Laser gas measurement method and product based on nonlinear output of laser

Abstract

The invention discloses a laser gas measurement method and product based on nonlinear output of a laser, and aims to solve the problem that a traditional TDLAS harmonic method cannot accurately measure gas concentration when laser output by a mid-infrared laser and a mid-far infrared laser has an obvious nonlinear effect. The method comprises the following steps: acquiring an infrared laser signal penetrating through a to-be-detected gas, and converting the infrared laser signal into an electric signal to obtain a voltage signal of the laser signal; wherein the initial frequency of the infrared laser signal is f; synchronously acquiring a voltage signal, and performing phase-locked amplification on a 4f frequency band signal of the voltage signal to obtain a voltage curve of the 4f signal; the absorption amplitude is calculated according to the peak-to-peak value, and the gas concentration is obtained through calibration coefficient and temperature and gas pressure correction. When the method provided by the invention is applied to a nonlinear output laser, the gas concentration can be accurately inversed and calculated.

Description

Technical Field

[0001] The present application relates to the technical field of gas detection, and in particular to a laser gas measurement method. Background Art

[0002] Tunable Diode Laser Absorption Spectroscopy (TDLAS) scans the laser output wavelength periodically and modulates the sawtooth and sine wave signal intensity. This method is called Wavelength Modulation Spectroscopy (WMS). The general sine wave modulation frequency is f, and the demodulation frequency is 2f, which is called second harmonic demodulation technology. This absorption measurement method uses the peak-to-peak value of the second harmonic waveform as the measurement value to invert and calculate the gas concentration.

[0003] The second harmonic 2f detection method assumes that the power P of the laser light and the driving current I, the output wavelength λ and the driving current I are all linearly related. Under this assumption, it is believed that the second harmonic is only related to the gas concentration, so the gas concentration can be inverted and calculated.

[0004] For near-infrared DFB lasers, the power P and drive current I, and the output wavelength λ and drive current I are very close to a linear relationship, so this assumption is valid and has become the main solution for current WMS technology. However, for mid-infrared and mid-to-far-infrared lasers, the power P and drive current I, and the output wavelength λ and drive current I are nonlinear, and this assumption does not hold. If calculated in this way, the gas concentration cannot be accurately measured. Therefore, a method for measuring gas concentration in the case of nonlinear laser output beam is urgently needed. Summary of the invention

[0005] The present application provides a laser gas measurement method and product based on the nonlinear output of the laser, which aims to solve the problem that the traditional TDLAS harmonic method cannot accurately measure the gas concentration when the laser output of the mid-infrared and mid-to-far-infrared lasers has obvious nonlinear effects.

[0006] In a first aspect, a laser gas measurement method based on nonlinear output of a laser comprises:

[0007] Obtaining an infrared laser signal passing through the gas to be measured and converting it into an electrical signal to obtain a voltage signal of the laser signal; the initial frequency of the infrared laser signal is f;

[0008] The voltage signal is collected synchronously, and the 4f frequency band signal of the voltage signal is phase-locked and amplified to obtain a voltage curve of the 4f signal; wherein the amplitude of the 4f frequency band signal is only related to the gas concentration;

[0009] The absorption amplitude is calculated according to the peak-to-peak value, and the gas concentration is obtained after correction by the calibration coefficient, temperature and air pressure.

[0010] In the above scheme, optionally, the infrared laser signal is a mid-infrared, mid-to-far-infrared laser signal or a near-infrared laser signal with poor linearity.

