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

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.

Control of waveguide properties by tuning femtosecond laser induced compositional changes

“…In this irradiation conditions, the waveguides are formed due to the balance between heat accumulation and thermal diffusion 22 effects, as can be observed by comparing the cross-section dimensions ( Fig. 1) with the spot size of the aspheric lens ($1 lm).…”

Femtosecond laser microstructuring: an enabling tool for optofluidic lab‐on‐chips

“…The high-frequency regime was initially accessed using Ti:sapphire oscillators with the cavity length stretched by a telescope or a multipass cell (5-20-MHz repetition rate) [49][50][51][52]. Recently, Yb-based bulk or fiber lasers, working at repetition rates from a few hundreds kHz to a few MHz, have become increasingly widespread and have provided the best fabrication results [53][54][55][56]. Two different writing geometries are possible, longitudinal and transverse, in which the sample is translated, respectively, along and perpendicularly to the beam propagation direction.…”

Ultrafast-laser-inscribed 3D integrated photonics: challenges and emerging applications

“…Furthermore, low numerical aperture (NA) focusing (NA < 0.6) is used, thus maximizing the vertical real-estate available because of the longer physical working distance of such microscope objectives. In the athermal regime, there is sufficient time between two consecutive pulses, that is, longer than the thermal diffusion time of~1 µs in glass, for the generated heat to diffuse out of the focal volume before the next pulse arrives [41,42]. This results in a repetitive, pulseby-pulse modification of the material.…”