Optical Pulse Train Propagation in Sodium Vapor
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We present here results of a theoretical and experimental study of the propagation of infinite pulse trains through a two-level absorber. Our model is based on the semiclassical theory used in the description of single-pulse self-induced transparency (SIT).1 Previous theoretical studies of infinite pulse trains have been based on analytical solutions found with the assumption of an infinitely long relaxation time for the atomic system.2 In an experiment, however, relaxation by spontaneous emission, collisions, or other mechanisms is present. We find that for long pulse trains, even weak relaxation has important consequences and must be included in the theory.3 In particular, relaxation provides a mechanism for the atomic system to come to equilibrium with a repetitive pulse train. At equilibrium, the atomic variables are also fully repetitive in time. The repetitive polarization causes reshaping of the pulse train as it propagates through the atomic medium. In fact, our computer simulations show that the characteristic features of single-pulse SIT, including pulse breakup, peaking, delays, and low-loss transmission are, in theory, observable on a pulse train at equilibrium. Moreover, our experimental results reported here demonstrate this to be true in practice.
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