(As featured in Utility Products)
 
By Don Allard

Fiber optic cable provides a low loss medium for high-speed communications.  While the continuous fiber cable itself is low loss, terminations at each access point provide a potential Achilles heal.  The biggest cause of signal loss across fiber optic connectors is contamination.  Dust, dirt, oils, and other common contaminates can cause hours of grief for network service technicians.  The good news is proper cleaning tools and techniques allow trained technicians to effectively remove contaminates and get networks back in service.

Fiber Size: A quick note regarding size helps to visualize the impact of dust and dirt.  An average human hair is around 85 microns in diameter.  The core or signal carrying portion of the most widely used optical fibers is 62.5 microns and 9 microns.  It does not take many dust particles to block a 9 micron “window”.  A blocked window means light is blocked and the network experiences signal loss.

Inspection: A fiber optic microscope with proper magnification is necessary to view fiber connector ends for cleanliness.  Typical magnifications used are 200x and 400x.  When selecting a fiber optic microscope, keep safety in mind.  Some microscopes are all optical and can direct laser light into a user’s eye.  Others are available with no optical path to the eye.  While all manufacturers caution users not to view live fibers, it is easy for a technician to inadvertently view a live fiber because laser emissions are invisible to the human eye.

Cleaning: Properly armed with a fiber optic microscope, one can determine if connectors are dirty, cracked, or pitted.  If the fiber end is cracked or pitted, the connector will need to be repolished or replaced.  More likely, the connector end will be dirty.  One’s first inclination may be to wipe the fiber end on a shirtsleeve and clean it like a set of glasses.  This is not helpful.  Lint, dust and other partials big enough to block a 9 micron light path are on cloths regardless of how clean they may look.

Proper cleaning of fiber optic connectors requires fiber optic grade cleaning materials.  Materials vary from optical grade wipes to cassette cleaners such as the widely used Cletop® brand cleaners.  Technicians often use these materials alone to “dry clean” connectors.  While this is often effective, wiping a connector end-face across a dry wipe can add a static charge to the end-face.  During the summer months when the relative humidity is high, static charge may not present much issue.  However, dry winter months – especially in northern climates – and in air-conditioned areas allow static charge to be a significant hindrance to the connector cleaning process.

Static charge build up on connectors attracts charged particles to the connector end-face.  This can effectively negate the dry cleaning process.  A recent study by iNEMI (1), documented the tendency of particles to be drawn to the foremost part of connector end-faces during mating and unmating.  Since fiber optic connectors are polished to have a radius end-face with the fiber core at the center, static charge causes particles to migrate to the optimal signal blocking position - not good.

Static charge is not ESD.  Optical fiber is glass.  Single fiber connector ferules are ceramic.  There is no conductive path from the connector end-face to the fiber jacket.  Thus, ESD protection such as wrist or ankle straps will not eliminate static charge from connector end-faces.  How does one minimize static charge then? 

Fiber optic cleaning fluids are static dissipative.  With an optical grade-cleaning wipe, a technician can wet a corner of the wipe with an appropriate cleaning fluid and drag the connector end-face from the wet area into the dry area.  The fluid will neutralize static charge and help to release particles from the end-face.

IPA Alcohol has long been the fiber optic cleaning standard.  Technicians regularly have 99% IPA in a pump dispenser bottle.  IPA is static dissuasive.  However, it is far from an ideal optical cleaning fluid.  To begin with, it is hydrophilic – meaning it absorbs water.  IPA in a dispenser bottle will absorb atmospheric moisture over time.  This is bad for the cleaning process for two reasons.  First, water slows down the drying process and requires more air to evaporate off all the fluid.  Since most fiber optic technicians are not working in a clean room environment, more air brings more dust particles.  Water absorption into IPA is also bad as water can leave streaks on the connector end-face.

IPA alcohol and some alcohol derivatives bring health and safety concerns.  Alcohol is flammable.  Transportation is expensive, as it is a regulated product.

Fortunately, there are safer alternatives available today.  These newer cleaning fluids dry faster than alcohol, do not absorb water, are non-hazardous, and not regulated.  As always, it pays to do your homework and not just grab the “ole standby”. 

Thus far, we have discussed cleaning jumper end-faces.  This access to the connector is not always possible.  For example, when removing a connector from a patch panel, the mating connector on the backside may not be accessible.  In this situation, a common option is to use a fiber optic cleaning stick to clean the end-face inside the adaptor.  One inserts the stick until it makes contact with the end-face.  Then the stick is twisted approximately ten times and discarded.  The twisting motion can generate a static charge on the end-face.  Therefore, a quick drying static dissipative cleaning fluid is a plus. 

Recommended Approach to Cleaning:

• Always assume connectors are dirty prior to mating. 
• A fiber optic termination has two connectors – always clean both!  Cleaning one and remating it to a dirty connector is not productive.
• Use optical quality cleaning materials to clean fiber end-faces.
• Use an optical cleaning fluid to minimize the static build up.
• Use a fiber optic microscope with built in eye-protection to inspect connectors prior to mating.

Cleaning Procedures: It is important to standardize fiber optic cleaning procedures and train technicians on what to do and equally important, what not to do.  This can save time and frustration in the field.  

About the author:
Don Allard is a product manager with AFL Telecommunications. AFL offers a full range of fiber optic products and fiber optic certification courses.
www.afltele.com
donald.allard@afltele.com 

1. iNEMI, Accumulation of Particles Near the Core during Repetitive Fiber Connector Matings and De-matings, Presented at NFOEC March 29, 2007, http://thor.inemi.org/webdownload/newsroom/Presentations/OFC_NFOEC_2007/Accumulation_of_Particles_Near_Core.pdf
 
Note: Cletop is a registered trademark of NTT