Plane–Parallel Resonator

The plane–parallel or flat–flat resonator, which can be considered a special case of the large-radius mirror configuration (R₁ = R₂ =∞), is extremely sensitive to perturbation. However, in an active resonator, that is, a resonator containing a laser crystal, this configuration can be quite useful. Heat extraction leads to thermal lensing in the active medium, this internal lens has the effect of transforming the plane–parallel resonator to a curved mirror configuration as explained later in this section. Therefore the thermally induced lens in the laser material brings the flat– flat resonator into geometric stability.