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
This Brief includes a Technical Support Package (TSP).
Optical Comb From a Whispering Gallery Mode Resonator for Spectroscopy and Astronomy Instruments Cal
(reference NPO-48135) is currently available for download from the TSP library.
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Overview
The document presents a technical support package from NASA's Jet Propulsion Laboratory (JPL) detailing an innovative optical comb technology developed for high-precision wavelength calibration in astronomical and astrophysical spectroscopic observations. This technology, referred to as an optical comb from a whispering gallery mode resonator (WGMR), aims to significantly enhance the accuracy of Doppler measurements, achieving precision better than 10 cm/s—an improvement of one hundred times over current methods.
The optical comb serves as a reference wavelength source, crucial for calibrating spectrographs used in various astronomical applications, including the search for exoplanets and the study of cosmological dynamics. The ideal calibration spectrum produced by this technology is characterized by a series of lines with known wavelengths, high resolution, uniform frequency spacing, and stability over long periods, all while maintaining a high signal-to-noise ratio.
The document outlines the technical aspects of the WGMR, which is compact and low-power, with a disk-shaped resonator capable of generating a 50 GHz spaced comb that is less than a millimeter in diameter. The entire system, including the resonator, a small diode laser, and a temperature control module, is designed to fit within a volume of less than one liter. The anticipated cost of the instrument is relatively low, estimated to be in the range of a couple of thousand dollars, making it accessible for various applications.
To achieve simultaneous locking of two frequency components of the comb to different atomic references, the document discusses the implementation of dispersion management techniques. This involves using mechanical deformation and temperature control to stabilize the frequencies, ensuring that the two control parameters remain uncoupled. The saturation absorption technique is employed to achieve frequency stability of better than 1x10^-10, equating to approximately 3 cm/s at near-infrared wavelengths.
Overall, this document highlights a significant advancement in optical technology that promises to enhance the capabilities of astronomical observations, providing researchers with a powerful tool for exploring the universe and improving our understanding of fundamental physics.
