resonate with the optical resonator, thereby improving the coupling efficiency of light. PDH phase-locked frequency locks the frequency of the laser to the resonant frequency of the optical resonator, reducing frequency noise and enhancing the stability and accuracy of the measurement.

[0036] (3) While ensuring the convenience of the device, the combination of optical feedback and PDH phase-locked technology further enhances the stability of the device, enabling the CRDS technology to provide high-precision and high-sensitivity measurement results in a wider range of application scenarios, while achieving the effectiveness of accurate, real-time, and long-term measurements of a variety of trace gases.

[0037] (4) A piezoelectric ceramic (PZT) is installed on the rear mirror of the optical resonator to dynamically adjust the cavity length so that the optical resonator maintains synchronous resonance with the laser. The EOM and PZT in the PDH phase-locked system work simultaneously, but in different frequency ranges: the EOM is used to quickly respond to high-frequency interference and generate error signals without direct adjustment; the PZT is used to slowly respond to low-frequency drift and directly adjust the laser cavity. The advantage of this division of labor is that the laser frequency can be effectively controlled within a wide frequency range and maintained locked with the optical resonator. During the initial locking, the PZT is adjusted first to make the laser frequency close to the cavity resonance frequency; after locking, the PZT and EOM work together to automatically maintain the lock. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 A schematic diagram of important components of the measuring device provided in an embodiment of the present invention.

[0039] Figure 2 A schematic diagram of the overall structure of a measuring device provided in an embodiment of the present invention.

[0040] Figure 3 Schematic diagram of the front structure of a hexagonal high-reflectivity lens adjustment frame for an optical resonant cavity provided by an embodiment of the present invention.

[0041] Figure 4 Schematic diagram of the back structure of the hexagonal high-reflectivity lens adjustment frame of the optical resonant cavity provided by an embodiment of the present invention.

[0042] Figure 5 Schematic diagram of the optical feedback system structure of the measurement device provided by the embodiment of the present invention

[0043] Figure 6 A schematic diagram of the structure of a PDH phase-locked system of a measurement device provided in an embodiment of the present invention.

[0044] Figure 7 Schematic diagram of reflected signals according to an embodiment of the present invention.

[0045] Figure 8 2 is a schematic diagram of a PDH error signal according to an embodiment of the present invention.

[0046] Figure 9 FIG. 4 is a frequency stability comparison diagram of an embodiment of the present invention.

[0047] Figure 10 4 is a signal-to-noise ratio comparison diagram of an embodiment of the present invention.

[0048] Figure 11 Schematic diagram of a mode field matching optical module according to an embodiment of the present invention.

[0049] Figure 12 This is a schematic diagram of the TLV3501 ultra-high-speed voltage comparator circuit according to an embodiment of the present invention.

[0050] Figure 13 Schematic diagram of the threshold triggering of the TLV3501 ultra-high-speed voltage comparator according to an embodiment of the present invention.

[0051] Figure 14 FIG. 4 is a diagram of a CRDS ring-down signal according to an embodiment of the present invention. DETAILED DESCRIPTION

[0052] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0053] Depend on Figures 1-4 As shown, the CRDS multi-component trace gas measurement device combining optical feedback and PDH technology in this embodiment includes: a light source generation system for generating signal light, an optical feedback system for stabilizing the frequency and power of the light source, an optical resonant cavity 9 for the signal light to be injected and reciprocated, a PDH phase-locking system for high-precision frequency locking and noise suppression, and a signal processing system for monitoring the optical signal at the output end of the optical resonant cavity.

[0054] The light source generating system includes a laser module, a laser interface 4, and a mode field matching optical module 10. The laser module includes three DFB lasers 1 with different wavelengths and corresponding three electro-optical modulators 2 and a beam combiner 3 with different wavelengths. The wavelengths (center wavelengths) of the three DFB lasers 1 are 1392nm, 1605nm, and 1654nm, respectively. The three electro-optical modulators 2 with different wavelengths are denoted as EOM1, EOM2, and EOM3, which respectively perform laser modulation on the output lasers of the three DFB lasers 1 with different wavelengths. The three laser beams after laser modulation are combined into one laser beam by the beam combiner 3 and transmitted to the mode field matching optical module 10 through the laser interface 4. After mode matching by the mode field matching optical module 10, they are emitted into the optical resonant cavity 9. After reciprocating reflection in the optical resonant cavity 9, the output light of the optical resonant cavity 9 enters the optical feedback system.

