CN115524302A - Gas detection method based on single-cavity double-comb light source - Google Patents
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Abstract
The invention relates to the technical field of gas detection, and discloses a gas detection method based on a single-cavity double-comb light source, which comprises the following steps: s1: introducing gas to be detected into a gas pool; s2: a first optical comb and a second optical comb which have a repetition frequency difference and are emitted by a single-cavity double-comb light source, wherein the first optical comb and the second optical comb respectively pass through a gas pool in an opposite direction and repeat for a plurality of times; s3: the first optical comb and the second optical comb are split into a first split beam and a second split beam after beam combination and beat frequency of a beam combiner; s4: receiving the first split beam of light and the second split beam of light by a first photodetector and a second photodetector, respectively; s5: and analyzing the electric signals of the first photoelectric detector and the second photoelectric detector to obtain a gas detection result. The gas is detected by the single-cavity double-light comb light source, the problem that the cavity needs to be locked by the independent double-light comb in the traditional double-light comb spectrum technology is solved, the system complexity is reduced, the cost is reduced, and the high practicability is achieved.
Description
Technical Field
The invention relates to the technical field of gas detection, in particular to a gas detection method based on a single-cavity double-comb light source.
Background
The gas detection technology is widely applied to various production scenes at present, and can help maintenance managers to monitor and detect the running condition of equipment. In practical applications such as monitoring the operation of electrical equipment and detecting toxic gases in the environment during chemical production, most of the detected gases have low concentration, so that a high-sensitivity gas monitoring means is required. Because gas molecules have strong absorption in an infrared band and the absorption peak has strong characteristics, the types and the concentrations of the gas molecules can be determined by measuring the absorption spectral lines of the gas, so the method has important application value in the field of gas detection. At present, gas spectrum detection methods include a non-dispersive infrared method, a Fourier transform infrared spectroscopy, a differential optical absorption spectroscopy, a tunable diode laser absorption spectroscopy and the like.
The non-dispersive infrared method utilizes a broad-spectrum light source to detect gas to be detected, filters out a specific infrared band through a narrow-band filter, measures the intensity of infrared light entering an infrared sensor, and selects absorption characteristics to analyze the molecular components of the gas based on the gas. The method has the advantages that the specific gas molecule analysis is rapid and accurate; but the measurement band is narrow, the precision is low, and the complex multi-component gas is difficult to measure.
The fourier transform infrared spectroscopy converts an interference pattern of signal light and intrinsic light into an infrared spectrogram through fourier transform, can perform qualitative and quantitative analysis on a measurement gas, has high measurement accuracy and low noise, is commonly used for monitoring atmospheric pollutants, but has low sensitivity, is difficult to detect gas concentrations of ppm (parts per million) and below, is limited by mechanical movement speed, and consumes long time for measurement.
The differential optical absorption spectrum is obtained by detecting the narrow-band absorption characteristics of gas and deducing the species and concentration information of trace gas according to the Lambert beer law. The technical equipment is cheap and simple, and the precision is high; however, the method is mainly used for a non-scattering system with uniformly distributed optical media, so that the application range of the method is greatly limited.
The tunable diode laser absorption spectrum technology utilizes a tunable semiconductor laser light source to obtain the absorption spectrum of gas molecules through frequency scanning, and then utilizes the wavelength and height of an absorption peak to obtain information such as the type and concentration of gas. The technology has high sensitivity and high resolution, but has slow response speed, is limited by the tuning range of the light source, and has limited application range.
Therefore, the conventional gas spectrum detection technology generally has the problems of time consumption, low precision, limited detection sensitivity, difficulty in quantitative analysis of multi-component gas and the like.
The femtosecond optical frequency comb has the characteristics of wide frequency spectrum, narrow pulse width, stable repetition frequency and the like, and provides a better choice for high-resolution broadband infrared spectrum measurement. The double-optical comb spectrum gas detection technology is a novel spectrum detection technology developed on the basis of the optical comb technology. The principle is similar to the Fourier transform spectrum technology, and the difference is that two optical frequency combs with a tiny repetition frequency difference are used as a fully static interference light source, and a mechanical movable arm of an interferometer in the Fourier transform spectrum technology is not needed any more, so that the stability, the resolution, the precision and the measurement speed of a measurement system are greatly improved. However, the optical comb has a wide spectrum and low energy of single comb teeth, so that the detection sensitivity of the method is low, and the sensitivity is generally required to be further improved by combining with a cavity enhancement technology. The cavity enhanced infrared optical comb spectrum is characterized in that an optical resonant cavity is used as a gas pool, and light oscillates back and forth in the cavity, so that the effective optical path of gas absorption is increased, and the sensitivity of spectral measurement is improved. The existing double-optical comb spectrum gas detection technology uses two independent optical combs, and noise suppression processing is needed to ensure the coherence of the double optical combs. The technology needs elaborate cavity locking equipment and signal enhancement equipment to carry out cavity locking and signal enhancement processing on the double-optical comb, and the system cost is high. And the double-light comb is sensitive to the external environment, is not beneficial to outdoor use and has the problem of poor practicability.
