# Improve Apex Offset and Angle Measurements with APC Polishing

## Introduction

Of the two standard ferrule shapes involved in APC polishing – step and conical– the step ferrule is definitely the easiest with regards to controlling geometries. Step ferrules are comparatively expensive. However, from only process-control point of view, step ferrules are undeniably the easiest and best choice. If one is new to APC polishing, it is highly recommended to choose step ferrules over conical. Step ferrules are a lot easier to work with when it comes to developing and altering polishing processes, and are less sensitive to minor process and material variations. Thus, using step ferrules results in considerably better first-pass yields.

To ensure best performance between mated pairs of APC connectors, it is vital that the ferrule endface geometries match or surpass industry-accepted endface geometry standards1. The most typical issues that polishing process engineers come across regarding APC polishing geometry usually include the Apex Offset and Angle measurements. There is quite a bit of intricate trigonometry to mathematically “prove” the geometrical consequences behind the forming and measuring of a curved angle across a cylindrical or conical object (the ferrule). But it is not essential to delve too much into complex math. Simplified diagrams are adequate to help polishing process engineers visualize the elementary principles at work, enabling them to properly control their polishing procedure to meet the product’s geometry requirements.

To comprehend APC polishing, it is helpful to begin by reviewing PC polishing. The mechanisms at work impacting Apex Offset and Angle are the same, but it is simpler and more intuitive when explaining PC polishing.

## Apex Offset and Angles in PC Polishing

Apex Offset is quite a simple concept to understand in PC polishing. With PC ferrules, the idea during polishing is to hold the ferrule at a 0.0 degrees vertical angle (perpendicular to the polishing surface). Given that the ferrule is perfectly perpendicular to the polishing surface, the Apex (highest point) of the radiused endface will be the precise center of the ferrule, and Apex Offset value will be zero2 (refer Figures 1a and 1b).

In case the ferrule is not perpendicular to the polishing surface, the Apex of the curved endface will not be centered on the ferrule: an Apex Offset value > 0 (refer Figures 2a and 2b).

The more the ferrule is angled away from perpendicular, the farther away the Apex of the Radius will be from the ferrule’s center – therefore the larger the Apex Offset value will be. Apex Offset is directly proportional to the angle with which the ferrule was polished. Actually, Apex Offset and Angular Offset are two ways to state the same thing; they only use diverse units of measurement. (Apex Offset is measured in microns from the fiber center, while Angular Offset is measured in degrees from the fiber center axis.) Since Angular Offset is a common reason of Apex Offset measurement problems, polishing process engineers can identify the following as a basic Geometry Process Rule:

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

Additionally, since the endface is polished with a radiused (domed) surface, it is known that the surface angle of any two points along the curve will not be equal. Specifically, the Angle at Apex point and the Angle at ferrule centerline point can never be equal (refer Figure 3). (It is assumed the dome created is perfectly spherical, which is unlikely in reality but is not applicable to the discussion.)

Another vital concept to bear in mind: The tangent angle at Apex point will always be precisely the angle at which the ferrule was held during polishing. If the ferrule was held at precisely 0.4 degrees from perpendicular during polishing, this will create an angle at my Apex point of precisely 0.4 degrees from horizontal. This concept is crucial to remember when talking about APC polishing.

## Apex Offset and Angles in APC Polishing

All of the above also relates to polishing of APC ferrules – the only variance is that, with APCs, the ferrule is meant to be held at an angle to the fiber/ferrule axis during polishing (normally 8 degrees) instead of perfectly vertical (0 degrees). This adds other factors that have substantial influence on Apex Offset and Angle measurement values – such as ferrule Shape, Key Error, and End-Face Radius.

Note that Key Error is a big component of measured Apex Offset values. Key Error is the “rotational component” of Apex Offset. For instance, if the adapter keyway slot in the interferometer is considerably wider than the key width on the APC connector, it is possible to rotate the connector somewhat during measurement, which causes erroneous measurement values. It is a complex topic to completely explain and requires its own article. Due to this complexity – and the fact that Key Error issues are less common than Angular problems – Key Error will not be discussed in great detail in this article.

First, the ferrule shape is looked at. There are two standard types of APC ferrules in terms of shape: step and conical.

The conical ferrule (Image 1), obviously, 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.

