Photo-thermal spectrum device and method based on orthogonal pseudo-random code modulation

Abstract

The invention discloses a photo-thermal spectrum device and method based on orthogonal pseudo-random code modulation, and the device comprises a control and signal processing unit, a pseudo-random code generator, a laser driver, a tunable laser, a collimator, an air chamber, a focusing lens, a quartz tuning fork, a pre-amplifier, a data collector, and a related demodulator. A pseudo-random code sequence with good self-correlation and cross-correlation characteristics is used for carrying out binary orthogonal modulation on laser intensity, and weak photo-thermal signals are extracted from strong noise through a correlation demodulation technology. The method has the advantages that the method is insensitive to narrow-band noise due to the broadband modulation characteristic, any noise irrelevant to pseudo-random codes can be effectively suppressed by relevant calculation, and the method has extremely high anti-interference capability; related processing is equivalent to long-time energy accumulation, and the noise bandwidth can be compressed; the broadband characteristic of the pseudo-random code enables the pseudo-random code to cover the drift range of the resonant frequency of the quartz tuning fork, and the system is higher in adaptability to environmental changes and higher in stability.

Description

Technical Field

[0001] This invention relates to a photothermal spectroscopy device and method, specifically to a photothermal spectroscopy device and method based on orthogonal pseudo-random code modulation. Background Technology

[0002] Photothermal spectroscopy, a highly sensitive method for detecting trace gases, works by modulating a laser to excite the molecules of the target gas. The molecules then convert the absorbed light energy into heat energy through a non-radiative relaxation process, generating a localized periodic temperature field. This temperature field drives a micromechanical sensor (such as a quartz tuning fork) to vibrate via a thermoelastic effect. The gas concentration can be determined by detecting the vibration signal from the sensor. Quartz tuning forks are the preferred sensing element due to their extremely high quality factor (Q value) and excellent piezoelectric properties. However, efficiently extracting extremely weak photothermal signals from strong background noise and maintaining the long-term stability of the system under complex environments remains a core challenge in this field.

[0003] Currently, photothermal spectroscopy generally employs a technique combining single-frequency sinusoidal wave modulation with lock-in amplification. A typical approach involves utilizing a frequency that resonates with the inherent resonant frequency of a quartz tuning fork (…). A matched sinusoidal signal is used to modulate the intensity of the laser. The modulated laser irradiates the gas and excites a quartz tuning fork to vibrate. The weak piezoelectric signal generated by the tuning fork is sent to a lock-in amplifier. Using this sinusoidal wave as a reference frequency, the lock-in amplifier extracts the signal component at that specific frequency through narrowband filtering, thereby achieving noise suppression and signal detection. This technique is essentially a narrowband detection method, where the signal energy is concentrated at a single frequency point. The above relies on long-term integration to improve the signal-to-noise ratio.

[0004] Although the above-mentioned technical solutions can achieve high sensitivity in an ideal laboratory environment, they have the following three significant drawbacks in real-world complex application scenarios:

[0005] (1) The electrical noise, mechanical vibration and other periodic interferences that are common in the environment are mostly narrowband in nature, and their frequencies are easily resonant with the frequency of the quartz tuning fork. These interference signals overlap or are close to each other. Lock-in amplifiers amplify these interference signals synchronously, resulting in a severe drop in the signal-to-noise ratio and significant performance degradation of the system in industrial or urban environments.

[0006] (2) The resonant frequency of a quartz tuning fork It will drift due to changes in temperature, pressure, and gas composition. Traditional lock-in amplifiers typically have a fixed reference frequency. If a frequency mismatch occurs, it will lead to decreased excitation efficiency and signal amplitude attenuation, resulting in distorted measurement results. Therefore, the system requires complex automatic frequency tracking circuitry or frequent manual calibration, which is not conducive to long-term continuous monitoring.

