...at JILA, which is a joint institute of the University of Colorado and the National Institute for Standards and Technology
Pulse Propagation Effects
"Breathing" of a dispersion managed soliton as it propagates [from Q. Quraishi, S.T. Cundiff, B. Ilan, and M.J. Ablowitz, Phys. Rev. Lett. 94, 243904 (2005)]
A mode-locked laser is a pulsed laser that emits a periodic sequence of optical pulses where the pulses are spaced by the cavity round-trip time (the group delay). These pulses take the form of solitons. In general, the main feature of solitons is that they propagate for a long time without visible changes. Mathematically, a soliton is a localized solution of a partial differential equation describing the evolution of a nonlinear system with an infinite number of degrees of freedom. In dissipative systems, however, the pulse dynamics depend drastically on the system parameters. The evolution can be periodic or chaotic or solitons can be switched from one stable state to another.
For the solitons propagating in mode-locked Ti:sapphire lasers, we have shown that the dominant factor in the pulse dynamics is the equilibrium established between the Kerr nonlinearity and the linear dispersion. The competition between these aspects results in dynamics such as "breathing" of the pulse as it propagates (see figure above). We used an asymptotic theory developed for the perturbed nonlinear Schroedinger equation to predict the dynamics of these solitons using only one fitting parameter in the model to generate the theoretical curves. This study showed that the dispersion management concepts originally developed in fiber communications apply in a much broader context.
In addition, we have measured and modeled nonlinear polarization evolution in microstructure fiber. Comparison between measurement and theory show that chromatic dispersion plays an important role of actually enhancing the nonlinear polarization evolution while decreasing other nonlinearities. In particular, the shape of the final spectrum depends on third-order dispersion.
More recently, in collaboration with Curtis Menyuk of the University of Maryland Baltimore County, we have begun to study the quantum noise properties of mode-locked lasers. In order to be able to predict noise due to amplified spontaneous emission, the fundamental source of noise in all lasers, one has to understand the linear response of the laser. This varies widely from laser to laser and cannot easily be predicted from first principles. So, in order to calculate the noise properties, we had to experimentally characterize a Ti:sapphire laser's linear response. The results of that will be used to predict the quantum-noise limited linewidth of comb lines, a sort of analogue to the Schawlow-Townes linewidth in cw lasers.
Postdoc Jared Wahlstrand and graduate student John Willits are currently involved in this research.