Figures 4a and 4b show cross-section diagrams of the two ferrule types. The gray areas are the ferrule material, the center is the fiber, and the red-dashed lines highlight the centerline of the ferrule diameter.

The results of cutting an 8-degree angle through both of these shapes need to be considered - first the conical ferrule shape.

## Conical Shape and Apex3

When a straight line is cut through the conical ferrule, it can be seen that – because of the cone shape – any angle cut flat across the surface leaves us with uneven distances from the center to the outside edges of the ferrule.

In Figure 5, it is clearly seen the distance from the center to the left (lower) edge is larger than the distance from the center to the right (higher) edge:

If a straight line is drawn across the angled surface, its mid-point will not be – and can never be – in the center of the ferrule (Figure 6).

As the material is polished more and more from the cone shape, the center point moves farther and farther away from the centerline. This mid-point will never be in the center of the ferrule, and will move farther and farther away from center as more material is removed – and cut the ferrule shorter and shorter (Figure 7).

Looking at what happens when that surface has a radiused face instead of being perfectly flat. In Figure 8, 5 mm radii have been added to two different locations on the ferrule. As estimated, the ferrule Apex will be even farther from center than the mid-point of the cut. Plus, it will continue to move farther and farther from the ferrule center as more material is taken away.

Now if a conical ferrule is polished at 8 degrees – then measure in an interferometer (which also holds the ferrule at 8 degrees) – it is virtually impossible to obtain good Apex Offset measurements. Thus, here is another Geometry Process Rule:

• Conical APC ferrules must be polished at an angle greater than 8.0 degrees (normally either 8.2 or 8.3 degrees).

Because ferrules are always held at precisely 8.0 degrees when measuring in an interferometer, if the conical ferrules are polished at 8.3 degrees, the measured Apex will be closer to the center of the ferrule. The shape of the ferrule remains constant – the “ferrule Apex” is still far off center. But since the measurement is done from a different relative angle, the “measured Apex” will be a different point4 (refer Figures 9a, 9b, 9c).

By using a larger angle to polish than regularly used to measure, much better Apex Offset measurements was achieved. But it still holds that, because of the conical shape, the more material removed, the worse the measured Apex Offset value will be (the ferrule Apex will continue to move “to the left” or toward the lower side of the ferrule). This provides another useful Geometry Process Rule:

• For any given Angle and Radius with conical ferrules, the more it is polished, the more Apex Offset will move to the direction of the lower side of the angled ferrule.

## Conical Shape and Angle

As discussed earlier (refer Figure 5), each point along a domed surface has a different angle to horizontal – the tangent to the Radius at that point. At the ferrule Apex, the Angle will be exactly the same as the Angle the ferrule was held at during polishing. But for interferometer measurements, the Angle at the fiber region: the center of the ferrule. And the Angle at the ferrule center (the fiber region, where Angle values are measured) will always be less than the Angle at which polished (refer Figure 10) when the Apex is on the “low side” of the angle, and will always be greater than the Angle at which polished (not drawn) when the Apex is on the “high side” of the angle.

As more and more material is removed and cut farther down the conical ferrule, the Angle at Apex stays constant, while the Angle at centerline (the fiber area) gets smaller and smaller. And since only measuring of the Angle at the fiber region of the ferrule is required here, another Geometry Process Rule can be concluded:

• For any given Radius and polishing Angle, the more polishing done to a conical ferrule, the smaller the angle in the fiber region will become.
• If the measured angle is too big, it can be fixed by just polishing more material away (more polishing time).
• If the measured angle is too small, it cannot be fixed by polishing more – it would require modification of the process to remove less material (less time, or less aggressive polishing films) and begin again.

The effect Radius value has on Angle in the fiber region. It is a well-known fact that, because the surface is spherical, the Angle at the fiber region will always be different than the Angle at the ferrule Apex (refer Figure 10). But the magnitude of that difference in Angle relies on how curved the Radius is. A larger Radius (“flatter”) will have a smaller difference; a smaller Radius (“rounder”) will have a larger difference (see Figures 11a and 11b).

This gives two more Geometry Process Rules:

• Increasing Radius will increase the Angle at fiber region. (This applies to step and conical ferrules.)
• Increasing Radius will decrease the Apex Offset. (This applies to step and conical ferrules.)

