Mid-infrared multilayer optical coatings for bio-medical laser applications

Abstract

Multilayer optical components for ultrafast laser applications operating in the mid-infrared (MIR) spectral range from 3 to 15 μm exhibit optical and physical properties essentially different from those ones of the coatings operating in the visible-near-infrared spectral range, namely, they should (i) comprise layers from special MIR thin-film materials; (ii) contain much thicker layers; (iii) deal with adhesion problems due to high stresses; (iv) deal with regions of O-H absorption in the MIR range; (v) exhibit positive group delay dispersion; (vi) be ultra-broadband in order to support short pulses. The mid-infrared thin film materials Ge, ZnS, YbF3, and LaF3 were carefully characterized. ZnS/YbF3 and Ge/YbF3 dichroic filters specified in the visible, near- and mid-infrared spectral ranged were designed, produced, and characterized. The filters operate at Brewster angle 67.5°. Almost one-octave broadband dispersive mirrors operating in the mid-infrared spectral range were developed for the first time. The mirrors consist of Ge and YbF3 layers, which have not been used before for manufacturing of multilayer dispersive optics. The mirrors compensate group delay dispersion of ultrashort laser pulses accumulated by propagation through 4 mm ZnSe windows and additional residual phase modulation of an ultrashort laser pulse. Using the mirrors, the 86 fs pulses were compressed up to 55 fs.