Page 197 - Sensors and Control Systems in Manufacturing

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Cha p te r

more light than a 40-μm fiber, the historical telecommunication indus-
try standard.
Computer data communications systems commonly employ
62.5-μm fiber because of its ability to support very high data rates
(up to 100 Mbaud) while offering increased power and ease of han-
dling. Factory bandwidth requirements for most links are typically
one or more orders of magnitude. Therefore, fiber core size may be
increased to 200 μm to gain the benefits of enhanced power and
handling ease, the decrease in bandwidth being of no consequence
(Fig. 3.31).

3.12.1 Optical Power Budget
An optical power budget examines the available optical power and
how it is used and dissipated in a fiber-optic system. It is important
to employ the highest possible optical power budget for maximum
power margin over the detector requirement. A budget involves four
factors:

• Types of light source
• Optical fiber acceptance cone
• Receiver sensitivity
• Splice, coupling, and connector losses
Laser-diode sources are generally not economically feasible or
necessary in industrial systems. Light-emitting diodes are recom-
mended for industrial applications. Such systems are frequently
specified with transmitted power at 50 μW or greater if 200-μm-core
fiber is used.
Successful communication in industry and commercial applica-
tions is determined by the amount of light energy required at the
receiver, specified as receiver sensitivity. The higher the sensitivity, the
less light required from the fiber. High-quality systems require power
only in the hundreds of nanowatts to low microwatts range.
Splice losses must be low so that as little light as possible is
removed from the optical-fiber system. Splice technology to repair
broken cable is readily available, permitting repairs in several loca-
tions within a system in a short time (minutes) and causing negligible
losses. Couplers and taps are generally formed through the process of
glass fusion and operate on the principle of splitting from one fiber to
several fibers. New active electronic couplers replenish light as well
as distribute the optical signal.
An example of an optical power budget follows:
1. The optical power injected into a 200-μm-core fiber is 200 μW
(the same light source would inject approximately 40 μW into
a 100-μm core fiber).