[0055] In this embodiment, the DFB laser integrates TEC, butterfly packaging, and has a frequency tuning function, and the power of the three DFB lasers 1 is 20 mW.

[0056] The mode field matching optical module 10 includes an acousto-optic modulator (AOM), a collimator, an optical isolator, a focusing lens, and a reflector. The mode field matching optical module 10 is used to control the beam waist position of the signal light to match the beam waist position of the resonant cavity and support multi-longitudinal mode operation. In this embodiment, Figure 11Schematic diagram of the beam waist matching of the mode field matching optical module 10. Figure 11 As shown in the figure, F represents a focusing lens with a focal length of f; n is the refractive index of the resonant cavity front mirror; R is the reflectivity of the resonant cavity front mirror; w1 is the beam waist of the signal light; w0 is the beam waist of the resonant cavity; d is the thickness of the resonant cavity front mirror, which is a known fixed value; L1 is the distance from the output light of the focusing lens to the resonant cavity front mirror; L2 is the distance from the beam waist of the signal light to the output light of the focusing lens; and L0 is the distance from the resonant cavity front mirror to the beam waist of the resonant cavity. Before beam waist matching, matrix operations must be performed to calculate the distance L1 from the output light of the collimator to the resonant cavity front mirror. Poor coupling will produce high-order modes, thereby reducing the signal-to-noise ratio of the system.

[0057] Matrix operations are as follows:

[0058]

[0059]

[0060] The optical feedback system includes a beam splitter, a polarizer, and a photodetector 13. The beam splitter is a beam splitting prism of appropriate wavelength. After passing through the beam splitter, the outgoing light of the optical resonant cavity 9 is divided into three paths, which correspond to 1392nm, 1605nm, and 1654nm respectively, so as to separate the outgoing light of different wavelengths, and then output them to the photodetectors of the corresponding wavelengths through the polarizer respectively; the optical feedback system includes three photodetectors 13 corresponding to three different wavelengths (1392nm, 1605nm, and 1654nm). In this embodiment, Figure 5 As shown in the figure, the optical feedback process specifically includes: 1. Beam separation: Use a polarizer to separate about 20% of the light intensity from the output light of the optical resonator as the feedback beam; 2. Polarization matching: Adjust the collimator and polarizer to make the polarization direction of the feedback beam consistent with the polarization direction of the laser output beam; 3. Feedback optimization: Adjust the polarization angle and incident angle of the feedback beam to improve the feedback efficiency, and adjust the feedback light intensity to keep it in the range of 10% to 30% of the laser output intensity, so as to obtain a suitable feedback effect and avoid instability.

[0061] The PDH phase-locked system includes three phase-locked amplifiers corresponding to three different wavelengths (1392nm, 1605nm, and 1654nm); the phase-locked amplifiers are respectively connected to the photodetectors and light source generation systems of the corresponding wavelengths, and generate PDH error signals based on the detection signals of the photodetectors and the reference signals provided by the light source generation systems. The laser frequency of the corresponding wavelength is adjusted in real time based on the PDH error signals to match the laser frequency with the resonant frequency of the optical resonant cavity. In this embodiment, Figure 6As shown, the specific locking of the PDH phase-locked system is as follows: the three wavelengths of laser light pass through the electro-optical modulator (EOM) respectively, and a 10 MHz RF modulation signal is introduced. By adjusting the modulation depth, the laser modulation amplitude of each wavelength meets the PDH phase-locked requirement. The output light of the optical resonator 9 passes through the beam splitter and is introduced into the photodetector 13 of the corresponding wavelength. The detection signal of the photodetector 13 is input into the phase-locked amplifier of the corresponding wavelength, and is demodulated using the RF reference signal to generate a PDH error signal. Finally, the PDH error signal is input into the frequency control module (laser driver) of the laser of the corresponding wavelength to adjust the frequency of the laser in real time to match the resonance frequency of the optical resonator 9.

