It is a fundamental property, which follows from quantum mechanics, that photonic amplifiers also add noise (often called spontaneous emission noise) to the signal they amplify. This noise is, e.g., responsible for such diverse things as the non-zero linewidth of lasers and the limited distance of amplified fiber transmission links. Phase-sensitive parametric amplifiers are an exception to this rule, however. At the price of amplifying one quadrature, and de-amplifying the other quadrature of a signal, no noise needs to be added to it. The noise figure, which relates the signal-to-noise ratio at the input and output of amplifiers, is ideally 0 dB for such phase-sensitive amplifiers. However, a number of imperfections usually prevent the noise figure from reaching 0 dB. For example, spontaneous Raman scattering, intensity noise on the pump waves, or, as is shown in this article, higher order nonlinear mixing products might limit the performance. Usually such higher order products are neglected, but in a novel theoretical approach, the authors quantify their effect on the gain and the noise properties of the amplifier. It is shown that noise figures below 1 dB are still possible with proper design of the parametric amplifier.
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