Yb-doped LMA triple-clad fiber for power amplifiers

Abstract

High-energy pulsed narrow-linewidth diffraction-limited ytterbium-doped power amplifiers in the 1030 to 1100 nm wavelength range and in the nanosecond regime require large mode area (LMA) fibers to mitigate stimulated Brillouin scattering (SBS). However, typical LMA fibers with mode-field diameters larger than 20 mm are inherently multimode. To achieve a diffraction-limited output, several techniques are available such as low core numerical aperture, fiber coiling and selective doping. The triple-clad fiber design takes advantage of the three techniques. The first clad located next to the core allows a reduction and a better control of the effective numerical aperture for high ytterbium doping that is difficult to achieve with the standard double-clad fiber design. Also, the thickness of the first clad gives an extra degree of freedom that allows either a nearly bending-insensitive output or mode filtering through bending losses that can be enhanced by a depressed-clad design. Incorporating to the triple-clad design an optimized selective rare-earth doping of the core favors the fundamental mode over higher-order modes by the gain differential. Using the right dopants, it can also favor SBS suppression by reducing the overlap between optical and acoustic field distributions. Ytterbium-doped LMA triple-clad fibers with a large depressed first clad and selective ytterbium doping are tested in a power amplifier configuration. Also, ytterbium-doped polarization-maintaining LMA triple-clad fibers with a thin first clad are tested for SBS.