IAN LANG ELECTRONICS

Optoelectronics

Bending Loss.

The usual cause in bending losses is cables being bent in too sharp a manner. This is known as macrobend loss. A cable should be bent in a smooth curve, and not at an angle. The radius of the curve is important too. As we saw when we looked at refraction, the normal is always at right angles to the surface of the core and so wherever the core changes direction so will the normal. The ray can now find itself entering a section of cable at less than the critical angle and escaping the core. Cables come with a specified minimum bend radius but most technicians see a 50 mm or 2 inch radius as the absolute safe minimum regardless of the cable specs as the tighter the bend the more the loss. A rule of thumb for unknown cable specifications is to make the bend radius ten times the outer diameter of the cable, or if this is less then 50 millimetres then make the radius 50 mm.
Macrobend losses are not a manufacturing defect. They are solely caused by the installer, and most will use a radial guide for the type of cable they are installing.

The other type of bending loss is a microbend loss, which is almost always a manufacturing defect. The radius of a microbend loss is equal to or less than the diameter of the fiber and is inside the cable. A typical cause is differential expansion of outer layers and optic fibre. If one is too hot or too cold they will expand and contract at different rates and the core cladding is likely to "kink" and cause a microbend. the answer is to carefully control the temperature in manufacturing and storage of the cable.

Fresnel Loss.

Fresnel losses are due to reflections when sections of cable are coupled together. Some light is always reflected back the way it came, and this leads to an attenuation at the receiving end. We can reduce Fresnel loss by not having surfaces that are too reflective.

These four constitute the means of attenuation in fiber optics and we will now turn our attention to distortion.