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.
Eaton
Shane M
SM
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
Zhang
Haibin
H
Ng
Mi Li
ML
Li
Jianzhao
J
Chen
Wei-Jen
WJ
Ho
Stephen
S
Herman
Peter R
PR
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