Methods

The Flashlight Test

A simple continuity test for short-to-medium length fiber optic links is to shine a flashlight into a cleaved or connectorized link and observe if light comes out of the other end. On short lengths, it may be necessary to cleave only the end where the flashlight injects light into the fiber.

This simple check can be made on cable lengths of up to a mile and more. If cable ends are outdoors, sunlight may be used. NOTE: on longer lengths, the light observed at the opposite end may appear red in color. This is normal and is caused by the filtering of light within the fiber.

CAUTION: NEVER LOOK DIRECTLY INTO A FIBER CONNECTED TO LIGHT LAUNCHING EQUIPMENT. THIS CAN CAUSE PERMANENT EYE DAMAGE.

Optical Power Measurements

When an optical cable has been installed, all splices made and connectors attached, it must be determined if the system is capable of delivering the required power. The simplest test requires a light source of the same type, wavelength and approximate power as that of the equipment to be used. The system equipment itself is often a satisfactory source.

The first step is to obtain an approximate measure of system launch power. A short test cable with the same fiber and connector style as the installed cable can be used for this procedure. One end of the short cable is connected to the light-launching equipment. The other end is connected to an optical power meter.

After the initial reading is taken on the short length of test cable, a second similar reading is taken with the installed cable in place. The difference between the two readings indicates the additional power losses due to fiber length and differences in optical qualities of connectors. Because approximate fiber losses are known, losses greater than 1.0 to 1.5 dB above fiber losses might indicate an inferior connection - requiring either repolishing or replacement.

Optical Power Meters

Power meters often read directly in power units, such as dBm and dBµ. By using connector adapters and light sources of the same wavelength as the installed equipment, an accurate measure of link losses with connectors and splices may be obtained.

The Optical Time Domain Reflectometer (OTDR)

OTDRs are typically used to measure distance and attenuation over the entire fiber link. They are also used to identify specific points along the link where losses occur, such as splices.

An OTDR is an optical radar which measures time of travel and the return strength of a short pulse of light launched into an optical fiber. Small reflections occur throughout the fiber, becoming weaker as power levels drop with distance. At major breaks, large reflections occur and appear as strong peaks on an oscilloscope.

Testing of short and medium distance fiber optic systems seldom requires an OTDR. In smaller systems, optical power meter tests are faster and more useful.

Many instrument rental companies are now offering OTDR's as well as other fiber optic splicing and test equipment.

Magnifying Glasses and Microscopes

Because the naked eye cannot detect scratches or defects in optical fibers, use of magnification equipment is required. For most routine inspections, and ordinary battery-powered illuminated microscope of 30x to 100x can produce satisfactory results.

Some microscopes are available with special adapters specifically designed for use with fiber optic connectors.