Efficient second harmonic generation in femtosecond laser written optical waveguides on periodically poled lithium niobate

The authors report on the fabrication of buried waveguides in both lithium niobate and periodically poled lithium niobate. First a low insertion loss waveguide is fabricated in z-cut lithium niobate using femtosecond laser waveguide inscription. To fabricate a waveguide exhibiting both low propagation and coupling losses, we used the multiscan fabrication technique to control the size of the waveguide cross section. We measured coupling losses of 1.1 dB/facet and propagation losses as low as 0.6 dBcm^-1. Optical waveguides have been also inscribed in periodically poled lithium niobate by femtosecond laser pulses with the same multiscan technique. Second harmonic generation experiments from a fundamental wavelength of 1567 nm demonstrate that the nonlinear optical coefficient in the waveguides is preserved, yielding a conversion efficiency of 18% W^-1.