Volume grating induced by a self-trapped long filament of femtosecond laser pulses in silica glass

A self-trapped filament of ultrashort laser pulses can induce a 10-500 mum-long region of refractive-index change in silica glass. The translation of filament perpendicular to the optical axis induces a layer of refractive-index change with a thickness of 2 mum. We stacked the layers periodically and fabricated volume gratings. We irradiated a He-Ne laser beam at the wavelength of 632.8 nm onto the gratings at the Bragg angle. The maximum diffraction efficiency was 74.8% with the grating that had the period of 3 mum and the thickness of 150 mum.