Transmission volume phase holographic gratings in photo-thermo-refractive glass written with femtosecond laser Bessel beams

Transmission volume phase holographic gratings were fabricated in bulk photothermo- refractive glass using zero-order femtosecond laser Bessel beams and subsequent thermal treatment. Microstructures composed of nano-sized crystals were observed in the exposed regions. The concentration of nano-crystals depended on the writing power, but the size of the nano-crystals was found to be quasi power-independent. Low writing power led to sparse nano-crystals, while optimized writing power achieved dense nano-crystals distribution and high refractive index change. Effects of the gratings thickness, writing laser power and thermal treatment on the diffraction efficiency were investigated. A maximum diffraction efficiency of 94.73% was achieved (at 532 nm testing wavelength) with 1 mm grating thickness at period of 5 μm.