[0007] (3) In order to distinguish the real photothermal signal from electrical crosstalk, optical background and other interference, traditional methods usually require interrupting the detection and performing two measurements, one with light and one without light, to perform background subtraction. This asynchronous background correction method not only cannot achieve true real-time continuous monitoring, but also makes it difficult to eliminate the transient interference newly introduced during the measurement process, which limits its application in dynamic processes and high temporal resolution detection. Summary of the Invention

[0008] To overcome the shortcomings of the prior art, the present invention provides a photothermal spectroscopy device and method based on orthogonal pseudo-random code modulation. The device utilizes a pseudo-random code sequence with good autocorrelation and cross-correlation characteristics to perform binary orthogonal modulation on the laser intensity, and extracts weak photothermal signals from strong noise through correlation demodulation technology.

[0009] The objective of this invention is achieved through the following technical solution:

[0010] A photothermal spectroscopy device based on orthogonal pseudo-random code modulation includes a control and signal processing unit, a pseudo-random code generator, a laser driver, a tunable laser, a collimator, a gas cell, a focusing lens, a quartz tuning fork, a preamplifier, a data acquisition unit, and a correlation demodulator. The control and signal processing unit controls the pseudo-random code generator to generate two orthogonal pseudo-random code sequences. and ; The signal is fed to the laser driver to control the output intensity of the tunable laser, achieving binary amplitude modulation. The modulated laser, after being collimated by a collimator, enters the gas chamber, interacts with the gas under test, and then radiates through a focusing lens to the root surface of the quartz tuning fork, thereby exciting the quartz tuning fork to vibrate. The weak piezoelectric signal output by the quartz tuning fork is initially amplified by a preamplifier and then sent to a correlation demodulator for demodulation by a data acquisition unit. The control and signal processing unit drives the correlation demodulator to... Using the reference code, cross-correlation operations are performed on the input signal to extract the pure photothermal signal. The control and signal processing unit then inverts the concentration of the gas to be measured based on the extracted signal amplitude.

[0011] A photothermal spectroscopy method based on orthogonal pseudorandom code modulation using the above-mentioned device includes the following steps:

[0012] Step 1: The control and signal processing unit starts the pseudo-random code generator, generates and outputs two orthogonal pseudo-random code sequences. and ;

[0013] Step Two: The input laser driver drives the tunable laser to output a laser beam modulated by a pseudo-random code intensity. After being collimated by a collimator, the laser beam passes through an air cell and is focused by a focusing lens to irradiate the surface of the root of the quartz tuning fork.

[0014] Step 3: The gas molecules in the gas chamber absorb the modulated laser energy, generating a periodic photothermal effect, which excites the quartz tuning fork to vibrate. The quartz tuning fork generates a weak piezoelectric signal due to the piezoelectric effect. This piezoelectric signal is amplified by a preamplifier and then converted into a digital signal by a data acquisition unit.

[0015] Step 4: The control and signal processing unit processes the acquired digital signals and The signal is fed into a correlation demodulator for cross-correlation calculation to obtain the correlation output curve. ;

[0016] Step 5: The control and signal processing unit identifies and extracts the peak values ​​of relevant curves. Based on the pre-calibrated gas concentration curve, the concentration value of the gas to be measured is calculated and output.

[0017] Compared with the prior art, the present invention has the following advantages:

[0018] 1. Its wideband modulation characteristics make it insensitive to narrowband noise, and its correlation calculation can effectively suppress any noise unrelated to the pseudo-random code, giving it a strong anti-interference capability.

[0019] 2. Related processing is equivalent to long-term energy accumulation and can compress noise bandwidth; theoretically, the signal-to-noise ratio improvement is proportional to... ( For sequence length, (with bandwidth), it has high sensitivity.

[0020] 3. The broadband characteristics of pseudo-random codes enable them to cover the drift range of the resonant frequency of quartz tuning forks, making the system more adaptable to environmental changes and more stable. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of a photothermal spectroscopy device based on orthogonal pseudo-random code modulation.

