Controlling light-emission properties of power scalable fiber lasers.

Abstract

We present several methods controlling light-emission properties in power-scalable fiber lasers, including modal quality, polarization, spectra, and optical nonlinearities. The thesis contains three major parts. In the first part we present high power fiber lasers and high energy fiber amplifiers based on LMA fibers, which need external mode management techniques to maintain diffraction-limited output quality. The 810W high power CW fiber laser, which produces the highest diffraction-limited fiber laser power in 2004, uses coiling to maintain only one mode propagating in 20 mum-core LMA fibers. Thermal analysis shows pump power is not a limiting factor for further power scaling, and according to numerical modeling results, nonlinear threshold can be increased to >2kW. A 405W single-polarization fiber laser uses coiling method onto PM LMA fibers, in which the polarization is totally controlled by coiling, relying on no free-space polarization components. One fiber Bragg grating (FBG) is used both to stabilize output spectrums and to increase SRS threshold. A monolithic high-energy nanosecond fiber amplifier, based on single-mode excitation tapering, demonstrates 26-28dB gain with diffraction-limited output beam quality. Chirally-Coupled-Core fibers are presented in the second part for effectively single-mode performance, which means only one mode is allowed to propagate even when core sizes are large (V>>2.405). The theoretical concepts and analysis of CCC fibers are given. The experimental measurements of our 35 mum-core CCC fibers show consistency with numerical model. Monolithic operation such as mode-preserving splicing and fiber pigtailing is enabled by using CCC fibers, and the suppression of inter-modal scattering allows better performance than conventional LMA fibers. The third part contains the experimental characterization of SBS suppression. With the additional acoustic layer structure we experimentally measure ∼2.5 times SBS suppression. The method is compatible with other techniques, such as longitudinal distribution of tensile strain and dopant concentration, and we expect better SBS suppression when this method is used with those techniques.