Optical communication systems can be broadly classified into two categories based on the concept of guiding such as guided and unguided.
In Guided optical communication systems, the optical beam emitted by the transmitter remains spatially confined. This is done practically by using optical fibers, which can be generally either glass or plastic. Since all guided optical communication systems currently use optical fibers, the commonly used term for them is fiber optic communication systems. Some people prefer to call fiber optic system as lightwave system.
In unguided optical communication systems, the optical beam emitted by the transmitter travels in space. The light beam spreads in space as microwave signals spreads in a microwave communication system. Unguided optical systems are less suitable for broadcasting applications than microwave systems because optical beams spread mainly in the forward direction. Lightwave systems use shorter wavelengths in electromagnetic spectrum and hence the beam spread happens more in the forward direction.