A Cooled Optics Spectrometer for High Resolution Measurements in Support of Atmospheric Research

A high resolution, 3 meter focal length grating spectrometer utilizing liquid nitrogen cooled order sorting optics has been developed for laboratory spectroscopy of atmospheric constituents in the 2 μm to 30 μm spectral region, the spectrometer uses a Littrow optical design modified to allow either two or four diffractions off the 25 cm echelle grating. The theoretical resolution is 0.011 cm−1 for double pass and 0.006 cm−1 for four pass operation. The fundamental limitation in achieving high resolution in infrared grating spectroscopy has been the problem of high noise associated with high room temperature background. In order to perform measurements with sufficient signal to noise, grating spectrometers must generally be operated with slit widths substantially greater than the diffraction limit. This results in an increase in signal but with a loss in resolution. In the near infrared, where relatively low blackbody background photon fluxes occur, spectrometers with resolutions as high as 0.015 cm−1 have been used. In the thermal infrared, however, the resolution of grating spectrometers has been limited to about 0.05 to 0.10 cm−1 by the high blackbody backgrounds occurring in this region.