High power pulsed fiber lasers at near-IR optical wavelengths

Among fiber-based lasers, the availability of high energy and high peak-power pulsed sources is of critical importance for a variety of application ranging from remote sensing, material processing to laser-plasma EUV generation. For most of the applications, high average-power is required in preferably diffraction-limited beams. In this dissertation, we demonstrate usage of very large-mode-area (LMA) fibers to significantly exceed fundamental pulse energy and peak-power limitations of standard single-mode fibers and explore the limit of core size scaling in LMA fibers. We show systematic studies in both theoretical modeling and experimental characterization of beam quality control in extremely large core LMA fibers, including coiling induced higher-order-mode filtering effect in low numerical aperture (NA) Yb-doped fiber amplifiers and fundamental mode excitation in high NA Er-doped fiber amplifiers. We report achieving good beam qualities (M2∼1.3) in 80-mum Yb-doped and 70-mum Er-doped fiber amplifiers, corresponding to the largest mode diameters of conventional step-index LMA fibers with nearly diffraction-limited output for applications at ∼1-mum and ∼1.55mum wavelengths, respectively.; Large core size ensures higher energy extraction and increases thresholds for the onset of optical damage and detrimental nonlinear effects. We demonstrate in this work that millijoule energies and megawatt peak-powers in nanosecond pulses can be achieved in nearly diffraction-limited beam using 80-mum core Yb-doped fiber amplifier, and also show the average-power scalability of this pulsed fiber laser up to 85-W. Comparable pulse energies and peak-powers are believed to be achievable in 70-mum core Er-doped fiber amplifier for applications at ∼1.55-mum wavelength. Here, we report demonstration of average-power scaling of nanosecond and ultrashort pulsed fiber lasers using 25-mum core Er/Yb co-doped fiber amplifier. In particular, high average-power (∼28-W) ultrashort (∼1.4-ps) pulsed fiber laser was realized in fiber-based chirped pulse amplification (FCPA) system using novel chirped volume Bragg grating (CVBG) pulse stretchers/compressors. These results indicate that fiber laser technology has significant potential of replacing conventional pulsed solid-state lasers and is a better alternative for applications that require high average-powers.