Chapter 7 gives the theory of a laser, which is a self-sustained oscillator consisting of an active medium in a Fabry-Perot or ring cavity. The laser output frequencies, imposed by the self-consistent laser equations, are compromises between the atomic and cavity natural frequencies. In this chapter, we discuss another situation involving a nonlinear medium in Fabry-Perot and ring cavities, but with two major changes: 1) the cavity output depends on an injected signal for its energy and output frequency, and 2) the medium is passive, i.e., it absorbs and/or provides an index change — but for two-level media, the upper state is not pumped. The name optical bistability comes from the characteristic of such systems that for a single input intensity, two (or more) stable output intensities are often possible, one large and one small. The system is like an electronic flip-flop except that it is all-optical.
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