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 SiO2 are nSi=3.47 and nSi02=1.46 respectively. The access waveguides (W1=0.45 μm) have been designed to propagate just the fundamental mode, TE0. The optimum width for the second waveguide was chosen to achieve the phase-matching condition for the TE1 mode, which corresponds to W2=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 LP01 mode from the SSMF (Standard Single-Mode Fiber) to the TE0 mode in the SOI waveguide; thus a typical design for a SOI coupler can be used. However, the output coupler must simultaneously couple the TE0 and TE1 modes in the SOI wide waveguide to the LP01 and LP11 modes in the FMF (Few-Mode Fiber). Input gratings are designed to have an area of 12x12 μm2 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 μm2) to correctly couple the LP01 and LP11 modes in the FMF.
David Garcia-Rodriguez, Juan L. Corral, Amadeu Griol, and Roberto Llorente, "Mode-converter and multiplexer based on SOI technology for few-mode fiber at 1550 nm," Proc. SPIE 10130, Next-Generation Optical Communication: Components, Sub-Systems, and Systems VI, 1013008 (Presented at SPIE OPTO: February 01, 2017; Published: 28 January 2017); https://doi.org/10.1117/12.2252137.
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