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该方法最早由英国南安普敦大学K.I Martin等人提出[32],通过在谐振腔内插入倍频晶体,实现相邻纵模的自抑制作用,从而实现单纵模输出。其原理简述为:在只考虑两个纵模(ω1, ω2) 振荡的情况下,假设频率分别为 ω1和 ω2的主纵模及其相邻纵模(边模)的光强分别为 I(ω1) 和 I(ω2),在倍频晶体中,除ω1与ω2 各自频率的倍频过程外,还存在这两个频率的和频过程。将和频与倍频过程等效为两个纵模的损耗,其表达式为:
$$ \begin{split} \\ \frac{{{{L}}({\omega _1})}}{{{{L}}({\omega _2})}} = \frac{{k{{I}}({\omega _1}) + 2k{{I}}({\omega _2})}}{{k{{I}}({\omega _2}) + 2k{{I}}({\omega _1})}} \end{split}$$ (1) 这里的k是和非线性强度相关的常数,由于两个相邻纵模的间隔远小于晶体的可接收带宽,所以两个频率的k值近似相等。由于主模式的光强远大于边模光强,也就是I(ω2)<< I(ω1),上述等式趋近于1/2,表明光强更强的主振荡模式所受的损耗只有相邻弱振荡模式的一半。因此,通过腔内引入倍频晶体,可以有效增加腔内相邻模式的净增益差,稳定激光器的单纵模运转。
2019年,麦考瑞大学的杨学宗等人利用一台最高功率321 W,光谱半高全宽3.3 GHz的准连续1064 nm多纵模Nd:YAG激光器作为泵浦源,通过在金刚石驻波谐振腔中插入倍频晶体LBO,得到了输出功率为11.8 W的稳定单纵模1240 nm一阶斯托克斯光输出,同时获得了输出功率38 W的准连续单纵模620 nm倍频光输出[33],实验装置如图10所示。在除倍频晶体外,未采取其他稳定单纵模特性措施的情况下,在最高输出功率下除偶尔跳模外,激光器仍保持稳定的单纵模运转。值得注意的是,即便泵浦光为多纵模激光,只要其线宽小于金刚石晶体的拉曼增益线宽,仍然可以得到单纵模斯托克斯光输出,这极大地提高了泵浦源的灵活性。实验中作者研究了倍频晶体相位失配程度对多纵模自抑制效果的影响。如图11所示,LBO晶体的最佳相位匹配温度为40 ℃,当LBO晶体温度在39.2~41.5 ℃范围内时,拉曼激光器能够在最高泵浦功率下维持单纵模运转。随着LBO晶体温度过度失谐,相位匹配程度进一步劣化,激光器变为多纵模运转。随后,作者利用该非线性纵模自抑制技术,先后实现了22 W连续波单纵模589 nm金刚石拉曼钠导星激光器[3]和8 W连续波单纵模590~615 nm可调谐金刚石拉曼激光器[34]。此外,作者也对单纵模金刚石拉曼激光器无跳模调谐和单纵模线宽测量进行了研究,如图12所示,通过非线性纵模自抑制技术,实现了倍频光3.7 GHz范围的连续无跳模调谐,利用法布里-珀罗扫描干涉仪测得倍频光单纵模线宽约为8.5 MHz。综上所述,通过腔内插入倍频晶体,可有效提高单纵模金刚石拉曼激光的输出功率和稳定性,得益于倍频晶体(LBO、BBO)和金刚石晶体的高损伤阈值,该方法有望大幅提升连续波单纵模拉曼激光的输出功率水平。
图 12 (a) 输出光频移和腔长的关系;(b) 通过法布里-珀罗扫描干涉仪得到的589 nm单频激光的光谱特性
Figure 12. (a) Frequency shift as a function of cavity length for output light; (b) Scanning Fabry–Perot interferometer trace of the single-frequency 589 nm laser
虽然实验证实非线性多纵模自抑制技术可应用在单纵模拉曼激光器中,但目前还没有理论模型对拉曼激光器内的这一机制作出定量描述。在粒子数反转激光器领域,2014年,山西大学的卢华东等人根据粒子数反转激光器的增益饱和性质,提出了非线性多纵模自抑制效应的定量描述[35],将谐振腔内线性损耗与非线性损耗,以及激光晶体的光谱带宽和非线性晶体的非线性可接收带宽等变量,与谐振腔的单纵模谐振和多纵模谐振条件定量联系。其研究对二次谐波的多纵模抑制效应在拉曼增益激光器中的理论模型推导具有重要参考价值。
Single-frequency continuous-wave diamond Raman laser (Invited)
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摘要: 受激拉曼散射增益具有无空间烧孔特性,通过简单的腔型设计,拉曼激光器即可实现稳定单纵模运转。同时拉曼激光波长灵活,因此拉曼激光器在特殊波长单频激光领域具有重要技术优势。近年来,随着金刚石合成技术的突破,以金刚石作为增益介质的单纵模、高功率拉曼激光器获得了快速发展。文中简要介绍了拉曼激光器的单纵模运行机制;总结了单纵模金刚石拉曼激光器的研究进展;并对单纵模金刚石拉曼激光器的发展趋势进行了展望。Abstract: Stimulated Raman scattering is a mature technology that provides laser outputs with flexible wavelengths. Stable single-longitudinal-mode (SLM) Stokes is able to be achieved in a simply designed oscillator due to the nature of spatial hole burning free of Raman gain. Therefore, the Raman laser is considered as an attractive and potential method to generate SLM output with a particular