Yellow lasers have applications in industrial, medical, and scientific fields. In addition, the demand for yellow lasers has gradually increased in astronomy, spectroscopy, and similar fields. Common methods for generating yellow lasers include using semiconductor lasers, nonlinear frequency conversion, and direct pump laser gain media doped with appropriate ions. These methods have yielded relatively high yellow lasers and Q-switched-pulse yellow lasers. However, the principle and process of nonlinear frequency conversion are complex. With the development of blue laser diode (LD) pumping technology, visible lasers can be obtained by directly pumping a gain medium doped with rare-earth ions. The Dy³⁺ ion has an energy-level emission of ⁴F_9/2 to ⁶H_13/2 corresponding to the yellow emission at 574 nm, and an energy-level absorption of ⁶H_15/2 to ⁴I_15/2 corresponding to the absorption peak at 450 nm. Thus, Dy³⁺ ion-doped crystals are potential yellow laser gain materials for being directly pumped by blue LDs. Co-doping with Tb³⁺ ions has also been reported as an effective method for quenching the lower level ⁶H_13/2 of Dy³⁺, which can lead to fast depopulation of the population in the lower laser level and reduce the pumping threshold. Therefore, a yellow laser performance with the output power of 55 mW was obtained in Dy-Tb∶LuLiF₄ crystal in 2014. The results can be further optimized through the development of pumping technologies. Black phosphorus (BP), a two-dimensional (2D) material, has a direct bandgap of 0.3?2 eV. The direct bandgap theoretically indicates that BP is a potential broadband saturable absorber in the visible to mid-infrared regions. Therefore, with BP as a suitable saturable absorber, a pulsed yellow laser can be realized using Q-switching technology. Hence, in this study, with a Dy-Tb∶LuLiF₄ crystal as the gain material, continuous wave (CW) and passively Q-switched yellow lasers are generated with single-emitter and double-emitter blue LDs as the pump sources, respectively.

The pump sources used in the experiment were single-emitter and double-emitter blue LDs, under the same conditions [Fig.1(a) and Fig.2(a)]. The laser resonator consisted of two concave mirrors (M1 and M2) with a radius of curvature of 50 mm each. Two mirrors were coated to generate a yellow laser. The distance between M1 and M2 was optimized to approximately 50 mm. Finally, the pump beam was focused onto the Dy-Tb∶LuLiF₄ crystal using a planoconvex lens. The laser crystal was polished and mounted onto a copper holder equipped with circulating cool water. The BP sample was fabricated via chemical vapor deposition (CVD) using sapphire as the substrate. The transmission spectra in the visible range and the Raman spectrum of the sample were measured (Fig.3). To investigate the saturable absorption of the BP sample, a self-administered Z-scan test system was employed with a pump source at 532 nm. The variation in the normalized transmittance with incident intensity was presented and fitted using the saturable absorption equation (Fig. 4 and equation 1). To generate a Q-switched yellow laser, the BP sample was inserted into the cavity and placed at the minimum possible distance from the Dy-Tb∶LuLiF₄ crystal.

With the laser setup of the single-emitter blue LD, a CW laser was generated at a threshold absorbed pump power of 0.74 W. When the absorbed pump power increases to 2.12 W, the maximum output power of the yellow laser is obtained with a corresponding slope efficiency of 11.3% [Fig.1(b)]. With the double-emitter-blue-LD setup, a maximum output power of 297 mW is generated under the absorbed pump power of 3.0 W with a corresponding slope efficiency of 12.3% [Fig.2(b)]. To verify the Q-switching performance, variations in the pulse width and repetition rate as functions of the absorbed pump power were obtained (Fig.5). As the pump power increases, the pulse width decreases to 766.8 ns, and the repetition rate increases from 9.4 to 26.2 kHz with the increase in the absorbed pump power. Thus, the pulse energy and peak power can be estimated. When the absorbed pump power is 3 W, the maximum pulse energy is 2.1 μJ, and the maximum peak power is 2.7 W. The temporal waveform of the pulse is also provided, which verifies the stable Q-switching behavior.

In this paper, we report a yellow laser pumped by single- and double-tubed blue LDs. Combined with a temperature control system to deal with the heat generated by the crystal, a 573.9 nm yellow laser is generated. The maximum output power is 297 mW at an absorbed pump power of 3.0 W, and the corresponding slope efficiency is 12.3%. A multilayer BP sample is used as a saturable absorber to generate a Q-switched pulsed yellow laser. When the absorption pump power is 2.8 W, the average output power of the pulsed yellow laser is 54 mW, with the corresponding pulse width of 766.8 ns and the corresponding pulse energy of 2.1 μJ.