Arbitrary coupling ratio multimode interference couplers in   Silicon-on-Insulator
  In this paper we present the design, manufacturing, characterization and analysis of the coupling ratio spectral response for Multimode Interference (MMI) couplers in Silicon-on-Insulator (SOI) technology. The couplers were designed using a Si rib waveguide with SiO 2 cladding, on a regular 220 nm film and 2 {\mu}m buried oxide SOI wafer. A set of eight different designs, three canonical and five using a widened/narrowed coupler body, have been subject of study, with coupling ratios 50:50, 85:15 and 72:28 for the former, and 95:05, 85:15, 75:25, 65:35 and 55:45 for the latter. Two wafers of devices were fabricated, using two different etch depths for the rib waveguides. A set of six dies, three per wafer, whose line metrology matched the design, were retained for characterization. The coupling ratios obtained in the experimental results match, with little deviations, the design targets for a wavelength range between 1525 and 1575 nm, as inferred from spectral measurements and statistical analyses. Excess loss for all the devices is conservatively estimated to be less than approximately 2 dB. All the design parameters, body width and length, input/output positions and widths, and tapers dimensions are disclosed for reference. 
Jos\'e David Dom\'enech, Javier S. Fandi\~no, Bernardo Gargallo,   Pascual Mu\~noz
physics.optics