With conical ferrules, the depth to which a cut is made can be seen – how far down the ferrule can be cut and how much material is to be removed – needs to be properly controlled if the aim is to achieve controlling measured Apex Offset and Angle values. The step ferrule does not require such focus on depth of cut.

## Step Shape and Apex

The effects of cutting a radiused Angle through a step ferule are a lot easier to explain and comprehend. Due to its cylindrical shape, any Angle cut flat across the surface will have precisely the same distance from the center to the outside edge as illustrated in Figure 12.

Since the constant diameter of the cylinder, the centerline of the Angle will always be in the center of the ferrule, irrespective of how much material is taken away in the polishing process (refer Figure 13).

When polished with a Radius, it can be seen that, like conical ferrules (refer Figure 8), the ferrule Apex will also always be offset from the ferrule center.

However, due to the symmetry of the cylinder shape, this offset will be greatly less than with conical ferrules as shown in Figure 14.

An inherent Apex Offset is present for polishing a domed surface on a step ferrule, but this offset is minor and a standard eight-degree polishing fixture can be employed.

## Step Shape and Angle

It is known that the Angle at the fiber region will always be less than the Angle that is being polished – this is true of both step and conical ferrule shapes. But as with Apex, the magnitude of the difference is a lot less when using step ferrules. Since the ferrule Apex is closer to center, the difference in Angle will be considerably less – insignificant, in fact. And, as with Apex, the Angle at the ferrule center will not vary irrespective of how much or how little polish is done.

This effect is precisely the same as with conical ferrules (refer Figures 11a and 11b), and the same rules provided in that section relate to step ferrules.

## Conclusions

Controlling Apex and Angle on conical ferrules is a lot more difficult than with step ferrules, specifically because they need to control the “depth of cut” – the quantity of ferrule material they have remove during the polishing process. This means it must guarantee the polishing films used have excellent lot-to-lot abrasive consistency. Likewise, since the abrasive cut-rate of polishing films decreases the more times the film is used, the number of times the films are reused in the process (this will increase polishing costs) is reduced. Because the step ferrule is not influenced by depth of cut, abrasive cut-rate variation has a lot less effect on the results.

With both step and conical ferules, the Radius generated has a similar effect on measured Apex and Angle. On the whole, targeting a Radius at the lower end of tolerance limits will enhance chances of acceptable Apex Offset and Angle measurements.

Reference

1. Telcordia GR-326; IEC 61755-3-2

2. This statement is a bit of an oversimplification. There are other factors that certainly can and do affect Apex Offset and Angle values on both PC and APC polishing such as imperfections in material or equipment, errors in measurement, process errors made by operators, etc. Our discussion is focused on the concept of generating a radiused Angle through an APC ferrule and how our polishing process, in ideal conditions, affects the resulting Apex Offset and Angle measurement values. So, for simplicity, we will ignore such non-process-related factors.

3. The images of the conical and step ferrules show unpolished ferrules – there is no Angle yet generated on the tips. Both types of ferrules are available to purchase as “pre-angled,” where the ferrule manufacturer cuts the angle already. This significantly reduces polishing time and costs for the cable assembly. Whether using pre-angled ferrules or not, however, the same factors affecting Angle and Apex discussed in this paper apply.

4. The term “Apex” is generally understood to mean “the highest point along a surface.” But we need to understand that “highest point” is a RELATIVE term. Highest, in relation to what? Let’s use an example of an orange traffic cone. Imagine placing the cone on the floor and standing over it, so you are looking straight down on the cone. From this position, you could say the Apex of the cone is the point closest to your face. But what happens when you kick the cone over onto its side – essentially tilting it 90 degrees? Now the point closest to your face is not the tip of the cone, instead, it’s the edge of the cone’s base. You did not change the location of the Apex point on the object (the cone), you only changed its relative position in space from where you are measuring (your eyes). Thus, in our discussion, we need to be careful to distinguish between the FERRULE’S Apex (the highest point on its Radius, relative to the Angle to which it was polished) and the MEASURED Apex (the point on the ferrule surface closest to the interferometer camera during testing). This is particularly important when discussing conical ferrule polishing, as the angle at which we polish the ferrule and the angle at which we measure the ferrule are not the same – we essentially “kick the cone over” a bit during measurement.

This information has been sourced, reviewed and adapted from materials provided by Fiber Optic Center, Inc.