[0022] Figure 2 The results are for identifying the relevant demodulator. Detailed Implementation

[0023] The technical solution of the present invention will be further described below with reference to the accompanying drawings, but it is not limited thereto. Any modifications or equivalent substitutions to the technical solution of the present invention that do not depart from the spirit and scope of the technical solution of the present invention should be covered within the protection scope of the present invention.

[0024] The core of this invention lies in: using a pseudo-random code sequence with good autocorrelation and cross-correlation characteristics to perform binary orthogonal modulation on the laser intensity, and extracting weak photothermal signals from strong noise through correlation demodulation technology.

[0025] Pseudo-random code sequence It is a binary sequence with an amplitude of either "1" or "0", exhibiting good autocorrelation properties. Its autocorrelation function... Approximately Dirac function:

[0026]

[0027] in, For signal energy, This is the time delay variable in cross-correlation operations.

[0028] Laser is pseudo-random code Modulation: When the code value is 1, the laser modulation amplitude is When the code value is 0, the modulation amplitude is Modulated laser (in The average light intensity. (To modulate the depth) irradiation of the gas excites the quartz tuning fork to vibrate. The signal generated by the tuning fork... Photothermal response With system shock response The convolution, mixed with noise. :

[0029]

[0030] At the demodulation end, using and Orthogonal pseudo-random codes , and cross-correlation function right Perform the relevant calculations:

[0031]

[0032] in, The result of the cross-correlation operation is the final output signal value, which can be used to calculate the gas concentration.

[0033] because and Orthogonal, and With noise Since they are irrelevant, the above correlation operations can suppress noise to the greatest extent and extract the relevant information. The photothermal response associated with the encoded excitation signal is used to significantly improve the signal-to-noise ratio.

[0034] This invention designs a photothermal spectroscopy device based on orthogonal pseudo-random code modulation, such as... Figure 1 As shown, the device includes a control and signal processing unit 1, a pseudo-random code generator 2, a laser driver 3, a tunable laser 4, a collimator 5, a gas cell 6, a focusing lens 7, a quartz tuning fork 8, a preamplifier 9, a data acquisition unit 10, and a correlation demodulator 11. The control and signal processing unit 1 controls the pseudo-random code generator 2 to generate two orthogonal pseudo-random code sequences. and . The signal is transmitted to the laser driver 3 to control the output intensity of the tunable laser 4, achieving binary amplitude modulation. The modulated laser, after being collimated by the collimator 5, enters the gas chamber 6, interacts with the gas to be measured, and then radiates through the focusing lens 7 onto the root surface of the quartz tuning fork 8, thereby exciting the quartz tuning fork 8 to vibrate. The weak piezoelectric signal output from the quartz tuning fork 8 is initially amplified by the preamplifier 9 and then sent by the data acquisition unit 10 to the correlation demodulator 11 for demodulation. The control and signal processing unit 1 drives the correlation demodulator 11 to... Using the reference code, cross-correlation is performed on the input signal to extract the pure photothermal signal. The control and signal processing unit 1 then uses the extracted signal amplitude to invert the gas concentration. The specific implementation process is as follows:

[0035] Step 1: Control and signal processing unit 1 starts pseudo-random code generator 2, generating and outputting two orthogonal pseudo-random code sequences. and .

[0036] Step Two: The input laser driver 3 drives the tunable laser 4 to output a laser beam modulated by a pseudo-random code intensity. After being collimated by the collimator 5, the laser beam passes through the gas chamber 6 and is focused by the focusing lens 7 to irradiate the root surface of the quartz tuning fork 8.

[0037] Step 3: The gas molecules in the gas chamber 6 absorb the modulated laser energy, generating a periodic photothermal effect, which excites the quartz tuning fork 8 to vibrate. The quartz tuning fork 8 generates a weak piezoelectric signal due to the piezoelectric effect. This piezoelectric signal is amplified by the preamplifier 9 and then converted into a digital signal by the data acquisition unit 10.

[0038] Step 4: Control and signal processing unit 1 processes the acquired digital signals and The signal is fed into the correlation demodulator 11 for cross-correlation calculation to obtain the correlation output curve. .

