Cavity-enhanced Infrared Gas Detection Technique And Application

The national medium and long-term science and technology development plan outline,the"Thirteenth Five-Year Plan"national science and technology innovation plan,and the environmental field all propose to focus on the research of environmental monitoring and early warning technology,and the development of major environmental protection equipment suitable for national conditions.Furthermore,environmental pollution in China,especially air pollution,is becoming increasingly serious,requiring real-time online monitoring of air quality.Therefore,the development of ultra-high sensitivity and ultra-fast response gas sensors with independent intellectual property rights is of great social value and scientific significance for ensuring environmental quality and industrial safety production.Gas sensors based on cavity enhanced absorption spectroscopy(CEAS)meet these requirements due to its high sensitivity,good selectivity,and real-time detection capability.This paper carried out two types of CEAS researches,including incoherent broadband cavity-enhanced absorption spectroscopy(IBBCEAS)and off-axis integrated cavity output spectroscopy(OA-ICOS).The two techniques were studied in terms of optical,electrical,mechanical,algorithm simulation and system structure.A ppbv level methane/ethane detection system for vehicle-mounted natural gas leakage inspection was developed.The main research contents are summarized as follows:Firstly,the IBBCEAS gas detection technique was studied,and the specific works were as follows:(1)The majority of IBBCEAS were used to detect trace gas species with strong absorption in the visible spectral range.Due to the weak absorption intensity of gas molecules in the near-infrared band,its high-resolution detection was relatively difficult.Therefore we demonstrated a tungsten-halogen lamp-based IBBCEAS sensor system in combination with a high-resolution Fourier-transform spectrometer(FTS)in the near-infrared(NIR)region for high-sensitivity methane(CH₄)detection.A wavelet denoising method was introduced in the concentration retrieval to reduce the noise level,which improved the measurement precision with an enhancement factor of 2.(2)The commonly used infrared broadband light sources in IBBCEAS include supercontinuum sources(SC)and xenon lamps,etc.,which were usually expensive,large in size and high-power consumption.In order to overcome these limitations and take advantage of a low-cost,power-efficient,compact,and long-lived near-infrared light-emitting diode(LED),we demonstrated a novel NIR-LED-based portable IBBCEAS system for multi-gas detection.In this work,a NIR-LED with m W-level output was used in IBBCEAS as the broadband light source.A portable cage-based cavity was designed,and a variety of IBBCEAS signal processing approaches were studied,such as multi-spectral nonlinear fitting,etc.For the field sensing application,simultaneous detection of CH₄and acetylene(C₂H₂)was conducted combined with a lock-in amplifier and a scanning monochromator to prove the multigas sensing ability.A field deployment of the sensor system for rapid and accurate CH₄leakage monitoring was carried out using the fiber-based spectrometer to verify the usability of the system.Secondly,the OA-ICOS gas detection technique was studied,and the specific works were as follows:(1)In order to improve the detection sensitivity and multi-gas capability of the OA-ICOS technique,a compact cage-based meter-level near-infrared OA-ICOS was developed.An effective length of 9.28 m can be achieved with the short cavity length of 0.06 m.Two gas detection methods of laser direct absorption spectroscopy(LDAS)and wavelength modulation spectroscopy(WMS)were used.Compared with LDAS,the signal-to-noise ratio(SNR)of WMS was increased by 10times,and the detection sensitivity was increased by 9 times.(2)By combining frequency division multiplexing-assisted wavelength modulation spectroscopy(FDM-WMS),a dual-gas OA-ICOS sensor was demonstrated for simultaneous detection of C₂H₂and CH₄.With two laser wavelengths(C₂H₂:1532 nm,CH₄:1653 nm),two effective optical path lengths in the same cavity(C₂H₂:9.28 m,CH₄:8.56 m)was achieved.An Allan deviation of 700 parts-per-billion in volume(ppbv)for C₂H₂ and850 ppbv for CH₄ was achieved for this two gas species.(3)In order to achieve high-sensitivity measurement of atmospheric trace components in the near-infrared range,a kilometer-level optical path-based OA-ICOS sensor was developed.An absorption cell with an effective length of 2150 m was fabricated with a length of 0.6 m and a reflectivity of 99.972%.Sensor response time of 0.8 s was obtained and Allan analysis yielded a detection limit of 2.7 ppbv for CH₄detection.For field application,dual-gas sensing was implemented for simultaneous measurement of CH₄ and water vapor(H₂O).Also,a three-day continuous monitoring of atmospheric CH₄ concentration levels in Changchun city was carried out to verify the validity and reliability of the demonstrated ppbv-level CH₄ sensor system.Thirdly,in order to further improve the detection sensitivity and expand the application range,three novel OA-ICOS gas detection techniques were proposed,and the specific works were as follows:(1)In order to suppress the cavity mode noise in OA-ICOS,a novel dual-input