We introduce and demonstrate a new design for mode multiplexing in proton-exchange lithium niobate waveguides using asymmetric Y-junctions. The coupling between the lowest two modes in the device is suppressed by modifying the Y-junction shape. Without increasing the device length, the mode contrast is enhanced by >3.4dB compared to conventional designs. The new design improves the performance of an important functional block in integrated optics.
We propose a method to compensate for group-velocity mismatch (GVM) effects that limit the efficiency-bandwidth product in optical frequency (OF) mixers. Integrated wavelength-dependent delay lines are introduced in a waveguide containing a series of QPM gratings. Appropriate choice of the time delays can compensate for GVM. We have demonstrated a two-stage quasi-group-velocity matching (QGVM) device in a periodically-poled lithium niobate waveguide. Two approximately 150-fs-long pulses generated 6 ps apart by SHG in two QPM gratings were re-synchronized by the fixed delay line, and their relative phase fine controlled by temperature tuning. The technique, which can be iterated to more than two segments, enables OF mixers of higher efficiency-bandwidth product than would be possible in a single grating short enough to evade GVM effects.