Simple High-Bandwidth Sideband Locking with Heterodyne Readout

TL;DR

This paper introduces a robust, high-bandwidth sideband laser locking method using heterodyne readout, suitable for low probe power applications, achieving over 3.5 MHz feedback bandwidth and potential for exceeding 6 MHz.

Contribution

It demonstrates a novel sideband locking technique with high bandwidth and simplified setup, eliminating the need for a second laser or additional modulators.

Findings

Achieved 3.5 MHz feedback bandwidth with a single integrator.

Model predicts potential bandwidth exceeding 6 MHz with optimized design.

Uses off-resonance optical carrier for alignment-free heterodyne readout.

Abstract

We present a robust sideband laser locking technique that is ideally suited for applications requiring low probe power and heterodyne readout. By feeding back to a high-bandwidth voltage controlled oscillator, we lock a first-order phase-modulation sideband to a table-top high-finesse Fabry-Perot cavity, achieving a feedback bandwidth of 3.5 MHz with a single integrator, limited fundamentally by the signal delay. The directly measured transfer function of the closed feedback loop agrees with a model assuming ideal system components, and from this we suggest a modified design that should realistically achieve a bandwidth exceeding 6 MHz with a near-causally limited feedback gain of $4\times 10^7$ at 1 kHz. The off-resonance optical carrier is used for alignment-free heterodyne readout, alleviating the need for a second laser or additional optical modulators.