Experimental Observation of the Fundamental Dark Soliton in Optical Fibers

Abstract

We present evidence of soliton propagation by 185-fsec dark pulses at a wavelength of 0.62 /zm in a 1.4-m length of single-mode optical fiber. Our experiments utilize specially shaped, antisymmetric input pulses, which closely correspond to the form of the fundamental dark soliton. At appropriate power levels the dark pulses propagate 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 in optical fibers. PACS numbers:42.50.Qq, 42.65.Re, 42.81.Dp Although soliton phenomena arise in many distinct areas of physics, the single-mode optical fiber has been found an especially convenient medium for their study. Hasegawa and Tappert proposed in 1973 that the nonlinear refractive index in glass optical fibers could be utilized to compensate for group velocity dispersion (GVD), resulting in optical solitons which could propagate without distortion. 1 Since then, soliton propagation of bright optical pulses has been verified in a number of elegant experiments performed in the negative GVD region of the spectrum (X>\.3jjmin standard singlemode fibers) 2 ; most recently, transmission of 55-psec optical pulses through 4000 km of fiber was achieved, by use of a combination of nonlinear soliton propagation to avoid pulse spreading and Raman amplification to overcome losses. 3 For positive dispersion (A,<1.3//m), bright pulses cannot propagate as solitons, and the interaction of the nonlinear index with GVD leads to spectral and temporal broadening of the propagating pulses. These effects form the basis for the fiber-and-grating pulse compressor, 4 ' 5 which was utilized to produce the shortest optical pulses (6 fsec) ever reported. 6 For both signs of GVD, the experimental results are in quantitative agreement with the predictions of the nonlinear Schrodinger equation (NLSE).Although bright solitons are allowed only for negative dispersion, the NLSE admits other soliton solutions for positive GVD. 1,7 These solutions are "dark-pulse solitons," consisting of a rapid dip in the intensity of a broad pulse or a cw background. The fundamental dark soliton, for which we report here the first experimental observation, is predicted to be an antisymmetric function of time, with an abrupt K phase shift and zero intensity at its center. Other dark solitons with a reduced contrast and a lesser, more gradual phase modulation also exist. Throughout the text we will use the terms "black" and "gray" soliton, respectively, to refer to the fundamental and to the lower-contrast dark-soliton solutions.As a result of difficulty in generating the required input dark pulses, previous experimental evidence for dark-soliton propagation in fibers is limited. Krokel et al* reported the evolution of an even-symmetry, 300fsec dark pulse into a complementary pair of lowcontrast dark pulses, which they interpreted as gray solitons. Emplit et al. 9 performed experiments utilizing odd-symmetry dark pulses -...

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