The development of tera-hertz sources and receivers recently has given rise to a desire to do spectroscopy on molecular rotational spectra in this region. In particular the use of semiconductor photo-mixers provides practical packaging concepts and good sensitivity for building absorption spectrometers. This paper deals with some data processing concepts associated with absorption spectra from such spectrometers. Specifically, the concepts of multivariate analysis developed and applied over the last 25 years to infrared spectroscopy appear to be very useful to the analysis of tera-hertz absorption spectra. These concepts are particularly useful to address problems associated with: spectra baseline variations, species concentration estimates and spectral cross talk assessments when multiple species are present in the measurement sample. This latter element is of particular importance in tera-hertz spectroscopy when exploring multiple species with overlapping rotation spectral bands.
In this paper some basics of multivariate analysis are reviewed. The baseline signatures or backgrounds of our photo-mixer absorption spectrometer are described and processed. Algorithm results are presented that remove backgrounds and other instrument spectral artifacts. Finally, a simulated example of separating overlapping spectra is presented.
For many years Hughes Danbury Optical Systems has ben developing algorithms for detecting trace gases in the atmosphere using hyperspectral data processing techniques. We have shown in the past that our orthogonal background suppression (OBS) algorithms are effective for measuring the column density-thermal radiance contrast product of a gas plume in the atmosphere at some distance from a passive thermal-IR emission spectrometer. The algorithm facilitates the detection of the target signal in the presence of low signal to spectral clutter ratio.Our current work shows that using the non-linear absorption features of a target gases' spectral signature, coupled with our OBS algorithm, we can separate column density-thermal radiance contrast product and obtain absolute plume column density and plume temperature. The OBS algorithms are straight forward and allow detection near theoretical random noise limits. The efficacy of our novel technique is demonstrated using simulations and field data.
Electromagnetic windows generally require low dc resistance for radar performance and high optical throughput for visibility. For obtaining high optical throughput and desired EMI attenuation, the use of metal grids on windows should be considered. This paper reviews many of the properties of EMI grids that are useful to an EMI window designer.
A model of back illuminated CCDs is presented that concentrates on the fields that can exist at or near the illuminated surface. The parameters that influence these fields and how they affect the device quantum efficiency are explored. We also present a design for a thinned CCD using ion implantation and KrF excimer laser anneal that is tuned for enhanced UV response.
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