High-repetition rate femtosecond lasers are shown to drive heat accumulation processes that are attractive for rapid writing of low-loss optical waveguides in transparent glasses. A novel femtosecond fiber laser system (IMRA America, FCPA muJewel) providing variable repetition rate between 0.1 and 5 MHz was used to study the relationship between heat accumulation and resulting waveguide properties in fused silica and various borosilicate glasses. Increasing repetition rate was seen to increase the waveguide diameter and decrease the waveguide loss, with waveguides written with 1-MHz repetition rate yielding ~0.2-dB/cm propagation loss in Schott AF45 glass. A finite-difference thermal diffusion model accurately tracks the waveguide diameter as cumulative heating expands the modification zone above 200-kHz repetition rate.

New high repetition rate picosecond lasers offer possibility for high efficiency structuring of transparent conductors on glass and other substrates. The results of ablation of the indium-tin oxide (ITO) layer on glass with picosecond lasers at various wavelengths are presented. Laser radiation initiated ablation that formed trenches in ITO. Profile of the trenches was analyzed with a phase contrast optical microscope, a stylus type profiler, SEM and AFM. Clean removal of the ITO layer with the 266 nm radiation was observed when laser fluence was above the threshold of 0.20 J/cm 2 , while for the 355 nm radiation the threshold was higher, above 0.46 J/cm 2 . The glass substrate was damaged in the area where the fluence was higher than 1.55 J/cm 2 . The 532 nm radiation allowed getting well defined trenches, but a lot of residues in the form of dust were generated on the surface. Use of UV laser radiation with fluences close to the ablation threshold made it possible to minimize the recast ridge formation and surface contamination during the process. The latter was confirmed by the scanning Auger spectroscopy. The processing speed of up to 0.5 m/s was achieved when using high repetition rate picosecond lasers in the UV range.

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The nonlinear refractive index, n2, of sapphire was experimentally measured in the 550-1550-nm wavelength range by use of a picosecond Z-scan technique. It was found that in this spectral region the value of n2 decreases monotonically from approximately 3.3 x 10(-16) to approximately 2.8 x 10(-16) cm2/W. An empirical expression for the wavelength dependence of the nonlinear refractive index in the 270-1550-nm range was obtained.

Waveguides were written in fused silica using both a femtosecond fiber laser with a 1MHz pulse repetition rate and a femtosecond amplified Ti:sapphire laser with a 1kHz repetition rate. Confocal Raman and fluorescence microscopies were used to study structural changes in the waveguides written with both systems. A broad fluorescence band, centered at 650nm, associated with nonbridging oxygen hole center (NBOHC) defects was observed after waveguide fabrication with the megahertz laser. With the kilohertz laser system these defects were only observed for pulse energies above 1μJ. Far fewer NBOHC defects were formed with the megahertz laser than with kilohertz writing, possibly due to thermal annealing driven by heat accumulation effects at 1MHz. When the kilohertz laser was used with pulse energies below 1μJ, the predominant fluorescence was centered at 550nm, a band assigned to the presence of silicon clusters (Eδ′). We also observed an increase in the intensity of the 605cm−1 Raman peak relative to the total Raman intensity, corresponding to an increase in the concentration of three-membered rings in the lines fabricated with both laser systems.