Disclosure of Invention
The invention aims to provide a gas detection method based on a single-cavity double-light comb light source, which is used for detecting gas through the single-cavity double-light comb light source, solves the problem that the traditional double-light comb spectrum technology needs to lock an independent double-light comb, reduces the system complexity and the cost, and has high practicability.
The technical scheme provided by the invention is as follows: a gas detection method based on a single-cavity double-comb light source comprises the following steps:
s1: introducing gas to be detected into a gas pool;
s2: a single-cavity double-comb light source emits a first optical comb and a second optical comb which have a repetition frequency difference, wherein the first optical comb and the second optical comb respectively pass through a gas pool in an opposite direction and repeat for a plurality of times;
s3: the first optical comb and the second optical comb are split into a first split beam and a second split beam after beam combination and beat frequency of a beam combiner;
s4: receiving the first split beam of light and the second split beam of light by a first photodetector and a second photodetector, respectively;
s5: and analyzing the electric signals of the first photoelectric detector and the second photoelectric detector to obtain a gas detection result.
The working principle and the advantages of the invention are as follows: the method aims at the problems of the existing gas spectrum detection technology in the aspects of measurement resolution, sensitivity, precision, cost and the like. The invention sends out a first optical comb and a second optical comb with a repeated frequency difference to respectively pass through the gas pool in an opposite direction and repeat for a plurality of times, then the first optical comb and the second optical comb are combined by the beam combiner to carry out beam splitting after beat frequency, and the first photoelectric detector and the second photoelectric detector respectively receive and convert optical signals into electric signals, and the electric signals are analyzed to obtain a gas detection result. The gas detection method based on the single-cavity double-comb light source does not need mechanical or spectral scanning, and the second-level measurement speed of the traditional spectrum method is improved to the millisecond level, so that the quick response and the spectrum formation are realized. The single-cavity double-comb mode does not need a second optical comb light source, has high coherence, does not need to inhibit noise processing, and solves the coherence problem of the traditional double-spectrum technology. Therefore, the requirements on cavity locking equipment and technology are reduced, extra cavity locking and frequency control are not needed, the complexity of the system is reduced, the cost is reduced, and the practicability is high.
Further, after the S4 receiving the first split beam light and the second split beam light by the first photodetector and the second photodetector respectively, the method further includes: and collecting a time domain interference signal and converting the time domain interference signal into an electric signal.
The fixed detector is used for collecting time domain interference signals and converting the time domain interference signals into electric signals so as to realize the conversion of optical signals.
Further, the first photoelectric detector and the second photoelectric detector are respectively electrically connected with a differential amplifier, and the differential amplifier is electrically connected with a frequency spectrograph;
and S5 is as follows:
s5-1: common-mode noise of the electric signals of the first photoelectric detector and the second photoelectric detector is suppressed and amplified through a differential amplifier;
s5-2: analyzing the amplified electric signal by a frequency spectrograph to obtain an absorption spectrum of the gas molecules;
s5-3: the type and concentration of the gas are identified from the absorption spectrum.
The double-optical comb passes through the photoelectric detector, optical signals are converted into electric signals, the electric signals of the first photoelectric detector and the second photoelectric detector are subjected to common-mode noise suppression and amplification through the differential amplifier, and the amplified electric signal data are recorded and analyzed by the frequency spectrograph, so that a hyperfine spectrogram of gas molecules, namely an absorption spectrum of the double-optical comb, is obtained. Compared with an oscilloscope, the frequency spectrograph can directly convert the electric signals into the intensity signals of a frequency domain without Fourier transform.
Further, the single-cavity double-comb light source comprises a pumping light source and a resonant cavity, wherein the resonant cavity comprises a high-reflection mirror, an output mirror, a first reflecting mirror, a second reflecting mirror and a gain medium; the high-reflection mirror, the gain medium and the output mirror are sequentially arranged on the same straight line, the first reflecting mirror, the gas pool and the second reflecting mirror are sequentially arranged on the same straight line, and the pumping light source faces the high-reflection mirror.
The S2 comprises the following steps:
s2-1: turning on a pumping light source, and enabling the light source to be incident to the gain medium through a high-reflection mirror to form a first optical comb and a second optical comb which are opposite in direction; the first optical comb is reflected by the output mirror and the first reflector, passes through the gas cell, and then is reflected by the second reflector and the high-reflection mirror to enter the gain medium; the second optical comb passes through the gas cell in an opposite direction of the first optical comb through the reflection of the high-reflection mirror and the second reflection mirror, and then is incident to the gain medium through the reflection of the first reflection mirror and the output mirror; the first optical comb and the second optical comb are respectively incident to the gain medium, then are reflected again, pass through the gas cell and repeat for a plurality of times;
s2-2: until the optical comb completes oscillation in the cavity, the first optical comb passes through the gas pool, then is reflected by the second reflector and the high-reflection mirror, and is emitted out through the output mirror; meanwhile, the second optical comb passes through the gas cell, is reflected by the first reflector and is emitted through the output mirror.
