APC Polishing Advice to Improve Apex Offset and Angle Measurements

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Of the two common ferrule shapes
involved in APC polishing – conical and step – the step ferrule is by far the
easiest in terms of controlling geometries. Step ferrules are more expensive
than conical ferrules. However, from a purely process-control point of view,
step ferrules are absolutely the easiest and best choice. If you are new to APC
polishing, it is highly recommended to use step ferrules over conical.
Step ferrules are less sensitive to minor process and material variations, and
they are much easier to work with when it comes to developing and modifying
polishing processes. Thus, using step ferrules results in significantly
improved first-pass yields.

 

To ensure optimum performance between
mated pairs of APC connectors, it is important that the ferrule endface
geometries meet or exceed industry-accepted endface geometry standards1. The
most common issues that polishing process engineers face regarding APC
polishing geometry usually involve the Apex Offset and Angle measurements.
There’s quite a bit of complex trigonometry to mathematically “prove” the
geometrical consequences involved in forming and measuring a curved angle
across a conical or cylindrical object (the ferrule). But it’s not necessary to
delve too deeply into complex math. Simplified diagrams are sufficient to help
polishing process engineers visualize the basic principles at work, enabling
them to better control their polishing process to meet the product’s geometry
needs.

 

To understand APC polishing, it is
helpful to start by reviewing PC polishing. The mechanisms at work affecting
Apex Offset and Angle are the same, but it is simpler and more intuitive when
describing PC polishing.

 

Apex Offset and Angles in PC polishing

 

Apex Offset is a fairly simple concept
to understand in PC polishing. With PC ferrules, the intention is to hold the
ferrule at a 0.0 degrees vertical angle during polishing (perpendicular to the
polishing surface). As long as the ferrule is perfectly perpendicular to the
polishing surface, the Apex (highest point) of the radiused endface will be the
exact center of the ferrule, and Apex Offset value will be zero2 (Figures 1a
and 1b).

 

The more the ferrule is angled away from
perpendicular, the farther away the Apex of the Radius will be from the
ferrule’s center – thus the larger the Apex Offset value will be. Apex Offset
is directly proportional to the angle with which the ferrule was polished. In
fact, Apex Offset and Angular Offset are two ways to express the same thing,
they just use different units of measurement. (Apex Offset is measured in
microns from the fiber center, whereas Angular Offset is measured in degrees
from the fiber center axis.) Since Angular Offset is a common cause of Apex
Offset measurement issues, polishing process engineers can identify the
following as a general Geometry Process Rule:

 

All other factors being equal, the
bigger the difference between the intended polishing angle and the actual polishing
angle, the bigger the measured Apex Offset (or Angular Offset) value will be.

 

Furthermore, since the endface is
polished with a radiused (domed) surface, we know that the surface angle of any
two points along the curve will not be equal.That is, the Angle at my Apex
point and the Angle at my ferrule centerline point can never be equal (Figure
3). (We’re assuming the dome created is perfectly spherical, which is unlikely
in reality but is not relevant to our discussion.)

 

Here’s another important concept to
keep in mind: The tangent angle at my Apex point will always be exactly the
angle at which I held the ferrule during polishing. If I held my ferrule at
exactly 0.4 degrees from perpendicular during polishing, this will produce an
angle at my Apex point of exactly 0.4 degrees from horizontal. This concept is
important to remember when we discuss APC polishing.

 

Apex Offset and Angles in APC polishing

 

All of the above also applies when
polishing APC ferrules – the only difference is that, with APCs, we are
intending to hold the ferrule at an angle to the fiber/ferrule axis during
polishing (typically 8 degrees) instead of perfectly vertical (0 degrees). This
introduces other factors that have significant impact on Apex Offset and Angle
measurement values – namely ferrule SHAPE, end-face RADIUS, and KEY ERROR.

 

Note that Key Error is a major
component of measured Apex Offset values. Key Error is the “rotational
component” of Apex Offset. For example, if the adapter keyway slot in my
interferometer is significantly wider than the key width on my APC connector,
you can imagine it is possible for me to rotate the connector slightly during
measurement, resulting in erroneous measurement values. It is a complicated
topic to fully explain and is worthy of its own article. Because of this
complexity – and the fact that Key Error problems are less common than Angular
problems – I will not discuss Key Error in great detail in this article.

 

First, let’s look at ferrule shape.
There are two common types of APC ferrules in terms of shape: conical and step.

 

The conical ferrule (Image 1),
naturally, has a cone shape to the polishing area of the ferrule.  The step ferrule (Image 2) has as cylindrical
shape to the polishing area of the ferrule.