飞秒光纤激光器利用补偿板提升三倍频紫外输出效率超短脉冲激光源，尤其是紫外超快激光，在微纳加工、超快光子学、量子光学和研究等方面都有着广泛的应用。紫外光单光子能量高，可用于加工更大带隙的材料，同时可聚焦到更小的光斑从而允许加工精细的结构，对实际应用具有重要意义。目前，产生紫外激光的主要技术手段之一是通过非线性频率变换，尤其是通过近红外超快激光产生三倍频。较常用的三倍频结构是倍频晶体与和频晶体共线串联，但对于飞秒激光，由于其脉冲宽度极窄，因群速度失配而导致的时间走离效应会极大影响基频光和倍频光在时域上的重叠程度，从而降低飞秒三倍频效率。传统的解决方法是在倍频之后利用双色镜将基频光和倍频光分离，然后利用延迟线来控制二者在和频晶体内的再次重合。虽然延迟线原理简单，但由于尺寸大、设计复杂、存在损耗等缺陷，导致系统复杂难调并且对外界环境过于敏感。一种有效的替代方法是引入一个或者多个具有等效反常色散的补偿板来补偿时空走离。补偿板具有结构简单、体积小、损耗低等优点，采用简单的串联结构就能达到较高的转化效率，并且双折射效应的补偿板可以同时补偿时间走离和空间走离。近年，补偿板已被用于钛宝石激光的三倍频系统中，相对于不补偿情况下的转化效率有所提升。但是，钛宝石激光器庞大的结构和高昂的价格使其难以满足工业应用上的实际需求。随着光纤激光器的高速发展，高功率、紧凑小巧、结构稳定且价格低廉的光纤激光器日益受到工业应用的青睐。因此，基于光纤激光器的紧凑、稳定、高效的飞秒紫外激光源具有重要的应用价值。天津大学胡明列教授课题组在Chinese Optics Letters的第19卷第3期上（M Zhang, et al., Efficient generation of third harmonics in Yb-doped femtosecond fiber laser via spatial and temporal walk-off compensation）展示了基于自制掺镱飞秒光纤激光器，利用补偿板实现了超过2 W的紫外激光输出，重复频率为1 MHz。该工作第一次将补偿板与光纤飞秒激光器的优势相结合，通过设计补偿板的厚度和切割角度，实现了时间和空间的同时补偿，从而使得基频光和倍频光在和频晶体内重合。为了实现最优的三倍频效率输出，该研究分别使用了5种具有不同补偿能力的补偿板。通过优化补偿板的参数和入射光功率密度，最终实现了最高为2.23 W的紫外功率输出，近红外到紫外的转化效率高达23%。利用了补偿板的情况下，串联三倍频结构的紫外转化效率是不补偿的1.78倍。相比于传统延迟线，补偿板的方法利用更简单的结构实现了更高的功率输出和更好的稳定性。此项工作展示了利用补偿板的方法，基于飞秒光纤激光器以实现紧凑小巧、稳定、高效的紫外飞秒激光源。基于补偿板的级联三倍频结构对实际工业应用具有长远意义。未来工作将专注于在加热非线性晶体时利用补偿板的方法实现更高质量的紫外光束输出和更高的功率稳定性。三倍频系统中补偿板时空走离补偿原理图。红色：基频光脉冲；绿色：倍频光脉冲；紫色：紫外光脉冲基于时空走离补偿板的掺镱飞秒光纤激光三倍频装置图。飞秒光纤激光器经过半波片( λ/2 )和薄膜偏振片(TFP)调节入射光强，被透镜F1聚焦后打到倍频晶体(SHG)上。倍频后的绿光和剩余基频光被透镜F2同时准直，入射到补偿板(CP)上补偿时空走离，然后经过双波长波片(DW)旋转基频光的偏振与倍频光重合，再经过透镜F3聚焦后入射到和频晶体(SFG)产生紫外光，之后通过分光镜(DM)和紫外反射镜(M2)分离出紫外。Efficient generation of third harmonics in Yb-doped femtosecond fiber laser via spatial and temporal walk-off compensationThe ultra-short laser sources, especially in ultraviolet (UV) region, are widely used for many different kinds of applications such as microfabrication, ultrafast spectroscopy, quantum optics, scientific research and so on. The UV pulses with high single-photon energy allows rapid absorption and a small focal spot, potentially enabling high machining accuracy and patterning ability even in wide bandgap materials.At present, the nonlinear frequency up-conversion, especially the third harmonic generation (THG), of near-infrared ultra-short lasers is the prevalent technology to generate UV pulses. For the THG process, the most widely used method is collinear sum-frequency mixing (SFM) configuration, which works well for continuous wave or relative long laser pulse. However, this approach is not suitable for the TH generation in the femtosecond regime due to the extremely narrow pulses durations. The temporal walk-off effect caused by group velocity mismatch will reduce the overlap of three interacting waves, thus decrease the conversion efficiency. Traditionally, a delay line is used to control the temporal overlap of the unconverted pump and second harmonic (SH) beams after they are separated using a dichroic mirror. The most common drawbacks of such a delay line system lie in their bulky size and complicated system design. Besides, the separated and recombined beams is difficult to align and oversensitive to minor disturbances in the environment.An alternative way to compensate the temporal walk-off effect is to introduce one or multiple