CN111781153B - Wavelength modulation active laser heterodyne spectrum gas remote measuring method - Google Patents

Abstract

The invention provides a wavelength modulation active laser heterodyne spectrum gas remote measurement method. After the semiconductor laser is used as the laser light source for linear frequency sweep and modulation, the emitted light is divided into two paths, namely signal light and local oscillator light. The signal light is passed through the laser After being collimated, it is injected into the measured space, and the light scattered back by the actual terrain target is received by the laser echo receiving device and then guided into the fiber coupler. The fiber coupler combines the returned signal light with the local oscillator light. After interference, the photoelectric Detector detection: The output signal of the photoelectric detector is band-pass filtered and detected by the Schottky diode detector, and then sent to the lock-in amplifier for harmonic detection to obtain the measured gas concentration. The wavelength modulation active laser heterodyne spectrum gas telemetry method of the invention can effectively reduce noise interference and amplify return signals, and has the advantages of strong detection ability, high sensitivity and long detection distance.

Description

A wavelength modulated active laser heterodyne spectroscopy gas remote sensing method

Technical Field

The invention belongs to the field of gas remote sensing, and in particular relates to a wavelength modulated active laser heterodyne spectroscopy gas remote sensing method.

Background Art

Remote sensing of gas concentration is a highly sought-after capability. Modern industrial production often requires the mining, manufacturing, transportation, storage and use of various flammable, explosive and toxic gases. If the emission and leakage of these dangerous gases can be remotely sensed, the life safety of the workers can be guaranteed. For some common environmental gases, such as carbon dioxide, which is the main component of greenhouse gases and the main part of human exhaled gas, if carbon dioxide gas can be remotely sensed at a long distance, then it is easy to evaluate the emission of factory exhaust and automobile exhaust, evaluate the effect of indoor exhaust systems in public places such as classrooms and shopping malls, and detect camouflaged and hidden personnel in security and anti-terrorism.

The current laser spectroscopy gas concentration remote sensing technology is mainly divided into through-beam remote sensing technology and non-cooperative target remote sensing technology. Through-beam remote sensing technology includes tunable laser absorption spectroscopy (TDLAS), chirped laser dispersion spectroscopy (CLaDS), dual comb spectroscopy (DCS), etc. These spectroscopy technologies require the deployment of reflectors on site and can only be measured at fixed positions. It is not convenient to detect gases at different positions, and their use is very limited. Non-cooperative target spectroscopy remote sensing technology includes differential absorption lidar (DIAL), passive laser heterodyne spectroscopy (LHR), wavelength modulation spectroscopy (WMS), active laser heterodyne spectroscopy (ALHS), etc. These spectra use the terrain and obstacle surfaces behind the gas to reflect the laser backward, or use passive detection methods for detection. Differential absorption lidar relies on complex optical components and large and heavy equipment, which makes it very difficult to carry and use, and cannot cope with the various required usage scenarios; passive heterodyne laser spectroscopy relies on sunlight and cannot be used in places without sunlight; while systems made with wavelength modulation spectroscopy technology and active heterodyne spectroscopy technology are easy to carry and use, and are not limited by sunlight, making them suitable for remote sensing of gases in multiple scenarios, but their maximum detection distances are relatively short, about 10 meters and 40 meters respectively, which cannot fully meet measurement needs.

The main limitation of laser spectroscopy for gas remote sensing is that the noise is strong and the return signal is weak during long-distance measurement, which limits the available measurement distance. As for non-cooperative target spectroscopy technology, since the terrain reflection is usually diffuse reflection, the system can receive fewer reflected signals. When a 10mW laser is used to illuminate the ground at a distance of 5 meters, the diffuse scattered light power collected by the system is in the nW range, which greatly limits the measurement distance. The wavelength modulation-active laser heterodyne spectroscopy of the present invention can extend the limit measurement distance by about tens of meters or even hundreds of meters due to the combination of noise suppression and signal amplification capabilities, making remote sensing of gas concentration more convenient and having a wider range of applications.

Summary of the invention

In view of this, the present invention aims to propose a wavelength modulated active laser heterodyne spectroscopy gas telemetry method to solve the problem that the existing telemetry method has strong noise and weak return signal during long-distance measurement, resulting in limited available measurement distance.

To achieve the above object, the technical solution of the present invention is achieved as follows:

A wavelength modulated active laser heterodyne spectroscopy gas remote sensing method comprises the following steps:

Step 1: Configure the laser so that the laser wavelength sweeps over the target gas spectrum;

Step 2: The light beam emitted by the laser is divided into signal light and local oscillator light. The signal light is sent to the target gas through the laser collimation emission device, and the local oscillator light is introduced into the optical fiber coupler.

