CN111829981A - TDLAS-based gas heterodyne detection device and detection method - Google Patents
TDLAS-based gas heterodyne detection device and detection method Download PDFInfo
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Abstract
The invention discloses a TDLAS-based gas heterodyne detection device and a detection method, which specifically comprise the following steps: the system comprises a DFB laser, an optical fiber mode coupler, an optical fiber acousto-optic device, a gas absorption cell, a first single-mode optical fiber coupler, a second single-mode optical fiber coupler, a photoelectric detector, a division circuit and a demodulation and analysis module; the invention provides a TDLAS-based gas heterodyne detection device, which applies heterodyne interference to TDLAS gas detection, and simultaneously eliminates the influence of laser intensity modulation by using a division circuit, thereby obviously improving the anti-noise capability and the measurement precision and achieving the purpose of gas concentration detection.
Description
Technical Field
The invention relates to the technical field of gas detection, in particular to a TDLAS-based gas heterodyne detection device.
Background
In the detection of the concentration of a gas, it is desirable to be able to measure the gas component and the concentration corresponding thereto quickly and accurately. The tunable semiconductor laser absorption spectroscopy (TDLAS) is used as a method for detecting the concentration of gas, and the concentration information of the gas to be detected can be obtained by adjusting the output wavelength of a narrow-line-width laser to continuously scan the characteristic absorption spectral line of the gas to be detected and utilizing the Lambert-beer law. However, in the TDLAS system, the laser output wavelength is scanned by the injection current, and the laser power is also changed to generate intensity modulation. On the other hand, TDLAS uses a direct detection method, and only amplitude information of the optical wave can be detected.
The heterodyne detection technology is an optical frequency coherent detection technology, is realized based on the principle of frequency mixing of coherent reference light and incident signal light, is very suitable for detection of weak signals, has sensitivity which is improved by several orders of magnitude compared with direct detection, and can not only respond to amplitude information of signals, but also respond to frequency and phase information of the signals. However, the heterodyne detection technique not only requires that the polarization directions of the carrier beam and the probe beam are parallel and the transmission directions are the same, but also requires that the frequencies of the two beams are different, that is, there is a frequency shift between the two beams. The traditional heterodyne detection generally needs a spatial optical frequency shifter or other frequency shifting devices, so that the whole heterodyne system is complex and tedious.
In summary, finding an efficient and compact TDLAS-based gas heterodyne detection apparatus has become a concern for researchers.
Disclosure of Invention
In order to solve the technical problem, the detection device and the detection method for the gas heterodyne based on the TDLAS are provided, heterodyne interference is applied to TDLAS gas detection by the detection device, meanwhile, the influence of laser intensity modulation is eliminated by a division circuit, the anti-noise capability and the measurement precision are obviously improved, and the purpose of gas concentration detection is achieved.
In order to achieve the above object, the present invention provides a TDLAS-based gas heterodyne detection apparatus, which is characterized by comprising: the optical fiber coupler comprises a DFB laser, an optical fiber mode coupler, an optical fiber acousto-optic device, a single-mode optical fiber coupler and a photoelectric detector;
the DFB laser is connected with a fiber mode coupler, the fiber mode coupler comprises a single mode fiber and a few-mode fiber, and the DFB laser is coupled through a fiber modeThe combiner is divided into two paths L1And L2One path enters L through the single mode fiber1One path and the other path enter L through the few-mode optical fiber2A post-path reconnection of an optical fiber acousto-optic device, L2The path is connected with the photoelectric detector through the single-mode fiber coupler.
Preferably, the single-mode optical fiber comprises a single-mode optical fiber input end and a single-mode optical fiber output end; the few-mode optical fiber comprises a few-mode optical fiber input end and a few-mode optical fiber output end; the input end of the single-mode optical fiber is connected with the DFB laser; and the few-mode optical fiber output end is connected with the optical fiber acousto-optic device.
Preferably, the DFB laser is used as a gas detection light source, and DFB lasers with different central wavelengths can detect different gases.
