CN111781153B - Wavelength modulation active laser heterodyne spectrum gas remote measuring method - Google Patents
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Abstract
The invention provides a wavelength modulation active laser heterodyne spectrum gas telemetering method, which is characterized in that a semiconductor laser is used as a laser light source to perform linear frequency sweep and modulation, emitted light is divided into two paths, namely signal light and local oscillator light, the signal light is collimated by laser and then emitted into a measured space, the light scattered back by an actual terrain target is received by a laser echo receiving device and then guided into an optical fiber coupler, the optical fiber coupler combines the returned signal light and the local oscillator light, and the light is detected by a photoelectric detector after interference occurs; the output signal of the photoelectric detector is sent to a phase-locked amplifier for harmonic detection after being subjected to band-pass filtering and detection by a Schottky diode detector, so that the concentration of the gas to be detected is obtained. The wavelength modulation active laser heterodyne spectrum gas telemetering method can effectively reduce noise interference and amplify return signals, and has the advantages of strong detection capability, high sensitivity and long detection distance.
Description
Technical Field
The invention belongs to the field of gas remote measurement, and particularly relates to a wavelength modulation active laser heterodyne spectrum gas remote measurement method.
Background
Remote telemetry of gas concentrations is a highly sought after capability. The production of modern industries often requires the exploitation, manufacture, transportation, storage and use of various flammable, explosive and toxic gases. If the discharge and leakage of the dangerous gases can be remotely measured, the life safety of workers can be ensured. And some common environmental gases, such as carbon dioxide, are the main components of greenhouse gases and the main part of gases exhaled by human bodies, if the carbon dioxide gas can be remotely and conveniently telemetered, the emission conditions of factory waste gas and automobile exhaust can be easily evaluated, the effect of an indoor exhaust system can be evaluated in public places such as classrooms and markets, and disguised and hidden personnel can be detected in security and anti-terrorism.
The current laser spectrum gas concentration remote measuring technology is mainly divided into a correlation remote measuring technology and a non-cooperative target remote measuring technology. The correlation type telemetry technology comprises tunable laser absorption spectroscopy (TDLAS), chirped laser dispersion spectroscopy (CLaDS), double-light comb spectroscopy (DCS) and the like, wherein reflectors need to be arranged on site, measurement can be carried out only at fixed positions, gases at different positions cannot be conveniently detected, and the use is very limited. Non-cooperative target spectral telemetry includes differential absorption laser radar (DIAL), passive laser heterodyne spectroscopy (LHR), wavelength Modulation Spectroscopy (WMS), active Laser Heterodyne Spectroscopy (ALHS), etc., which utilize the topography behind the gas and the surface of obstacles to retroreflect laser light or use passive detection for detection. The differential absorption laser radar depends on complex optical elements and bulky and heavy equipment, is very difficult to carry and use, and cannot cope with various required use scenes; the passive heterodyne laser spectrum depends on sunlight and cannot be used in the occasions without sunlight; the systems manufactured by the wavelength modulation spectrum technology and the active heterodyne spectrum technology are convenient to carry and use, are not limited by sunlight, are suitable for remotely measuring gas in multiple scenes, but have short limit detection distances of about 10 meters and about 40 meters respectively, and cannot completely meet the measurement requirements.
The main limitations of laser spectroscopy on gas telemetry are that the noise is strong and the return signal is weak for long distance measurements, resulting in a limited range of measurement available. And as the topographic reflection is generally diffuse reflection, the system can receive fewer reflected signals, and when the ground is irradiated by using laser with the power of 10mW at the distance of 5 meters, the power of diffuse scattered light collected by the system is within the nW range, so that the measurement distance is greatly limited. The wavelength modulation-active laser heterodyne spectrum disclosed by the invention combines the noise suppression and signal amplification capabilities, so that the limit measurement distance can be prolonged by about tens of meters or even hundreds of meters, the remote measurement on the gas concentration is more convenient, and the application range is wider.
