Page 327 - Introduction to Information Optics

312 6. Interconnection with Optics

Fig. 6.11, Two-dimensional light intensity profiles for a polymer-based waveguide with TE 0 mode
excitation.

6.3. THIN-FILM WAVEGUIDE COUPLERS

Efficient input and output coupling is an important factor to be addressed
for reasonable performance from optoelectronic interconnects. Employment of
conventional coupling techniques utilizing prisms and lenses tends to be costly,
bulky, and very inconvenient from the packaging point of view and often puts
severe restrictions on planarization. To efficiently couple optical signals from
vertical cavity surface-emitting lasers (VCSELs) to polymer waveguides and
then from waveguides to photodetectors, two types of waveguide couplers,
grating couplers and 45° waveguide-coupling mirrors, can be employed. There
are tilted-profile grating couplers and 45° waveguide micromirrors. This
section gives a brief description of research results for both approaches.

6.3.1. SURFACE-NORMAL GRATING COUPLER
DESIGN AND FABRICATION

For the targeted applications, the planarized surface-normal coupler is the
most crucial element that needs thorough investigation. There are a large
number of publications about grating design [16-20]. However, the surface-
normal coupling scenario in optical waveguides has not been carefully inves-
tigated thus far. We are interested in a tilted grating profile in a planar
structure within a thin waveguide layer. The tilted grating profile greatly
enhances coupling efficiency in a desired direction. The gratings under investi-
gation are fabricated by well-established planar microfabrication processes,
such as photo and holographic lithographs. It is very important to conduct a
theoretical investigation first to determine the required grating profile par-