Introduction to Data Communications
Previous	8d. Transmission Media - Guided (cont'd)	Next

1. Optical Transmission Modes

There are 3 primary types of transmission modes using optical fibre. They are

1. Step Index
2. Grade Index
3. Single Mode

Step Index has a large core the light rays tend to bounce around, reflecting off the cladding, inside the core. This causes some rays to take a longer or shorted path through the core. Some take the direct path with hardly any reflections while others bounce back and forth taking a longer path. The result is that the light rays arrive at the receiver at different times. The signal becomes longer than the original signal. LED light sources are used. Typical Core: 62.5 microns.

Step Index Mode

Grade Index has a gradual change in the Core's Refractive Index. This causes the light rays to be gradually bent back into the core path. This is represented by a curved reflective path in the attached drawing. The result is a better receive signal than Step Index. LED light sources are used. Typical Core: 62.5 microns.

Grade Index Mode

Note: Both Step Index and Graded Index allow more than one light source to be used (different colours simultaneously!). Multiple channels of data can be run simultaneously!

Single Mode has separate distinct Refractive Indexes for the cladding and core. The light ray passes through the core with relatively few reflections off the cladding. Single Mode is used for a single source of light (one colour) operation. It requires a laser and the core is very small: 9 microns.

Single Mode

Introduction to Data Communications
Previous	Table of Contents	Next