[0039] Step 5: Control and signal processing unit 1 identifies and extracts the peak values ​​of relevant curves. Based on the pre-calibrated gas concentration curve, the concentration value of the gas to be measured is calculated and output.

[0040] In this invention, the control and signal processing unit 1 serves as the core of the system. It employs an embedded processor or FPGA and is responsible for coordinating the work of each component, generating control timing sequences, and executing relevant demodulation algorithms and concentration inversion.

[0041] In this invention, the pseudo-random code generator 2 generates two channels with periods of orthogonal pseudorandom code sequences and Preferred Sequence or Gold sequence, sequence length To ensure good related characteristics.

[0042] In this invention, the laser driver 3 is based on The injection current of the tunable laser 4 is dynamically adjusted to achieve binary modulation of the laser intensity. The wavelength of the tunable laser 4 is matched with the absorption line of the target gas, and the output power is greater than 20mW.

[0043] In this invention, the collimator 5 controls the laser divergence angle to within 0.3 mrad.

[0044] In this invention, the air chamber 6 has an optical window and ensures good airtightness.

[0045] In this invention, the focusing lens 7 precisely focuses the emitted laser onto the root surface of the quartz tuning fork 8 to enhance the efficiency of photothermal action.

[0046] In this invention, the standard resonant frequency of the quartz tuning fork 8 is 32.768 kHz, and the quality factor Q > 10000.

[0047] In this invention, the gain of the preamplifier 9 is adjustable from 10 to 1000 times, and the equivalent input noise is less than 1 μV.

[0048] In this invention, the sampling rate of the data acquisition unit 10 is not less than 10 times the pseudo-random code chip rate, and the resolution is not less than 16 bits.

[0049] In this invention, the correlation demodulator 11, driven by the control and signal processing unit 1, performs the following cross-correlation operation:

[0050]

[0051] in, The signal input to the data acquisition unit 10, The integration time is typically several pseudo-random code periods. This is achieved by finding the relevant output curve. peak This allows us to obtain a signal amplitude that is proportional to the gas concentration.

[0052] Example:

[0053] This embodiment provides highly sensitive and rapid-response monitoring for potential methane gas leaks at pipeline valves. The device employs a distributed feedback laser, with the output wavelength precisely tuned to the near-infrared absorption line of methane at 1650.96 nm. A pseudo-random code generator produces two orthogonal lasers of length [missing information]. =1023 The sequence and chip rate were set to 32.768 kHz. The laser was driven by a pseudo-random code, where the laser power was 25 mW when the code value was 1 and 5 mW when the code value was 0. The modulated laser passed through a 20 cm long open air chamber and was then focused by a focusing lens onto the root of a standard 32.768 kHz quartz tuning fork. The weak piezoelectric signal generated by the tuning fork was first amplified by a preamplifier with a gain of 1000 times and an equivalent input noise of 0.8 μV, and then digitized by an 18-bit data acquisition card with a sampling rate of 500 kSPS. An FPGA was used as the control and signal processing unit. The acquired signal was cross-correlated with another orthogonal reference pseudo-random code, and the integration time was set to 0.312 seconds (corresponding to 10 complete pseudo-random code cycles). In a complex industrial environment with mechanical vibration and electrical noise, the correlation demodulator experienced time delays. A clear correlation peak with an amplitude of 3.28 mV was identified at the chip location, as shown below. Figure 2 As shown, based on the pre-calibrated methane concentration curve in the laboratory (concentration (ppm) = 10 × peak voltage (mV)), the methane concentration at the current sampling point was calculated to be 32.8 ppm, demonstrating the device's powerful ability to achieve rapid and highly sensitive gas leak detection in harsh industrial environments.

