Speaker
Description
Ultrastable lasers locked to Fabry-Perot resonators are an important part of optical clocks, providing narrow bandwidth radiation for the excitation of clock transitions and acting as flywheel during deadtimes. The best systems operated at both room and cryogenic temperatures are limited by Brownian thermal noise of the dielectric mirror coatings [1-2]. Crystalline AlGaAs mirror coatings due to their low mechanical loss reduce this limit. However, the original birefringence of these coatings is unexpectedly modified by light which introduces technical photo-birefringent noise due to laser power fluctuation [3-4] as well as power driven spontaneous fluctuations of the birefringence (i.e. birefringent noise) [3,5]. After suppression of these noise contributions, the achieved frequency stability is still above the expected low thermal noise floor [3,5]. The source of this noise (global excess noise) is still yet to be further investigated.
We have further investigated the birefringent property of these coatings on a 48-cm long ULE cavity at room temperature, in particular on how light at different wavelengths influences the static birefringence of the crystalline coatings. Two independent lasers with wavelength of 1542 nm are locked to fast and slow polarization eigenmodes of this cavity respectively as in previous investigations. A step in intracavity power modifies the static birefringence of the mirror coatings on timescales of a second, with faster response at higher final intracavity power [3,6]. We also investigate the modification of the coating birefringence from illumination by external LED light at different photon energies. Our results point to a two photon mechanism for photon energy below the bandgap of GaAs/AlGaAs or single photon mechanism at energies above the bandgap. By compensating the birefringent response with the polarization-independent thermal response on fast polarization mode, a fractional frequency instability of 4.8 x 10-17 (expressed in modified Allan deviation) could be demonstrated from a three-corned hat comparison with two other independent lasers locked to ultrastable cryogenic silicon cavities.
Our findings might help to understand the physical mechanisms of the photo-induced birefringent effect of these coatings, and thus enable improved designs of crystalline coatings, finally realizing their envisioned low Brownian thermal noise.
[1] M. Schioppo et al., Nat. Commun., 13, 212 (2022)
[2] D. G Matei et al., Phys. Rev. Lett., 118, 263202 (2017)
[3] J. Yu et al., Phys. Rev. X, 13, 041002 (2023)
[4] B. Kraus et al., Opt. Lett. 50, 658-661 (2025)
[5] D. Kedar et al., Optica, 10, 464 (2023)
[6] C. Y. Ma et al., J. Phys.: Conf. Ser. 2889, 012055 (2024)
