Nonreciprocal phase shifts in a femtosecond dye laser

Beating is observed when the two output beams from a colliding-pulse mode-locked laser are recombined outside the cavity. This beating is attributed to nonlinear dephasing in the saturable absorber. An explanation in terms of the soliton period variation is given. The experimental results show good agreement with the predictions of the nonlinear Schrödinger equation.