Transverse-mode coupling effects in scanning cavity microscopy

Abstract

Tunable open-access Fabry-Pérot microcavities enable the combination of cavity enhancement with high resolution imaging. To assess the limits of this technique originating from background variations, we perform high-finesse scanning cavity microscopy of pristine planar mirrors. We observe spatially localized features of strong cavity transmission reduction for certain cavity mode orders, and periodic background patterns with high spatial frequency. We show in detailed measurements that the localized structures originate from resonant transverse-mode coupling and arise from the topography of the planar mirror surface, in particular its local curvature and gradient. We further examine the background patterns and find that they derive from non-resonant mode coupling, and we attribute it to the micro roughness of the mirror. Our measurements and analysis elucidate the impact of imperfect mirrors and reveal the influence of their microscopic topography. This is crucial for the interpretation of scanning cavity images, and could provide relevant insight for precision applications such as gravitational wave detectors, laser gyroscopes, and reference cavities.

Achievements and perspectives of optical fiber Fabry–Perot cavities

“…More recently, sharp variations in finesse at specific cavity lengths have been well modelled in position and magnitude as mirror-profile-dependent crossings of the fundamental cavity mode with higher-order transversal modes. Deviations of the cavity mirrors from the idealized spherical mirror surface have also been identified as the origin of the birefringence, which are observed in many fiber cavity systems [9][10][11].…”

“…11a), use a fiber mirror as a scanning probe above a reflective substrate. They can be facilitated to map the surface [9,11] or particles located on it [85].…”

Achievements and Perspectives of Optical Fiber Fabry-Perot Cavities

“…More recently, sharp variations in finesse at specific cavity lengths have been well modelled in position and magnitude as mirror-profile-dependent crossings of the fundamental cavity mode with higher order transversal modes. Deviations of the cavity mirrors from the idealized spherical mirror surface have also been identified as the origin of the birefringence, which are observed in many fiber cavity systems [7][8][9].…”

“…11 (a)), use a fiber mirror as a scanning probe above a reflective substrate. They can be facilitated to map the surface [7,9] or particles located on it [80]. Measurements including multiple higher order cavity modes are thereby used to increase the spatial resolution below the mode waist size of the fundamental mode.…”

Observation of mode-mixing in the eigenmodes of an optical microcavity

“…Optical cavities support fundamental and higher-order transverse modes. At certain cavity lengths, some modes become frequency degenerate and hence couple [10][11][12][13]. The coupling of optical modes in analogous to two pendulums connected by a spring as depicted in Fig.…”