Hailiang Shi

Raman spectroscopic detection using a two-dimensional spatial heterodyne spectrometer

Abstract. Spatial heterodyne Raman spectroscopy (SHRS) is a type of method for the detection of Raman spectra and can achieve a very high spectral resolution. SHRS has no moving parts and can be built with rugged, compact packages, making it extremely suitable for planetary exploration. However, if a high spectral resolution is needed, a traditional one-dimensional spatial heterodyne spectrometer cannot achieve a broad bandpass because it is limited by the number of pixels of the detector. In order to solve this, two-dimensional (2-D) SHRS can be used to broaden the bandpass. A breadboard of 2-D SHRS has been designed and built, and some artificial and natural targets have been tested to learn about the detection ability of 2-D SHRS. The results show that 2-D SHRS can be used to detect Raman signals scattered from liquid and solid targets. When the Raman scattered signal is strong, it can even detect targets in containers. The detection of anti-Stokes Raman shift for sulfur and carbon tetrachloride has also been tried, and the results show that 2-D SHRS has the ability to detect anti-Stokes Raman shift below 500  cm−1. The research may have a general implication in chemical analysis and planetary exploration.

Raman Spectroscopic Detection for Simulants of Chemical Warfare Agents Using a Spatial Heterodyne Spectrometer

Raman spectroscopic detection is one of the suitable methods for the detection of chemical warfare agents (CWAs) and simulants. Since the 1980s, many researchers have been dedicated to the research of chemical characteristic of CWAs and simulants and instrumental improvement for their analysis and detection. The spatial heterodyne Raman spectrometer (SHRS) is a new developing instrument for Raman detection that appeared in 2011. It is already well-known that SHRS has the characteristics of high spectral resolution, a large field-of-view, and high throughput. Thus, it is inherently suitable for the analysis and detection of these toxic chemicals and simulants. The in situ and standoff detection of some typical simulants of CWAs, such as dimethyl methylphosphonate (DMMP), diisopropyl methylphosphonate (DIMP), triethylphosphate (TEP), diethyl malonate (DEM), methyl salicylate (MES), 2-chloroethyl ethyl sulfide (CEES), and malathion, were tried. The achieved results show that SHRS does have the ability of in situ analysis or standoff detection for simulants of CWAs. When the laser power was set to as low as 26 mW, the SHRS still has a signal-to-noise ratio higher than 5 in in situ detection. The standoff Raman spectra detection of CWAs simulants was realized at a distance of 11 m. The potential feasibility of standoff detection of SHRS for CWAs simulants has been proved.

The Upper Atmosphere Wind Measurement Based on Doppler Asymmetric Spatial Heterodyne Interferometer

The wind detection in the upper atmosphere plays an important role in understanding the dynamics and actinochemistry, building the atmospheric dynamic model, offering the long term weather forecast and guaranteeing the development of aeronautics and astronautics. Doppler Asymmetric Spatial Heterodyne (DASH) technique is proposed to detect the wind speed in the upper atmosphere, depending on its wide field of view, large etendue, high spectrum resolution, static measurement and multiline capability. In this article the relationship between DASH optical path difference and the phase sensitivity and visibility is discussed. Then the principle prototype with large offset to detect Doppler shift of O2 A-band airglow is described. The interferometric system is simulated and designed. The system noise and temperature change of the environment is simulated and analyzed quantitatively. After that the detailed parameters of the interferometer system is designed according to the requirement. Based on the instrument an experimental system is established in the laboratory. The wind simulation detection is conducted to validate the performance of the instrument. From a group of measurements under different wind speed, a wind precision of 3 m/s is obtained. The result shows that DASH interferometer is suited for passive wind detection.

Optical Design and Simulation of Spatial Heterodyne Spectroscopy (SHS) for Mesospheric Temperature

The mesospheric region exists a lot of complex photochemistry reactions and dynamical processes, which plays an important role in affecting the dynamics in global circulation models and the safety for aircrafts. And gravity wave is a significant dynamical factor in thermodynamic structure and circulation structure. However, because of rare atmosphere in this region, the high-precision measurements on features of mesospheric temperature are insufficient due to a lack of measurement techniques. Design and performance parameters for a high spectral resolution, high spatial resolution in vertical direction, robust and CubeSat-scale spatial heterodyne spectrometer for mesospheric temperature detection are reported. According to the measured value of in-orbit payloads, the subsystem parameters of instrument are described in detail by using spatial heterodyne spectroscopy (SHS) in the O2 atmospheric-band (A-band) (0-0). By optical design and simulation for the instrument with high spectral resolution and spatial resolution in one dimension, we can get the quantitative relationship between the spectral stability and the retrieved temperature precision. For 0.5% relative spectral intensity stability, it has a theoretical temperature precision of 2 K by fitting three peaks, and the signal to noise ratio will be improved with a higher spectral resolution and more emission spectral peaks under the same equivalent noise condition. Finally, orbit attitude control system requirements can be estimated by considering orbit height and the instrument parameters. The scan is not accomplished through controlled nodding or rotation of the spacecraft during short exposure time, and the spectral background signals due to narrow Fraunhofer features and O2-A band absorption lines below 90 km is detectable at SHS spectral resolution. These results will provide a new method to get the global distribution of airglow in wide altitude range (40–130 km) information with simultaneously imaging technique for each view slice of the scene. It will provide basic data for studying the mesospheric temperature and gravity wave activity in the mesospheric atmosphere circulation models.

Study on the Performance of Spatial Heterodyne Spectrometer Influenced by the Field Widened Prism

Spatial heterodyne spectrometer(SHS) is based on Fourier transform spectroscopy, and field widening techniques of conventional FTS interferometers can also be applied to Spatial heterodyne Spectrometer(SHS) system. The field widened prism and spacers of SHS system should be adjusted so the reduction in the fringe contrast resulting from the fringe frequency on off-axis angle is less than 0.36, and it is one of the most important advantages to use field widened prism to increase the throughput for SHS system with high spectral resolution. Unlike the original method for the prism and spacer, the new method combines original field widened method and offers fabrication flexibility. Using this new method, we evaluate the theoretical modulation efficiency of the field widened interferometer for an experimental set up, including effect of the field view on the fringe frequency and subsequently on the spectral calibration accurate. To minimize the influence of the fringe frequency errors, the dispersion effects introduced by the prism are considered in the analysis. Finally, the new method and analysis are validated through a serial of simulations and experiments, and it gives a theoretical explanation of the spectrum shift and stretch in previous processing to recover the spectrum.