Takuya Hirano

We report on the waveguide-based generation of pulsed squeezed light at 795 nm, suitable for quantum enhanced measurements with rubidium atoms. Pulsed ultraviolet second harmonic light with a power of more than 400 mW is produced using a periodically poled LiNbO₃ (PPLN) waveguide and is injected into another PPLN waveguide to generate quadrature squeezing. We find that the phase of the second harmonic pulse is shifted within a pulse, and we attribute the shift to heating due to blue-light induced infrared absorption (BLIIRA) from a comparison between the experiment and a numerical simulation. A squeezing level of −1.5(1) dB is observed in homodyne detection when we apply a linear phase shift to the local oscillator. The experiment and simulation imply that the squeezing level can be further improved by reducing BLIIRA.

Second-harmonic generation (SHG) using periodically poled material in the high-conversion regime is investigated experimentally and theoretically. In the experiment, we use nanosecond pulses and periodically poled MgO:LiNbO3 waveguides with two lengths, 8.3 and 3.6 mm. In both waveguides, the conversion efficiency reaches 80% with increasing pump power and then decreases. The reduction in efficiency is more prominent for the long waveguide. For a peak power of the fundamental wave exceeding 140 W, stronger SHG is achieved by using the short waveguide. To understand these phenomena, we numerically investigate the effect of the cascaded nonlinear phase shift caused by the quasi-phase-matched SHG. The nonlinear phase shift induces an energy backflow to the fundamental wave even when effective phase matching is satisfied, and it greatly reduces the conversion efficiency, at the same level of power as the experiment.