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按照上述Tm,Ho:LLF激光器实验装置进行设计并搭建光路,当腔内没有激光运转时,晶体对泵浦光的吸收效率约为33.7%,当有激光运转时,且氧化石墨烯没有插入谐振腔内,晶体的吸收效率约为59.48%。而在腔内插入可饱和吸收体时,晶体的吸收率略有变化,在透过率为3%的输出耦合镜下,晶体的吸收效率约为53.8%,输出耦合镜透过率为5%时,晶体吸收率约为53.6%。经过分析得知,这是由于腔内插入氧化石墨烯可饱和吸收体增加了腔内损耗,从而导致晶体的吸收效率降低。Tm,Ho:LLF激光器连续运转下输出功率与泵浦功率变化的关系如图2所示。当输出耦合镜的透过率分别为3%、5%和9%时,输出激光的阈值分别为2.3 W、2.4 W和2.6 W,倾斜效率分别为5.63%、9.08%和10.76%。当泵功率为20 W时,最大连续光输出功率为954 mW、1522 mW和1793 mW。
为了选择最佳输出镜透过率,进行了理论模拟。谐振腔最佳透过率与泵浦功率之间的关系如公式(1)所示:
$$T = \sqrt {\frac{{4\sigma \tau_ {\rm{f}}\lambda_ {\rm{p}}P_{\rm in}\left[ {1 - \exp (1 - \alpha _ {\rm{p}}L)} \right] \times \delta_ 0}}{{\pi hc({{\overline {W_{\rm p}} }^2} + W_{\rm o}^2)}}} - \delta_ 0$$ (1) 式中:
$\sigma $ 为晶体发射截面;${\tau _{\rm{f}}}$ 为发射寿命;${\lambda _{\rm p}}$ 为泵浦光波长;${P_{{\rm{in}}}}$ 为泵浦功率;${\alpha _{\rm p}}$ 为吸收系数;$L$ 为晶体长度;${\delta _0}$ 为腔内损耗;$W_{\rm p}$ 为平均泵浦光斑半径;$W_{\rm o}$ 为振荡光斑半径。通过MATLAB软件模拟计算公式(1)得到的曲线如图3所示。可以看出,在泵浦功率为20 W时,谐振腔的最佳透过率T=0.0896,所以选取透过率为0.09的输出镜较为合适,同时在连续光实验下,透过率为9%的OC输出功率最高,理论与实验结果相符合。因此,下面主要在透过率为9%的输出耦合镜下进行研究调Q以及调Q锁模实验,这样就可以在进行调Q锁模实验时得到较高的调Q锁模激光。
图 3 谐振腔最佳透过率随泵浦功率变化图
Figure 3. Relationship between the best transmittance of the resonant cavity and the pump power
当在腔内插入GO-SA后,透过率为9%的输出耦合镜下的出光阈值增加到2.5 W,当泵浦光功率高于5 W时,激光进入调Q运转状态,且在示波器中没有观察到寄生脉冲。随着泵浦功率的提高,调Q包络的脉冲宽度变得越来越窄,脉冲重复频率变得越来越高,调Q脉冲重复频率和脉冲宽度随泵浦功率的变化如图4所示。可以看出,当抽运功率从5 W增加到7.25 W时,调Q脉冲重复频率从60 kHz增加到139 kHz,相应地脉冲宽度从16.3 μs减小到5.5 μs。
图 4 调Q的脉冲重复频率和脉冲持续时间与吸收的泵浦功率的变化关系
Figure 4. Q-switched pulse repetition rate and pulse duration versus the absorbed pump power
继续增加泵浦光,腔内功率密度逐渐增大,并且仔细调节氧化石墨烯可饱和吸收体到M3的距离,随后激光器进入调稳定的Q锁模运转。当泵浦功率达到20 W时,在透过率为9%的输出镜下锁模运转的最高输出功率为1052 mW,斜效率为5.1%,且没有发现GO表面发生损伤。输出功率与泵浦功率关系如图5所示。
图 5 加入GO-SA后Tm,Ho:LLF激光器输出功率随泵浦功率变化
Figure 5. After adding GO-SA, Tm, Ho: LLF laser output power changes with pump power
实验采用透过率为9%的输出耦合镜,当泵浦功率为10 W、输出功率为433 mW时,通过使用光谱分析仪(AvaSpec-NIR 256-2.5 TEC)获得调Q锁模脉冲光谱,如图6所示。输出脉冲信号的中心波长为1 895 nm,光谱的半高宽
$\Delta {\lambda _{}}$ 为15 nm。随着功率的提升以及脉冲宽度的变窄,并没有观察到激光器的输出光谱发生明显漂移,其保持在1 895 nm。图7所示是由快速光电二极管(ET-500)连接的数字示波器(RIGOL, DS4034)测量出的脉冲序列,获得该图的实验条件与图6相同。扫描时间分别为1 ms/div(上)和10 ns/div(下)时所获得调Q锁模脉冲序列,根据扫描时间为1 ms/div的脉冲示波图可以看出其为调Q包络,由扫描时间为10 ns/div 的脉冲序列图可以看出两个脉冲序列之间的间隔为10 ns,其为调Q包络下的锁模脉冲序列,经计算可知其与实验中所用的2819 mm腔长相匹配。调Q包络下锁模脉冲的重复频率为53.19 MHz, 分析调Q包络脉冲的锁模脉冲,调制深度接近100%。实验中激光器稳定运行,调Q锁模运转的最大单脉冲能量为19.77 nJ。
