Holographic Measurement of Dark Conductivity in LiNbO3:Ti:Fe Planar Optical Waveguides

LiNbO3 is a promising photorefractive material for many applications [1]. Phase holograms formed in strip and planar waveguides may be used for phase conjugation, narrow-band holo-graphic filters, or light amplifiers [2]. In most cases Fe2=3 serves as the photorefractive center. On the one hand, the maximum value of the light-induced refractive index changes has been found to be proportional to the concentration of Fe3 [3]. On the other hand, there are several hints that iron doping also has a significant influence on the dark conductivity of the material [4], thereby limiting the storage time. In waveguides the Fe concentration can be easily varied by indiffusion. In this letter we will show that the dark conductivity of heavily iron-doped planar waveguides in LiNbO3 increases exponentially with the iron concentration, and this correlation may also be applicable to samples with a much lower doping. Planar photorefractive waveguides are fabricated by two subsequent surface diffusions using commercially available optical grade y-cut LiNbO3 wafers of congruently melting composition. At first, an 80 nm-thick evaporated titanium layer is indiffused at 1000 C for 24 h in wet argon atmo-sphere to increase the surface refractive indices of LiNbO3. In a second step, evaporated iron layers of different thicknesses (30, 40, 60, 80, 100, and 120 nm) are indiffused at 1000 C for 18 h in