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/PC 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/PC 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 LuxLink^© is the FC/PC style.   The FC/PC connector has a central pin that is the same size (and made to the same tolerances) as the ST/PC but the locking mechanism is completely different. Instead of a spring, the FC/PC 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 all.

The FC/PC style connector is standard connector for LuxLink^© single mode products.   These models are designated by the '-7' suffix.  If you want a single mode unit for use with ST/PC connectors, which is not recommend,  you would have to utilize a custom fiber patch cable, which has FC/PC on one end and ST/PC on the other.  

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