Laser frequency stepped scanning method applied to cavity ring-down spectroscopy

Abstract

The invention belongs to the technical field of frequency scanning of cavity ring-down spectroscopy, and particularly relates to a laser frequency stepped scanning method applied to cavity ring-down spectroscopy. According to the invention, through accurate voltage stepping and step control, stable acquisition of cavity mode signals on each step is ensured, and errors in frequency scanning are reduced. The processing of superposed signals and the application of cut-off signals improve the stability of the system, so that the frequency scanning process is more reliable. And through fine stepped scanning and data processing, the resolution of spectral measurement is improved, so that the concentration inversion analysis is more accurate.

Description

Technical Field

[0001] The invention belongs to the technical field of frequency scanning of cavity ring-down spectroscopy, and in particular relates to a laser frequency step-by-step scanning method applied to cavity ring-down spectroscopy. Background Art

[0002] Cavity Ring-Down Spectroscopy (CRDS) is a highly sensitive spectral measurement technology that is widely used in gas analysis, environmental monitoring and chemical reaction research. The core of this technology is to obtain the spectral characteristics of the sample by measuring the decay time of light in the cavity. Accurate control and scanning of laser frequency are the key to achieving high-resolution measurement. However, laser frequency scanning in the prior art is prone to instability and large frequency deviation, which makes it impossible to accurately control the frequency of light during the measurement process, thereby affecting the accuracy of the measurement results; the frequency scanning resolution of the prior art is not high, and the spectrum cannot be measured with high precision, which limits its application in high-resolution brightness analysis and precise environmental monitoring; due to the inconsistency between the laser mode and the cavity mode signal, mode jumping often occurs in the prior art, reducing the reliability of the overall measurement. Summary of the invention

[0003] The present invention provides a laser frequency step scanning method applied to cavity ring-down spectroscopy, aiming to improve the accuracy and stability of laser frequency scanning, optimize the data acquisition process, and thus improve the resolution and accuracy of spectral measurement.

[0004] To achieve the above object, the present invention adopts the following technical solutions:

[0005] A laser frequency step scanning method for cavity ring-down spectroscopy, using a gas detection system based on CRDS technology, includes the following steps:

[0006] Step 1, determine the voltage parameters: turn off the function generator, use the LabVIEW program to give a sawtooth wave to scan the DFB laser drive, adjust the starting voltage and step voltage given by the LabVIEW program to ensure that in each step, the laser mode in the cavity corresponds to the cavity mode signal, so as to avoid data loss or error caused by mode jumping. After determining the starting voltage and step voltage, save the data to the LabVIEW program, turn off the sawtooth wave and turn on the function generator;

[0007] Step 2, signal generation and superposition: The LabVIEW program generates a step-increasing voltage signal, which changes in a step-like manner on the time axis. Each step represents a fixed frequency increment. The triangular wave signal (used for modulating the laser frequency) and the step voltage signal are combined and superimposed with the chopping signal generated by the digital pulse generator through an adder. The chopping signal is used to cut off the laser pulse from the cavity in order to measure the attenuation of light. The superimposed composite signal is scanned and transmitted to the laser driver, thereby achieving precise control and modulation of the laser frequency;

[0008] Step 3, data acquisition and scanning: In each step, the system performs data acquisition at a set voltage, collects the decay time of the cavity mode signal by cutting off the signal, and records the cavity mode signal strength at this frequency. After the acquisition is completed, the voltage will be increased to the next step for a new round of data acquisition;

[0009] Step 4, data processing and analysis: perform data fitting on the data points collected at each step to obtain the absorption line spectrum of the cavity mode signal, perform concentration inversion analysis on the obtained absorption line spectrum, and calculate the concentration or other relevant parameters of the sample through the spectral data.

[0010] Furthermore, the gas detection system based on CRDS technology includes a laser driver, a DFB laser, an optical resonant cavity, an adder, a function generator and a computer terminal:

[0011] The output end of the laser driver is connected to the input end of the DFB laser, the output end of the DFB laser emits laser light, and the laser light passes through an isolator, a collimating lens, a focusing lens and a reflector in sequence and enters an optical resonant cavity, and a transmission signal of the optical resonant cavity is detected by a detector, a first output end of the detector is connected to a first input end of an adder via a digital pulse generator, a second output end of the detector is connected to an input end of a computer via a low-pass filter, an output end of the computer is connected to a second input end of the adder, an output end of the function generator is connected to a third input end of the adder, and an output end of the adder is connected to an input end of the laser driver, and a LabVIEW program is installed in the computer for recording and processing data.

[0012] Furthermore, a total of 15 steps are set in the LabVIEW program, and after the 15 steps are completed, the program will return to the first step and continue the cycle.

[0013] Compared with the prior art, the present invention has the following advantages:

[0014] (1) Through precise voltage stepping and step control, the stable acquisition of cavity mode signals on each step is ensured, reducing the error in frequency scanning.

[0015] (2) The processing of superposition signals and the application of chopping signals improve the stability of the system and make the frequency scanning process more reliable.

[0016] (3) Through fine step scanning and data processing, the resolution of spectral measurement is improved, making the concentration inversion analysis more accurate. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a schematic diagram of a gas detection system based on CRDS technology;

[0018] Figure 2 This is the 24-hour variation curve of methane gas concentration. DETAILED DESCRIPTION

[0019] In order to further illustrate the technical solution of the present invention, the present invention is further described below through embodiments.

