Basic concepts of coherence

As a last point in this chapter we will address the coherence properties of hght The concept of coherence is related to the stabihty, or predictability, of the phase of an electromagnetic wave. Therefore, in the broadest sense, coherence is defined as the property of waves (or wave-like states like wave packets) that 

In interference, at least two wave states are combined and, depending on the relative phase between them, they can add constructively or subtract destructively. There are two basic properties of coherent interference of waves temporal coherence describes the correlation or predictable relationship between signals observed at different moments in time, and spatial coherence describes the correlation between signals at different points 

Temporal coherence is the measure of the average correlation between wave trains separated by a delay T. The delay over which the phase or amplimde diverges significantly (and hence the correlation function (t) decreases by a specified fraction) is defined as 

There are different ways to quantify the degree of coherence, using amongst others  

Mathematically, the most common approach is the use of correlation functions T, which are related to the complex degree of coherence y and 

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