Abstract
We report on a type of 2 μm h-shaped pulse generation in a thulium-holmium-doped fiber laser and experimentally investigate its cavity birefringence and pump power dependences. An asymmetric nonlinear optical loop mirror is employed as an artificial saturable absorber, which incorporates dispersion-shifted fiber and ultra-high numerical aperture fiber to enhance the nonlinearity. The h-shaped pulse shows both a polarization state (PS) and pump power related evolutions, even when randomly weak birefringence fibers are employed. By further incorporating different lengths of high birefringence polarization-maintaining fiber (PMF), i.e., introducing different amounts of linear cavity birefringence, much larger pulse tuning ranges can be realized. In particular, when the PMF is lengthened to through manipulating the PS, the achieved longest pulse duration of can almost cover the whole repetition period of , corresponding to a pulse duty circle of , the largest ever reported from a fiber laser, to the best of our knowledge. We demonstrate the related characteristics in detail.
© 2018 Optical Society of America
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