compensation plates (CPs) with relative anomalous dispersion, which enables tabletop UV laser systems with compact configuration and low loss. The cascaded THG scheme with a CP can achieve high conversion efficiency. The CP can be used to reverse both the spatial and temporal walk-off due to its birefringence effect. In recent years, the CPs have been used in the THG of Ti:Sapphire lasers to achieve efficiency improvements compared to no CPs. However, the bulky design and high price of Ti:Sapphire lasers make them less competitive compared with fiber lasers. In particular, the rapid development of Yb-doped femtosecond fiber lasers has brought about a new generation of high-power, compact, stable, and cost effective laser sources for scientific research and industrial applications. Therefore, this compact, stable, and efficient femtosecond UV source based on femtosecond fiber lasers with compensation plates will benefit various practical applications.The research group led by Prof. Minglie Hu from Tianjin University presented a cascaded THG scheme with CPs based on a home-made Yb-doped femtosecond fiber laser in Chinese Optics Letters, Volume 19, Issue 3, 2021 (M Zhang, et al., Efficient generation of third harmonics in Yb-doped femtosecond fiber laser via spatial and temporal walk-off compensation). The UV source at 345 nm based on Yb-doped femtosecond fiber laser with the CP provided output power >2 W at 1 MHz. This work is the first time to combine the advantages of the CP and the femtosecond fiber laser. The thickness and cutting angle of the CP are optimized to reverse both the spatial and temporal walk-off so that the fundamental and the SH pulses coincide in the SFG crystal.To obtain optimum SFG efficiency, five kinds of CPs with different compensation capabilities were applied respectively. Each CP is distinguished for different delay times around the theoretical delay time induced by each component because the temporal walk-off dominates before SFG. By optimizing the parameters of the CP and incident light intensity, a maximum UV output of 2.23 W is obtained corresponding to a single-pass near-infrared-to-UV conversion efficiency as high as 23%. Over 1.78 times improvement of conversion efficiency is achieved comparing with the configuration without the CP. Moreover, compared with the traditional delay line, the CP achieved a higher power output and better power stability with a compact structure.This work presented a compact, stable, and efficient femtosecond UV source based on femtosecond fiber lasers with CPs. The cascaded THG scheme with CPs will benefit various practical applications. Future work will focus on a systematic research of the heated nonlinear crystals by the CP method to obtain better beam quality and power stability.Spatial and temporal walk-off effects and their optimization using a CP. Red, fundamental pulses; green, SH pulses; violet, UV pulses.Experimental setup. λ/2, half-wave plate; TFP, thin-film polarizer; M1, plano mirror at 1030 nm; F1-F4, lenses; SHG, 3-mm LBO crystal; CP, compensation plate; DW, dual-wavelength waveplate; SFG, 3-mm BBO crystal; DM, dichroic mirror; M2, plano mirror at 345 nm.