Fabrication and Characterization of Microlasers by the Sol-Gel Method

Citation

Yang, Lan (2005) Fabrication and Characterization of Microlasers by the Sol-Gel Method. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/HHQ8-VC25. https://resolver.caltech.edu/CaltechETD:etd-06032005-115306

Abstract

The present study explores the application of new materials systems for low threshold microlasers, and characterization of the microcavities. The sol-gel method is used for gain functionalization of high-Q microcavities. A detailed procedure for preparation of the sol-gel films by the spin-on or dip-coating method is presented. The effect of different process conditions on the properties and microstructure of the thin films is investigated through Fourier Transform Infrared (FTIR) Spectroscopy, Scanning Electron Microscopy (SEM), and etching rate test.

Surface gain functionalization of microsphere cavities is fabricated by coating the microsphere with a thin layer of Er³⁺-doped sol-gel films. The optical gain is due to the population inversion of rare earth ions in the sol-gel films. A fiber taper is used to both couple the pump power into and extract the laser power out of the microsphere laser. The laser dynamics change between continuous-wave and pulsating operation by varying the doping concentration and the thickness of the sol-gel films outside the microsphere.

Surface functionalization is also achieved on the microtoroid on a single silicon chip, which can be fabricated in parallel using wafer-scale processing and has characteristics that are more easily controlled than microsphere. The microtoroid can be selectively coated only at the periphery by making use of the variation of etching rate (in buffered HF) of sol-gel films with different degrees of densification. The laser performance of the gain functionalized microtoroids is investigated. Highly confined whispering gallery modes make possible single-mode microlasers. This work also shows that the high Q microtoroid laser has a linewidth much lower than 300 kHz.

The thesis explores fabrication of high Q microcavities directly from the sol-gel silica films deposited on a single silicon wafer. Quality factor as high as 2.5 x 10⁷ at 1561 nm is obtained in toroidal microcavities formed of silica sol-gel, which allows Raman lasing at absorbed pump power below 1 mW. Additionally, Er³⁺-doped microlasers are fabricated from Er³⁺-doped sol-gel layers with control of the laser dynamics possible by varying the erbium concentration of the starting sol-gel material. Continuous lasing with a record threshold of 660 nW for erbium-doped microlaser on a silicon wafer is also obtained.

Analytic formulas are derived to predict the laser performance, such as the laser output power, the threshold power, and the differential quantum efficiency, under different loading condition, i.e. the air gap between the fiber-taper coupler and the cavities. The effect of Er3+ concentration on the minimum threshold is also investigated. In addition, we present a theoretical model in which we include paired ions as the saturable absorber. It shows that self-pulsing operation can be expected with paired-ions-induced quenching in the system. The pulsation frequency increases linearly with the square root of the pumping level, which is consistent with the experimental observation.

Thesis Files