Modulation and demodulation method for noise suppression of multi-reverse air chamber

Abstract

The invention relates to a modulation and demodulation method for noise suppression of multiple reverse air chambers, and belongs to the technical field of gas detection. The method comprises the following steps: selecting a DFB laser with specific wavelength and current modulation rate; the sawtooth wave and the sine wave are superposed to generate an original modulation signal; according to a comparison relationship between a sawtooth wave scanning spectrum range and a gas spectrum absorption peak, determining a Gaussian signal amplitude, and generating a Gaussian signal; combining the generated Gaussian signal with the original modulation signal to obtain a variable stepping modulation signal; inputting the variable stepping modulation signal as a driving current into a laser, and driving the laser to emit a laser signal; the laser signal passes through the gas to obtain a gas spectral signal. According to the invention, the Gaussian line type signal similar to the gas absorption peak is superposed on the sawtooth wave signal, so that the current stepping is increased and the scanning time ratio is reduced in the non-absorption section, the current stepping is reduced and the scanning time ratio is increased in the absorption section, and the signal absorption intensity is effectively improved under the condition that the current scanning range is not changed.

Description

Technical Field

[0001] The invention belongs to the technical field of gas detection, relates to PPM-level high-resolution laser methane gas detection, and particularly relates to a modulation and demodulation method for multi-reverse gas chamber noise suppression. Background Art

[0002] According to data from the United Nations Framework Convention on Climate Change, coal mine methane emissions account for about 10% to 15% of global methane emissions, while wind exhaust gas accounts for more than 70% of coal mine methane emissions. Accurate metering equipment for coal mine wind exhaust gas is an important guarantee for the advancement of coal mine methane emission reduction work, and high-precision laser methane detection devices are an important part of this equipment. With the increase in CH4 control efforts, there will be a continued increase in market demand for high-precision laser methane detection devices.

[0003] PPM-level high-resolution laser methane gas detection technology is an important support for the accurate measurement of coal mine wind exhaust gas. The current high-resolution laser methane gas detection technology uses multiple reflection chambers to increase the absorption optical path to achieve the purpose of improving measurement resolution. However, due to the existence of multiple reflections, each reflection has some backscattered light, and the superposition of the optical interference effect is multiplied relative to the incident optical path, and the noise generated seriously affects the accuracy of low-concentration methane gas detection.

[0004] In the current modulation and demodulation method, the current scan is superimposed by sawtooth wave and sine wave, where the sawtooth wave is a linear scan with a fixed step. From the output light intensity curve of the laser, it is a straight line with a positive slope, as shown in Figure 1. Sawtooth wave scanning is easy to implement through hardware means, and after the output wavelength drifts, it shows the characteristics of overall drift of the harmonic signal. The signal identification and compensation technology are easy. However, since the current step amplitude of the absorption section and the non-absorption section of the sawtooth wave scanning spectrum is the same, the effective absorption accounts for a low proportion in the signal, and the overall signal-to-noise ratio is not high. Figure 2 shown. Summary of the invention

[0005] In view of this, the purpose of the present invention is to provide a modulation and demodulation method for multi-reverse gas chamber noise suppression, superimposing a Gaussian linear signal similar to the gas absorption peak on the sawtooth wave scanning signal, increasing the current step in the non-absorption section and reducing the scanning time ratio, and reducing the current step in the absorption section and increasing the scanning time ratio, thereby effectively improving the absorption signal strength without changing the current scanning range.

[0006] In order to achieve the above object, the present invention provides the following technical solutions:

[0007] A modulation and demodulation method for multi-reverse air chamber noise suppression, the method comprising:

[0008] S1, selecting a DFB laser with a specific wavelength and current modulation rate;

[0009] S2, superimposing the sawtooth wave and the sine wave to generate the original modulation signal;

[0010] S3, determining the Gaussian signal amplitude according to the comparison relationship between the sawtooth wave scanning spectrum range and the gas spectrum absorption peak, and generating a Gaussian signal;

[0011] S4, combining the generated Gaussian signal with the original modulation signal to obtain a variable step modulation signal;

[0012] S5, inputting the variable step modulation signal as a driving current into the DFB laser to drive the DFB laser to emit a laser signal;

[0013] S6. The laser signal passes through the gas to obtain a gas spectrum signal.

