-
兼顾脉宽、线宽和输出功率,主振荡器选用了40%中等初始透过率的Cr4+:YAG和短于10 mm的腔长,在1 kHz、峰值功率30 W、占空比20%的脉冲泵浦下,经光纤耦合半导体激光器泵浦的Nd:YAG/Cr4+:YAG键合晶体,得到输出功率106 mW、脉宽810 ps、线宽34 pm的种子激光脉冲输出。图3为采用响应时间25 ps、带宽15 GHz的 ET-3500 InGaAs快速光电探测器和1 GHz-Tektronix的数字示波器,测量得到的脉冲波形及脉冲宽度。为确定种子激光的稳定性,实验还采用示波器余晖累积方法,测量了输出脉冲波形的稳定度,得到如图4所示的余晖累积图,经计算时间抖动性小于±2.9%,幅值不稳定度为±2.7%。
从图3、4可以看出种子激光幅值稳定性和时间抖动性较好且输出脉冲波形无前后尾脉冲,为后续高效率稳定放大提供了可靠种子激光。随后,利用Solar Laser Systems波长计测量得到输出线宽为34 pm,其测量精度为±3 pm,测量结果如图5所示。采用Spiricon M2-200S光束质量分析仪对种子光光束质量进行测量,经测量得到M2=1.5。
-
对于激光功率放大器,可以利用迭代方法来模拟激光的放大过程[10-11],其单程放大后输出能量可表示为:
$$ {E_1} = {E_S}{\rm{l}}n\left\{ {1 + \left[ {{\rm{exp}}\left( {\frac{{{E_0}}}{{{E_S}}}} \right) - 1} \right]{\rm{exp}}\left( {{g_0}l} \right)} \right\} $$ (1) 式中:E0为放大器的输入能量密度;ES为饱和能量密度;E1为输出能量密度;g0为小信号增益系数;l为增益介质长度。其中g0是由泵浦条件,增益介质等诸多因素所确定的,根据固体激光工程可得:
$$ {g_0}l = {{{{K}}}}{E_P} $$ (2) $$ {{{{K}}}} = {{\rm{\eta }}_T}{\eta _a}{\eta _S}{\eta _Q}{\eta _B}{\eta _{ST}}{\eta _{ASE}}/A{E_S} $$ (3) $$ {g_0} = {{\rm{\eta }}_T}{\eta _a}{\eta _S}{\eta _Q}{\eta _B}{\eta _{ST}}{\eta _{ASE}}{E_P}/A{E_S}l $$ (4) 式中:ηT表示泵浦耦合效率;ηa表示增益介质的吸收效率;ηS表示Stokes效率;ηQ表示量子效率;ηB表示光斑与激光棒横截面之比;ηST表示上能级储存效率;ηASE表示放大自发辐射损耗;EP表示泵浦能量密度;ES表示饱和能量密度;A为光束在增益介质中的有效横截面积。从公式(4)中可以看出小信号增益系数g0与诸多能量转换过程密切相关,且这些能量转换过程都是影响小信号增益的重要因素,且在实际过程中,上面这些参数彼此相关又复杂,很难进行清晰的测量。因此,可在单通放大的实验中推导得出g0,进而在双通放大实验中验证输出结果。实验获得双模块单通输出1.5 W代入公式(1)中可推导得出g0l=2.44。
双程放大器的输出能量密度表达式:
$$ {E_2} = {E_S}{\rm{l}}n\left\{ {1 + \left[ {{\rm{exp}}\left( {\frac{{{E_1}}}{{{E_S}}}} \right) - 1} \right]{\rm{exp}}\left( {{g_0}^{'}l} \right)} \right\} $$ (5) $$ {g_0}^{'} = \left( {1 - {\eta _1}} \right){g_0} $$ (6) $$ {\eta _1} = \left( {{E_1} - {E_0}} \right)/{g_0}l{E_S} $$ (7) 联立以上公式(5)~(7),当入射光平均功率为106 mW,计算得到双模块双通放大的理论输出功率为13.8 W。
-
在获得亚纳秒种子激光输出后,将其导入双模块进行双通放大,实验中将所用侧泵模块的电源与种子光的泵浦驱动电源做了同步驱动控制。在模块泵浦脉宽200 μs,电流80 A时,经两个特性相同的Nd:YAG侧泵模块双通放大,通过调整光路最后获得了功率10.1 W、单脉冲能量10.1 mJ、脉宽816 ps的亚纳秒激光输出,图6为放大输出的脉冲波形。
采用Spiricon M2-200S光束质量分析仪测量了双通放大后的光束质量,获得M2=1.8的测量结果,光斑分布如图7所示。利用Solar Laser Systems的波长计测量其输出线宽39 pm,所得结果如图8所示。所得输出功率随泵浦电流变化曲线如图9所示。
从实验结果来看,实验所得输出功率为10.1 W比理论计算值13.8 W要小;输出脉冲宽度和线宽与种子光相比,没有明显的变化;输出光束质量略有变差。分析放大输出功率低于理论设计值的原因在于,退偏补偿不能100%补偿,退偏光未经TFP2反射输出而是经光隔离器后从TFP1反射输出,进而降低了双通放大从TFP2处的输出功率。
Research on 1 kHz high-power sub-nanosecond all-solid-state laser amplifier
-
摘要: 高功率全固态亚纳秒激光器具有体积小、成本低、线宽窄、峰值功率高等优势,在诸多领域具有重要的应用价值。为获得高功率亚纳秒激光输出,首先通过被动调Q激光器得到亚纳秒种子激光,然后利用LD侧泵模块,采用双模块双通放大的实验设计,在重复频率为1 kHz时,获得了平均功率达10 W,脉冲宽度816 ps,线宽39 pm,光束质量M2小于1.8的激光输出,放大器整体放大倍率达95倍以上。Abstract: High-power all-solid-state sub-nanosecond lasers have the advantages of small size, low cost, narrow line width and high peak power. They have important application value in many fields. In order to obtain high-power sub-nanosecond laser output, a sub-nanosecond seed laser was first obtained through a passive Q-switched laser, and then an LD side pump module was used to design a dual-module dual-pass amplification experiment. At a repetition rate of 1 kHz, a laser output with an average power of 10 W, a pulse width of 816 ps, a line width of 39 pm, and a beam quality M2 of less than 1.8 was obtained. The overall magnification the amplifier was over 95 times.
