Slot waveguide by using double high-mesa structure for optical absorption sensing

Slot waveguide, of which the optical field profiles mainly in the slot region, is attractive for sensing application by optical absorption because it has a high portion of optical field inside the slot region. A slot waveguide itself can be structured by utilizing buried hetero, ridge, rib, strip, and highmesa structures. Among them, slot waveguide by using double high-mesa structure is attractive because it offers higher portion of optical field profile inside the slot region compared to the others. One critical issue is its propagation condition, as the slot waveguide structure may allow "directional-coupler" mode, which prevents proper utilization especially to the sensing application. To get rid of the directional-coupler mode propagation, here we propose a scheme by utilizing leaky mode for each high-mesa part. Under the condition of leaky mode for each high-mesa part, it is successfully confirmed that there is a propagation condition of which optical field mainly profiles in the slot region. Based on this propagation condition, more than 85% of the optical field profile can be achieved in the slot region. Moreover, under this condition, it is also confirmed that a very small radius of curvature of 10 mm is available, which is useful for long length integration into a compact area. We have also discussed about a mode-size converter to inject optical field into this double high-mesa slot waveguide from regular single high-mesa waveguide. Only 1.08 dB excess loss has been reported at mode-size converter.