Custom designed and unstrained SESAMs for fiber based shortpulse lasers

脉冲长度在飞秒范围内的超短脉冲激光器用于工业和研究，例如作为材料加工、生物光子学和量子传感中的种子激光器。为了从固态和光纤激光器产生超短脉冲，可饱和吸收镜（SESAM）通常用于无源模式耦合。SESAM的优点是其对各种激光系统的通用适应性以及易于集成到激光腔中，因为作为无源元件，它们不需要任何进一步的控制。然而，对于在1550 nm左右波长范围内工作的基于光纤的超短脉冲激光器，有两个关键挑战迄今为止阻碍了SESAM的使用或导致该领域的过早激光故障。首先，锁模超短脉冲激光器的动态行为是复杂的，即，原则上非常接近混沌状态。为了使激光器在所需的良好定义的工作状态下保持鲁棒性和稳定性，锁模SESAM必须被调谐到特定的激光器。要做到这一点，整个激光系统必须充分了解，不仅在锁模脉冲产生的稳定状态，但特别是在启动阶段。对于波长在1550 nm左右的光纤耦合脉冲激光器，尚未达到这一成熟阶段。相反，目前的工作与经验确定的要求，并指定专门选择的SESAM到一个特定的激光器。第二，在标准的GaAs衬底上用于SESAM，没有应变和长期稳定的吸收材料为1550 nm。对于该波长，所需的快速吸收体由In0.47Ga0.53As组成，其对GaAs衬底具有+3.5%的强晶体偏置。虽然可以通过掺入少量的氮来降低偏差，但所得的稀氮化物尚未显示出具有长期稳定性。在商业系统中，1550 nm SESAM的故障率比1030 nm SESAM高40倍。然而，市售的1550 nm SESAM是基于GaAs的。这两个限制方面都在该项目中得到解决，以克服与TOPTICA Photonics AG密切合作使用1550 nm光纤超快激光器的障碍。有一个DFG的前体项目。使用SESAM锁模光纤激光器的数值模型，确定用于光纤激光器的期望单脉冲操作的优化的激光器和吸收器参数。然后，这些优化的吸收体将在弗劳恩霍夫研究所生长，并在TOPTICA和汉堡大学的指定光纤激光器中进行启动行为，动力学和老化测试。将运行其中几个周期以实现整体优化的系统。

Ultrashort pulse lasers with pulse lengths in the femtosecond range are used in industry and research, e.g. as seed lasers in materials processing, biophotonics and quantum sensing. To generate ultrashort pulses from solid-state and fiber lasers, saturable absorber mirrors (SESAMs) are typically used for passive mode coupling. The advantages of SESAMs are their versatile adaptability to a wide variety of laser systems and their ease of integration into the laser cavity, since as passive components they do not require any further control. However, for fiber-based ultrashort pulse lasers operating in the wavelength range around 1550 nm, there are two key challenges that have so far hampered the use of SESAMs or led to premature laser failure in the field. First, the dynamic behavior of mode-locked ultrashort pulse lasers is complex, i.e., in principle very close to a chaotic state. To keep the laser robust and stable in a desired well-defined operating state, the mode-locking SESAM must be tuned to the specific laser. To do this, the entire laser system must be fully understood not only in the stable state of mode-locked pulse generation, but especially in the startup phase. This stage of maturity has not yet been reached for fiber-coupled pulse lasers with wavelengths around 1550 nm. Instead, one currently works with empirically determined requirements and assigns specially selected SESAMs to a particular laser.Second, on the standard GaAs substrate used for SESAMs, there is no strain-free and long-term stable absorber material for 1550 nm. For this wavelength, the required fast absorber consists of In0.47Ga0.53As with strong crystal bias of +3.5% to the GaAs substrate. Although the bias can be reduced by incorporating small amounts of nitrogen, the resulting dilute nitrides have not been shown to have long-term stability. In commercial systems, the failure rate of 1550 nm SESAMs is increased 40-fold compared to 1030 nm SESAMs. Nevertheless, commercially available 1550 nm SESAMs are based on GaAs. Both limiting aspects are addressed in this project to overcome the hurdles for the use of fiber-based ultrafast lasers at 1550 nm in close collaboration with TOPTICA Photonics AG. There is a DFG precursor project for both. Using a numerical model of the SESAM mode-locked fiber laser, optimized laser and absorber parameters are to be determined for the desired single-pulse operation of the fiber laser. These optimized absorbers will then be grown at the Fraunhofer Institute and tested at TOPTICA and University of Hamburg in designated fiber lasers for startup behavior, dynamics and aging. Several of these cycles will be run to achieve an overall optimized system.