Cryogenic silicon Fabry-Perot resonator with Al0.92Ga0.08As/GaAs mirror coatings

authored by
Jialiang Yu
supervised by
Christian Lisdat

The sensitivity and stability of today's most precise optical interferometers, like gravitational wave detectors and ultra-stable lasers, are fundamentally limited by thermodynamically induced length fluctuations of high-reflectivity mirror coatings. Among them, Brownian thermal noise related to internal friction is the dominant contribution and can be reduced by using coating materials with lower mechanical loss. Owing to their low mechanical losses, AlGaAs/GaAs crystalline mirror coatings are expected to reduce this limit set by conventional dielectric coatings as demonstrated from a room temperature measurement. However, due to the high noise contributions from other resonator constituents in previous study, accurate characterization of the noise of crystalline coatings has yet been possible. In this work, the first detailed study on the spatial and temporal noise properties of crystalline coatings at an unprecedented level of precision is presented. This was achieved by using these novel coatings in a cryogenic silicon Fabry-Perot resonator operating at a temperature of 124 K and at a wavelength of 1.5 µm. To observe the expected low fractional frequency instability of mod σ_y=1x10^-17 imposed significant challenges in suppressing technical noise contributions. With methods and experimental setups described in this work, technical noise contributions were suppressed to a level well below the predicted coating noise. Nevertheless, the measured frequency was significantly higher than the predicted thermal and the total technical noise, which indicates the existence of excess noise in crystalline coatings. To disentangle the different excess noise sources, a sophisticated interrogation scheme, which investigates spatiotemporal correlations between different cavity eigenmodes by stabilizing two independent lasers simultaneously on the resonator, was developed. With this interrogation scheme, noise mechanisms related to the large birefringence mode splitting in these coatings were discovered. Upon a step change of optical power, anticorrelated frequency transient responses between the two birefringence-induced polarization eigenmodes of the silicon resonator were measured. The frequency noise induced by power fluctuations from this photo-birefringent effect was reduced to a neglectable level by active stabilization of optical power. However, anticorrelated spontaneous frequency fluctuations between the two polarization eigenmodes were still observed, indicating intrinsic birefringence fluctuations. To cancel this dominant excess noise - birefringent noise - in the crystalline coating, a dual-frequency locking technique was developed to stabilize the laser to the average of both polarization eigenmodes. With this technique, the expected low Brownian thermal noise was verified, but at the same time, this revealed another novel global excess noise with a correlation length larger than the mode diameter of 1 mm. This excess noise currently limits the frequency stability of the new cryogenic silicon resonator at a level comparable to dielectric coatings. Due to its large correlation length, increasing the beam size will only marginally reduce the noise level. In future ultra-sensitive interferometers using similar coatings based on semiconductor materials, these novel noise contributions discovered in this thesis must be carefully considered.

QUEST-Leibniz Research School
Doctoral thesis
No. of pages
Publication date
Publication status
Electronic version(s) (Access: Open)

Details in the research portal "Research@Leibniz University"