Compact, Robust Technology for Next-Generation Ultrafast High-Power Fiber Lasers.

Abstract

Fiber lasers are an attractive alternative to bulk solid-state systems due to their potential for compactness and robustness, as well as their having diffraction-limited output even at high average powers. Combined with the technique of chirped-pulse-amplification (CPA), a new generation of ultrafast lasers can be engineered providing reliable high average power and ultrahigh peak power for applications in high-field research, novel radiation sources, spectroscopy, and materials processing. However, current fiber CPA systems still rely on large stretchers and compressors with free-space bulk diffraction gratings, which are incompatible with fiber laser benefits.

Clearly, the bulk diffraction grating stretchers and compressors need to be replaced by much smaller and simpler devices. Chirped volume Bragg gratings (CVBGs) are simple slabs of glass with quasi-periodic indices of refraction that can chirp ultrafast pulses to hundreds of picoseconds and back down to the sub-picosecond level in only a few centimeters of material and with easy alignment. Proof-of-principle experiments using CVBGs for stretchers and compressors in fiber CPA systems have previously been performed, but several issues need to be resolved before they are deployed for mainstream use.

This thesis presents a quantitative analysis of the performance of CVBGs at high average powers, which is backed by experimental data wherein the gratings are exposed to a record high 200 W of input power. Due to the grating’s bandwidth and thermal properties, the pulses are recompressible to 350 fs, indicating high fidelity operation. Extrapolation from the model predicts that kW operation, a major goal for all fiber CPA lasers, will be feasible with this technology.

Moreover, the fundamental performance of the CVBGs, both spatial and temporal, is characterized. A new fabrication technique has allowed for the elimination of spatial chirp, a previous limitation on the beam quality. Measurements clearly show the new improvement. The fundamental temporal performance is evaluated using numerical and analytical theories, and CVBG stretchers and compressors are shown to have a negligible difference in group delay responses for sub-ps range bandwidths and can be further enhanced through the technique of apodization.