A method and system for simultaneously measuring the resonant frequency and concentration of a chirp-modulated photothermal spectrum

Abstract

The application discloses a kind of chirp modulation photo-thermal-optical spectrum resonance frequency and concentration synchronous measurement method and system, belong to photo-thermal-optical spectrum field, comprising: S1, generating chirp modulation signal driving laser, produces modulated laser beam;S2, the modulated laser beam is inducted after passing through gas to the surface of quartz tuning fork excitation photo-thermal-optical signal demodulation obtains first harmonic signal;S3, using lorentz line function fitting the first harmonic signal;S4, according to fitting result, the center resonance frequency position of quartz tuning fork is calculated;S5, whether the center resonance frequency position of the quartz tuning fork is located in the center of first harmonic signal;If not, adjust chirp modulation signal initial frequency repeats S1-S5, until the center resonance frequency position of quartz tuning fork is located in the center of first harmonic signal;S6, according to fitting result, response coefficient is calculated, and inversion obtains gas concentration information.The application can realize high-precision measurement of gas concentration and real-time correction of resonance frequency.

Description

Technical Field

[0001] This invention belongs to the field of photothermoelastic spectroscopy and relates to a method for synchronously measuring resonant frequency and concentration, and more particularly to a method and system for synchronously measuring resonant frequency and concentration in chirped modulated photothermoelastic spectroscopy. Background Technology

[0002] The core sensor in photothermoelastic spectroscopy (LITES) is the quartz tuning fork (QTF). The frequency response curve of the QTF can be described by a Lorentz linear function, and its characteristic parameters are highly sensitive to the external environment. The thermal expansion coefficient of the QTF substrate material causes the QTF resonant frequency to decrease linearly with increasing temperature, and the quality factor exhibits complex fluctuations with temperature. According to currently used wavelength modulation techniques, due to the narrow frequency response bandwidth of the QTF, even small frequency drifts have a significant impact on the spectral response intensity, severely affecting the long-term stability of the LITES system. Currently, most methods use electrical excitation to measure the resonant frequency of the quartz tuning fork. This involves generating sinusoidal signals of different frequencies to excite the quartz tuning fork, measuring and processing the output signal amplitude at different frequencies to obtain a precise resonant frequency. This method offers high accuracy but is time-consuming, taking several minutes. Furthermore, current wavelength modulation techniques cannot achieve simultaneous measurement of the resonant frequency and gas concentration, making it difficult to meet the real-time compensation requirements for resonant frequency in concentration measurement applications. Summary of the Invention

[0003] This invention provides a method and system for synchronously measuring the resonant frequency and concentration of a chirped modulated photothermoelastic spectrum, thereby overcoming the shortcomings of existing technologies.

[0004] To achieve the above objectives, the present invention adopts the following technical solution:

[0005] In a first aspect, the present invention provides a method for synchronously measuring the resonant frequency and concentration of a chirped modulated photothermoelastic spectrum, comprising the following steps:

[0006] S1. Generate a chirped modulation signal to drive the laser and produce a modulated laser beam;

[0007] S2. The modulated laser beam passes through the gas and is incident on the surface of the quartz tuning fork to excite the photothermal elastic signal. The photothermal elastic signal is collected and demodulated to obtain the first harmonic (1f) signal.

[0008] S3. Fit the first harmonic signal using the Lorentz linear function;

[0009] S4. Calculate the location of the center resonant frequency (i.e., the resonant frequency) of the quartz tuning fork based on the fitting results;

[0010] S5. Determine whether the center resonant frequency of the quartz tuning fork is located at the center of the first harmonic signal.

[0011] If not, adjust the initial frequency of the chirp modulation signal and repeat S1 to S5 until the center resonant frequency of the quartz tuning fork is located at the center of the first harmonic signal.

[0012] S6. Calculate the response coefficient based on the fitting results and invert the gas concentration information.

[0013] Furthermore, in S1, the chirped modulation signal S chirp The expression is as follows:

[0014] S chirp =acos[(2π(bt+f0))t+φ]+c;

[0015] In the formula, a is the modulation amplitude of the chirped modulation signal, b is the frequency change rate of the chirped modulation signal, t is time, f0 is the initial frequency of the chirped modulation signal, φ is the initial phase of the chirped modulation signal, and c is the bias of the chirped modulation signal.

[0016] Furthermore, in S1, the chirped modulation signal generates a modulated laser beam through the injection current of the modulated laser.

