Study and control of the intermodal energy transfer in high-power fiber laser systems

激光系统的功率提取过程中会产生不必要的热量，从而导致激光性能的下降。由于其极长的几何形状，纤维对热光学效应具有特别的弹性，由于其大的表面体积比，它允许有效的热排出。尽管如此，纤维并不是完全不受热光效应的影响。实际上，当光纤激光系统的输出平均功率达到一定阈值时，原本稳定的高质量输出光束开始波动，其强度分布发生畸变。产生这种现象的原因是在光纤中产生热致折射率光栅的辅助下，在有源光纤中不同横模之间产生了不稳定的能量传递。这种效应被称为横向模不稳定性（TMI），它已迅速成为世界范围内光纤激光系统进一步扩大输出平均功率的最大限制。这在一定程度上解释了这种效应最近在科学界引起的强烈兴趣。最近，申请人展示了一种新的TMI缓解战略。它是基于通过调制泵浦功率来减弱热致折射率光栅。然而，在进行这些实验时，很明显，泵浦调制也导致了光纤中横向模式之间的周期性能量转移。这是一个极其重要的观察结果，因为这是第一次证明能量流的方向可以受到外界的影响。因此，学习控制这样的能量流是这个项目的主要目标。如果该项目成功，它将是将光纤激光技术中最具破坏性的问题之一转变为优势的第一步，因为它将导致这些系统将所有能量发射到单个（可选择的）横向模式，而不管输出功率如何，这对于应用来说是非常有趣的，在任何其他激光技术中都是闻所未闻的。

The power extraction in laser systems is accompanied by the unwanted generation of heat, which can lead to the degradation of the laser performance. Fibers are particularly resilient against thermo-optic effects due to their extremely elongated geometry, which allows for an efficient heat evacuation due to the large surface to volume ratio. In spite of this, fibers are not completely immune against thermo-optic effects. In fact, when the output average power of a fiber laser system reaches a certain threshold, the formerly stable high-quality output beam starts to fluctuate and its intensity profile is distorted. The reason for this behavior is the unstable energy transfer between different transverse modes in the active fiber assisted by the creation of a thermally-induced index grating in the fiber. Such an effect is called transverse mode instabilities (TMI) and it has quickly become the strongest limitation for the further scaling of the output average power of fiber laser systems worldwide. This, in part, explains the strong interest that such effect has attracted recently in the scientific community.Very recently, a new mitigation strategy for TMI has been demonstrated by the applicant. It is based on weakening the thermally-induced index grating by modulating the pump power. However, while doing these experiments, it became apparent that the pump modulation also led to a periodic energy transfer between the transverse modes in the fiber. This is an extremely important observation, since this is the first time that it has been demonstrated that the direction of the energy flow can be influenced from the outside. Thus, it is the main goal of this project to learn to control such energy flow. If the project is successful it can be the first step towards turning one of the most damaging problems in fiber laser technology into an advantage, since it will lead to these systems emitting all their energy into a single (selectable) transverse mode regardless of the output power, something extremely interesting for the applications and unheard of in any other laser technology.