Ultracompact LiNbO3 ridge waveguide multi-mode interferometer based on low-effective-index mode interference

Abstract

Waveguides fabricated in crystals, created by utilizing techniques such as ion implantation, femtosecond laser inscription, and proton exchange, have low refractive index contrast with the crystal substrate, which limits their potential development to create compact passive devices, such as waveguide interferometers. In this work, a new waveguide multi-mode interferometer fabrication strategy based on low-effective-index mode interference is presented. Numerical tools have been used for the analysis of this new guided-wave approach used for the device design. The research has demonstrated that a series of high-order modes with an effective refractive index much lower than the substrate can exist in trench-cladded ridge crystal waveguides that have a tiny index contrast in the vertical direction. Simple trench-cladded tapered waveguide configurations to excite such modes and to realize compact waveguide multi-mode interferometers with lengths of several tens and up to hundreds of micrometers are presented. The waveguide multi-mode interferometer design is compact in size, easy to modulate, and with low insertion loss. Furthermore, refractive index sensing is realized, with a sensitivity of ∼490 nm/RIU for aqueous solution samples. The novel multi-mode interference phenomenon present here offers new possibilities and significant opportunities for waveguide modulation and, thus, the development of compact waveguide refractometers.