The pumping light source forms a first optical comb and a second optical comb which have repeated frequency difference in the resonant cavity, and then the first optical comb and the second optical comb directly face each other in the resonant cavity to pass through the gas pool and repeat for a plurality of times. The mode of intracavity reinforcing has been realized through above-mentioned structure, has removed traditional technical means from and has need carry out the step and the corresponding equipment of signal reinforcing behind the lock chamber to independent two optical combs, has optimized the structure. Through the resonant cavity structure, the pumping light source generates bidirectional light, the bidirectional light vibrates for many times, the double optical combs are generated after mode locking, the first optical comb and the second optical comb repeatedly pass through the gas pool after being reflected for many times in the cavity and are partially absorbed by gas to be detected, and therefore the optical combs can carry fingerprint information of a gas absorption spectrum. In addition, the invention combines the characteristic of high coherence of a single-cavity double-comb mode and the structure enhanced in the cavity, so that the absorption of gas molecules on the double-optical comb with corresponding wave bands can be further enhanced, the double-optical comb can fully carry the absorption information of a multi-component gas sample, and accordingly, weaker absorption spectral lines can be detected, the detection sensitivity is improved, and the detection capability of the complex multi-component gas in the power equipment fault gas can be improved.
Further, an extra-cavity optical path of the resonant cavity comprises a third reflector, and the beam combiner is a beam combining sheet;
the S3 comprises the following steps:
s3-1: arranging a beam combining sheet on a straight line along the output mirror, and enabling the first optical comb to shoot to the beam combining sheet after passing through the output mirror; a third reflector is arranged on a light path emitted by the second optical comb through the output mirror, so that the second optical comb is reflected to the beam combining sheet through the third reflector after passing through the output mirror; the beam combining sheet and the two beams of incident light are arranged at an angle of 45 degrees;
s3-2: the first optical comb and the second optical comb are split into first split light and second split light after beam combination and beat frequency of the beam combination pieces.
The beam combining piece and the two beams of incident light are placed at an angle of 45 degrees, the first optical comb and the second optical comb can be effectively combined into one beam of light through the beam combining piece, the double combs are used for spatially combining the light beams, and the first split beam of light and the second split beam of light after splitting are respectively incident on the two photoelectric detectors, so that the light signals after combining are fully received.
Further, the first split beam of light includes transmitted light of the first optical comb and reflected light of the second optical comb, and the second split beam of light includes reflected light of the first optical comb and transmitted light of the second optical comb.
After the first optical comb and the second optical comb are fully combined and subjected to beam beat frequency by the beam combiner, the first split beam of light comprises transmission light of the first optical comb and reflection light of the second optical comb, and the second split beam of light comprises reflection light of the first optical comb and transmission light of the second optical comb.
Further, the center wavelengths of the first optical comb and the second optical comb are 1550nm, and the gain medium is erbium-doped glass.
The central wavelength of the first optical comb and the second optical comb is related to the gain medium used by the light source, and the spectrum of the light source needs to cover the absorption peak of the gas to be measured. A single-cavity light source enters a gain medium to generate a double-optical comb with central wavelengths of 1550nm through stimulated radiation, and the double-optical comb is arranged in the same resonant cavity, so that common-mode noise is effectively inhibited, and high coherence between optical frequency combs is realized. The erbium-doped glass is used as a gain medium in an optical path and can effectively amplify optical signals.
Further, the resonant cavity comprises a wavelength division multiplexer, a polarization controller, a semiconductor saturable absorber mirror, a first optical fiber coupler, a first optical fiber collimator and a second optical fiber collimator, and the beam combiner is the second optical fiber coupler; the first optical fiber collimator, the wavelength division multiplexer, the gain medium, the polarization controller, the semiconductor saturable absorption mirror, the first optical fiber coupler and the second optical fiber collimator are sequentially in optical fiber connection, the pump light source faces the wavelength division multiplexer, the gas cell is arranged between the first optical fiber collimator and the second optical fiber collimator, the first optical fiber coupler is in optical fiber connection with the second optical fiber coupler, and the second optical fiber coupler is in optical fiber connection with the first photoelectric detector and the second photoelectric detector respectively;
the S2 comprises the following steps:
s2-1: opening a pumping light source, enabling the light source to enter a resonant cavity through a wavelength division multiplexer and generate a first optical comb and a second optical comb, and respectively emitting the light through a first optical fiber collimator and a second optical fiber collimator and repeatedly passing through a gas pool after the first optical comb and the second optical comb pass through a polarization controller and a semiconductor saturable absorption mirror;
s2-2: the first optical comb and the second optical comb pass through the gas pool, then enter the optical fiber through the first optical fiber collimator and the second optical fiber collimator, and are output through the first optical fiber coupler;
the S3 comprises the following steps:
s3-1: the first optical comb and the second optical comb are output to the second optical fiber coupler through the first optical fiber coupler;
s3-2: the first optical comb and the second optical comb are split into first split light and second split light after beam combination and beat frequency of the second optical fiber coupler.
The structure of the gas detection system can be further optimized by conducting optical signals through the optical fiber, the requirements of equipment technology and environment are reduced, and the gas detection system is favorable for being applied to detection of fault gas of outdoor power equipment.
Further, the gain medium is an erbium-doped fiber.
The erbium-doped fiber is used as a gain medium in the fiber, can effectively amplify optical signals, is compatible with a wavelength division multiplexing system, and has high pumping efficiency and stable working performance.
Further, the central wavelength of the pumping light source is 980nm, and the wavelength division multiplexer is a 980/1550nm wavelength division multiplexer.
