The laser performs aerial ballet—Rotary disk laser with optically driven maglev motion

Schematic diagram of rotary disk laser levitated above the magnet. Inset: Photograph of maglev system.

The thermal load accumulated in the gain medium induces thermal lensing which results in thermal aberration to the laser cavity, which would degrade the performance of solid state lasers. When the gain medium is rotated, the heat accumulation can be shifted laterally such that the disturbance to the laser performance is minimized.

Currently, a majority of the rotary disk lasers require the intervention of an electric motor to drive the gain medium disk, as well as a servo system to correct the wiggle of gain medium. As a result, the structure of lasers increased in complexity

The group led by Prof. Jianlang Li, from Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, proposed a maglev and rotary disk laser by combining the rotary disk laser technology with an optically driven maglev motion. The details of the study are reported in Chinese Optics Letters, Vol. 13, No. 12, 2015 (H. X. Han, J. L. Li, A Maglev and Rotating Disk Laser)

Diamagnetic materials such as pyrolytic graphite (PG) have negative magnetic susceptibility. When exposed to an external magnetic field, the diamagnetic material generates an internal magnetic field in the opposite direction, implying that it can be levitated above the external magnetic field at room temperature. Under the illumination of a laser beam, the magnetic susceptibility of the diamagnetic material changes and subsequently the local temperature variation enables the rotation of the levitated diamagnetic material with high stability and precision.

In their proof-of-concept investigation, a thin-disk Nd:YAG laser crystal was attached to a PG plate, and was levitated above a stack of permanent magnets. The pump irradiation from a fiber-coupled laser diode was absorbed by the laser crystal and its residual power was utilized to activate the rotation of PG plate. The levitated compound comprising the laser crystal and the PG disk was heavy, therefore the compressed gas was used to assist the rotation. In the experiment, a single-mode output of 17.7 mW was obtained from this compound, and the initial results verified the validity of the proposed laser scheme.

According to the authors, this study proposed a new rotary laser scheme for power scaling and generation of a high brightness beam. "In the future, with further optimization to laser gain media and diamagnetic materials, maglev and optically-driven rotating disk laser will be more realizable, and it may bring in breakthroughs in high-power solid-state laser and planar waveguide laser," said Prof. Jianlang Li.

The following work will be focused on reducing the weight of the maglev compound by the planar waveguide structure deposited on thin diamagnetic materials, and building maglev and rotary disk laser which is driven only by unabsorbed pump light.

激光器的空中芭蕾—激光操纵的磁悬浮旋转激光器

磁悬浮转盘激光器实验示意图。插图为磁悬浮系统照片

固体激光器中的废热累积会严重影响激光器的性能。通过转动激光增益介质盘片可以有效减少其内部的热累积，显著提高激光器的功率和光束质量。在现有的转盘激光器中，增益介质盘片的旋转须靠电驱动（马达）等方式，同时需要伺服系统克服增益介质的摇摆，这导致激光器结构比较复杂。

中国科学院上海光学精密机械研究所李建郎研究员课题组将转盘激光器技术和磁悬浮光驱动旋转技术结合起来，提出了全新的磁悬浮光驱动转盘激光器方案。相关研究成果发表在Chinese Optics Letters 2015 年第12 期上（H. X. Han, J. L. Li, A Maglev and Rotating Disk Laser）。

反磁性材料（比如热解石墨）因具有负的磁化率，在外磁场环境下会产生反向磁场，因而在室温时可产生磁悬浮现象。如果将激光照射到磁悬浮热解石墨薄片的边缘位置，热致磁导率的变化形成转动力矩，热解石墨薄片会在磁场中转动。由于采用了光驱动，此种方式易于精确控制并具有良好的稳定性。

该方案利用反磁性材料（热解石墨片）将激光增益介质盘片悬浮在磁场中，同时由半导体激光器抽运激光增益介质盘片并提供反磁性材料转动所需的能量。该团队的科研人员通过热解石墨片将Nd:YAG激光晶体悬浮在磁铁之上，在悬浮体重量过重的情况下以气体辅助推动旋转，获得了17.7 mW的基模激光输出，初步验证了他们的设想。

这项研究揭示了一种面向高功率、高亮度激光输出的磁悬浮、光驱动旋转激光器技术。李建郎研究员表示，随着对激光增益介质和反磁性材料等的进一步研究，光驱动的磁悬浮转盘激光器将变得愈发可行，该技术有望在高功率激光器和平面波导激光器方面取得突破。

后续工作主要是利用平面波导结构减小增益介质和热解石墨片的整体重量，建造一台基于剩余抽运光驱动的磁悬浮自旋转激光器。