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Q: Fachverband Quantenoptik und Photonik

Q 24: Präzisionsmessungen II

Q 24.9: Talk

Tuesday, March 3, 2009, 18:30–18:45, Audi-A

High-order modes for reference cavities in optical clocks — •Björn Stein, Tanja E. Mehlstäubler, Ivan Sherstov, Maksim Okhapkin, Burghard Lipphardt, Christian Tamm, and Ekkehard Peik — Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany

Optical clocks have reached a performance level where the thermal noise of ULE reference cavities becomes a limitation to the observed instability. Options for improvement include cooling, low mechanical loss materials, elongating the resonator and enlarging the on-mirror spot area: By sampling a larger area, thermally driven mechanical fluctuations of the mirror surface are averaged down. Practical considerations of resonator size and fabrication tolerances prohibit a significant increase of the TEM₀₀ spot size. We investigate the use of high-order TEM_m,n modes to increase the effective spot area.

In existing reference resonators made from ULE, the mirror substrate is the dominant source of thermal noise. An improvement of the short-term instability by at least a factor of two seems feasible. Several new resonators now use different materials and the noise of the mirror coating is expected to be the future limitation in short term stability. Since the mirror coating noise is spatially uncorrelated, a much larger improvement in stability can be expected from using a high-order mode in such future ultra-stable resonators.

We present calculations on coating and substrate noise suppression for high-order Gauss-Hermite modes and report on the selective excitation of such modes in an optical resonator.