Disclosure of Invention
In view of the above, the present invention is directed to a wavelength modulation active laser heterodyne spectroscopy gas telemetry method, so as to solve the problem that the available measurement distance is limited due to strong noise and weak return signal in long-distance measurement in the conventional telemetry method.
In order to achieve the purpose, the technical scheme of the invention is realized as follows:
a wavelength modulation active laser heterodyne spectrum gas telemetry method comprises the following steps:
the method comprises the following steps: configuring a laser to enable laser wavelength to scan and cover a target gas spectral line;
step two: dividing a light beam emitted by a laser into signal light and local oscillator light, wherein the signal light is transmitted to target gas through a laser collimation emitting device to emit the signal light, and the local oscillator light is guided into an optical fiber coupler;
step three: the emitted signal light is absorbed by target gas and reflected by a reflecting surface to form return signal light, the return signal light is focused by a laser receiving and focusing device and then guided into an optical fiber coupler, the optical fiber coupler combines the return signal light and a local oscillator light to generate a beat signal, and the beat signal is connected into an photoelectric detector;
step four: the output signal of the photoelectric detector is filtered by a band-pass filter to remove noise, then is subjected to envelope detection by a Schottky diode detector to output an envelope curve, is led into a phase-locked amplifier to be subjected to harmonic detection, and outputs a first harmonic signal and a second harmonic signal;
step five: and acquiring harmonic signals by using a signal acquisition card, and guiding the harmonic signals into a computer to calculate gas concentration data.
Further, the specific process of configuring the laser in the first step is as follows: the signal generating device generates a sawtooth signal and a sine signal, the sawtooth signal and the sine signal are superposed and then are connected to the laser controller, the laser is tuned and modulated, and the laser controller directly controls the output wavelength of the laser after receiving the signal of the signal generating device.
Further, the light intensity relationship between the emission signal light and the return signal light conforms to the Lambert-Beer law:
where ρ is the reflectivity of the reflecting surface, I 1 Is the intensity of the emitted signal light; i is 2 Is the return signal light intensity; p is pressure (atm); s (T) is the absorption intensity (cm) of the spectral line -2 ·atm -1 )；
Is a linear function of gas absorption; l is the laser passing gas length (cm); x is the gas concentration.
Further, the specific method for dividing the light beam emitted by the laser into the signal light and the local oscillator light pair in the second step is as follows: when a single laser is adopted, the laser splits light through an optical fiber coupler, wherein one part of the light is used as signal light, and the other part of the light is used as local oscillation light; when the dual laser is adopted, the dual laser comprises a laser A and a laser B, wherein laser emitted by the laser A is used as signal light, and laser emitted by the laser B is used as local oscillation light.
Further, the expression formula of the beat signal is as follows:
where G is the photoelectric gain coefficient, ω 0 、ω 2 The angular frequency of the local oscillation light and the return signal light, t is a time variable, rho is the reflectivity of the reflecting surface, and P is pressure (atm); s (T) is the absorption intensity (cm) of the spectral line -2 ·atm -1 )；
Is a linear function of gas absorption; l is the laser passing gas length (cm); x is the gas concentration.
Further, the calculation process in the fifth step is as follows: the computer reads the first harmonic signal and the second harmonic signal at the corresponding position according to the central wavelength of the target gas spectral line, and calculates the ratio of the second harmonic to the first harmonic at the position, and the ratio and the gas concentration have the following conversion relation:
wherein i is a linear intensity modulation coefficient of the laser and is determined by the characteristics of the laser which is actually used; r 21 The ratio of the second harmonic to the first harmonic at the center wavelength of the target gas spectral line; a is modulation depth (cm) -1 ) (ii) a Theta is a phase angle and is equal to 2 pi ft, wherein f is a modulation frequency; x is the gas concentration.
Compared with the prior art, the wavelength modulation active laser heterodyne spectrum gas telemetry method has the following advantages:
(1) The wavelength modulation active laser heterodyne spectrum gas telemetering method combines two spectrum technologies, can effectively reduce noise interference and amplify return signals by extracting and demodulating the wavelength modulation return signals from beat signals, has stronger detection capability on weak light, and has farther sensitivity and detection distance, namely, has the advantages of the two technologies of wavelength modulation spectrum and active laser heterodyne spectrum.
