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Journal of the Optical Society of America B

Excess noise in gain-guided amplifiers

Ivan Deutsch, J. C. Garrison, Ewan M. Wright

Published Jun 1, 1991• 35 citations
Publisher

Abstract

A second-quantized theory of the radiation field is used to study the origin of the excess noise observed in gain-guided amplifiers. We find that the reduction of the signal-to-noise ratio is a function of the length of the amplifier, and thus the enhancement of the noise is a propagation effect arising from longitudinally inhomogeneous gain of the noise rather than from an excess of local spontaneous emission. We confirm this conclusion by showing that the microscopic rate of spontaneous emission into a given non-power-orthogonal cavity mode is not enhanced by the Petermann factor. In addition, we illustrate the difficulties associated with photon statistics for this and other open systems by showing that no acceptable family of photon-number operators corresponds to a set of non-power-orthogonal cavity modes.

DOI
10.1364/josab.8.001244

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A second-quantized theory of the radiation field is used to study the origin of the excess noise observed in gain-guided amplifiers. We find that the reduction of the signal-to-noise ratio is a function of the length of the amplifier, and thus the enhancement of the noise is a propagation effect arising from longitudinally inhomogeneous gain of the noise rather than from an excess of local spontaneous emission. We confirm this conclusion by showing that the microscopic rate of spontaneous emission into a given non-power-orthogonal cavity mode is not enhanced by the Petermann factor. In addition, we illustrate the difficulties associated with photon statistics for this and other open systems by showing that no acceptable family of photon-number operators corresponds to a set of non-power-orthogonal cavity modes.
We theoretically investigate the quantum dynamics of an unstable resonator laser. Compared to a stable cavity laser of equal gain and loss it exhibits a K-fold enhanced linewidth. This excess noise factor K is a measure of the nonorthogonality of the resonator eigenmodes and amounts to an enlargement of the quantum vacuum fluctuations. Using a quantum treatment starting from first principles based on the nonorthogonal eigenmodes, we put previous theoretical predictions onto a more firm ground. While we find a position-dependent enhancement of the spontaneous emission rate into an empty mode of only K,\sqrt{K},K​, the constructive quantum interference of the spontaneous emission with a single oscillating mode lets the Petermann excess noise factor K reappear in the phase diffusion (laser linewidth). Hence locally enhanced spontaneous emission as well as noise enhanced by interference (amplified spontaneous emission) play an equal role in the origin of excess noise.
The results of a quantum-electrodynamical theory of excess spontaneous emission noise in lossy resonators will be presented. The " Petermann K factor" does not enter into the spontaneous emission rate of a single atom in the cavity. The QED theory allows different interpretations of the K factor and we use this fact to justify semiclassical analyses and to provide in one example a simple derivation of K in terms of the amplification of the quantum vacuum field entering the resonator through its mirrors.
The basic equations governing noise phenomena are derived from first principles and applied to examples in optical communications. Quantum noise arises from two sources, the momentum fluctuations of electrons at optical frequencies and the uncertainty-related fluctuations of the electromagnetic field. Shot noise results from the beating of the noise sources with the signal field. In high-gain amplifiers, the spontaneous-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 spontaneous emission as once thought, but to single-pass amplification. The noise fields and spontaneous currents have Gaussian distributions, while nonlasing modes have exponential photon-number distributions. Low-frequency intensity fluctuations arise from the electric current driving the laser and can be sub-Poissonian, in contrast to shot noise, which has a Poissonian distribution. The calculational tools are a wave equation for the field operator and a rate equation for the carrier-number operator, each containing spontaneous current noise sources. The correlation functions of these sources are determined by the fluctuation-dissipation theorem. [S0034-6861(96)00503-X]
The results of a quantum-electrodynamical theory of excess spontaneous emission noise in lossy resonators will be presented. The Petermann K factor'' does not enter into the spontaneous emission rate of a single atom in the cavity. The QED theory allows different interpretations of the K factor, and we use this fact to justify semiclassical analyses and to provide in one example a simple derivation of K in terms of the amplification of the quantum vacuum field entering the resonator through its mirrors. 17 refs.
The gain-guided nature of an amplifier can lead to more noise in the output than would be predicted based on the quantum noise limit of 1 photon per mode. We report the results of an experiment which demonstrate that the gain-guided nature of a focused-gain Raman amplifier can lead to excess noise in the amplified field. The results are compared with a theoretical analysis of the output of an amplifier which incorporates the effects of gain guiding as well as the focused nature of the gain profile.
Previous investigations of Raman amplification in a multipass cell showed that a plane-wave transient theory matched the experimental data. In this work the output energy of a Raman amplifier was measured when no input was present. To model the output we initially used a fully quantum mechanical plane wave theory, which was scaled to take into account both the focusing and the multiple passes with the noise source being the usual one photon per mode. While the theory adequately describes the exponential growth for large gain, for gz < 10 there is a substantial deviation between the theory and experiment. To account for this excess noise the Stokes output is modeled by using a set of non-power-orthogonal modes. These modes arise naturally from the non-Hermitian nature of the gain guided process. It is the inherent non-power-orthogonality of these modes that can lead to excess noise, as noted first by Petermann. We find that the output from this model matches qualitatively the observed experimental output.