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Mode-locked operation of a ring laser has been observed to switch direction at rates that did not seem to correspond to any cavity parameter.[1] A clockwise circulating pulse alternated with a counterclockwise circulating pulse at rates of the order of tens of Hz. A weak saturable absorber jet could balance the tendency to unidirectional operation and force bidirectional operation, with pulses crossing in the saturable absorber jet. If the coupling between the pulses circulating in opposite sense in the cavity is small, the carrier frequency of each pulse is seen to follow the resonance of the cavity [2]. The difference in carrier frequency can be measured as a beat note between the two output pulse trains [3]. A very simple model of the complex operation of this laser is proposed, in which the two cavities corresponding to opposite senses of circulation are viewed as coupled resonators. The model leads to a description of this laser as a simple two-level system interacting with a resonant electromagnetic wave. In this analogy, the energy separation between the two level is the beat frequency, the driving electromagnetic field is replaced by the coupling parameter r between the two field. Rabi oscillations are predicted between the two directions, at a rate proportional to the coupling parameter r bidirectional laser. An experimental demonstration of the ``Rabi cycling'' will lead to a convenient and extremely sensitive measure of very small (r Expe4 <<10^{-8}) backscattering coefficients.
Experimental Setup
Bidirectional modelocked ring laser is achieved with dye jet, the
pulses are crossing at the interface and dye jet. The position of
elements in the cavity are carefully chosen so that the separation
between successive arrows is a quarter of the cavity perimeter.
References:
[1] Matthew~J. Bohn and Jean-Claude Diels.
Bidirectional kerr-lens mode-locked femtosecond ring laser.
Opt.~Comm., 141:53--58, 1997.
[2] J.-C. Diels, Jason Jones, and Ladan Arissian.
Applications to sensors of extreme sensitivity.
In Jun Ye and Stephen Cundiff, editors, Femtosecond Optical
Frequency Comb: Principle, Operation and Applications, chapter~3, pages
333--354. Springer, New York, NY, 2005.
[3] Scott Diddams, Briggs Atherton, and Jean-Claude Diels.
Frequency locking and unlocking in a femtosecond ring laser with the
application to intracavity phase measurements.
Applied Physics B, 63:473--480, 1996.
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