CN113777073B - Gas detection method and system based on optical phase amplification - Google Patents
Gas detection method and system based on optical phase amplification Download PDFInfo
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- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
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Abstract
The invention relates to the field of gas detection, in particular to a gas detection method and system based on optical phase amplification. According to the invention, the gas to be detected absorbs the heating laser to generate heat, and the generated heat can modulate the phase of the detection laser; the photo-thermal phase modulation of the detection laser is amplified by the optical resonant cavity; the phase of the detection laser light exiting the optical resonator is measured and analyzed to obtain parameters of the gas to be detected. The invention can effectively amplify the tiny photo-thermal phase modulation received by the detection laser, and improves the signal-to-noise ratio of detection. The invention can realize the detection of trace gas parameters with the lower limit of volume fraction detection of the order of trillion, and has universality for the gas capable of absorbing visible light wave band, near infrared wave band and middle infrared wave band.
Description
Technical Field
The invention relates to the field of gas detection, in particular to a gas detection method and system based on optical phase amplification.
Background
The high-sensitivity gas detection technology has wide application in the fields of environmental pollutant monitoring, respiratory gas disease diagnosis, atomic and molecular physics and the like. The gas detection method based on the absorption spectrum technology and the derivative technology has the characteristics of high gas parameter detection sensitivity, good gas type selectivity and the like. According to the beer-lambert law, when incident light of a specific wavelength passes through a gas to be detected, a gas molecule absorbs a part of optical power, so that the transmitted optical power is reduced. The parameters of the gas can be determined according to the intensity of the absorption of the optical power by the gas molecules and the characteristic absorption wavelength. Another technique is to detect gas parameters using a photothermal interferometer based on the photothermal effect caused by light absorption. When the gas molecules absorb the incident light power, heat is released, and the gas temperature is changed. The photothermal interferometer measures the amplitude of the optical phase modulation caused by the change of the gas temperature to characterize the gas parameter. The optical resonance module is utilized to enhance the interaction between the light and the gas molecules, so that the gas detection sensitivity is improved.
The optical absorption detection technology based on the enhancement of the hollow micro-nano structure fiber resonant cavity is limited by the multimode transmission characteristics of the hollow micro-nano structure fiber, and the system is greatly influenced by mode interference noise, and has low detection sensitivity and poor stability. The optical fiber resonant cavity based on the hollow micro-nano structure can theoretically improve the sensitivity of light-heat interference gas detection. The technology fixes the detection laser wavelength at the position with the maximum transmission spectrum slope of the resonant cavity, and improves the gas detection sensitivity by improving the detection efficiency of the photothermal phase modulation. However, at the position with the maximum transmission spectrum slope of the resonant cavity, the phase noise of the detection laser is converted into intensity noise, so that the overall noise level of the detection system is increased, the detection sensitivity of the system is improved to a limited extent, and the requirements of practical application cannot be met.
In summary, the sensitivity of detecting gas parameters in the prior art is low.
Accordingly, there is a need for improvement and advancement in the art.
Disclosure of Invention
In order to solve the technical problems, the invention provides a gas detection method and a system based on optical phase amplification, which solve the problem of low sensitivity of detecting gas parameters.
In order to achieve the above purpose, the present invention adopts the following technical scheme:
in a first aspect, the present invention provides a gas detection method based on optical phase amplification, including:
injecting heating laser and detection laser into an optical resonant cavity, wherein the optical resonant cavity contains a gas sample containing gas to be detected, the absolute value of the difference between the wavelength of the heating laser and the wavelength corresponding to the center of an absorption line of a gas molecule to be detected is smaller than a first set value, the wavelength of the detection laser is matched with one resonant wavelength of the optical resonant cavity, the absolute value of the difference between the wavelength of the detection laser and the wavelength corresponding to the center of the absorption line of the gas molecule to be detected is larger than a second set value, and the second set value is larger than the first set value;
Acquiring detection emergent light information emitted by the detection light after amplified photo-thermal phase modulation of the optical resonant cavity for containing the gas sample containing the gas to be detected;
And obtaining the parameter of the gas to be detected according to the phase modulation amplitude corresponding to the detection emergent light information.
In one implementation, the detection-exit light includes detection-reflected light of the detection light reflected by the optical resonator and detection-transmitted light of the detection light transmitted by the optical resonator; the step of obtaining the parameter of the gas to be detected according to the phase modulation amplitude of the detection emergent light comprises the following steps:
Acquiring a database, wherein the database comprises parameters of calibration gas and phase modulation amplitude of calibration laser matched with the parameters of the calibration gas; the calibration laser corresponds to the detection emergent light, the phase modulation amplitude of the calibration laser corresponds to the phase modulation amplitude of the detection emergent light, and the calibration gas corresponds to the gas sample; the calibration gas refers to a uniform and stable gas sample formed by mixing the gas to be measured with another gas of different types according to a determined volume fraction under the condition that the physical parameters of the gas sample are known.
And obtaining the parameter of the gas to be detected according to the phase modulation amplitude of the detection emergent light, the phase modulation amplitude of the calibration laser corresponding to the phase modulation amplitude of the detection emergent light and the parameter of the calibration gas matched with the phase modulation amplitude of the calibration laser.
