Experimental studies of dark soliton propagation in fibers

Abstract

Solitons are intense ultrashort pulses which propagate without distortion in optical fibers. Bright solitons, which exist for negative group velocity dispersion (GVD), have been investigated extensively over the past decade. Dark solitons, which consist of a short hole superimposed on a broad background pulse and which occur for positive GVD, have been studied much less, in part due to the lack of a convenient technique for producing the required specially shaped input dark pulses. We have developed a technique for precise synthesis of femtosecond optical waveforms, and we apply this technique to investigate nonlinear propagation of femtosecond dark pulses in single-mode fibers. Our experiments utilize carefully tailored antisymmetric input pulses, which closely correspond to the form of the fundamental dark soliton. At a power level consistent with theory, the dark pulse propagates without broadening. Our measurements are in quantitative agreement with numerical solutions to the nonlinear Schrodinger equation and constitute the first clear observation of the fundamental dark soliton. At significantly higher powers, we discover a dark soliton self-frequency shift, characterized by a spectral blue shift of the output dark pulse. We show that numerical solutions to a modified NLSE which includes Raman gain are consistent with our experimental findings.