[0011] In a second aspect, a laser gas measurement system includes: a mid-infrared laser, a long optical path gas chamber, a mid-infrared detector, a current-to-voltage amplifier module, a phase-locked amplifier module, an embedded control system, a laser current drive module, and a laser temperature control module;

[0012] The output optical path of the mid-infrared laser is connected to the input end of the long optical path gas chamber;

[0013] The long optical path gas chamber is arranged on the optical path of the mid-infrared laser, the input end of which receives the laser from the mid-infrared laser, and the output detection end is connected to the input detection end of the mid-infrared detector;

[0014] The mid-infrared detector has an input detection end connected to the output detection end of the long optical path gas chamber, and an output end connected to the input end of the current-to-voltage amplification module;

[0015] The current-to-voltage amplification module has an input end connected to the output end of the mid-infrared detector, and an output end connected to the input end of the phase-locked amplification module;

[0016] The phase-locked amplifier module, whose input end is connected to the output end of the current-to-voltage amplifier module and whose output end is connected to the first signal end of the embedded control system, is used to phase-lock and amplify the 4f frequency of the signal;

[0017] The embedded control system, whose first signal end is connected to the output end of the phase-locked amplifier module, whose second signal end is connected to the first signal end of the laser temperature control module, and whose output end is connected to the laser current driving module; is used to control the laser temperature control module and the laser current driving module to provide temperature control and drive current modulation of the mid-infrared laser, and to adjust the drive current according to the sawtooth wave and sine wave output f frequency; and to perform inversion calculation on the 4f frequency signal collected by the phase-locked amplifier module;

[0018] The laser current driving module has an input end connected to the second signal end of the embedded control system and an output end connected to the first control end of the mid-infrared laser;

[0019] The first signal end of the laser temperature control module is connected to the second signal end of the embedded control system, and the output end is connected to the second control end of the mid-infrared laser.

[0020] In a third aspect, an embedded control system comprises a memory, a processor and a computer program stored in the memory, wherein the processor executes the computer program to implement the steps of the method described above.

[0021] In a fourth aspect, a computer-readable storage medium stores a computer program thereon, wherein the computer program implements the steps of the above method when executed by a processor.

[0022] In a fifth aspect, a computer program product comprises a computer program / instruction, wherein the computer program / instruction implements the steps of the method described above when executed by a processor.

[0023] Compared with the prior art, this application has at least the following beneficial effects:

[0024] This application is based on further analysis and research of the prior art problems, and recognizes that the TDLAS harmonic method is based on the signal demodulation method of the second harmonic. The second harmonic 2f detection method assumes that the power P of the laser light and the driving current I, the output wavelength λ and the driving current I are all linearly related. Under this assumption, it is believed that the second harmonic is only related to the gas concentration, so the gas concentration can be inverted and calculated. However, for mid-infrared and mid-to-far-infrared lasers, the power P and the driving current I, the output wavelength λ and the driving current I are nonlinear, and this assumption does not hold. If calculated in this way, the gas concentration cannot be accurately measured.

[0025] Assuming that the laser output power P and the driving current I, and the output wavelength λ and the driving current I are all linearly related, reference [1] explored the second and fourth harmonics. The center amplitude intensity ratio of the fourth harmonic and the second harmonic is only related to the modulation depth value. A modulation depth function is established. The modulation depth value under the current environment is calculated through the modulation depth function, and then the modulation frequency amplitude is adjusted to optimize the modulation depth, ensuring that the amplitude of the second harmonic signal 2f is maintained at the maximum value, thereby improving the signal-to-noise ratio and measurement accuracy.

[0026] This application conducts nonlinear theoretical analysis through nonlinear laser output cases, proposes a method for measuring gas concentration using fourth harmonic signals, and proposes system composition and corresponding algorithms. The 4f demodulation method proposed in this application aims to calculate the gas concentration when the laser outputs nonlinearly, rather than optimizing the modulation depth coefficient to obtain the optimal 2f result. The 4f signal demodulation method proposed in this application is applied to nonlinear output lasers to obtain a 4f signal related only to the gas concentration, which can accurately invert and calculate the gas concentration. It is not only applicable to mid-infrared ICL and QCL lasers, but also to other near-infrared DFB lasers with poor linearity.

[0027] References [1] Chen Hao, Ju Yu, Han Li. Study on the relationship between modulation depth and high-order harmonic center amplitude in TDLAS wavelength modulation method [J]. Spectroscopy and Spectral Analysis, 2021, 41(12): 3676-81. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 A nonlinear relationship diagram between laser input current and output wavelength provided in one embodiment of the present application;

[0029] Figure 2 A nonlinear relationship diagram between laser input current and output light intensity provided in one embodiment of the present application;

[0030] Figure 3 A structure diagram of a laser gas measurement system provided for one embodiment of the present application;

[0031] Figure 4 A program flow chart of a laser gas measurement system provided for one embodiment of the present application;

[0032] Figure 5 FIG. 4 is a diagram showing the internal structure of a computer device in one embodiment.