wavelength. As the single-crystal diamond synthetic technology matures, high-power continuous-wave SLM diamond Raman lasers have been widely investigated for the past few years. In this review, the mechanism of Raman SLM operation in Raman oscillator and the state art of the SLM diamond Raman lasers are summarized. At last, the future investigations of continuous-wave SLM diamond Raman lasers are proposed.
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Key words:
- diamond /
- solid state laser /
- single-longitudinal mode /
- stimulated Raman scattering
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图 5 利用H-C锁腔方法实现单纵模输出的拉曼激光器实验装置图。BS为光束采样镜,HWP为半波片,FL为聚焦透镜,IC为输入耦合镜,OC为输出耦合镜,PZT为带有压电陶瓷的位移台。DM为二向色镜,QWP为四分之一波片,PBS为偏振分束器,PD1与PD2为光电探测器[27]
Figure 5. Schemematic layout of the Raman laser by using H-C locking strategy, BS: beam sampler, HWP : half- wave plate, FL: focusing lens, IC: input coupler, OC: output coupler, PZT: piezoelectric translation stage, DM: dichroic mirror, QWP: quarter-wave plate, PBS: polarizing beam splitter, PD1 and PD2: photodetectors[27]
图 6 基于泵浦谐振单向环形腔的拉曼激光器实验装置图。泵浦源是M Squared公司的SolsTis钛蓝宝石激光器,HR是高反射镜,PR是部分反射镜,IC/OC是输入/输出耦合镜,DM是对斯托克斯光高反射,对泵浦光高透过的二向色镜,BS是光束采样镜,BM是光挡,λ/2,λ/4分别是半波片和四分之一波片[29]
Figure 6. Schematic layout of resonant pumping Raman laser by direction control of field in the ring cavity. The Pump source is the SolsTis Ti:Sapphire laser from M Squared Lasers Ltd. HR: high reflector; PR: partial reflector; IC/OC: input/output coupler; DM: dichroic mirror, HR at Stokes, HT at pump; BS: uncoated beam sampler; BM: beam dump; λ∕2: half-wave plate; λ∕4: quarter-wave plate[29]
7 (a) 谐振腔自由运转时前向和后向斯托克斯光的时域特性;(b) 通过法布里-珀罗扫描干涉仪得到谐振腔自由运转时的斯托克斯谱线;(c) 使用反射镜反馈后,得到腔内斯托克斯光单向运行时的前向与后向斯托克斯光时域特性,插图显示了通过法布里-珀罗扫描干涉仪得到的稳定单纵模斯托克斯光运转[29]
7. (a) Temporal behavior of the forward Stokes and backward Stokes of the free-running Raman laser; (b) The Stokes trace of the free-running Raman laser by a scanning Fabry–Perot interferometer; (c) Temporal behavior of a unidirectional Raman laser using a feedback mirror. The inset shows stable single-mode operation of a scanning Fabry–Perot interferometer[29]
图 9 通过腔长锁定实现环形腔内二阶斯托克斯输出的光路图,插图为M1腔镜的镀膜曲线,点P、S、SS分别代表腔镜对泵浦光、一阶斯托克斯光与二阶斯托克斯光的反射率[31]
Figure 9. Schematic layout of the experimental setup of the second Stokes operation in ring cavity by cavity locking. Inset is the coating curve of the M1 mirror, where spot P, S and SS means the reflectivity of the M1 for pump, 1st Stokes and 2nd Stokes fields[31]
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