Mode-converter and multiplexer based on SOI technology for few-mode fiber at 1550 nm

摘要

The Asymmetric Directional Coupler (ADC) based on SOI (Silicon-on-Insulator) technology converts and couples the fundamental mode to the first higher order mode. The ADC is designed to achieve phase-matching condition, which is accomplished when both propagation constants are equal in each waveguide arm. Devices are fabricated in a SOI wafer with a 220 nm thick silicon layer. The refractive indexes of Si and SiO_2 are n_(Si)=3.47 and n_(SiO2)=1.46 respectively. The access waveguides (W_1=0.45 μm) have been designed to propagate just the fundamental mode, TE_0. The optimum width for the second waveguide was chosen to achieve the phase-matching condition for the TE_1 mode, which corresponds to W_2=0.962 μm. The coupling to the input and output waveguides is achieved through grating couplers. The input grating coupler will need to couple the LP_(01) mode from the SSMF (Standard Single-Mode Fiber) to the TE_0 mode in the SOI waveguide; thus a typical design for a SOI coupler can be used. However, the output coupler must simultaneously couple the TE_0 and TE_1 modes in the SOI wide waveguide to the LP_(01) and LP_(11) modes in the FMF (Few-Mode Fiber). Input gratings are designed to have an area of 12x12 μm~2 and a period of Λ=610 nm in order to maximize the optical power coupled between the fiber and the waveguide for an incident angle of 10 degrees. Output gratings are designed with the same period but distinct area (12.5x12.5 μm~2) to correctly couple the LP_(01) and LP_(11) modes in the FMF.