Abstract
We report on measurements and modeling of the mode structure of tunable Fabry–Pérot optical microcavities with imperfect mirrors. We find that non-spherical mirror shape and finite mirror size leave the fundamental mode mostly unaffected, but lead to loss, mode deformation, and shifted resonance frequencies at particular mirror separations. For small mirror diameters, the useful cavity length is limited to values significantly below the expected stability range. We explain the observations by resonant coupling between different transverse modes of the cavity and mode-dependent diffraction loss. A model based on resonant state expansion that takes into account the measured mirror profile can reproduce the measurements and identify the parameter regime where detrimental effects of mode mixing are avoided.
Highlights
Fabry-Perot optical microcavities built from micro-machined concave mirrors [1, 2, 3, 4, 5, 6, 7] offer a powerful combination of small mode cross section, high finesse, and open access
This has proven to be beneficial for experiments covering a broad range of topics, including cavity quantum electrodynamics with cold atoms [8, 9], ions [10, 11], and solid-state-based emitters [12, 13, 14, 15, 16, 17, 18], as well as cavity optomechanics [19, 20, 21] and scanning cavity microscopy [22]
We study the cavity performance by measuring the finesse for each accessible axial mode order
Summary
Fabry-Perot optical microcavities built from micro-machined concave mirrors [1, 2, 3, 4, 5, 6, 7] offer a powerful combination of small mode cross section, high finesse, and open access. In this work we study the consequences of finite mirror size and non-ideal shape on the performance of laser-machined, fiber-based Fabry-Perot microcavities [1] Their mirrors are characterized by surface profiles with low microroughness in the range of 1 − 2 ̊A, a near-spherical central part, and an overall shape that is well approximated by a Gaussian. Dashed lines: Gaussian fit. (d) Residuals of the fits in (c). (e) Residual of a 2D parabolic fit to the region of the profile typically covered by the cavity mode
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