Preferably, the splitting ratio of the single-mode fiber and the few-mode fiber in the fiber mode coupler is adjusted by the attenuation flange when the output wavelengths of the DFB laser are different1The road power is such that L1The output power ratio of the path to the fiber acousto-optic device reaches 1: 1; said L1The working mode of the optical signal of the path is LP01A mode; said L2The working mode of the optical signal of the path is LP11Mode, LP after passing through said fiber optic acousto-optic device11Mode conversion to LP01A mode; the single mode fiber supports only one mode, the fundamental mode, LP in the operating band01Molding;
preferably, a gas absorption cell is arranged between the single-mode fiber output end and the single-mode fiber coupler.
Preferably, the optical fiber acousto-optic device comprises an aluminum cone, a vibration generating device and a radio frequency signal transmitting device; the radio frequency signal transmitting device is connected with the vibration generating device and then is connected with the aluminum cone; the radio frequency signal transmitting device comprises a signal generator and a voltage amplifier, wherein the signal generator generates a radio frequency signal, and the radio frequency signal is amplified by the voltage amplifier and then loaded into the vibration generating device; the vibration generating device is a piezoelectric ceramic piece, and the vibration of the piezoelectric ceramic piece drives the vibration of the aluminum cone to cause an acousto-optic grating effect.
Preferably, the single mode fiber coupler is divided into a first single mode fiber coupler and a second single mode fiber coupler.
Preferably, the single-mode fiber output end is connected with a gas absorption cell, and L1And L2The two paths of optical signals have the same polarization direction, and the two paths of optical signals are input to the second single-mode fiber coupler to generate heterodyne interference.
Preferably, said L2The path light signal passes through the optical fiber acousto-optic device and then is divided into two paths L by the first single-mode optical fiber coupler3And L4Said L is4The circuit is connected with a photoelectric detector;
preferably, said L1The road light signal enters a gas absorption cell through a single mode fiber, L2The path light signal enters the optical fiber acousto-optic device after passing through the few-mode optical fiber, the optical fiber acousto-optic device has a set optical signal frequency shift, and the optical signal frequency shift is divided into two paths L through the first single-mode optical fiber coupler3And L4；
Said L3Road light signal and L passing through gas absorption cell1The path light signals are all connected with the input end of a second single-mode fiber coupler to generate heterodyne interference, and the output end of the second single-mode fiber coupler and the L4The output ends of the circuits are respectively connected with one photoelectric detector;
the photoelectric detector has the functions of filtering and amplifying and converts optical signals into electric signals; the gas heterodyne detection device further comprises a division circuit, a demodulation analysis module, an output end of the second single-mode coupler and an L4The two paths of signals are converted into electric signals, connected with a division circuit, and then processed by a demodulation analysis module to invert concentration information of different gases.
The invention has the beneficial effects that:
(1) the invention combines TDLAS on the traditional heterodyne detection mode, the transmission directions of the two light paths are consistent, the invention has high conversion gain, good filtering characteristic and polarization discrimination capability, and heterodyne detection not only can respond to the amplitude information of signals, but also can respond to the frequency and phase information of the signals, thereby providing another idea for gas detection in the future.
(2) The device has the advantages of full optical fiber structure, low cost, simple relative manufacturing process and good sensitivity.
(3) The division circuit is added, so that the influence of light source intensity modulation in the TDLAS technology is eliminated, and the demodulation result is more accurate.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings needed to be used in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings without inventive exercise.
