Fabrication of high refractive index-modulation structures in fused silica by femtosecond laser pulses

摘要

We present the microfabrication of high refractive index-modulated structures written by line scan inside bulk of fused silica with a femtosecond laser at wavelength of 810 nm. The femtosecond laser beam, with duration between 130 fs and 500 fs at a repetition rate of 1 kHz, was focused through a microscope objective with numerical aperture (NA) of 0.10 or 0.25 into the sample. To fabricate high refractive index-modulated structures in fused silica, we investigated the dependence of refractive index change on laser pulse energy, pulse duration, scan speed, and scanning repetitions. The results showed a "triangle region", with pulse duration of 130 fs to 230 fs and pulse energy of 0.35 μJ to 1.5 μJ, for the fabrication of refractive index modulation structures. The refractive index modulation was increased to 3 x 10~(-3) after several scanning passes. Diffractive optical components such as grid, square, circle gratings and Fresnel zone plates have been fabricated by direct writing technique. The structures could be used as diffractive beam splitters, beam shaper and micro-lens.