In one implementation, the detecting the phase modulation amplitude of the outgoing light includes detecting a phase modulation amplitude of the transmitted light and detecting a phase modulation amplitude of the reflected light;
The phase modulation amplitude of the calibration laser comprises a calibration phase modulation amplitude corresponding to the phase modulation amplitude of the detection transmission light, or the phase modulation amplitude of the calibration laser comprises a calibration phase modulation amplitude corresponding to the phase modulation amplitude of the detection reflection light, or the phase modulation amplitude of the calibration laser comprises a calibration phase difference modulation amplitude corresponding to the detection light phase difference modulation amplitude formed by the phase modulation of the detection transmission light and the phase modulation of the detection reflection light;
The parameters of the calibration gas comprise the concentration of the calibration gas matched with the calibration phase modulation amplitude and the concentration of the calibration gas matched with the calibration phase difference modulation amplitude;
the parameter of the gas to be detected comprises the concentration of the gas to be detected in the gas sample;
The step of obtaining the parameter of the gas to be detected according to the phase modulation amplitude of the detection emergent light, the modulation amplitude of the calibration laser corresponding to the phase modulation amplitude of the detection emergent light and the parameter of the calibration gas matched with the phase modulation amplitude of the calibration laser, comprises the following steps:
Acquiring a calibration phase modulation amplitude corresponding to the phase modulation amplitude of the detection transmission light according to the phase modulation amplitude of the detection transmission light;
obtaining a calibration gas concentration matched with the calibration phase modulation amplitude according to the calibration phase modulation amplitude corresponding to the phase modulation amplitude of the detected transmitted light;
Obtaining the concentration of the gas to be detected in a gas sample according to the concentration of the calibration gas;
or obtaining a calibrated phase modulation amplitude corresponding to the phase of the detection reflected light according to the phase modulation amplitude of the detection reflected light;
Obtaining a calibration gas concentration matched with the calibration phase modulation amplitude according to the calibration phase modulation amplitude corresponding to the phase modulation amplitude of the detection reflected light;
Obtaining the concentration of the gas to be detected in a gas sample according to the concentration of the calibration gas;
Or according to the phase modulation of the detection transmission light and the phase modulation of the detection reflection light, obtaining a calibration phase difference modulation amplitude corresponding to a detection light phase difference modulation amplitude formed by the phase modulation of the detection transmission light and the phase modulation of the detection reflection light;
obtaining a calibration gas concentration matched with the calibration phase difference modulation amplitude according to the calibration phase difference modulation amplitude;
and obtaining the concentration of the gas to be detected in the gas sample according to the concentration of the calibration gas.
In one implementation, the gas detection method further includes tuning a wavelength of the detection laser to be equal to a standard resonant wavelength of the optical resonant cavity during gas detection, including:
and controlling the wavelength of the detection laser in real time so that the wavelength of the detection laser is equal to a selected resonant wavelength of the optical resonant cavity, wherein the selected resonant wavelength is one resonant wavelength of which the absolute value of the difference between the resonant wavelengths of the optical resonant cavity and the wavelength corresponding to the center of the absorption line of the gas molecule to be detected is larger than a second set value.
In one implementation, the heating laser is a periodic laser signal.
In a second aspect, an embodiment of the present invention further provides a gas detection system based on optical phase, where the gas detection system includes:
the heating laser module is used for outputting heating laser with the absolute value of the difference between the wavelength and the wavelength corresponding to the center of the absorption line of the gas molecules to be detected smaller than a first set value;
The detection laser module is used for outputting detection laser with the absolute value of the difference between the wavelength and the wavelength corresponding to the molecular absorption line center of the gas to be detected being larger than a second set value;
the optical phase amplifying and detecting module is used for containing a gas sample containing a gas to be detected, receiving heating laser and detecting laser, and amplifying and measuring photo-thermal phase modulation received by the detecting laser;
The signal processing module is used for demodulating the phase modulation amplitude of the detection emergent light measured by the optical phase amplification and detection module and obtaining the parameter of the gas to be detected;
The heating laser module and the detecting laser module are connected with the optical phase amplifying and detecting module, and the optical phase amplifying and detecting module is connected with the signal processing module.
In one implementation, the detection laser module includes a detection laser light source and a wavelength stabilizing component;
The detection laser light source is used for outputting the detection laser;
the wavelength stabilizing component is used for enabling the wavelength of the detection laser output by the detection laser source to be matched with one resonant wavelength of the optical resonant cavity, and enabling the absolute value of the difference between the wavelength of the detection laser output by the detection laser source and the wavelength corresponding to the absorption line center of the gas molecules to be detected to be larger than a second set value;
The output end of the detection laser light source is connected with the optical resonant cavity, the input end of the wavelength stabilizing component is connected with the optical resonant cavity, and the output end of the wavelength stabilizing component is connected with the wavelength control end of the detection laser light source.
In one implementation, the optical phase amplifying and detecting module comprises a wavelength division multiplexer, a circulator, a second optical coupler and an optical resonant cavity for containing a gas to be detected and receiving heating laser and detecting laser, wherein the optical resonant cavity comprises a micro-nano structure optical fiber and a gas chamber containing the micro-nano structure optical fiber;
The laser output end of the heating laser module is connected with the first port of the wavelength division multiplexer;
The laser output end of the detection laser source is connected with the first port of the circulator, and the second port of the circulator is connected with the second port of the wavelength division multiplexer;
The third port of the circulator is connected with the input end of the second optical coupler, the first output end of the second optical coupler is connected with the input end of the wavelength stabilizing component, and the output end of the wavelength stabilizing component is connected with the wavelength control end of the detection laser source;
The second output end of the second optical coupler is connected with the input end of the signal processing module.
In one implementation, the optical phase amplification and detection module further includes a third optical coupler and an optical phase control assembly for reducing environmental interference with phase detection;
The second output end of the second optical coupler is connected with the input end of the optical phase control assembly, the output end of the optical phase control assembly is connected with the first input end of the third optical coupler, the output end of the optical resonant cavity is connected with the second input end of the third optical coupler, and the output end of the third optical coupler is connected with the input end of the signal processing module.
In one implementation, the heating laser module includes a pump laser source and an optical power amplifier connected to an output of the pump laser source; the output end of the optical power amplifier is connected with the first port of the wavelength division multiplexer.
