Page 356 - Optical Communications Essentials

Manufacturing Issues

346 Chapter Twenty

Figure 20.10. Subassembly or platform and some
of its associated parts for mounting optoelectronic
components within a butterfly package.

blocks. The basic properties of these complex metallic subassemblies include a
heat dissipation ability, a thermal expansion match with the internal devices,
and an ability to weld or bond items such as metallized fibers, semiconductor
die carriers, and high-speed electronic circuitry. The basic platform structure
nominally consists of a Cu-W alloy and also may include other metals or metal-
lic alloys such as copper, nickel, a copper-molybdenum alloy, and Kovar.
Manufacturers of these butterfly packages often supply the housings with a
TE cooler already attached inside.

20.4.4. Hermetic sealing
The purpose of hermetic sealing is to prevent contaminants from entering a
device package. The items inside a package that require such protection include
electronics, electrodes, microelectromechanical system (MEMS) devices, epoxy
and die bonds, laser chips, and component surfaces. A key culprit is moisture,
which over time can corrode electrodes, cause mechanical shifts in items bonded
with water-sensitive epoxy, and damage the surfaces of fiber ends, mirrors, opti-
cal filters, or MEMS devices.
Hermetic packaging designs basically use the same packaging principles that
have proved very successful in the microelectronics and microwave industries.
The additional challenge in optical fiber components lies in how to create a her-
metic seal at the point where the optical fiber passes through the package wall.
When a fiber is inside a cable, the plastic buffer coating surrounding the cladding
is sufficient to protect the fiber from typical environmental effects and mechan-
ical disturbances that it may encounter. However, this coating is not adequate
to provide a tight moisture seal when the fiber traverses a package wall.
To provide such a seal, the elastic coating is stripped off the fiber down to the
cladding, and then a thin metal film is bonded to the bare fiber. The metal film
nominally consists of two layers. First the fiber is coated with a layer of nickel
to provide a strong adhesion to the glass and a stable soldering base. Then the
fiber is plated with an overcoat layer of gold to provide resistance to oxidization.
This creates a strong metal coating that is capable of withstanding the rigors of

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