How to Develop a Fiber Optic Polishing Process that Produces Consistent Results

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Frequently, my customers ask for
guidance to develop a fiber optic polishing process.

 

While there are many details to
consider, the process comes down to 5 basic steps.

 

Note that this article focuses on
single-fiber (and not MT) PC ferrules. While much of my advice applies to APC
polishing, those connectors require additional considerations.

 

 

 

Step 1. Define your objectives,
especially measurement criteria and process yields.

 

When developing a polishing process,
it’s important to clarify your acceptance criteria and targeted yields – and
have the ability to accurately measure and quantify pass/fail results. For
example, do your polished ferrules need to meet an end-face geometry
specification such as those specified in Telcordia GR-326? Also, virtually all
polished fiber optic ferrule end-faces must meet SOME visual appearance
specifications. What criteria will you use to determine if your polishing
results are acceptable or not?

 

When developing a polishing process, I
highly recommend that your initial targets are to meet GR-326 geometry
requirements and IEC 61300-3-35 visual requirements – with a minimum yield
expectation of 95% passing.

 

Step 2. Ensure you have the proper
equipment.

 

Of course, it would be ideal if you are
starting with all-new polishing equipment. The precision of polishing equipment
dimensions (both the fixturing as well as the machine itself) play a vital role
in determining your process capabilities. Properly maintained polishing
machines should last for decades, so be sure to follow the manufacturer’s
maintenance recommendations.

 

Polishing
fixtures wear out more quickly
depending, of course, on the amount of use. If you follow the recommendations
below and still find it impossible to get acceptable results (particularly in
Apex Offset), it’s very possible you may need to procure new fixtures.

 

Regardless of whether your customer or
product design requires you to meet geometry specifications, it is HIGHLY
recommended that you measure geometries during process development, using an
interferometer. This is an expensive piece of equipment (typically $20,000 or
more), but it is invaluable to developing a robust polishing process. If the
cost is prohibitive, look into renting or borrowing an interferometer.

 

Be sure your equipment is capable of
properly measuring what you need it to measure, particularly regarding visual
inspection. For example, if you need to meet single-mode requirements of IEC
61300-3-35, does your microscope have a high-resolution system as defined
within the spec? If you are inspecting APC ferrules, do you have an APC adapter
for your scope, or just the PC adapter? (Unfortunately, it’s fairly common to
find production lines inspecting APC ferrules with a PC adapter, which is
improper. With the APC ferrule end-face at an angle to the microscope optics,
the entire fiber end-face will never be in proper focus, making it very
difficult to detect small defects.)

 

When developing a polishing process, be
sure to use only brand-new rubber polishing pads. Rubber pads play a critical
role in polish results and are often overlooked. Having a consistent hardness
(durometer) across the surface of the rubber pad is CRITICAL in getting
consistent polishing results. Rubber hardness will change over time, so rubber
pads always need to be replaced periodically (once a year, at minimum).

 

Step 3. Conduct a trial run, following
our recommended procedures.

 

Fiber Optic Center offers Recommended
Polishing Procedures for Domaille and Seikoh Giken polishing machines – by far
the two most popular polishing platforms used today. However, due to variations
in ferrule material, connector design, and even in machine/fixturing
dimensions, it is likely that you will need to modify some of the recommended
machine settings. Use Fiber Optic Center’s recommended polishing settings as a
starting point, and budget time to modify as needed based on your results.

 

Step 4. To adjust your process, start
with a focus on Radius.

 

The ability to achieve desirable,
consistent Radius values is critical to developing a fiber optic polishing
process. Radius is the first geometry parameter to bring under control. Why?
Because the Radius value is an excellent indicator of consistency in polishing
pressure and the amount of ferrule material removed during the process –
important factors that highly impact all results, including visual appearance
and angle (particularly with conical-style APC ferrules). When all polished
ferrules have an identical Radius after polishing, this indicates that all
ferrules have undergone consistent polishing conditions.

 

After polishing a group of connectors,
use the interferometer to measure geometry and focus on the Radius values. As a
rough rule of thumb, a stable Radius variation on one plate of ferrules should
be less than 2mm. If the variation is higher, you likely have a process issue
or your polishing equipment is subjecting ferrules to varying pressure.

 

Generally speaking, the smaller your
average Radius value, the smaller the deviation in results will be. It is
usually a good idea to aim for the lower end of the Radius specification rather
than the middle or upper end of the spec. For example, if the Radius spec
limits are 7-25mm, you’ll usually have better and more consistent results if
you target 10-12mm as opposed to 18-20mm.

 

How to decrease or increase Radius
values:

 

Decrease Radius values by applying more
pressure or using a softer rubber pad.

Increase Radius values by applying less
pressure or using a harder rubber pad.

When your Radius values are under
control, look at another key geometry parameter: Apex (also called Apex
Offset). This reflects the angle of the polished end-face. If the Apex values
are too large, this means your ferrules are being polished at an angle that is
too large. All other conditions being equal, a smaller Radius value will
generally result in a better Apex value. This is another reason to aim for an
average Radius value toward the lower end of spec limits. If you’re already
dialed-in on desired Radius values and the Apex values remain unacceptable, the
cause is likely related to worn polishing fixtures or worn rubber pads. In both
cases, the only fix is to replace the worn fixture or rubber pad.

 

Step 5. Conduct a visual inspection.

 

Your customers will require some type
of visual inspection. (Clearly, you cannot ship a connector with a shattered
fiber.) After achieving desired geometry values, measure the visual appearance
of the end-face, looking for 2 types of defects: pits and scratches. Typically,
pitting can be resolved by increasing time during the intermediate (diamond)
polishing steps. Consistent appearance of a large quantity of fine scratches
(the “skating rink” appearance) is likely due to insufficient time and/or
pressure on the final polishing step. Sporadic appearance of a few scratches is
likely caused by contamination, due to insufficient cleaning.

 

When developing a polishing process,
carefully evaluate other major visual defects: shattered fibers, core cracks,
no visible fiber, etc. In many cases, the cause of the defect is not related to
your polishing process but rather to cleaving, product handling, curing
temperatures, and so forth. These defects are real and need to be corrected,
but you should address the root cause of these issues by preventing them from
happening and not relying on your polishing process to remove large fiber
defects, which only adds to the time, cost, and instability of your polishing
process.

 

Final thoughts:

 

Cleaning is critical – The act of
polishing removes material, similar to sanding wood. To prevent contamination,
it’s vital to thoroughly clean the fixture and ferrules between every polishing
step. Contamination causes scratching and will negatively impact visual yields.

Focus on preventive action, not
corrective action – As you develop your fiber optic polishing process, I
encourage you to implement a statistical process control system. For example,
over time, rubber pads and fixtures will wear out, and yields will begin to degrade.
Record the data, track it, and actively analyze it. This will help you to
prevent problems rather than fixing issues after they occur. Watch the trends
and take preventive action, and you’ll consistently see high yields.