-
按照上述实验装置图进行实验设计。连续光和调Q锁模输出功率随泵浦功率变化的关系如图2所示。选取透过率分别为3%、5%和9%的输出耦合镜,当腔内未加入氧化石墨烯可饱和吸收体时,实现连续光运转,出光阈值分别为716 mW,939 mW和1311 mW,对应的斜效率为4.63%,4.94%和4.35%;当泵浦功率为16 W时,最大输出功率为680 mW,714 mW和620 mW。为了获得最佳输出透过率,首先在理论上进行了模拟,其中谐振腔最佳透过率与泵浦功率之间的关系如公式(1)所示[28-29]:
$$ T=\sqrt{\dfrac{4{{\sigma \tau }_{f}}{{\lambda }_{P}}{{P}_{in}}\left[ 1-\exp \left( 1-{{\alpha }_{p}}L \right) \right]\times {{\delta }_{0}}}{\pi hc\left( \bar{W}_{p}^{2}+W_{0}^{2} \right)}}-{{\delta }_{0}} $$ (1) 式中:
$\sigma $ 为晶体发射截面;${\tau _f}$ 为发射寿命;${\lambda _P}$ 为泵浦光波长;${P_{in}}$ 为泵浦功率;${\alpha _p}$ 吸收系数;$L$ 为晶体长度;${\delta _O}$ 为腔内损耗;$\overline {{W_p}} $ 为平均泵浦光斑半径;${W_O}$ 为振荡光斑半径。通过模拟公式(1)得到图3的曲线图,从图中可以看出在泵浦功率为16 W时,谐振腔的最佳透过率T=0.045。因此,考虑到光学玻璃镀膜在加工中的实际情况,所以选取透过率为0.05的输出镜较为合适。图 3 连续光和锁模输出功率随泵浦功率变化图
Figure 3. The average output power of continuous wave and mode locking versus the pump power
将GO-SA加入激光腔中,选取透过率为3%的输出耦合镜,和连续光运转情况相比,它的出光阈值提高到1.68 W。当泵浦功率达到7.24 W时进入稳定调Q锁模运转,此时输出功率为68 mW,氧化石墨烯可饱和吸收体表面光斑半径约为160 μm, 计算可饱和吸收体表面功率密度约为25.94 μJ/cm2。继续增加泵浦功率,当泵浦功率达到16 W时,最大输出功率为160 mW,斜效率为1.04%。在5%输出耦合镜下的出光阈值功率为2.05 W,当泵浦功率达到8 W时,实现了稳定的调Q锁模运转,输出功率为104 mW,经过计算可饱和吸收体表面功率密度约为23.8 μJ/cm2,输出的最高功率为200 mW,斜效率为1.35%。而当输出镜为9%时腔内功率密度不足,无法实现调Q锁模。通过以上数据分析可知,当输出镜为5%时,拥有较高的输出功率与斜效率。因此,在调Q锁模运转中使用5%的输出镜,相信,通过不断优化激光腔体结构来获得较高功率的连续锁模运转。
实验的调Q锁模脉冲光谱通过(AvaSpec-NIR256-2.5 TEC)分析仪来获得。由图4可知,测量出来的脉冲信号的半高宽Δλ为11 nm,中心波长为2024 nm。
图5为锁模脉冲序列图。实验使用数字示波器(RIGOL, DS4034)来测量调Q锁模脉冲序列。当扫描时间分别为2 ms、100 µs和10 ns时,重复频率为108.7 MHz。调Q锁模脉冲的调制深度接近100%。
由于调Q锁模脉冲无法通过自相关仪直接去测量,因此利用以下公式理论上对Q包络中的锁模脉冲宽度进行估算[30]:
$${t_{\rm{m}}} = \sqrt {t_{\rm{p}}^2 + t_{\rm{r}}^{\rm{2}} + t_{\rm{o}}^{\rm{2}}} $$ (2) 式中:
${t_{\rm m}}$ 、${t_{\rm p}}$ 、${t_{\rm r}}$ 和${t_{\rm o}}$ 分别为实验中测量出来的锁模脉冲上升沿时间、光电二极管探测器的上升沿时间、锁模脉冲实际的上升沿时间以及数字示波器的上升沿时间。根据公式:
$${t_{\rm{o}}} \times {W_{\rm{B}}} = 0.35\sim 0.4$$ (3) 式中:示波器的带宽
${W_\rm{B}}$ 为200 MHz,根据公式(3)计算得到${t_{\rm o}}$ 为1 900 ps。实验中测得的${t_{\rm m}}$ 约为1 980 ps,${t_{\rm p}}$ 约为35 ps。将数据代入公式(2)可得锁模脉冲的实际上升沿时间${t_{\rm r}}$ 为556 ps,由于脉冲宽度约为${t_{\rm r}}$ 的1.25倍,所以实验中的调Q锁模脉冲宽度大约为695 ps。
Q-switched mode-locked all-solid-state Tm:LuAG ceramic laser
-
摘要: 采用垂直生长法制备的氧化石墨烯(Graphene oxide, GO)作为可饱和吸收体,利用典型“X”型折叠腔在全固态Tm:Lu3Al5O12(Tm:LuAG)陶瓷激光器中实现了调Q锁模运转。以790 nm激光二极管(Laser diode, LD)作为泵浦源,当泵浦功率大于8 W时,激光器进入稳定的调Q锁模状态。当输出镜透过率为5%时,连续光最高输出功率为714 mW,斜效率为4.94%。当泵浦达到16 W时,激光器最大输出功率为200 mW , 光谱中心波长为2024 nm,脉冲宽度约为695 ps,对应的锁模脉冲重复频率为108.7 MHz,调Q包络中锁模脉冲的调制深度接近100%。该2 μm超短脉冲激光器在生物医学和激光通讯等领域具有非常重要的应用。
-
关键词:
- Tm:LuAG陶瓷激光器 /
- 氧化石墨烯可饱和吸收体 /
- 调Q锁模 /
- 调制深度
Abstract: Using graphene oxide (GO) by vertical growth method as saturable absorber, an all-solid-state Q-switched mode-locked Tm: Lu3Al5O12 (Tm: LuAG) ceramic laser with typical 'X' folded cavity was firstly demonstrated . A 790 nm laser diode (LD) was used as the pumping source. When the pumping power was greater than 8 W, the laser entered a stable Q-switched mode-locked state. When the output mirror was 5%, the maximum output power of continuous light was 714 mW, and the oblique efficiency was 4.94%. When the pumping power reached 16 W, the maximum output power of the laser was 200 mW, the corresponding repetition frequency of mode-locked pulse was 108.7 MHz, and the modulation depth of mode locked pulse in Q-switched envelope was close to 100%. The 2 μm ultrashort pulsed laser has important applications in biomedicine and laser communication. -
