Nonlinear Amplifiers
Nonlinear amplifiers do not have a linear relation between their input and output; the maximum noise of a nonlinear amplifier cannot be much smaller than that of an idealized linear amplifier. This limit is determined by the derivatives of the mapping function; a larger derivative implies an amplifier with greater uncertainty. Examples include most lasers, which include near-linear amplifiers, operate close to their threshold and thus exhibit large uncertainty and nonlinear operation. As with the linear amplifiers, they may preserve the phase and keep the uncertainty low, but there are exceptions. These include parametric oscillators, which amplify while shifting the phase of the input.
Read more about this topic: Quantum Amplifier