Accumulating tools for troubleshooting a fiber optic link doesn’t have to be expensive. There are some basic pieces of test gear that can be added to a fiber technician’s tool box that cost under $500 dollars. One of these pieces of fiber optic test equipment is a Visual Fault Locator, often referred to as a VFL. The price range can start as low as $79.95 and vary between manufacturers to a high of $400 dollars.

Most all fiber optic visual fault locators or VFLs operate on replaceable “AA”, “AAA” or 9V batteries. Size of a VFL can be as small as a key chain accessory (2.8”x 1.4”x 0.06”) or as large as regular handheld meter (4” x 7” x 1.5”). The most popular size is that shaped as a MAG Flashlight (7.1” x 0.86”). It has a power and modulation switch located on the side of the outer body and adapter interface on top in place of the flashlight lens. It also has a protective cap or shroud. In the event the VFL is dropped the fiber optic interface adapter will not be damaged.

Visual Fault Locators are optimized for use on Singlemode optical fiber often called OS1 or OS2 but can be utilized on Multimode optical fiber as well as the families of OM1, OM2, OM3, OM4 and OM5 grades.

These VFLs emit a high intensity red laser (light) into the optical fiber through a terminated factory polished fiber optic connector. The wavelength of this high powered laser is often between 630nm-690nm which is visible light to the naked eye unlike many data transmission wavelengths that start at 850nm in multimode fiber increasing to 1550nm in singlemode optical fiber. The lower wavelengths such as 630nm or 635nm are very bright making fault locating easy but do not travel as far as the higher 670nm wavelength. Several manufacturers have chosen wavelength of 650nm for their visual fault locators with a specification of traveling distance of 5-8km inside the core of a singlemode fiber with minimal attenuation. In multimode fiber use of the VFL is slightly shorter with specifications referencing distance of 3-5km with minimal attenuation. Battery life on these trouble shooting fiber optic testers is very good. They range from 30-80hrs. Some have installed low battery indicators to alert technician to replace batteries relatively quick to continue fiber testing.

The interface adapter on a visual fault locator is often universal 2.5mm sleeve accommodating many styles of fiber optic connectors including ST, FC, FC/APC, SC and SC/APC. There are a couple manufacturers such as Fiber Instrument Sales that still offer fixed style adapter interfaces on top of handheld meters with SC, SC/APC, FC, FC/APC and LC style connectors. Small form factor connectors such as LC, MU, CS and MDC utilize a 1.25mm ferrule therefore to use a universal 2.5mm tester technicians have to purchase hybrid adapters, or use a hybrid patch cables and mating sleeves to allow testing of LC, LC/APC connectors or other connectors with 1.25mm ferrules.

Trouble shooting with a visual fault locator can be rewarding to any technician. They are a must have tool to closely locate nearby faults. When a test is conducted, the laser red light brightly shines round the fracture or break in optical fiber. This allows red light to escape into the cladding, fiber coatings, buffers or ultimately cable jacket. This red light allows the tech to identify the issue at hand.

Without directly looking into the laser light, technicians can verify continuity or through-put with the VFL. Simply plug into one side of the network link or cable and recognize if light is traveling to the opposite end. If the technician verifies continuity they can eliminate one potential fault of having a broken fiber. However, keep in mind although the cable with the red VFL light shows continuity, it may still have attenuation too high for the receiver to function properly, the VFL will not assist in that determination.

Utilize a visual fault locator to verify correct polarity on multi-fiber cables such as duplex patch cables. One fiber is designated transmit and other receive, if the fibers are crossed the network will not operate. Often identification of this issue can easily be corrected by switching connector positioning. Several manufacturers offer only single fiber VFLs which will require the use of patch cords and duplex mating sleeve troubleshooting LC duplex jumpers. Fiber Instrument Sales, Inc. manufactures duplex visual fault locators with features technicians desire such as duplex adapter interface and polarity toggling between A and B sides as well continuous or modulated lasers for quick identification of fiber.

Locating a bad splice or macro bend beyond fiber radius limitations inside a fusion splice tray is simple chore for a VFL. The red laser will bleed outside the core of the fiber into the cladding and provide visible target for failure. This macro bend will lead to increased attenuation in the form of a non-reflective event on large pieces of test equipment like an OTDR. A poor splice will require a fusion splicer to realign and re-fuse the two fibers. A heat shrink sleeve with metal rod should be used to keep the spliced area rigid to avoid further breakage. Tight bends within a splice tray can be rerouted or positioned so that red laser light is no longer visible.

Using a visual fault locator while installing mechanical splices or quick termination pre-polished mechanical connectors such as Corning Unicams, AFL FastConnect, FIS Bobtail or 3M NPC(no polish connector) style connectors is popular practice among installers. It often requires a patch cable or manufacturer assembly tool along with quick termination connector. This way a technician can visually see the red laser light illuminating the one side with VFL attached. When the incoming fiber is mating to existing fiber within a mechanical splice/connector the VFL red laser light will dissipate. Often not completely disappearing but minimally visible. Due to the strength of a visual fault locator and its close proximity to the mechanical splice common practice among installers is to use 5dB attenuator between VFL and the connector or mechanical splice under test. This will minimize false errors of poor mechanical splice.

Scattered optical fiber inside connector ferrules can be identified with a fiber optic visual fault locator. During the connectorization process at the factory or the field the fiber can be broken off below the physical surface of the ferrule end face. This occurs during the scribing process after the epoxy curing step. A technician can quickly indentify this occurrence. When fiber is scattered below the ferrule end face the entire end of the ferrule will glow red. When properly terminated and polished a technician will not see the ferrule end face illuminated with red laser light. Should this fault be identified the connector will need to be completely replaced.

Identifying a broken fiber under the jacket of patch cable or inside 900um buffer coating of cable is possible with visual fault locator. When the fiber is broken red laser light will escape and leak into jacket material for quick identification. Some colors prove difficult to see the red laser light of VFL such as brown, black or green. This fault locating display is a common practice in teaching environments to locate broken fiber.

Visual fault locators are available as an accessory on several different types of fiber optic test equipment. This includes Optical Time Domain Reflectometers (OTDRs), power meters, time of flight meters such as Fiber Ranger from Fiber Instrument Sales. It can also be found on some fiber optic fusion splicers.

Use extreme caution when using visual fault locators. Do not look directly into the red laser light. Be aware of unsafe visual fault locators sold at power levels dangerous to users and unneeded for communication fiber optic trouble shooting. Standard acceptable laser power is approximately 1 milliwatt. The use of clear adapter caps on patch cords may protect user from direct contact of laser. Mating sleeve manufacturers are now providing SC and LC connector styles of adapters with either external or internal shutters. These internal shutters lift up or retract down when the patch cable is plugged in. When the cable assembly is disengaged the shutter releases and covers the center alignment sleeve and prevents any active laser light from entering the fiber optic technician’s work space. Often these caps are often misplaced or lost during fiber optic network installations. Additional adapter caps or safety caps are available and can be purchased separately.

The fiber optic visual fault locator is must have for a fiber optic technician. It is a relatively inexpensive piece of test equipment that has several functions of trouble shooting a fiber optic network link or cable.