The beneficial effects are that: the invention generates heat by absorbing heating laser by the gas molecules to be detected, the generated heat can modulate the phase of the detection laser, the detection laser with modulated phase is emitted from the optical resonant cavity, and the phase of the modulated detection laser is analyzed to obtain the parameters of the gas to be detected. The specific process is as follows:
The invention firstly places a gas sample containing a gas to be detected in an optical resonant cavity, and then two laser beams are injected into the optical resonant cavity. The absolute value of the difference between the wavelength of the heating laser and the wavelength corresponding to the center of the absorption line of the gas molecules to be detected is smaller than a first set value, namely, the wavelength of the heating laser is close to the wavelength corresponding to the center of the absorption line of the gas molecules to be detected, so that the absorption of the gas to be detected to the heating laser is improved, the thermal effect of the gas to be detected is further improved, and finally, the accuracy of the acquired gas parameters to be detected is improved; the absolute value of the difference between the wavelength of the detection laser and the wavelength corresponding to the center of the absorption line of the gas molecule to be detected is larger than a second set value, namely, the wavelength of the detection laser is far away from the wavelength corresponding to the center of the absorption line of the gas molecule to be detected, so that the absorption of the gas to be detected to the detection laser is reduced, the detection transmission light and the detection reflection light formed by the detection light are obtained, and the sensitivity of the concentration detection of the gas to be detected is improved.
The optical resonant cavity is provided with a plurality of discontinuous resonant wavelengths, and the wavelength of the detection laser is far away from the wavelength corresponding to the center of the absorption line of the gas molecules to be detected, and is matched with one resonant wavelength of the plurality of resonant wavelengths of the optical resonant cavity so as to amplify the photo-thermal phase modulation received by the detection laser. Therefore, even if the volume fraction of the gas to be detected in the gas sample is very small, the concentration of the gas to be detected can be acquired very accurately because the invention can amplify the phase modulation to which the detection laser is subjected.
In summary, the invention can effectively amplify the photo-thermal phase modulation received by the tiny detection laser, and improve the signal-to-noise ratio of detection. The invention can realize the detection of trace gas parameters with the lower limit of volume fraction detection of the order of trillion, and has universality for the gas capable of absorbing visible light wave band, near infrared wave band and middle infrared wave band.
Drawings
FIG. 1 is a system block diagram of the present invention;
FIGS. 2A and 2B are block diagrams of optical resonators of the present invention;
FIG. 3 is a cross-sectional view of a micro-nano structured fiber according to the present invention;
FIGS. 4A and 4B are schematic diagrams showing the principle of amplifying the photo-thermal phase modulation of the detected reflected light and the detected transmitted light when the wavelength of the detected light is fixed at one resonance wavelength of the micro-nano structure fiber resonant cavity according to the present invention;
FIGS. 5A and 5B are reflection spectra of the inventive mirror;
Fig. 6 is a plot of the second harmonic signal of methane gas at a volume concentration of 1.8ppm (1 ppm = parts per million) measured by the method of the present invention.
The meaning of the reference symbols in the figures is as follows:
1. heating the laser module; 11. a pump laser light source; 12. a modulation component;
13. An optical power amplifier;
2. a detection laser module; 21. detecting a laser light source; 22. a wavelength stabilizing component;
3. An optical phase amplifying and detecting module; 31. an optical resonant cavity; 311. micro-nano structured optical fiber;
312. A first mirror; 313. a second mirror; 314. a first optical coupler; 32. a wavelength division multiplexer; 33. an optical phase control assembly; 34. a circulator; 35. a second optical coupler;
36. A third optical coupler;
4. a signal processing module; 41. a photodetector; 42. and a signal detection component.
Detailed Description
The technical scheme of the invention is clearly and completely described below with reference to the examples and the drawings. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
The high-sensitivity gas detection technology has wide application in the fields of environmental pollutant monitoring, respiratory gas disease diagnosis, atomic and molecular physics and the like. The gas detection method based on the absorption spectrum technology and the derivative technology has the characteristics of high gas parameter detection sensitivity, good gas type selectivity and the like. According to the beer-lambert law, when incident light of a specific wavelength passes through a gas to be detected, a gas molecule absorbs a part of optical power, so that the transmitted optical power is reduced. The parameters of the gas can be determined according to the intensity of the absorption of the optical power by the gas molecules and the characteristic absorption wavelength. Another technique is to detect gas parameters using a photothermal interferometer based on the photothermal effect caused by light absorption. When the gas molecules absorb the incident light power, heat is released, and the gas temperature is changed. The photothermal interferometer measures the amplitude of the photothermal phase modulation caused by the change of the gas temperature to characterize the gas parameter. Based on the hollow micro-nano structure optical fiber 311 resonant cavity, the detection laser wavelength is fixed at the position with the maximum transmission spectrum slope of the hollow micro-nano structure optical fiber 311 resonant cavity, and the sensitivity of the photo-thermal interference gas detection can be theoretically improved by improving the detection efficiency of the photo-thermal phase signal. However, at the position with the maximum transmission spectrum slope of the resonant cavity, the phase noise of the detection laser is converted into intensity noise, so that the overall noise level of the detection system is increased, the detection sensitivity of the system is improved to a limited extent, and the requirements of practical application cannot be met.
In order to solve the technical problems, the invention provides a gas detection method and a system based on optical phase amplification, which solve the problem of low sensitivity of detecting gas parameters. In specific implementation, the invention generates heat by absorbing the heating laser by the gas to be detected, the generated heat can modulate the phase of the detection laser, the detection laser after the phase modulation is emitted from the optical phase amplifying and detecting module 3, and the phase modulation amplitude of the detection laser after the phase modulation is analyzed to obtain the parameter of the gas to be detected. The optical resonant cavity 31 has a plurality of discontinuous resonant wavelengths, the wavelength of the detection laser of the invention is far away from the wavelength corresponding to the absorption line center of the gas molecules to be detected, and is matched with one of the resonant wavelengths of the optical resonant cavity 31 so as to amplify the photo-thermal phase modulation received by the detection laser, even if the volume fraction of the gas to be detected in the gas sample is very small, the invention can amplify the photo-thermal phase modulation received by the detection laser, improve the signal-to-noise ratio of detection and accurately acquire the concentration of the gas to be detected. The invention can realize the detection of trace gas parameters with the lower limit of volume fraction detection of the order of trillion, and has universality for the gas capable of absorbing visible light wave band, near infrared wave band and middle infrared wave band.
