Optimization of a passively Q-switched double clad Yb3+:Cr4+ all fibre laser

This paper reports on the optimization of a double-clad pumped, passively Q-switched (PQS) all-fibre laser built around spliced Yb³⁺ doped amplifier and Cr⁴⁺ doped saturable absorption fibers. The respective merits of near infrared emission of potential Q-switched all-fibre are investigated versus Cr⁴⁺ concentration in the saturable absorber (SA), depending on the available pump power. For the Yb:Cr system pumped at 980 nm and operated at 1064 nm, and for a chosen set of cavity parameters described by the cavity photon lifetime, the main control parameters are the maximum gain in the Yb-doped fiber and the saturable losses in the Cr-doped SA fiber, respectively. These are controlled by the absorbed pump power and the Cr⁴⁺ ion concentration in the SA, respectively. The PQS operation is observed between two pump power values: the lowest one is approximately equal to the laser threshold, the upper one corresponds to the PQS-to-CW operation transition.