调Q脉冲无法用自相关仪测量,但是根据理论可以估算脉冲宽度,根据公式(2)(其中tm为被测脉冲序列的上升沿时间,tr为实际锁模脉冲序列上升沿时间,tp为光电探测器上升沿时间,to为示波器的上升沿时间),实验中实际所测锁模脉冲的上升沿时间约为1 910 ps,光电探测器上升沿时间约为35 ps。再由公式(3)(其中WB是实验中所用示波器带宽为200 MHz)估算实验的to约为1750 ps, 联合公式(2)和(3)可以计算实验中所测脉冲的上升沿约为764 ps,又由于锁模脉冲的实际宽度是上升沿的1.25倍,因此实际锁模脉冲约为955 ps,在后期的工作中,将通过提高抽运功率获得更优质氧化石墨烯可饱和吸收体和降低吸收体的损耗,进一步降低锁模脉冲宽度,提升输出功率[16-17]。
$$ {{{t}}_{\rm{m}}} = \sqrt {t_{\rm{p}}^2 + t_{\rm{r}}^2 + t_{\rm{o}}^2} $$ (2) $$ {{{t}}_{\rm{o}}} \times {W_{\rm{B}}}{\rm{ = }}0.35 \sim 0.4 $$ (3)
High single pulse energy passively Q-switched mode-locked Tm, Ho: LLF laser
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摘要: 报道了一种采用氧化石墨烯作为可饱和吸收体的二极管泵浦的被动调Q和调Q锁模运转的Tm, Ho: LLF激光器。采用透过率分别为3%、5%和9%的输出镜,首先研究了Tm, Ho: LLF激光器的连续运转特性。实验和模拟结果均表明采用透过率为9%的输出镜输出特性最好,当最大泵浦功率为20 W时,连续光输出功率高至1793 mW。接着以氧化石墨烯为饱和吸收体,采用透过率为9%的输出镜研究了Tm, Ho: LLF激光器的调Q和调Q锁模特性。实验表明:当790 nm LD泵浦功率小于7.26 W时,激光处于单纯调Q运转状态;当大于7.26 W时,激光器进入稳定的调Q锁模状态,当最大泵浦功率为20 W时,最大输出功率为1052 mW,锁模重复频率为53.19 MHz,对应的平均单脉冲能量为19.77 nJ,该单脉冲能量是目前2 μm锁模激光器的最高指标,同时证实了氧化石墨烯材料在大能量高功率激光锁模中是发展潜力优良的二维锁模材料。
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关键词:
- 2 μm激光器 /
- 全固态激光器 /
- Tm, Ho: LLF晶体 /
- 高单脉冲能量 /
- 高功率激光器
Abstract: A LD pumped passively Q-switched and Q-switched mode-locked Tm, Ho: LLF laser using graphene oxide as saturable absorber was reported. Using output mirrors with transmittance of 3%, 5% and 9%, the continuous operation characteristics of Tm, Ho: LLF laser were studied. The experimental and simulation results show that the output mirror with 9% transmittance has the best output characteristics. When the maximum pump power is 20 W, the CW output power is as high as 1793 mW. Then, the Q-switched and Q-switched mode-locked characteristics of Tm, Ho: LLF laser were studied by using graphene oxide as saturable absorber under OC with 9%. The experimental results show that when the pump power of 790 nm LD is less than 7.26 W, the laser is in a simple Q-switched state. When the power is greater than 7.26 W, the laser operation enters into a stable Q-switched mode-locked state. When the maximum pump power is 20 W, the maximum output power is 1052 mW, the repetition rate of mode-locked is 53.19 MHz, and the corresponding average single pulse energy is 19.77 nJ. This average single pulse energy is currently the highest level of a 2 μm mode-locked laser. At the same time, it is confirmed that graphene oxide is a promising two-dimensional mode-locked material in high-energy mode-locked lasers.-
Key words:
- 2 μm laser /
- all solid-state laser /
- Tm, Ho: LLF crystal /
- high single pulse energy /
- high power laser
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