The length of laser's optical cavity. Multiple of wavelength?

[deccard]

In order to produce coherent laser light is it necessary that the length of optical cavity between the mirrors is a multiple of the wavelength of laser light? I cannot see how otherwise the laser been could bee coherent. On the other hand I don't see how it is possible to place the mirrors with nanometer precision so that for example thermal expansion doesn't change the length between the mirrors.

 

[Volcano]

as i know, no relationship. The mirrors for intensify the light.

 

[deccard]

Yes the mirrors are used to intesify the laser beam. And the coherence of the laser beam comes from stimulated emision from atoms in gain medium. But the question in hand raised when I thought how the light is reflected back and forth. But now I'm starting to be pretty confident that it doesn't matter whether the length between mirrors is or is not multiple of the laser waveleangth in order to retain the coherence of light. Especially if the electromagnetic field has to be zero at the reflection surface of mirror.

 

[monish]

Actually, there is some "give" in the frequency of the laser emission. It adjusts itself to find an operating point which involves a standing wave mode. So it turns out to be an integer number of wavelengths no matter what the actual length of the cavity.

 

[Manchot]

To elaborate on what monish said, the gain spectrum of a laser is not really perfectly discrete, and every peak will have a finite linewidth. As long as the longitudinal modes of a Fabry-Perot resonator are spaced sufficiently close together, the system will lase at one of them. You are right, though, in that if one were to construct a system whose linewidth was small compared to the mode spacing (e.g., by making an extremely short cavity), then it is possible for the gain spectrum to "miss" all of the longitudinal modes.