Thermal heating effects in writing optical waveguides with 0.1-5 MHz repetition rate

High-repetition rate (> 200 kHz) ultrafast lasers drive novel heat accumulation processes attractive for rapid writing of low loss optical waveguides in transparent glasses. Heat accumulation is significant at similar to 1 MHz when thermal diffusion is insufficient to remove the absorbed laser energy from the focal volume (< 10-mu m diameter) during the interval between pulses. A novel femtosecond fiber laser system (IMRA, FCPA mu Jewel) providing variable repetition rate between 100 kHz and 5 MHz was applied to waveguide writing in fused silica and various borosilicate glasses in order to investigate the relationship of such heat accumulation effects. Waveguides were formed with < 400-fs pulses at 1045-nm at pulse energies of 2.5 mu J at 100 kHz to > 150 nJ at 5 MHz. Wide variations in waveguide properties were encountered, particularly when processing 1737F and AF45 borosilicate, at repetition rates greater than 200 kHz. Waveguide characterization revealed unique material-dependent thresholds for cumulative and single pulse phenomenon. Of these materials, fused silica is unusual in resisting waveguide formation at the fundamental wavelength of 1045 nm, but amenable to waveguide writing at the second harmonic of 522 nm. Laser processing windows are presented for several silica-based glasses for creating symmetric waveguides with low insertion loss when coupled to standard optical fibers. The effects of material and laser parameters on thermal accumulation and waveguide characteristics are discussed.