Integrated Optics - Applied Optics Fundamentals and Device Applications - page 231

least reliable part of any electronic or optoelectronic package is often the
wire bonds. With a single monolithic integrated optic/optoelectronic chip
the number of wire bonds decreases and reliability improves. This can also
be translated to lower cost since the entire process will be subjected to well-
calibrated batch fabrication processes.
An additional advantage of increased flexibility exists at the 0.85 μm wave-
length. There are large research and development efforts in fabricating opti-
cal logic devices using AlGaAs/GaAs multiple quantum well structures.
These devices rely on a resonant excitonic absorption and must be operated
near the material band edge. This means that the wavelength must be in
the 0.8-0.9 μm range (depending on Al concentration). These devices have
shown the capability to operate at more than 10 GHz [13] with energies of less
than 5 pJ [14]. Several logic gates have been demonstrated including XOR,
NOR, and OR [15]. The capability of incorporating optical logic in a system
can significantly improve the flexibility of the entire system.
Smearing of quantum wells of AlGaAs by diffusion of Zn or implantation
of Si has been demonstrated and is under investigation in several labora-
tories. As the refractive index of the AlGaAs alloy is different than that of
the original multi quantum well structure this effect provides an additional
parameter that can be used for optical confinement, particularly in the lateral
direction.
5.5.3 Optical Throughput Loss
Many forms of loss encountered in the waveguide can be related to a number
of factors. These are material properties which include growth effects, the
processing of the material for waveguide fabrication, and simply the design
requirements. A reasonable fabrication but fairly stringent design tolerance
for a GaAs/GaAlAs waveguide system might be 1 dB cm −1 loss in straight
sections for the waveguide and 0.3 dB rad −1 loss in curved sections of the
waveguide. This requires careful design and fabrication to minimize all con-
tributors to loss, that is, interband absorption, free-carrier absorption, scat-
tering, and radiation.
The major loss to be dealt with in an AlGaAs system is band-edge or inter-
band absorption. In an ideal system, this loss can be overcome by simply
using a waveguide of slightly less energy than the bandgap of the waveguide
material less the energy of acceptors. In a real material with a source wave-
length close to the band edge, a certain amount of loss is unavoidable due
to deep level states. A large part of this loss can be attributed to band-edge
tailing resulting from various defects and impurity states in the material.
The quality of the material used for the waveguide can thus be directly cor-
related with the loss.
Using the early absorption data of Sturge [16] and Stoll et al. [17], one
would project that interband absorption loss would be greater than