[0017] Furthermore, in step S2, the chirped modulation signal is used as a reference signal to demodulate the photothermal elastic signal, thereby obtaining a first harmonic signal conforming to a Lorentz linear distribution.

[0018] Furthermore, in S3, the Lorentz linear function y used for fitting is expressed as:

[0019]

[0020] In the formula, y0 is the bias, k is the response coefficient (the value of k is related to the gas absorption coefficient), A is the area under the frequency response curve of the quartz tuning fork, w is the frequency response bandwidth of the quartz tuning fork, b is the frequency change rate of the chirped modulation signal, t is time, f0 is the initial frequency of the chirped modulation signal, and f c It is the center resonant frequency of the quartz tuning fork.

[0021] Furthermore, in S4, the center resonant frequency of the quartz tuning fork is the frequency corresponding to the maximum value of the fitted curve.

[0022] Furthermore, in step S5, if the center resonant frequency of the quartz tuning fork deviates from the center of the first harmonic signal, the method for adjusting the initial frequency of the chirped modulation signal is as follows: if the first harmonic signal is in a state of under-adjustment, the initial frequency of the chirped modulation signal is gradually increased, and steps S1 to S5 are repeated until the center resonant frequency of the quartz tuning fork is located at the center of the first harmonic signal; if the first harmonic signal is in a state of over-adjustment, the initial frequency of the chirped modulation signal is gradually decreased, and steps S1 to S5 are repeated until the center resonant frequency of the quartz tuning fork is located at the center of the first harmonic signal.

[0023] Furthermore, in step S5, after the first harmonic signal frequency adjustment is completed, the center resonant frequency of the quartz tuning fork, the frequency change rate of the chirped modulation signal, and the initial frequency of the chirped modulation signal conform to the following relationship:

[0024]

[0025] In the formula, f c denoted as the center resonant frequency of the quartz tuning fork, f0 as the initial frequency of the chirped modulation signal, b as the rate of change of the frequency of the chirped modulation signal, and t as time.

[0026] Furthermore, in step S6, the response coefficient is inversely proportional to the gas concentration. By measuring and calibrating the relationship between the gas concentration and the response coefficient, the gas concentration information is obtained by inversion.

[0027] Secondly, the present invention provides a chirped modulation photothermal elastic spectral resonant frequency and concentration synchronous measurement system for implementing the above-mentioned method; the system includes a microcontroller, a waveform generator, a laser diode, a gas measurement cell, a quartz tuning fork, a lock-in amplifier, and an analog-to-digital converter; the microcontroller controls the waveform generator to generate a chirped modulation signal; the chirped modulation signal drives the injection current of the laser diode to generate a modulated laser beam; the modulated laser beam passes through the gas measurement cell and converges on the surface of the quartz tuning fork to excite the photothermal elastic signal; the photothermal elastic signal is demodulated by the lock-in amplifier to obtain a first harmonic signal; the first harmonic signal is input to the analog-to-digital converter to realize data acquisition.

[0028] The beneficial effects of this invention are as follows: This invention provides a method and system for synchronously measuring the resonant frequency and concentration of a chirped-modulated photothermoelastic spectrum, which can achieve high-precision measurement of gas concentration and real-time correction of the resonant frequency, thus helping to improve the long-term stability of photothermoelastic spectroscopy in measurement applications. Specifically, this invention uses a chirped modulation signal to modulate the laser injection current. When the chirped-modulated laser beam is directly incident on the surface of a quartz tuning fork, a chirped modulation signal with the same parameters is used as a reference signal to demodulate the photothermoelastic signal output by the quartz tuning fork, obtaining the first harmonic signal. When the frequency scanning range of the chirped modulation signal covers the characteristic frequency range of the quartz tuning fork, the amplitude of the first harmonic signal exhibits a Lorentz linear function distribution with a maximum value. Lorentz linear fitting is performed on it, and the frequency corresponding to the maximum value of the curve is the center resonant frequency of the quartz tuning fork. If the frequency corresponding to the maximum value deviates from the center of the first harmonic signal, the initial frequency of the chirped modulation signal is adjusted to make the center resonant frequency of the quartz tuning fork consistent with the center of the first harmonic signal. The above analysis and adjustment methods enable real-time measurement and compensation of the central resonant frequency of a quartz tuning fork. Simultaneously, gas absorption in the measurement optical path causes overall attenuation of the first harmonic signal, allowing for the inversion of gas concentration information. Attached Figure Description

[0029] Figure 1 This is a block diagram of the chirped-modulated photothermoelastic spectral resonant frequency and concentration synchronous measurement system of the present invention;

[0030] Figure 2 This is a flowchart of the method for synchronously measuring the chirped modulated photothermoelastic spectral resonant frequency and concentration of the present invention;

[0031] Figure 3 This is a schematic diagram of the first harmonic signal in the chirped-modulated photothermoelastic spectral resonant frequency and concentration synchronous measurement method of the present invention;

[0032] Figure 4 The waveforms of the first harmonic signals with different response coefficients in the chirped modulated photothermoelastic spectral resonant frequency and concentration synchronous measurement method of the present invention are shown.

