Excess quantum noise is colored

Summary form only given. Spontaneous emission is a fundamental source of noise in a laser. In most lasers, this quantum noise amounts to a level of "one photon per mode", leading for instance to the well-known Schawlow-Townes limit to the laser linewidth. The enhancement factor K can become quite large, K/spl ap/500 has been demonstrated in recent experiments, and the possibility of K values larger than 10/sup 4/ has been predicted. This leads naturally to the question: what are the limitations to the concept of excess quantum noise? We demonstrate both theoretically and experimentally one such limitation, namely that excess quantum noise is colored. This is in contrast to the usual spontaneous emission noise in a laser with orthogonal eigenmodes, which is essentially white noise. The coloring can be attributed to the finite time it takes for the excess quantum noise to build up from the one-photon per mode level. Thus, the picture of simply having "K noise photons in the lasing mode" breaks down. The experiments were performed by measuring the intensity noise of a miniature He-Xe gas laser, operating at 3.5 /spl mu/m.