Claims

1. A photothermal spectrometer based on orthogonal pseudo-random code modulation, characterized in that... The device includes a control and signal processing unit, a pseudo-random code generator, a laser driver, a tunable laser, a collimator, a gas chamber, a focusing lens, a quartz tuning fork, a preamplifier, a data acquisition unit, and a correlation demodulator. The control and signal processing unit controls the pseudo-random code generator to generate two orthogonal pseudo-random code sequences. and ; The signal is fed to the laser driver to control the output intensity of the tunable laser, achieving binary amplitude modulation. The modulated laser, after being collimated by a collimator, enters the gas chamber, interacts with the gas under test, and then radiates through a focusing lens to the root surface of the quartz tuning fork, thereby exciting the quartz tuning fork to vibrate. The weak piezoelectric signal output by the quartz tuning fork is initially amplified by a preamplifier and then sent to a correlation demodulator for demodulation by a data acquisition unit. The control and signal processing unit drives the correlation demodulator to... Using the reference code, cross-correlation operations are performed on the input signal to extract the pure photothermal signal. The control and signal processing unit then inverts the concentration of the gas to be measured based on the extracted signal amplitude.

2. The photothermal spectrometer based on orthogonal pseudo-random code modulation according to claim 1, characterized in that... The control and signal processing unit employs an embedded processor or an FPGA.

3. The photothermal spectrometer based on orthogonal pseudo-random code modulation according to claim 1, characterized in that... The pseudo-random code sequence and for Sequence or Gold sequence, period is , .

4. The photothermal spectrometer based on orthogonal pseudo-random code modulation according to claim 1, characterized in that... The wavelength of the tunable laser is matched with the absorption line of the target gas, and the output power is greater than 20 mW.

5. The photothermal spectrometer based on orthogonal pseudo-random code modulation according to claim 1, characterized in that... The collimator controls the laser divergence angle to within 0.3 mrad.

6. The photothermal spectrometer based on orthogonal pseudo-random code modulation according to claim 1, characterized in that... The standard resonant frequency of the quartz tuning fork is 32.768 kHz, and the quality factor Q > 10000.

7. The photothermal spectrometer based on orthogonal pseudo-random code modulation according to claim 1, characterized in that... The gain of the preamplifier is adjustable from 10 to 1000 times, and the equivalent input noise is less than 1 μV.

8. The photothermal spectrometer based on orthogonal pseudo-random code modulation according to claim 1, characterized in that... The sampling rate of the data acquisition device is no less than 10 times the pseudo-random code chip rate, and the resolution is no less than 16 bits.

9. The photothermal spectrometer based on orthogonal pseudo-random code modulation according to claim 1, characterized in that... Under the drive of the control and signal processing unit, the correlation demodulator performs the following cross-correlation operation: in, The signal input to the data acquisition unit 10, For the integration time, find the relevant output curve. peak This allows us to obtain a signal amplitude that is proportional to the gas concentration.

10. A photothermal spectroscopy method based on orthogonal pseudorandom code modulation using the apparatus described in any one of claims 1-9, characterized in that... The method includes the following steps: Step 1: The control and signal processing unit starts the pseudo-random code generator, generating and outputting two orthogonal pseudo-random code sequences. and ; Step Two: The input laser driver drives the tunable laser to output a laser beam modulated by a pseudo-random code intensity. After being collimated by a collimator, the laser beam passes through an air cell and is focused by a focusing lens to irradiate the surface of the root of the quartz tuning fork. Step 3: The gas molecules in the gas chamber absorb the modulated laser energy, generating a periodic photothermal effect, which excites the quartz tuning fork to vibrate. The quartz tuning fork generates a weak piezoelectric signal due to the piezoelectric effect. This piezoelectric signal is amplified by a preamplifier and then converted into a digital signal by a data acquisition unit. Step 4: The control and signal processing unit processes the acquired digital signals and The signal is fed into a correlation demodulator for cross-correlation calculation to obtain the correlation output curve. ; Step 5: The control and signal processing unit identifies and extracts the peak values ​​of relevant curves. Based on the pre-calibrated gas concentration curve, the concentration value of the gas to be measured is calculated and output.

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