[0033] Among them, 1. mid-infrared laser; 2. long optical path gas chamber; 3. MCT mid-infrared detector; 4. current-to-voltage amplifier module; 5. phase-locked amplifier module; 6. embedded control system; 7. laser current drive module; 8. laser temperature control module. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical solution and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0035] In the description of this application: Unless otherwise specified, the terms "first", "second", etc. in this application are intended to distinguish the objects referred to, and do not have special meanings in terms of technical connotations (for example, they should not be understood as emphasizing the importance or order, etc.). Expressions such as "including", "comprising", "having", etc. also mean "not limited to" (certain units, components, materials, steps, etc.).

[0036] In one embodiment, a laser gas measurement method based on nonlinear output of a laser includes:

[0037] Obtain the infrared laser signal passing through the gas to be measured and convert it into an electrical signal to obtain a voltage signal of the laser signal; the initial frequency of the infrared laser signal is f;

[0038] The voltage signal is collected synchronously, and the 4f frequency band signal of the voltage signal is phase-locked and amplified to obtain the voltage curve of the 4f signal; wherein the amplitude of the 4f frequency band signal is only related to the gas concentration;

[0039] The absorption amplitude is calculated according to the peak-to-peak value, and the gas concentration is obtained after correction by the calibration coefficient, temperature and air pressure.

[0040] In one embodiment, the infrared laser signal is a mid-infrared, mid-to-far-infrared laser signal or a near-infrared laser signal with poor linearity.

[0041] The TDLAS harmonic method is based on the signal demodulation method of the second harmonic. Assuming that the laser light power P and the driving current I, and the output wavelength λ and the driving current I are linearly related, reference [1] explored the second and fourth harmonics. The center amplitude intensity ratio of the fourth harmonic and the second harmonic is only related to the modulation depth value. The modulation depth function is established. The modulation depth value under the current environment is calculated by the modulation depth function, and then the modulation frequency amplitude is adjusted to optimize the modulation depth, ensuring that the amplitude of the second harmonic signal 2f remains at the maximum value, thereby improving the signal-to-noise ratio and measurement accuracy.

[0042] This application also proposes a 4f demodulation method, the purpose of which is to calculate the gas concentration when the laser outputs nonlinearly, rather than optimizing the modulation depth coefficient to obtain the best 2f result.

[0043] After the mid-infrared ICL and QCL lasers are output, obvious nonlinear effects will occur, such as Figure 1 and Figure 2 As shown, when the laser current is sinusoidally modulated with a frequency of f, the output current is described as:

[0044] I(t)=I0+ΔI cosωt (1)

[0045] Where I0 represents the driving current corresponding to the gas absorption wavelength, ΔI represents the current value corresponding to the modulation wavelength, and ω=2πf.

[0046] The time variation of wavelength λ can be approximated by equation (2):

[0047]

[0048] In the formula, λ represents the laser wavelength; λ0 represents the constant term of the laser wavelength; I represents the current driving the infrared laser, and ΔI represents the current corresponding to the modulated wavelength; the changing wavelength and current parameters follow Figure 1 The current wavelength characteristics shown in .

[0049] Appear as Figure 1 , Figure 2When nonlinear effects occur, when the modulation wavelength range is very small, the time variation of the laser power P is approximated by equation (3). The nonlinear effect refers to the nonlinear relationship between the laser power P and the driving current I, and the output wavelength λ and the driving current I of the infrared laser.