The embodiment provides a gas detection system based on optical phase, as shown in fig. 1, the gas detection system includes the following components:
The heating laser module 1 is used for outputting heating laser with the absolute value of the difference between the wavelength and the wavelength corresponding to the center of the absorption line of the gas molecules to be detected smaller than a first set value;
The detection laser module 2 is used for outputting detection laser with the absolute value of the difference between the wavelength and the wavelength corresponding to the center of the absorption line of the gas molecules to be detected being larger than a second set value;
The optical phase amplifying and detecting module 3 is used for containing a gas sample containing a gas to be detected, receiving heating laser and detecting laser, and amplifying and measuring the photo-thermal phase modulation received by the detecting laser;
the signal processing module 4 is used for demodulating the phase modulation amplitude of the detection emergent light measured by the optical phase amplification and detection module 3 to obtain the parameter of the gas to be detected. The following description will be given of each:
as shown in fig. 1, the heating laser module 1 comprises a modulation component 12, a pump laser light source 11 with an input end connected with an output end of the modulation component 12, and an optical power amplifier 13 with an input end connected with an output end of the pump laser light source 11.
As shown in fig. 1, the detection laser module 2 includes a detection laser light source 21, and a wavelength stabilizing member 22 whose output end is connected to a wavelength control end of the detection laser light source 21.
When the optical phase amplifying and detecting module 3 detects the photo-thermal phase modulation of the amplified detection laser using the optical fiber mach-zehnder interferometer, in order to ensure the maximum phase modulation detection efficiency, it is necessary to stabilize the phase direct current component difference Φ 0 of the detection reflected light and the detection transmitted light passing through the optical phase amplifying and detecting module 3 constituted by the micro-nano structured optical fiber 311 at a phase point of +90° or-90 °. In the present application, an optical phase control assembly 33 is employed. Specifically, the optical phase control module 33 performs low-pass filtering on the phase difference of the detection reflected light and the detection transmitted light to obtain a direct-current component Φ of the phase difference of the detection reflected light and the detection transmitted light, wherein the bandwidth of the low-pass filter is smaller than the heating laser modulation frequency. The optical phase control assembly 33 changes the direct current component of the optical phase difference of the probe transmitted light or the probe reflected light transmitted along the optical fiber by comparing the difference of Φ and Φ 0 and generating a corresponding feedback signal to change the phase of the probe light transmitted along the optical fiber after passing through the optical phase control assembly 33 such that Φ=Φ 0.
As shown in fig. 1, the optical phase amplifying and detecting module 3 includes a wavelength division multiplexer 32, a circulator 34, a second optical coupler 35, a third optical coupler 36, an optical resonator 31 for holding a gas sample containing a gas to be detected and receiving heating laser light and detecting laser light, and an optical phase control assembly 33 for reducing environmental interference of the optical phase amplifying and detecting module 3.
The output of the optical power amplifier 13 is connected as a laser output of the heating laser module 1 to a first port of the wavelength division multiplexer 32.
The laser output of the detection laser source 21 is connected to a first port of a circulator 34, and a second port of the circulator 34 is connected to a second port of the wavelength division multiplexer 32.
The third port of the circulator 34 is connected to an input of a second optical coupler 35, a first output of the second optical coupler 35 is connected to an input of a wavelength stabilizing member 22, and an output of the wavelength stabilizing member 22 is connected to a wavelength control end of the detection laser light source 21.
The second output of the second optical coupler 35 is connected to the input of the optical phase control assembly 33, the output of the optical phase control assembly 33 is connected to the first input of the third optical coupler 36, and the output of the optical resonant cavity 31 is connected to the second input of the third optical coupler 36.
As shown in fig. 2A, the optical resonator 31 includes a first mirror 312, a micro-nano structured fiber 311, and a second mirror 313, which are disposed in order along the optical path direction.
Alternatively, as shown in fig. 2B, the optical resonator 31 includes a micro-nano structured optical fiber 311 and a first optical coupler 314, and the micro-nano structured optical fiber 311 and the first optical coupler 314 form a closed optical path structure.
The micro-nano structured fiber 311 of the present embodiment has low transmission loss at the wavelength of the detection light and the wavelength of the pumping laser; the micro-nano structure optical fiber 311 can be any one of solid micro-nano structure optical fiber 311 based on interaction of light evanescent field and gas, such as D-type optical fiber, solid photonic crystal optical fiber, micro-nano optical fiber, suspended core optical fiber and the like; or hollow micro-nano structure fiber 311 based on interaction of light guide mode and gas, such as any one of hollow capillary, hollow photonic crystal band gap fiber, antiresonant hollow fiber, kagome fiber, etc.
In this embodiment, the micro-nano structured optical fiber 311 is an anti-resonant hollow-core optical fiber as shown in fig. 3, and fig. 3 is a schematic cross-sectional view of a typical micro-nano structured hollow-core optical fiber based on the anti-resonant principle provided in this embodiment. The antiresonant hollow fiber is made of silicon dioxide and is uniformly and circumferentially arranged by a plurality of capillaries. The anti-resonant hollow-core fiber of FIG. 3 has a capillary diameter of tens of microns and a wall thickness of about several hundred nanometers; the core of an antiresonant hollow-core fiber has an inscribed circle diameter of several tens micrometers. The antiresonant hollow-core optical fiber has a very wide low-loss spectral transmission window. An antiresonant optical fiber with a suitable transmission window can be selected according to the intrinsic absorption wavelength that the actual detection gas has. The antiresonant hollow-core fiber used in this embodiment has a low loss transmission window for laser light having a wavelength of 1500nm to 1700 nm.
