Dual-mode photoacoustic spectrometry temperature drift suppression method based on dual-frequency modulation

Abstract

The invention discloses a dual-mode photoacoustic spectrum temperature drift suppression method based on dual-frequency modulation, and relates to the field of gas detection.The dual-mode photoacoustic spectrum temperature drift suppression method based on dual-frequency modulation comprises the following steps that S1, a dual-mode photoacoustic spectrum temperature drift suppression system is constructed, the system comprises a laser controller, a tunable laser, a photoacoustic cell, a microphone, a lock-in amplifier and a data processing unit, compared with the prior art, the device and the method have the beneficial effects that different acoustic modes in the same photoacoustic cell are used for carrying out differential compensation on the physical difference of temperature sensitivity, so that the detection precision is remarkably improved; double-frequency simultaneous modulation and demodulation are adopted, and real-time response and compensation of the gas temperature effect are achieved; an additional temperature control device or an expensive quick response temperature sensor does not need to be added, so that the complexity and the cost of the system are reduced; the intrinsic attribute of the gas is utilized, and the method has better universality and robustness compared with pure data fitting.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of gas detection, and particularly to a dual-mode photoacoustic spectroscopy temperature drift suppression method based on dual-frequency modulation. BACKGROUND

[0002] Photoacoustic spectroscopy gas detection technology is widely used in trace gas detection field due to its high sensitivity, zero background noise and wide dynamic range. Its basic principle is to use the thermal expansion of the measured gas molecules to absorb modulated light energy, and then excite sound waves, and the gas concentration is inversely calculated by detecting the sound wave intensity.

[0003] In practical application, the change of environmental temperature is one of the main factors limiting the detection accuracy and long-term stability of photoacoustic spectroscopy. Temperature drift will bring two influences: resonance frequency drift: the speed of sound is proportional to the square root of temperature, and the change of temperature will cause the resonance frequency of the photoacoustic cell to shift. If the laser modulation frequency is fixed, the shift of the resonance frequency will cause the photoacoustic signal amplitude to drop significantly. System response coefficient changes: the amplitude of the photoacoustic signal is not only related to the resonance frequency, but also related to the thermal physical parameters (such as thermal diffusion coefficient) of the gas and the quality factor (Q value) of the photoacoustic cell. Even if the resonance frequency tracking solves the frequency drift problem, the change of background temperature will still directly cause the change of the detected photoacoustic signal amplitude, thereby introducing measurement error.

[0004] The existing temperature compensation techniques mainly include: External sensor compensation method: a temperature sensor (such as a thermistor) is attached to the wall of the photoacoustic cell to detect the temperature and make corrections. However, the sensor measures the wall temperature, which has a thermal conduction lag with the gas temperature in the cell, and cannot accurately reflect the real temperature fluctuation of the gas in real time.

[0005] Algorithm compensation method: a temperature-signal model is established for software correction. However, this depends on the accuracy of the model, and cannot eliminate the physical interference caused by rapid temperature fluctuation.

[0006] In summary, the existing technology cannot suppress temperature drift in real time from the physical mechanism, and needs to be improved. SUMMARY

[0007] The purpose of the present application is to provide a dual-mode photoacoustic spectroscopy temperature drift suppression method based on dual-frequency modulation to solve the problems raised in the background art.

[0008] To achieve the above purpose, the present application provides the following technical solutions: A dual-mode photoacoustic spectroscopy temperature drift suppression method based on dual-frequency modulation, comprising the following steps: Step S1, constructing a dual-mode photoacoustic spectrum temperature drift suppression system, the system comprising a laser controller (or a dual-frequency signal source), a tunable laser, a photoacoustic cell, a microphone, a lock-in amplifier and a data processing unit; the photoacoustic cell is designed as a cylindrical structure capable of supporting longitudinal resonant mode and radial resonant mode at the same time; Step S2, controlling the system to perform gas detection and temperature drift suppression, step S2 comprising: Step S21, controlling the laser controller to generate a modulation signal containing two frequency components, driving the tunable laser to output a dual-frequency intensity-modulated light beam, the light beam being incident into the photoacoustic cell; Step S22, the light beam is absorbed by the gas to be detected in the photoacoustic cell, and a mixed acoustic signal containing longitudinal and radial resonant modes is excited at the same time; Step S23, the microphone receives the mixed acoustic signal and converts it into an electrical signal, and outputs the electrical signal to the lock-in amplifier; Step S24, the lock-in amplifier uses a reference signal with the same frequency as the modulation frequency to perform dual-channel synchronous demodulation on the electrical signal, and outputs a first signal amplitude V L corresponding to the longitudinal mode and a second signal amplitude V R corresponding to the radial mode in real time; Step S25, the data processing unit reads V L and V R , and performs a weighted difference operation V final =V L -k*V R to obtain a final signal V final that has been suppressed in temperature drift, wherein k is a pre-calibrated weighting coefficient (the purpose of the weighted difference operation is to use the difference in sensitivity of the longitudinal mode and the radial mode to temperature changes to eliminate the common influence of temperature changes on the gas concentration measurement signal at the physical level through difference processing); Step S26, based on the pre-calibrated monotonic correspondence between the final signal V final and the gas concentration, the gas concentration value is obtained by inversion (with very high temperature drift stability) and output.

