With the goal of creating high refractive index modulation structures in a bulk of fused silica, we investigated the refractive index change by modulating the pulse duration of a femtosecond laser pulse. The femtosecond laser, with duration between 130 and 500 fs at a repetition rate of 1 kHz, was focused inside fused silica through a microscope objective with effective numerical aperture (NA) of 0.08. The results provided a 'triangle region', with pulse duration of 130-230 fs and pulse energy of 0.35-1.5 muJ, for the fabrication of refractive index modulation structures. The refractive index modulation structures inside fused silica were obtained only when the pulse duration was less than 230 fs under this focus condition.
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