Slow light based devices require large delay compared to pulse width and small distortion. We describe methods for characterizing delay-induced pulse distortion and distortion management in slow-light systems.
We study analytically pulse distortion in linear slow light systems, and provide some useful limits on these devices. Additionally, we also show that the contributions of phase and amplitude broadening can be de-coupled and quantified. It is observed that phase broadening is generally smaller than amplitude broadening in conventional slow light media (lorentzian gain profile) except for very large fractional delays, where it becomes larger. Upon these expressions, we may envisage new strategies to minimize the distortion in the delaying of pulses, depending on the specific application and the required fractional delay. To overcome the residual distortion, we show that nonlinear systems can lead to a resharpening of the pulses and a re-generation of the filtered frequencies.
We study numerically all-optical slow-light delays in room-temperature single-mode optical fibers induced by stimulated Brillouin scattering. We consider the propagation of a pulse through a cw-pumped Brillouin fiber amplifier, where the carrier frequency of the pulse is tuned near the Stokes resonance. Pulse delay and broadening of the Stokes pulse are studied in the small-signal and gain-saturation regimes. Pulse delay is shown to be limited by saturation of the Brillouin amplifier. In the small-signal regime, both time delay and pulse broadening increase with increasing gain. In the gain-saturation regime, both time delay and broadening decrease with increasing gain, and the pulse even achieves advancement. Time delay of more than one pulse-width is observed with modest pulse distortion, and over one pulse-width advancement can be obtained with larger pulse distortion in the gain-saturation regime.
We describe a methodology to maximize slow-light pulse delay subject to a constraint on the allowable pulse distortion. We show that optimizing over a larger number of physical variables can increase the distortion-constrained delay. We demonstrate these concepts by comparing the optimum slow-light pulse delay achievable using a single Lorentzian gain line with that achievable using a pair of closely-spaced gain lines. We predict that distortion management using a gain doublet can provide approximately a factor of 2 increase in slow-light pulse delay as compared with the optimum single-line delay. Experimental results employing Brillouin gain in optical fiber confirm our theoretical predictions.