Frequency doubling in surface periodically poled lithium niobate waveguides: Competing effects

We fabricated α-phase pro ton-exchanged (PE) lithium niobate (LN) channel waveguides quasi phase-matched (QPM) via surface periodic poling (SPP) and carried out the first experimental demonstration of second harmonic generation (SHG) in such devices.[1] Experiments were performed by employing an optical parametric amplifier/oscillator producing 25 ps pulses in the range 1.1-1.6 μm with a line-width less than 2cm−1 and a repetition rate of 10Hz. SHG measurements were performed either at a fixed wavelength by varying the fundamental frequency (FF) input power or by scanning the FF wavelength, ratioing the second harmonic (SH) output to the FF input to obtain the conversion efficiency. By repeating the SHG scan versus peak power, we observed that the wavelength for maximum conversion (the FF resonance for SHG) shifted with excitation, as visible in Fig. 1a. Otherwise stated, when performing a power scan at a fixed FF wavelength, the conversion efficiency first increased with power (as expected) and then decreased. By adjusting the wavelength, conversely, we observed a linear dependence in maximum conversion to SH (see Fig. 1c), as well as in peak SHG wavelength (see Fig. 1b).