Progress in active fibers

Abstract

Progress in active fibers for high power fiber sources is discussed. The waveguiding geometry naturally lends itself to high-power diffraction-limited operation. Further refinements allow for the control of nonlinearities and wavelength. Fabrication advances mitigate photodarkening.OCIS codes: (060.2280) Fiber design and fabrication, (060.2320) Fiber optics amplifiers and oscillators Low-loss rare-earth doped silica fibers, first realized using MCVD and solution-doping in mid 1980s, made erbium-doped fiber amplifiers (EDFAs) [1] possible. These revolutionized optical communications. After successful implementation of EDFAs in communications, high-power cladding pumped fiber lasers and amplifiers have become a major field of operation of rare-earth doped fibers. Ytterbium-doped fiber lasers have been powerscaled beyond the kW level at ~1.1 mm. Also other fiber devices, including those at other wavelengths and with different temporal and spectral properties have seen rapid progress. This presentation will review the progress in high power fiber lasers and amplifiers, highlighting the advances in the active fibers seen to date and those that will be required in the future.The output power of fiber lasers and amplifiers has rapidly grown over the past years and has now reached a point where they are looking to become the laser of choice in a wide range of applications beyond optical communications such as materials processing, micro-machining, medicine, and LIDAR to name a few. The attractions of cladding-pumped fiber lasers and amplifiers derive from the high beam quality, high power, high gain, and broad gain bandwidth. Thermal properties are crucial for high-power operation with high beam quality Fiber lasers benefit from a geometry that makes the thermal management less critical than in bulk lasers. Thus, the generated heat is distributed over a long length, which reduces the risk of thermal damage of the fiber. Waveguiding of the signal as well as the pump is essential for making long-length devices. Moreover, the output beam quality in a fiber laser is greatly determined by the waveguiding properties of the core in the fiber and can be tailored to generate a single-mode output beam. The relatively low thermal load per unit length mitigates thermal distortions of the waveguiding even at high powers. Recent demonstrations of Yb-doped fiber lasers with excellent beam quality at power levels of > 1 kW in the 1100 nm regime [2], and Yb sensitized Er-and Tm-doped fiber lasers with output powers of 188 W [3] and 75 W [4] in the 1550 and 2000 nm wavelength range respectively, show the potential of the fiber-based sources. An important point here is the considerable amounts of heat generated by these devices. Advances in both the pump waveguide (NA, size) of double-clad fibers, which enable the multimode diode pump sources with continuously increasing output power to be used, and the core (large mode area and low numerical aperture even at high dopant concentrations), lie behind the current rapid progress.In a master os...