Rami Arieli: "The Laser Adventure"

Rami Arieli: " The Laser Adventure" Chapter 1.2.3 Extra

Coherence
Coherence is a complicated subject, and is used in special applications of the laser, like interference and holography. It is explained in chapter 10.

There is a difference between time coherence, and space coherence, of a wave, and there is a difference between:

Self Coherence - of the wave with itself, meaning that over a long time or space, the wave continue to retain its properties, and its phase does not change.

Coherence between different sources - deals with the relative phase between two sources. There is a possibility of coherence between two waves which themselves are not coherent, but the relative phase between them remain the same in time and space. Every change in phase of one wave appears in the other.