Exploring the nature of excess quantum noise

Summary form only given.We report here on a series of experiments aimed at a better understanding of excess quantum noise. Our experimental system is a miniature HeXe laser (L=5 cm, /spl lambda/=3.51 /spl mu/m). We start by comparing the quantum phase noise of a geometrically unstable and a geometrically stable HeXe cavity, with the same diffraction loss rate (realized by using a sufficiently small outcoupling mirror for the stable laser). Experimentally, we find K/sub unstable//spl sim/220 and K/sub stable//spl sim/5, showing that losses are not the key issue. In fact, these values are in good agreement with the mode-nonorthogonality theory. The data for K/sub stable/ are best fitted by the curve K/sub trans/K/sub long/, showing that the excess quantum noise can be factorized, as is expected within the paraxial approximation. We observed this factorizability also for K=K/sub trans/K/sub pol/, using a cavity with nonorthogonal transverse and polarization eigenmodes; nonorthogonality of the latter was realized by dissipative coupling of two circularly polarized modes, using a quasi-Brewster-plate in the HeXe cavity. We also report the appearance of the K factor in the intensity noise of an unstable HeXe laser (all reports so far dealt with phase noise). We measured intensity noise spectra, both below and above threshold. We extract K from these data using a phenomenological laser model in which one of the spontaneous emission decay channels, namely, the laser mode, has been given a K times larger weight than the other channels. K values deduced from intensity noise were found to be consistent with those derived from phase noise.