Nonlinear pulse propagation in dispersion decreasing fibers Deepak Gupta * , Gautam Kumar, K. Thyagarajan * Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110 016, India Received 21 February 2003; received in revised form 16 March 2004; accepted 2 April 2004 Abstract By numerically solving the nonlinear Schr€ odinger equation, we analyze pulse propagation through dispersion de- creasing optical fibers (DDFs) in which the core radius varies along the length of the fiber; this in turn leads to a variation of dispersion as well as mode effective area along the length of the fiber. We calculate an optimum core radius profile required to create a local balance between group velocity dispersion and self-phase modulation, so that the DDF can support the propagation of fundamental ‘‘soliton’’ pulses. The variation in the optimum profile corresponding to variations in the fiber parameters has also been analyzed. Ó 2004 Elsevier B.V. All rights reserved. PACS: 42.81.Dp Keywords: Dispersion decreasing fiber; Solitons; Nonlinearity; Dispersion; Pulse propagation 1. Introduction A normal optical fiber has a constant core ra- dius throughout its length, as a result of which the dispersion coefficient remains constant. However, if the core radius can be made to vary along the fiber length, without changing the refractive index profile, the dispersion coefficient would also vary. If the core radius decreases, the waveguide con- tribution to dispersion increases, and in the anomalous region of operation the total dispersion decreases, and such fibers are called dispersion decreasing fibers (DDFs). Practically, these fibers can be realized by varying the pulling speed of the fiber during its fabrication from the preform. By controlling the variation of the pulling speed, various radius profiles, namely linear, exponential, parabolic, etc. can be achieved. These DDFs have been used extensively in higher-order soliton pulse compression for gener- ating femtosecond pulses [1–3] as well as in su- percontinuum generation [4,5]. But another interesting area in which DDFs can be used is the propagation of fundamental soliton pulses. People have reported fabrication of DDFs in which the dispersion coefficient decreases almost exponen- tially in the same fashion as pulse power, thereby maintaining a local balance between group veloc- ity dispersion (GVD) and self-phase modulation * Corresponding authors. Tel.: +91-11-26596579; fax: +91-11- 26581114. E-mail addresses: d_gupta10@rediffmail.com (D. Gupta), ktrajan@physics.iitd.ernet.in (K. Thyagarajan). 0030-4018/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2004.04.006 Optics Communications 237 (2004) 309–317 www.elsevier.com/locate/optcom