Biomedical Engineering Reference
In-Depth Information
is being fabricated and provides greater design freedom for the problem of
contacting. One detrimental feature of this structure is that it has less lateral
confinement, making it more lossy for waveguide bends if not very care-
fully designed. Numerical examples in another section illustrate this point.
The fundamental constraint on the operational wavelength of this wave-
guide is normally that the light be confined in a single mode. With a GaAs
substrate, the clear choice for the waveguide structure is the AlGaAs/GaAs
system. The AlGaAs system will guide light over a range of wavelengths
from less than 0.8 μm to greater than 10.6 μm. For AlGaAs waveguides, the
wavelengths that have received the most attention are 0.85 and 1.3 μm. Since
wavelengths longer than about 0.87 μm are not possible with an integrated
source in the AlGaAs material, the final constraint becomes the wavelength
of the source. If operation speed is to be in excess of 3 Gb s −1 , performance can
be improved by integrating the lasers and electronic drive circuitry with the
rest of the optical components.
The one performance parameter that probably will not exceed that which
can be done at 1.3 μm is optical waveguide loss. Losses in AlGaAs waveguides
are typically 1 dB cm −1 while losses at 0.85 μm are often 2-4 dB cm −1 due to
band-edge material loss. As available material quality improves the optical
losses will improve. One must consider not only waveguide absorption and
scattering loss but also laser to waveguide coupling loss and the advantages
to be gained from monolithic integration. Without anti-reflection coating
applied to the facets of the waveguide, it is difficult to achieve better than 50%
coupling efficiency into an AlGaAs waveguide since reflection loss itself is
30%. End fire or butt coupling losses will be dramatically reduced when the
laser is integrated on the substrate. The AlGaAs multilayer material system
is much more developed than the InGaAaP so that one would expect lower
values of waveguide loss with the former when lasers are monolithically inte-
grated. Although use of the InGaAsP lasers at 1.3 μm wavelength as sources
for waveguides formed in AlGaAs can achieve lower values of loss, one gives
up the potential for monolithic integration of lasers with integrated optical
devices. Another method that can be used to compensate for optical loss is
to integrate an optical amplifier on the substrate. Optical amplifiers exhibited
gains in excess of 20 dB [12]. Although this demonstration was at a wavelength
of 1.5 μm, it could also be achieved in AlGaAs waveguides at 0.85 μm.
The 1.3 μm lasers consist of InGaAsP layers grown on InP substrates.
This wavelength is a good choice for specialized long haul communications
(distances longer than 10 km); however, because GaAs is transparent at this
wavelength, the fabrication of lasers and detectors on the same substrate
with GaAs electronics would be extremely difficult. In addition, there is a
large lattice mismatch between InP and GaAs.
Higher reliability is achieved by reducing the number of components via
larger scale integration. Since sources and detectors can be readily integrated
on GaAs substrates the number of hybrid components can be reduced. The
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