Harmonic Mode Locking in a Soliton Fiber Laser Maintaining Polarization: Stabilization and Higher Pulse Repetition Rate

The paper describes an erbium fiber laser mode-locked with a semiconductor saturable-absorber mirror (SESAM) and made up entirely of polarization-maintaining fiber components. With the focal spot correctly adjusted on the semiconductor mirror, the laser operates in the harmonic mode-locking (HML) regime available over the entire pump power range up to similar to 355 mW with the supermode suppression level of less than 25 dB. In the HML regime, the laser can generate linear polarized pulses and provide a pulse repetition rate up to similar to 1145 MHz. We have shown experimentally that by injecting radiation from an external continuous laser directly into the fiber cavity we can improve the stability of laser operation in the HML regime while the level of supermode suppression is increased by 20 or 30 dB. In addition, it is shown that external injection can extend the pump power range available for the laser in the HML regime increasing the maximum pulse repetition rate up to similar to 2195 MHz. It is important to note that optical injection does not affect the high purity of the laser's polarization state. The presented results of numerical modeling can qualitatively explain the effects observed experimentally.