Method and device for improving laser gas sensing measurement precision based on interference suppression

Abstract

The embodiment of the invention discloses a method and device for improving laser gas sensing measurement precision based on interference suppression, and the method comprises the steps: providing 1f-frequency sine wave driving current signal frequency for laser driving current during TDLAS second harmonic detection, and synchronously providing high-frequency sine wave driving current signal frequency; wherein f is the set sine wave frequency of the driving current of the laser, and the harmonic signal is demodulated according to 2f frequency during demodulation. According to the embodiment of the invention, the interference of the gas absorption spectrum caused by optical interference of the long-optical-path gas chamber is solved, the interference of the amplitude of interference fringes can be effectively reduced, the interference fringes in the measurement gas absorption spectrum period are mutually counteracted by externally adding a periodic high-frequency modulation signal, and the effect that the integral is zero is realized; the influence of interference fringes on a gas absorption spectrum is inhibited, and the gas measurement accuracy is improved.

Description

Technical Field

[0001] The embodiments of the present invention relate to the technical field of laser gas sensor measurement, and specifically to a method and device for improving the measurement accuracy of laser gas sensor based on interference suppression. Background Art

[0002] TDLAS (Tunable Diode Laser Absorption Spectroscopy) periodically scans the laser output wavelength, usually with a sine wave modulation frequency of f and a demodulation frequency of 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 long optical path gas chamber used in this technology is a low-concentration gas measurement method, which can effectively expand the lower limit of concentration measurement. When designing a long optical path gas chamber, it is necessary to perform n reflections as many times as possible within the limited gas chamber length L to achieve an optical path of nL. According to Beer-Lambert's gas absorption law, the lower limit of gas measurement can be greatly reduced by increasing the optical path level. When designing a long optical path gas chamber, a high-reflectivity optical lens (reaching a reflectivity of more than 95%) is used. The laser beam is reflected multiple times on an optical surface with a limited area. Due to the poor interference in the optical system, its transmittance is modulated by the wavelength and overlaps with each other, causing interference. When periodic interference fringes appear, the fringes are affected by temperature and pressure changes, superimposed on the gas absorption spectrum and move randomly, interfering with the measurement of gas concentration and making gas measurement inaccurate. Summary of the invention

[0004] To this end, an embodiment of the present invention provides a method and device for improving the measurement accuracy of laser gas sensors based on interference suppression, so as to solve the technical problem that periodic interference fringes appear in the prior art, which are affected by temperature and pressure changes, overlap with the gas absorption spectrum, and interfere with the gas concentration measurement.

[0005] In order to achieve the above purpose, the embodiment of the present invention provides the following technical solutions:

[0006] According to a first aspect of an embodiment of the present invention, a method for improving the measurement accuracy of laser gas sensing based on interference suppression is provided, the method comprising:

[0007] Obtain the voltage signal of driving the laser and convert it into driving current;

[0008] Regulating the temperature and modulating the driving current, adjusting the driving current according to the first preset laser driving current sinusoidal wave frequency 1f and the high-frequency sinusoidal wave driving current signal frequency, and generating a modulated current;

[0009] The gas to be detected is input into the long optical path gas chamber through the gas inlet pipeline;

[0010] Obtaining the modulation current and outputting the modulated wavelength and optical power, while irradiating the light beam to the collimator, the collimator converts the light beam into a spatial light beam and irradiates the light beam into the long optical path gas chamber;

[0011] The gas to be detected in the long optical path gas chamber absorbs the energy of the spatial light beam and converts the optical signal into a current signal, and then into a voltage signal;

[0012] Performing second harmonic demodulation processing on the voltage signal to extract a second harmonic signal related to gas concentration;

[0013] The accurate gas concentration is obtained by inverting the Beer-Lambert gas absorption law and the second harmonic signal.

[0014] Furthermore, the frequency of the high-frequency sinusoidal wave driving current signal is at least 10 times the first preset laser driving current sinusoidal wave frequency 1f; the interference of interference fringes is reduced by the high-frequency sinusoidal wave driving current signal frequency.