[0020] like Figure 1 As shown, the gas detection system based on CRDS technology of this embodiment includes a laser driver, a DFB laser, an optical resonant cavity, an adder, a function generator and a computer terminal:

[0021] The output end of the laser driver is connected to the input end of the DFB laser, the output end of the DFB laser emits laser light, and the laser light passes through an isolator, a collimating lens, a focusing lens and a reflector in sequence and enters an optical resonant cavity, and a transmission signal of the optical resonant cavity is detected by a detector, a first output end of the detector is connected to a first input end of an adder via a digital pulse generator, a second output end of the detector is connected to an input end of a computer via a low-pass filter, an output end of the computer is connected to a second input end of the adder, an output end of the function generator is connected to a third input end of the adder, and an output end of the adder is connected to an input end of the laser driver, and a LabVIEW program is installed in the computer for recording and processing data.

[0022] A laser frequency step scanning method applied to cavity ring-down spectroscopy in this embodiment uses the above-mentioned gas detection system based on CRDS technology, including the following steps:

[0023] Step 1, determine the voltage parameters: turn off the function generator, use the LabVIEW program to give a sawtooth wave to scan the DFB laser drive, adjust the starting voltage and step voltage given by the LabVIEW program to ensure that in each step, the laser mode in the cavity corresponds to the cavity mode signal, so as to avoid data loss or error caused by mode jumping. After determining the starting voltage and step voltage, save the data to the LabVIEW program, turn off the sawtooth wave and turn on the function generator;

[0024] Step 2, signal generation and superposition: The LabVIEW program generates a step-increasing voltage signal, which changes in a step-like manner on the time axis. Each step represents a fixed frequency increment. The triangular wave signal and the step voltage signal are combined and superimposed with the chopping signal generated by the digital pulse generator through an adder. The superimposed synthetic signal is scanned and transmitted to the laser driver, thereby realizing precise control and modulation of the laser frequency.

[0025] Step 3, data acquisition and scanning: In each step, the system performs data acquisition at a set voltage, collects the decay time of the cavity mode signal by cutting off the signal, and records the cavity mode signal strength at this frequency. After the acquisition is completed, the voltage will be increased to the next step for a new round of data acquisition; a total of 15 steps are set in the LabVIEW program of this embodiment. After these 15 steps are completed, it will return to the first step and continue to cycle.

[0026] Step 4, data processing and analysis: perform data fitting on the data points collected at each step to obtain the absorption line spectrum of the cavity mode signal, perform concentration inversion analysis on the obtained absorption line spectrum, and calculate the concentration or other relevant parameters of the sample through the spectral data.

[0027] Taking methane gas as an example, the 24-hour change curve of methane gas concentration measured by this method is as follows: Figure 2 As shown, the performance indicators of the continuous automatic monitoring system for greenhouse gases in ambient air (Type I) in the "Technical Specifications for Environmental Protection Product Certification" formulated by the state have been met.

[0028] The above shows and describes the main features and advantages of the present invention. It is obvious to those skilled in the art that the present invention is not limited to the details of the above exemplary embodiments, and that the present invention can be implemented in other specific forms without departing from the spirit or essential features of the present invention. Therefore, no matter from which point of view, the embodiments should be regarded as exemplary and non-restrictive. The scope of the present invention is defined by the appended claims rather than the above description, and it is intended that all changes that fall within the meaning and scope of the equivalent elements of the claims are included in the present invention.

[0029] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims

1. A laser frequency step scanning method applied to cavity ring-down spectroscopy, characterized in that: Using a gas detection system based on CRDS technology includes the following steps: Step 1, determine the voltage parameters: turn off the function generator, use the LabVIEW program to give a sawtooth wave to scan the DFB laser drive, adjust the starting voltage and step voltage given by the LabVIEW program to ensure that in each step, the laser mode in the cavity corresponds to the cavity mode signal, so as to avoid data loss or error caused by mode jumping. After determining the starting voltage and step voltage, save the data to the LabVIEW program, turn off the sawtooth wave and turn on the function generator; Step 2, signal generation and superposition: The LabVIEW program generates a step-increasing voltage signal, which changes in a step-like manner on the time axis. Each step represents a fixed frequency increment. The triangular wave signal and the step voltage signal are combined and superimposed with the chopping signal generated by the digital pulse generator through an adder. The superimposed synthetic signal is scanned and transmitted to the laser driver, thereby realizing precise control and modulation of the laser frequency. Step 3, data acquisition and scanning: In each step, the system performs data acquisition at a set voltage, collects the decay time of the cavity mode signal by cutting off the signal, and records the cavity mode signal strength at that frequency. After the acquisition is completed, the voltage will be increased to the next step for a new round of data acquisition; Step 4, data processing and analysis: perform data fitting on the data points collected at each step to obtain the absorption line spectrum of the cavity mode signal, perform concentration inversion analysis on the obtained absorption line spectrum, and calculate the concentration or other relevant parameters of the sample through the spectral data.

2. The laser frequency step scanning method for cavity ring-down spectroscopy according to claim 1, characterized in that: The gas detection system based on CRDS technology includes a laser driver, a DFB laser, an optical resonant cavity, an adder, a function generator and a computer terminal: The output end of the laser driver is connected to the input end of the DFB laser, the output end of the DFB laser emits laser light, the laser light passes through an isolator, a collimating lens, a focusing lens and a reflector in sequence and enters an optical resonant cavity, the transmission signal of the optical resonant cavity is detected by a detector, the first output end of the detector is connected to the first input end of an adder via a digital pulse generator, the second output end of the detector is connected to the input end of a computer via a low-pass filter, the output end of the computer end is connected to the second input end of the adder, the output end of the function generator is connected to the third input end of the adder, the output end of the adder is connected to the input end of the laser driver, and a LabVIEW program is installed in the computer end.

3. The laser frequency step scanning method for cavity ring-down spectroscopy according to claim 1, characterized in that: A total of 15 steps are set in the LabVIEW program. After completing these 15 steps, the program will return to the first step and continue the cycle.

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