Rectangular Pulse Generation From a Mode Locked Raman Fiber Laser

Rectangular and step-like pulses are generated in a mode-locked Raman fiber laser with a nonlinear optical loop mirror. The Raman laser has a typical figure-8 configuration. Stable rectangular pulses with a fundamental repetition rate of 733 kHz are obtained. As the pump power increases, the Raman pulse evolves to a step-like rectangular pulse. The maximum pulse energy is 64.1 nJ with a pulsewidth of 25.1 ns. After spectrally resolved measurements, the step-like pulse is proved to be induced by the dual wavelength oscillation. It is the first report of rectangular pulsed output in the mode-locked Raman fiber laser to the best of our knowledge.