[0014] Furthermore, in step S1, the wavelength of the laser should be a characteristic wavelength of methane gas, such as 1550nm, 1654nm, 1653nm, etc.

[0015] Furthermore, in step S2, the frequency of the sine wave should be at least 100 times the frequency of the sawtooth wave.

[0016] The original modulation signal generated by the superposition of sawtooth wave and sine wave is:

[0017]

[0018] In the formula, F 锯齿 (n) represents sawtooth wave, F 正弦 (n) represents a sine wave, A 偏 Indicates the offset, A 锯齿 Indicates the sawtooth wave amplitude, f 锯齿 represents the sawtooth wave frequency, G represents the sampling frequency, A 正弦 Represents the amplitude of the sine wave, f 正弦 represents the frequency of the sine wave, and n represents the total number of samples.

[0019] Furthermore, in step S4, the generated Gaussian signal is multiplied by the original modulation signal to obtain a variable step modulation signal, as shown below:

[0020]

[0021] In the formula, D represents the amplitude of the Gaussian signal, b represents the center position of the Gaussian signal, and c represents the standard deviation (also called the Gaussian RMS width), which corresponds to the half-peak width. By incorporating the Gaussian signal for modulation, the current modulation is no longer limited to uniform step changes, but according to the characteristics of the Gaussian function, a finer or faster scan is achieved in a specific area, thereby improving the sensitivity and resolution of the detection system.

[0022] The beneficial effect of the present invention is that the present invention superimposes a Gaussian linear signal similar to the gas absorption peak on the sawtooth wave scanning signal, increases the current step in the non-absorption section, reduces the scanning time ratio, and reduces the current step in the absorption section, and increases the scanning time ratio, thereby effectively improving the absorption signal strength without changing the current scanning range. Compared with the traditional step modulation technology, the present invention can significantly enhance the strength and amplitude of the absorption signal, thereby greatly improving the signal-to-noise ratio of methane detection, making the detection result more accurate and reliable.

[0023] Other advantages, objectives and features of the present invention will be described in the following description to some extent, and to some extent, will be obvious to those skilled in the art based on the following examination and study, or can be taught from the practice of the present invention. The objectives and other advantages of the present invention can be realized and obtained through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention will be described in detail below in conjunction with the accompanying drawings, wherein:

[0025] Figure 1 Typical sawtooth wave, sine wave and modulation waveform;

[0026] Figure 2 is the laser signal after methane absorption;

[0027] Figure 3 It is a schematic diagram of the process of the method of the present invention;

[0028] Figure 4 is a schematic diagram of a Gaussian signal;

[0029] Figure 5 It is a variable step sawtooth wave and its modulation waveform;

[0030] Figure 6 Comparison of laser signals after methane absorption under different modulation and demodulation methods. DETAILED DESCRIPTION

[0031] The following describes the embodiments of the present invention by specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments only illustrate the basic concept of the present invention in a schematic manner, and the following embodiments and features in the embodiments can be combined with each other without conflict.

[0032] The present invention provides a modulation and demodulation method for multi-reflector gas chamber noise suppression. The method mainly utilizes a distributed feedback (DFB) laser with a central wavelength precisely set to 1550nm. The laser exhibits excellent wavelength / current modulation performance, that is, each milliampere current change corresponds to a wavelength shift of 0.01nm, and the optical power changes at a rate of 0.3mW / mA with respect to current, thereby achieving dual precise control of laser wavelength and light intensity.

[0033] like Figure 3 As shown, the method described in the present invention is specifically as follows:

[0034] A waveform modulation strategy is introduced to optimize the detection effect. Specifically, a sawtooth wave is used as the basic scanning waveform, which has settable frequency, amplitude and offset parameters to increase the flexibility of spectral scanning. On this basis, a sine wave modulation is further superimposed. The sine wave also has independent frequency and amplitude settings to introduce additional frequency components and enrich signal characteristics. The sampling rate ensures high-precision recording of the modulation process.

