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More noise than expected

Limiting the sensitivity of interferometers and ultra-stable lasers

PTBnews 3.2023
Especially interesting for

Laser technology

Gravitational wave detection

Nano-optomechanical systems

Highly reflecting mirrors are an essential component of the most precise optical interferometers, which are used, for example, for the detection of gravitational waves or in optical atomic clocks (to reduce the linewidth of lasers). These applications are inherently limited by Brownian noise, where the thermal motion of the particles in the mirror leads to fluctuations of the reflected light wave. It is expected that novel GaAs/AlGaAs Bragg reflectors will substantially reduce this noise contribution. Due to newly discovered noise processes, however, the improvement at low temperatures is only small compared to conventional mirrors.

The strong anti-correlation of the fluctuations of a mirror surface for light with polarization along the fast and slow axes of the crystalline mirror coating is clearly visible. Its average shows a clearly reduced coating noise which, however, is still higher than the expected Brownian noise level. The photo shows the silicon resonator used for the measurements.

Das Foto zeigt den für die Messungen verwendeten Siliziumresonator.

The length fluctuations due to Brownian noise increase with the mechanical damping of the materials used in the interferometer. Established dielectric mirror coatings have a considerably higher damping than the other materials used. We therefore expect mirror coatings made of novel materials with small damping (e.g. monocrystalline GaAs/AlGaAs Bragg reflectors) to show a clear reduction of these length fluctuations.

In cooperation between JILA (Boulder, USA) and PTB, these mirrors were tested at a wavelength of 1.5 μm in cryogenic optical resonators at temperatures of 124 K, 16 K and 4 K. Monocrystalline silicon was used as a spacer. These systems allowed us to reduce technical noise sources to such an extent that the only remaining limit on the length and frequency stability of the resonators can be assumed to be the Brownian noise of the conventional mirrors. Comparable systems with conventional dielectric mirror coatings are already being used for frequency stabilization of the best lasers currently available.

Although a reduction of the Brownian noise could be confirmed for the novel crystalline mirrors, we observed other, unexpectedly large fluctuations. These opposing, strongly anti-correlated fluctuations of the two polarization eigenmodes of the resonators could be traced to fluctuations of the intrinsic birefringence of these mirror coatings. Averaging the fluctuations of the two modes reduces the noise, but it still remains clearly higher than the Brownian fluctuations of the novel mirror coatings.

The physical causes for the two new noise processes are still unknown. Initial evidence points to the semiconducting properties of the coatings.


Uwe Sterr
Department 4.3
Quantum Optics and Unit of Length
Phone: +49 531 592-4310
Opens local program for sending emailuwe.sterr(at)ptb.de

Scientific publication

D. Kedar, J. Yu, E. Oelker, A. Staron, W. R. Milner, J. M. Robinson, T. Legero, F. Riehle, U. Sterr, J. Ye: Frequency stability of cryogenic silicon cavities with semiconductor crystalline coatings. Optica, 10, 464–470 (2023)

Opens external link in new windowhttps://dx.doi.org/10.1364/OPTICA.479462