Coherence Measurement of Square-Pulses in Passively Mode-Locked Fiber Laser

Square-pulses in passively mode-locked fiber lasers are generally assumed to be the manifestation of the dissipative soliton resonance (DSR). The DSR is a particular solution of the nonlinear propagation equation which does not suffer from any wave-breaking. As a consequence both the energy and the width of the pulses increase linearly with the pumping power while the peak power remains clamped to a fixed value. Another important characteristics of DSR pulses is that they exhibit square shape with a perfect temporal coherence. In this communication we investigate the coherence of square-pulses in a passively mode-locked double-clad fiber laser. We demonstrate that although almost all the characteristics of DSR pulses are verified (energy and width scaling with pump power, peak power clamping, optical spectrum), the square-pulses are not temporally coherent thus demonstrating that square-pulses are not necessarily DSR pulses.