Integrated Optics - Applied Optics Fundamentals and Device Applications - page 245

To obtain efficient coupling between optical fibers and waveguides very
accurate alignment of the fiber and waveguide is required. Fiber position-
ing systems are available commercially that provide repeatable accuracy of
.1 μm. Automated, production-type, alignment machines that provide 0.1 μm
resolution are also available.
5.8.3 Laser Diode to Waveguide Coupling
Hybrid coupling of a discrete laser diode to a semiconductor waveguide can
be efficiently done by means of the butt coupling technique. Theoretical stud-
ies of laser-waveguide butt coupling [36] indicate that coupling efficiency to a
single mode waveguide can exceed 90% if the refractive indices and effective
cross-sectional areas of the laser light emitting layer and the waveguide are
well matched. Alignment tolerance of less than 0.1 μm is required; however,
commercially available piezoelectrically driven X-Y-Z micropositioners are
available at reasonable cost that easily meet this requirement. Reliable hybrid
butt coupling of an AlGaAs laser diode to lithium niobate waveguide has
been demonstrated in the Integrated Optic RF Spectrum Analyzer program
[37]. Butt coupling between single mode semiconductor lasers and strip
waveguides with a theoretical coupling efficiency of greater than 95% has
been predicted by Emkey [38].
Because of the convenience and demonstrated effectiveness of end-butt
coupling between the laser and waveguide, it should not be necessary to con-
sider more complicated methods, such as those involving mirrors, prisms,
and lenses, for most applications. In addition to being an effective method
for coupling laser sources for testing and evaluating waveguide devices,
end-butt coupling represents a close approximation to the ultimate coupling
of a laser diode monolithically fabricated on the same substrate as the semi-
conductor waveguide. Thus, evaluation of the characteristics of the hybrid-
coupled laser/waveguide combination should provide a good indication of
what to expect in the monolithic case.
5.8.4 Waveguide to Detector Coupling
A photodiode detector can be butt coupled to the semiconductor waveguide
in the same way described for the laser diode. Ideally one would like an
edge-oriented photodiode such as would be used in a monolithically fabri-
cated OIC. Thus, for convenience in testing waveguide devices it is often best
to use commercially available, surface-oriented germanium photodiodes.
These photodiodes generally have surface areas that are relatively large com-
pared to the cross sectional area of the waveguide; hence, direct end butt cou-
pling results in low coupling efficiency. In this case an intervening lens can
be used to compensate for the area mismatch to produce efficient coupling.
The 0.7 eV bandgap of Ge results in an absorption edge at 1.55 μm. Thus, a Ge
photodiode can be used throughout the wavelength range from 0.8 to 1.3 μm.