The optical resonator 31 of the present embodiment is a resonator formed by the micro-nano structure optical fiber 311, and may be a linear resonator formed by the micro-nano structure optical fiber 311 and a mirror, or a ring resonator formed by the micro-nano structure optical fiber 311 and an optical coupler. Fig. 2A and 2B are schematic structural views of an optical resonator 31 according to the present embodiment.
The mirror has a high reflectivity at the probe wavelength, as shown in FIG. 5A the mirror has a low reflectivity at the pump wavelength; as shown in fig. 5B, the mirror has high reflectivity at both the probe wavelength and the pump wavelength.
In this embodiment, a linear resonant cavity composed of a mirror and a micro-nano structured fiber 311 as shown in fig. 2A is selected. As shown in fig. 5A, the mirror has a high reflectivity at the detection light wavelength (λ Detection of =1550 nm) and a low reflectivity at the pump light wavelength (λ pump with a pump body =1653 nm).
As shown in fig. 1, the signal processing module 4 includes a photodetector 41 having an input connected to the output of the third optical coupler 36, and a signal detecting element 42 having an input connected to the output of the photodetector 41.
The working process of the gas detection system of this embodiment is described by using methane gas as the gas to be detected:
The pump laser output by the heating laser module 1 enters the optical resonant cavity 31 through the wavelength division multiplexer 32, the optical resonant cavity 31 contains a gas sample of gas methane to be detected, the methane absorbs the pump laser to generate heat, the detection laser output by the detection laser light source 21 also enters the optical resonant cavity 31, the gas sample after the detection laser is heated carries out photo-thermal phase modulation and reflects out of the optical resonant cavity 31, at the moment, the methane concentration of the reflected detection laser is not measured, the reflected detection laser is input into the wavelength stabilizing component 22 through the circulator 34 and the second optical coupler 35, the wavelength stabilizing component 22 judges whether the reflected detection laser is matched with one selected resonant wavelength of the optical resonant cavity 31, and the absolute value of the difference between the detection light wavelength and the wavelength corresponding to the absorption line center of the gas molecule to be detected is larger than a second set value. If the detection laser is not matched with the optical resonant cavity 31, the wavelength of the detection laser output by the detection laser source 21 is adjusted through the wavelength stabilizing component 22 until the detection laser wavelength is matched with the selected resonant wavelength of the optical resonant cavity 31, the absolute value of the difference between the detection wavelength and the wavelength corresponding to the center of the absorption line of the gas molecule to be detected is larger than a second set value, and then the methane concentration is measured according to the phases of the reflected detection light and the transmitted detection light.
The second optical coupler 35 splits the detection reflected light reflected by the optical resonant cavity 31 into two paths, and one path of detection reflected light enters the wavelength stabilizing component 22 to adjust the wavelength of the detection laser output by the detection laser light source 21 so as to align with the selected resonant wavelength of the optical resonant cavity; the other path of the detection reflected light enters the signal processing module 4 through the optical phase control assembly 33 and the third optical coupler 36.
Part of the detection laser light output by the detection laser light source 21 also passes through the gas sample containing methane to be detected in the optical resonant cavity 31 to form detection transmission light, and the detection transmission light also enters the signal processing module 4. The signal processing module 4 obtains the concentration of methane gas from a database of detected transmitted light and detected reflected light and a database containing the phase difference modulation amplitude at which the gas concentration matches the gas concentration.
The embodiment also provides a gas detection method based on optical phase amplification, which comprises the following steps:
s100, heating laser and detection laser are emitted to an optical resonant cavity 31 for containing a gas sample containing a gas to be detected;
The absolute value of the difference between the wavelength of the heating laser and the wavelength corresponding to the center of the absorption line of the gas molecule to be detected is smaller than a first set value, namely the wavelength of the heating laser is equal to or close to the wavelength corresponding to the center of the absorption line of the gas molecule to be detected; the absolute value of the difference between the wavelength of the detection laser and the wavelength corresponding to the center of the absorption line of the gas molecule to be detected is larger than a second set value, namely the wavelength of the detection laser is far away from the wavelength corresponding to the center of the absorption line of the gas molecule to be detected; and the wavelength of the detection laser is matched to one of the resonant wavelengths of the optical resonant cavity 31.
In this embodiment, the wavelength of the detection laser is equal to one resonant wavelength of the optical resonant cavity 31, and the absolute value of the difference between the resonant wavelength and the wavelength corresponding to the center of the absorption line of the gas molecule to be detected is greater than the second set value. The optical resonant cavity 31 has a plurality of resonant wavelengths, and some resonant wavelengths are near to the wavelength corresponding to the center of the absorption line of the gas molecule to be detected and some resonant wavelengths are far from the wavelength corresponding to the center of the absorption line of the gas molecule to be detected. The wavelength of the detection laser light of the present embodiment is equal to one resonance wavelength of the optical resonator 31 and is far from the center of the absorption line of the gas molecule to be detected.
For example, the wavelength corresponding to the center of a certain gas molecule absorption line is 1650nm, and the optical resonant cavity 31 has a plurality of resonant wavelengths, and five resonant wavelengths are assumed to be: the wavelength of the detection laser selected by the embodiment is 1550nm, which is 1500nm, 1550nm, 1600nm, 1650nm and 1700nm, so that the wavelength of the detection laser is far away from the wavelength corresponding to the center of the absorption line of the gas molecule to be detected, and is equal to one of the resonant wavelengths, and the detection sensitivity can be improved to the greatest extent.
In this embodiment, the heating laser is a periodic pump laser, and if the pump laser is a continuous laser, the pump laser is periodically modulated before being input into the micro-nano structure optical fiber 311; wherein the periodic modulation is an intensity modulation, or a wavelength modulation.
