Optical parametric amplifiers constitute a category of amplifiers whose operation is based on a physical process that is quite distinct from other amplifiers discussed in the preceding chapters. The major difference is that an atomic population is not transferred to any excited state of the system, in the sense that the initial and final quantum-mechanical states of the atoms or molecules of the medium remain unchanged [1]. In contrast, molecules of the gain medium end up in an excited vibrational state, in the case of Raman amplifiers. Similarly, atoms are transferred from an excited electronic state to a lower energy state in the case of fiber amplifiers and semiconductor optical amplifiers.

Optical parametric amplification is a nonlinear process in which energy is transferred from a pump wave to the signal being amplified. This process was first used to make optical amplifiers during the 1960s and has proved quite useful for practical applications. In Section 8.1 we present the basic physics behind parametric amplification, and then focus on the phase-matching requirement in Section 8.2. In Section 8.3 fiber-based parametric amplifiers are covered in detail, because of the technological importance of such amplifiers. Section 8.4 is devoted to parametric amplification in birefringent crystals, while Section 8.5 focuses on how phase matching is accomplished in birefringent fibers.