Optical resonances of two-dimensional microcavities with circular and non-circular shapes.

摘要

Optical resonances of two-dimensional microcavities are experimentally studied by using elastic scattering from optical fibers with circular, square and deform shapes. A focused Gaussian beam from a wavelength-tunable diode laser is side-coupled perpendicularly to the optical fibers. The Gaussian beam side-coupled fibers are essentially μ-pillar cavities. Optical resonances are probed when the cavity round-trip wave is wavefront matched with the incident wave. The elastic scattering resonances are measured in the far field.; For circular microcavities, optical resonances are measured simultaneously along the fiber length for fiber sizing applications. By measuring the relative resonance wavelength shifts along the fiber length, the fiber diameter uniformity is measured in the order of nm over a cm length. Fiber surface roughness amplitude and size are also estimated.; For square microcavities, multimode resonances are observed. The multimode nature disagrees with the Fabry Perot-based concept of closed round-trip orbit phase matching. By using the wavefront-matching concept, the multi modes can be attributed to the open round-trip orbits that are wavefront matched with the incident wave. The multi modes can be represented by means of a 2-D k-space diagram with a total internal reflection critical angle cone.; Three assumptions are imposed to the k-space model in order to approximately match the calculated k space modes with the scattering resonances: (1) the incident wave is preferentially input coupled in the vicinity of the critical angle, (2) the input coupled modes have non-zero field amplitude along the input coupled sidewall, and (3) the square shape has an aspect ratio different from 1.; For deformed microcavities, the measured scattering spectra are complicated. The resonances depend sensitively on the wavelengths and the input and output coupling. High-Q quasi-periodic resonances are observed only under specific input and output coupling.; The complicated spectral features are modeled by a Fabry-Perot-based multiple ray interference model with round-trip length fluctuations. The round-trip wave with small and random phase shifts have quasi constructive interference. When the round-trip length fluctuations are small compared with the averaged round-trip length, the resonance spectra are quasi-periodic. Whereas the round-trip length fluctuations are large, the resonance spectra have irregular resonance wavelengths and peak heights.