Dispersive mirror in AlGaAs designed by inverse spectral theory

Abstract

We introduce a new mathematical method, based on the inverse spectral theory of Gel'fand and Levitan, of designing dispersive coatings for use in femtosecond lasers. We fabricated an example in AlGaAs by metal-organic chemical-vapor deposition. The mirror has a high value of group delay dispersion over a bandwidth of 10 nm, reaching an extreme of -1200 fs(2) at the center (805 nm) and falling to -800 fs(2) at the edge of this range. In the same band the reflectivity remains over 95%. We created the design by identifying parameters in the spectral domain, numerically optimizing these parameters, and solving the resulting inverse problem to recover the refractive-index profile. Because we performed the optimization in the spectral domain, we needed only four parameters to obtain a good result. Numerical analysis shows that the excess optical delay at the red end of the stop band can be achieved by use of a mild optical resonance, with the optical field approximately twice the magnitude of that at the blue end.

Double-chirped semiconductor mirror for dispersion compensation in femtosecond lasers

“…The most common way to provide negative dispersion in a cavity is to insert a prism pair; 1 alternatives to that are dispersive mirror structures. Gires-Tournois interferometer ͑GTI͒ structures 2 have been used in various devices, including a semiconductor mirror with negative dispersion 3 and even a semiconductor saturable absorber mirror ͑SESAM͒ with negative dispersion. 4 GTI structures are simple and can provide very large dispersion, but only in a limited bandwidth.…”