OPTICAL QUANTUM-NOISE TREATED WITH CLASSICAL ELECTRICAL NETWORK THEORY

A field noise spectral density matrix for the noise from a linear opti cal device, modeled as an optical multiport, is derived semi-classical ly. The noise is formulated in the scattering parameters, population i nversion factor, and internal efficiency of the device. From this nois e expression, a new equation for the amplified spontaneous emission sp ectral density from an optical amplifier is derived. This equation is more general than those previously published. The derivations are acco mplished with the aid of methods from ordinary (classical) electrical network theory and only one quantum mechanical result regarding the no ise from a conductance. As a result, the first-mentioned noise express ion also includes the so-called vacuum field fluctuations. Further, th e current noise spectrum of a square-law detector is derived with clas sical methods. The optical input to the detector includes the vacuum f ield fluctuations, which are shown to be the cause of the detector sho t noise. Here, a second quantum mechanical result has to be employed, viz., that the vacuum field fluctuations cannot be detected alone. The results in this paper agree with the special cases found in the liter ature. The noise expressions are well suited for examination of the op tical noise performance of arbitrary linear optical networks including , e.g., amplifiers, attenuators, isolators, reflections, filters, and couplers.