[0015] Further, the voltage signal is subjected to second harmonic demodulation processing to extract a second harmonic signal related to the gas concentration, including:

[0016] Performing phase-locked amplification of the voltage signal at a phase-locked amplification frequency of 2f, acquiring a corresponding 2f signal curve, and using the voltage signal to draw an absorption curve;

[0017] The voltage of the high-frequency sinusoidal wave driving current signal frequency is adjusted to minimize the 2f signal amplitude and make the measurement wavelength correspond to the absorption curve one by one. At this time, the 2f signal curve is the second harmonic signal related to the gas concentration.

[0018] Furthermore, accurate gas concentration is obtained by inverting the Beer-Lambert gas absorption law and the second harmonic signal, including:

[0019] The peak value of the second harmonic signal is proportional to the gas concentration, and the accurate gas concentration can be calculated using the second harmonic signal.

[0020] Furthermore, the sine wave of the high-frequency sine wave driving current signal frequency is consistent with the clock of the phase-locked amplification frequency 2f.

[0021] Furthermore, the sine wave of the high-frequency sine wave driving current signal frequency is consistent with the clock of the phase-locked amplification frequency 2f, and further includes:

[0022] During high-frequency modulation, when the modulation wavelength changes very little, the interference fringes are approximately sinusoidal waveforms. When driven by the high-frequency sinusoidal drive current signal frequency, the integral value of the interference fringe change is made zero through phase change, eliminating the influence of fringe interference.

[0023] Furthermore, the voltage amplitude of the high-frequency sinusoidal wave driving current signal frequency is adjustable within a voltage amplitude range of 0.1 to 10 times the first preset laser driving current sinusoidal wave frequency 1f, which is used to optimize the interference fringe shift phase and the suppression effect.

[0024] According to a second aspect of an embodiment of the present invention, a device for improving the measurement accuracy of laser gas sensing based on interference suppression is provided, the device comprising:

[0025] An embedded control module, used to control the drive signal addition module and provide a first preset laser drive current sine wave frequency 1f, receive the second harmonic signal to perform inversion calculation, and obtain accurate gas concentration;

[0026] A driving signal adding module, used for providing a voltage signal for driving the laser;

[0027] A laser current driving module is used to convert the voltage signal driving the laser into a current signal to provide current modulation for the laser;

[0028] Laser temperature control module, providing temperature control for the laser;

[0029] A laser is used to output a modulated wavelength by modulating current;

[0030] A collimator, used for converting an incident light beam into a spatial light beam and irradiating the spatial light beam into a long optical path gas chamber;

[0031] The gas absorbs the energy of the beam emitted by the laser in the long optical path gas cell;

[0032] Inlet and outlet pipelines are used to input and output the gas that needs to be tested;

[0033] Photodetector, which converts light signals into current signals;

[0034] TIA voltage conversion module, converting the current signal converted by the photodetector into a voltage signal;

[0035] The lock-in amplifier module is used to perform second harmonic demodulation processing on the voltage signal to extract the second harmonic signal related to the gas concentration.

[0036] The embodiments of the present invention have the following advantages:

[0037] In the TDLAS second harmonic detection, the embodiment of the present invention provides a 1f frequency sine wave drive current signal frequency for the laser drive current, and synchronously provides a high-frequency sine wave drive current signal frequency, wherein f is the set laser drive current sine wave frequency, and demodulates the harmonic signal according to the 2f frequency during demodulation. The embodiment of the present invention solves the gas absorption spectrum interference caused by the optical interference of the long optical path gas chamber, and can effectively reduce the interference of the interference fringe amplitude. By adding a periodic high-frequency modulation signal, the interference fringes within the measurement gas absorption spectrum period are offset, and the integral is zeroed, thereby suppressing the influence of the interference fringes on the gas absorption spectrum and improving the gas measurement accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] In order to more clearly illustrate the implementation methods of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for the implementation methods or the description of the prior art. Obviously, the drawings in the following description are only exemplary, and for ordinary technicians in this field, other implementation drawings can be derived from the provided drawings without creative work.

[0039] The structures, proportions, sizes, etc. illustrated in this specification are only used to match the contents disclosed in the specification so as to facilitate understanding and reading by persons familiar with the technology. They are not used to limit the conditions under which the present invention can be implemented, and therefore have no substantial technical significance. Any structural modification, change in proportion or adjustment of size shall still fall within the scope of the technical contents disclosed in the present invention without affecting the effects and purposes that can be achieved by the present invention.

