High-Power Acoustol-Optic Q-Switched Nd:YLF Laser Operating at 1314 nm

Objective Lasers emitted in the 1. 3 mu m spectral region have received significant attention owing to increasing applications in remote sensing, timing systems, dermatologic procedures, and nonlinear frequency conversion. It is well known that Nd:YLF is a promising material for generating high-energy 1. 3 mu m pulsed laser because of its extended upper-laser-level lifetime. However, the power scaling of 1. 3 mu m Nd:YLF lasers is challenging because of their small stimulated emission cross-section and low thermal fracture limit. An end-pumped scheme with a broadband 880 nm laser diode (LD) is investigated to overcome these limitations. However, the power stability of the 1314 nm laser is reduced by the thermal wavelength shift and linewidth fluctuation of the broadband LD. When the broadband LD is used as the pump source, it is difficult to simultaneously improve the pump absorption efficiency, enhance the mode-to-pump overlap efficiency, and reduce the thermal stress of the laser crystal. Therefore, the high-power, high-efficiency laser output is greatly restricted. This paper introduces a wavelength-locked narrowband 880 nm LD as the pump source for generating a stable, efficient, and powerful 1314 nm laser. Methods Figure 1 shows the experimental setup. The pump source is a fiber Bragg grating (FBG) locked narrowband fiber-coupled LD with a numerical aperture of 0. 22 mu m and a core diameter of 200 mu m. Its center wavelength is stabilized at 879. 9 nm with a narrow spectral bandwidth of 0. 2 nm. A pair of coupling lenses with 1:5 magnification is used to re-image the pump beam with a spot diameter of approximately 1 mm into the gain medium. An a-cut 1. 0% (atomic fraction) Nd:YLF crystal with a size of 3 mmx3 mmx30 mm is selected as the gain medium, which is coated for high transmission at 880 nm and 1047-1321 nm on the entrance surface and high transmission at 1047-1321 nm and partial reflectivity at 880 nm (reflectivity R approximate to 60%) on the rear surface. Under non-lasing conditions, the pump absorption efficiency exceeds 90%. During the experiments, the gain medium is wrapped with indium foil and closely packed using a water-cooled copper holder at 16 degrees C. The Q-switched device is a 46-mm-long acousto-optic modulator plated with a 1314 nm antireflection coating on both surfaces and driven by a 27. 12-MHz ultrasonic frequency generator operating at a 100 W radio frequency. The linear resonator is composed of a plano-concave mirror M1 with a radius of curvature of 500 mm and a plane output coupler M2. The input mirror M1 is coated for high transmission at 880 nm and 1047-1053 nm and high reflection at 1314-1321 nm, whereas the plane mirror M2 coated for partial reflectivity at 1314 nm (coupling output rate T-OC=5%, 10%) is employed as the output coupler. Considering the thermally induced diffraction loss and energy transfer upconversion (ETU) effect, the optimized mode-to-pump ratio is approximately 0. 84. Consequently, the physical length of the resonator is set to approximately 250 mm based on the ABCD matrix theory. Results and Discussions When T-OC=5%, the maximum continuous-wave output power reaches 20. 4 W under an incident pump power of 70 W, resulting in optical-to-optical conversion efficiency of 29. 1% and a slope efficiency of 32. 5% (Fig. 2). Under the full output power, the beam quality factors are M-x(2)=1. 65 and M-y(2)=1. 81 (Fig. 3), and the power stability (root mean square) is 0. 1% within 1 h. In addition, when T-OC=10%,the maximum output power reaches 19 W with an optical-to-optical efficiency of 27. 2% and a slope efficiency of 32% (Fig. 2). After inserting an acousto-optic Q-switcher, when T-OC=5%, the average output power increases from 9. 8 W at a pulse repetition frequency (PRF) of 1 kHz to 16. 5 W at a PRF of 20 kHz, corresponding to a decrease in pulse energy from 9. 8 mJ to 0. 82 mJ (Fig. 6). The pulse duration increases from 119 ns at 1 kHz to 433 ns at 20 kHz, decreasing the peak power from 82. 3 kW to 1. 8 kW (Fig. 7). Under the full output power, the corresponding power stability (root mean square) within 1 h is 1. 2%. Conclusions A high-power end-pumped Nd:YLF laser operating at 1314 nm is demonstrated using a wavelength-locked narrowband 880 nm laser diode. The optimized mode-to-pump ratio is approximately 0. 84 considering the thermal and ETU effects. The Nd:YLF laser delivers the maximum continuous-wave output power of 20. 4 W with an optical-tooptical conversion efficiency of 29. 1% and a slope efficiency of 32. 5%. After Q-switching with an acousto-optic modulator, the laser system generates the maximum average output power of 16. 5 W at 20 kHz and the maximum pulse energy of 9. 8 mJ at 1 kHz. To the best of our knowledge, we demonstrate the highest average power and highest pulse energy from Q-switched end-pumped single-crystal 1. 3 mu m Nd:YLF lasers. Future upgrades to achieve higher output power and pulse energy will involve a multisegment-doped or diffusion-bonded Nd:YLF crystal and a double-end pumping scheme.