[0050] P(t)=P0+Δ P cosωt+Δ 2P cos 2 ωt (3)

[0051] In the formula, P(t) represents the relationship between laser power and time, and P0 represents the laser power; Represents the second harmonic generated by the nonlinearity of laser power; represents the third harmonic generated by the nonlinearity of laser power;

[0052] According to the Beer-Lambert absorption law, the power of the beam absorbed by the gas is approximately expressed as (4) under small signal conditions:

[0053] P T (t)=P(t)exp[-α(t)]=P(t)[1-α(t)] (4)

[0054] Where α(t) represents the gas absorption coefficient at the gas absorption wavelength; P T (t) represents the beam power absorbed by the gas; P(t) represents the relationship between the laser power and time;

[0055] When measuring gas, the output wavelength of the infrared laser is scanned near the gas absorption wavelength α, so the absorption coefficient also changes with time, as shown in (5):

[0056] α(t)=α0+α1 cosωt+α2 cos 2 ωt (5)

[0057] Where α0 represents the absorption coefficient constant; and Represents the dynamically changing part of the absorption coefficient;

[0058] According to formulas (3)-(5), we can get:

[0059] P T (t)=(P0+Δ P cosωt+Δ 2P cos 2 ωt)(1-α0-α1cosωt-α2cos 2 ωt) (6)

[0060] Where P T (t) represents the beam power absorbed by the gas; P0 represents the laser power; ΔP Indicates the second harmonic generated by the nonlinearity of laser power; Δ 2P represents the third harmonic generated by the nonlinearity of laser power; α1 and α2 represent the dynamic change part of the absorption coefficient;

[0061] When the voltage amplitude of the photodetector receiving signal is less than the voltage amplitude of the photodetector cutoff signal, and the voltage amplitude of the photodetector receiving signal is greater than the voltage amplitude of the noise, the photodetector receiving signal is proportional to the transmission power: s(t) = ξP T (t), where ξ represents the efficiency of the photodetector. Using trigonometric identities, the signal can be expressed in the following form as (7):

[0062]

[0063] Where s(t) represents the absorption signal of the photodetector; ξ represents the efficiency of the photodetector; P T (t) represents the beam power absorbed by the gas; P0 represents the laser power; Δ P Indicates the second harmonic generated by the nonlinearity of laser power; Δ 2P represents the third harmonic generated by the nonlinearity of laser power; α0 represents the absorption coefficient constant; α1 and α2 represent the dynamic change part of the absorption coefficient; ω, 2ω and 3ω represent the first harmonic, second harmonic and third harmonic generated by gas absorption, respectively;

[0064] Conventional WMS measurements are performed at the second harmonic. In the case of the current-power relationship as characterized by equation (3), the value of the constant component of equation (7) and the amplitudes of the first harmonic ω, second harmonic 2ω, and third harmonic 3ω generated by gas absorption are complex combinations of various parameters. The amplitude of the second harmonic 2ω depends not only on the gas concentration, but also on Δ 2P Parameters. From equation (7), it can be obtained that the amplitude of the fourth harmonic 4ω is directly proportional to α2 and proportional to the gas concentration. Therefore, in the above case, choosing the 4f signal is better than the 2f signal, and a more accurate gas concentration can be obtained.

[0065] The laser gas measurement method based on the nonlinear output of the laser in the present application is applicable not only to mid-infrared ICL and QCL lasers, but also to other near-infrared DFB lasers with poor linearity.

[0066] In one embodiment, reference Figure 3 , a laser gas measurement system, the system comprising: a mid-infrared laser, a long optical path gas chamber, a mid-infrared detector, a current-to-voltage amplifier module, a phase-locked amplifier module, an embedded control system, a laser current drive module and a laser temperature control module;

[0067] A mid-infrared laser, the output optical path of which is connected to the input end of the long optical path gas cell;

[0068] The long optical path gas chamber is arranged on the optical path of the mid-infrared laser, the input end of which receives the laser light from the mid-infrared laser, and the output detection end is connected to the input detection end of the mid-infrared detector;

[0069] A mid-infrared detector, whose input detection end is connected to the output detection end of the long optical path gas chamber, and whose output end is connected to the input end of the current-to-voltage amplification module;

[0070] A current-to-voltage amplifier module, whose input end is connected to the output end of the mid-infrared detector, and whose output end is connected to the input end of the phase-locked amplifier module;

