Fe²⁺: ZnSe and graphene Q-switched singly Ho³⁺-doped ZBLAN fiber lasers at 3 μm

μm Ho³⁺-doped ZBLAN fiber lasers passively Q-switched by a Fe²⁺:ZnSe crystal and graphene saturable absorbers were investigated, respectively. 800 ns pulses at 2.93 μm with an energy of 460 nJ and repetition rate of 105 KHz were obtained when a Fe²⁺:ZnSe crystal was inserted into a free space collimating and focusing setup. A more compact and reliable Q-switched fiber laser was achieved when a graphene coated fiber mirror was butt-coupled to the angle-cleaved end of the gain fiber. 1.2 μs pulses with an energy of 1 μJ and repetition rate of 100 KHz were achieved. More than 100 mW average output power was obtained at the maximum available pump power. Our experiments demonstrate that both Fe²⁺:ZnSe crystal and graphene are promising saturable absorbers for pulse generation in the 3 μm wavelength region.