Step 3: The transmitted signal light is absorbed by the target gas and reflected by the reflective surface to form a return signal light. The return signal light is focused by the laser receiving and focusing device and then introduced into the fiber coupler. The fiber coupler combines the return signal light with the local oscillator light to generate a beat signal, which is then connected to the photodetector.

Step 4: The output signal of the photodetector is filtered through a bandpass filter, and then the envelope is detected by a Schottky diode detector to output the envelope line, which is then introduced into a lock-in amplifier for harmonic detection, and the first harmonic and second harmonic signals are output;

Step 5: Use the signal acquisition card to collect the harmonic signal, import it into the computer, and calculate the gas concentration data.

Furthermore, the specific process of configuring the laser in step one is: generating a sawtooth signal and a sine signal through a signal generating device, superimposing the sawtooth signal and the sine signal and connecting them to a laser controller, tuning and modulating the laser, and the laser controller directly controls the output wavelength of the laser after receiving the signal from the signal generating device.

Furthermore, the intensity relationship between the transmitted signal light and the returned signal light conforms to the Lambert-Beer law:

为气体吸收线型函数；L为激光经过气体长度(cm)；X为气体浓度。Where ρ is the reflectivity of the reflecting surface, I ₁ is the intensity of the transmitted signal; I ₂ is the intensity of the returned signal; P is the pressure (atm); S(T) is the absorption intensity of the spectral line (cm ^-2 ·atm ^-1 );

is the gas absorption line function; L is the length of the laser passing through the gas (cm); X is the gas concentration.

Furthermore, the specific method of dividing the light beam emitted by the laser into a signal light and a local oscillator light pair in the step 2 is: when a single laser is used, the laser is split through a fiber coupler, one part of which is used as the signal light and the other part is used as the local oscillator light; when a dual laser is used, including laser A and laser B, the laser emitted by laser A is used as the signal light, and the laser emitted by laser B is used as the local oscillator light.

Furthermore, the beat signal is expressed as:

为气体吸收线型函数；L为激光经过气体长度(cm)；X为气体浓度。Where G is the photoelectric gain coefficient, ω ₀ and ω ₂ are the angular frequencies of the local oscillator light and the return signal light, t is the time variable, ρ is the reflectivity of the reflection surface, P is the pressure (atm); S(T) is the spectral line absorption intensity (cm ^-2 ·atm ^-1 );

is the gas absorption line function; L is the length of the laser passing through the gas (cm); X is the gas concentration.

Furthermore, the calculation process in step 5 is as follows: the computer reads the first harmonic signal and the second harmonic signal values at the corresponding position according to the central wavelength of the target gas spectrum, and calculates the ratio of the second harmonic to the first harmonic at this position. The ratio has the following conversion relationship with the gas concentration:

Where i is the linear intensity modulation coefficient of the laser, which is determined by the actual characteristics of the laser used; R ₂₁ is the ratio of the second harmonic to the first harmonic at the central wavelength of the target gas spectrum; a is the modulation depth (cm ^-1 ); θ is the phase angle, which is equal to 2πft, where f is the modulation frequency; and X is the gas concentration.

Compared with the prior art, the wavelength modulated active laser heterodyne spectroscopy gas telemetry method described in the present invention has the following advantages:

(1) The wavelength modulated active laser heterodyne spectroscopy gas telemetry method described in the present invention combines two spectral technologies. By extracting the wavelength modulated return signal from the beat signal and demodulating it, it can effectively reduce noise interference and amplify the return signal. It has a stronger detection capability for weak light, so it has a longer sensitivity and detection distance, that is, it has the advantages of both wavelength modulation spectroscopy and active laser heterodyne spectroscopy.

(2) The wavelength modulation active laser heterodyne spectroscopy gas telemetry method described in the present invention adopts wavelength modulation-active laser heterodyne spectroscopy (WM-ALHS). Compared with independent wavelength modulation spectroscopy and active laser heterodyne spectroscopy, it has higher sensitivity and longer limit measurement distance. Wavelength modulation spectroscopy uses high-frequency signals to modulate lasers and finally uses harmonics for detection, which can filter out low-frequency noise and greatly improve the signal-to-noise ratio of the system. The output power of active laser heterodyne spectroscopy is proportional to the signal light and the local oscillator light power. When the reflected signal light is weak, the reflected signal light power can be amplified by several orders of magnitude by high-power local oscillator light, which greatly improves the sensitivity of the system.

