Application Note 05
Hazards of using ST connectors with single-mode fiber
The light-carrying core diameter of single-mode optical fiber is only 8 to 10
microns (approximately 0.0004 inches) in diameter. This means that only light
impinging on this tiny core can enter the fiber and be properly transmitted. As
a result, tolerances are extremely critical.
The popular multimode optical ST connector is available for use with single-mode
fiber as well and the only difference between it and its multimode cousin is in
the tolerance of the center pin. But this is not the real problem. The ST
connector is a spring-loaded connector, a feature that allows it to be quickly
mated and unmated, much like the electrical BNC. The spring arrangement also
allows the fiber-carrying center pin to move when strain is put on the cable.
When used with multimode fiber with a much larger light-carrying core, a slight
movement of the center pin is usually not a problem. However, with
single-mode fiber any movement increases optical losses. In some cases these
losses can be so severe that a data link actually becomes intermittent or even
stops working completely. Such applications as aerial cables (strung between
telephone poles for example), or cables in an environment that is subject to
constant vibration are particularly prone to this sort of situation.
The solution in these cases is to simply use a different optical connector and
the connector we recommend at Liteway™ is the FCPC style. The
FCPC connector has a central pin that is the same size (and made to the same
tolerances) as the ST but the locking mechanism is completely different. Instead
of a spring, the FCPC connector is secured in place with a threaded collar. This
results in no movement of the fiber-carrying pin at all, regardless of the
strain on the fiber cable. In summary, whenever using single-mode fiber
you should always consider the operating environment. If the connector is not
chosen properly, the fiber optic link may become intermittent or not function at
The FCPC style connector is standard connector for Liteway™ single
mode products. These models are designated by the '-7' suffix.
By special request we will manufacture our single mode products with ST
connectors. Single mode ST models are designated the '-8' suffix.
If your still unsure, or have questions please call us at Liteway™.
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