The central wavelength of the pump light source is 980nm, the output of the 980nm pump light source is mostly unpolarized light, and a 980/1550nm wavelength division multiplexer is selected for matching with the output optical fiber of the light source. Due to the fact that the interval between 980nm and 1550nm is large, the coupling coefficient difference is large, wavelength division multiplexing of the coupler is easy to achieve, and 980nm light is almost completely coupled into the optical fiber.
Drawings
FIG. 1 is a schematic structural diagram of a first embodiment of a gas detection method based on a single-cavity double-comb light source according to the present invention;
FIG. 2 is a schematic diagram of intracavity absorption enhancement of a gas detection method based on a single-cavity double-comb light source according to the present invention;
FIG. 3 is a diagram of optical frequency comb teeth of a double optical comb beat frequency of a gas detection method based on a single-cavity double-comb light source according to the present invention;
FIG. 4 is a beat signal diagram of the double optical comb beat frequency of the gas detection method based on the single cavity double comb light source of the present invention;
fig. 5 is a schematic structural diagram of a second embodiment of the gas detection method based on a single-cavity double-comb light source of the present invention.
Detailed Description
Embodiments of the present invention will be described in detail below with reference to the accompanying drawings. The following examples are only for illustrating the technical solutions of the present invention more clearly, and therefore are only examples, and the protection scope of the present invention is not limited thereby.
It is to be noted that, unless otherwise specified, technical or scientific terms used herein shall have the ordinary meaning as understood by those skilled in the art to which the invention pertains.
The reference numbers in the drawings attached hereto include: the device comprises a pumping light source 1, a gas pool 2, a first photoelectric detector 3, a second photoelectric detector 4, a differential amplifier 5, a frequency spectrograph 6, a beam combining sheet 7, a high-reflection mirror 8, an output mirror 9, erbium-doped glass 10, a first reflection mirror 11, a second reflection mirror 12, a third reflection mirror 13, a wavelength division multiplexer 14, an erbium-doped optical fiber 15, a polarization controller 16, a semiconductor saturable absorber 17, a first optical fiber collimator 18, a second optical fiber collimator 19, a first optical fiber coupler 20 and a second optical fiber coupler 21.
The first embodiment is as follows:
the mixed gas of CH4, CO2, CO, C2H4 and the like is common important characteristic gas in the operation safety monitoring and early warning of the power equipment, and can be used for representing system faults, operation safety and eliminating fault sources. For example, CH4, C2H4 gas corresponds to an oil overheating fault type, CH4, CO2, CO, C2H4 gas corresponds to an oil and paper overheating fault type, and so on. However, the characteristic spectral lines of the gases are distributed in different spectral bands, the absorption is weak, and the gases are difficult to detect simultaneously, wherein the CH4 spectral band is 1630-1750nm, the CO2 spectral band is 1754-1828nm, the C2H4 spectral band is 1510-1540nm, and the CO spectral band is 1578-1585nm.
In this example, mixed gas such as CH4, CO2, CO, and C2H4 is detected.
In this embodiment, the gas detection method based on the single-cavity double-comb light source includes an adopted apparatus including the single-cavity double-comb light source, a gas cell 2, a beam combiner, a first photodetector 3, a second photodetector 4, a differential amplifier 5, and a spectrometer 6. The single-cavity double-comb light source comprises a pumping light source 1 and a resonant cavity, wherein the resonant cavity comprises a high reflecting mirror 8, an output mirror 9, a first reflecting mirror 11, a second reflecting mirror 12 and a gain medium. The high reflecting mirror 8, the gain medium and the output mirror 9 are sequentially arranged on the same straight line, the first reflecting mirror 11, the gas pool 2 and the second reflecting mirror 12 are sequentially arranged on the same straight line, and the pumping light source 1 faces the high reflecting mirror 8. The first photodetector 3 and the second photodetector 4 are respectively electrically connected to a differential amplifier 5, the differential amplifier 5 is electrically connected to a spectrometer 6, and the extra-cavity optical path of the resonant cavity includes a third mirror 13. In this embodiment, the beam combiner is a beam combining sheet 7, the central wavelength of the pump light source 1 is 980nm, and the gain medium is erbium-doped glass 10.
As shown in fig. 1, this embodiment discloses a gas detection method based on a single-cavity double-comb light source, which specifically includes the following steps (in this solution, the numbering of each step is only used for distinguishing the steps, the specific execution sequence of each step is not limited, and each step may also be performed simultaneously):
s1: the gas to be measured is introduced into the gas cell 2. The gas pool 2 is used for storing trace multi-component gas, is provided with a gas inlet and a gas outlet, and is used for a light-transmitting window for ultra-sensitive infrared spectrum detection. The gas to be detected is guided into the gas pool 2 through the gas inlet, the gas inlet and the gas outlet are closed, and the gas in the gas pool is uniformly diffused.
