Diode-pumped power scalable Kerr-lens mode-locked Yb:CYA laser
On the top: Schematic diagram of the high-power Kerr-lens mode-locked Yb:CYA laser by introducing an extra Kerr-medium to separate the gain material and Kerr material. On the bottom: (a) Continuous wave and mode-locked spectra with an inset of the beam profile at the maximum average power of 1.5 W. (b) Measured (black dash) and fitted (red) intensity autocorrelation trace.
The generation of high power femtosecond pulses directly from a solid-state laser oscillator is currently one of the frontiers of ultrafast optics, with their extensive applications ranging from nonlinear microscopy to ultrafast spectroscopy as well as from frequency conversion to high-field physics. Shortening the pulse duration generated from the laser diode (LD) pumped all-solid-state Yb-based lasers to less than 100 fs or even 50 fs relies on advanced mode-locking techniques such as passive mode-locking with new saturable absorbers or Kerr-lens mode-locking (KLM).
Passive mode-locking faces the challenges of complex absorber preparation, high loss and low damage threshold, which makes it difficult to obtain the sub-100 fs pulse with high power. Traditional KLM falls in serious trouble in enhancing the Kerr nonlinear effect and mode-matching, especially for the LD pumped solid-state lasers. As a result, short pulse generation simultaneously with high average power is very difficult in either case.
Researchers from a joint research team of Xidian University, Institute of Physics (CAS), and Jiangsu Normal University proposed a new scheme to generate high power ultrashort femtosecond pulses from LD pumped all-solid-state lasers using a modified KLM method. They separated the laser gain medium and the Kerr medium in a double confocal cavity to release the restrain imposed on the laser crystal which usually acts as double-role of gain medium and Kerr medium. Related results are published in Photonics Research, Volume 6, No. 2, 2018 (W. Tian, et al., Diode-pumped power scalable Kerr-lens mode-locked Yb:CYA laser).
An extra Kerr medium of 2-mm-thick quartz plate is introduced in the laser cavity. An immediate result is that the laser gain medium, a 2-mm-thick 8-at.% doped Yb:CYA crystal, is no longer necessary to act as the Kerr medium and as a consequence, higher pump power with larger beam spot size is allowed to focus on the laser crystal. At the same time, the beam waist inside the quartz is designed to be small enough to ensure the appropriate Kerr nonlinearity. By doing so, a multi-mode LD pumped high power KLM Yb:CYA laser is demonstrated for the first time (to the best knowledge of the authors). The output power up to 1.5 W is obtained with the pulse duration down to 68 fs.
Compared with the traditional single-confocal KLM Yb lasers, the output power has been increased by an order of magnitude, which proves a feasible technique for obtaining short pulses with high average power. Researchers from this team believe that multi-watt power sub-100 fs pulses could be obtained based on this cavity geometry.
激光二极管抽运功率可提升的克尔透镜锁模Yb:CYA激光器
上图：利用双共焦腔结构分离增益介质和克尔介质实现高功率KLM Yb:CYA激光器运转的示意图。下图: (a)连续激光和锁模激光光谱, 插图为最大输出功率为1.5 W时的近场光斑分布。 (b) 测量和拟合的强度自相关曲线。
固体激光器能够直接产生高功率飞秒脉冲，在许多领域——从非线性显微成像到超快光谱学，从非线性频率变换到强场物理——都具有广泛的应用，是目前超快光学领域的前沿课题之一。而欲将激光二极管（LD）抽运的Yb全固态激光器产生的激光脉冲宽度缩短至100 fs以内，甚至低于50 fs，通常需要依靠先进的锁模技术，如采用新型可饱和吸收体的被动锁模技术或克尔透镜锁模技术（KLM）。
目前众多新型可饱和吸收体制备工艺往往比较复杂，而且具有高损耗、低损伤阈值的缺点，故而很难实现高功率的亚100 fs脉冲。而传统的KLM技术，特别是对于多模LD抽运的固体激光器，在增强克尔非线性效应和模式匹配方面也陷入严重困境。因此，直接从Yb全固态激光器中产生同时具有高平均功率和短脉冲宽度的飞秒脉冲面临很大的挑战。在传统的KLM激光器中，激光晶体通常既充当增益介质又作为克尔介质，因此为了实现KLM对激光晶体和谐振腔型提出了很高的要求。
西安电子科技大学、中国科学院物理研究所和江苏师范大学的联合研究团队提出了一种从LD抽运全固态激光器中产生高功率超短激光脉冲的新方案。研究人员将激光增益介质和克尔介质在双共焦腔中进行了分离，从而大大降低了对激光晶体和谐振腔型的要求，实现了高功率窄脉冲KLM运转。相关成果发表在Photonics Research 2018年第6卷第2期上(W.Tian, et al., Diode-pumped power scalable Kerr-lens mode-locked Yb:CYA laser)。
实验中，研究人员在激光腔中插入一个2 mm厚的石英片作为克尔介质。而作为增益介质的2 mm厚、8 at.%掺杂的Yb:CYA晶体，不再担任克尔介质的角色，因此可以允许具有更大的激光光斑尺寸以实现与多模LD最好的模式匹配，从而实现高功率激光输出。同时，设计石英片内的束腰大小满足提供合适的克尔非线性效应。采用这样的设计，据该团队所知，他们首次实现了多模LD抽运的高功率KLM Yb:CYA激光器运转。激光脉冲宽度为68 fs，平均输出功率达1.5 W。
与传统的单共焦KLM激光器相比，该项研究中的激光输出功率提高了一个数量级，证实了这种双共焦腔结构在获得高平均功率短脉冲激光运转的可行性。该团队的研究人员认为，利用这个方案有望获得数瓦级的亚100fs激光脉冲。