Fig. 1 is a schematic view of a TDLAS-based gas heterodyne detection apparatus according to embodiment 1 of the present invention;
FIG. 2 is a schematic diagram of mode conversion of the fiber acousto-optic device in embodiment 1 of the present invention;
fig. 3 is a schematic view of a TDLAS-based gas heterodyne detection apparatus according to embodiment 2 of the present invention;
fig. 4 is a schematic view of a TDLAS-based gas heterodyne detection apparatus according to embodiment 3 of the present invention;
FIG. 5 is a waveform diagram of the heterodyne detection output of ammonia gas in embodiment 1 of the present invention;
fig. 6 is a waveform diagram of an output optical signal of a drop path after the first single-mode fiber coupler is split according to embodiment 2 of the present invention;
fig. 7 is a diagram of an output waveform of a division circuit and a demodulated second harmonic in embodiment 3 of the present invention;
in the figure: the device comprises a 1-DFB laser, a 2-optical fiber mode coupler, a 3-optical fiber acousto-optic device, a 4-aluminum cone, a 5-vibration generating device, a 6-radio frequency signal transmitting device, a 7-gas absorption pool, an 8-first single-mode optical fiber coupler, a 9-second single-mode optical fiber coupler, a 10-photoelectric detector, an 11-division circuit, a 12-demodulation analysis module, a 2.1-single-mode optical fiber input end, a 2.2-single-mode optical fiber output end, a 2.3-few-mode optical fiber input end and a 2.4-few-mode optical fiber output end.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
In order to make the aforementioned objects, features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in further detail below.
Example 1
Referring to fig. 1-2, embodiment 1 provides a TDLAS-based gas heterodyne detection apparatus, which specifically includes:
the system comprises a DFB laser 1, an optical fiber mode coupler 2, an optical fiber acousto-optic device 3, a gas absorption cell 7, a second single-mode optical fiber coupler 9 and a photoelectric detector 10; wherein 2.1 is the input end of the single mode fiber, 2.2 is the output end of the single mode fiber, 2.3 is the input end of the few-mode fiber, and 2.4 is the output end of the few-mode fiber. The optical fiber mode coupler 2 is formed by coupling a single mode optical fiber and a few-mode optical fiber in a set fiber core mode; the laser beam output by the DFB laser 1 is divided into two optical signals by the fiber mode coupler 2, and one optical signal is output as LP by the single mode fiber output end 2.201Mode, which is transmitted as signal light to the gas absorption cell 7, L in FIG. 11A way; the output of the other route few-mode optical fiber output end 2.4 is LP11Mode, which is transmitted as carrier light to the fiber optic acousto-optic device 3 and converted to LP01Modes, e.g. L in FIG. 12A way; the output ends of the gas absorption cell 7 and the optical fiber acousto-optic device 3 are respectively connected with the input end of a second single-mode optical fiber coupler 9, and the output end of the second single-mode optical fiber coupler 9 is connected with a photoelectric detector. The optical fiber acousto-optic device 3 comprises an aluminum cone 4, a vibration generating device 5 and a radio frequency signal transmitting device 6; the radio frequency signal transmitting device comprises a signal generator and a voltage amplifier, wherein the signal generator generates a signalThe frequency signal is amplified by a voltage amplifier and then applied to the vibration generating device 5.
The vibration generating device 5 is a piezoelectric ceramic piece, and the vibration of the piezoelectric ceramic piece drives the aluminum cone 4 to vibrate to cause an acousto-optic grating effect, so that LP is finally realized11Mode to LP01And the mode is converted, and the set optical signal frequency shift is carried out, so that the heterodyne interference condition is formed.
The operation steps specifically comprise:
1) the laser beam output by the DFB laser 1 is divided into two optical signals by the fiber mode coupler 2, the two optical signals have the same polarization direction, and one LP01The pattern is used as signal light for detecting gas absorption spectrum signals.
The DFB laser is used as a gas detection light source, the line width is narrow, and different gases can be detected by DFB lasers with different central wavelengths.
The output splitting ratio of the fiber mode coupler 2 is 1: 1.
2) The other path LP11The mode as carrier light is converted into LP by the optical fiber acousto-optic device 301Mode, with set optical signal frequency shift, two way LP01The optical signal of the mode is input to the second single-mode fiber coupler 9 to generate heterodyne interference, and the second single-mode fiber coupler 9 is connected with the photodetector 10 to convert the optical signal into an electrical signal.
