Spectral filtering of femtosecond laser pulses by interference filters

Phase properties of optical thin film interference filters are discussed fr om the aspect of their usage for phase-error free wavelength separation of broadband femtosecond laser pulses. It is shown that both transmissive or r eflective interference filters with high contrast ratios exhibit high cubic phase shifts on transmission or reflection, respectively, causing intolera ble distortion in the temporal pulse shape. We show, however, that high eff iciency wavelength separation of broadband femtosecond laser pulses can be achieved by using low contrast, properly designed reflective optical interf erence filters directly built into the cavity of the broad spectrum, femtos econd pulse lasers or parametric oscillators. For demonstrative purposes, w e implemented the idea for a Kerr-lens mode-locked Ti:sapphire laser, and o btained two-color, inherently synchronized, unchirped, femtosecond pulse ou tputs from a single laser oscillator.