A dual-cavity master-oscillator power-amplifier (MOPA) offers laser beams with high power and a time-dependent tailored intensity profile!

Schematic diagram of the three-stage Nd:YVO₄ amplifier incorporating a Nd:YAG dual-cavity master-oscillator is shown in the figure. The adaptable beam profiles were well-maintained in the amplifiers (bottom left) and the output beam profiles could be modulated in the temporal domain (bottom right).

Tailoring laser beam profiles is very attractive in material processing applications since the intensity distribution at the work piece dictates the resultant pattern of the induced damaged zone, and the effects on the zone surrounding it. Moreover temporal modulation of the laser beam profiles should be very useful for numerous applications in areas, such as sophisticated management of the laser processing quality, and particle manipulation in the time and space domains.

Recently Prof. Ji Won Kim’s research group, from Hanyang University ERICA in Korea, proposed an innovative laser system, so called a dual-cavity laser configuration. The innovation in this configuration is that it can produce the laser output with the targeted beam profile from the primary cavity simply by adjusting the round-trip loss of the secondary cavity independently. This architecture had already proved its validity in generation of the laser outputs with the TEM₀₀ or LG_0n mode only, or tailored beam profiles ranging from a pure donut to a quasi-top-hat shape. Despite its simplicity, flexibility, and low cost, power scaling of the laser output was rather challenging.

Afterwards the authors overcame this difficulty and demonstrated efficient power scaling of the laser output from a Nd:YAG dual-cavity master-oscillator using a carefully-designed three-stage end-pumped Nd:YVO4 amplifier system. Temporal modulation of the laser beam profiles in a repetition rate of 1 Hz to 100 Hz was also achieved by modulating the diffraction loss of the acousto-optic modulator (AOM). It is reported in Chinese Optics Letters Volume 15, No.12, 2017 (S. H. Noh et al., High power generation of adaptive laser beams in a Nd:YVO₄ MOPA system).

"This laser configuration will benefit a range of applications requiring laser beams with high power and a bespoke intensity profile, or time-dependent spatial intensity distribution because of its simplicity and flexibility along with low cost," said Prof. Ji Won Kim from this group.

The following works will be focused on pulsed modes of operation, such as Q-switching and mode-locking, to produce high-peak pulsed outputs and the investigation of how fast mode-switching can be achieved.

Schematic diagram of the three-stage Nd:YVO₄ amplifier incorporating a Nd:YAG dual-cavity master-oscillator is shown in the figure. The adaptable beam profiles were well-maintained in the amplifiers (bottom left) and the output beam profiles could be modulated in the temporal domain (bottom right).

双腔主振荡功率放大器产生空间与时间域均可控高功率和强度激光束

和Nd:YAG双腔主振荡器组合的三级Nd:YVO₄放大器。所采用的光斑截面在放大器中得很好的保持（左下图），同时输出光束强度截面可以被时域调制（右下图）。

在材料加工应用领域，到达加工件的激光束强度分布决定了诱导损伤区的结构以及对损伤区周围的影响，因此对针对激光束的强度截面进行剪裁将很具吸引力。此外，如果能对激光束的强度截面施加时间上的调制，这在激光加工质量的精细化管理、以及在时间域和空间域上对粒子精细操控等诸多领域内有重要价值。

最近，韩国汉阳大学ERICA校区的Ji Won Kim教授课题组提出了一种双腔激光结构的新型激光系统。这种结构的创新之处在于，通过简单调节整副腔的振荡损失，它的主腔可以产生具有期望强度截面的激光输出。他们的实验已可产生TEM₀₀或LG_0n模式的激光输出，也可以通过调节副腔得到甜甜圈状和类似平顶帽状的激光束输出，这证明了该激光器结构的有效性。这种结构简单灵活且成本低，但激光功率的标定仍然是个挑战。

之后，作者团队克服了这个困难，利用精心设计的三级端面抽运Nd:YVO4放大系统对Nd:YAG双腔主振荡器的激光输出进行了有效的定标。通过调整声光调制器的衍射损失，在1-100Hz重复频率范围内对实现了输出激光束的强度截面的时间调制。相关成果发表在Chinese Optics Letters 2017年第12期上(S. H. Noh , et al., High power generation of adaptive laser beams in a Nd:YVO₄ MOPA system)。

他们接下来的工作将会集中在所建议结构的脉冲的模式运转，例如通过调Q和锁模，以产生高峰值的脉冲输出，并探索获得快速横向模式切换的途径。