The heterodyne interference passes through the photodetector 10, and the current passing through the intermediate frequency band pass filter is:
wherein the content of the first and second substances,
and
indicating the amplitude of the signal light and the carrier light,
and
representing the frequencies of the signal light and the carrier light,
and
indicating the initial phases of the signal light and the carrier light; if light of a particular frequency is absorbed by a gas,
the value becomes small, and the overall current value becomes small;
and
varies linearly with the laser injection current, but
The difference does not change. The heterodyne detection apparatus responds to both amplitude information and frequency information of a signal, where t is the amount of time and m is a scaling factor determined by the performance of the photodetector. Generally, the initial phase is zero, and the response signal of the heterodyne detection without gas absorption is:
I1＝mEL1EL2cos[2π(fL1-fL2)t](2)
example 2
Referring to fig. 3, a TDLAS-based gas heterodyne detection apparatus according to embodiment 2 of the present invention is provided. As shown in fig. 3, the optical fiber laser comprises a DFB laser 1, a fiber mode coupler 2, a fiber acousto-optic device 3, a first single mode fiber coupler 8 and a photodetector 10. The optical fiber mode coupler 2 is formed by coupling a single mode optical fiber and a few-mode optical fiber in a set fiber core mode; the laser beam output by the DFB laser 1 is divided into two optical signals by the optical fiber mode coupler 2, and one optical signal is divided into a few modesThe output of the optical fiber output end 2.4 is LP11Mode, which is transmitted as carrier light to the fiber optic acousto-optic device 3 and converted to LP01A mode; LP01The mode signal light is divided into two paths through the first single mode fiber coupler 8, and the upper path is L in fig. 33Path, lower path light beam L after light splitting4The photoelectric detector is connected, and the splitting ratio of the first single-mode fiber coupler 8 is 1:1, so the response signal of the detector and L3The same way is:
example 3
As shown with reference to figure 4 of the drawings,
The splitting ratio of the optical fiber mode coupler 2 and the first single-mode optical fiber coupler 8 is 1: 1.
The demodulation analysis module 12 includes an oscilloscope, a lock-in amplifier and a computer.
In embodiment 3 of the present invention, the division circuit divides the two paths of response signals, and
as can be seen from equations (2) and (3), the output of the divider circuit is:
Iout＝I1/I2＝4cos[2π(fL1-fL2)t](4)
from the above equation, the divider circuit can eliminate the influence of the laser intensity modulation. After the light is absorbed by gas, the Lambert-beer law is satisfied, and the expression is as follows:
It＝exp[-σ(λ)CL]I0(5)
wherein, ItIs the intensity of transmitted lightDegree, sigma (lambda) is the absorption cross section of the gas and is related to the temperature, pressure and absorption frequency in the gas absorption cell, C is the concentration of the gas to be measured in the gas absorption cell, L is the optical path length of the gas absorption, and I0Is the incident light intensity. Therefore, the output of the division circuit after gas absorption is as follows:
Itand
the output of the division circuit can still eliminate the influence of laser intensity modulation after gas absorption.
The filtered and amplified electric signal is subjected to a division circuit to eliminate the influence of laser intensity modulation, and then is accessed into a demodulation analysis module, and is connected with an oscilloscope to check the absorption peak of the heterodyne detected gas and store data; then input into a phase-locked amplifier, and set the reference frequency of the phase-locked amplifier to
The second harmonic can be demodulated, and the concentration information of the gas can be inverted according to the harmonic amplitude.
As shown in fig. 4, the TDLAS-based gas heterodyne detection apparatus is implemented by dividing a center wavelength output by a DFB laser 1 into two optical signals with a splitting ratio of 1:1 by an optical fiber mode coupler 2, wherein the center wavelength is 1512.20 nm; one path is connected with a gas absorption pool as a signal light, and the inside of the gas absorption pool contains ammonia gas with the concentration of 500 ppm; the other path is used as a carrier optical connection optical fiber acousto-optic device and has a set optical signal frequency shift; the output of the optical fiber acousto-optic device is connected with the first single-mode optical fiber coupler and divided into two paths of light beams, the light beam on the upper path of the optical fiber acousto-optic device and the light beam absorbed by the gas absorption cell have specific frequency shift, the two paths of light signals have the same polarization direction, and the two paths of light signals are combined together through the second single-mode optical fiber coupler, so that the optical heterodyne detection of ammonia is realized.
