Multicore Fibers (MCF), the integrated alternative to spatially separated amplification in fibers, are a promising laser architecture thanks to their capability to deliver high pulse energy and average power in a compact format. The introduction of 7x7 MCF laser systems represents a significant advancement of this technology, bringing us closer to realizing multi-kilowatt and J-class fiber laser amplifiers. In this context, Ytterbium-doped MCFs have already demonstrated power scalability proportional to the number of amplifying cores. Using 4x4 MCFs already showcases high pulse energy and high average power and, if coherently combined, offers nearly diffraction-limited beam quality. This work complements the Coherent Beam Combination (CBC) testbed by Incoherent Beam Combination (IBC). IBC emerges as a straightforward and robust solution, providing the opportunity to achieve performance capabilities equivalent to Multimode Fibers (MMFs) while demonstrating a better beam quality. These IBC systems are appealing for various applications, including pumping solid-state lasers and incoherent frequency conversion towards shorter wavelengths, e.g., to the Extreme Ultraviolet (EUV) or soft-Xray through laser-produced plasma sources.
We present the first polarization-maintaining, Ytterbium-doped, rod-type, multicore fiber with 5x7 cores. This fiber is drawn from a drilled 7x7 preform in which the outermost columns were filled with Boron-doped stress-rods, resulting in a stress-field strong enough to induce linear birefringence in all the cores simultaneously. Experimentally we have observed the preservation of linear polarization in the cores, albeit to a different degree depending on the core positions.
In this work we will cover the basic considerations of incoherent combination of fiber laser systems and, in particular of multicore fibers, since these are ideal sources for this type of combination. Our analysis will have a special emphasis on what separates incoherent combination from its coherent counterparts. Additionally, we will discuss the scalability of the beam quality with the number and arrangement of the cores in a MCF, and we will compare the incoherently combined emission from a multicore fiber with that of a multimode fiber.
Both transverse mode instability (TMI) and non-linear effects hinder the power scaling in fiber laser amplifiers with a diffraction-limited beam profile. In this context core size scaling is a key design parameter to counteract nonlinear effects and, therefore, it is of outmost importance to understand the impact of this parameter on the TMI threshold. In this work we present the first systematic experimental studies that investigate the impact of core size scaling on TMI characteristics. In this study, the unique characteristics of large pitch photonic crystal fibers is exploited to ensure comparability between fibers with different core sizes. This has been done by manufacturing a set of test fibers (Ytterbium-doped LPFs) with different core sizes which maintain their modal characteristics when operated under the same conditions. Furthermore, our experiments reveal a TMI power threshold of 570 W, the highest ever reported in a rod-type fiber with a very large core diameter.
We present an Ytterbium-doped, multicore fiber with 7×7 cores. The fiber is realized in a rod-type geometry with step index cores and an air-cladding for pump guiding. Using a segmented-mirror beam splitter followed by a double-pass multicore pre-amplifier and a main-amplifier of 1 m length, the stretched femtosecond input pulses emitted from the frontend system are amplified. The cores of the main amplifier have a mode-field diameter of 28 μm. Operating at 10 MHz repetition rate, a high average power in excess of 1 kW with near single-mode operation is achieved.
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