[0033] The labels in the attached diagram are: 1. Microcontroller; 2. Waveform generator; 3. Laser diode; 4. Gas measuring cell; 5. Quartz tuning fork; 6. Lock-in amplifier; 7. Analog-to-digital converter. Detailed Implementation

[0034] The specific embodiments of the present invention will be described below with reference to the accompanying drawings.

[0035] This invention provides a method and system for synchronously measuring the resonant frequency and concentration of a chirped modulated photothermoelastic spectrum.

[0036] like Figure 1As shown, the system includes a microcontroller 1, a waveform generator 2, a laser diode 3, a gas measuring cell 4, a quartz tuning fork 5, a lock-in amplifier 6, and an analog-to-digital converter 7.

[0037] Microcontroller 1 controls waveform generator 2 to generate a chirped modulation signal, S. chirp Expressed using the following formula:

[0038] S chirp =acos[(2π(bt+f0))t+φ]+c;

[0039] In the formula, a is the modulation amplitude of the chirped modulation signal, b is the frequency change rate of the chirped modulation signal, t is time, f0 is the initial frequency of the chirped modulation signal, φ is the initial phase of the chirped modulation signal, and c is the bias of the chirped modulation signal.

[0040] The generated chirped modulation signal drives the injection current of the laser diode 3 to generate a modulated laser beam. After passing through the gas measurement cell 4, the modulated laser beam converges on the surface of the quartz tuning fork 5 to excite the photothermal elastic signal. After conditioning the output signal of the quartz tuning fork, it is demodulated by the lock-in amplifier 6 to obtain the first harmonic signal. Finally, it is input to the analog-to-digital converter 7 to realize the data acquisition of the harmonic signal.

[0041] like Figure 2 As shown, the method is as follows: First, a photothermal elastic signal is acquired, and then the chirped modulation signal is used as a reference signal to demodulate and obtain the 1f signal. The obtained 1f signal conforms to the Lorentz linear distribution. The acquired 1f signal is fitted using the Lorentz linear function, and the function used for fitting is expressed as follows:

[0042]

[0043] In the formula, y0 is the bias, k is the response coefficient (the value of k is related to the gas absorption coefficient), A is the area under the frequency response curve of the quartz tuning fork, w is the frequency response bandwidth of the quartz tuning fork, and f c It is the center resonant frequency of the quartz tuning fork.

[0044] Based on the Lorentz linear function fitting results, the center resonant frequency f of the quartz tuning fork can be obtained. c And the response coefficient k. Figure 3 This demonstrates the frequency under-tuning, full-tuning, and over-tuning of the 1f signal. If the 1f signal is in an under-tuned state, the initial frequency f0 of the chirped modulation signal is gradually increased, and the above steps are repeated until the resonant frequency f is reached. c Located at the center of the 1f signal; if the 1f signal is in a frequency over-adjustment state, gradually decrease the initial frequency f0 of the chirped modulation signal and repeat the above steps until the resonant frequency f is reached. cLocated at the center of the 1f signal. After the 1f signal frequency is adjusted, the center resonant frequency f of the quartz tuning fork is... c The rate of change of the frequency b of the chirped modulated signal and the initial frequency f0 of the chirped modulated signal conform to the following relationship:

[0045]

[0046] At this point, the number of effective data points is the highest during the Lorentz linear fitting process, resulting in the most accurate fitting result. The k value of the 1f signal is then calculated based on the Lorentz linear function fitting result. Figure 4 The waveforms of the 1f signal with different k values ​​are shown. The k value of the 1f signal is inversely proportional to the gas concentration. Based on the concentration calibration curve, the current gas concentration can be calculated.

[0047] In this invention, unless otherwise stated, scientific and technical terms used herein have the meanings commonly understood by those skilled in the art. Furthermore, the equipment and operating procedures used herein are those widely used and conventional in the relevant fields.

