Grey solitons in doped nonlinear fibers
  Grey solitons, with complex envelope, are obtained as exact solutions of the coupled system describing doped optical nonlinear fibers. The phase degree of freedom plays a crucial role in removing the restrictive equal frequency conditions on the tanh-sech paired pulses, identified earlier [1]. The cross-phase modulation is found to control the speed of the grey soliton, which has an upper bound. The coupled soliton solution obtained here was found to be stable in a wide parameter range. 
Challenger Mishra, T. N. Dey and P. K. Panigrahi
quant-ph