Frequency stability of cryogenic silicon cavities with semiconductor crystalline coatings

TL;DR

This study investigates the frequency stability of cryogenic silicon cavities with crystalline coatings, revealing birefringent noise issues and demonstrating noise cancellation techniques, showing potential for improved optical oscillator performance.

Contribution

It provides the first measurements of birefringent noise in fully crystalline cryogenic cavities and introduces a method to cancel this noise, advancing the understanding of crystalline coating stability.

Findings

Birefringent noise causes anti-correlated frequency fluctuations.

A noise cancellation scheme effectively reduces birefringent noise.

Crystalline coatings achieve stability comparable to dielectric coatings.

Abstract

State-of-the-art optical oscillators employing cryogenic reference cavities are limited in performance by the Brownian thermal noise associated with the mechanical dissipation of the mirror coatings. Recently, crystalline Al$_{1-x}$Ga$_{x}$As/GaAs coatings have emerged as a promising candidate for improved coating thermal noise. We present measurements of the frequency noise of two fully crystalline cryogenic reference cavities with Al$_{0.92}$Ga$_{0.08}$As/GaAs optical coatings. We report on previously unmeasured birefringent noise associated with anti-correlated frequency fluctuations between the polarization modes of the crystalline coatings, and identify variables that affect its magnitude. Comparing the birefringent noise between the two cryogenic reference cavities reveals a phenomenological set of scalings with intracavity power and mode area. We implement an interrogation scheme that cancels this noise by simultaneous probing of both polarization modes. The residual noise remaining after this cancellation is larger than both cavities thermal noise limits, but still lower than the instabilities previously measured on equivalent resonators with dielectric coatings. Though the source of these novel noise mechanisms is unclear, we demonstrate that crystalline coatings can provide stability and sensitivity competitive with resonators employing dielectric coatings.