Transition from thermal diffusion to heat accumulation in high repetition rate femtosecond laser writing of buried optical waveguides.
A variable (0.2 to 5 MHz) repetition rate femtosecond laser was applied to delineate the role of thermal diffusion and heat accumulation effects in forming low-loss optical waveguides in borosilicate glass across a broad range of laser exposure conditions. For the first time, a smooth transition from diffusion-only transport at 200 kHz repetition rate to strong heat accumulation effects at 0.5 to 2 MHz was observed and shown to drive significant variations in waveguide morphology, with rapidly increasing waveguide diameter that accurately followed a simple thermal diffusion model over all exposure variables tested. Amongst these strong thermal trends, a common exposure window of 200 mW average power and approximately 15-mm/s scan speed was discovered across the range of 200 kHz to 2 MHz repetition rates for minimizing insertion loss despite a 10-fold drop in laser pulse energy. Waveguide morphology and thermal modeling indicate that strong thermal diffusion effects at 200 kHz give way to a weak heat accumulation effect at approximately 1 microJ pulse energy for generating low loss waveguides, while stronger heat accumulation effects above 1-MHz repetition rate offered overall superior guiding. A comprehensive characterization of waveguide properties is presented for laser writing in the thermal diffusion and heat accumulation regimes. The waveguides are shown to be thermally stable up to 800 degrees C and can be written in a convenient 520 microm depth range with low spherical aberration.
Edward S. Rogers Department of Electrical and Computer Engineering and Institute for Optical Sciences, University of Toronto, 10 King's College Road, Toronto, ON M5S-3G4, Canada. shane.eaton@utoronto.ca
Shane M
SM
Eaton
Haibin
H
Zhang
Mi Li
ML
Ng
Jianzhao
J
Li
Wei-Jen
WJ
Chen
Stephen
S
Ho
Peter R
PR
Herman
D003198:Computer Simulation; D004867:Equipment Design; D019544:Equipment Failure Analysis; D005898:Glass / Q000737:chemistry* / Q000528:radiation effects; D006358:Hot Temperature; D007834:Lasers; D008027:Light; D008962:Models, Theoretical; D055095:Optics and Photonics / Q000295:instrumentation*; D012542:Scattering, Radiation; D017640:Silicates / Q000737:chemistry* / Q000528:radiation effects; D013499:Surface Properties
D017640:Silicates