[0062] Depend on Figure 7-10 As shown, in this embodiment, by defining basic parameters such as frequency range, cavity width (full width at half maximum), and modulation frequency, the following is obtained: Figure 7 The Lorentz line shape is used to represent the reflection characteristics in the cavity, specifically the reflection signal with intensity changes under different conditions; the following is obtained: Figure 8 The error signal is represented by a modulated weighted sine wave, specifically a PDH error signal with varying signal amplitudes under different conditions; Figure 9 The frequency stability shown and Figure 10 The signal-to-noise ratio (SNR) shown in the figure is: No Feedback indicates no optical feedback, With Feedback indicates optical feedback, With PDH indicates PDH phase locking, and With Both indicates both optical feedback and PDH phase locking. Figure 7 The horizontal axis represents the frequency offset, that is, the frequency difference relative to the cavity resonance frequency, and the vertical axis represents the reflection intensity, that is, the intensity of the laser after reflection in the cavity, which is expressed by Figure 7 It can be seen that both optical feedback and PDH phase locking can enhance the coupling between the laser and the cavity, allowing more light to enter the cavity and thus reducing reflection. Figure 8 The horizontal axis represents the frequency offset, and the vertical axis represents the error signal intensity, which is used to represent the offset of the laser frequency relative to the cavity resonance, and is expressed by Figure 8 It can be seen that after using optical feedback and PDH technology, the amplitude of the error signal becomes smaller, indicating that the laser frequency is closer to the cavity resonance frequency and the system locking effect is better. Figure 9 It shows that optical feedback and PDH technology significantly improve frequency stability, especially when the two are combined, the performance is best. Figure 10 It means that optical feedback and PDH technology can not only reduce noise, but also significantly increase the ratio of signal to noise (i.e., signal-to-noise ratio), thereby improving system performance.

[0063] The signal processing system includes a voltage comparator, an acquisition card 14 and a computer 15. The photodetector 13 is connected to the voltage comparator and the acquisition card 14, respectively, and sends the detection signal to the voltage comparator and the acquisition card 14 of the corresponding wavelength. The voltage comparator and the acquisition card 14 are connected to the computer 15, respectively. The voltage comparator outputs a control signal to the computer 15 according to the detection signal, and the computer 15 controls the acousto-optic modulator (AOM) in the light source generating system according to the control signal, thereby turning off the incident light. The acquisition card 14 sends the collected data to the computer 15 for processing. In order to obtain the ring-down signal, the present invention needs to quickly generate a low-level signal to trigger the acousto-optic modulator (AOM) through the voltage comparator when the light intensity reaches a certain threshold. The photodetector 13 converts the optical signal into an electrical signal and then splits it into two paths. One path enters a voltage comparator, which compares the signal with a preset threshold voltage and outputs a pulse signal, i.e., a control signal, to trigger an acousto-optic modulator (AOM). The AOM shuts off the optical path via an external modulation signal. The other path enters an acquisition card 14 for real-time acquisition of the detection signal, i.e., the ring-down signal. The timing of triggering the acquisition is determined by the pulse signal output by the trigger circuit. All control and acquisition are performed by a computer 15.

[0064] In this embodiment, the voltage comparator is a TLV3501 ultra-high-speed voltage comparator, and the propagation delay time is 4.5 ns. Figure 12 This is the circuit schematic diagram of the TLV3501 ultra-high-speed voltage comparator. Figure 13 The diagram of the threshold trigger of the TLV3501 ultra-high-speed voltage comparator is shown in FIG. The TLV3501 ultra-high-speed voltage comparator outputs a 5V high level when the set threshold is not reached, and outputs a low pulse signal when the set threshold is reached. It is used to perform waveform shaping on the fundamental mode signal output by the detector 13 and output a TTL level signal. The TLV3501 ultra-high-speed voltage comparator circuit can be combined with the acousto-optic modulator AOM to turn off the incident light, thereby generating the following: Figure 14 The ring-down signal.

[0065] The optical resonant cavity 9 is provided with an air inlet 901 and an air outlet 902 respectively; a filter and a ball valve 6 are provided on the air inlet connecting pipe in sequence, and the gas to be measured 5 passes through the ball valve 6 after passing through the filter to control the flow rate of the gas to be measured; a pressure gauge 7 and a vacuum pump 8 are provided on the air outlet connecting pipe in sequence.

[0066] The optical resonant cavity 9 includes a high-reflection mirror, a window lens, a cavity 906 , and a high-reflection mirror adjustment frame 907 .

