William Loh, Adam A. Green, Frederick N. Baynes, Daniel C. Cole, Franklyn J. Quinlan, Hansuek Lee, Kerry J. Vahala, Scott A. Diddams
The tunable narrow-linewidth laser is a revolutionary metrology tool that is critical in precision spectroscopy and the determination of fundamental physical constants, displacement measurements at the 10-20 level, and the development of the most advanced optical atomic clocks. Further applications of narrow-linewidth lasers include LIDAR, coherent communications, novel frequency synthesis, trace gas sensing, and a variety of precision sensors that transduce the relevant physical parameter (e.g. strain, mechanical motion, temperature, etc.) into optical frequency or phase. While all of these measurements benefit from improved frequency noise performance, many applications also require a frequency-stable laser that is robust, compact and amenable to integration. Here we demonstrate a new paradigm for a narrow-linewidth laser that takes advantage of a chip-integrable silica microresonator to generate tunable laser light in the 1550 nm spectral region via low-noise stimulated Brillouin scattering (SBS). This SBS source is then frequency stabilized to a silica microrod resonator having quality factor of 0.9 billion, that acts to reduce the frequency noise two orders of magnitude to below 15 Hz/√Hz at 10 Hz offset. Beyond the demonstrated noise performance, which enters a new regime for a microresonator-based laser, our approach has the advantages of tunability across terahertz of bandwidth and the potential for full chip-level integration without compromising the performance. We demonstrate the utility of our dual microcavity laser by performing optical spectroscopy with hertz-level resolution.