Lasers without photons ? or should it be lasers with too many photons?

Abstract

Expanding the fields of a laser cavity in a set of orthonormal modes is a standard technique in laser theory. Expansion in a normal mode set is also the basis of the concept of “photons”. A substantial number of practical lasers do not, however, support any kind of normal or orthogonal cavity modes, and thus, their fields cannot be represented (at least not easily) in terms of normal modes, or photons. This leads to a number of unusual results, including situations in which the lowest-order mode of a cavity can contain substantially more energy than the total energy in the cavity, as well as enhanced quantum spontaneous emission far stronger than the “single extra photon” level characteristic of an ordinary laser oscillator. We review the theoretical origins of these unusual effects and present experimental confirmation of greatly enhanced Schawlow-Townes fluctuations in an unstable-resonator laser with a Petermann-noise enhancement factor of several hundred times.