Coupling and tunability of optical whispering-gallery modes: a basis for coordinate meter

Mechanical tunability and quality-factor of normal modes in coupled optical whispering-gallery microresonators are analysed theoretically and tested experimentally. It is shown that the position-dependent normal mode splitting in two spherical resonators coupled via evanescent field provides significant mechanical tuning of WG modes. Additional radiative losses due to perturbation of partial mode configurations are shown to be small enough to allow preservation of the quality-factor at the level of Q> 1 × 10 9. Combination of high Q and mechanical tunability allows to propose this system as a novel type of optical parametic transducer: the twin-ball sensor of small displacements. The predicted ultimate sensitivity in terms of displacement per unit phase shift is 8 × 10 −12cm/rad at a wavelength λ=1.55 μm (minimum of losses in fused silica), higher than in best Fabry-Pérot interferometers. In preliminary experiments with coupled microspheres of fused silica at λ=0.63 μm, phase sensitivity 3 × 10 −9 cm/rad is demonstrated, equal to that of an FP interferometer with a supercoating mirror finesse F=1 × 10 4. The proposed tunable optical dielectric cavities (single sphere mass down to 10 −9g can be used in small probe mass experiments such as QND coordinate measurement. Ponderomotive interaction between two spheres can be a basis for QND photon-number measurement.