How does a Resonator enhance coherence? (LASER)

[Anton Alice]

Hello,

the resonator of a laser system is said to have 3 functions: Amplification in one direction, selection of one or few frequencies, and enhancement of coherence.
I have a question about the coherence part. I see that the stimulated emission process produces phase matched light.

Initially a spontaneous emission generates a weak standing wave with a certain phase inside the resonator.
Then the stimulated emission contributes to that standing wave by adding photons of same phase. Thus, that particular standing wave can grow further.

But the resonator also allows for the coexistence of other standing waves with different phases, right? I mean this is simply superposition.

A moment later, another spontaneous emission occurs, sowing the seed for another standing wave, with a somewhat different phase. This one is again amplified by stimulated emission.

The laser can therefore emit a superposition of light components, whereby each of the components is coherent taken by itself, but the superposition is not, because of the fact, that the laser cultivates standing waves of random phase relation due to initial spontaneous emissions.

Where is the catch?

 

[mfb]

A few different phases are still better than the case without resonator, where you need 10^whatever spontaneous emissions with random phases as seeds.

 

[Andy Resnick]

Hello,

the resonator of a laser system is said to have 3 functions: Amplification in one direction, selection of one or few frequencies, and enhancement of coherence.
I have a question about the coherence part. I see that the stimulated emission process produces phase matched light.

Not exactly- using a resonator helps when the gain is low; the gain is a media-dependent thing. Solid state lasers work with high gain media and generally don't require a resonant cavity. Resonators *do* select for resonance frequencies, and the frequencies can refer either to the emitted light (longitudinal modes) or pulse rate- this leads to 'mode locking'. Also remember that lossy resonators do not have a single resonance frequency but a finite spectral width. This is the connection with coherence length- a finite spectral width has a finite coherence length.
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