Transition from thermal diffusion to heat accumulation in high repetition rate femtosecond laser writing of buried optical waveguides

TLDR

A variable‑rate (0.2–5 MHz) femtosecond laser was used to probe thermal diffusion versus heat accumulation in forming low‑loss optical waveguides in borosilicate glass, and the study presents a comprehensive characterization of waveguide properties across the diffusion and accumulation regimes. Waveguide morphology transitions smoothly from diffusion‑dominated at 200 kHz to heat‑accumulation‑dominated at 0.5–2 MHz, with diameter growth following a diffusion model, and an optimal exposure window of 200 mW average power and ~15 mm s⁻¹ scan speed yielding low loss across this range; stronger heat accumulation above 1 MHz improves guiding, while waveguides remain thermally stable up to 800 °C and can be fabricated at 520 µm depth with low spherical aberration.

Abstract

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.