The most accurate astronomical data is available from space-based observations that are not impeded by the Earth’s atmosphere. Such measurements may require spectral samples taken as long as decades apart, with the 1 cm/s velocity precision integrated over a broad wavelength range. This raises the requirements specifically for instruments used in astrophysics research missions — their stringent wavelength resolution and accuracy must be maintained over years and possibly decades. Therefore, a stable and broadband optical calibration technique compatible with spaceflights becomes essential. The space-based spectroscopic instruments need to be calibrated in situ, which puts forth specific requirements to the calibration sources, mainly concerned with their mass, power consumption, and reliability.

A high-precision, high-resolution reference wavelength comb source for astronomical and astrophysics spectroscopic observations has been developed that is deployable in space. The optical comb will be used for wavelength calibrations of spectrographs and will enable Doppler measurements to better than 10 cm/s precision, one hundred times better than the current state-of-the-art.

The concept leverages the progress of wide-span frequency comb generation in frequency standards and metrology. The source consists of a crystalline whispering gallery mode (WGM) microresonator, a near-IR tunable single-frequency CW (continuous wave) laser, an FM (frequency modulated) spectroscopy unit, and control and stabilization electronics. The coupling in and out of the resonator is fiber-based through the evanescent waves. This approach is based on an external laser coupled to the Kerr-media WGM resonator.

This novel precision comb provides a new generation of super-stable, evenly spaced, and wideband wavelength calibration sources. In addition, this source does not age as the lamps do. Presently, this approach allows users to achieve an absolute accuracy of better than 10-12 per day when referenced to a suitable atomic transition.

The improved Doppler measurement accuracy and resolution will significantly enhance the current astronomy observation capability in exoplanet search and the study of cosmology dynamics.

This work was done by Dmitry V. Strekalov, Nan Yu, and Robert J. Thompson of Caltech for NASA’s Jet Propulsion Laboratory. NPO-48135

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Optical Comb From a Whispering Gallery Mode Resonator for Spectroscopy and Astronomy Instruments Cal

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This article first appeared in the February, 2012 issue of NASA Tech Briefs Magazine.

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