The condition which must be satisfied to ensure cavity stability, as discussed previously, is shown below:
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.
Looking at the values for R1 and R2 for this experiment, we have:
R2 = 60 cm
Plugging these values into the equation above we have a stability condition for the HeNe laser:
To test this theory we can look at a graph of laser intensity versus the cavity length:
It is clear that as the value of L approaches 60 cm, the upper boundary for a stable cavity, the stability of the cavity and thus the power of the laser beam diminish.