The Application of VFL

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 is visual fault locators.

Visual Fault Locator Description

The visual fault locator launches 635 nm or 650 nm visible laser diode light into the fiber. When light encounters a break or sharp bend, it scatters, and the scattered light can be observed emerging from the cable.

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. A visual fault locator is also much less expensive than an OTDR. However, they are not recommended for use with dark-colored or armored cables.

Finding Faults

Fiber optic visual fault locators are available in four sizes: pen size, pocket size, bench top, and a high power non-contact style bench top unit. The pocket size fault locators can be operated in either continuous wave (CW) mode or in pulse modulation mode. Pulse modulation aids in locating faults under high ambient light conditions and improves battery life. 2 Hz modulation is easy to detect by the naked eye, while 270 Hz and 2 kHz pulse modulation modes are used for fiber identification by detectors. The pocket size fault locator comes with a carrying pouch and belt clip. Pen size fault locators are CW and 2 Hz modulation internally, as are high power bench top units, including the non-contact version.

Another use for visual fault locators is to check connector quality. Often a connector may appear to be perfect, even when viewed with a microscope, but inside the connector ferrule itself, poor gluing or dirt may create a microbend in the fiber. This microbend will produce excess insertion losses or return losses, and may result in premature failure of the connector. If one launches visible light through the fiber, so that it emerges from the connector in question, one can readily see the distortion as a series of rings superimposed on a normal output (See Figure 1). Bending or twisting the fiber may affect the overall intensity pattern, but not the ring pattern itself.

Figure 1: Using a Fault Locator To Find Connector Microbend

Applications:
• Singlemode and multimode fiber testing
• Fiber identifier applications
• Locating breaks and bends in fibers and connectors
• Identifying fibers and tracing optical signals by using modulated signals
• Optimizing splices