Taking methane gas as an example, the pumping wavelength is chosen to be λ pump with a pump body =1653 nm, corresponding to the center of one methane gas absorption line. The pump laser wavelength is wavelength modulated at 13 kHz. The pump laser after periodic modulation is coupled into the micro-nano structure optical fiber 311, and then interacts with the gas molecules to be detected to generate photo-thermal phase modulation. The detection light is emitted by another detection laser, and the wavelength of the light wave can be selected to be around 1550nm, so as to detect the photo-thermal phase modulation generated by the interaction of the pump light and the gas molecules. The wavelength of the detection light is fixed on the resonance wavelength of a resonant cavity formed by the micro-nano structure optical fiber 311, the reflected detection light and the transmitted detection light passing through the resonant cavity are respectively subjected to destructive interference and constructive interference at the resonance wavelength, and the photo-thermal phase modulation of the reflected detection light and the transmitted detection light is amplified. Further, the phase difference of the amplified reflected probe light and the transmitted probe light can be measured by using an optical fiber Mach-Zehnder interferometer
For example, the pump laser (heating laser) wavelength is tuned near the center of the absorption line of the gas molecule to be detected. The pump laser modulated periodically is absorbed by the gas molecules to be detected, and a part of light energy is converted into heat energy, so that the temperature of the gas sample containing the detection gas is changed, the refractive index of the gas sample is changed, and the phase of the detection laser is modulated. The concentration of the gas to be detected can thus be obtained by detecting the phase modulation of the detection laser light passing through the gas sample containing the gas to be detected.
S200, measuring detection emergent light emitted by the detection laser by the cavity of the optical resonant cavity 31 for containing the gas sample containing the gas to be detected;
In this embodiment, the detection emergent light emitted from the optical resonator 31 includes detection transmitted light and detection reflected light, and the concentration of the gas to be detected can be obtained by measuring the detection transmitted light and the detection reflected light, and the principle is as follows:
when the micro-nano structure optical fiber 311 is transmitted in the fundamental mode, the pumping laser intensity is approximately Gaussian distributed along the cross section direction of the optical fiber. The heat distribution generated by absorption of the pump laser by the gas molecules to be detected can be expressed as:
Wherein, P pump is the total power of pump laser, w pump is the field radius of the pump laser in the micro-nano structure optical fiber 311, alpha is the peak absorption coefficient corresponding to the gas concentration to be detected of 100%, and C is the gas concentration to be detected. Correspondingly, the temperature field change T in the micro-nano structured fiber 311 due to the photo-thermal effect can be obtained by the heat conduction equation:
where ρ is the density of the gas sample containing the gas to be detected, C P is the specific heat of the gas sample containing the gas to be detected, and k is the thermal conductivity of the gas sample containing the gas to be detected. Since Q is proportional to C, the T thus induced is also proportional to C.
Then, the refractive index change deltan of the gas to be detected due to the gas temperature change can be obtained by the clausius-moxidect equation
Wherein n 0 is the refractive index of the gas under normal temperature and pressure, and T abs is the ambient temperature, i.e. the initial temperature of the gas sample containing the gas to be detected. Since T is proportional to C, Δn is also proportional to C.
The one-round phase change of the probe laser light in the resonant cavity constituted by the micro-nano structured fiber 311 caused by the change of the gas refractive index is ΔΦ 0:
Δφ0＝4πΔnL/λ Detection of
Where lambda Detection of is the detection laser wavelength and L is the length of micro-nano structured fiber 311.
The detected reflected light and the detected transmitted light of the optical resonant cavity 31 formed by the micro-nano structure optical fiber 311 respectively generate destructive interference and constructive interference at the resonance wavelength of the optical resonant cavity 31, the detected transmitted light phase delta phi t and the detected reflected light phase delta phi r generate severe changes along with the changes of delta phi 0, the phase amplification function is achieved, and the amplification factor is as follows:
kt＝2F/π
Where F is the finesse of the optical cavity 31, t 1 is the transmission coefficient of the first mirror 312, and r 1 and r 2 are the reflection coefficients of the first mirror 312 and the second mirror 313, respectively. The amplified phase modulation amplitudes of the detection reflected light and the detection projected light are respectively as follows:
Δφr-krΔφ0
Δφt＝ktΔφ0
Fig. 4A and 4B are schematic diagrams of the principle of phase modulation amplification of the detection reflected light and the detection transmitted light when the wavelength of the detection light is fixed at one resonance wavelength of the micro-nano structure fiber resonant cavity 31 according to the present invention.
S300, obtaining parameters of the gas to be detected according to the phase information of the detection emergent light.
In this embodiment, the parameter of the gas to be detected is the volume fraction of the gas to be detected in the gas sample containing the gas to be detected. The method comprises the following steps:
S301, acquiring a database, wherein the database comprises parameters of calibration gas and phase modulation amplitude of calibration laser matched with the parameters of the calibration gas; the calibration laser corresponds to the detection emergent light, the phase modulation amplitude of the calibration laser corresponds to the phase modulation amplitude of the detection emergent light, and the calibration gas corresponds to a gas sample containing the gas to be detected;
s302, obtaining parameters of the gas to be detected according to the phase modulation amplitude of the detection emergent light, the phase modulation amplitude of the calibration laser corresponding to the phase modulation amplitude of the detection emergent light and the parameters of the calibration gas matched with the phase modulation amplitude of the calibration laser.
In this embodiment, the detecting the phase modulation amplitude of the outgoing light includes detecting the phase modulation amplitude of the transmitted light and detecting the phase modulation amplitude of the reflected light; the phase modulation amplitude of the calibration laser comprises a calibration phase difference modulation amplitude corresponding to a detection light phase difference modulation amplitude formed by the phase modulation of the detection transmission light and the phase modulation of the detection reflection light; the parameters of the calibration gas include the concentration of the calibration gas matched with the modulation amplitude of the calibration phase difference, and the specific steps of step S302 are as follows:
step S3021, obtaining a detection light phase difference modulation amplitude formed by the phase modulation of the detection transmission light and the phase modulation of the detection reflection light according to the phase modulation of the detection transmission light and the phase modulation of the detection reflection light.
