ORTHOGONALITY PROPERTIES OF OPTICAL-RESONATOR EIGENMODES

The transverse eigenmodes of optical resonators are not in general orthogonal to each other, but rather are biorthogonal to an adjoint set of functions which turn out to be the transverse eigenmodes for propagation in the reverse direction around the same resonator. We present a fairly general derivation of this property which is valid for ring or standing-wave and for stable or unstable resonators with arbitrary output mirror profiles. We also point out certain unusual consequences of these properties for high-magnification unstable resonators. © 1979.