Elimination of photorefractive grating writing instabilities in iron-doped lithium niobate

Two types of instabilities, one slow (seconds) and one fast (nanoseconds), are observed while writing volume gratings in bulk crystals of iron-doped lithium niobate using contra-directional two-beam coupling along the c axis. A method to eliminate the fast instability was achieved by coating the z surfaces of the crystal with either dielectric (anti-reflection) or transparent conductive coatings. For the second case, electrically shorting the crystal eliminated the slow instability.