Bound state of hundreds pulses in the Er:Yb-doped double-clad fiber laser

Multiple pulsing is a very usual behaviour of passively mode-locked fiber lasers in the either normal or anomalous dispersion regime. This occurs independently of the exact optical configuration and with different doping ions. In general, the pulses are randomly distributed along the optical cavity and their amplitudes and phases are not correlated. Sometimes, self-organization of pulses occurs and they are identical and equally spaced within the cavity. This corresponds to the well-known passive harmonic mode-locking. A few pulses can interact then leading to the formation of bound states where all pulses are identical and the phase difference between two neighbouring pulses remain constant. These states have been greatly investigated both theoretically and experimentally. Bound state generally concerns few pulses, typically 2 or 3 and more rarely up to 10 pulses. In this communication we report on the observation of bound states of several hundreds of pulses. The experiment has been performed with a Er:Yb doped double-clad fiber laser passively mode-locked with the nonlinear polarization rotation technique. In addition to the double-clad fiber, the optical cavity contains a piece of dispersion shifted fiber, a polarizing cube and several phase plates. The cavity operates in the anomalous dispersion regime which favours multiple pulsing. By a suitable adjustment of the phase plates, the fiber laser delivers a bound state of about 350 pulses. In an autocorrelation trace and a typical optical spectrum, the optical correlator does not permit the visualization of all pulses because the distance between two neighbouring pulses is about 23 ps leading to a total temporal interval of about 8 ns which largely exceeds the scan of the apparatus (about 200 ps). Several measurements have been done to confirm that the pulses are effectively bound.