Investigation of optical inhomogeneity of MgO:PPLN crystals for frequency doubling of 1560 nm laser

The asymmetrical phase-matching temperature tuning curves of periodically-poled 5%-mol MgO-doped lithium niobate (MgO:PPLN) bulk crystals for frequency doubling of 1560 nm telecom laser have been observed experimentally and analyzed theoretically. We find that the asymmetrical phenomena are resulted from the variation in the extraordinary refractive index by the doping concentration inhomogeneity of the crystals. The critical parameters of phase-matching temperature mismatching γ(γ=δTpm/ΔT, in which δTpm is the change of the phase-matching temperature due to the doping concentration inhomogeneity, and ΔT   is the temperature acceptance bandwidth) and the thermal refractive index ∂(Δn)/∂T∂(Δn)/∂T are investigated. Our analysis method is not only a reasonable explanation to the experiment, but also can be a simple evaluation way of optical quality of the given MgO:PPLN bulk crystals. Moreover, the MgO:PPLN bulk crystal is used in single-pass configuration for frequency doubling a 4.4 W Erbium-doped fiber amplifier boosted 1560 nm telecom laser, and 300 mW of tunable 780 nm laser is obtained, corresponding to a conversion efficiency of 6.9%. This source provides a simple and compact laser system for rubidium atom cooling and manipulation experiments.