System designers must keep in mind that specifying less than 0.5 dB insertion loss from a connector manufacturer does not mean that the system in the field will have less than 0.5 dB. Connector manufacturers guarantee less than 0.5 dB insertion loss relative to their reference cable. Two connectors that have 0.5 dB insertion loss relative to a reference cable might have 0.8 dB when mated together. Imagine the loss of these two connectors when mated in the field or after five years of thermal cycling. Some system designers feel that they can avoid this by specifying low-loss connectors with less than 0.1 dB insertion loss. However, this only works if the low-loss connectors can withstand the environment they are being used in.
Another consideration when designing a fiber optic system is field-reparability and spares. Certain connectors are field-installable, but they offer less reliability and lower performance than factory-installed connectors. Some systems use spare, terminated fibers, which are used to replace fibers that are damaged during installation or fail during life. Spare fibers also offer future expandability of the system.
4. Select Type of Fiber
There are two main types of fiber to choose from — multimode and singlemode. The fiber type is predominantly determined by the data rate of the system (see Table 1). Multimode fibers cannot handle data rates as high as singlemode fibers because of modal dispersion. A simple explanation of modal dispersion is that the different modes traveling down a multimode fiber arrive at the receiver in a different order than they were sent (see Figure 1). There are two subsets of singlemode fibers — polarization maintaining (PM) and polarization fiber (PZ). PM fibers maintain the polarity of light as it travels down the fiber while PZ fibers polarize unpolarized light as it travels down the fiber. PM and PZ fibers are expensive to terminate because they require the fiber to be axially and radially aligned in the connector.
It is easier and cheaper to terminate multimode fibers with their large core diameters than singlemode fibers (see Figure 2). If the core of a 100 mm multimode fiber is off by 2 mm there is much less loss than if the core of a 9 mm singlemode fiber is off by 2 mm.
5. Select Optoelectronic Packages
Optoelectronic packages house the electronics that convert electrical impulses to optical signals or optical signals to electrical impulses. They can be divided into two groups: those that are used commercially, and those that are suitable for harsh environments. Optoelectronic packages that are used commercially include components in fixed devices and either single port or multiport components with fixed interfaces. For harsh environments, optoelectronic packages with pigtails are required to meet the environmental demands. Pigtails allow the optoelectronic packages to be isolated from the harsh environmental inputs transmitted by the fiber optic connector.
6. Select Type of Connector
The main connector choices for commercial applications are DIN, E2000, FC, LC, SC, SMA and ST for simplex cables. For multichannel cables, the connector choices are Duplex E200, Duplex LC, MT, MT-RJ and Duplex SC. The choice of commercial connectors is usually driven by the connectors already utilized in the system or by the optoelectronic device interface.