After passing through the photoelectric detector, the filtered and amplified electric signal and the electric signal converted by the next light beam are connected with a division circuit, the output is accessed to a demodulation analysis module, specific data is stored, then 1200 sampling points are obtained to perform linear fitting to obtain a continuous signal, and the continuous signal is expressed by the output wavelength of the corresponding laser. The heterodyne detection output waveform of ammonia in embodiment 1 is shown in fig. 5, the output waveform of the drop optical signal after the first single-mode fiber coupler 8 is split in embodiment 2 is shown in fig. 6, the DFB laser 1 has an absorption peak at an absorption wavelength near 1512.25nm, corresponding to an obvious depression in the waveform, and according to embodiment 3, the output waveform and the demodulated second harmonic of the division circuit are shown in (r) and (r) in fig. 7.
In the description of the present invention, it is to be understood that the terms "longitudinal", "lateral", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", and the like, indicate orientations or positional relationships based on those shown in the drawings, are merely for convenience of description of the present invention, and do not indicate or imply that the referenced devices or elements must have a particular orientation, be constructed and operated in a particular orientation, and thus, are not to be construed as limiting the present invention.
The above-described embodiments are merely illustrative of the preferred embodiments of the present invention, and do not limit the scope of the present invention, and various modifications and improvements of the technical solutions of the present invention can be made by those skilled in the art without departing from the spirit of the present invention, and the technical solutions of the present invention are within the scope of the present invention defined by the claims.
Claims (10)
1. A TDLAS-based gas heterodyne detection apparatus, comprising: the optical fiber acousto-optic device comprises a DFB laser (1), an optical fiber mode coupler (2), an optical fiber acousto-optic device (3), a single-mode optical fiber coupler and a photoelectric detector (10);
the DFB laser (1) is connected with the optical fiber mode coupler (2), the optical fiber mode coupler (2) comprises a single-mode optical fiber and a few-mode optical fiber, and the DFB laser (1) is divided into two paths of L through the optical fiber mode coupler (2)1And L2One path enters L through the single mode fiber1One path and the other path enter L through the few-mode optical fiber2A fiber acousto-optic device (3) is connected after the path, L2The path is connected with the photoelectric detector (10) through the single-mode fiber coupler.
2. TDLAS-based gas heterodyne detection apparatus as set forth in claim 1, wherein the single-mode fiber comprises a single-mode fiber input end (2.1) and a single-mode fiber output end (2.2); the few-mode optical fiber comprises a few-mode optical fiber input end (2.3) and a few-mode optical fiber output end (2.4); the single-mode optical fiber input end (2.1) is connected with the DFB laser (1); the few-mode optical fiber output end (2.4) is connected with the optical fiber acousto-optic device (3).
3. The TDLAS-based gas heterodyne detection apparatus as recited in claim 1, wherein the DFB lasers (1) are used as gas detection light sources, and the DFB lasers (1) with different central wavelengths are capable of detecting different gases.
4. TDLAS-based gas heterodyne detection apparatus as claimed in claim 1, wherein the splitting ratio of the single-mode fiber and the few-mode fiber in the fiber mode coupler (2) is adjusted by an attenuation flange at different output wavelengths of the DFB laser (1)1The road power is such that L1The output power ratio of the path to the fiber acousto-optic device (3) reaches 1: 1; said L1The working mode of the optical signal of the path is LP01A mode; said L2The working mode of the optical signal of the path is LP11Mode, LP after passing through said fiber optic acousto-optic device (3)11Mode conversion to LP01A mode; the single mode fiber supports only one mode, the fundamental mode, LP in the operating band01And (5) molding.
