Ultrastable Laser Based on a Room Temperature ULE Cavity with Crystalline AlGaAs Coatings

Brownian thermal noise of dielectric mirror coatings limits the fundamental frequency stability of both table top ultra-stable lasers based on Fabry-Perot optical resonators and of large-scale gravitational wave detectors. Crystalline AlGaAs mirror coatings with low mechanical losses reduce this limit. However, these birefringent coatings lead to a splitting of the cavity eigenmodes. At temperatures from 124 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{K}$</tex> to 297 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{K}$</tex> a significant dependence on intracavity power and external illumination was observed [1]–[3] (photo-birefringent effect). In addition, unexpected noise contributions above the thermal noise level have been observed on cryogenic silicon resonators [1], [4], and so far, no optical resonators with AlGaAs coatings have come close to the Brownian thermal noise limit [5]–[7].