In this embodiment, the phase modulation ΔΦ r of the detected reflected light and the phase modulation ΔΦ t of the detected transmitted light amplified by the photo-thermal phase modulation are measured by using the interference of the detected light and the detected reflected light or the detected transmitted light, respectively. Or, the phase difference delta phi r-Δφt of the detection reflected light and the detection transmitted light amplified by photo-thermal phase modulation is measured by utilizing the interference of the detection reflected light and the detection transmitted light.
Step S3022, obtaining the calibration phase difference modulation amplitude corresponding to the phase difference modulation amplitude in the database according to the detected optical phase difference modulation amplitude.
Step S3023, obtaining a calibration gas concentration matched with the calibration phase difference modulation amplitude according to the calibration phase difference modulation amplitude;
step S3024, obtaining the concentration of the gas to be detected according to the concentration of the calibration gas.
The advantageous effects of the present invention are illustrated by the following examples:
for measurement of methane gas, methane was first mixed in pure nitrogen to obtain a methane gas sample having a volume fraction of 1.8 ppm; the gas sample is filled into the micro-nano structure optical fiber 311 in a natural diffusion mode; the pump laser wavelength was chosen to be 1653nm. The pump laser adopts a wavelength modulation mode, and the modulation frequency is 13kHz. The signal processing module 4 detects a second harmonic signal of the detected light phase modulation caused by the photo-heat of the methane gas. The time constant of the signal processing module 4 was set to 1s, the filter slope of the signal processing module 4 was set to 18dB/0ct, and the second harmonic signal obtained by periodically scanning the wavelength of the pumping light around the methane absorption peak was as shown in fig. 6. And filling pure nitrogen into the micro-nano structure optical fiber 311 air chamber, fixing the wavelength of the pumping light at the center of a methane absorption line, and measuring the noise value obtained by the change of a second harmonic signal along with time. When the power of the pumping laser entering the anti-resonance hollow micro-nano structure optical fiber 311 with the length of 10cm is 100mW, the signal to noise ratio obtained by calculating the peak value and noise of the second harmonic signal is 14912, which corresponds to the methane detection sensitivity with the volume fraction of 0.12 ppb. Using longer time constants for the averaging process, the detection sensitivity of the detection system for methane can be on the order of 1ppt (1 ppt = parts per trillion). The measuring device effectively solves the problems of long response time and insufficient sensitivity in the existing optical fiber gas sensing system.
Finally, it should be noted that: the above embodiments are only for illustrating the technical solution of the present invention, and are not limiting; although the invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical scheme described in the foregoing embodiments can be modified or some technical features thereof can be replaced by equivalents; such modifications and substitutions do not depart from the spirit and scope of the technical solutions of the embodiments of the present invention.
Claims (7)
1. A gas detection method based on optical phase amplification, the gas detection method comprising:
Injecting heating laser and detection laser into an optical resonant cavity, wherein the optical resonant cavity contains a gas sample containing gas to be detected, the absolute value of the difference between the wavelength of the heating laser and the wavelength corresponding to the center of an absorption line of a gas molecule to be detected is smaller than a first set value, the wavelength of the detection laser is matched with one resonant wavelength of the optical resonant cavity, the absolute value of the difference between the wavelength of the detection laser and the wavelength corresponding to the center of the absorption line of the gas molecule to be detected is larger than a second set value, and the second set value is larger than the first set value;
Acquiring detection emergent light information emitted by the detection light after amplified photo-thermal phase modulation of the optical resonant cavity for containing the gas sample containing the gas to be detected;
obtaining parameters of the gas to be detected according to the phase modulation amplitude corresponding to the detection emergent light information;
Obtaining parameters of the gas to be detected according to the phase modulation amplitude corresponding to the detection emergent light information, wherein the parameters comprise:
Acquiring a database, wherein the database comprises parameters of calibration gas and phase modulation amplitude of calibration laser matched with the parameters of the calibration gas; the calibration laser corresponds to the detection emergent light, the phase modulation amplitude of the calibration laser corresponds to the phase modulation amplitude of the detection emergent light, and the calibration gas corresponds to the gas sample; the calibration gas refers to a uniform and stable gas sample formed by mixing the gas to be measured into another gas of different types according to the determined volume fraction under the condition that the physical parameters of the gas sample are known;
Obtaining parameters of the gas to be detected according to the phase modulation amplitude of the detection emergent light, the phase modulation amplitude of the calibration laser corresponding to the phase modulation amplitude of the detection emergent light and the parameters of the calibration gas matched with the phase modulation amplitude of the calibration laser;
the detection emergent light comprises detection reflected light of the detection light reflected by the optical resonant cavity and detection transmitted light of the detection light transmitted by the optical resonant cavity; the detection of the phase modulation amplitude of the emergent light comprises detection of the phase modulation amplitude of the transmitted light and detection of the phase modulation amplitude of the reflected light;
The phase modulation amplitude of the calibration laser comprises a calibration phase modulation amplitude corresponding to the phase modulation amplitude of the detection transmission light, or the phase modulation amplitude of the calibration laser comprises a calibration phase modulation amplitude corresponding to the phase modulation amplitude of the detection reflection light, or the phase modulation amplitude of the calibration laser comprises a calibration phase difference modulation amplitude corresponding to the detection light phase difference modulation amplitude formed by the phase modulation of the detection transmission light and the phase modulation of the detection reflection light;
The parameters of the calibration gas comprise the concentration of the calibration gas matched with the calibration phase modulation amplitude and the concentration of the calibration gas matched with the calibration phase difference modulation amplitude;
the parameter of the gas to be detected comprises the concentration of the gas to be detected in the gas sample;
the obtaining the parameter of the gas to be detected according to the phase modulation amplitude of the detection emergent light, the phase modulation amplitude of the calibration laser corresponding to the phase modulation amplitude of the detection emergent light, and the parameter of the calibration gas matched with the phase modulation amplitude of the calibration laser, includes:
Acquiring a calibration phase modulation amplitude corresponding to the phase modulation amplitude of the detection transmission light according to the phase modulation amplitude of the detection transmission light;
obtaining a calibration gas concentration matched with the calibration phase modulation amplitude according to the calibration phase modulation amplitude corresponding to the phase modulation amplitude of the detected transmitted light;
Obtaining the concentration of the gas to be detected in a gas sample according to the concentration of the calibration gas;
Or obtaining a calibration phase modulation amplitude corresponding to the phase modulation amplitude of the detection reflected light according to the phase modulation amplitude of the detection reflected light;
Obtaining a calibration gas concentration matched with the calibration phase modulation amplitude according to the calibration phase modulation amplitude corresponding to the phase modulation amplitude of the detection reflected light;
Obtaining the concentration of the gas to be detected in a gas sample according to the concentration of the calibration gas;
or according to the phase modulation amplitude of the detection transmission light and the phase modulation amplitude of the detection reflection light, obtaining a calibration phase difference modulation amplitude corresponding to the detection light phase difference modulation amplitude formed by the phase modulation of the detection transmission light and the phase modulation of the detection reflection light;
obtaining a calibration gas concentration matched with the calibration phase difference modulation amplitude according to the calibration phase difference modulation amplitude;
and obtaining the concentration of the gas to be detected in the gas sample according to the concentration of the calibration gas.
