Passive mode locking resulting from weak polarization dependence based on evanescent field interaction with a monolayer graphene absorber

Weak polarization dependence based on evanescent field interaction with a monolayer graphene absorber is demonstrated. By covering single-layer graphene on the top of a fiber taper, the symmetry of the light field propagation was destroyed. Polarization-dependent loss (PDL) of 2 dB was obtained, and no saturable absorption was observed. Passive mode locking was achieved by using the graphene-covered fiber taper in a fiber laser. It is found that the weak PDL down to 2 dB is capable for achieving mode locking in a fiber laser. Therefore, the claim of saturable absorption of any graphene or two-dimensional materials in a fiber laser needs to be made with care.