Design of Multilayer Dielectric Mirrors Optimized for Femtosecond Laser Pulses

Abstract: A general method for modelling reflection in a multilayer structure is developed. This is used to simulate how a Gaussian Ti:sapphire femtosecond pulse reflects off of a periodic SiO2/TiO2 dielectric stack at normal incidence. The method is based on a transfer matrix approach combined with the fast Fourier transform in MATLAB. Both a simple Bragg reflector and a linearly chirped mirror is investigated for three different pulse durations using mirrors with 5, 10, 20 and 40 layer pairs. It is demonstrated that a Bragg reflector is not suitable for reflecting a pulse shorter than 10 femtoseconds due to the limited reflectance bandwidth. The chirped mirror features a wider reflectance bandwidth but a reflected pulse exhibits strong group delay oscillations with wavelength, emanating from unwanted high-order dispersion in the mirror. Designs based on refined versions of this mirror type can however be used for reflection and dispersion compensation of pulses as short as 5 femtoseconds.