Investigation of the cracks on the surface of proton-exchanged LiNbO3 wave-guides

In order to investigate the generation mechanism of the cracks on the surface of proton-exchanged(PE) wave-guides on the x-cut lithium niobate substrate and the influence on the characteristics of wave-guide.The crystal microstructure,the intensity of the cracks on the surface,the existing manner of hydrogen ion and the variation of density of the annealing proton-exchanged(APE) wave-guide are studied by means of contrast experiments,observation on the surface of wave-guide,XRD(X-ray diffraction) analysis of the crystal phase in the wave-guide,infrared absorption spectra analysis of wave-guide,and so on.The results show that the cracks are made by the stress parallel with the principal cleavage plane of lithium niobate,which have four characteristics:made in the PE process,parallel with each others,deteriorated in proportion to the time of PE process and being ameliorated somewhat in the annealing process.It is also shown that the cracks are the macroscopical representation of the stress and that the annealing process could influence the existing manner of hydrogen ion and its density variation,alter the crystal phases and the stress in the wave-guide,and improve the performance of the wave-guide.It is concluded that multiple-phase coexistence and the difference of the crystal lattice coefficient resulted in the stress and then lead to the crackand that the smallest distortion in crystal lattice,the lowest stress and the α phase wave-guide may have the lowest possibility to bring out crack,and can improve the performance of the wave-guide.