[0040] Figure 1 A schematic diagram of the logical structure of a device for improving the measurement accuracy of laser gas sensing based on interference suppression provided by an embodiment of the present invention;

[0041] Figure 2 A schematic flow chart of a method for improving the measurement accuracy of laser gas sensing based on interference suppression provided by an embodiment of the present invention;

[0042] Figure 3 A schematic diagram of the effect of fringe interference on the absorption curve of a method for improving the measurement accuracy of laser gas sensing based on interference suppression provided by an embodiment of the present invention;

[0043] Figure 4 A schematic diagram of the absorption wavelength of a CO2 laser in a method for improving the measurement accuracy of laser gas sensing based on interference suppression provided in an embodiment of the present invention;

[0044] Figure 5 A schematic diagram comparing the effects before and after turning on 100f in a method for improving laser gas sensor measurement accuracy based on interference suppression provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0045] The following is a description of the implementation of the present invention by specific embodiments. People familiar with the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. Obviously, the described embodiments are 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 creative work are within the scope of protection of the present invention.

[0046] When measuring low-concentration gases, the gas absorption cell uses a long optical path cell, which requires multiple reflections in a limited space. Although the optical system of the cell is highly reflective, according to optical theory, slight interference will occur. When measuring low-concentration gases, this interference will affect the gas measurement results, usually increasing the measurement value.

[0047] Assuming that the laser beam enters the long optical path gas chamber at an angle α, the transmittance formula of the optical system is:

[0048]

[0049] Where T is the transmittance, R is the reflectivity of the optical lens, and δ can be expressed by the following formula:

[0050]

[0051] Where λ is the input laser wavelength. Δβ represents the phase during reflection, which is Δβ=π when propagating in air. The thickness of the optical lens in the system is d, and n is the refractive index of the lens material. is the phase, which represents the thickness uncertainty of the optical lens and the phase change caused by its thermal expansion.

[0052] This phase change will produce a small wavelength change in the gas absorption wavelength modulation. Figure 3 Interference fringes 1 and 2, when the external temperature, vibration, etc. change, a phase change occurs, causing the phase of the waveform to change.

[0053] Combined with the gas absorption, according to the Beer-Lambert absorption law, the beam power absorbed by the gas is approximately expressed as follows under small signal conditions:

[0054] P T (t)=P(t)exp[-α(t)]=P(t)[1-α(t)]

[0055] α(t) is the gas absorption coefficient at the absorption wavelength.

[0056] When there is gas absorption, the Figure 3 The waveform is shown as the black solid absorption curve in the middle.

[0057] When the photoelectric detector collects signals, it receives superimposed signals, such as Figure 3 From the dotted superposition curves 1 and 2, it can be observed that the peak value of the absorption curve after superposition is interfered by the superposition of interference fringes, which increases the peak value of the absorption peak and causes the measured gas concentration value to be too large.

[0058] In order to suppress the influence of such interference fringes on gas absorption measurement, this method provides a high-frequency modulation method. When the TDLAS second harmonic method is used for detection, a 1f frequency sine wave drive current signal frequency is required for the laser drive current, where f is the set laser drive current sine wave frequency, and the harmonic signal is demodulated according to the 2f frequency during demodulation. This method not only provides a 1f drive current frequency, but also synchronously provides a high-frequency sine wave drive current signal frequency of ≥100f. The demodulation is carried out according to the normal method. The effect of this method is that the 100f high-frequency signal modulation reduces the interference of interference fringes.

[0059] According to the high-frequency driven sine wave modulation laser output laser wavelength λ, when the wavelength change is very small, when high-frequency modulation is achieved, the fringe interference fringes are similar to sin waveforms. When driven at 100f, The integral value of the interference fringe change is zero, so the fringe interference effect can be eliminated by this method:

[0060]

[0061] in, For phase.

[0062] In order to solve the technical problem that the periodic interference fringes mentioned above appear, the fringes are affected by temperature and pressure changes, overlap with the gas absorption spectrum, and interfere with the gas concentration measurement.

