Cavity Stability of the HeNe Laser:

In the case of a cavity having mirrors 1 and 2 which have radii of curvature R1 and R2 respectively, that are separated by a distance L there are requirements that govern cavity stability. If a ray is traced as it is reflected back and forth between the mirrors in the cavity, it becomes apparent that not all combinations of radii and cavity lengths result in stable cavities. If a ray is initially directed parallel to the cavity axis, it may diverge from its original trajectory after repeated reflections. If divergence occurs, the cavity sustains very high diffraction loss and is said to be unstable. The condition which must be satisfied for the cavity to be considered stable is shown below:

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where R1 and R2 are the radii of curvature for the two mirrors and L is again the separation between them. Outside of this condition, the laser cavity is not stable because the reflected rays diverge from a path parallel to the cavity axis.

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