(2) According to the wavelength modulation active laser heterodyne spectrum gas telemetering method, wavelength modulation-active laser heterodyne spectrum (WM-ALHS) is adopted, compared with independent wavelength modulation spectrum and active laser heterodyne spectrum, the method has high sensitivity and longer limit measurement distance, the wavelength modulation spectrum modulates laser by using high-frequency signals, and finally uses harmonic waves for detection, so that low-frequency noise can be filtered, and the signal-to-noise ratio of a system is greatly improved; the output power of the active laser heterodyne spectrum is in direct proportion to the power of signal light and local oscillator light, and when the reflected signal light is weak, the reflected signal light power can be amplified by several orders of magnitude through the high-power local oscillator light, so that the sensitivity of the system is greatly improved.
(3) The wavelength modulation active laser heterodyne spectrum gas telemetry method has two implementation modes, wherein one mode uses a single laser, and the other mode uses a double laser, and the two modes aim to adjust the signal phase by different modes, but the finally achieved measurement effect is the same, so that the method has stronger universality.
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The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an embodiment of the invention and, together with the description, serve to explain the invention and not to limit the invention. In the drawings:
FIG. 1 is a flow chart of gas telemetry using a single laser according to an embodiment of the present invention;
FIG. 2 is a flow chart of gas telemetry using twin lasers according to an embodiment of the present invention.
Detailed Description
It should be noted that the embodiments and features of the embodiments may be combined with each other without conflict.
In the description of the present invention, it is to be understood that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", and the like, indicate orientations or positional relationships based on those shown in the drawings, and are used only for convenience in describing the present invention and for simplicity in description, and do not indicate or imply that the referenced devices or elements must have a particular orientation, be constructed and operated in a particular orientation, and thus, are not to be construed as limiting the present invention. Furthermore, the terms "first," "second," and the like are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or to implicitly indicate a number of the indicated technical features. Thus, a feature defined as "first," "second," etc. may explicitly or implicitly include one or more of that feature. In the description of the present invention, "a plurality" means two or more unless otherwise specified.
In the description of the present invention, it should be noted that, unless otherwise explicitly specified or limited, the terms "mounted," "connected," and "connected" are to be construed broadly, e.g., as meaning either a fixed connection, a removable connection, or an integral connection; can be mechanically or electrically connected; they may be connected directly or indirectly through intervening media, or they may be interconnected between two elements. The specific meaning of the above terms in the present invention can be understood by those of ordinary skill in the art through specific situations.
The present invention will be described in detail below with reference to the embodiments with reference to the attached drawings.
A method for wavelength-modulated active laser heterodyne spectroscopy gas telemetry, as shown in fig. 1 and 2, comprising the steps of:
the method comprises the following steps: configuring a laser to enable laser wavelength to scan and cover a target gas spectral line;
step two: dividing a light beam emitted by a laser into signal light and local oscillator light, wherein the signal light is transmitted to target gas through a laser collimation emitting device to emit the signal light, and the local oscillator light is guided into an optical fiber coupler;
step three: the emitted signal light is absorbed by target gas and reflected by a reflecting surface to form return signal light, the return signal light is focused by a laser receiving and focusing device and then guided into an optical fiber coupler, the optical fiber coupler combines the return signal light and a local oscillator light to generate a beat signal, and the beat signal is connected into an photoelectric detector;
step four: the output signal of the photoelectric detector is filtered by a band-pass filter to remove noise, then envelope detection is carried out by a Schottky diode detector to output an envelope curve, the envelope curve is led into a phase-locked amplifier to carry out harmonic detection, and a first harmonic signal and a second harmonic signal are output;
step five: and acquiring harmonic signals by using a signal acquisition card, and guiding the harmonic signals into a computer to calculate gas concentration data.
