Self-starting mode-locking in an all-PM Yb-doped fiber laser oscillator enabled by a 3x3 nonlinear optical lossy loop mirror

Ultrafast fiber oscillators based on loop mirror saturable absorbers often suffer from a problematic self-starting mode-locking operation. It is usually necessary to provide an additional phase shift in the loop, guaranteeing repeatable initiation of the pulsed regime. 3x3 even splitting ratio fiber couplers have recently introduced a required phase shift in an all-fiber loop mirror architecture. Until now, mode-locking induced by a 3x3 fiber coupler has been associated only with nonlinear amplifying loop mirrors. Here, we present a self-starting ultrafast dispersion-managed all-polarization-maintaining Yb-doped oscillator that utilizes a nonlinear optical lossy loop mirror instead. We show three ways of inserting asymmetric losses in a loop via a variable optical attenuator, a fiber coupler, and a very simple lossy splice. Complete characterization of all output ports of the oscillator proves significant spectral and temporal breathing of the pulse when circulating through the net normal dispersion cavity, which can deliver nJ-level pulse energy. The system guarantees excellent stability, low noise and performance comparable to nonlinear amplifying loop mirrors while being simpler, cheaper, and providing more usable output ports with different pulse characteristics.