<title>Acousto-optic spectrometers for millimeter and submillimeter wavelength radio astronomy</title>

Abstract

Radiometer spectrometers are used in millimeter and submillimeter wavelength radio astonomy for the spectral measurement of molecular transitions. The spectrum of interest spans 10's of GHz and the measurement time is large in order to obtain adequate signal-to- noise ratio. The low power/channel and simplicity of acousto-optic technology has led to the current development of acousto-optic spectrometers (AOS) with 1 GHz bandwidth and 1,000 channels in small, low-power consuming packages. The recent development of multichannel AOS's, utilizing multichannel Bragg cell technology, provides increased spectral coverage, reduction in overall data acquisition time, and accommodation and multibeam antennas. This paper describes the design features utilized in this current generation AOS's that provide them with the stability needed to compete favorably with more conventional filter bank spectrometers. Specific attention is given to the design of the key optoelectronic elements in these AOS' including thermally stabilized single-mode semiconductor laser diode light sources, polarization switching single and multichannel Bragg cells, photodetector arrays, and optics, as well as to the design of the system data acquisition electronics. Suppression of system noise, including laser speckle, is also discussed. The performance of these AOS's for specific radio astronomy spectroscopy applications is also described.