-
[1] Lu Xianyang, Li Xuebin, Qin Wubin, et al. Retrieval of horizontal distribution of aerosol mass concentration by micro pulse lidar [J]. Optics and Precision Engineering, 2017, 25(7): 1697-1704. (in Chinese) [2] Zeng Haomin, Li Song, Zhang Zhiyu, et al. Risley-prism-based beam scanning system for mobile lidar [J]. Optics and Precision Engineering, 2019, 27(7): 1444-1450. (in Chinese) doi: 10.3788/OPE.20192707.1444 [3] Chen Y P, Zhai J P, Xu X T, et al. Mode-locked thulium-doped fiber laser based on 0.3 nm diameter single-walled carbon nanotubes at 1.95 μm [J]. Chinese Optics Letters, 2017, 15(4): 041403. doi: 10.3788/COL201715.041403 [4] Wang Caili, Xie Shiyong, Liu Hui, et al. Theoretical study 2 μm Tm:YAG laser with wavelength switchable accurately for lidar [J]. Infrared and Laser Engineering, 2018, 47(8): 0830003. (in Chinese) doi: 10.3788/IRLA201847.0830003 [5] Ling W J, Xia T, Dong Z, et al. Passively mode-locked Tm, Ho: LLF laser at 1895 nm [J]. Journal of Optics, 2019, 48(2): 209-213. doi: 10.1007/s12596-019-00528-y [6] Wang Y C, Xie G Q, Xu X D, et al. SESAM mode-locked Tm:CALGO laser at 2 µm [J]. Optical Materials Express, 2015, 6(1): 131-136. doi: https://doi.org/10.1364/OME.6.000131 [7] Tan W D, Su C Y, Knize R J, et al. Mode locking of ceramic Nd: yttrium aluminum garnet with graphene as a saturable absorber [J]. Applied Physics Letters, 2010, 96(3): 031106. doi: https://doi.org/10.1063/1.3292018 [8] Popa D, Sun Z, Torrisi F, et al. Sub 200 fs pulse generation from a graphene mode-locked fiber laser [J]. Applied Physics Letters, 2010, 97(20): 203106. doi: https://doi.org/10.1063/1.3517251 [9] Cizmeciyan M N, Kim J W, Bae S, et al. Graphene mode-locked femtosecond Cr: ZnSe laser at 2500 nm [J]. Optics Letters, 2013, 38(3): 341. doi: 10.1364/OL.38.000341 [10] Wei C, Jiang S, Xu S, et al. Graphene saturable absorber for diode pumped Yb: Sc2SiO5 mode-locked laser [J]. Optics & Laser Technology, 2015, 65: 1-4. [11] Zhu H T, Zhao L, Jie L, et al. Monolayer graphene saturable absorber with sandwich structure for ultrafast solid-state laser [J]. Optical Engineering, 2015, 55(8): 081304. doi: 10.1117/1.OE.55.8.081304 [12] Zhu H T, Liu J, Jiang S Z, et al. Diode-pumped Yb, Y: CaF2 laser mode-locked by monolayer graphene [J]. Optics & Laser Technology, 2015, 75: 83-86. [13] Xu Jinlong, Li Xianlei, Wu Yongzhong, et al. Graphene saturable absorber mirror for ultra-fast-pulse solid-state laser [J]. Optics letters, 2011, 36(10): 1948-1950. doi: 10.1364/OL.36.001948 [14] Zhao C, Huang Q Q, Mohammed A A, et al. Observation of chaotic polarization attractors from a graphene mode locked soliton fiber