Theory of the dispersive modulator

The recent observations by Loy of amplitude modulation and compression of infrared laser pulses caused by passage through the dispersive modulator (a cell containing dilute NH3 vapor with a Stark-modulated resonant frequency) are explained. It is shown that there is a formal equivalence between (i) the passage of a near-resonant light beam through a vapor with a time-dependent resonant frequency, and (ii) the passage of a frequency-modulated light beam through a vapor with a constant resonant frequency. For low-intensity light this equivalence reduces the problem to the optical analog of chirp radar.