Growth and Characterization of Ytterbium Doped Silicate Crystals for Ultra-Fast Laser Applications

Diode-pumped solid-state lasers (DPSSL) have predominated over waveguide lasers and fiber lasers when considering the efficiency and operability since the first realization of laser-diode pumped Yb-doped laser at room temperature (Lacovara et al., 1991). As a rule of thumb, DPSSL are preferable for devices operating with high peak power, whereas low-threshold and high-gain operation is much easier to be achieved with waveguide lasers and amplifiers. Besides the application in the fields of double-frequency, remote sensing and biomedical, ultra-fast DPSSL with diversified wavelength and stable system is widely exploited in the fields of mechanics, micro-electrics and ultra-fast photocommunication. DPSSL are composed of laser resonator which is mostly formed with discrete laser mirrors placed around gain medium with an air space in between. Bulk crystals or glasses doped either with rare earth ions or transition-metal ions are adopted as gain medium. With the development of DPSSL industries, the demand for laser crystals with the advantageous physicochemical properties such as efficient energy absorption, high optical uniformity and favorable thermal behavior has dramatically increased over the past few decades (Keller, 2003). With the rapid development of InGaAs laser diodes emitting from 900nm to 980nm, Yb3+ doped laser crystals are expected to alternate the traditional Nd3+ doped for generating efficient broad tunable and ultra-fast DPSSL in near-IR spectral range (Krupke, 2000). Yb3+ ion with simple quasi-three energy level scheme of 2F7/2 and 2F5/2 is provided with high quantum efficiency, long lifetime of metastable 2F5/2 level and large crystal-field splitting which is beneficial for reducing thermal load and enhancing Yb3+ doping level bringing about the realization of compact device without luminescence quenching caused by cross relaxation and excited-state absorption (Giesen & Speiser, 2007; Pelenc et al., 1995). However, the strong re-absorption at emission wavelengths leads to high pump threshold since the thermally populated terminal level of Yb3+ lasers at the ground state manifold is contemporary the laser terminal level. To reduce the re-absorption losses at laser emission wavelengths, strong splitting of ground sublevels of 2F7/2 in Yb3+ ion is required to form a quasi-four-level system as that of Nd3+. Thus, laser crystal hosts with low symmetry structure and strong crystal field splitting are the central issues in exploiting new Yb doped gain media (Du et al., 2006). Crystal hosts such as aluminum, tungsten, oxides, fluorides and