S2-1: turning on a pumping light source 1, and enabling the light source to enter bait-doped glass 10 through a high-reflection mirror 8 to form a first optical comb and a second optical comb which are opposite in direction; the first optical comb passes through the gas cell 2 after being reflected by the output mirror 9 and the first reflecting mirror 11, and then is incident to the bait-doped glass 10 after being reflected by the second reflecting mirror 12 and the high reflecting mirror 8; the second optical comb is reflected by the high reflecting mirror 8 and the second reflecting mirror 12, passes through the gas cell 2 in an opposite direction to the first optical comb (in fig. 1, the solid line arrow indicates the propagation direction of the first optical comb, and the dotted line arrow indicates the propagation direction of the second optical comb), and is reflected by the first reflecting mirror 11 and the output mirror 9 to enter the bait doped glass 10. Wherein the first optical comb and the second optical comb are respectively incident to the bait-doped glass 10, and then pass through the gas cell 2 after being reflected for a plurality of times again, and are repeated for a plurality of times.
In the process, the pump light source 1 generates bidirectional 1550nm light at the position of the bait-doped glass 10, vibrates for many times, and generates a double-light comb after mode locking. The first optical comb and the second optical comb are arranged in the cavityAnd the part of the gas passes through the gas cell 2, and is absorbed by the gas to be detected, so that the optical comb carries the fingerprint information of the gas absorption spectrum. Through arranging gas cell 2 in the resonant cavity, need not extra lock chamber, reduced the requirement to external environment to strengthen the absorption of gas molecule to two optical combs through increasing effective absorption length, improved detectivity. Wherein the first and second optical combs have repetition frequencies of f r1 ＝100MHz，f r2 ＝100MHz+1Hz。
The relationship between the absorption of the transmitted light by the gas molecule to be measured and the passing times is shown in FIG. 2, and the final average passing times n B In relation to the precision F of the cavity enhancement, the relationship with the reflectivity of the cavity mirror is:
r 2 and r 4 Respectively the reflectivity of the high mirror 8 and the output mirror 9. The change expression of the transmitted light power P caused by the gas to be measured in the cavity is as follows:
α (f) is an absorption coefficient, and d is a light absorption path.
S2-2: until the optical comb completes oscillation in the cavity, the first optical comb passes through the gas pool 2, then is reflected by the second reflector 12 and the high reflector 8, and is emitted out through the output mirror 9; meanwhile, the second optical comb passes through the gas cell 2, is reflected by the first reflector 11, and is emitted through the output mirror 9. In the process, the first optical comb and the second optical comb pass through the gas cell 2 for multiple times in the resonant cavity, collect gas absorption information and respectively emit the gas absorption information through the output mirror 9.
S3-1: a beam combining sheet 7 is arranged on a straight line along the output mirror 9, so that the first optical comb passes through the output mirror 9 and then emits to the beam combining sheet 7; a third reflector 13 is arranged on a light path emitted by the second optical comb through the output mirror 9, so that the second optical comb is reflected by the third reflector 13 after passing through the output mirror 9 and then emitted to the beam combining sheet 7; the beam combining sheet 7 and the two beams of incident light are arranged at an angle of 45 degrees.
S3-2: the first optical comb and the second optical comb are split into first split light and second split light after beam combination and beat frequency of the beam combination piece 7. Wherein the first split beam of light comprises transmitted light of the first optical comb and reflected light of the second optical comb and the second split beam of light comprises reflected light of the first optical comb and transmitted light of the second optical comb.
And S4, receiving the first split beam light and the second split beam light by the first photoelectric detector 3 and the second photoelectric detector 4 respectively, collecting a time domain interference signal and converting the time domain interference signal into an electric signal.
In the process, the first optical comb and the second optical comb are respectively output from an output mirror 9 of the resonant cavity, the second optical comb is reflected on the beam combining piece 7 through a third reflecting mirror 13 to beat frequency with the first optical comb, and due to different gas absorption peaks, generated beat frequency signals are different, so that simultaneous measurement of a plurality of gas molecule absorption peaks is realized, and then the gas molecules are split through the beam combining piece 7 and respectively incident on the two photoelectric detectors. The first and second split beams are received by the first and second photodetectors 3 and 4, respectively, and time-domain interference signals are collected and converted into electrical signals
S5-1: the electrical signals of the first photodetector 3 and the second photodetector 4 are suppressed in common mode noise and amplified by the differential amplifier 5.
S5-2: the spectrometer 6 analyzes the amplified electrical signal to obtain an absorption spectrum of the gas molecules.
Common-mode noise of the electric signals of the first photoelectric detector 3 and the second photoelectric detector 4 is suppressed and amplified through the differential amplifier 5, the amplified electric signal data is recorded and analyzed through the frequency spectrograph 6, and the frequency spectrograph 6 can directly convert interference signals of double-optical comb beat frequency into intensity signals of frequency domain, so that a hyperfine spectrogram of gas molecules, namely a double-optical comb absorption spectrum, is obtained.
S5-3: the type and concentration of the gas are identified from the absorption spectrum.
