Our paper on ultra-compact waveguide taper in SOI appeared in Optical Express 
https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-9-10196
A novel design of large bandwidth, fabrication tolerant, CMOS-compatible compact tapers (15 µm) have been proposed and experimentally demonstrated in silicon-on-insulator. The proposed taper along with linear grating couplers for spot-size conversion exhibits no degradation in the coupling efficiency compared to a standard focusing grating in 1550 nm band. A single taper design has a broadband operation over 600 nm that can be used in O, C and L-band. The proposed compact taper is highly tolerant to fabrication variations; ±80 nm change in the taper width and ±200 nm in end waveguide width varies the taper transmission by <0.4 dB. The footprint of the device i.e. taper along with the linear gratings is ~250 μm2; this is 20X smaller than the adiabatic taper and 2X smaller than the focusing grating coupler.