[0063] refer to Figure 1 The embodiment of the present invention discloses a device for improving the measurement accuracy of laser gas sensing based on interference suppression, and the device includes: an embedded control module 1; a drive signal addition module 2; a laser current drive module 3; a laser temperature control module 4; a laser 5; a collimator 6; a long optical path gas chamber 7; 8a an air inlet pipeline, 8b an air outlet pipeline; a photodetector 9; a TIA voltage conversion module 10; and a phase-locked amplifier module 11.

[0064] The embedded control module 1 controls the driving signal addition module 2 to provide a voltage signal to drive the laser, which is converted into a laser current drive by the laser current driving module 3. The laser current driving module 3 and the laser temperature control module 4 provide temperature control and driving current modulation for the laser 5. The driving current is adjusted according to the sine wave f frequency and the 100f frequency. The laser 5 outputs the modulated wavelength λ according to the modulated current, and the optical power P also changes accordingly. The light beam irradiates the collimator 6, which is converted into a spatial beam by the collimator 6 and irradiates into the long optical path gas chamber 7.

[0065] The gas absorbs the energy of the light beam emitted by the laser, converts the light signal into a current signal at the photodetector 9, converts it into a voltage signal through the TIA voltage conversion module 10, performs 2f frequency phase-locked amplification through the phase-locked amplifier module 11, amplifies the 2f signal excited by f, collects the 2f signal curve through the embedded control module 1, and performs inversion calculation through the software process to obtain the gas concentration.

[0066] The embedded control module 1 provides a sinusoidal voltage signal with a frequency of 1f, and synchronously provides a sinusoidal voltage signal with a frequency much greater than 1f, which is generally set to a frequency of 100f. The driving signal addition module 2 adds the voltage to obtain a superimposed voltage signal, and the laser current driving module 3 converts the voltage signal into a current signal to perform current driving of the laser 5.

[0067] The voltage amplitude of the 100f signal is set to be adjustable, and the adjustable range is 0.1 to 10 times the voltage amplitude of the 1f signal, which is used to optimize the moving phase of the interference fringes and optimize the suppression effect.

[0068] Corresponding to the above-disclosed device for improving the measurement accuracy of laser gas sensors based on interference suppression, an embodiment of the present invention further discloses a method for improving the measurement accuracy of laser gas sensors based on interference suppression. The following describes in detail a method for improving the measurement accuracy of laser gas sensors based on interference suppression disclosed in an embodiment of the present invention in combination with the above-described device for improving the measurement accuracy of laser gas sensors based on interference suppression.

[0069] refer to Figure 2The present invention discloses a method for improving the measurement accuracy of laser gas sensing based on interference suppression, the method comprising: obtaining a voltage signal of a driving laser and converting it into a driving current; regulating the temperature and modulating the driving current, adjusting the driving current according to a first preset laser driving current sinusoidal wave frequency 1f and a high-frequency sinusoidal wave driving current signal frequency to generate a modulated current; inputting the gas to be detected into a long optical path gas chamber through an air intake pipeline; obtaining the modulated current and outputting the modulated wavelength and optical power after modulation, and at the same time irradiating the light beam to a collimator, the collimator converts the light beam into a spatial light beam and irradiates it into the long optical path gas chamber; the gas to be detected in the long optical path gas chamber absorbs the energy of the spatial light beam and converts the light signal into a current signal, and then converts it into a voltage signal; performing second harmonic demodulation processing on the voltage signal, and extracting a second harmonic signal related to the gas concentration; performing inversion calculation based on the Beer-Lambert gas absorption law and the second harmonic signal to obtain an accurate gas concentration.

[0070] Furthermore, the frequency of the high-frequency sinusoidal wave driving current signal is at least 10 times the first preset laser driving current sinusoidal wave frequency 1f; the interference of interference fringes is reduced by the high-frequency sinusoidal wave driving current signal frequency.

[0071] It is particularly important to note that the frequency of the high-frequency sinusoidal wave drive current signal proposed in the embodiment of the present invention only needs to be much greater than the frequency f, and the effect described in the embodiment of the present invention can be achieved from 10f to more, and 100f in the embodiment of the present invention is only for example.