5. TDLAS-based gas heterodyne detection apparatus as claimed in claim 1, wherein a gas absorption cell (7) is provided between the single-mode fiber output (2.2) and the single-mode fiber coupler.
6. The TDLAS-based gas heterodyne detection apparatus as recited in claim 1, wherein the fiber acousto-optic device (3) comprises an aluminum cone (4), a vibration generation device (5), and a radio frequency signal emission device (6); the radio frequency signal transmitting device (6) is connected with the vibration generating device (5) and then is connected with the aluminum cone (4); the radio frequency signal transmitting device (6) comprises a signal generator and a voltage amplifier, wherein the signal generator generates a radio frequency signal, and the radio frequency signal is amplified by the voltage amplifier and then loaded into the vibration generating device (5); the vibration generating device (5) is a piezoelectric ceramic piece, and the vibration of the piezoelectric ceramic piece drives the vibration of the aluminum cone (4) to cause an acousto-optic grating effect.
7. TDLAS-based gas heterodyne detection apparatus as claimed in claim 1, wherein the single-mode fiber couplers are divided into a first single-mode fiber coupler (8) and a second single-mode fiber coupler (9).
8. TDLAS-based gas heterodyne detection apparatus as claimed in claim 7, characterized in that the single-mode fiber output (2.2) is connected to a gas absorption cell (7), L1And L2The two paths of optical signals have the same polarization direction, and the two paths of optical signals are input into a second single-mode fiber coupler (9) to generate heterodyne interference.
9. The TDLAS-based gas heterodyne detection apparatus of claim 7, wherein the L is2The path light signal passes through the optical fiber acousto-optic device (3) and then passes through the first single-mode optical fiber coupler (8) to be divided into two paths L3And L4Said L is4The circuit is connected with the photoelectric detector (10).
10. The TDLAS-based gas heterodyne detection apparatus of claim 9, wherein the L is1The road light signal enters a gas absorption cell (7) through a single mode fiber, L2Optical signal path through few-mode optical fiberThen enters the optical fiber acousto-optic device (3), the optical fiber acousto-optic device (3) has set optical signal frequency shift, and is divided into two paths L through a first single-mode optical fiber coupler (8)3And L4；
Said L3Road light signal and L passing through gas absorption cell (7)1The path light signals are connected with the input end of a second single-mode fiber coupler (9) to generate heterodyne interference, and the output end of the second single-mode fiber coupler (9) and the L4The output ends of the circuits are respectively connected with one photoelectric detector (10);
the photoelectric detector (10) has the functions of filtering and amplifying, and converts an optical signal into an electric signal; the gas heterodyne detection device further comprises a division circuit (11) and a demodulation analysis module (12), wherein the output end of the second single-mode coupler (9) and the L4The two paths of signals are converted into electric signals, are connected with a division circuit (11) and then are processed by a demodulation analysis module (12), and the concentration information of different gases is inverted.
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| CN112394014A (en) * | 2020-11-02 | 2021-02-23 | 河北地质大学 | Real-time monitoring device and method for ammonia escape concentration of flue gas |
| CN113686812A (en) * | 2021-07-23 | 2021-11-23 | 复旦大学 | Gas sensing method and system for eliminating different-path interference noise |
| CN114184568A (en) * | 2021-11-04 | 2022-03-15 | 合肥学院 | All-fiber mid-infrared laser heterodyne radiometer system |
| CN114069377A (en) * | 2021-11-17 | 2022-02-18 | 上海大学 | Mode control system based on acousto-optic device |
| CN114069377B (en) * | 2021-11-17 | 2023-12-26 | 上海大学 | Mode control system based on acousto-optic device |
| CN114034660A (en) * | 2021-12-03 | 2022-02-11 | 福建美营自动化科技有限公司 | Gas detection system and method based on TDLAS |
| CN114034660B (en) * | 2021-12-03 | 2024-03-08 | 福建美营自动化科技有限公司 | Gas detection system and method based on TDLAS |
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