To the user, the term "coherence" is refering to two different things :

- Temporal coherence

- Spatial coherence

Temporal coherence

Light is temporally coherent in one location P if its phase in P can be predicted without uncertainty at any time in the future. In other words, the field in P is oscillating very regularly, and predicting its phase in the future is just a matter of "counting cycles". Of course, this is easiest if the field contains only one frequency : monochromatic beams are temporally coherent. The closest thing to a perfectly temporally coherent light is a beam from a single longitudinal mode of a continuous laser.

Spatial coherence

Let P1 and P2 be two spatially separated points in a electromagnetic field (light). The field is said to be spatially coherent if the difference of the phases is P1 and P2 remains constant throughout time. Just as temporal coherence allowed us to predict the value of the phase throughout time, spatial coherence allows us to predict the phase some distance away. One may show that this means that light is then passing through P as a single plane wave. So, just as temporal coherence meant "light oscillating at a single frequency", spatial coherence means "light coming from a single direction".

If we get the two concepts together, one sees that :

a) A single longitudinal mode laser emits light that is both temporally and spatially coherent.

b) A light bulb emits light that is neither temporally, not spatially coherent.

c) Polychromatic light emanating from a very small pinhole is not temporally coherent, but is approximately spatially coherent.

d) Light across the exit slit of a monochromator is not spatially coherent, but is approximately temporally coherent.

As a matter of fact, the notion of coherence requires a mathematical treatment for being complete, but the explantations above should give you a good starting point.

Expert in lasers


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