From ultra-noisy to ultra-stable: optimization of the optoelectronic laser lock

TL;DR

This paper presents a compact, thermally noise-limited method for locking a semiconductor laser to an ultrastable cavity, achieving unprecedented noise suppression and phase stability suitable for integrated photonics applications.

Contribution

It introduces a novel optoelectronic laser locking technique with enhanced noise suppression and a feedforward noise correction method surpassing current models.

Findings

Over 140 dB suppression of laser noise at 10 Hz offset.

Achieved phase noise level of -120 dBc/Hz at 200 kHz offset.

Demonstrated 60 dB additional noise rejection with feedforward correction.

Abstract

We demonstrate thermal-noise-limited direct locking of a semiconductor distributed feedback (DFB) laser to a sub-1 mL volume, ultrastable optical cavity, enabling extremely compact and simple ultrastable laser systems. Using the optoelectronic laser locking method, we realize over 140 dB suppression of the DFB free-running laser noise at 10 Hz offset, a level we estimate to be ~ 70 dB greater than Pound-Drever-Hall locking can provide, and reach a phase noise level of -120 dBc/Hz at 200 kHz offset. We also demonstrate a new feedforward noise correction method that improves the quality of the heterodyne beat with an optical frequency comb by providing another 60 dB of laser noise rejection - a level that is 15 dB greater than predicted by current models. With feedforward, we transfer the cavity thermal noise limit across the comb spectrum despite the fact that the cavity-locked laser itself is noisy. These results establish a simple, low noise, compact approach to ultrastable laser locking that is compatible with integrated photonics, with applications in low phase noise microwave generation, sensing, and satellite ranging.