Abstract

The mechanism of passive mode locking in a fiber laser containing a birefringence fiber and polarizer is well known. In this case passive mode locking occurs because of the nonlinear rotation of the polarization plane resulting in generation of soliton-like pulses if the polarizer axis is oriented in a proper way with respect to the principal axis (x,y) of the fiber. In this report we show than another passive mode locking mechanism can exist in such a laser configuration. It can generate high-frequency sequences short pulses which are similar to vector solitons. Orthogonal (x,y) components of a pulse in a vector soliton have slightly differing carrier frequencies (f_x, f_y) to compensate a difference in group velocities of the components arising due to fiber birefringence by the group velocity dispersion of the fiber. Qualitatively, the new mechanism of passive mode locking arises as a result of nonlinear phase modulation with the beat frequency Δf = |f_x - f_y| due to the nonlinear interaction between the orthogonal pulse components. Strict theoretical calculations using the nonlinear Schrodinger equation have shown that optical losses in the laser resonator become minimal if the laser radiation is in the form of sequences of very short pulses with the repetition frequency equal to Δf, and the pulse polarization direction during propagation through the polarizer coincides with she polarizer axis orientation. The efficiency of this mechanism has different angular dependences on the polarizer axis orientation compared with the mechanism of nonlinear polarization plane rotation. The new mechanism allows generation with various Δf(Δf = 1-10³ GHz) in case of a proper choice of parameters of birefringence and group velocity dispersion of a fiber.

© 1996 IEEE