2. The optical phase amplification-based gas detection method according to claim 1, further comprising:
and controlling the wavelength of the detection laser in real time so that the wavelength of the detection laser is equal to a selected resonant wavelength of the optical resonant cavity, wherein the selected resonant wavelength is one resonant wavelength of which the absolute value of the difference between the resonant wavelengths of the optical resonant cavity and the wavelength corresponding to the center of the absorption line of the gas molecule to be detected is larger than a second set value.
3. The optical phase amplification based gas detection method according to any one of claims 1-2, wherein said heating laser is a periodic laser signal.
4. A gas detection system based on optical phase amplification, the gas detection system comprising:
the heating laser module is used for outputting heating laser with the absolute value of the difference between the wavelength and the wavelength corresponding to the center of the absorption line of the gas molecules to be detected smaller than a first set value;
The detection laser module is used for outputting detection laser with the absolute value of the difference between the wavelength and the wavelength corresponding to the center of the absorption line of the gas molecules to be detected being larger than a second set value, and comprises a detection laser light source and a wavelength stabilizing component;
the optical phase amplifying and detecting module is used for containing a gas sample containing a gas to be detected, receiving heating laser and detecting laser, and amplifying and measuring the photo-thermal phase modulation received by the detecting laser;
The optical phase amplifying and detecting module comprises a wavelength division multiplexer, a circulator, a second optical coupler and an optical resonant cavity, wherein the optical resonant cavity is used for containing gas to be detected and receiving heating laser and detecting laser and comprises micro-nano structure optical fibers and an air chamber containing the micro-nano structure optical fibers;
The laser output end of the heating laser module is connected with the first port of the wavelength division multiplexer;
The laser output end of the detection laser source is connected with the first port of the circulator, and the second port of the circulator is connected with the second port of the wavelength division multiplexer;
The third port of the circulator is connected with the input end of the second optical coupler, and the first output end of the second optical coupler is connected with the input end of the wavelength stabilizing component;
the second output end of the second optical coupler is connected with the input end of the signal processing module;
The signal processing module is used for demodulating the phase modulation amplitude of the detection emergent light measured by the optical phase amplification and detection module to obtain parameters of the gas to be detected, and the detection emergent light comprises detection reflected light of the detection light reflected by the optical resonant cavity and detection transmitted light of the detection light transmitted by the optical resonant cavity; the detection of the phase modulation amplitude of the emergent light comprises detection of the phase modulation amplitude of the transmitted light and detection of the phase modulation amplitude of the reflected light;
The heating laser module and the detecting laser module are connected with the optical phase amplifying and detecting module, and the optical phase amplifying and detecting module is connected with the signal processing module.
5. The optical phase amplification based gas detection system according to claim 4, wherein:
The detection laser light source is used for outputting the detection laser;
The wavelength stabilizing component is used for enabling the wavelength of the detection laser output by the detection laser source to be matched with the resonance wavelength of the optical resonant cavity, and enabling the absolute value of the difference between the wavelength of the detection laser output by the detection laser source and the wavelength corresponding to the absorption line center of the gas molecules to be detected to be larger than a second set value;
The output end of the detection laser light source is connected with the optical phase amplifying and detecting module, the input end of the wavelength stabilizing component is connected with the optical phase amplifying and detecting module, and the output end of the wavelength stabilizing component is connected with the wavelength control end of the detection laser light source.
6. The optical phase amplification based gas detection system according to claim 4, wherein the optical phase amplification and detection module further comprises a third optical coupler and an optical phase control assembly for reducing environmental interference with phase detection;
The second output end of the second optical coupler is connected with the input end of the optical phase control assembly, the output end of the optical phase control assembly is connected with the first input end of the third optical coupler, the output end of the optical resonant cavity is connected with the second input end of the third optical coupler, and the output end of the third optical coupler is connected with the input end of the signal processing module.
7. The optical phase amplification based gas detection system according to any one of claims 4-6, wherein said heating laser module comprises a pump laser source and an optical power amplifier connected to the output of the pump laser source; the output end of the optical power amplifier is connected with the first port of the wavelength division multiplexer of the optical phase amplifying and detecting module.
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| CN113777073A (en) | 2021-12-10 |
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