[0035] In one embodiment of the present invention, a sawtooth wave scanning frequency is f 锯齿 =1Hz, amplitude A 锯齿 =2V, offset A 偏 =1V, the superimposed sine wave frequency is f 正弦 =100Hz, amplitude A 正弦 =0.2V, sampling rate G=2000Hz.

[0036] The sawtooth wave in one cycle is:

[0037] The sine wave is:

[0038] The modulation signal in one cycle is:

[0039]

[0040] In the process of generating the modulation signal, a Gaussian signal with a specific amplitude and center position is integrated into the sawtooth wave signal to form a variable step current modulation signal. This design makes the current modulation no longer limited to uniform step changes, but based on the characteristics of the Gaussian function, it can achieve finer or faster scanning in a specific area, thereby improving the sensitivity and resolution of the detection system.

[0041] In one embodiment of the present invention, the Gaussian signal amplitude used is D=1, the center b=1000, c=1000, such as Figure 4 As shown, its expression is

[0042] The Gaussian signal is superimposed on the sawtooth wave signal to obtain a variable step current modulation signal, such as Figure 5 shown.

[0043]

[0044] When the above modulated laser signal passes through methane gas, its absorption characteristics are accurately captured and reflected in the output signal, such as Figure 6 As shown, Figure 6 In the figure, the black waveform is the detection waveform without absorption, the red waveform is the detection waveform after equal step modulation absorption, and the green waveform is the detection waveform after variable step modulation absorption. By comparing and analyzing the difference between the output signal and the initial signal under the equal step and variable step modulation methods, it can be clearly observed that under the premise of keeping the sawtooth wave scanning range unchanged, the variable step wavelength modulation technology significantly enhances the intensity and amplitude of the absorption signal, thereby greatly improving the signal-to-noise ratio of methane detection, making the detection results more accurate and reliable.

[0045] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solution, which should be included in the scope of the claims of the present invention.

Claims

1. A modulation and demodulation method for multi-reverse air chamber noise suppression, characterized in that: Selecting a DFB laser with a specific wavelength and current modulation rate; The sawtooth wave and the sine wave are superimposed to generate the original modulation signal; According to the comparison between the sawtooth wave scanning spectrum range and the gas spectrum absorption peak, the Gaussian signal amplitude is determined to generate a Gaussian signal; Combining the generated Gaussian signal with the original modulation signal to obtain a variable step modulation signal; Inputting the variable step modulation signal as a driving current into the DFB laser to drive the DFB laser to emit a laser signal; A gas spectrum signal is obtained after the laser signal passes through the gas.

2. The method according to claim 1, characterized in that: The frequency of the sine wave is 100 times or more of the frequency of the sawtooth wave.

3. The method according to claim 1 or 2, characterized in that: The original modulation signal generated by the superposition of sawtooth wave and sine wave is: In the formula, F 锯齿 (n) represents sawtooth wave, F 正弦 (n) represents a sine wave, A 偏 Indicates the offset, A 锯齿 Indicates the sawtooth wave amplitude, f 锯齿 represents the sawtooth wave frequency, G represents the sampling frequency, A 正弦 Represents the amplitude of the sine wave, f 正弦 represents the frequency of the sine wave, and n represents the total number of samples.

4. The method according to claim 1, characterized in that: The generated Gaussian signal is multiplied by the original modulation signal to obtain a variable step modulation signal, as shown below: In the formula, A 偏 Indicates the offset, A 锯齿 Indicates the sawtooth wave amplitude, f 锯齿 represents the sawtooth wave frequency, G represents the sampling frequency, A 正弦 Represents the amplitude of the sine wave, f 正弦 represents the frequency of the sine wave, n represents the total number of samples, D represents the amplitude of the Gaussian signal, b represents the center position of the Gaussian signal, and c represents the standard deviation, which corresponds to the half-peak width.

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