Compared with an independent wavelength modulation spectrum and an active laser heterodyne spectrum which have high sensitivity and longer limit measurement distance, the wavelength modulation spectrum modulates laser by using a high-frequency signal and finally detects by using harmonic waves, so that low-frequency noise can be filtered, and the signal-to-noise ratio of the system is greatly improved; the output power of the active laser heterodyne spectrum is in direct proportion to the power of signal light and local oscillator light, and when the reflected signal light is weak, the reflected signal light power can be amplified by several orders of magnitude through the high-power local oscillator light, so that the sensitivity of the system is greatly improved.
The specific process of configuring the laser in the first step is as follows: the sawtooth signal and the sine signal are generated by the signal generating device, are superposed and then are accessed to the laser controller to tune and modulate the laser, and the laser controller directly controls the output wavelength of the laser after receiving the signal of the signal generating device.
Setting a sawtooth signal to be 10Hz, and enabling laser wavelength to scan and cover a target gas spectral line by adjusting the temperature; the sinusoidal signal was set to 3000Hz so that the output wavelength of the laser was high frequency modulated.
The specific method for splitting the laser in the second step is as follows: when a single laser is adopted, the laser splits light through the optical fiber coupler A, wherein one part of the light is used as signal light, and the other part of the light is used as local oscillation light;
the split signal light part points to the direction of the target gas through a laser collimation emitting device to send emitted signal light, the emitted signal light is emitted to the target gas, and after being absorbed by the gas, the emitted signal light is reflected back by a reflecting surface to form returned signal light; the local oscillator light is partially guided into an optical delay generator, and the optical delay generator conducts delay processing on the local oscillator light and adjusts the signal phase, and then the local oscillator light is guided into an optical fiber coupler B;
when the dual laser is adopted, the dual laser comprises a laser A and a laser B, wherein laser emitted by the laser A is used as signal light, laser emitted by the laser B is used as local oscillator light, and the signal light emitted by the laser A is transmitted to a target gas direction through a laser collimation transmitting device and is used for obtaining an absorption spectrum line of the target gas; the emitted signal light is absorbed by gas and then is reflected back by the reflecting surface to form returned signal light; and local oscillation light emitted by the laser B is guided into the optical fiber coupler by the optical fiber. The two modes aim to adjust the signal phase in different modes, but the finally achieved measurement effect is the same, so that the method is more universal.
The light intensity relation of the emission signal light and the return signal light conforms to the Lambert-Beer law:
where ρ is the reflectivity of the reflecting surface, I 1 Is the intensity of the emitted signal light; I.C. A 2 Is the return signal light intensity; p is pressure (atm); s (T) is the absorption intensity (cm) of the spectral line -2 ·atm -1 )；
Is a linear function of gas absorption; l is the laser passing gas length (cm); x is the gas concentration.
The return signal light is received by the laser receiving focusing device, because the optical path of the return signal light is different from that of the local oscillator light, the wavelength of the return signal light is different when the return signal light reaches the coupler, the interference phenomenon can occur, because the bandwidth of the photoelectric detector is limited, the sum frequency part can not be collected, the direct current part is filtered out through intermediate frequency filtering, finally, the photoelectric detector only collects the difference frequency part, which is called as a beat signal or a heterodyne signal, and the beat signal is expressed as:
where G is the photoelectric gain coefficient, ω 0 、ω 2 The angular frequency of the local oscillation light and the return signal light, t is a time variable, rho is the reflectivity of the reflecting surface, and P is pressure (atm); s (T) is the absorption intensity (cm) of the spectral line -2 ·atm -1 )；
Is a linear function of gas absorption; l is the laser passing gas length (cm); x is the gas concentration.
When a single laser is adopted, the optical delay generator is not adjusted at the moment, and the sinusoidal modulation phases of the signal light and the local oscillator light are different, so that the beat frequency is too high and exceeds the bandwidth of the photoelectric detector. Therefore, the optical delay generator needs to be continuously adjusted to delay the local oscillator light so that the phase difference between the modulation signal of the local oscillator light and the modulation signal of the return signal light is close to 0 until a beat signal with more stable frequency is obtained;
when the double laser is adopted, the phase of the sinusoidal signal is continuously adjusted in the signal generator for controlling the local oscillator light, so that the sinusoidal signal is close to the phase of the returned signal light until a beat signal with more stable frequency is obtained.