As shown in FIG. 3, the optical comb in this embodiment is an infrared single-cavity dual-optical comb light source, the spectrum range is 1000-14000nm, since the spectrum of the optical comb is distributed with N equally-spaced frequency teeth, N is any integer, and usually N is 10 5 Left and right. Wherein each optical frequency tooth is equivalent to a beam of single longitudinal mode laser, and the frequency of the first comb tooth of the optical comb is J 0 Adjacent frequency tooth spacing of f r Absolute of nth combFrequency is denoted as f n ＝(f 0 +nf r ) Wherein N is more than 0 and less than N, and the solid line of the optical frequency comb teeth is the first optical comb f in FIG. 3 n(1) ＝f 0 +nf r1 The dotted line is a second optical comb f n(2) ＝f 0 +nf r2 . In addition, because the double optical combs are generated in the same resonant cavity, the repetition frequencies of the double optical combs are slightly different due to different sequences of the optical devices, so that the double optical combs have good coherence, extra frequency control is not needed, the requirement on a frequency-locked phase-locked system is reduced, and the cost is reduced.
As shown in FIG. 4, the expression of the beat signal of the dual optical comb at the nth comb tooth is f b ＝|(f 0 +nf r2 )-(f 0 +nf r1 ) I= n Δ f. The zero frequencies of the light sources of the double optical combs are both f 0 The repetition frequencies are respectively f r1 And f r2 And the difference in repetition frequency is Δ f = f r2 -f r1 . The absorption lines follow the Lambert beer law, I (v) = I v (v)exp[-σ(v)Nd]Where I denotes light intensity, v denotes light frequency, N denotes the number concentration of light-absorbing substance molecules, d denotes a light-absorbing path, and σ (v) denotes a medium absorption cross section, and is related to the wavelength of incident light or the like at a constant temperature.
The beat frequency method converts the interaction information of the optical frequency and the gas into a radio frequency domain, thereby greatly reducing the difficulty of detecting the spectral signals. For example, if the optical comb detects a CH4 absorption signal at the nth comb tooth, a frequency f is generated at the corresponding comb tooth b Of beat signals, i.e. f b ＝n|f n(2) -f n(1) And if | = n Δ f, wherein Δ f is a known quantity, then converting the radio frequency domain into the optical frequency domain, obtaining gas component information by comparing absorption peak positions of corresponding gas molecules through simulation, and calculating the gas concentration by using the Lambert beer law.
After the measurement is finished, the gas pool 2 is vacuumized through the gas outlet, and the original residual sample gas is discharged.
The system for detecting the fault gas in the transformer oil based on the single-cavity double-comb does not need mechanical or spectrum scanning, and the second-level measuring speed of the traditional spectrum method is increased to the millisecond level, so that the quick response spectrum forming is realized. Through the mode of single chamber double comb, do not need second light comb light source, itself has high coherence, need not to restrain noise processing, has solved the coherence problem that traditional two spectral technique exists. Therefore, the requirements on cavity locking equipment and technology are reduced, extra cavity locking and frequency control are not needed, the complexity of the system is reduced, the cost is reduced, and the practicability is high. In addition, the invention combines the characteristic of high coherence of a single-cavity double-comb mode and an enhanced structure in the cavity, and can further enhance the absorption of gas molecules on the double-optical comb with corresponding wave bands, so that the double-optical comb fully carries the absorption information of a multi-component gas sample, thereby being capable of detecting a weaker absorption spectrum line and improving the detection sensitivity. The structure of the reinforcing in the cavity is used, the requirements on cavity locking equipment, technology and environment are reduced, and the detection of fault gas of outdoor power equipment is facilitated. The beat frequency method is adopted to convert the information of the interaction between the light frequency and the gas into the radio frequency domain, and the information is directly measured by the frequency spectrograph 6 or the oscilloscope, so that the difficulty of detecting the spectrum signal is reduced, and the measurement precision is improved.
Example two:
as shown in fig. 5, the present embodiment is different from the first embodiment in that:
the resonant cavity comprises a wavelength division multiplexer 14, a polarization controller 16, a semiconductor saturable absorber mirror 17, a first fiber coupler 20, a first fiber collimator 18 and a second fiber collimator 19. In this embodiment, the combiner is a second fiber coupler 21, and the gain medium is an erbium-doped fiber 15. The first optical fiber collimator 18, the wavelength division multiplexer 14, the gain medium, the polarization controller 16, the semiconductor saturable absorber mirror 17, the first optical fiber coupler 20 and the second optical fiber collimator 19 are sequentially connected through optical fibers, the pump light source 1 faces the wavelength division multiplexer 14, the gas cell 2 is arranged between the first optical fiber collimator 18 and the second optical fiber collimator 19, the first optical fiber coupler 20 is connected with the second optical fiber coupler 21 through optical fibers, and the second optical fiber coupler 21 is respectively connected with the first photoelectric detector 3 and the second photoelectric detector 4 through optical fibers. The central wavelength of the pumping light source 1 is 980nm, and the wavelength division multiplexer 14 is a 980/1550nm wavelength division multiplexer 14. The first fiber coupler 20 and the second fiber coupler 21 are each 2 x 2 fiber couplers.
S1: the gas to be measured is introduced into the gas cell 2.
