QUANTUM-NOISE IN PHOTONICS

The basic equations governing noise phenomena are derived from first p rinciples and applied to examples in optical communications. Quantum n oise arises from two sources, the momentum fluctuations of electrons a t optical frequencies and the uncertainty-related fluctuations of the electromagnetic field. Shot noise results from the beating of the nois e sources with the signal field. In high-gain amplifiers, the spontane ous-emission noise dominates shot noise and results in a noise figure of at least 3 dB. It is shown explicitly how, at high power, both the laser field and the laser noise source become classical. The increase in noise in lasers with open cavities is not due to enhanced spontaneo us emission as once thought, but to single-pass amplification. The noi se fields and spontaneous currents have Gaussian distributions, while nonlasing modes have exponential photon-number distributions. Low-freq uency intensity fluctuations arise from the electric current driving t he laser and can be sub-Poissonian, in contrast to shot noise, which h as a Poissonian distribution. The calculational tools are a wave equat ion for the field operator and a rate equation for the carrier-number operator, each containing spontaneous current noise sources. The corre lation functions of these sources are determined by the fluctuation-di ssipation theorem.