dual-output(DIDO)coupling scheme of OA-ICOS was demonstrated by splitting the laser beam into two at the same time and coupled into the cavity independently.Multiple light beams travelled back and forth in the cavity without interference,which can increase the cavity mode density,smooth the cavity mode structure,and reduce the light intensity fluctuation caused by the coherent oscillation,thus it can suppress the cavity mode noise and improve the SNR.The influences of beam splitting ratio and light reflection number to the output intensity and cavity mode linewidth were theoretically studied.Experimental investigation on the suppression of cavity mode noise was carried out through CH₄ measurements.Compared to the regular single laser beam coupled-in-cavity(single-input single-output,R-SISO)approach,the SNR of DIDO was improved by a factor of?2.5 and the sensitivity was improved by a factor of?2.2.(2)Multi-input multi-output(MIMO)configuration was further numerically studied to verify the noise suppression ability of this detection concept.The incident laser beam was divided into multiple beams,and each laser beam was coupled into the cavity through a collimating fiber independently,and the multiple cavity output were coupled to the same detector simultaneously.Detailed theoretical investigation was performed to explore the relation among the MIMO parameters.The noise suppression factor of the MIMO scheme and the noise level and dominated noise in the cavity were studied using light and mode field simulation.Under the selected cavity parameters,a maximum SNR enhancement factor of 13 was achieved when compared to the R-SISO approach.(3)In order to realize remote gas measurement for explosion-proof places,a novel all-fiber coupled-OA-ICOS(FC-OA-ICOS)/cavity-reflected WMS(CR-WMS)technique was proposed.The low-loss optical fiber coupling method was used to separate the electrical and optical modules,which realized the separation between measurement and control,and avoided the intrinsic safety and explosion-proof design of the electrical modules,therefore realizing long distance,anti-static,and safe gas detection.Furthermore,by feeding back the reflected light from the first cavity mirror to a WMS-based single-pass gas cell via a multi-mode fiber,two different techniques were achieved in the sensor system with a wide dynamic range from?15 ppmv to?12%for CH₄ detection.Field monitoring of CH₄ leakage was performed under static and mobile states using the developed gas sensor.Finally,in order to deploy the developed gas sensor for out-of-laboratory,on site application,a set of OA-ICOS detection system for vehicle-mounted natural gas leakage inspection was developed.Ppbv level methane/ethane detection was achieved using the time division multiplexing technique.Alos this system has the functions of gas pretreatment and leakage source location,etc.Using a cavity mirror with a reflectivity of 99.99%,an integrated cavity with an effective optical path of 3.5 km was developed.Furthermore,combining with a low-noise electrical design,the detection limit of methane/ethane in the near-infrared reached 3.4 ppbv and 25 ppbv,respectively.Based on the above-mentioned research work,the innovations of this paper are summarized as follows:(1)A power-efficient near-infrared LED based IBBCEAS technique.In order to overcome the disadvantages of commonly used infrared broadband light sources(such as xenon lamps)of expensive,large in size and high-power consumption,and take advantage of a low-cost,power-efficient,compact,and long-lived near-infrared light-emitting diode(LED),a novel NIR-LED-based portable IBBCEAS technique was demonstrated for multi-gas detection.Fast-identification and high-resolution detection of various gases in a wide spectrum range were realized.Multi-gas concentration retrieval algorithm was applied for high-efficiency infrared gas detection.(2)A novel OA-ICOS technique based on the MIMO laser-cavity coupling method.A novel MIMO method was proposed to suppress the cavity mode noise and to achieve a denser and smoother cavity mode structure.DIDO pattern showed that the SNR was increased by 2.5 times and the sensitivity was increased by 2.2 times compared to the R-SISO pattern.Theoretical research of the MIMO pattern showed that the SNR can be increased by more than 13 times.(3)A novel FC-OA-ICOS/CR-WMS technique.In order to realize remote gas measurement for explosion-proof places,the low-loss optical fiber coupling method was used to separate the measurement and control modules,and avoided the intrinsic safety and explosion-proof design of the electrical modules.Furthermore,by feeding back the reflected light from the first cavity mirror to a WMS-based single-pass gas cell via a multi-mode fiber,two different techniques were achieved in the sensor system with a wider dynamic range,which realized the remote,variable-range gas measurement.(4)Development and application of gas detection system based on CEAS technique.Integrated cavities with different optical path lengths ranging from meter-to kilometer-level were developed,and the application of multi-gas sensing such as CH₄,C₂H₆,C₂H₂ and H₂O were realized.A vehicle-mounted natural gas inspection system was developed,and indoor and outdoor system function verification tests and applications was carried out.