Noise of Quantum-Mechanical Amplifiers

M. W. P. Strandberg

M. W. P. Strandberg

Kyo Inoue

Noise is one of the basic characteristics of optical amplifiers. Whereas there are various noise sources, the intrinsic one is quantum noise that originates from Heisenberg’s uncertainty principle. This chapter describes quantum noise in optical amplifiers, including population-inversion–based amplifiers such as an Erbium-doped fiber amplifier and a semiconductor optical amplifier, and optical parametric amplifiers. A full quantum mechanical treatment is developed based on Heisenberg equation of motion for quantum mechanical operators. The results provide the quantum mechanical basis for a classical picture of amplifier noise widely used in the optical communication field.

Dmitri A. Kouznetsov, Daniel Rohrlich

William Bialek

Quantum mechanics limits the noise performance of linear amplifiers, and these limits, in turn, restrict the effects of active feedback on the noise level of any device which uses feedback from such an amplifier. I present a simple quantum-mechanical characterization of feedback, and use this method to determine the limiting noise levels of active filters and oscillators. The results clarify the role of the amplifier in the measurement process and illustrate the limits of a Langevin treatment of quantum noise.

R. C. Swanson, Philip R. Battle, John L. Carlsten

Quantum mechanics fundamentally requires any linear amplifier to introduce quantum noise onto the amplified signal.1 In the case of the linear Raman amplifier, the fully quantum-mechanical Raman equations give a precise prediction of how much noise is introduced.2 Understanding and testing this prediction is important not only from a practical point of view, where one might be interested in the conditions for which a signal can be amplified without distortion, but also from the fundamental point of view of understanding the quantum-mechanical Raman process. To test these predictions, we consider the specific case of a Mach-Zehnder interferometer with a Raman amplifier in each leg of the interferometer. The input signal is provided by a separate Raman generator. As the input signal is varied from a small input to a large input, the fringe visibility varies from essentially zero, indicating that the noise dominates the output pulse, to nearly unity, as the amplified input signal dominates the noise. Using the Raman equations, we find that surprisingly few photons are predicted to be necessary for the signal to dominate the noise. The present work is devoted to measuring these fringes experimentally.