Passively mode locked pulses are obtained from an optical fiber laser by using a nonlinear loop mirror in the positive group velocity dispersion (+GVD) regime. The laser used in the experiment consists of a Nd-doped fiber as an active medium with a dichroic mirror at one end, and a nonlinear fiber loop mirror at the other end.
In the nonlinear fiber loop mirror, two optical pulses propagate in the opposite directions with different peak powers, so that they experience different self phase modulation(SPM) while they propagate in this loop mirror. An efficient pulse shortening is obtained when the two counter-propagating optical pulses interfere with proper phase offset. The phase offset is generated such that the optical signals at the peaks of the pulses interfere constructively while destructive interference occurs at the wings of the pulses. In other words, since the high intensity portion of the pulse will be reflected more efficiently by the nonlinear loop mirror than the low intensity portion, pulse narrowing occurs.
The pulse width obtained in this work is less than 4ns with 20 - 50 peak power. The pulse width obtained in this experiment is the detection limited value. We expect that the pulse width may be shorter than we measured in the experiment. The computer simulation results show that the pulse shortening occurs in our system in the +GVD regime. The repeatition rate of the pulse depends on the length of the loop mirror. It is about 641.7kHz in our system.