Highly dispersive mirror in Ta2O5/SiO2 for femtosecond lasers designed by inverse spectral theory

A highly dispersive mirror for dispersion compensation in femtosecond laser s is designed by inverse spectral theory. The design of a simple quarter-wa ve Bragg reflector can be modified by moving the poles in the optical imped ance found in the photonic stop band. These spectral quantities are used as independent variables in the numerical optimization because they have no e ffect on the location of the photonic stop band, and so the design requirem ents to obtain a high reflectivity and a specific delay spectrum are decoup led. The design was fabricated by ion-beam sputtering. A group delay disper sion of - 300 fs(2) was measured over a bandwidth of 28 nm, with a remainin g reflectivity of greater than 99% in this range. The mirrors were used to make two Ti:sapphire lasers with 10- and 4-mm-long crystals, both of which generated near-transform-limited pulses of 35-fs duration. Because of the h igh dispersion of the mirrors, the laser cavities needed only five and thre e bounces from the mirrors, thus keeping reflection losses to a minimum. (C ) 1999 Optical Society of America.