[0072] Furthermore, the voltage signal is subjected to second harmonic demodulation processing to extract a second harmonic signal related to the gas concentration, including: performing phase-locked amplification of the voltage signal at a phase-locked amplification frequency of 2f, acquiring a corresponding 2f signal curve, and drawing an absorption curve using the voltage signal; adjusting the voltage magnitude of the high-frequency sinusoidal wave driving current signal frequency to minimize the 2f signal amplitude while making the measurement wavelength correspond one-to-one with the absorption curve. At this time, the 2f signal curve is the second harmonic signal related to the gas concentration.

[0073] During demodulation, the lock-in amplifier demodulates the 2f harmonic signal without demodulating the 100f signal. It only needs to observe the 2f harmonic signal amplitude and interference, increase or decrease the 100f voltage amplitude, optimize the interference fringe shift phase, and obtain the absorption curve signal with the minimum 2f signal amplitude and no interference.

[0074] Furthermore, accurate gas concentration is obtained by inverting the Beer-Lambert gas absorption law and the second harmonic signal, including: the peak value of the second harmonic signal is proportional to the gas concentration, and the accurate gas concentration is obtained by calculating the second harmonic signal.

[0075] Furthermore, the sine wave of the high-frequency sine wave driving current signal frequency starts and ends synchronously with the phase-locked amplification frequency 2f, wherein the synchronization ends according to the end time of the 2f signal, and the high-frequency sine wave ends synchronously when the 2f signal ends.

[0076] Furthermore, the synchronous start and synchronous end of the sine wave of the high-frequency sinusoidal wave driving current signal frequency and the phase-locked amplification frequency 2f also include: during high-frequency modulation, when the change in the modulation wavelength is extremely small, the interference fringes at this time are approximately sinusoidal waveforms, and when driven by the high-frequency sinusoidal wave driving current signal frequency, the integral value of the interference fringe change is made zero through phase change, thereby eliminating the influence of fringe interference.

[0077] Furthermore, the voltage amplitude of the high-frequency sinusoidal wave driving current signal frequency is adjustable within a voltage amplitude range of 0.1 to 10 times the first preset laser driving current sinusoidal wave frequency 1f, which is used to optimize the interference fringe shift phase and the suppression effect.

[0078] The voltage amplitude of the 100f signal is set to be adjustable, and the adjustable range is 0.1 to 10 times the voltage amplitude of the 1f signal, which is used to optimize the moving phase of the interference fringes and optimize the suppression effect.

[0079] Taking CO2 gas absorption as an example, there are three CO2 gas absorption peaks at 4242nm~4248nm. Figure 4 For example, the following Figure 4 The X-axis units are shown as nm and the Y-axis units are shown as absorbance (1 ppm-m@296K).

[0080] When there is no interference suppression according to the present invention, the measured absorption curve is as follows: Figure 5 As shown, it is marked "not turned on" above. When the gas concentration is 1ppm, the absorption curve is disturbed by the interference fringes, causing the curve to fluctuate, and the second wavelength cannot be distinguished. When the embodiment of the present invention is turned on to suppress interference, it can be seen that the curve amplitude decreases because the interference fringes amplitude is reduced. At the same time, the measured wavelength and the absorption curve can correspond one to one, which shows that the embodiment of the present invention suppresses the interference fringes and realizes accurate gas concentration measurement.

[0081] Although the present invention has been described in detail above by general description and specific embodiments, it is obvious to those skilled in the art that some modifications or improvements can be made to the present invention. Therefore, these modifications or improvements made without departing from the spirit of the present invention all belong to the scope of protection claimed by the present invention.

Claims

1. A method for improving the measurement accuracy of laser gas sensing based on interference suppression, characterized in that: The method comprises: Obtain the voltage signal of driving the laser and convert it into driving current; Regulating the temperature and modulating the driving current, adjusting the driving current according to the first preset laser driving current sinusoidal wave frequency 1f and the high-frequency sinusoidal wave driving current signal frequency, and generating a modulated current; The gas to be detected is input into the long optical path gas chamber through the gas inlet pipeline; Obtaining the modulation current and outputting the modulated wavelength and optical power, while irradiating the light beam to the collimator, the collimator converts the light beam into a spatial light beam and irradiates the light beam into the long optical path gas chamber; The gas to be detected in the long optical path gas chamber absorbs the energy of the spatial light beam and converts the optical signal into a current signal, and then into a voltage signal; Performing second harmonic demodulation processing on the voltage signal to extract a second harmonic signal related to gas concentration; The accurate gas concentration is obtained by inverting the Beer-Lambert gas absorption law and the second harmonic signal.

