Multimode fibers provide a means of scaling the peak power of ultrafast fiber lasers by orders of magnitude. While large mode area (LMA) fibers have been widely utilized in fiber amplifiers, these fibers often sacrifice practical benefits of fiber, such as flexibility, and increase system cost and complexity. In addition, the mode-field area of effectively single-mode LMA fibers is smaller than what can be achieved in multimode fibers. Recent work has shown that nonlinear interactions in multimode graded-index fiber can cause a highly multimode field to self-organize into the fundamental mode in a condensation-like process that is robust even with fiber perturbations or disorder.
Building on this, we developed a series of Yb:fiber, mode-locked lasers utilizing normal-dispersion, multimode graded-index fiber. In experiments, we observe that the transition from continuous wave lasing to mode-locking is characterized by a beam cleaning process, whereby the highly multimode (speckled) beam of the continuous wave field transforms into a low-order mode beam. Remarkably, experiments and numerical simulations show that the pulses can consist not just the fundamental mode, but can even comprise multiple transverse modes. Our theoretical analysis shows this to be a consequence of a surprising kind of mode-locking – spatiotemporal mode-locking - which relies on strong intermode interactions and spatial filtering. Our initial experiments yield MW-power pulses after external compression, rivaling the best results with flexible LMA fibers. Meanwhile, simulations show that nearly GW peak powers should be possible, making spatiotemporal mode-locking extremely attractive for high-power ultrafast laser development.
Logan Wright, Demetrios N. Christodoulides, and Frank W. Wise, "High power ultrafast laser sources based on multimode fiber (Conference Presentation)," Proc. SPIE 10518, Laser Resonators, Microresonators, and Beam Control XX, 105181G (Presented at SPIE LASE: February 01, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2290550.5751433524001.
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