[0067] The cavity 906 is constructed of electrolytically polished stainless steel, with high-reflection mirror adjustment mounts 907 mounted on both ends of the cavity 906. High-reflection mirrors are fixed to the inside of the high-reflection mirror adjustment mounts 907 via O-rings. The two high-reflection mirrors located in the two high-reflection mirror adjustment mounts at both ends of the cavity 906 are identical in size and shape. Both mirrors are provided with apertures for laser input / output, and serve as the front cavity mirror 103 and rear cavity mirror 104 of the optical resonator 9, respectively. Window lenses are fixed to the outside of the high-reflection mirror adjustment mounts 907 via O-rings, serving as sealing covers for the cavity 906 and isolating it from the outside air. Both mirrors in the two high-reflection mirror adjustment mounts at both ends of the cavity 906 are coated with high-reflection and anti-reflection coatings for the corresponding wavelengths (1392nm, 1605nm, and 1654nm). Correspondingly, the window lenses are coated with anti-reflection coatings for the corresponding wavelengths (1392nm, 1605nm, and 1654nm) to increase the incidence rate. In this embodiment, the reflectivity of the two high-reflection mirrors is greater than 99.99%, and the length of the cavity 906 is 20 cm.

[0068] A piezoelectric ceramic (PZT) is installed on the rear mirror 104 of the cavity 906 to dynamically adjust the cavity length so that the optical resonant cavity 9 maintains synchronous resonance with the laser. In the PDH phase-locked system, the EOM and PZT will work simultaneously, but in different frequency ranges: the EOM is used to quickly respond to high-frequency interference and generate error signals without direct adjustment; the PZT is used to slowly respond to low-frequency drift and directly adjust the laser cavity. The advantage of this division of labor is that the laser frequency can be effectively controlled within a wide frequency range and maintained locked with the optical resonant cavity. During the initial locking, the PZT is adjusted first to make the laser frequency close to the cavity resonance frequency; after locking, the lock is automatically maintained through the joint work of the PZT and EOM.

[0069] The high-reflectivity mirror adjustment frame 907 is an embedded hexagonal high-reflectivity mirror adjustment frame equipped with spring columns. The frame includes six fixing holes, three of which are equipped with spring columns 9073 with a high spring coefficient. The spring columns 9073 are fixed by a set screw 9071. The other three fixing holes are equipped with 304 stainless steel fine-thread hexagon socket screws 9072. The two different types of fixing holes alternate with each other. By squeezing the spring columns 9073, the optical axes of the two high-reflectivity mirrors are brought into coaxial alignment. The use of 304 stainless steel fine-thread hexagon socket screws 9072 enhances adjustment accuracy. In this embodiment, the spring columns 9073 are spherical heavy-duty spring columns with a load range of 3.73N-6.96N.

[0070] In this embodiment, the mode field matching optical module 10 , the optical resonant cavity 9 , the optical feedback system, and the PDH phase-locked system are all disposed in a thermally insulating housing 11 .

[0071] In this embodiment, the device also includes a temperature control module and a pressure control module. The temperature control module includes a temperature controller, a temperature sensor, and a feedback control loop; the pressure control module includes a pressure control device, a pressure sensor, and a feedback control loop. The temperature controller and pressure controller are used to control the temperature and pressure within the optical resonant cavity 9. The temperature sensor and pressure sensor are both arranged within the insulated housing 11 and are used to collect the temperature and pressure within the optical resonant cavity 9. The collected temperature and pressure data are fed back to the temperature controller and pressure controller respectively via the feedback control loop, thereby controlling the temperature and pressure within the resonant cavity 9 to remain within the required range.

[0072] The CRDS multi-component trace gas measurement method combining optical feedback and PDH technology of this embodiment includes the following steps:

[0073] S1, according to the optical cavity ring-down principle, when laser light is incident on an optical resonant cavity and the incident light is shut off by the TLV3501 ultra-high-speed voltage comparator and the AOM, the light intensity decays exponentially with time. By measuring the ring-down time when there is or is not the gas to be measured in the cavity, the gas absorption coefficient α(v) can be obtained. τ(v) is used to represent the ring-down time when there is the gas to be measured in the cavity, and τ0 is used to represent the ring-down time when there is no gas to be measured. The absorption coefficient can be expressed by the formula Represented by: wherein v represents the wavelength of the gas to be measured (selected absorption line).

