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Applied Physics Letters

Theory of the dispersive modulator

D. Grischkowsky, M. M. T. Loy

Published Feb 15, 1975• 17 citations
Publisher

Abstract

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.

DOI
10.1063/1.88124

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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.
By sweeping the frequency of a diode laser beam near the cesium resonance frequency, followed by propagation through a short length of cesium vapor, we obtained periodic peaks with peak-to-average ratios near 5 at pulse repetition rates up to a gigahertz. Light in the wings of a strong atomic absorption line of an alkali metal vapor encounters large group velocity dispersion. A frequency modulation in an off-resonance cw beam passing through the vapor consequently develops an amplitude modulation. For certain combinations of laser and vapor parameters, a sinusoidally frequency modulated beam can be made to develop peaks. We generated pulse trains in a frequency modulated single mode diode laser tuned to the cesium absorption line near 852 nm, which we observed by using a fast p-i-n photodiode and a sampling oscilloscope. The frequency of a diode laser was sinusoidally modulated with a sweep of about a gigahertz and a period of a few nansoseconds by introducing a small modulation into the laser pump current. We observed the formation of sharp peaks with a width of about one-tenth of the modulation period and a peak-to-background ratio of approximately 5.
The narrow-line output of a dye laser on the low-frequency side of the 2P12^{2}P_{\frac{1}{2}}2P21​​ resonance line (7948 \AA{}) of rubidium was self-defocused by passage through dilute rubidium vapor. The defocusing was caused by the electronic nonlinearity associated with "adiabatic following" of the laser field by the pseudomoment of the resonant atoms. By using the corresponding nonlinear dielectric constant, the wave equation was solved numerically and gave excellent quantitative agreement with experiment.