S2-1: opening the pump light source 1, enabling the pump light source 1 with the wavelength of 980nm to enter the resonant cavity through the 980/1550nm wavelength division multiplexer 14 and generating a first optical comb and a second optical comb, wherein the first optical comb is emitted out through the first optical fiber collimator 18 and repeatedly passes through the gas cell 2; the second optical comb passes through the polarization controller 16 and the semiconductor saturable absorber mirror 17, and then exits through the second fiber collimator 19 and repeatedly passes through the gas cell 2 (in fig. 5, the solid line arrow indicates the propagation direction of the first optical comb, and the dotted line arrow indicates the propagation direction of the second optical comb). The first and second optical combs have a repetition frequency of f r1 ＝100MHz，f r2 ＝100MHz+1Hz。
S2-2: the first optical comb and the second optical comb pass through the gas cell 2, then enter the optical fiber through a first optical fiber collimator 18 and a second optical fiber collimator 19, and are output through a first optical fiber coupler 20;
s3-1: the first optical comb and the second optical comb are output to a second optical fiber coupler 21 through a first optical fiber coupler 20;
s3-2: the first optical comb and the second optical comb are split into a first split beam and a second split beam after being combined and subjected to beat frequency by a second optical fiber coupler 21.
In the above process, the first optical comb is emitted through the first optical collimator 18 and repeatedly passes through the gas cell 2, then is re-coupled into the optical fiber through the second optical collimator 19, and is emitted again through the first optical collimator 18 after passing through the first optical coupler 20, the semiconductor saturable absorber 17, the polarization controller 16 and the erbium-doped optical fiber 15. The second optical comb passes through the polarization controller 16, the semiconductor saturable absorber mirror 17 and the first optical fiber coupler 20, then exits through the second optical fiber collimator 19, passes through the gas cell 2 repeatedly, is re-coupled into the optical fiber through the first optical fiber collimator 18, then passes through the erbium-doped optical fiber 15, and returns to the polarization controller 16 (in fig. 5, the solid line arrow indicates the propagation direction of the first optical comb, and the dotted line arrow indicates the propagation direction of the second optical comb). After passing through the gas cell 2 for multiple times, the first optical comb and the second optical comb are respectively output to the second optical fiber coupler 21 through the first optical fiber coupler 20. The beams are combined and beat-frequency-modulated by the second optical fiber coupler 21 and then split into a first split beam and a second split beam.
And S4, receiving the first split beam light and the second split beam light by the first photoelectric detector 3 and the second photoelectric detector 4 respectively, collecting a time domain interference signal and converting the time domain interference signal into an electric signal.
S5-1: the electrical signals of the first photodetector 3 and the second photodetector 4 are suppressed in common mode noise and amplified by the differential amplifier 5.
S5-2: the spectrometer 6 analyzes the amplified electrical signal to obtain an absorption spectrum of the gas molecules.
S5-3: the type and concentration of the gas are identified from the absorption spectrum.
The first split beam light and the second split beam light are respectively received by the first photoelectric detector 3 and the second photoelectric detector 4, and time domain interference signals are collected and converted into electric signals. Common-mode noise of electric signals of the first photoelectric detector 3 and the second photoelectric detector 4 is suppressed and amplified through the differential amplifier 5, the frequency spectrograph 6 records and analyzes data of the amplified electric signals, and the frequency spectrograph 6 can directly convert interference signals of double-optical-comb beat frequency into intensity signals of frequency domains, so that a hyperfine spectrogram of gas molecules, namely a double-optical-comb absorption spectrum, is obtained. And obtaining gas component information by comparing absorption peak positions of corresponding gas molecules in a simulation mode, and calculating the gas concentration according to the Lambert beer law.
After the measurement is finished, the gas pool 2 is vacuumized through the gas outlet, and the original residual sample gas is discharged.
The foregoing are merely exemplary embodiments of the present invention, and no attempt is made to show structural details of the invention in more detail than is necessary for the fundamental understanding of the art, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice with the teachings of the invention. It should be noted that, for those skilled in the art, without departing from the structure of the present invention, several changes and modifications can be made, which should also be regarded as the protection scope of the present invention, and these will not affect the effect of the implementation of the present invention and the practicability of the patent. The scope of the claims of the present application shall be determined by the contents of the claims, and the description of the embodiments and the like in the specification shall be used to explain the contents of the claims.
Claims (10)
1. A gas detection method based on a single-cavity double-comb light source is characterized by comprising the following steps:
s1: introducing gas to be detected into a gas pool;
s2: a first optical comb and a second optical comb which have a repetition frequency difference and are emitted by a single-cavity double-comb light source, wherein the first optical comb and the second optical comb respectively pass through a gas pool in an opposite direction and repeat for a plurality of times;
s3: the first optical comb and the second optical comb are split into a first split beam and a second split beam after beam combination and beat frequency of a beam combiner;
s4: receiving the first split beam of light and the second split beam of light by a first photodetector and a second photodetector, respectively;
s5: and analyzing the electric signals of the first photoelectric detector and the second photoelectric detector to obtain a gas detection result.
2. The gas detection method based on the single-cavity double-comb light source as claimed in claim 1, wherein: after the S4 receives the first split beam light and the second split beam light by the first photodetector and the second photodetector, respectively, the method further includes: and collecting a time domain interference signal and converting the time domain interference signal into an electric signal.