[0071] A phase-locked amplifier module, whose input end is connected to the output end of the current-to-voltage amplifier module and whose output end is connected to the first signal end of the embedded control system; and is used to phase-lock and amplify the 4f frequency of the signal;

[0072] An embedded control system, wherein the first signal end is connected to the output end of the phase-locked amplifier module, the second signal end is connected to the first signal end of the laser temperature control module, and the output end is connected to the laser current driving module; the embedded control system is used to control the laser temperature control module and the laser current driving module to provide temperature control and drive current modulation of the mid-infrared laser, and to adjust the drive current according to the f frequency of the sawtooth wave and the sine wave output; and to perform inversion calculation on the 4f frequency signal collected by the phase-locked amplifier module;

[0073] A laser current driving module, whose input end is connected to the second signal end of the embedded control system and whose output end is connected to the first control end of the mid-infrared laser;

[0074] The first signal end of the laser temperature control module is connected to the second signal end of the embedded control system, and the output end is connected to the second control end of the mid-infrared laser.

[0075] refer to Figure 3 , the embedded control system 6 controls the laser current driving module 7 and the laser temperature control module 8 to provide temperature control and driving current modulation of the mid-infrared laser 1, and adjusts the driving current according to the sawtooth wave and the sine wave f frequency. The mid-infrared laser 1 outputs the modulated wavelength λ according to the modulated current, and the optical power P also changes accordingly. The light beam is irradiated into the long optical path gas chamber 2, and the gas absorbs the energy of the light beam. The light signal is converted into a current signal at the current-to-voltage amplifier module 4, and then converted into a voltage signal through the current-to-voltage amplifier module 4. The 4f frequency phase-locked amplification is performed through the phase-locked amplifier module 5, and the 4f signal excited by f is amplified. The curve of the 4f signal is collected by the embedded control system 6, and the gas concentration is obtained by inversion calculation through the software process. Among them, the mid-infrared laser 1 is an ICL or QCL mid-infrared laser, which directly emits light through TO.

[0076] In one embodiment, a control method for a laser gas measurement system is also provided. The method is based on a laser gas measurement system. Figure 4 Under the control of the laser temperature control module 8, the laser temperature control is normal (that is, the output wavelength λ0 corresponds to the gas absorption wavelength), and the embedded control system 6 controls the laser current driving module 7 to output a sawtooth wave and a sine wave with a frequency of f. The current-to-voltage amplification module 4 synchronously collects the corresponding 4f signal, and after scanning once, obtains the voltage curve of the 4f signal, calculates the absorption amplitude according to the peak-to-peak value, and obtains the gas concentration after correction by the calibration coefficient and temperature and pressure.

[0077] Combination Figure 4 In this embodiment, the laser gas measurement system is initialized, and the laser temperature control module 8 is used to drive the mid-infrared laser 1 for temperature control to determine whether the temperature control is normal (i.e., determine whether the output wavelength λ0 corresponds to the gas absorption wavelength). When the temperature control is abnormal, the process is terminated and the laser temperature control module 8 is readjusted; when the temperature control is normal, the laser current drive module 7 is controlled to output sawtooth waves and sine waves at a frequency of f, and the laser passes through the long optical path gas chamber 2 and is detected by the MCT mid-infrared detector 3. The current-to-voltage amplifier module 4 synchronously collects the signal. After scanning once, the voltage curve of the signal is obtained, and the phase-locked amplifier module 5 is used to phase-lock amplify the frequency band with an output wavelength of 4f, and the embedded control system 6 is used to collect the voltage value of the 4f harmonic signal; it is determined whether the sawtooth wave array is scanned, and the data is repeatedly collected when the scan is not completed; when the scan is completed, the peak-to-peak value of the 4f signal within the sawtooth wave scanning period is calculated, and the gas concentration is calculated by inversion combined with the calibration coefficient, and the calculated result is corrected for temperature and pressure and output.

