Low-noise laser frequency locking with a directly modulated microresonator

It is a common requirement to lock the frequency of a laser to an optical cavity resonance in experimental physics research. While the Pound-Drever-Hall technique is among the most popular methods, it requires the use of auxiliary phase modulators, which increases the complexity and cost. Moreover, the laser-locking noise caused by unwanted residual amplitude modulation within the modulator often limits the achievable locking stability. In this work, we demonstrate an alternative approach that negates the need for any external modulators, and inherently mitigates the residual amplitude noise. We directly apply fast electro-optic modulation to a monolithic lithium niobate microresonator to efficiently generate laser sidebands at frequencies much higher than the optical resonance bandwidth. Such a modulation allows us to generate low-noise laser-locking signals after appropriate photodetection and demodulation. Our noise characterization and frequency instability measurement show that the direct modulation technique renders a locking instability that is one order of magnitude better than that of the conventional Pound-Drever-Hall technique. The residual locking instability is also numerically analyzed and experimentally verified.