Second-harmonic generation using an a axis Nd:MgO:LiNbO₃ single crystal fiber with Mg-ion indiffused cladding

We report the implementation of a low-loss core-cladding waveguide structure of an a axis Nd:MgO:LiNbO₃ single-crystal fiber with a step refractive index profile and the demonstration of a frequency-doubled laser made from such a cladded crystal fiber. The laser-heated pedestal-grown a axis single-crystal fiber, with an elliptical cross-sectional area of 200×150 μm, is further shown with a Mg ion indiffusion process. Electron probe microanalysis is used to measure the Mg ion concentration distribution in the Mg-diffused layer. After several trials, diffusion parameters suitable for achieving a core-cladding waveguide structure with a step refractive index profile are obtained. The optical and structural properties of the cladded crystal fibers are also characterized. Room-temperature second-harmonic generation with such a cladded crystal fiber is demonstrated. At room temperature, cw green laser output with a power of 10 μW at the wavelength of 0.532 μm is achieved. The origin of the relatively low conversion efficiency is discussed.