Ultrastable lasers: investigations of crystalline mirrors and closed cycle cooling at 124 K

TL;DR

This study explores the stability of crystalline AlGaAs/GaAs optical coatings at various cryogenic temperatures, revealing effects beyond known photo-thermo-optic phenomena and demonstrating a low-maintenance cooling system for ultra-stable lasers.

Contribution

It provides new insights into the temperature-dependent optical properties of crystalline coatings and introduces a closed cycle cooling system for ultra-stable laser cavities at 124 K.

Findings

Resonance frequency responses depend on intracavity light intensity.

Photo-modified birefringence observed above GaAs bandgap.

Successful implementation of a low-maintenance cryogenic cooling system.

Abstract

We have investigated crystalline AlGaAs/GaAs optical coatings with three ultra-stable cavities operating at 4 K, 16 K, 124 K and 297 K. The response of the resonance frequencies of cavities to variations in optical power indicates effects beyond the photo-thermo-optic effect observed in dielectric coatings. These effects are strongly dependent on the intensity of the intracavity light at 1.5~\textmu m. When the rear side of the mirrors is illuminated with external light, we observe a prominent photo-modified birefringence for photon energies above the GaAs bandgap, which points to a possible mechanism relating our observations to the semiconductor properties of the coatings. Separately, we also present a low maintenance evolution of our 124 K silicon cavity system where the liquid nitrogen based cooling system is replaced with closed cycle cooling from a pulse-tube cryo-cooler.