(3) The wavelength modulated active laser heterodyne spectroscopy gas telemetry method described in the present invention has two implementation methods, one using a single laser and the other using dual lasers. The two methods aim to adjust the signal phase in different ways, but ultimately achieve the same measurement effect, making the method more versatile.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the accompanying drawings:

FIG1 is a flow chart of gas remote sensing using a single laser according to an embodiment of the present invention;

FIG. 2 is a flow chart of gas remote sensing using dual lasers according to an embodiment of the present invention.

DETAILED DESCRIPTION

It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside" and the like indicate positions or positional relationships based on the positions or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first", "second", etc. are only used for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, features defined as "first", "second", etc. may explicitly or implicitly include one or more of the features. In the description of the present invention, unless otherwise specified, "multiple" means two or more.

In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood by specific circumstances.

The present invention will be described in detail below with reference to the accompanying drawings and in conjunction with embodiments.

A wavelength modulated active laser heterodyne spectroscopy gas remote sensing method, as shown in FIG1 and FIG2, comprises the following steps:

Step 1: Configure the laser so that the laser wavelength scan covers the target gas spectrum;

Step 2: The light beam emitted by the laser is divided into signal light and local oscillator light. The signal light is sent to the target gas through the laser collimation emission device, and the local oscillator light is introduced into the optical fiber coupler.

Step 3: The transmitted signal light is absorbed by the target gas and reflected by the reflective surface to form a return signal light. The return signal light is focused by the laser receiving and focusing device and then introduced into the fiber coupler. The fiber coupler combines the return signal light with the local oscillator light to generate a beat signal, which is then connected to the photodetector.

Step 4: The output signal of the photodetector is filtered through a bandpass filter, and then the envelope is detected by a Schottky diode detector to output the envelope line, which is then introduced into a lock-in amplifier for harmonic detection, and the first harmonic and second harmonic signals are output;

Step 5: Use the signal acquisition card to collect the harmonic signal, import it into the computer, and calculate the gas concentration data.

The wavelength modulation-active laser heterodyne spectroscopy (WM-ALHS) has higher sensitivity and longer limit measurement distance than independent wavelength modulation spectroscopy and active laser heterodyne spectroscopy. The wavelength modulation spectroscopy uses high-frequency signals to modulate the laser and finally uses harmonics for detection, which can filter out low-frequency noise and greatly improve the signal-to-noise ratio of the system. The output power of the active laser heterodyne spectroscopy is proportional to the signal light and the local oscillator light power. When the reflected signal light is weak, the high-power local oscillator light can amplify the reflected signal light power by several orders of magnitude, greatly improving the sensitivity of the system.

The specific process of configuring the laser in step 1 is: generating a sawtooth signal and a sine signal through a signal generating device, superimposing the sawtooth signal and the sine signal and connecting them to a laser controller, tuning and modulating the laser, and the laser controller directly controls the output wavelength of the laser after receiving the signal from the signal generating device.

The sawtooth signal is set to 10 Hz, and the temperature is adjusted so that the laser wavelength scan covers the target gas spectrum. The sine signal is set to 3000 Hz so that the output wavelength of the laser is modulated at high frequency.

The specific method of splitting the laser in step 2 is: when a single laser is used, the laser is split through the optical fiber coupler A, one part of which is used as signal light and the other part is used as local oscillator light;

After the splitting, the signal light part is sent to the target gas direction through the laser collimation emission device. The signal light is emitted to the target gas, and after being absorbed by the gas, it is reflected back by the reflection surface to form the return signal light. The local oscillation light part is introduced into the optical delay generator, which delays the local oscillation light, adjusts the signal phase, and then introduces the local oscillation light into the optical fiber coupler B.

When dual lasers are used, including laser A and laser B, the laser emitted by laser A is used as signal light, and the laser emitted by laser B is used as local oscillator light. The signal light emitted by laser A is sent through a laser collimation emission device and pointed in the direction of the target gas to obtain the absorption spectrum of the target gas; after the emission signal light is absorbed by the gas, it is reflected back by the reflection surface to form a return signal light; the local oscillator light emitted by laser B is introduced into the fiber coupler by the optical fiber. The two methods aim to adjust the signal phase in different ways, but the final measurement effect is the same, making this method more versatile.

