Large-Mode-Volume Fiber Devices for High-Power and High-Energy Applications

This thesis explores a new type of gain medium for fiber lasers and amplifiers, active tapered double-clad fibers (TDCF), and describes their distinct properties, design, and applications. The TDCF technology is based on fabrication of axially non-uniform active optical fibers, with the aim to provide highly practical and cost-effective alternatives to the widely used fiber devices of today. While retaining all the flexibility associated to present-day double-clad fiber (DCF) instruments, these fibers offer the added benefits of low-brightness end pumping combined with high output brightness, a robust method for mode area scaling, and mitigation of certain deleterious optical effects. The TDCF technology was first established as a proof of concept during this dissertation work, followed by gradual power scaling to near-kW range in the continuous-wave (CW) regime, and to multi-mJ energies in pulsed operation. Given the preceding, practically nonexistent, high-power fiber laser (HPFL) research at the home university, this progress required substantial developments in thermal management, pumping techniques, and fiber design. The characteristics of the asymmetric and non-reciprocal active fiber waveguides were found to be very distinct from regular, cylindrical fibers. Their special features have been thoroughly studied within this work. The ytterbium-doped flared active DCFs studied in this dissertation work were applied as gain fibers in several types of laser cavities, as amplifiers in master oscillator – power amplifier (MOPA) configurations, and as pump sources for nonlinear processes. The appended publications demonstrate all these applications, and the related TDCF characteristics are discussed in the thesis. Given the directly-applicable wavelength versatility provided by manifold rare-earth dopants, the established technological platform appears particularly feasible for realization of compact MOPA systems for mid-power materials processing at 1 µm, as well as light detection and ranging (LIDAR) and medical applications at 1.5 µm.