Kerry Vahala's current research is focused on a whole range of resonator structures and applications that have been made possible by a new process developed by his team that eliminates the requirement to reflow (melt) the silica to smooth out the dielectric surface. Kerry's team has demonstrated resonators having Q factors of nearly 1 billion. Even more important is that size and shape control are now possible using this design. This has opened up access to a wide range of applications. Also, the seminconductor processing method developed to create these UHQ devices lends itself to a new class of waveguides with record low optical loss.

Kerry Vahala and his team created the highest Q-factor chip-based resonators. Like a tuning fork for light, optical resonators have a characteristic set of frequencies at which it is possible to confine light waves. At these frequencies, optical energy can be efficiently stored for lengths of time characterized by the resonator Q factor, roughly the storage time in cycles of oscillation. In the last ten years there has been remarkable progress in boosting this storage time in micro and millimeter-scale optical resonators. Chip-based devices have attained Q factors of nearly 1 billion and micro-machined crystalline devices have achieved Qs exceeding 100 billion. The long, energy-storage time and small form factor of these ultra-high-Q (UHQ) resonators enable access to an amazingly wide range of nonlinear phenomena and creation of laser devices with remarkable properties. Also, new science results from radiation-pressure coupling of optical and mechanical degrees-of-freedom in the resonators themselves.

During the first part of this two-workshop series on Optical Frequency Combs for Space Applications, the participants identified 29 applications and mission concepts where frequency combs might play a critical, enabling role for achieving new science goals. The second workshop, that took place on February 8-11, 2016 focused on some of these applications and explored the performance requirements and technology development needed for both the OFC and, where appropriate, the overall spacecraft and mission design. An emphasis was placed on near-term applications to guide the OFC development path, while still identifying technology development goals for longer-term missions.

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.