Abstract
Ultralow-noise yet tunable lasers are a revolutionary tool in precision spectroscopy, displacement measurements at the standard quantum limit, and the development of advanced optical atomic clocks. Further applications include lidar, coherent communications, frequency synthesis, and precision sensors of strain, motion, and temperature. While all applications benefit from lower frequency noise, many also require a laser that is robust and compact. Here, we introduce a dual-microcavity laser that leverages one chip-integrable silica microresonator to generate tunable 1550 nm laser light via stimulated Brillouin scattering (SBS) and a second microresonator for frequency stabilization of the SBS light. This configuration reduces the fractional frequency noise to 7.8×10^(−14) 1/√Hz at 10 Hz offset, which is a new regime of noise performance for a microresonator-based laser. Our system also features terahertz tunability and the potential for chip-level integration. We demonstrate the utility of our dual-microcavity laser by performing spectral linewidth measurements with hertz-level resolution.
Highlights
The stabilization of a continuous-wave laser to an isolated, high-Q Fabry–Perot cavity is the primary technique today for achieving optical frequency references exhibiting the highest levels of spectral purity
The pump laser we use is a commercial integrated planar external-cavity diode laser assembled within a standard 14-pin butterfly package that we couple to the microdisk through a fiber taper
The frequency tuning of the stimulated Brillouin scattering (SBS) laser is accomplished by sending the pump through a commercial fiber-pig-tailed semiconductor optical amplifier (SOA) operating in saturation, whose output power is controllable via the SOA operating current [40]
Summary
The stabilization of a continuous-wave laser to an isolated, high-Q Fabry–Perot cavity is the primary technique today for achieving optical frequency references exhibiting the highest levels of spectral purity. Taken together with advanced photonic heterogeneous integration [20], it appears feasible that active laser and electro-optic components could be integrated with high-Q microresonators to realize a frequency-stabilized and narrow-linewidth laser on a silicon chip. It is within this context that we demonstrate a prototype dual-microcavity Brillouin laser that is robust, chip-compatible, and sufficiently low noise for hertz-level applications. We achieve a frequency noise of 200 Hz2∕Hz at 10 Hz offset, which corresponds to a singlesideband phase noise of 0 dBc/Hz on the 193 THz carrier
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