The relationship between the intensity of the transmitted signal light and the returned signal light conforms to the Lambert-Beer law:

为气体吸收线型函数；L为激光经过气体长度(cm)；X为气体浓度。Where ρ is the reflectivity of the reflecting surface, I ₁ is the intensity of the transmitted signal; I ₂ is the intensity of the returned signal; P is the pressure (atm); S(T) is the absorption intensity of the spectral line (cm ^-2 ·atm ^-1 );

is the gas absorption line function; L is the length of the laser passing through the gas (cm); X is the gas concentration.

The return signal light is received by the laser receiving and focusing device. Since the optical path of the return signal light is different from that of the local oscillator light, the wavelength when it reaches the optical fiber coupler is also different, and interference will occur. Due to the limited bandwidth of the photodetector, the sum frequency part cannot be collected. Then the DC part is filtered out through the intermediate frequency filter. Finally, the photodetector only collects the difference frequency part, which is called the beat signal or heterodyne signal. The beat signal is expressed as:

为气体吸收线型函数；L为激光经过气体长度(cm)；X为气体浓度。Where G is the photoelectric gain coefficient, ω ₀ and ω ₂ are the angular frequencies of the local oscillator light and the return signal light, t is the time variable, ρ is the reflectivity of the reflection surface, P is the pressure (atm); S(T) is the spectral line absorption intensity (cm ^-2 ·atm ^-1 );

is the gas absorption line function; L is the length of the laser passing through the gas (cm); X is the gas concentration.

When a single laser is used, the optical delay generator is not adjusted, and the sinusoidal modulation phases of the signal light and the local oscillator light are different, which will make the beat frequency too high and exceed the bandwidth of the photodetector. Therefore, it is necessary to continuously adjust the optical delay generator to delay the local oscillator light so that the phase difference between its modulation signal and the return signal light modulation signal is close to 0, until a beat signal with a relatively stable frequency is obtained;

When dual lasers are used, the phase of the sinusoidal signal is continuously adjusted in the signal generator that controls the local oscillator light to make it close to the phase of the return signal light until a beat signal with a relatively stable frequency is obtained.

The process of calculating the gas concentration data by the computer in step 5 is as follows: the computer reads the first harmonic signal and the second harmonic signal values at the corresponding position according to the central wavelength of the target gas spectrum, and calculates the ratio of the second harmonic to the first harmonic at this position. The ratio has the following conversion relationship with the gas concentration:

Among them, i is the linear intensity modulation coefficient of the laser, which is determined by the actual characteristics of the laser used; R ₂₁ is the ratio of the second harmonic to the first harmonic at the central wavelength of the target gas spectrum; a is the modulation depth (cm ^-1 ); θ is the phase angle, which is equal to 2πft, where f is the modulation frequency; and X is the gas concentration. In this way, the gas concentration can be calculated.

A wavelength modulated active laser heterodyne spectroscopy gas telemetry method.

The specific process when using a single laser is as follows: a sawtooth signal and a sine signal are generated by a single signal generating device, the sawtooth signal and the sine signal are superimposed and then connected to a single laser controller, the single laser is tuned and modulated, and the single laser controller directly controls the output wavelength of the single laser after receiving the signal from the single signal generating device; the single laser is split by an optical fiber coupler A, one part of which is used as signal light and the other part as local oscillator light; the local oscillator light is introduced into an optical delay generator, the optical delay generator delays the local oscillator light, adjusts the signal phase, and then introduces the local oscillator light into an optical fiber coupler B; the split signal light is directed to the optical fiber coupler B through a laser collimation transmitting device. The transmission signal light is sent in the direction of the target gas. After being absorbed by the target gas, the transmission signal light is reflected back by the reflection surface to form the return signal light. The return signal light is focused by the laser receiving and focusing device and then introduced into the fiber coupler B. The fiber coupler B combines the return signal light with the local oscillator light to generate a beat signal and connects to the photodetector. The output signal of the photodetector is filtered by a bandpass filter, and then the envelope is detected by a Schottky diode detector to output the envelope line, which is introduced into the phase-locked amplifier for harmonic detection and outputs the first harmonic and second harmonic signals. The harmonic signal is collected by a signal acquisition card and imported into a computer to obtain the gas concentration data through calculation.

