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emerge rapidly. The first, optical alignment, has no equivalent in conductive technologies,
while the second, the choice of optical waveguide material, can be roughly compared to
aluminum versus copper interconnects in that the former was a well-established
technology, while the latter is a critically better conductor, but implementation problems
such as oxidation barriers, electromigration, electrochemical deposition, and planarization
have to be systematically solved. The optical signal distribution architecture, whether
interboard or backplane, for example, is yet to be determined. Each approach has its
advantages and shortcomings.

6.3.1 Optical Alignment
The difficulty in optical alignment is determined, quite simply, by the choice of
optoelectronic components. There are two integration approaches. The first makes use of
available components, most notably vertical cavity surface-emitting lasers (VCSELs)
and planar photodiodes, and attempts to steer the optical beam from the vertical emission
direction into a planar, horizontal waveguide and then out of the waveguide and back
into a vertical propagation direction in order to couple into a top-viewing photodiode.
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Each out-of-plane 90 turn generally requires a 45 mirror and collimating lens and incurs
mode coupling losses. Nevertheless, the vast majority of the integration effort around
the world is done using this cumbersome approach, in no small part due to the fact that
since the mid-1980s all high-speed laser sources operating in the 850- to 980-nm range
are of the VCSEL type. Prior to that period, GaAs edge-emitting lasers were available at
this wavelength. The 850- to 980-nm range happens to coincide with an absorption
minimum for most optical polymers known today, as discussed in section III B.
The guiding principle for the second integration approach, practiced exclusively by
Georgia Tech, is that of keeping light propagation strictly planar. Thus edge-emitting
lasers and edge-viewing photodetectors (EVPD) are end-coupled to the waveguide. No
lenses or mirrors are used in this integration process [5], and a coupling efficiency of laser-
to-waveguide of 70 percent has been reported [7]. For the present time, edge-emitting
laser sources at 1310 nm and 10 Gb/s offer a good compromise between speed, availability,
and optical absorption. These are generally used for local area network applications. As
of this date, the only one edge photodetector available commercially is a refractive-type
monitor photodetector that operates between 1300 and 1600 nm at 1 Gb/s. Georgia
Tech has designed an EVPD with an epitaxial PIN structure grown on a mesa sidewall
and operating at 850 nm. For reference it is pointed out that Hitachi has reported an edge-
emitting laser coupling to polymer optical waveguides as a means of coupling laser
emission into optical fibers for local area network application [8].

6.3.2 Key Physical and Optical Properties of Thin-Film Optical
Waveguide Materials
All optical waveguides are composed of organic polymer material with only one exception
known to the authors. The exception is PPC Electronic AG, which uses glass laminates
both for electric multilayer circuit boards and for optical waveguide circuits [9].
From the perspective of performance and fabrication the desirable properties of
organic polymer waveguides can be summarized as follows:
1. From a survey of published data (Table III), a practical lower limit for the
absorption coefficient seems to be about 0.01 dB/cm at wavelengths roughly
between 800 and 1000 nm, about 0.1 dB/cm between 1000 and 1300 nm, and