-
Key words:
- amplifier /
- sub-nanosecond /
- narrow linewidth /
- side pump
-
-
[1] 段加林, 李旭东, 武文涛, 等. LD泵浦Nd:YAG 1.06 μm脉冲串激光及放大研究[J]. 红外与激光工程, 2019, 48(1): 0105003. Duan Jialin, Li Xudong ,Wu Wentao, et al. Research on LD pumped 1.06 μm burst-mode laser and the amplification systems [J]. Infrared and Laser Engineering, 2019, 48(1): 0105003. (in Chinese) [2] 刘秋武, 陈亚峰, 王杰, 等. 差分吸收NO2激光雷达波长漂移和能量波动对浓度反演的影响[J]. 光学 精密工程, 2018, 26(2): 253−260. doi: 10.3788/OPE.20182602.0253 Liu Qiuwu, Chen Yafeng, Wang Jie, et al. Effects of wavelength shift and energy fluctuation on inversion of NO2differential absorption lidar [J]. Optics and Precision Engineering, 2018, 26(2): 253−260. (in Chinese) doi: 10.3788/OPE.20182602.0253 [3] 赵志龙, 吴谨, 王海涛, 等. 微弱回波条件下差分合成孔径激光雷达成像实验演示[J]. 光学 精密工程, 2018, 26(2): 276−283. doi: 10.3788/OPE.20182602.0276 Zhao Zhilong, Wu Jin, Wang Haitao, et al. Experimental demonstration of differential synthetic aperture ladar imaging at very low return level [J]. Optics and Precision Engineering, 2018, 26(2): 276−283. (in Chinese) doi: 10.3788/OPE.20182602.0276 [4] 刘国军, 薄报学, 曲轶, 等. 高功率半导体激光器技术发展与研究[J]. 红外与激光工程, 2007, 36(S1): 4−6. Liu Guojun, Bo Baoxue, Qu Yi, et al. High power semiconductor lasers [J]. Infrared and Laser Engineering, 2007, 36(S1): 4−6. (in Chinese) [5] Cerny P, Jelinkova H, Zverev P G, et al. Solid state lasers with Raman frequency conversion [J]. Prog in Quant Electr, 2004, 28(2): 113−143. doi: 10.1016/j.pquantelec.2003.09.003 [6] Agnesi A, Dallocchio P, Pirzio F, et al. Sub-nanosecond single-frequency 10-KHz diode-pumped MOPA laser [J]. Applied Physics B, 2010, 98: 727−741. [7] 颜凡江, 杨策, 陈檬, 等. 高重频高峰值功率窄线宽激光放大器[J]. 红外与激光工程, 2019, 48(2): 0206002. Yan Fanjiang, Yang Ce, Chen Meng, et al. High repetition, high peak power and narrow line-width laser amplifie [J]. Infrared and Laser Engineering, 2019, 48(2): 0206002. (in Chinese) [8] Yukio Kyusho, Motohiro Arai, Katsuji Mukaihara, et al. High-energy subnanosecond compact laser system with diode-pumped, Q-switched Nd:YVO4 laser [J]. Advanced Solid-State Laser, 1996, 1: 382−385. [9] Wandt C, Klingebiel S, Siebold M, et al. Generation of 220 mJ nanosecond pulses at a 10 Hz repetition rate with excellent beam quality in a diode-pumped Yb: YAG MOPA system [J]. Opt Lett, 2008, 33: 1111−1113. doi: 10.1364/OL.33.001111 [10] 克希耐尔W. 固体激光工程[M]. 孙文, 江泽文, 程国祥, 译. 北京: 科学出版社, 2002. Koechner W. Solid State Laser Engineering[M]. Sun W, Jiang H W, Cheng G X, transl. Beijing: Science Press, 2002.(in Chinese) [11] Lee M Franz, John S Nodvik. Theory of pulse propagation in a laser amplifier [J]. Applied Physics, 1963, 2349: 8.