Xiaojuan Cui

A Quantum Cascade Laser-Based Optical Sensor for Continuous Monitoring of Environmental Methane in Dunkirk (France)

A room-temperature continuous-wave (CW) quantum cascade laser (QCL)-based methane (CH4) sensor operating in the mid-infrared near 8 μm was developed for continuous measurement of CH4 concentrations in ambient air. The well-isolated absorption line (7F2,4 ← 8F1,2) of the ν4 fundamental band of CH4 located at 1255.0004 cm−1 was used for optical measurement of CH4 concentration by direct absorption in a White-type multipass cell with an effective path-length of 175 m. A 1σ (SNR = 1) detection limit of 33.3 ppb in 218 s was achieved with a measurement precision of 1.13%. The developed sensor was deployed in a campaign of measurements of time series CH4 concentration on a site near a suburban traffic road in Dunkirk (France) from 9 to 22 January 2013. An episode of high CH4 concentration of up to ~3 ppm has been observed and analyzed with the help of meteorological parameters combined with back trajectory calculation using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model of NOAA.

Reconstruction of combustion temperature and gas concentration distributions using line-of-sight tunable diode laser absorption spectroscopy

Abstract. Spatial temperature and gas concentration distributions are crucial for combustion studies to characterize the combustion position and to evaluate the combustion regime and the released heat quantity. Optical computer tomography (CT) enables the reconstruction of temperature and gas concentration fields in a flame on the basis of line-of-sight tunable diode laser absorption spectroscopy (LOS-TDLAS). A pair of H2O absorption lines at wavelengths 1395.51 and 1395.69 nm is selected. Temperature and H2O concentration distributions for a flat flame furnace are calculated by superimposing two absorption peaks with a discrete algebraic iterative algorithm and a mathematical fitting algorithm. By comparison, direct absorption spectroscopy measurements agree well with the thermocouple measurements and yield a good correlation. The CT reconstruction data of different air-to-fuel ratio combustion conditions (incomplete combustion and full combustion) and three different types of burners (one, two, and three flat flame furnaces) demonstrate that TDLAS has the potential of short response time and enables real-time temperature and gas concentration distribution measurements for combustion diagnosis.

Development of a tunable diode laser absorption sensor for online monitoring of industrial gas total emissions based on optical scintillation cross-correlation technique.

We report the first application of gas total emission using a DFB diode laser for gas concentration measurements combined with two LEDs for gas velocity measurements. In situ gas total emissions and particle density measurements in an industrial pipeline using simultaneous tunable diode laser absorption spectroscopy (TDLAS) and optical scintillation cross-correlation technique (OSCC) are presented. Velocity mean values obtained are 7.59 m/s (OSCC, standard deviation is 1.37 m/s) and 8.20 m/s (Pitot tube, standard deviation is 1.47 m/s) in a steel plant pipeline for comparison. Our experiments demonstrate that the combined system of TDLAS and OSCC provides a new versatile tool for accurate measurements of total gas emissions.

A photocoustic spectroscopy system for gas detection based on the multi-pass cell

Photo-acoustic spectroscopy gas detection technology has the advantages of high sensitivity, good selectivity, small size and real time monitoring and has been widely used in environmental monitoring, industrial production, medical diagnosis, biological technology and monitoring of power facilities. In this paper, a method to improve the sensitivity of photo-acoustic spectroscopy system is presented, which is combined with the technique of Herriott type multiple pass cell. In this experimental apparatus, the design of the experimental device can make the beam pass the cell 18 times. By comparing the signal of one time pass through the photoacoustic cell and the signal of 18 times passes pass through the photoacoustic cell, we can confirm that the signal is increased and this method is feasible.

Detection of gas temperature using a distributed feedback laser at O 2 absorption wavelength 760 nm

Direct absorption spectroscopy for gas temperature monitoring in a tube furnace is proposed over the temperature range 300–900 K with intervals of 100 K. This detecting technique is based on the relationship between two lines’ absorption strength and temperature. The gas temperature can be inferred from the ratio of the integrated spectral area of the oxygen absorption features measured with a distributed feedback diode laser near 760 nm. Compared with the thermocouple measured results, the direct absorption spectroscopy approach also provides a temporal resolution. The results show that the accuracy is better at low temperatures than at high temperatures. In the future, we hope to improve the detection accuracy and demonstrate the utility of the diode laser absorption sensors operating for active combustion diagnostics and optimizations, simultaneously.

A broadly tunable mid-infrared laser system based on difference frequency generation in periodically poled lithium niobate

A room-temperature broadly tunable mid-infrared laser system based on quasi-phase-matched (QPM) difference frequency generation (DFG) in a periodically poled LiNbO3(PPLN) crystal is developed. The continuous wave mid-infrared coherent radiation from 3.2 to 3.7µm with a maximum output power of about 1µW has been generated. Performance characteristics of the mid-infrared laser system has been carried out.

Development of a broadly tunable mid-infrared laser source for highly sensitive multiple trace gas detection

A room-temperature broadly tunable mid-infrared difference frequency laser source for highly sensitive trace gas detection has been developed recently in our laboratory. The mid-infrared laser system is based on quasi-phase-matched (QPM) difference frequency generation (DFG) in a multi-grating, temperature-controlled periodically poled LiNbO3 (PPLN) crystal and employs two near-infrared diode lasers as pump sources. The mid-infrared coherent radiation generated was tunable from 3.2 to 3.7 mum with an output power of about 100 muW. By changing one of the pump laser head with another wavelength range we can readily obtain other needed mid-infrared wavelength range cover. Performance of the mid-infrared laser system and its application to highly sensitive spectroscopic detection of CH4, HCI, CH2O and NO2 has been carried out. A multi-reflection White cell was used in the experiment gaining ppb-level sensitivity. The DFG laser system has the features of compact, room-temperature operation, narrow line-width and broadly continuous tunable range for potential applications in industry and environmental monitoring.