Abstract
The quantum noise properties of the laser were studied intensively after its invention in the early sixties. The conclusion of these studies was that the laser output approaches quite closely the noise properties of a coherent state, provided the laser is operated well above threshold and only short time intervals (high frequencies) are considered. That is the spectral variance of the photon number and phase fluctuations at high enough Fourier frequencies will be at the Poissonian or quantum noise limit (QNL) [1] (also referred to as the shot noise level). These predictions were subsequently confirmed experimentally [2]. At lower frequencies excess noise, both intrinsic and technical is predicted. None-the-less, under ideal conditions, the intensity noise of the laser tends to the QNL, for all frequencies, as the laser is pumped very far above threshold. On the other hand the phase of the laser is unconstrained and drifts as a function of time, so-called laser phase diffusion [3]. As a result even under ideal conditions the phase fluctuations show excess noise at low frequencies.
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