Page 346 - Introduction to Information Optics

6.4. Integration of Thin-Film Photodetectors

Fig. 6.31. A SEM picture of a mirror coupler integrated with a polymer channel waveguide and
input light coupling in a planar polyimide waveguide with a waveguide mirror.

nearly 100% for a waveguide mirror coupler due to total internal reflection.
Considering the implementation of thin-film vertical cavity surface-emitting
lasers and Si-photodetectors onto a board involving 3-D interconnection
layers, we need to use a very short working distance (~ few /an to tens of /an)
in the surface-normal direction. This restriction makes the 45° waveguide
mirrors the best approach for this purpose.

6.4. INTEGRATION OF THIN-FILM PHOTODETECTORS

In the context of board-level optical interconnect applications, we determine
the thin-film MSM photodetector to be the most appropriate because it can
provide a very high demodulation speed, due to the fast transit time of
electron hole pairs, and it can be directly integrated onto target systems.
Moreover, compared to the vertical structure of a PIN diode, the structure of
the MSM photodetector makes it much easier to integrate it with microwave
planar waveguide circuits. Recent research on distributed traveling-wave MSM
photodetectors reveals the possibility of making optoelectronic hybrid inte-
grated circuits, where optical channel waveguide and microwave coplanar
waveguide (CPW) coexist on the same substrate [25]. This is an important
feature for the architectural design of optical interconnects, because it can
provide better integration level and simplify the configuration.
Further, MSM structure can function as a switching device by biasing it
appropriately [26]. This specific attribute can be extremely helpful to recon-
figure the interconnect topology. The switching capability is a consequence of
the energy-band structure of the MSM junction, in which a potential well