[0009] As a further scheme of the present application: in step S1, the tunable laser is a distributed feedback (DFB) laser, and the center wavelength of the distributed feedback laser is aligned with the absorption line of the target gas.

[0010] As a further scheme of the present application: in step S1, the photoacoustic cell is an H-shaped longitudinal-radial composite resonant photoacoustic cell.

[0011] As a further scheme of the present application: in step S21, the modulation signal is in the form of:

[0012] I represents the laser driving current, represents the longitudinal modal modulation amplitude, represents the radial modal modulation amplitude, represents the longitudinal resonance frequency, represents the radial resonance frequency.

[0013] As a further scheme of the present application: in step S24, the phase-locked amplifier works in a double-mode demodulation mode, and can demodulate two signal components of different frequencies in parallel.

[0014] As a further scheme of the present application: in step S25, the weighting coefficient k is determined through the following pre-calibration steps: Step S251, in a controllable temperature environment, a standard gas of a known concentration is introduced, and the longitudinal signal S L and the radial signal S R measured at different temperatures T are recorded, and the temperature response functions F L and F R of the two modes are fitted; Step S252, the weighting coefficient k is calculated, since the loss mechanisms of the longitudinal and radial acoustic fields are different (the longitudinal is greatly affected by the viscous boundary layer, and the radial is greatly affected by the thermal conduction boundary layer), the temperature dependence of the two is different, and a compensation formula is set as S comp =S L - k * S R , the value of k is determined so that the variance in the test temperature range is minimum (i.e., not sensitive to temperature).

[0015] As a further scheme of the present application: the controllable temperature environment realizes the suppression of background temperature drift through physical methods.

[0016] Compared with the prior art, the present application has the beneficial effects that: the present application utilizes the physical differences in temperature sensitivity of different acoustic modes in the same photoacoustic cell for differential compensation, fundamentally suppressing the measurement error caused by environmental temperature fluctuations, significantly improving the detection accuracy; using double-frequency simultaneous modulation and demodulation, without switching the measurement mode, and without waiting for the thermal equilibrium of the external temperature sensor, realizing real-time response and compensation for the temperature effect of the gas; without increasing additional temperature control devices or expensive fast-response temperature sensors, but only by improving the modulation and demodulation method and algorithm, the system complexity and cost are reduced; the intrinsic properties of the gas (sound speed, viscosity, thermal conductivity, etc. change with temperature) are utilized, which has better universality and robustness than simple data fitting. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 is a schematic diagram of a dual-mode photoacoustic spectroscopy temperature drift suppression system.

[0018] Figure 2 is a flowchart of controlling the operation of the dual-mode photoacoustic spectroscopy temperature drift suppression system. DETAILED DESCRIPTION

