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Optoelectronic light sources require packaging that
protects the chip and allows light through to the chip. Window lens designs can be tailored to
provide higher response on the optical axis of the lens, greater directional
sensitivity, and a larger aperture (with less resolution). In commercial devices, the lens is usually an
integral part of the package and tight controls of optical tolerances are
somewhat compromised in order to optimize protection of the chip with a hermetic
seal. Devices with integral lenses tend
to exhibit response variations that are significantly greater than those without
integral lenses. Devices without integral lenses are recommended for
applications requiring quality, external optics, high gain, and highly
directional optical systems. Light source package designs are either plastic or
hermetic, and are available with or without a lens. The optical axis is typically perpendicular
to the leads in plastic packages but is parallel to the leads in hermetic
packages. The hermetic package is more
robust than plastic in providing protection in harsh environments and although
normally more expensive, can nevertheless be cost effective.
Optical fiber is typically wrapped in a
protective coating (cable) which is available in a number of
fiber-optic cable configurations.
MIL-PRF-85045 is an excellent guideline for the selection of an
appropriate fiber optic cable.
Optical connectors provide a mechanical coupling
mechanism that minimizes optical
loss through a path within a fiber optic link from source to detector. A high degree of tolerance control associated
with fiber optic connectors tends to allow a small amount of misalignment
between two fibers being joined at a junction.
Optical connectors today offer insertion losses in the range of 0.3 to
0.5 dB, which is below the 0.75 dB allowed by network standards. To provide mechanical security and optical
alignment, connectors contain a ferrule, which secures and aligns the fiber.
Stainless steel ferrules offer higher durability that ferrules made of ceramics
or polymers, because of steel’s rugged physical characteristics. Connectors
designed to meet the requirements of military specification MIL-C-83522 are
ideally suited for aircraft, shipboard, and land-based applications. The connector types of most interest in fiber
optic link applications are either a ST or SC style:
ST style connectors utilize a quick release bayonet
style plug, which requires only a half turn for mating. The unique construction
design of the ST connector makes it less sensitive to cable movement and
prevents accidental disengagement. ST
connectors can be designed for single-mode or multi-mode light duty tactical
SC connectors employ a push pull mechanism for mating
and are a popular design for many less severe applications.
Detector packages for both photodiodes and
phototransistors are designed to allow light onto the light sensitive region of
the chip. Phototransistors are typically
assembled in transparent packages. The
manufacturer’s data sheets are useful for determining the appropriate package
Optocouplers are packaged for either commercial or
high reliability applications. The
commercial devices are normally available in plastic, dual in-line packages (PEM
/DIP). When applications require a higher reliability than can be obtained with
a plastic encapsulated package, hermetically sealed optocoupler package designs
are typically available as DIPs or metal cans (TO… Series).