- Klaus Petermann reported from a phenomenological consideration that the spontaneous emission into a gain-guided mode is enhanced by a factor K given by
- He wrote, "The spontaneous emission factor not only increases with decreasing volume of the active layer but also increases with increasing astigmatism", where the former effect is nothing but the Purcell effect. The latter is the Petermann's effect, which comes from bi-orthogonality of modes in non-Hermitian systems.