Abstract

Astronomical spectroscopy in the far infrared (FIR) between 150 and 400 μm has previously relied on instruments that use interferometric techniques and are consequently wavelength sensitive. Such spectrometers are generally limited to resolutions below 1 part in 10⁴ because of physical size constraints and the extended angular sizes of observed sources. Heterodyne spectrometers, on the other hand, are frequency sensitive and not subject to these resolution constraints. Radio astronomers have long used heterodyne techniques to achieve resolutions exceeding 1 part in 10⁶, which are required to resolve velocity-broadened profiles of atomic and molecular lines in interstellar gas clouds. The development of laser heterodyne techniques now makes equivalent observations in the FIR possible. Spectrometers using optically pumped FIR lasers as local oscillators (LOs), and GaAs diodes as mixers now operate regularly aboard NASA’s Kuiper Airborne Observatory. The laser LOs oscillate on fixed molecular transitions, but spectroscopic tunability can be achieved within the intermediate frequency output bandwidth of the mixer (generally up to 15 GHz). Heterodyne spectroscopy has proved valuable for astronomical observations of the fine-structure line emission from neutral atomic carbon (809 GHz) and ionized carbon (1900 GHz) as well as the J = 7-6 (807-GHz) and J = 17-16 (1956-GHz) rotational lines of CO.

© 1988 Optical Society of America