Abstract

In recent years it has been shown that quasi-phase-matched wavelength conversion in periodically-poled lithium-niobate (PPLN) crystals can have important applications in various fields. Unfortunately the presence of the photorefractive effect represents a serious drawback, limiting a wide utilization in guided-wave optics and in high intensity operations [1]. Resistance to photorefractivity can be considerably increased by adding to lithium niobate (LN) an appropriate dopant, such as MgO (at concentrations of 5÷6 mol%). Recent experiments performed by our group have given clear evidence that also congruent LN (cLN) doped with Hf0₂ exhibit a strong resistance to the photorefractive damage [2], The optical quality and the homogeneity of the crystals is a key issue in view of their utilization in real nonlinear devices. As the incorporation of dopants into the matrix can be very critical, it’s advantageous to keep their concentration as low as possible. To this end, it is essential to detennine the dopant threshold concentration, which is the value beyond which the photorefractive resistance doesn’t further improve. It should be noted that the theoretical “threshold concentration” of dopant, in a congruent crystal, should correspond to the minimum amount needed to remove all the Nb-ions in the Li-sites [1], To investigate the effective threshold in Hf:LN we measured two different parameters: i) the photorefractive effect as a function of the intensity 1 of the illuminating beam, ii) the phase-matching (PM) temperature for second hannonic generation (SHG) at fixed pump wavelength for different Hf concentration.

© 2007 IEEE