Research And Development Of Gas Detection System Based On Mid-infrared Semiconductor Lasers

Methane, ethane and carbon monoxide are detected using mid-infrared semiconductor lasers and three different gas detection methods(direct absorption technique, tunable diode laser absorption spectroscopy and photoacoustic spectroscopy) in this dissertation. The detection sensitivity and stability of these sensors are also tested and discussed respectively.First of all, the interband cascade laser(ICL) and tunable diode laser absorption spectroscopy(TDLAS)-based methane and ethane(C2H6) sensor are introduced. Both of them are employing a multi-pass gas cell(MPC) and a continuous wave, room temperature interband cascade laser(ICL). The optical integration based on an advanced folded two layers optical path design and an efficient ICL control system with appropriate electrical power management results in a methane sensor with a small footprint(32 × 20 × 17 cm~3) and low-power consumption(6W). Additionally, polynomial and least-squares fit algorithms are employed to remove the baseline of the spectral scan and retrieve CH4 concentrations, respectively. An Allan-Werle deviation analysis shows that the measurement precision can reach 10.5 ppb for a 1 s averaging time and can reach 1.4 ppb for a 60 s averaging time. Continuous measurements lasting seven days were performed to demonstrate the stability and robustness of the reported methane sensor. While the C2H6 sensor employed single layer optical path design, the physical size is 35.5×18×12.5 cm~3. It including a continuous-wave(CW) interband cascade laser(ICL) emitting at 3.34 μm, a multi-pass gas cell(MGC, physical size is 17×6.5×5.5 cm~3) with 54.6 m optical path length. The ICL was employed for targeting an interference-free strong C2H6 line at 2996.88 cm-1 in the fundamental absorption band of C2H6. The sensor performance, including the minimum detection limit(MDL) and the stability were improved by reducing the effect of laser power drift by means of the 2f/1f-WMS technique. The minimum detection limit(MDL) of ~1.2 parts per billion(ppbv) for 2f-WMS and ~1.0 ppbv for 2f/1f-WMS are achieved, respectively, with a measurement time of 4 s. The MDL is improved from 299 pptv(@108s for 2f-WMS) to 239 pptv(@208s for 2f/1f-WMS), based on Allan deviation analysis, which reflects the enhancement of the sensor sensitivity. In addition, dynamic measurements were performed with the two detection schemes to determine both accuracy and precision of the C2H6 sensor.Secondly, a quartz-enhanced photoacoustic spectroscopy(QEPAS) and ICL-based ethane sensor is introduced. The system calibration, determine the sensitivity and stability test were completed. Gas pressure and laser frequency modulation depth were optimized to be 100 Torr and 0.106 cm-1, respectively, for maximizing the 2f signal amplitude. Additionally, the averaged noise of the system was determined to be 0.347 V through 1 hour continuous measurement to the background noise, and the signal to noise ratio of 28.56 was obtained, this also determines the sensitivity of this ethane sensor is ~39.9 ppb.At last, the quantum cascaded laser(QCL) and direct absorption spectroscopy(DAS)-based methane(CH4) and carbon monoxide(CO) gas sensors are introduced. Using the fundamental absorption characteristics at the wavelength of 7.5 μm of CH4 molecule, a mid-infrared QCL and multi-pass gas cell(MPC)-based methane gas sensor was designed according to the principle of infrared absorption spectrum,. This sensor uses a thermoelectrically cooled QCL which central wavelength is 7.5 μm. Meanwhile, a compact MPC(40 cm long and 800 ml sampling volume) was utilized to achieve an effective optical path length of 16 meters. Additionally, the beam quality was effectively improved, the noise which caused by the fluctuation of QCL was reduced and the detection sensitivity of this instrument was also improved under the guidance of differential optical absorption spectroscopy method. It indicates that the stability of this instrument is good by means of multiple measurements to the methane gas which with different concentrations. The system configuration and detection method of CO system is similar to CH4 sensor. It employs a QCL which central emitting wavelength is 4.75 μm, and effective detection to the concentrations of CO have been carried out at the positon of 2103.3 cm-1, finally, a detection limit of 5 ppm was realized. Moreover, this part also introduces a low temperature drift power supply of QCL, the effects of driving current come from temperature changes of integrate chips and resistive components are analyzed in detail. These kinds of effects can be reduced by proceeding relevant compensation in software.