We successfully fabricated a dispersion-shifted 20-um-core LMA fiber by incorporating four resonant side cores. This fiber was designed to operate at 1640-nm eye-safe wavelength and has been used in a pulsed Raman amplifier to overcome modulational instability. A peak power of 100-kW was obtained from the Raman amplifier.
Hybrid microstructured fibers, utilizing both air holes and high index cladding structures, provide important advantages over conventional fiber including robust fundamental mode operation with large core diameters (>30μm) and spectral filtering (i.e. amplified spontaneous emission and Raman suppression). This work investigates the capabilities of a hybrid fiber designed to suppress stimulated Brillouin scattering (SBS) and modal instability (MI) by characterizing these effects in a counter-pumped amplifier configuration as well as interrogating SBS using a pump-probe Brillouin gain spectrum (BGS) diagnostic suite. The fiber has a 35 μm annularly gain tailored core, the center doped with Yb and the second annulus comprised of un-doped fused silica, designed to optimize gain in the fundamental mode while limiting gain to higher order modes. A narrow-linewidth seed was amplified to an MI-limited 820 W, with near-diffraction-limited beam quality, an effective linewidth ~ 1 GHz, and a pump conversion efficiency of 78%. Via a BGS pump-probe measurement system a high resolution spectra and corresponding gain coefficient were obtained. The primary gain peak, corresponding to the Yb doped region of the core, occurred at 15.9 GHz and had a gain coefficient of 1.92×10-11 m/W. A much weaker BGS response, due to the pure silica annulus, occurred at 16.3 GHz. This result demonstrates the feasibility of power scaling hybrid microstructured fiber amplifiers
A nanoparticle (NP) doping technique was used for making erbium-doped fibers (EDFs) for high energy lasers. The nanoparticles were doped into the silica soot of preforms, which were drawn into fibers. The Er luminescence lifetimes of the NP-doped cores are longer than those of corresponding solution-doped silica, and substantially less Al is incorporated into the NP-doped cores. Optical-to-optical slope efficiencies of greater than 71% have been measured. Initial investigations of stimulated Brillouin scattering (SBS) have indicated that SBS suppression is achieved by NP doping, where we observed a low intrinsic Brillouin gain coefficient, of ~1× 10-11 m/W and the Brillouin bandwidth was increased by 2.5x compared to fused silica.
Ytterbium-doped large mode area all-solid photonic bandgap fiber amplifiers were used to demonstrate <400 W of
output power at 1064 nm. In an initial set of experiments, a fiber with a core diameter of ~50 μm, and a calculated
effective area of 1450 μm2 in a straight fiber, was used to generate approximately 600 W. In this case, the input
seed was modulated using a sinusoidal format at a frequency of 400 MHz. The output, however, was multimode as
the fiber design did not allow for single-mode operation at this wavelength. A second fiber was then fabricated to
operate predominantly in single mode at 1064 nm by having the seed closer to the short wavelength edge of the
bandgap. This fiber was used to demonstrate 400 W of single-frequency output with excellent beam quality. As the
signal power exceeded 450 W, there was significant degradation in the beam quality due to the modal instability.
Nevertheless, to the best of our knowledge, the power scaling results obtained in this work far exceed results from
prior state of the art all-solid photonic bandgap fiber lasers.
An 1178 nm seeded and 1069 nm pumped Raman laser system where the second Stokes is amplified in a 1121 nm resonator defined by high reflector fiber Bragg gratings (FBGs) has the potential of producing high output power of narrow linewidth 1178 nm. However, 1121 nm power leakage out of the resonator cavity around the gratings was found to impact the performance of the laser and needs to be dealt with in order to obtain high 1178 nm output power levels. In order to address this problem, the causes of linewidth broadening must be understood. A fully nonlinear model has been built which involves propagation of the spectral wave shape via the nonlinear Schrödinger equation in addition to the Raman processes. It was found that increases in 1121 nm cavity power, fiber Bragg grating bandwidth, and the nonlinear index of refraction n2, as well as a decrease in group velocity dispersion β 2 leads to an increase in linewidth broadening. It is concluded that the magnitude of linewidth broadening seen experimentally can only be explained if the spectral components outside the bandwidth of the FBGs are being amplified. Experimentally, 1121 nm power leakage can be handled by using a three wavelength WDM on either side of the rare earth doped amplifier. In addition, usage of a fiber having a high value for group velocity dispersion and/or a low value for nonlinear index of refraction n2 in addition to narrower bandwidth fiber Bragg gratings may help reduce the amount of linewidth broadening.
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