1.Institute of Physics and Optoelectronics Technology, Baoji University of Arts and Sciences, Baoji 721016, China 2.Institute of Laser Technology, Tianshui Normal University, Tianshui 741001, China 3.School of Arts and Sciences, Shaanxi University of Science and Technology, Xi'an 710021, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 61564008, 11774257, 11647008, 11504416), the International Science & Technology Cooperation and Exchanges Project of Shaanxi, China (Grant No. 2018KW-016), the Key Sciences and Technology Project of Baoji City, China (Grant No. 2015CXNL-1-3), and the Key Research Project of Shaanxi University of Science & Technology, China (Grant No. 2018WLXY-01-01).
Received Date:19 December 2018
Accepted Date:15 March 2019
Available Online:01 May 2019
Published Online:20 May 2019
Abstract:A watt-level passive Q-switched mode-locked operation in Tm: LuAG all-solid-state laser is realized for the first time by using graphene oxide (GO) saturable absorber as a mode-locked starting element. The laser is pumped by a wavelength tunable Ti: sapphire laser operating at 794.2 nm. In this experiment, the maximum continuous-wave (CW) output power of 1440 mW, 2030 mW and 2610 mW are obtained by 1.5%, 3% and 5% output coupled (OC) mirrors respectively, in which the corresponding slope efficiencies are 22.3%, 32.6% and 40.6%, respectively. When the GO is inserted into the cavity, the laser bump threshold is further increased due to more intracavity loss. With a 1.5% OC mirror, the absorbed pump threshold is as low as 325 mW, the maximum output power is 787 mW, and the corresponding slope efficiency is 12.5%. With a 3% OC mirror, the absorbed bump threshold is 351 mW, the maximum output power is 1740 mW, and corresponding slope efficiency is 30.3%. With a 5% OC mirror, the QML operation is not realized due to the increase of intracavity loss. Although the laser pump threshold power of 3% OC mirror differs from that of 1.5% OC mirror by 26 mW, the output power is more than twice higher than that of 1.5% OC mirror. For these reasons, we use a 3% OC mirror in our experiment. In this case, a stable QML operation with a threshold of 3420 mW is obtained. When the pump power reaches 8.1 W, the corresponding maximum output power is 1740 mW, the central wavelength is 2023 nm, the repetition frequency is 104.2 MHz, the maximum single pulse energy is 16.7 nJ, and the modulation depth is close to 100%. According to the symmetrical shape of the mode locked pulse and considering the definition of rise time, we can assume that the duration of the pulse is approximately 1.25 times the pulse rise time. So the width of the mode locked pulse is estimated at about 923.8 ps. The results show that the GO is a promising high power saturable absorber in 2 μm wavelength for the QML solid-state laser. In the next stage, we will increase the pump power, optimize the quality of the GO material, and compensate for the dispersion in the cavity. It is expected to achieve a CW mode-locked operation and femtosecond pulse output. Keywords:high power/ Tm: LuAG laser/ Q-switched mode-locking/ graphene oxide
4.实验结果分析与讨论采用上述Tm: LuAG被动锁模激光实验装置, 获得实验数据如图3(a)所示, 激光晶体对泵浦光的吸收率在激光器出光和非出光状态下有很明显的差异. 当腔内无激光运转时, 激光晶体对泵浦光的吸收效率仅65.0%, 当实现连续光运转时, 激光晶体的吸收效率达到了85%左右. 之后在腔内插入氧化石墨烯时, 选用1.5%和3%的输出镜, 激光晶体的吸收效率变化不大, 约为84.5%. 图 3 (a)晶体吸收变化图; (b)连续光和锁模输出功率随吸收抽运功率的变化 Figure3. (a) The change of crystal absorbed power; (b) Relation between average output power and absorbed pump power under continuous-wave and mode-locked operation.
图 1 氧化石墨烯可饱和吸收体 (a)拉曼光谱图; (b)扫描电镜图; (c)实物图
图 2 Tm: LuAG被动锁模激光实验装置图
图 3 (a)晶体吸收变化图; (b)连续光和锁模输出功率随吸收抽运功率的变化
图 4 锁模脉冲信号的输出光谱
图 5 扫描时间为1 ms, 100 μs, 2 μs和10 ns的锁模脉冲序列