Up and down UV-trimming of planar directional waveguide couplers: Modeling

In contrast to etched channel waveguide couplers, directional couplers UV-written into a photosensitive layer can be both down and up trimmed by UV post-illumination of the waveguide cores and of the separating photosensitive region, respectively. Theoretically, the coupling length can be corrected by 10%-40% several times, before the third supermode starts to propagate.