Actively Q-switched ring Tm-doped fiber laser with free space structure
Schematic diagram of actively Q-switched ring Tm-doped fiber laser (TDFL) setup based on free space structure. A type of free space acousto-optic modulator was employed to generate pulsed laser stably in 2 μm region. The maximum pulse energy output of ring TDFL thus built was 150 μJ at a repetition rate of 100 Hz with a pulse width of 382 ns (cavity optical length: 12 m).
Measurement accuracy is one of the most important indexes to estimate the capability of LIDAR systems. For Doppler LIDAR, the emission source with a proper long pulse width is beneficial to measurement accuracy. The pulse width is mainly controlled by slave-laser for emission source of LIDAR system using injection-locking technology. With the advantage of unidirectional operation and long pulse output, pulsed fiber ring laser can be used as an ideal slave laser for injection locking laser.
At present, the reports of pulsed fiber ring laser operating in 2 μm region mainly focus on passively Q-switched lasers, which have simple structures. However, the poor control over characteristics of output pulses make passively Q-switched laser unfit for injection-locking technology.
To sidestep drawbacks of the passive mode, the research group led by Prof. Youlun Ju at School of Astronautics, Harbin Institute of Technology, used one type of free space acousto-optic modulator (AOM) to build an actively Q-switched ring Tm-doped fiber laser (TDFL), which was able to operate in 2 μm region stably. This work was published in Chinese Optics Letters, Volume 14, No. 9, 2016 (W. Liu, et al., Actively Q-switched ring Tm-doped fiber laser with free space structure).
In contrast to passively Q-switch technology, actively Q-switched fiber ring lasers can provide more control over the characteristics of output pulses, such as repetition rate and pulse width. AOM is a conventional choice for actively Q-switching. Presently, there are two types of AOMs: fiber coupled and free space. Compared with fiber coupled AOM, the higher damage threshold of AOM with a free space structure makes it more suitable for further research on high pulse energy output of TDFL in the 2 μm region.
The maximum pulse energy output of ring TDFL thus built was 150 μJ at a repetition rate of 100 Hz with a pulse width of 207 ns (cavity optical length: 6.68 m). With the same output energy, a pulse width of 382 ns has been achieved by increasing the cavity optical length of the ring TDFL to 12 m.
"The present work was the first step to resolving the problem of narrow pulse width output bottleneck of slave-laser for injection locking laser in the 2 μm region," said Prof. Youlun Ju.
Further work will be focused on optimizing the parameters of ring TDFL in the hope that this platform can be put into use for the Doppler LIDAR sensing system.
提高激光雷达精度的利器——主动调Q掺铥光纤环形激光器
图片说明：基于自由空间结构的主动调Q掺铥光纤环形激光器实验装置示意图。实验采用自由空间Q开关实现了2 μm波段稳定脉冲激光输出，在100 Hz重复频率下，腔长为12 m时，这一激光器系统获得了单脉冲能量为150 μJ、脉冲宽度为382 ns的脉冲激光输出。
探测精度可谓是激光雷达（LIDAR）系统的命脉所在，高精度一直是LIDAR技术不渝的追求。对于多普勒相干激光雷达，提高探测精度的有效途径之一便是实现其发射源的长脉冲输出。在采用注入锁频技术的激光雷达发射源中，脉冲宽度主要受从激光器(slave laser)控制。脉冲光纤环形激光器具有单向运行、长脉冲输出的特点，因此可作为理想的从激光器在注入锁频激光器中使用。
目前，在2 μm波段有关脉冲光纤环形激光器的报道多采用被动调Q方式。被动调Q激光器具有结构简单的优点，但其脉冲重复率和脉冲宽度往往不可控，这导致其不适于在注入锁频技术中使用。
为了避免被动调Q技术的上述缺陷，哈尔滨工业大学航天学院鞠有伦教授课题组采用主动调Q方式，利用自由空间声光调制器实现了稳定工作在2 μm波段的掺铥光纤环形激光器。相关研究成果发表在Chinese Optics Letters 2016年第9期上（W. Liu, et al., Actively Q-switched ring Tm-doped fiber laser with free space structure）。
与被动调Q技术相比，主动调Q激光器在重复频率、脉冲宽度等激光器性能上都可得到较好的稳定控制。而声光调制器是实现主动调Q的惯常选择，它主要有光纤耦合式和自由空间式两种类型。考虑到自由空间声光调制器在2 μm波段具有更高的损伤阈值，更有利于实现高能量脉冲输出，该课题组利用其搭建了掺铥光纤环形激光器。在100 Hz重复频率下，腔长为6.68 m时，这一激光器系统获得了单脉冲能量150 μJ、脉冲宽度207 ns的脉冲激光输出；在将激光器的腔长通过传导光纤增加至12 m后，在相同脉冲能量下，输出脉冲宽度达到了382 ns。
鞠有伦教授表示，该项研究结果初步解决了在2 μm波段注入锁频激光器中从激光器输出脉冲宽度较窄的问题。下一阶段的工作将对掺铥光纤环形激光器各项参数进一步优化，使其最终有望应用于多普勒相干激光雷达系统。