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.