[0078] In one embodiment, an embedded control system is provided, whose internal structure diagram can be shown as follows: Figure 5 As shown. The computer device includes a processor, a memory, a communication interface, and a human-computer interaction interface (such as a combination of a display, a keyboard, and a mouse, and a touch screen, etc.) connected through a system bus. Among them, the processor of the computer device is used to provide computing and control capabilities, and the communication interface is used to communicate with an external terminal in a wired or wireless manner. The wireless method can be achieved through WIFI, an operator network, NFC (near field communication) or other technologies. The computer device implements the above-mentioned laser gas measurement method based on the nonlinear output of the laser by loading and running a computer program.

[0079] Those skilled in the art will understand that Figure 5The structure shown in the figure is only a block diagram of a part of the structure related to the solution of the present application, and does not constitute a limitation on the computer device to which the solution of the present application is applied. The specific computer device may include more or fewer components than those shown in the figure, or combine certain components, or have a different arrangement of components.

[0080] In one embodiment, a computer-readable storage medium is further provided, on which a computer program is stored. When the computer program is executed by a processor, the steps of the method in the above embodiment are implemented.

[0081] In one embodiment, a computer program product is also provided, including a computer program / instruction, which implements the steps of the above-mentioned embodiment method when executed by a processor.

[0082] The technical features of the above embodiments may be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

Claims

1. A laser gas measurement method based on nonlinear output of a laser, characterized in that: include: Obtaining an infrared laser signal passing through the gas to be measured and converting it into an electrical signal to obtain a voltage signal of the laser signal; the initial frequency of the infrared laser signal is f; The voltage signal is collected synchronously, and the 4f frequency band signal of the voltage signal is phase-locked and amplified to obtain a voltage curve of the 4f signal; wherein the amplitude of the 4f frequency band signal is only related to the gas concentration; The absorption amplitude is calculated according to the peak-to-peak value, and the gas concentration is obtained after correction by the calibration coefficient, temperature and air pressure.

2. The laser gas measurement method based on laser nonlinear output according to claim 1, characterized in that: include: The infrared laser signal is a mid-infrared, mid-to-far-infrared laser signal or a near-infrared laser signal with poor linearity.

3. A laser gas measurement system, characterized in that: include: Mid-infrared laser, long optical path gas chamber, mid-infrared detector, current-to-voltage amplifier module, phase-locked amplifier module, embedded control system, laser current drive module and laser temperature control module; The output optical path of the mid-infrared laser is connected to the input end of the long optical path gas chamber; The long optical path gas chamber is arranged on the optical path of the mid-infrared laser, the input end of which receives the laser from the mid-infrared laser, and the output detection end is connected to the input detection end of the mid-infrared detector; The mid-infrared detector has an input detection end connected to the output detection end of the long optical path gas chamber, and an output end connected to the input end of the current-to-voltage amplification module; The current-to-voltage amplification module has an input end connected to the output end of the mid-infrared detector, and an output end connected to the input end of the phase-locked amplification module; The phase-locked amplifier module, whose input end is connected to the output end of the current-to-voltage amplifier module and whose output end is connected to the first signal end of the embedded control system, is used to phase-lock and amplify the 4f frequency of the signal; The embedded control system has a first signal end connected to the output end of the phase-locked amplifier module, a second signal end connected to the first signal end of the laser temperature control module, and an output end connected to the laser current driving module; Used to control the laser temperature control module and the laser current driving module to provide temperature control and driving current modulation of the mid-infrared laser, and to adjust the driving current according to the sawtooth wave and sine wave output f frequency; And performing inversion calculation on the 4f frequency signal collected by the phase-locked amplifier module; The laser current driving module has an input end connected to the second signal end of the embedded control system and an output end connected to the first control end of the mid-infrared laser; The first signal end of the laser temperature control module is connected to the second signal end of the embedded control system, and the output end is connected to the second control end of the mid-infrared laser.

4. An embedded control system, comprising a memory, a processor and a computer program stored in the memory, characterized in that: The processor executes the computer program to implement the steps of the method of claim 1.

5. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of the method according to claim 1 are implemented.

6. A computer program product comprising a computer program / instructions, characterized in that When the computer program / instructions are executed by a processor, the steps of the method according to claim 1 are implemented.

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