Explicit analytic design rules are derived for both 3dB and full adiabatic couplers. The design rules are in excellent agreement with numerical calculations using the beam propagation method (BPM). It is shown that the length scaling for 3 dB couplers compared to full couplers makes the former more difficult to design. The design for each case is optimized to obtain the upper limit of performance and a comparison is carried out between two different design geometries for both 3 dB and full adiabatic couplers.
Tarek A. Ramadan, Robert Scarmozzino, and Richard M. Osgood, "Adiabatic Couplers: Design Rules and Optimization," J. Lightwave Technol. 16, 277- (1998)