2. A method for improving the measurement accuracy of laser gas sensing based on interference suppression as claimed in claim 1, characterized in that: The frequency of the high-frequency sinusoidal wave driving current signal is at least 10 times the first preset laser driving current sinusoidal wave frequency 1f; the interference of the interference fringes is reduced by the high-frequency sinusoidal wave driving current signal frequency.

3. A method for improving the measurement accuracy of laser gas sensing based on interference suppression as claimed in claim 2, characterized in that: Performing second harmonic demodulation processing on the voltage signal to extract a second harmonic signal related to the gas concentration includes: Performing phase-locked amplification of the voltage signal at a phase-locked amplification frequency of 2f, acquiring a corresponding 2f signal curve, and using the voltage signal to draw an absorption curve; The voltage of the high-frequency sinusoidal wave driving current signal frequency is adjusted to minimize the 2f signal amplitude and make the measurement wavelength correspond to the absorption curve one by one. At this time, the 2f signal curve is the second harmonic signal related to the gas concentration.

4. A method for improving the measurement accuracy of laser gas sensing based on interference suppression as claimed in claim 3, characterized in that: Through the inversion calculation of Beer-Lambert gas absorption law and the second harmonic signal, accurate gas concentration is obtained, including: The peak value of the second harmonic signal is proportional to the gas concentration, and the accurate gas concentration can be calculated using the second harmonic signal.

5. A method for improving the measurement accuracy of laser gas sensing based on interference suppression as claimed in claim 4, characterized in that: The sine wave of the high-frequency sine wave driving current signal frequency starts and ends synchronously with the phase-locked amplification frequency 2f, wherein the synchronization ends according to the end time of the 2f signal, and the high-frequency sine wave ends synchronously when the 2f signal ends.

6. A method for improving the measurement accuracy of laser gas sensing based on interference suppression as claimed in claim 5, characterized in that: The synchronous start and synchronous end of the sine wave of the high-frequency sine wave driving current signal frequency and the phase-locked amplification frequency 2f also include: During high-frequency modulation, when the modulation wavelength changes very little, the interference fringes are approximately sinusoidal waveforms. When driven by the high-frequency sinusoidal drive current signal frequency, the integral value of the interference fringe change is made zero through phase change, eliminating the influence of fringe interference.

7. A method for improving the measurement accuracy of laser gas sensing based on interference suppression as claimed in claim 6, characterized in that: The voltage amplitude of the high-frequency sinusoidal wave driving current signal frequency is adjustable within a voltage amplitude range of 0.1 to 10 times the first preset laser driving current sinusoidal wave frequency 1f, and is used to optimize the interference fringe shift phase and the suppression effect.

8. A device for improving the measurement accuracy of laser gas sensing based on interference suppression, characterized in that: The device comprises: An embedded control module is used to control the drive signal addition module and provide a first preset laser drive current sine wave frequency 1f, receive the second harmonic signal for inversion calculation, and obtain accurate gas concentration; A driving signal adding module, used for providing a voltage signal for driving the laser; A laser current driving module is used to convert the voltage signal driving the laser into a current signal to provide current modulation for the laser; Laser temperature control module, providing temperature control for the laser; A laser is used to output a modulated wavelength by modulating current; A collimator, used for converting an incident light beam into a spatial light beam and irradiating the spatial light beam into a long optical path gas chamber; The gas absorbs the energy of the beam emitted by the laser in the long optical path gas cell; Inlet and outlet pipelines are used to input and output the gas that needs to be tested; Photodetector, which converts light signals into current signals; TIA voltage conversion module, converting the current signal converted by the photodetector into a voltage signal; The lock-in amplifier module is used to perform second harmonic demodulation processing on the voltage signal to extract the second harmonic signal related to the gas concentration.

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