Bismuth nanosheets based saturable-absorption passively <i>Q</i>-switching mid-infrared single-crystal fiber laser
Authors: Hao Qian-Qian , Zong Meng-Yu , Zhang Zhen , Huang Hao , Zhang Feng , Liu Jie , Liu Dan-Hua , Su Liang-Bi , Zhang Han ,   Shandong Provincial Engineering and Technical Center of Light Manipulations, Shandong Provincial Key Laboratory of Optics and Photonic Device, School of Physics and Electronics, Shandong Normal University, Jinan 250358, China,   Collaborative Innovation Center of Light Manipulations and Applications, Shandong Normal University, Jinan 250358, China,   CAS Key Laboratory of Transparent and Opto-Functional Inorganic Materials, Synthetic Single Crystal Research Center, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899, China,   College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China
Published: December 2020

As a new two-dimensional material, bismuth nanosheet is an effective modulator for realizing a mid-infrared pulsed laser, which benefits from its suitable band gap, higher carrier mobility and better room temperature stability, as well as its excellent electrical and optical properties. The mid-infrared single-crystal fiber is a preferable gain medium for high-power laser because of its advantages of both crystal and fiber. In this paper, a bismuth nanosheet saturable absorber is successfully prepared by the ultrasonic method and used for the first time in a diode-pumped Er:CaF<sub>2</sub> single-crystal fiber mid-infrared passively <i>Q</i>-switching pulsed laser. A compact concave planar linear resonator is designed to study the <i>Q</i>-switching Er:CaF<sub>2</sub> single-crystal fiber laser with bismuth nanosheets serving as saturable absorbers. The pump source is a fiber-coupled semiconductor laser with a core diameter of 105 μm, a numerical aperture of 0.22, and a central emission wavelength of 976 nm. The pump light is focused onto the front end of the gain medium through a coupled collimating system with a coupling ratio of 1∶2. The gain medium is a 4 at.% Er<sup>3+</sup>:CaF<sub>2</sub> single-crystal fiber grown by the temperature gradient method, and this fiber has two polished but not coated ends, a diameter of 1.9 mm, and a length of 10 mm. To reduce the thermal effect, the single-crystal fiber is tightly wrapped with indium foil and mounted on a copper block with a constant temperature of 12 ℃. The input mirror has a high reflection coating at 2.7–2.95 μm and an antireflection coating at 974 nm, with a curvature radius of 100 mm. A group of partially transmitting plane mirrors are used as output couplers, respectively, with transmittances of 1%, 3%, and 5% at 2.7–2.95 μm. The total length of the resonant cavity is 26 mm. By inserting the bismuth nanosheet into the resonator and carefully adjusting its position and angle, a stable mid-infrared <i>Q</i>-switching laser is obtained. At the absorbed pump power of 1.52 W, a pulsed laser with an average output power of 190 mW is obtained for an output mirror with a transmittance of 3%. The shortest pulse width is 607 ns, the repetition frequency is 58.51 kHz, and the corresponding single pulse energy and peak power are 3.25 μJ and 5.35 W, respectively.