Today fiber optic network installations have become rather standardized with BICSI, TIA and IEEE all creating standards for installers to follow.  While current telecom installations follow these standards, the field technicians of today must also deal with installations from the past.  Installations from years past don’t always follow the standards that are in place today.  Being able to navigate unlabeled and undocumented fiber optic installation is a daunting challenge for technicians unfamiliar with troubleshooting.  Among the various tools available to the field technicians, few are as simplistic and effective  as the Visual Fault Locator.

The Visual Fault Locator or VFL (as it is commonly know as) offers a simplistic option to technician who need to troubleshoot and trace cables within an installation.  The VFL is truly a compact, powerful tool which is highly mobile and adaptable to many situations.  Commonly using the 650nm visible red light, VFLs are able to travel up to 5-7km through Singlemode fiber.  The 650nm red light is very useful since it is highly visible and tends to glow through any event or section of fiber which does not transmit light efficiently.

Visual Fault Locator

Visual Fault Locator

Visual Fault Locators can be used to trace fibers between panel locations, identify bends in fiber or patch cables, identify bad fusion splices  and poorly performing connectors.  Since the red 650nm light bleeds through patch cable jackets, identifying bends is very easy under most circumstances. Connectors which have poor performance can sometimes be seen glowing in patch panels or have red light glowing through the jacket before or after the connectors.  Inspection of patch panel with VFLs running can also help identify which port a fiber terminates at.

Generally speaking a bad fusion splice is highly visible in the splice tray when using a VFL as red light tends to glow through the clear fusion splice sleeve.  If a technician is trying to identify a bad splice within an OSP or ISP enclosure, it is a great idea to put a VFL on the fiber before opening the enclosure.  ISP and OSP splice points tend to have a high volume of fiber contained within a small footprint.   The combination of high fiber counts and small foot prints tend to make splice enclosures a likely spot for bends.  By using a VFL in this scenario, a technician should be able to identify a bad fusion splice and a bend as soon as the enclosure is opened.   This practice can greatly reduce the time associated with fixing bad fusion splices and bends.

Simplicity Prevails

Several years ago while at an extremely isolated location in the desert a customer had an issue with their phone service which was transmitted over fiber optic cable.  The network infrastructure at this location was installed by random contractors over several year with almost no quality control or documentation.  At one point we were asked to consult on a troubleshooting project which had been in progress for nearly 2 weeks already, with absolutely no progress.  Needless to say, patience was at an all time low. Contractor and end user could not come to an agreement over the issue.

During our initial investigation of the situation we found a Cisco switch running an SFP module with two Singlemode LC/UPC connectors installed.
The run of fiber was approximately 4000 feet long and went from an office building to a cell tower.  Based on the OTDR test performed by the current contractor, the span of fiber was losing 8.5 dB over the link.  The current diagnosis from the contractor on site was that the office needed a bigger switch.  The contractor repeatedly told the customer that 8.5 dB was too much power for the system to lose; although they had seen a system loose 23 dB and still work.   When we asked to see the OTDR trace to examine their findings, we were quickly denied.  The contractor quickly cited several security protocols and told us this issue wasn’t our concern.  This contractor was completely unwilling to work with us and jumped at every opportunity to discredit us.

Finally our customer told the contractor the he was going to be required to provide us with several tools for troubleshooting the system without him.  This request was quickly denied and the contractor left to go get a purchase requisition slip to be signed for a new switch.  At this point the customer told us he didn’t care what security protocol were about to be broken, this link was going to get fixed.  Armed only with a connector cleaner, inspection microscope and two visual fault locators we set out to find the issue at hand.

A technician connects a VFL to a patch panel using a patch cable

Technician connecting a VFL to the patch panel using a short patch cable

We decided to trace this fiber from it’s origin first and eliminate any potential issues within the building. Our first stop took us up to the second floor into a telecom closet that would make any technician cringe.  Cables were strung this way and that way, 20 meter patch cables were tied in knots to secure the excess slack, color codes were not followed, connectors were glowing red light through the mating sleeves when the lights were turned out and a thick film of dust coated everything in sight.  Once we located our fibers glowing in the panel we turned the VFL off to inspect the connectors. When we looked at the connectors in a microscope the fact that they even transmitted light was amazing.  Cleaning was clearly not a priority.   “If it passes light its alright” was a major theme around this place.  We cleaned the connectors which did help the transmission of light somewhat, but this was not the problem at hand.

Next we headed for the cell tower to examine the state of the connectors at the active equipment.  We found two connectors based on the customer verifying the location.  Both were in decent shape compared to the previous connectors in the closet.  Once the connectors were inspected and considered  “acceptable” we turned the VFL back on to evaluate the intensity of the red light.  Nothing came out…

At this point we were confident that the switch and SFP were not the immediate issue as the contractor had repeatedly insisted.  As we returned to the office the  contractor was almost done filling out the purchase requisition.  After a few moments of the contractor speaking with the customer things got very heated as the contractor was tired of being questioned.  At this point we stepped in and pointed out that if red light of the VFL did not make it to the cell tower, there was no way that his OTDR trace was accurate; they hadn’t reached the true end of the fiber.   Suddenly, the contractor back tracked and started in again about security protocols.  The customer then stepped back in to abruptly reminded the contractor that he had the high security clearance of anyone in the room and that the phone still wasn’t fixed.   On his heels the contractor said “Someone must have unplugged you down at the MDU.”

A quick trip the MDU 2800′ away produced major results as the customer and contractor found two Singlemode connectors dangling from panel with red light being emitted from both connectors.   A quick clean and re-connect and the phone system was up and running without a problem as soon as the network  rebooted.   Later that week the customer informed us that the 4000′ fiber run had been routed through 14 different connection points before it terminated at the cell tower, all connectors; not a single fusion splice to be found.  We all got a pretty good laugh out of that fact.  “8.5 dB through 14 connection points isn’t that bad when you motto is if it passes light it’s alright” we joked.


Troubleshooting fiber optic networks can be one of the most difficult tasks cabling technicians face on a regular basis.  While the OTDR is the ultimate troubleshooting tool in the hands of a skilled technician,  it cannot provide answers to every situation especially when someone doesn’t know how to use it.  Many technicians we speak with don’t have OTDR’s to troubleshoot with on a daily basis.  If you fall into this category remember that the VFL is one of the most basic, affordable and useful tools you can use for troubleshooting.  In our case,  the VFL was the only piece of equipment  we had to work with and it solved a two week old problem in less than two hours.  Sometimes simplicity does prevail.