Reference Guide To Fiber Optics
|Topic: Visual Tracing And Fault Location||Table of Contents: |
The FOA Reference Guide To Fiber Optics
Visual Tracing And Fault LocationTwo simple, inexpensive devices allow tracing and testing fibers for continuity or proper connections and one even is used to optimize splices or prepolished/splice connectors.
Many of the problems in connection of fiber optic networks are related to making proper connections. Since the light used in systems is invisible infrared light (IR) beyond the range of the human eye, one cannot see the system transmitter light. By injecting the light from a visible source, such as a LED, laser or incandescent bulb, one can visually trace the fiber from transmitter to receiver to ensure correct orientation and check continuity besides. The simple instruments that inject visible light are called fiber tracers or visual fault locators.
Fiber Optic Tracer
If a powerful enough visible light ,such as a red HeNe laser or visible diode laser at 635-650 nm is injected into the fiber, high loss points can be made visible. Most applications center around short cables such as used in telco central offices to connect to the fiber optic trunk cables. However, since it covers the range where OTDRs are not useful because of the dead zone of the OTDR, it is complementary to the OTDR in cable troubleshooting. This method will work on buffered fiber and even jacketed single fiber cable if the jacket is not opaque to the visible light. The yellow jacket of singlemode fiber and orange of multimode fiber will usually pass the visible light. Most other colors, especially black and gray, will not work with this technique, nor will most multifiber cables. However, many cable breaks, macrobending losses caused by kinks in the fiber , bad patchcords, spices etc. can be detected visually. Since the loss in the fiber is quite high at visible wavelengths, on the order of 9-15 dB/km, this instrument has a short range, typically 3-5 km.
The higher power of a visual fault locator can find breaks in fibers or high losses around connectors in simplex cables. The light that escapes at a break, for example, will be visible through the jacket of the cable as shown below. This is extremely helpful in finding cable faults near the end of a cable where the dead zone of the OTDR makes it impossible to resolve faults. It also allows finding cracked fibers or bad splices in splice closures where an OTDR cannot resolve faults.
Optimizing prepolished connectors (high loss at left, optimized at right.
Optimizing mechanical splices (high loss at left, optimized at right.