The process of calculating the gas concentration data by the computer in the fifth step is as follows: the computer reads the first harmonic signal and the second harmonic signal at the corresponding position according to the central wavelength of the target gas spectral line, and calculates the ratio of the second harmonic to the first harmonic at the position, and the ratio and the gas concentration have the following conversion relation:
wherein i is a linear intensity modulation coefficient of the laser and is determined by the characteristics of the laser which is actually used; r 21 The ratio of the second harmonic to the first harmonic at the center wavelength of the target gas spectral line; a is modulation depth (cm) -1 ) (ii) a Theta is a phase angle and is equal to 2 pi ft, wherein f is a modulation frequency; x is the gas concentration. Thus, the gas concentration can be calculated.
A wavelength modulation active laser heterodyne spectrum gas telemetering method,
the specific process when a single laser is adopted is as follows: generating a sawtooth signal and a sine signal by a single signal generating device, overlapping the sawtooth signal and the sine signal, then accessing the sawtooth signal and the sine signal into a single laser controller, tuning and modulating the single laser, and directly controlling the output wavelength of the single laser after the single laser controller receives the signal of the single signal generating device; the single laser splits light through an optical fiber coupler A, wherein one part of the light is used as signal light, and the other part of the light is used as local oscillation light; the local oscillator light is guided into an optical delay generator, and the optical delay generator conducts delay processing and signal phase adjustment on the local oscillator light and then guides the local oscillator light into an optical fiber coupler B; the split signal light part is transmitted to the direction of the target gas through a laser collimation transmitting device, and the transmitted signal light is absorbed by the target gas and then is reflected back by a reflecting surface to form returned signal light; the return signal light is focused by the laser receiving focusing device and then guided into the optical fiber coupler B, and the optical fiber coupler B combines the return signal light and the local oscillator light to generate a beat signal and is connected into the photoelectric detector; the output signal of the photoelectric detector is filtered by a band-pass filter to remove noise, then envelope detection is carried out by a Schottky diode detector to output an envelope curve, the envelope curve is led into a phase-locked amplifier to carry out harmonic detection, and a first harmonic signal and a second harmonic signal are output; acquiring harmonic signals by using a signal acquisition card, guiding the harmonic signals into a computer, and calculating to obtain gas concentration data;
the specific process when the double laser is adopted is as follows: the dual lasers are respectively named as a laser A and a laser B, a sawtooth signal and a sine signal are generated through a signal generating device B, the sawtooth signal and the sine signal are superposed and then are accessed to a laser controller B, the laser B is tuned and modulated, the laser controller B directly controls the output wavelength of the laser B after receiving the signal of the signal generating device B, laser emitted by the laser B is used as local oscillation light, and the local oscillation light emitted by the laser B is guided into an optical fiber coupler through an optical fiber; generating a sawtooth signal and a sine signal by a signal generating device A, overlapping the sawtooth signal and the sine signal, then accessing the signal into a laser controller A, tuning and modulating the laser A, directly controlling the output wavelength of the laser A after the laser controller A receives the signal of the signal generating device A, and transmitting signal light by directing the signal light emitted by the laser A to the direction of target gas through a laser collimation transmitting device for obtaining the absorption spectrum line of the target gas; the emitted signal light is absorbed by gas and then is reflected back by the reflecting surface to form returned signal light; the return signal light is focused by the laser receiving focusing device and then guided into the optical fiber coupler, the optical fiber coupler combines the return signal light and the local oscillator light to generate a beat signal, and the beat signal is connected into the photoelectric detector; the output signal of the photoelectric detector is filtered by a band-pass filter to remove noise, then envelope detection is carried out by a Schottky diode detector to output an envelope curve, the envelope curve is led into a phase-locked amplifier to carry out harmonic detection, and a first harmonic signal and a second harmonic signal are output; and acquiring harmonic signals by using a signal acquisition card, guiding the harmonic signals into a computer, and calculating to obtain gas concentration data.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.
