Confocal optical resonator
= Confocal optical resonator
A symmetric <confocal optical resonator> has two concave mirrors of equal curvature $R$, separated by $R$. Its symmetric fundamental <Gaussian beam> has a central <beam waist> and <Rayleigh range> $R/2$. Its ideal transverse spectrum has degeneracies, so transmission at one resonance need not certify exclusive excitation of the fundamental <normal mode>.