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 $\sim 52.7\mathrm{m}$ dispersion-shifted fiber and $\sim 3.8\mathrm{m}$ 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 $\sim 2.3\mathrm{m}$ through manipulating the PS, the achieved longest pulse duration of $\sim 318.14\mathrm{ns}$ can almost cover the whole repetition period of $\sim 323.96\mathrm{ns}$, corresponding to a pulse duty circle of $\sim 98.2\%$, 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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