laser [J]. Chinese Optics Letters, 2019, 17(2): 60-64. [15] Ma J, Xie G Q, Lv P, et al. Graphene mode-locked femtosecond laser at 2 μm wavelength [J]. Optics Letters, 2012, 37(11): 2085-2087. doi: 10.1364/OL.37.002085 [16] Wang Y C, Chen W D, Mero M, et al. Sub-100 fs Tm: MgWO4 laser at 2017 nm mode locked by a graphene saturable absorber [J]. Optics Letters, 2017, 42(16): 3076-3079. doi: 10.1364/OL.42.003076 [17] Cheng Chen, Wei Jiafeng, Liu Bowen, et al. Experiment of mode-locked laser using graphene oxide [J]. Physics Experimentation, 2014(1): 1-4. (in Chinese) [18] Zhu H T, Cai W, Wei J F, et al. 763 fs Passively mode-locked Yb: Y2SiO5 laser with a graphene oxide absorber mirror [J]. Optics & Laser Technology, 2015, 68: 120-123. [19] Zhang G, Wang Y G, Chen Z D, et al. Graphene oxide based reflective saturable absorber for Q-switched and mode-locked YVO4/ Nd: YVO4/ YVO4 laser [J]. Journal of Optics, 2018, 20(5): 055505. [20] Ling Weijun, Xia Tao, Dong Zhong, et al. Passively Q-switched mode-locked low threshold Tm, Ho:LiLuF4 laser with a graphene Oxide saturable absorber [J]. Chinese Journal of Lasers, 2018, 45(3): 0301006. (in Chinese) [21] Beil K, Fredrich-Thornton S T, Tellkamp F, et al. Thermal and laser properties of Yb: LuAG for kW thin disk lasers [J]. Optics Express, 2010, 18(20): 20712-20722. doi: 10.1364/OE.18.020712 [22] Feng T, Yang K, Zhao J, et al. 1.21 W passively mode-locked Tm: LuAG laser [J]. Optics Express, 2015, 23(9): 11819-11825. doi: 10.1364/OE.23.011819 [23] Yang K J, Luan C, Zhao S Z, et al. Diode-pumped mode-locked Tm: LuAG 2 µm laser based on GaSb-SESAM[C]//The European Conference on Lasers and Electro-Optics, Optical Society of America, 2017: CAP 27. [24] Yan D Y, Liu P, Xu X D, et al. Eye-safe Nd: LuAG ceramic lasers [J]. Optical Materials Express, 2017, 7(4): 1374-1380. doi: 10.1364/OME.7.001374 [25] Zhou Z Y, Huang X X, Guan X F, et al. Continuous-wave and passively Q-switched Tm3+-doped LuAG ceramic lasers [J]. Optical Materials Express, 2017, 7(9): 3441-3447. doi: 10.1364/OME.7.003441 [26] Wang Y C, Lan R J, Mateos X, et al. Thulium doped LuAG ceramics for passively mode locked lasers [J]. Optics Express, 2017, 25(6): 7084-7091. doi: 10.1364/OE.25.007084 [27] Hu Xing, Cheng Dejiang, Guo Zhiyan, et al. Highly efficient RTP electro-optic Q-switched Nd:YVO4 laser by end-pumping at 914 nm [J]. Infrared and Laser Engineering, 2019, 48(1): 0105001. (in Chinese) doi: 10.3788/IRLA201948.0105001 [28] Paolo M B. Design criteria for mode size optimization in diode-pumped solid-state lasers [J]. IEEE Journal of Quantum Electronics, 1991, 27(10): 2319-2326. doi: 10.1109/3.97276 [29] Feng Y, Song F, Zhao L J, et al. Upconversion in Nd: YVO4 crystal under LD pump and its influence [J]. Acta Physica Sinica, 2001, 50(2): 335-360. [30] Li Z Y, Zhang B T, Yang J F, et al. Diode-pumped simultaneously Q-switched and mode-locked Nd: GdVO4/LBO red Laser [J]. Laser Physics, 2010, 20(4): 761-765. doi: 10.1134/S1054660X10070170