부산대학교 도서관

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 $f_{\text{rep}}$ of the OFC to the cavity, while the carrier envelope offset frequency of the comb is locked to an external frequency reference. Changing $f_{\text{rep}}$ in incremental steps allows to scan the cavity mode profile with the OFC. To acquire these cavity mode spectra, the cavity transmission is measured by a Fourier-transform spectrometer with sub-nominal resolution [6]. We fit the individual cavity modes with a Lorentzian to extract their positions, from which we calculate dispersion-induced cavity mode shifts for vertically- and horizontally-polarized light, also fitting a polynomial (see Fig. 1(a)-(d)). From these fits, we calculate the birefringent splitting of the mode shifts (Fig. 1(e)), as well as the group delay dispersion (GDD, see Fig. 1(f)). This method allows us to resolve the birefringent cavity mode splitting and temperature induced shifts over a spectral range of 1535–1565 nm (6390–6515 cm −1 ), as the onefold statistical measurement uncertainty is