The index changes of waveguides fabricated by light irradiation in LiNbO3:Fe crystals

The refractive index changes of waveguides fabricated by light irradiation in LiNbO3 : Fe crystals were investigated by using ordinary and extraordinary laser beams at wavelengths of 632.8nm and 532nm. The experimental results show that the waveguide fabricated by ordinary light is better than that by extraordinary light, as the ordinary light causes much less light-induced scattering. The illuminated time must be strictly controlled when the region with a positive index change, formed by a single thin laser beam, is used as waveguide structure. This is beacuse the waveguide properties may be influenced by the strong noise grating and the shift of the waveguide region after the LiNbO3 : Fe crystal was irradiated for a long time. For avoiding the influence of the noise grating, the symmetric waveguide structure with high quality can be fabricated by sheet beam illumination employing the sandwich method at a relatively low exposure intensity. Moreover, waveguides with different index distributions and different dimensions can be fabricated by changing the width of the irradiating beam and the distance between twice irradiations.