Claims (4)
1. A wavelength modulation active laser heterodyne spectrum gas telemetry method is characterized in that: the method comprises the following steps:
the method comprises the following steps: configuring a laser to enable laser wavelength to scan a target gas spectral line; the specific process is as follows: generating a sawtooth signal and a sine signal by a signal generating device, overlapping the sawtooth signal and the sine signal, then accessing the signals into a laser controller, tuning and modulating the laser, and directly controlling the output wavelength of the laser after the laser controller receives the signals of the signal generating device;
step two: splitting emitted light of a laser into signal light and local oscillator light, wherein the signal light is transmitted to target gas through a laser collimation transmitting device and the local oscillator light is guided into an optical fiber coupler;
step three: the emitted signal light is absorbed by target gas and reflected by a barrier or a terrain surface to form echo return signal light, the return signal light is focused by a laser receiving focusing device and then guided into an optical fiber coupler, the optical fiber coupler combines the return signal light and a local oscillator light to generate a beat signal, and the beat signal is incident into an photoelectric detector;
step four: the output signal of the photoelectric detector is filtered by a band-pass filter to remove noise, then is subjected to envelope detection by a Schottky diode detector to output an envelope curve, is led into a phase-locked amplifier to be subjected to harmonic detection, and outputs a first harmonic signal and a second harmonic signal;
step five: collecting harmonic signals by using a signal acquisition card, guiding the harmonic signals into a computer, and calculating gas concentration data; the specific calculation process is as follows: the computer reads the first harmonic signal and the second harmonic signal at the corresponding position according to the central wavelength of the target gas spectral line, and calculates the ratio of the second harmonic to the first harmonic at the position, and the ratio and the gas concentration have the following conversion relation:
wherein i is a linear intensity modulation coefficient of the laser and is determined by the characteristics of the laser which is actually used; r is 21 The ratio of the second harmonic to the first harmonic at the center wavelength of the target gas spectral line; a is modulation depth cm -1 (ii) a Theta is a phase angle and is equal to 2 pi ft, wherein f is a modulation frequency; x is the gas concentration.
2. The wavelength modulation active laser heterodyne spectroscopy gas telemetry method as claimed in claim 1, wherein: the light intensity relation between the emission signal light and the return signal light conforms to the Lambert-Beer law:
where ρ is the reflectivity of the reflecting surface, I 1 Is the intensity of the emitted signal light; i is 2 Is the return signal light intensity; p is pressure atm; s (T) is the absorption intensity cm of the spectral line -2 ·atm -1 ；
Is a gas absorption line function; l is the length cm of laser passing through the gas; x is the gas concentration.
3. The wavelength modulation active laser heterodyne spectroscopy gas telemetry method as claimed in claim 1, wherein: the specific method for dividing the light beam emitted by the laser into the signal light and the local oscillator light pair in the second step is as follows: when a single laser is adopted, the laser splits light through an optical fiber coupler, wherein one part of the light is used as signal light, and the other part of the light is used as local oscillation light; when the dual laser is adopted, the dual laser comprises a laser A and a laser B, wherein laser emitted by the laser A is used as signal light, and laser emitted by the laser B is used as local oscillation light.
4. The wavelength modulation active laser heterodyne spectroscopy gas telemetry method as claimed in claim 1, wherein: the expression formula of the beat signal is as follows:
where G is the photoelectric gain coefficient, ω 0 、ω 2 The angular frequency of the local oscillation light and the return signal light is shown, t is a time variable, rho is the reflectivity of the reflecting surface, and P is pressure atm; s (T) is the absorption intensity cm of the spectral line -2 ·atm -1 ；
Is a linear function of gas absorption; l is the length cm of laser passing through the gas; x is the gas concentration. />
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| CN112924415B (en) * | 2021-01-26 | 2023-01-06 | 北京无线电计量测试研究所 | Data processing method and system for laser methane telemetering equipment |
| WO2023079459A1 (en) * | 2021-11-03 | 2023-05-11 | Onpoint Technologies, Llc | Heterodyne backscatter sensor and spectroscopy |
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