Abstract

Substrate-transferred crystalline mirror coatings have improved the quality of high-finesse cavities by significantly reducing thermal noise contributions when compared to amorphous dielectric coatings [1]. However recent work on the coating noise of GaAs/AlGaAs multilayers has drawn attention to previously reported intrinsic coating birefringence [2]. We report the temperature dependent measurement of birefringent cavity mode splitting in an ultrastable high-finesse optical cavity (F~300 000 at 1542 nm), comprised of two crystalline mirrors on fused silica substrates separated by a 30 cm-long ultra-low expansion glass spacer [3]. We employ a recently-developed method to determine dispersion-induced cavity mode shifts [4,5], where a continuous wave (CW) laser is locked to the cavity, while a fraction of its light is used to create a beat note with a single tooth of an Er:fiber optical frequency comb (OFC). This indirectly locks the repetition rate f_rep of the OFC to the cavity, while the carrier envelope offset frequency of the comb is locked to an external frequency reference. Changing f_rep in incremental steps allows to scan the cavity mode profile with the OFC.

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