[0048] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for synchronously measuring the resonant frequency and concentration of a chirped-modulated photothermoelastic spectrum, characterized in that: Includes the following steps: S1. Generate a chirped modulation signal to drive the laser and produce a modulated laser beam; S2. The modulated laser beam passes through the gas and is incident on the surface of the quartz tuning fork to excite the photothermal elastic signal. The photothermal elastic signal is collected and demodulated to obtain the first harmonic signal. S3. Fit the first harmonic signal using the Lorentz linear function; The Lorentz linear function used for fitting The expression is as follows: In the formula, For bias, For the response coefficient, The area under the frequency response curve of a quartz tuning fork. The frequency response bandwidth of the quartz tuning fork. The rate of change of the frequency of the chirped modulated signal. For time, The initial frequency of the chirped modulation signal. This is the center resonant frequency of the quartz tuning fork; S4. Calculate the position of the center resonant frequency of the quartz tuning fork based on the fitting results; S5. Determine whether the center resonant frequency of the quartz tuning fork is located at the center of the first harmonic signal. If not, adjust the initial frequency of the chirp modulation signal and repeat S1~S5 until the center resonant frequency of the quartz tuning fork is located at the center of the first harmonic signal; S6. Calculate the response coefficient based on the fitting results and invert the gas concentration information. The response coefficient is inversely proportional to the gas concentration. By measuring and calibrating the relationship between the gas concentration and the response coefficient, the gas concentration information is inverted.

2. The method for synchronously measuring the chirped modulated photothermoelastic spectral resonant frequency and concentration according to claim 1, characterized in that: In S1, the chirped modulation signal The expression is as follows: ； In the formula, The modulation amplitude of the chirped signal. The rate of change of the frequency of the chirped modulated signal. For time, The initial frequency of the chirped modulation signal. The initial phase of the chirped modulated signal. This is the bias of the chirped modulated signal.

3. The method for synchronously measuring the chirped modulated photothermoelastic spectral resonant frequency and concentration according to claim 1, characterized in that: In step S1, the chirped modulation signal generates a modulated laser beam through the injection current of the modulated laser.

4. The method for synchronously measuring the chirped modulated photothermoelastic spectral resonant frequency and concentration according to claim 1, characterized in that: In step S2, the chirped modulation signal is used as a reference signal to demodulate the photothermal elastic signal, thereby obtaining a first harmonic signal that conforms to the Lorentz linear distribution.

5. The method for synchronously measuring the chirped modulated photothermoelastic spectral resonant frequency and concentration according to claim 1, characterized in that: In S4, the center resonance frequency of the quartz tuning fork is the frequency corresponding to the maximum value of the fitted curve.

6. The method for synchronously measuring the chirped modulated photothermoelastic spectral resonant frequency and concentration according to claim 1, characterized in that: In step S5, if the center resonant frequency of the quartz tuning fork deviates from the center of the first harmonic signal, the method for adjusting the initial frequency of the chirp modulation signal is as follows: If the first harmonic signal is in a state of under-tuning, gradually increase the initial frequency of the chirped modulation signal and repeat S1~S5 until the center resonant frequency of the quartz tuning fork is located at the center of the first harmonic signal. If the first harmonic signal is in a state of frequency over-adjustment, the initial frequency of the chirped modulation signal is gradually reduced, and S1~S5 are repeated until the center resonant frequency of the quartz tuning fork is located at the center of the first harmonic signal.

7. The method for synchronously measuring the chirped modulated photothermoelastic spectral resonant frequency and concentration according to claim 6, characterized in that: In step S5, after the first harmonic signal frequency adjustment is completed, the center resonant frequency of the quartz tuning fork, the frequency change rate of the chirped modulation signal, and the initial frequency of the chirped modulation signal conform to the following relationship: In the formula, The center resonant frequency of the quartz tuning fork. The initial frequency of the chirped modulation signal. The rate of change of the frequency of the chirped modulated signal. For time.

8. A system for synchronously measuring the resonant frequency and concentration of a chirped modulated photothermoelastic spectrum, characterized in that: Used to implement the method as described in any one of claims 1 to 7; The system includes a microcontroller, waveform generator, laser diode, gas measurement cell, quartz tuning fork, lock-in amplifier, and analog-to-digital converter; The microcontroller controls the waveform generator to generate a chirped modulation signal; the chirped modulation signal drives the injection current of the laser diode to generate a modulated laser beam; the modulated laser beam passes through the gas measurement cell and converges on the surface of the quartz tuning fork to excite a photothermal elastic signal; the photothermal elastic signal is demodulated by a lock-in amplifier to obtain a first harmonic signal; the first harmonic signal is input to an analog-to-digital converter to realize data acquisition.

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