The specific process when using dual lasers is as follows: the dual lasers are named laser A and laser B respectively, a sawtooth signal and a sine signal are generated by the signal generator B, the sawtooth signal and the sine signal are superimposed and connected to the laser controller B, laser B is tuned and modulated, laser controller B directly controls the output wavelength of laser B after receiving the signal from the signal generator B, the laser emitted by laser B is used as the local oscillator light, and the local oscillator light emitted by laser B is introduced into the fiber coupler by the optical fiber; a sawtooth signal and a sine signal are generated by the signal generator A, the sawtooth signal and the sine signal are superimposed and connected to the laser controller A, laser A is tuned and modulated, laser controller A directly controls the output wavelength of laser A after receiving the signal from the signal generator A, The signal light emitted by laser A is sent through a laser collimation emission device in the direction of the target gas to obtain the absorption spectrum of the target gas; after being absorbed by the gas, the emitted signal light is reflected back by the reflection surface to form a return signal light; the return signal light is focused by a laser receiving and focusing device and then introduced into a fiber coupler, which combines the return signal light with the local oscillator light to generate a beat signal and connects to a photodetector; the output signal of the photodetector is filtered through a bandpass filter, and then envelope detection is performed through a Schottky diode detector to output an envelope line, which is then introduced into a phase-locked amplifier for harmonic detection, and the first harmonic and second harmonic signals are output; the harmonic signal is collected using a signal acquisition card, imported into a computer, and the gas concentration data is obtained through calculation.

The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims (4)

1. A wavelength modulated active laser heterodyne spectroscopy gas remote sensing method, characterized in that it comprises the following steps: Step 1: Configure the laser so that the laser wavelength sweeps across the target gas spectrum. The specific process is: generate a sawtooth signal and a sine signal through a signal generator, superimpose the sawtooth signal and the sine signal and connect them to a laser controller to tune and modulate the laser. After receiving the signal from the signal generator, the laser controller directly controls the output wavelength of the laser. Step 2: Split the laser emission light into signal light and local oscillator light, the signal light is sent to the target gas through the laser collimation emission device, and the local oscillator light is introduced into the optical fiber coupler; Step 3: The transmitted signal light is absorbed by the target gas and reflected by the obstacle or terrain surface to form an echo return signal light. The return signal light is focused by the laser receiving and focusing device and then introduced into the fiber coupler. The fiber coupler combines the return signal light with the local oscillator light to generate a beat signal, which is then connected to the photodetector. Step 4: The output signal of the photodetector is filtered through a bandpass filter, and then the envelope is detected by a Schottky diode detector to output the envelope line, which is then introduced into a lock-in amplifier for harmonic detection, and the first harmonic and second harmonic signals are output; Step 5: Use the signal acquisition card to collect harmonic signals, import them into the computer, and calculate the gas concentration data; the specific calculation process is: the computer reads the first harmonic signal and the second harmonic signal value at the corresponding position according to the central wavelength of the target gas spectrum, and calculates the ratio of the second harmonic to the first harmonic at this position. The ratio has the following conversion relationship with the gas concentration:

Where i is the linear intensity modulation coefficient of the laser, which is determined by the actual characteristics of the laser used; R ₂₁ is the ratio of the second harmonic to the first harmonic at the central wavelength of the target gas spectrum; a is the modulation depth cm ^-1 ; θ is the phase angle, which is equal to 2πft, where f is the modulation frequency; and X is the gas concentration. 2. A wavelength modulated active laser heterodyne spectroscopy gas remote sensing method according to claim 1, characterized in that the intensity relationship between the transmitted signal light and the returned signal light conforms to the Lambert-Beer law:

Where ρ is the reflectivity of the reflecting surface, I ₁ is the intensity of the transmitted signal; I ₂ is the intensity of the returned signal; P is the pressure in atm; S(T) is the absorption intensity of the spectral line in cm ^-2 ·atm ^-1 ;

is the gas absorption line function; L is the length of the laser passing through the gas in cm; X is the gas concentration. 3. The wavelength modulated active laser heterodyne spectroscopy gas telemetry method according to claim 1 is characterized in that: the specific method of dividing the light beam emitted by the laser into a signal light and a local oscillator light pair in the step 2 is: when a single laser is used, the laser is split through an optical fiber coupler, one part of which is used as the signal light and the other part is used as the local oscillator light; when a dual laser is used, including laser A and laser B, the laser emitted by laser A is used as the signal light, and the laser emitted by laser B is used as the local oscillator light. 4. A wavelength modulated active laser heterodyne spectroscopy gas remote sensing method according to claim 1, characterized in that: the beat signal is represented by the formula:

Where G is the photoelectric gain coefficient, ω ₀ and ω ₂ are the angular frequencies of the local oscillator light and the return signal light, t is the time variable, ρ is the reflectivity of the reflection surface, P is the pressure atm; S(T) is the spectral line absorption intensity cm ^-2 ·atm ^-1 ;

is the gas absorption line function; L is the length of the laser passing through the gas in cm; X is the gas concentration.

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