Fibre Optic Connector Endface Geometry

The endface geometry of a PC polished connector has long been known to play an important role in the long-term performance of a singlemode connection. The primary function of the PC polish design is to insure core to core contact between mating connectors while taking into account the effects of time, temperature, vibration and pressure.

The three most important parameters to be controlled in order to achieve this are:

  1. Radius of curvature
  2. Fibre Height
  3. Apex Offset
endface-fig-1

Fig.1

1. Radius of curvature

The endface of ferrules are domed to insure that the contact area between mating connectors is at the centre of the ferrule where the fibre core will be located. The radius of this dome is called the "Radius of curvature". Acceptable radius of curvatures for 2.5mm ferrules is in the range 10mm to 25mm. If the radius is too low, there will be a smaller contact area thus putting more force on the fibre during mating. If the radius is too high, physical contact between the too fibres may not be achieved because there will be a larger contact area resulting in less ferrule deformation. Measurement is performed over a 250 micron fitting area, by calculating the best fitting sphere over this area.

2. Fibre Height (Spherical)

Spherical fibre height is the difference in height between the centre of the fibre and the theoretical height of the ferrule where the centre would be when considered a continuous sphere (Fig. 1). In the past the allowable spherical fibre height was a fixed value, e.g. +/- 50nm or +/-100nm depending on the standard used. This resulted in an unfairly tight tolerance for connectors with low radius of curvatures because the projected centre of the ferrule was higher than for ferrules with low radius of curvatures. To correct this both EIA/TIA and Bellcore have changed to a variable value for allowable fibre height based on the measured radius of curvature. The value is calculated by the following relationship:

Max. allowable height = -0.02R + 1.3R+ 31R + 325

max-allowable-height

Fig.2

3. Apex Offset

Apex offset is a measure of the distance between the highest point of the convex of the polished endface and the centre of the fibre. The objective is for the centre of the fibre to be the highest point on the endface, thus guaranteeing contact between mating fibres. The maximum apex offset allowed is 50 microns. Offset can also be defined as angular offset. Angular offset is the angle between the radial line through the fibre centre and the radial line through the apex of the polish.

apex-offset

Fig.3

100% inspection of endface geometry is vital. Variations in ferrule length, hardness, positioning and polishing component differences all have an influence on endface geometry quality. Nothing can be taken for granted. Fibrepulse inspect 100% of all connector endfaces for the parameters described above. Results are directly entered and stored under the assembly serial number and can be traced back from the cable assembly serial number at any time in the future. Reports are available showing batch results.

For angled connectors there are two more parameters. These are:

1. Angle.

There is no international standard tolerance for the angle of an APC connector because if the apex offset is good when the connector is oriented at the desired angle, then it doesn't matter because point of contact will be at the core for that angle. However, Fibrepulse uses a tolerance 0.2.

2. Key Error.

This is an indication of the orientation of the angle to the connector key. If this is too high it means that the angle will not square up properly to a mated connector, causing an air-gap between the two. The tolerance Fibrepulse uses is 0.25.