Optical Fiber Taper Coupled Glass Microsphere Resonators

Citation

Cai, Ming (2001) Optical Fiber Taper Coupled Glass Microsphere Resonators. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/JB4C-VR78. https://resolver.caltech.edu/CaltechTHESIS:10212010-100027618

Abstract

This thesis studies optical fiber taper coupled dielectric microsphere resonators and their applications. Fundamental properties including ideal coupling and critical coupling in an optical fiber taper to fused silica glass microsphere coupling system is investigated both theoretically and experimentally. A symmetrical dual-taper coupling configuration is proposed to obtain highly efficient power transfer from the taper coupler to the microsphere resonator. Applications as channel add/drop filters and microsphere lasers are also demonstrated.

The physical essence of the fiber taper to silica microsphere is analyzed using a two-dimensional model. The relationship between the coupling strength and the cavity loss is unveiled. Adiabatic tapers and high-quality microspheres are fabricated and used to demonstrate actual coupling systems. Perfect agreement between the experimental results and the theoretical prediction is presented.

Power transfer from the taper to a microsphere resonator has been significantly improved by employing a dual-taper symmetrically coupling configuration. Up to -28 dB extinction at the central resonant wavelength has been measured.

We then propose a device application of the taper-sphere-taper structure as a channel add/drop filter in the wavelength division multiplexing systems. For a filter with a bandwidth of 3.8 GHz and a dropping channel isolation of 26 dB, the bit-error-rate measurement shows no power penalty at 2.5 Gbit/s.

A 1.5 µm wavelength single-frequency fiber laser is also demonstrated using a single tapered fiber coupling to a highly doped erbium:ytterbium phosphate glass microsphere. The fiber taper serves the dual purpose of transporting optical pump power into the sphere and extracting the resulting laser emission. As low as 60 µW pump threshold and fiber-coupled output power as high as 3 µW with single mode operation are obtained. Imaging of photoluminescence from the sphere at visible wavelengths reveals the pump power is resonantly coupled into semiclassical orbits due to the strong absorption damping in the phosphate glass. A bi-sphere laser system consisting of two microspheres attached to a single fiber taper is also demonstrated.

Finally, a novel hybrid fiber taper, made from a combination of a 980 nm single mode fiber and a 1550 nm single mode fiber, is proposed and demonstrated as the microsphere laser coupler. Both the pump wave and laser emission are found to be more efficiently coupled to and from, respectively, the sphere modes. As high as 112 µW single-frequency laser output power is measured with a differential quantum efficiency of 12%.

Thesis Files