Direct laser writing combined with a phase-delay probe.

We develop on lithium niobate crystals a photorefractive direct-laser-writing approach, in which we combine in one beam both direct writing and phase-delay probing functionalities to extract the in situ information of the refractive index or the electrostatic field. The phase-delay signal, predicted well by the photorefractive theory, is used as feedback for tuning the exposure time or scanning speed of the focused laser in order to control the refractive index change (Δn) at single points and scanning lines. Different features found in creating Δn at the points and lines are explained by the different photorefractive responses in the two cases.