Light propagation in high-power multicore fibers

光纤激光器系统作为平均功率可扩展的激光器架构获得了良好的声誉。然而，高功率光纤激光器系统的性能缩放受到与光纤折射率的非线性行为或热效应相关的几种现象的阻碍。一种解决方案是利用并行化的概念。由此，待放大的信号被分成N个子束，其将在尽可能多的并行放大通道中被同时放大。上述概念的经典实现方式（即，使用分离的放大器）的主要问题在于，系统复杂性、成本和占地面积随着放大通道的数量线性地缩放。这使得将这些系统缩放到大于几十个发射器的通道计数是不切实际的。为了打破这种线性依赖性，从而允许更大的通道数，可以将所有放大通道集成在单个光纤内。这就构成了所谓的多芯光纤概念。然而，多芯光纤不仅仅是将放大通道集成在设备中。它们构成了高度复杂的波导，在其中可以发生电磁辐射的复杂相互作用。最重要的是，大通道数（例如100到1000）的集成自然导致多芯光纤的尺寸比激光工艺中涉及的光的波长大几个数量级。光在这种大结构中的传输超出了目前已知的波导中的电磁传播。事实上，预计光将在引导和自由空间传播之间的区域中传播，这是以前没有研究过的。本计画旨在研究光在大型多芯光纤中的传播与电磁相互作用。特别是，这包括研究泵浦光沿着包层的传播，偏振光在光纤芯中的传播以及复杂的芯内和芯间模式相互作用，包括多芯光纤中的横模不稳定性的影响。事实上，这个应用程序被认为是为了帮助实现海森堡项目中设定的目标，因此，它与该项目的活动密切相关。

Fiber laser systems have gained a solid reputation as average-power scalable laser architectures. However, the performance scaling of high-power fiber laser systems is hindered by several phenomena related to either the non-linear behavior of the refractive index of the fiber or to thermal effects. One solution is to exploit the concept of parallelization. Hereby the signal to be amplified is split in N sub-beams, that will be simultaneously amplified in as many parallel amplification channels. The main problem of the classic implementation of the concept described above (i.e. using separated amplifiers) is that the system complexity, cost and footprint, scale linearly with the number of amplification channels. This makes the scaling of these systems to channel counts larger than a few tens of emitters impractical. In order to break this linear dependency and, therefore, to allow for larger channel counts, it is possible to integrate all the amplification channels inside of a single fiber. This constitutes the so-called multicore fiber concept. Multicore fibers are not simply a mere integration of amplification channels in a device, though. They constitute highly sophisticated waveguides in which complex interactions of the electromagnetic radiation can take place. On top of that, the integration of a large channel count (e.g. 100 to 1000) leads naturally to multicore fibers with dimensions that are several orders of magnitude larger than the wavelength of the light involved in the laser process. The transmission of light in such large structures is beyond what is currently known about electromagnetic propagation in waveguides. In fact, it is expected that the light will travel in a regime that is between guidance and free-space propagation, something that has not been studied before. This project aims at investigating the propagation and electromagnetic interaction of light in large multicore fibers. In particular, this includes studying the propagation of pump light along the cladding, the propagation of polarized light in the fiber cores and the complex intra- and inter-core modal interactions including the effect of transverse mode instability in multicore fibers. In fact, this application is conceived to help reaching the goals set in the Heisenberg project and it is, therefore, intimately intertwined with the activities of that project.