3. The gas detection method based on the single-cavity double-comb light source as claimed in claim 2, wherein: the first photoelectric detector and the second photoelectric detector are respectively electrically connected with a differential amplifier, and the differential amplifier is electrically connected with a frequency spectrograph;
and S5 is as follows:
s5-1: common-mode noise of the electric signals of the first photoelectric detector and the second photoelectric detector is suppressed and amplified through a differential amplifier;
s5-2: analyzing the amplified electric signal by a frequency spectrograph to obtain an absorption spectrum of the gas molecules;
s5-3: the type and concentration of the gas are identified from the absorption spectrum.
4. The gas detection method based on the single-cavity double-comb light source as claimed in claim 3, wherein: the single-cavity double-comb light source comprises a pumping light source and a resonant cavity, wherein the resonant cavity comprises a high-reflection mirror, an output mirror, a first reflecting mirror, a second reflecting mirror and a gain medium; the high-reflection mirror, the gain medium and the output mirror are sequentially arranged on the same straight line, the first reflecting mirror, the gas pool and the second reflecting mirror are sequentially arranged on the same straight line, and the pumping light source faces the high-reflection mirror.
The S2 comprises the following steps:
s2-1: turning on a pumping light source, and enabling the light source to be incident to the gain medium through a high-reflection mirror to form a first optical comb and a second optical comb which are opposite in direction; the first optical comb is reflected by the output mirror and the first reflector, passes through the gas pool, and then is reflected by the second reflector and the high-reflection mirror to be incident to the gain medium; the second optical comb is reflected by the high reflecting mirror and the second reflecting mirror and opposite to the first optical comb, passes through the gas pool, and is reflected by the first reflecting mirror and the output mirror to be incident to the gain medium; the first optical comb and the second optical comb are respectively incident to the gain medium, then are reflected again, pass through the gas cell and repeat for a plurality of times;
s2-2: until the optical comb completes oscillation in the cavity, the first optical comb passes through the gas pool, then is reflected by the second reflector and the high-reflection mirror, and is emitted out through the output mirror; meanwhile, the second optical comb passes through the gas cell, is reflected by the first reflector and is emitted out through the output mirror.
5. The gas detection method based on the single-cavity double-comb light source as claimed in claim 4, wherein: the external optical path of the resonant cavity comprises a third reflector, and the beam combiner is a beam combining sheet;
the S3 comprises the following steps:
s3-1: arranging a beam combining sheet on a straight line along the output mirror, and enabling the first optical comb to shoot to the beam combining sheet after passing through the output mirror; a third reflector is arranged on a light path emitted by the second optical comb through the output mirror, so that the second optical comb is reflected to the beam combining sheet through the third reflector after passing through the output mirror; the beam combining sheet and the two beams of incident light are arranged at an angle of 45 degrees;
s3-2: the first optical comb and the second optical comb are split into first split light and second split light after beam combination and beat frequency of the beam combination pieces.
6. The gas detection method based on the single-cavity double-comb light source as claimed in claim 5, wherein: the first split beam of light includes transmitted light of the first optical comb and reflected light of the second optical comb, and the second split beam of light includes reflected light of the first optical comb and transmitted light of the second optical comb.
7. The gas detection method based on the single-cavity double-comb light source as claimed in claim 6, wherein: the center wavelengths of the first optical comb and the second optical comb are 1550nm, and the gain medium is erbium-doped glass.
8. The gas detection method based on the single-cavity double-comb light source as claimed in claim 3, wherein: the resonant cavity comprises a wavelength division multiplexer, a gain medium, a polarization controller, a semiconductor saturable absorber mirror, a first optical fiber coupler, a first optical fiber collimator and a second optical fiber collimator, and the beam combiner is the second optical fiber coupler; the first optical fiber collimator, the wavelength division multiplexer, the gain medium, the polarization controller, the semiconductor saturable absorption mirror, the first optical fiber coupler and the second optical fiber collimator are sequentially in optical fiber connection, the pump light source faces the wavelength division multiplexer, the gas cell is arranged between the first optical fiber collimator and the second optical fiber collimator, the first optical fiber coupler is in optical fiber connection with the second optical fiber coupler, and the second optical fiber coupler is in optical fiber connection with the first photoelectric detector and the second photoelectric detector respectively;
the S2 comprises:
s2-1: opening a pumping light source, enabling the light source to enter a resonant cavity through a wavelength division multiplexer and generate a first optical comb and a second optical comb, and respectively emitting the light through a first optical fiber collimator and a second optical fiber collimator and repeatedly passing through a gas pool after the first optical comb and the second optical comb pass through a polarization controller and a semiconductor saturable absorption mirror;
s2-2: the first optical comb and the second optical comb pass through the gas pool, then enter the optical fiber through the first optical fiber collimator and the second optical fiber collimator, and are output through the first optical fiber coupler;
the S3 comprises the following steps:
s3-1: the first optical comb and the second optical comb are output to the second optical fiber coupler through the first optical fiber coupler;
s3-2: the first optical comb and the second optical comb are split into first split light and second split light after beam combination and beat frequency of the second optical fiber coupler.
9. The gas detection method based on the single-cavity double-comb light source as claimed in claim 8, wherein: the gain medium is an erbium-doped fiber.
10. The gas detection method based on the single-cavity double-comb light source as claimed in claim 9, wherein: the central wavelength of the pumping light source is 980nm, and the wavelength division multiplexer is a 980/1550nm wavelength division multiplexer.
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