Page 349 - Optical Communications Essentials

Manufacturing Issues

Manufacturing Issues 339

devices. However, this is being addressed through developments in areas such
as planar lightwave circuits and inexpensive passive athermal designs for ther-
mal wavelength stability. This section addresses those two issues. Other cost
reduction techniques are the use of manufacturing automation, as described in
Sec. 20.3, and improved packaging methods, which is the topic of Sec. 20.4.

20.2.1. Planar lightwave circuits
A significant saving in manufacturing cost may be realized by integrating the
functions of several optical functions (such as filtering, amplification, coupling,
and attenuation) on a single substrate. This substrate then can be placed in a
single package, thereby reducing the size required on a circuit card. This is the
aim of planar lightwave circuit (PLC) technology, which also is known as inte-
grated optics technology. Since PLC technology increases the density and func-
tionality of photonic devices per square centimeter, it leads to optical
transmission equipment designs that are simpler, cost less, and are not as
expensive to operate since they consume less electric power.
The manufacture of PLC devices is achieved by using tools and techniques
from the semiconductor industry, which allow high-performance components to
be made in high volume at low cost. At the core of this technology are tradi-
tional silicon-wafer processes and silica-on-silicon photonic circuit designs.
These processes are extremely flexible and allow a modular design approach.
Circuit configuration modifications within established design rules may be
implemented rapidly through mask changes. In addition to using waveguides
for passive optical functions, active control of light signals can be achieved
through the incorporation of thermooptic elements. Naturally, similar to the
fabrication of electronic components, extremely precise control must be exer-
cised in every manufacturing step.
Components that may be integrated include variable optical attenuators
(VOAs), optical switches, optical power splitters, optical couplers, arrayed wave-
guide gratings (AWGs), and rare earth–doped waveguides. As one example, sup-
pose an engineer wants to construct an eight-channel wavelength multiplexer
that has a variable output control on each channel. By using separately pack-
aged components, this can be done with an AWG and eight VOAs plus some con-
trol circuitry, as shown in Fig. 20.5. Here the discrete packages need to be
interconnected by short fiber jumpers. Integrating the AWG and VOA functions
into a single PLC device greatly reduces the size and cost of the final module.
In addition, the PLC design eliminates the external optical connections that
normally are needed to interconnect the AWG and the VOAs, which results in
a much lower optical power insertion loss.
As another example, consider the replacement of the erbium-doped fiber in a
conventional EDFA with an integrated erbium-doped waveguide. The resultant
erbium-doped waveguide amplifier (EDWA) is significantly smaller than its
EDFA counterpart. In the example from NKT Integration shown in Fig. 20.6,
eight EDWA devices are integrated on a single substrate. Each amplifier has

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