[0074] S2, using the Beer-Lambert law, the relationship between the absorption coefficient α(v) and the gas concentration C and the absorption cross section σ(v) is: α(v) = C·σ(v); where σ(v) represents the absorption cross section of the gas to be measured at a specific wavelength (cm 2 / molecule), which can be obtained from databases such as HITRAN or SpectraPlot; C represents the gas concentration (molecule / cm 3 ).

[0075] S3, gas concentration can be expressed as To perform calculations, if you need to express the concentration in common units (such as ppm, ppb), you can further convert: where k B represents the Boltzmann constant, T represents the gas temperature, and P represents the gas pressure.

[0076] S4, measure the ring-down time constant τ at different wavelengths λi , calculate the absorption coefficient α for each selected wavelength λi , establish the equations between absorption coefficient and concentration:

[0077]

[0078] in, is the jth gas at wavelength λ i The absorption cross section at j is the concentration of the jth gas;

[0079] S5, by solving the above equations, the concentration of each component gas (C1, C2, ..., C n ).

[0080] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. CRDS multi-component trace gas measurement device combining optical feedback and PDH, characterized in that: It includes a light source generating system for generating signal light, an optical resonant cavity for signal light to be injected and reflected back and forth, an optical feedback system for stabilizing the frequency and power of the light source, a PDH phase-locking system for frequency locking and noise suppression, and a signal processing system for monitoring the optical signal at the output end of the optical resonant cavity; The light source generating system provides signal light of two or more wavelengths, which is combined into a laser beam by a beam combiner and then emitted into the optical resonant cavity and reflected back and forth in the optical resonant cavity before being emitted to the optical feedback system; The optical feedback system includes a beam splitter, a polarizer and a photodetector; The outgoing light from the optical resonant cavity is split by a beam splitter to separate the outgoing light of different wavelengths, and then passes through a polarizer to output to the photodetector of the corresponding wavelength; There are two or more photoelectric detectors, each corresponding to the wavelength of the signal light; The PDH phase-locked system includes two or more phase-locked amplifiers, each corresponding to the wavelength of the signal light. The phase-locked amplifiers are respectively connected to a photodetector and a light source generation system of the corresponding wavelength, and generate a PDH error signal based on the detection signal of the photodetector and a reference signal provided by the light source generation system. The laser frequency of the corresponding wavelength is adjusted in real time based on the PDH error signal to match the laser frequency with the resonant frequency of the optical resonant cavity. The signal processing system includes a voltage comparator, an acquisition card, and a processing unit; the detection signal of the photodetector is sent to the voltage comparator and the acquisition card respectively; the voltage comparator outputs a control signal to the processing unit according to the detection signal, and the processing unit controls the light source generation system according to the control signal; the acquisition card collects the detection signal in real time and sends it to the processing unit for calculation; Two or more wavelengths of laser light are introduced into the RF modulation signal through the EOM respectively, and the modulation depth is adjusted so that the modulation amplitude of each wavelength of laser light meets the PDH phase-locking requirement; A piezoelectric ceramic, or PTZ, is installed on the rear mirror of the optical resonator to dynamically adjust the cavity length so that the optical resonator maintains synchronous resonance with the laser.

2. The CRDS multi-component trace gas measurement device combining optical feedback and PDH according to claim 1, characterized in that: The light source generation system includes a laser module, a laser interface and a mode field matching optical module; the laser module includes lasers with two or more wavelengths, electro-optical modulators (EOMs) corresponding to different wavelengths, and a beam combiner; the mode field matching optical module is used to control the beam waist position of the signal light to match the beam waist position of the resonant cavity, and includes an acousto-optic modulator (AOM), a collimator, an optical isolator, a focusing lens, and a reflector arranged in sequence along the light transmission direction; The lasers output by lasers of different wavelengths are modulated by the EOM of the corresponding wavelength, combined into a laser beam by a beam combiner, and then pass through the AOM, collimator, optical isolator, and reflector in sequence to enter the optical resonant cavity.

3. The CRDS multi-component trace gas measurement device combining optical feedback and PDH according to claim 1, characterized in that: The optical feedback process of the optical feedback system is as follows: Beam separation: Use a polarizer to separate a set proportion of light intensity from the outgoing light of the optical resonator as the feedback beam; Polarization matching: Adjust the collimator and polarizer to make the polarization direction of the feedback beam consistent with the polarization direction of the laser output beam; Feedback optimization: Adjust the polarization angle and incident angle of the feedback beam, and adjust the feedback light intensity to keep it within the set proportional range of the laser output intensity.

