Passively Q-Switched 3- $\mu$ m $\text{Ho}^{3+}$ -Doped Fiber Laser Based on Nonlinear Polarization Rotation

We report a 3-μm-waveband passively Q-switched Ho
³⁺
-doped ZBLAN fiber laser based on nonlinear polarization rotation (NPR) in the ring cavity for the first time, to the best of our knowledge. The laser can operate in stable single- and dual-wavelength regimes. The stable single-wavelength pulse train is obtained with a ~104 pm spectral linewidth at 2960.7 nm for 1.064-W launched pump power. By further increasing the launched pump power, the dual-wavelength pulsed laser can be observed, which is caused by the combination of intra-cavity birefringence and polarizer. The resulting dual-wavelength pulse has a narrow spectral linewidth of ~87 pm and ~131 pm at the central wavelength of 2966.8 nm and 2971.7 nm, respectively, under the maximum launched pump power of 3.86 W. The corresponding maximum output power and shortest pulse duration are 86.6 mW and 1.6 μs, respectively.