Mid-infrared (MIR) laser spectroscopy is a powerful analytical tool for trace gases sensing, since a number of atmospheric pollutants and greenhouse gases have strong fundamental vibrational transitions within this spectral range. Here, we report the development of mid-infrared spectroscopy techniques coupled with a broadband tunable external-cavity (EC) mode-hop-free quantum cascade laser (QCL) operating between 6.96 and 8.85 μm. The ECQCL sensor was evaluated for quantitative and qualitative analysis of volatile organic compound (VOC) components. For signal processing, a self-established spectral analysis model integrated with various algorithms was developed for VOC spectral analysis. A good agreement was obtained between the experimentally observed spectra and the standard reference spectra taken from Pacific Northwest National Laboratory (PNNL) database.
Quartz enhanced photoacoustic spectroscopy (QEPAS) is an extremely effective tool for the detection and quantification of trace gases, which offers advantages of fast response, high sensitivity and high resolution. In this paper, a gas sensor based on quartz-enhanced photoacoustic detection and an external cavity quantum cascade laser (ECQCL) was realized and characterized for acetone measurement. Photoacoustic signal dependence on gas pressure and laser operating parameters were studied to optimize sensor performance. In addition, potential approaches and detection schemes on improving sensor performance were also discussed.
An adaptive optics based non-null interferometry (ANI) is proposed for optical free form surfaces testing, in which an open-loop deformable mirror (DM) is employed as a reflective compensator, to compensate various low-order aberrations flexibly. The residual wavefront aberration is treated by the multi-configuration ray tracing (MCRT) algorithm. The MCRT algorithm based on the simultaneous ray tracing for multiple system models, in which each model has different DM surface deformation. With the MCRT algorithm, the final figure error can be extracted together with the surface misalignment aberration correction after the initial system calibration. The flexible test for free form surface is achieved with high accuracy, without auxiliary device for DM deformation monitoring. Experiments proving the feasibility, repeatability and high accuracy of the ANI were carried out to test a bi-conic surface and a paraboloidal surface, with a high stable ALPAOTM DM88. The accuracy of the final test result of the paraboloidal surface was better than 1/20 Μ PV value. It is a successful attempt in research of flexible optical free form surface metrology and would have enormous potential in future application with the development of the DM technology.
A new type of tunable diode spectroscopy sensor based on an external cavity quantum cascade laser (ECQCL) and a quartz crystal tuning fork (QCTF) were used for quantitative analysis of volatile organic compounds. In this work, the sensor system had been tested on different gasoline sample analysis. For signal processing, the self-established interpolation algorithm and multiple linear regression algorithm model were used for quantitative analysis of major volatile organic compounds in gasoline samples. The results were very consistent with that of the standard spectra taken from the Pacific Northwest National Laboratory (PNNL) database. In future, The ECQCL sensor will be used for trace explosive, chemical warfare agent, and toxic industrial chemical detection and spectroscopic analysis, etc.
Breath analysis is an attractive method for disease diagnosis and therapeutic monitoring, due to its high potential for non-invasive medical diagnostics. Among numerous analysis techniques, tunable diode laser-based absorption spectroscopy (TDLAS) is an excellent method for detection of gas concentration, since it presents advantages of high sensitivity, good selectivity, fast response and high temporal resolution. In this study, state-of-the-art quantum cascade laser based gas sensor is demonstrated as a promising new tool for noninvasive, real-time identification and quantification of trace gases in human breath for clinical uses. Details of selection of spectroscopic parameters and primary lab studies conducted on CO, H2O and N2O molecules in exhaled breath are presented, together with suggestions on the future direction of this challenging analytical field.
A sensitive open-path gas sensor employing a continuous-wave (CW) distributed feedback (DFB) quantum cascade laser (QCL) and direct absorption spectroscopy (DAS) was demonstrated for simultaneously measurements of atmospheric CO and N<sub>2</sub>O. Two interference free absorption lines located at 2190.0175 cm<sup>-1</sup> and 2190.3498 cm<sup>-1</sup> were selected for CO and N<sub>2</sub>O concentration measurements, respectively. The Allan variance analysis technique was performed to investigate the long-term performance of the QCL sensor system. The results indicate that a detection limit of 9.92 ppb for CO and 7.7 ppb for N<sub>2</sub>O with 1-s integration time were achieved, which can be further improved to 1.5 ppb and 1.1 ppb by increasing the average time up to 80 s.
A tunable diode laser absorption spectroscopy (TDLAS) system based on a broad band external cavity quantum cascade laser (ECQCL) near 7.78 μm was used to study volatile organic compounds (VOCs) measurements. Instead of using a standard infrared mercury cadmium telluride (MCT) detector, a quartz crystal tuning fork (QCTF) as a light detector was successfully used for laser signal detection. Fast Fourier transform (FFT) was used to extract vibration intensity information of QCTF. Primary results indicate that the new developed system has a good reproducibility, and a good agreement was obtained by comparing with data taken from standard spectroscopic database.
Mid-infrared laser spectroscopy provides an ideal platform for trace gas sensing applications. Despite this potential, early MIR sensing applications were limited due to the size of the involved optical components, e.g. light sources and sample cells. A potential solution to this demand is the integration of hollow fiber waveguide with novelty quantum cascade lasers.Recently QCLs had great improvements in power, efficiency and wavelength range, which made the miniaturized platforms for gas sensing maintaining or even enhancing the achievable sensitivity conceivable. So that the miniaturization of QCLs and HWGs can be evolved into a mini sensor, which may be tailored to a variety of real-time and in situ applications ranging from environmental monitoring to workplace safety surveillance. In this article, we introduce QCLs and HWGs, display the applications of HWG based on QCL gas sensing and discuss future strategies for hollow fiber coupled quantum cascade laser gas sensor technology.
We have developed a simple but robust method based on the Savitzky-Golay filter for real-time processing tunable diode laser absorption spectroscopy (TDLAS) signal. Our method was developed to resolve the blindness of selecting the input filter parameters and potential signal distortion induced in digital signal processing. By applying the developed adaptive Savitzky-Golay filter algorithm to the simulated and experimentally observed signal and comparing with the wavelet-based de-noising technique, the results indicate that the new developed method is effective in obtaining high-quality TDLAS data for a wide variety of applications including atmospheric environmental monitoring and industrial processing control.