Harmonic noise-like square pulses of a passively mode-locked Er/Yb double clad fiber laser in a figure-eight configuration

Abstract

Passively mode-locked fiber lasers are capable of generating a variety of optical pulses, depending of the pump power and cavity adjustments, which makes them attractive for research in multidisciplinary fields. In this paper, we analyze the experimental results of a fiber laser operating in passively mode-locked regime. The figure-eight laser cavity configuration uses 4 m of a co-doped Er/Yb double clad fiber, as a gain medium, pumped by a 976 nm diode laser. The cavity is completed by an isolator for ensure unidirectional propagation, two quarter-wave retarders (QWR), and a nonlinear optical loop mirror (NOLM) with 120 m of twisted standard single-mode fiber in the loop acting as saturable absorber. We used a couple of quarter-wave retarders (QWR) QWR1 at the NOLM input and QWR2 into the NOLM loop. By carefully adjusting of the QWRs the laser generates noise-like square pulses at the 1.54 μm range. With the increase of the pump power, the laser generates harmonics of the mode-locking fundamental mode with repetition frequency of 1.38 MHz, from which we can obtain the second, third and fourth harmonic in a controlled mode.