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Conference on Lasers and Electro-Optics Pacific Rim (CLEO-PR)

Group velocity of optical pulses in resonantly absorbing media

Yujiro FUKUI, Shingo Furue, K. Nakayama, T. Kohmoto, Yukio Fukuda, Masakazu Kunitomo

Published Nov 13, 2002• 0 citations
Publisher

Abstract

The propagation of femtosecond optical pulses in dye solutions is studied. The observed group delay of the transmitted pulses are compared with the new definition of the group velocity.
DOI
10.1109/cleopr.2001.970867

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Propagation of femtosecond light pulses in a resonant absorption medium is studied. The propagation time of the light pulses was measured in a dye solution by the optical-Kerr-gate method. Nonadherence to the conventional group velocity domega/dk , which is defined in weak absorption region, was found. The observed wavelength dependences of the group delay and the spectral change in the anomalous dispersion region are qualitatively in good agreement with the theoretical prediction obtained from the new definitions of group velocity, which can be clearly defined even in strong absorption region.
We have examined the propagation of femtosecond laser pulses in an absorbing dye solution through a short to a long range of propagation distance. The transmitted pulses show strong spectral shift and a superluminal to subluminal transition in the propagation velocity keeping its initial shape almost intact. It is verified that the peak velocity is well described by a modified group velocity v(S) defined within the framework of the saddle-point method as well as by a recent prediction of the net group delay of surviving frequency.
We experimentally demonstrate the direct measurement of net group and reshaping delays for arbitrary optical pulses in dispersive media, verifying the earlier prediction of Peatross et al. [Phys. Rev. Lett. 84, 2370 (2000)]. Incoherent pulse propagation in an absorptionless system is well described by net group delay; even the medium causes a great deal of deformation in the transmitted pulse. However, in the case of phase modulated chirping pulses in a resonant absorber, the so-called superluminal or subluminal propagation velocity is strongly influenced by the reshaping delay.