Optical Frequency Combs for Space Applications

An Optical Frequency Comb (OFC) is an optical spectrum consisting of uniformly spaced lines. The stability of the line spacing is controlled by a radio frequency or microwave source. Moreover, if the comb is spectrally broadened to encompass an octave of bandwidth, it is also possible to "self-reference" the comb, endowing it with absolute frequency stability at a Hertz level. It is accordingly a very precise spectroscopic tool for measuring different frequencies of light and is sometimes referred to as an optical ruler. Additionally, comb lines are phase coherent so that phase information can be transferred or measured across a very broad band of optical frequencies. OFCs are also sometimes referred to as optical clockworks because they can relate an optical frequency standard to an electronic one, enabling signal processing with fast electronics, yet with the precision afforded by optical frequencies. OFCs are now central to a new generation of optical clocks that are 100 times more accurate than today’s best time-keeping systems, enabling new capabilities in communication, navigation, and advancement of fundamental science. Because of the unique and powerful features of OFCs, the Keck Institute for Space Studies sponsored two workshops on Optical Frequency Combs for Space Applications at the California Institute of Technology in November of 2015 and February of 2016. The purpose of these workshops was to formulate space applications and mission concepts enabled by optical frequency comb technology and to identify high priority technology challenges and gaps that need to be addressed to implement these missions. This was accomplished by bringing together a diverse group of experts in OFC technology, space application specialists, and potential customers in the areas of astronomy and astrophysics, navigation, laser interferometry, Earth and planetary science, and instrumentation development. Workshop participants suggested 29 applications potentially enabled or significantly enhanced by the use of OFCs. These concepts spanned four general categories in the areas of spectroscopy, fundamental physics, astronomy, and technology. Four specific mission concepts were explored in more depth during the second workshop because of their potential science return. These concepts were 1) a Space-Time Observatory designed around a distributed network of optical clocks for use in gravitational wave detection, Dark Matter experiments, and a worldwide time standard for laboratory science; 2) The Alpha Centauri Reconnaissance Mission to demonstrate OFC technology on a small explorer-class spacecraft to achieve the highest possible Doppler shift measurement precision for radial velocity determination of exoplanetary mass and cosmological expansion, and also serve as a critical pathfinder for either a LUVOIR or HabEx observatory1; 3) a Comb Occultation Cubesat Observatory (COCO) for performing CubeSat-scale planetary atmospheric occultation measurements at Earth, Mars, and other solar system targets, thereby enabling fast, broadband, simultaneous measurement of multiple gas species with either active or passive illumination; and 4) Comb-enabled High Angular Resolution Imaging (CHARLI), a ground-based application using OFC-local oscillators for heterodyne detection and interferometry in the mid-infrared to allow imaging of complex scenes in astronomy on scales never imaged before.