[0019] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0020] Please refer to Figure 1 and Figure 2 , a dual-mode photoacoustic spectroscopy temperature drift suppression method based on dual-frequency modulation, comprising the following steps: Step S1, a dual-mode photoacoustic spectroscopy temperature drift suppression system is constructed, the system comprising a laser controller (or a dual-frequency signal source), a tunable laser, a photoacoustic cell, a microphone, a lock-in amplifier and a data processing unit; the photoacoustic cell is designed as a cylindrical structure capable of simultaneously supporting longitudinal resonant modes and radial resonant modes; Step S2, the system is controlled to perform gas detection and temperature drift suppression, and step S2 comprises: Step S21, the laser controller is controlled to generate a modulation signal containing two frequency components, to drive the tunable laser to output a dual-frequency intensity-modulated light beam, and the light beam is incident into the photoacoustic cell; Step S22, the light beam is absorbed by the gas to be detected in the photoacoustic cell, and simultaneously excites a mixed acoustic wave signal containing longitudinal and radial resonant modes; Step S23, the microphone receives the mixed acoustic wave signal and converts it into an electrical signal, and the electrical signal is output to the lock-in amplifier; Step S24, the lock-in amplifier uses a reference signal with the same frequency as the modulation frequency to perform dual-channel synchronous demodulation on the electrical signal, and real-time outputs a first signal amplitude V L corresponding to the longitudinal mode and a second signal amplitude V R corresponding to the radial mode; Step S25, the data processing unit reads V L and V R , and performs a weighted difference operation V final = V L -k*V R , to obtain a final signal V finalwhere k is a pre-calibrated weighting coefficient (the purpose of the weighted difference operation is to eliminate the common influence of temperature change on the gas concentration measurement signal at the physical level by using the difference between the longitudinal and radial modalities in the response sensitivity to temperature change); Step S26, based on the final signal V final The calibrated monotonic correspondence between the gas concentration is obtained by inversion (with very high temperature drift stability) and output.

[0021] In this embodiment: please refer to Figure 1 In step S1, the tunable laser is a distributed feedback (DFB) laser, and the center wavelength of the distributed feedback laser is aligned with the absorption line of the target gas.

[0022] The narrow linewidth and wavelength stability of the distributed feedback laser ensure that the light energy is accurately absorbed by the target gas, improving the detection sensitivity.

[0023] In this embodiment: please refer to Figure 1 In step S1, the photoacoustic cell is an H-shaped longitudinal-radial composite resonance photoacoustic cell.

[0024] The H-shaped structure can effectively separate and simultaneously enhance the longitudinal and radial acoustic resonance modes, providing a physical basis for dual-mode detection.

[0025] In this embodiment: please refer to Figure 2 In step S21, the form of the modulation signal is:

[0026] I represents the laser driving current, represents the longitudinal mode modulation amplitude, represents the radial mode modulation amplitude, represents the longitudinal resonance frequency, represents the radial resonance frequency.

[0027] The modulation signal is composed of two sine waves superimposed, which are accurately matched with the two inherent resonance frequencies of the photoacoustic cell to efficiently excite dual-mode acoustic waves.

[0028] In this embodiment: please refer to Figure 2 In step S24, the lock-in amplifier works in a dual-mode demodulation mode, which can demodulate two different frequency signal components in parallel.

[0029] The lock-in amplifier is set to a dual-mode demodulation mode, which can extract two frequency signals synchronously and independently, realizing real-time parallel acquisition of dual-mode signals.

[0030] In this embodiment: please refer to Figure 2, in step S25, the weighting coefficient k is determined by the following pre-calibration steps: Step S251, in a controllable temperature environment, a standard gas of known concentration is introduced, and the longitudinal signal S L and radial signal S R measured at different temperatures T are recorded L and F R are fitted to obtain the temperature response functions of the two modes Step S252, calculate the weighting coefficient k, since the loss mechanisms of the longitudinal and radial acoustic fields are different (the longitudinal is greatly affected by the viscous boundary layer, and the radial is greatly affected by the thermal conduction boundary layer), the dependence of the two on temperature is different, and the compensation formula is set as S comp =S L - k * S R Determine the value of k so that the variance in the test temperature range is minimized (i.e. not sensitive to temperature).

[0031] The value of k is calibrated by temperature control experiments, and the optimal compensation coefficient that minimizes the temperature variance of the difference signal is obtained by using the difference of the temperature response curves of the two modes.

[0032] In this embodiment: The controllable temperature environment physically suppresses the background temperature drift.

[0033] The physical method is used to suppress the background temperature drift and ensure the detection performance of the system. Temperature changes will change the speed of sound of the gas, causing the resonance frequency of the photoacoustic cell to drift, and may also change the sensitivity of the microphone. Physical operation: The laser is modulated at two different physical frequencies at the same time (for example, one corresponds to the first-order longitudinal mode, and the other corresponds to the second-order longitudinal mode). The system simultaneously detects the photoacoustic signals at these two frequencies. Since the influence of temperature on the two modes is correlated, by extracting the difference between the two signals through hardware circuit or physical model, the common variable temperature can be physically canceled out, thereby only retaining the information of the gas concentration.

