Plano-Concave Resonator

For a resonator with one flat mirror (R₁ =∞) and one curved mirror we obtain

......(1)

The beam waist w₀ occurs at the flat mirror (i.e., L₁ = 0 and L₂ = L). A special case of this resonator configuration is the hemispherical resonator. The hemispherical resonator consists of one spherical mirror and one flat mirror placed approximately at the center of curvature of the sphere. The resultant mode has a relatively large diameter at the spherical mirror and focuses to a diffractionlimited point at the plane mirror. In practice, one makes the mirror separation L slightly less than R₂ so that a value of w₁ is obtained that gives reasonably small diffraction losses.
In solid-state lasers, the small spot size can lead to optical damage at the mirror. A near hemispherical resonator has the best alignment stability of any configuration; therefore it is often employed in low-power lasers such as HeNe lasers.