Fiber Optic Polishing Fixture Maintenance

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To help ensure consistent polishing
results, maintenance of all fiber optic polishing equipment is essential.    The Preventive Maintenance program for polishing
fixtures should include periodic ultrasonic cleaning.  This is true for any fiber optic polishing
fixtures, regardless of connector or ferrule type:  connector fixtures with latching mechanisms
(LC, SC), ferrule-only fixtures with screw caps or cam-locks, MT ferrule
fixtures, etc.

 

All fixtures come from the factory with
very precisely milled ferrule “bores”, tailored specifically to the particular
ferrule which will be polished.  These
bores position the ferrule to precise angles to the polishing
surface—–90-degrees for PC polishing, other angles (typically 82-degrees, to
create an 8-degree finish) for various APC polishing.  (Note: 
I use the term “bore” for the circular holes milled into the fiber optic
polishing fixture which hold the round ferrule, but the same concepts apply to
fixture for rectangular ferrules such as MT. 
For simplification, this discussion will reference typical round,
PC-finished ferrules, but the concepts all apply to MT fixtures as well as APC
fixtures).

 

The level of precision to which the
fixture holds the ferrules determines many of the ferrule’s geometric
characteristics (Apex, Radius, Fiber Height, Ferrule Angle, etc).   Contamination of the fixture can negatively
affect this precision, by either pre-maturely wearing of the fixtures
dimensions, or by creating buildup in the fixture, which prevents the ferrule
from seating properly.

 

The contamination we are discussing
here is primarily a natural result of the normal polishing process.  Ferrules are polished on abrasive films or
flock-pile pads impregnated with abrasives. 
During polishing, some of these abrasive particles, along with the
ceramic or plastic material being removed from the ferrule itself, come free
and mix with the water used as lubricant within the process.  This results in an abrasive slurry that
contaminates the fixture and the ferrules themselves.  Common polishing practices require the
operator to clean the bottom of the fixture between polishing cycles, to remove
abrasive particles that may create fiber end-face scratches in subsequent
polishing steps.  However, this slurry
can (and will) migrate into crevices and the bores of the fixture—places
difficult or unlikely to be fully cleaned or flushed out during normal the
normal polishing process.  Allowed to
dry, it will harden and cake.

 

The 4 major effects of this
contamination to consider:

 

Contamination within the ferrule bore
is wearing the bore, creating larger bore diameter:  Ferrule bore wear is unavoidable over time—as
thousands of ceramic ferrules are inserted and removed from a stainless steel
fixture, the bore will wear.   However,
contamination of the bore, particularly with abrasive particles naturally
resulting from the polishing process (the “slurry” referred to above), can
accelerate this wear.  As the bore
diameter becomes larger, the ferrule is held to less than 90-degrees to the
polishing surface, resulting in larger Apex Offset and Angle values.

 

Contamination within the ferrule bore
is building up, “pushing” the ferrule to one side:  the slurry generated from polishing can
accumulate in the bore or other areas of the fixture, building up to prevent
the ferrule from seating at precisely 90 degrees to the polishing surface.  This can also result in large Apex Offset and
Angle values.  This buildup issue is
often seen in MT ferrule polishing fixtures.

 

Contamination within the ferrule bore is
preventing the ferrule from sliding freely within the bore:  This mainly applies to connector polishing
(as opposed to ferrule-only polishing) where the connector’s spring force is
relied on to present equal downward pressure to the ferrules during polishing.  The ferrules in a connector fiber optic
polishing fixture must be able to piston up-and-down freely in the
fixture.  If contamination of the ferrule
bore holds the ferrule too tightly, this may “lock up” the ferrules or prevent
them from pistoning consistently / smoothly within the bore during
polishing.  This, in turn, will result in
different pressures being applied to the end-faces during polishing—–which
creates variation in Radius values, FH values, and fiber end-face polish
quality (scratches).

 

Contamination on the fixture prevents
ferrules from protruding equal distance from the base of the fixture. This
mainly applies to fixtures for polishing ferrule only (as opposed to connector
polishing).  The ferrules must protrude
the same amount from the bottom of the fiber optic polishing fixture, to ensure
consistent pressure being applied to all ferrules.  Contamination at the top of the bore, where
the ferrule flange sits on, will cause the ferrules to sit higher and protrude
less.   If ferrules are protruding
different length from the bottom of the fixture, this results in unequal
pressures being applied to the ferrules during polishing, again resulting in
variation in Radius values, FH values, and fiber end-face polish quality
(scratches).

 

Fiber Optic Polishing Fixture
Maintenance Ultrasonic CleaningThe most effective way to reduce the effects of
such contamination is to thoroughly clean the fixtures between use, by rinsing
with distilled water immediately after polishing, to prevent the “surry” from
drying.   Even so, particularly with
fixture having many recessed faces or “nooks and crannies”, it is good practice
to give each fixture a more rigorous cleaning after use, when the fixture will
be not be in use for some time (such as at end of shift).  Ultrasonic cleaning, in combination with
brush-cleaning the bores, works very well for this type of cleaning, and it is
recommended that cleaning be part of a daily Preventive Maintenance program for
all fiber optic polishing fixtures.

 

Ultrasonic Cleaning:

 

Ultrasonic cleaners are a bath into
which the object to be cleaned is submerged. 
The bath is usually of distilled water, which can be augmented with
detergents or solvents or other cleaners. 
For cleaning of polishing fixtures, an ultrasonic bath of only distilled
water is sufficient, and recommended over other solutions.

The walls of an ultrasonic cleaner
vibrate and a particular high frequency, transferring pressure waves to the
water. These waves have the effect of pushing-and-pulling the water molecules
to create “cavitation”:  essentially
“tearing” the water molecules apart, creating a very short-lasting “bubble” of
vacuum within the water, which very quickly—and forcefully—implodes. The energy
of this collapse, which occurs near the surface of the submerged object, is
what creates the agitation to remove contaminates from the surface.

 

Because the cavitation occurs within
the water medium only, it is important that the entire object be submerged,
with no air bubbles trapped in areas to be cleaned.  As long as the surfaces to be cleaned are in
contact with the water, and because the cavitation happens on the molecular
level, the cleaning effect can reach into very small crevices and is ideal for
fiber optic polishing fixture cleaning.

 

It is important to note that the
cavitation is created by physical movement of the side walls of the cleaner,
and that most ultrasonic cleaners are calibrated such that most of the
cavitation power occurs somewhere in the middle of the bath.  It is important that objects to be cleaned
are suspended within the bath water, and do NOT contact the bottom or interior
walls of the ultrasonic cleaner.  
Objects contacting the bottom or sidewalls of the cleaner will reduce their
movement, and thus reduce the cavitation effect.

Following up the ultrasonic cleaning
with brushing of the ferrule bores and blowing dry with clean, compressed air
will help to ensure maximum life and performance of the fiber optic polishing
fixture.