Dependence of Lattice Constant Deviation and Refractive Index on Proton Concentration in Proton-Exchanged Optical Waveguides on a Single Crystal of LiNbO3

The lattice constant deviation and refractive index change of proton-exchanged optical waveguides fabricated on a single crystal of LiNbO3 are discussed in relation to the proton concentration. The exchange rate x of Li+ with H+ has been determined by nuclear reaction analysis and the lattice constant deviation has been measured by means of the double crystal X-ray diffraction method. The value of x in the samples proton-exchanged with the benzoic acid/lithium benzoate melts is in the range of 0.55<x<0.75. After annealing in air, x decreases with increasing annealing time. The lattice constant deviation is proportional to x in the range of x<0.55, while it is almost constant in the range of x>0.55. The dependence of the refractive index change on x is very similar to that of the lattice constant deviation. Reasons for the refractive index change caused by proton exchange are discussed.