Regardless what type of connectors (LC, SC or ST) and what size fiber (50, 62.5 or 9 micron) there are three ways to test the functionality of a new or existing fiber link. Technicians can use a flashlight/laser, Optical Loss Test Set (OLTS) or an Optical Time Domain Reflectometer (OTDR).
Each of these testers have their functions and advantages as well as disadvantages. A basic principle of fiber optics is that some form of visible light can pass through a fiber link end to end. This is a no-go test because if the light doesn’t reach the far end there is a break in the fiber link, which usually will indicate one or two bad connectors that need replacement. A standard flashlight can be used, but because it emits white light it can be hard to see at the far end if the fiber is long, and often it is difficult to align the fiber tip (the ferrule) onto a standard flashlight effectively. The visual red laser is much easier to use because they come equipped with adapters that allow the fiber tip to be fully inserted, and the red light is much easier to see at the far end.