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Measurement of electrooptic constants in proton-exchanged LiTaO3 optical waveguides

LiTaO3 is considered resistant to optical damage and it has a small dispersion. Thus, LiTaO3 is expected as a solution to such problems as dc drift and optical damage in an LiNbO3 waveguide. 
 
 
 
This paper describes a design method for a singlemode, three-dimensional optical waveguide by means of proton exchange and annealing on a Z-cut LiTaO3 substrate. A phase modulator with a different annealing time was designed and r33S was measured by the phase modulation method. It is found that the measured value of 6 pm/V immediately after exchange is recovered to 22 pm/V (about 70 percent of the bulk value) by annealing for more than 30 min at 400°C. 
 
 
 
By the resonance method, r33T was measured. the result is larger by 30 percent than the measured r33S. After annealing for 30 min, the value was almost equal to the bulk value. Further, by the SIMS (secondary ionmass spectroscopy) analysis, the change of the concentration profiles of Li and H+ before and after annealing is determined. It is found that Li returns to the surface from the interior of the substrate by annealing and H+ is diffused in the depth direction. This suggests that the crystallography of the proton-exchanged layer approaches that before exchange so that the degradation of r constants is recovered.