H\"ansch--Couillaud locking of Mach--Zehnder interferometer for carrier removal from a phase-modulated optical spectrum

TL;DR

This paper presents a method for stabilizing a Mach--Zehnder interferometer using H"ansch--Couillaud locking to effectively separate carrier and sidebands in phase-modulated lasers, with analysis and experimental validation.

Contribution

It introduces a detailed analysis of polarization-dependent phase delays in the interferometer and demonstrates a robust locking technique for carrier removal in phase-modulated spectra.

Findings

Successful stabilization of the interferometer with H"ansch--Couillaud method

Effective separation of carrier and sidebands achieved

Proposed extension using heterodyne detection

Abstract

We describe and analyse the operation and stabilization of a Mach--Zehnder interferometer, which separates the carrier and the first-order sidebands of a phase-modulated laser field, and which is locked using the H\"ansch--Couillaud method. In addition to the necessary attenuation, our interferometer introduces, via total internal reflection, a significant polarization-dependent phase delay. We employ a general treatment to describe an interferometer with an object which affects the field along one path, and we examine how this phase delay affects the error signal. We discuss the requirements necessary to ensure the lock point remains unchanged when phase modulation is introduced, and we demonstrate and characterize this locking experimentally. Finally, we suggest an extension to this locking strategy using heterodyne detection.