The Polarization Symmetric Laser

Abstract

Since those days in 1964, when Hermann Haken gave the essentials of modern laser theory /1/, a comprehensive understanding of the laser physics has been developed (cf. /2, 3/). Besides the high photon intensity I the most striking feature is the extremely small linewidth λ_u of the laser’s electric field u, measured with any interferometer. This field linewidth decreases with increasing pump rate p as λ_u ^αp⁻¹ in common single mode (SM) lasers. In contrast to the field linewidth λ_u the photon number fluctuates with a rather short correlation time λ_I ⁻¹, resulting in a large intensity linewidth λ_I ^αp. It is only near threshold (p=0) that the intensity-intensity correlation function shows slowing down. The physics behind this markedly different behaviour of λ_u and λ_I is the different origin of the fluctuations /1/: while the induced emission and absorption mechanism provides a strong restoring force to fix I at p, the restoring time being all the smaller the larger p is, the phase of u is completely free and undergoes a Brownian diffusion stimulated by the spontaneous emission processes. Since the available phase space for the diffusion is of order I, the decorrelation time is ~I, hence λ_u ^αp⁻¹.

Dedicated to Hermann Haken on his 60^th birthday.