[0034] The present application utilizes the physical difference in temperature sensitivity of different acoustic modes in the same photoacoustic cell for differential compensation, fundamentally suppressing the measurement error caused by environmental temperature fluctuations, significantly improving the detection accuracy; using dual-frequency simultaneous modulation and demodulation, without switching measurement modes or waiting for the external temperature sensor to reach thermal equilibrium, real-time response and compensation of the temperature effect of the gas is achieved; without the need for additional temperature control devices or expensive fast-response temperature sensors, it can be realized by improving the modulation and demodulation method and algorithm, reducing the system complexity and cost; using the intrinsic properties of the gas (sound speed, viscosity, thermal conductivity, etc. change with temperature), it has better universality and robustness than simple data fitting.

[0035] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and not restrictive.

[0036] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for suppressing temperature drift in a dual-mode photoacoustic spectrum based on dual-frequency modulation, characterized in that, The dual-mode photoacoustic spectral temperature drift suppression method based on dual-frequency modulation includes the following steps: Step S1: Construct a dual-mode photoacoustic spectral temperature drift suppression system. The system includes a laser controller, a tunable laser, a photoacoustic cell, a microphone, a lock-in amplifier, and a data processing unit. The photoacoustic cell is designed as a cylindrical structure that can simultaneously support longitudinal and radial resonant modes. Step S2: The control system performs gas detection and temperature drift suppression. Step S2 includes: Step S21: Control the laser controller to generate a modulation signal containing two frequency components, drive the tunable laser to output a beam modulated by dual-frequency intensity, and the beam is injected into the photoacoustic cell. In step S22, the light beam is absorbed by the gas to be tested in the photoacoustic cell, and at the same time, a mixed acoustic signal containing longitudinal and radial resonant modes is excited. Step S23: The microphone receives the mixed sound wave signal and converts it into an electrical signal, and outputs the electrical signal to the lock-in amplifier; In step S24, the lock-in amplifier uses a reference signal with the same modulation frequency to perform dual-channel synchronous demodulation of the electrical signal, and outputs the first signal amplitude V corresponding to the longitudinal mode in real time. L and the second signal amplitude V corresponding to the radial mode R ; Step S25, the data processing unit reads V L and V R And perform weighted difference operation V final =V L -k*V R The final signal V with temperature drift suppressed is obtained. final , where k is a pre-calibrated weighting coefficient; Step S26, based on the final signal V final The established monotonic correspondence between the gas concentration and the gas concentration is used to invert and output the gas concentration value.

2. The method for suppressing temperature drift in a dual-mode photoacoustic spectrum based on dual-frequency modulation according to claim 1, characterized in that, In step S1, the tunable laser is a distributed feedback laser, and the center wavelength of the distributed feedback laser is aligned with the absorption line of the target gas.

3. The method for suppressing temperature drift in a dual-mode photoacoustic spectrum based on dual-frequency modulation according to claim 1, characterized in that, In step S1, the photoacoustic cell is an H-shaped longitudinal-radial composite resonant photoacoustic cell.

4. The method for suppressing temperature drift in a dual-mode photoacoustic spectrum based on dual-frequency modulation according to claim 1, characterized in that, In step S21, the modulated signal takes the following form: ， I represents the laser drive current. Indicates the longitudinal mode modulation amplitude. Indicates the radial mode modulation amplitude. Indicates the longitudinal resonant frequency. This represents the radial resonant frequency.

5. The method for suppressing temperature drift in a dual-mode photoacoustic spectrum based on dual-frequency modulation according to claim 1, characterized in that, In step S24, the lock-in amplifier operates in dual-mode demodulation mode, which can demodulate two signal components of different frequencies in parallel.

6. The method for suppressing temperature drift in a dual-mode photoacoustic spectrum based on dual-frequency modulation according to claim 1, characterized in that, In step S25, the weighting coefficient k is determined through the following pre-calibration steps: Step S251: In a temperature-controlled environment, a standard gas of known concentration is introduced, and the longitudinal signal S measured at different temperatures T is recorded. L and radial signal S R The temperature response function F for two modes is obtained by fitting the curves of temperature change with temperature T. L and F R ; Step S252: Calculate the weighting coefficient k. Due to the different loss mechanisms of the longitudinal and radial sound fields, their dependence on temperature differs. The compensation formula is set as follows: S comp =S L -k*S R Determine the value of k such that the variance is minimized within the test temperature range.

7. The method for suppressing temperature drift in a dual-mode photoacoustic spectrum based on dual-frequency modulation according to claim 6, characterized in that, A temperature-controlled environment suppresses background temperature drift through physical methods.

Images