4. The CRDS multi-component trace gas measurement device combining optical feedback and PDH according to claim 2, characterized in that: The locking process of the PDH phase-locked system is as follows: After passing through the beam splitter and polarizer, the outgoing light from the optical resonant cavity is introduced into the photodetector of the corresponding wavelength. The detection signal of the photodetector is input into the phase-locked amplifier of the corresponding wavelength, demodulated using the RF reference signal to generate a PDH error signal. Finally, the PDH error signal is input into the frequency control module of the laser of the corresponding wavelength to adjust the frequency of the laser in real time to match the resonant frequency of the optical resonant cavity.

5. The CRDS multi-component trace gas measurement device combining optical feedback and PDH according to claim 2, characterized in that: In the signal processing system, the voltage comparator compares the voltage of the detection signal. If the voltage exceeds the set threshold voltage, the voltage comparator outputs a control signal to the processing unit. The processing unit triggers the AOM according to the control signal to shut down the optical path, thereby obtaining a ring-down signal.

6. The CRDS multi-component trace gas measurement device combining optical feedback and PDH according to claim 1, characterized in that: The optical resonant cavity is provided with an air inlet and an air outlet respectively; a filter and a ball valve are sequentially provided on the air inlet connecting pipe, and the flow rate of the gas to be measured is controlled by the ball valve after the gas to be measured passes through the filter; a pressure gauge and a vacuum pump are sequentially provided on the air outlet connecting pipe; The optical resonant cavity comprises a cavity, a high-reflection mirror, a window lens, and a high-reflection mirror adjustment frame; Both ends of the cavity are equipped with high-reflection mirror adjustment frames; a high-reflection mirror is fixed on the inner side of the high-reflection mirror adjustment frame via an O-ring, and the two high-reflection mirrors in the two high-reflection mirror adjustment frames at both ends of the cavity are exactly the same in size and shape, and both high-reflection mirrors are provided with small holes for laser injection / exit, and the two high-reflection mirrors serve as the front cavity mirror and the rear cavity mirror of the optical resonant cavity respectively; a window lens is fixed on the outer side of the high-reflection mirror adjustment frame via an O-ring, serving as a cavity sealing cover to isolate the outside air; the high-reflection mirror is coated with high-reflection films and anti-reflection films of two or more wavelength bands corresponding to the laser wavelength; the window lens is coated with anti-reflection films of two or more wavelength bands corresponding to the laser wavelength.

7. The CRDS multi-component trace gas measurement device combining optical feedback and PDH according to claim 1, characterized in that: The device includes three wavelengths of signal light, with center wavelengths of 1392nm, 1605nm, and 1654nm respectively.

8. The CRDS multi-component trace gas measurement device combining optical feedback and PDH according to claim 1, characterized in that: The device also includes a temperature control module and a pressure control module for controlling the temperature and pressure in the optical resonant cavity.

9. A CRDS multi-component trace gas measurement method combining optical feedback and PDH, characterized in that: The CRDS multi-component trace gas measurement device combining optical feedback and PDH according to any one of claims 1 to 8 is used, and the method comprises the following steps: S1, by measuring the ring-down time when there is a gas to be measured in the cavity and when there is no gas to be measured, the gas absorption coefficient α(v) is obtained; τ(v) represents the ring-down time when there is a gas to be measured in the cavity, and τ0 represents the ring-down time when there is no gas to be measured; the absorption coefficient Where, v represents the absorption wavelength of the gas to be measured; c is the speed of light; S2, using the Beer-Lambert law, the relationship between the absorption coefficient α(v) and the gas concentration C and the absorption cross section σ(v) is: α(v) = C·σ(v); S3, the calculation formula of gas concentration C is obtained: S4, measure the ring-down time τ at different wavelengths λi when there is no gas to be measured λi , calculate the absorption coefficient α of the gas to be measured for each wavelength λi λi , establish the equations between the absorption coefficient and gas concentration: in, is the jth gas to be measured at wavelength λ i The absorption cross section at j is the concentration of the jth gas.

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