Tailored fiber lasers and their use in nonlinear optics
Abstract: The objective of this thesis work was to develop tailored fiber lasers, which meet the pump requirement for efficient continuous-wave (cw) frequency conversion to the visible and the mid-infrared wavelength regimes: a stable, high-power, wavelength-tunable, narrow-linewidth output in a single polarization. As a first step, the prospect of tuning ytterbium fiber lasers over an unprecedented wavelength range, from 980nm to 1100nm, was investigated. The results further substantiate the enormous potential of fiber lasers to act as widely tunable pump sources for nonlinear frequency conversion schemes, allowing the design of coherent light sources in large parts of the optical spectrum. Subsequently, a method of flexible wavelength-tuning and -locking for high-power fiber oscillators was demonstrated, incorporating the use of a highly reflective transversely-chirped volume Bragg grating as cavity mirror. Through a simple lateral translation of the grating, continuous wavelength-tuning over 2.5 THz was achieved without sacrificing efficiency, spectral or spatial beam quality. As the latter free-space laser architecture relied on an intra-cavity polarization filter to ensure a linearly polarized output state, the filtered orthogonal polarization state was available for a secondary laser oscillation. Following this basic design idea, a high-power dual-wavelength laser with a tunable wavelength separation of up to 2 THz was demonstrated. With both signals separated in wavelength and polarization, gain competition was effectively suppressed and the presented source possessed the necessary stability for potential use in power-demanding applications such as difference frequency generation of cw THz radiation. After establishing a flexible and reliable fiber pump source, continuous-wave, quasi-phase matched, second harmonic generation experiments were performed on several crystals from the KTiOPO4 family. The power scaling limitations of these materials, when generating high-power signals in the visible spectral range were studied. Although, a conclusive identication of the optimal KTiOPO4 isomorph for this process was hindered due to varying crystal quality from different vendors, comparative transmission studies suggest that the arsenate isomorph, KTiOAsO4, could be best suited by virtue of its low linear absorption in the visible region. Finally, a singly-resonant optical parametric oscillator, pumped by the above narrowband fiber laser source, efficiently generated 11W of mid-infrared radiation at 3.4 µm and 19W single-frequency radiation at 1.55 µm. Stable, high-power operation of this parametric source was facilitated by a novel method of controlling the intra-cavity signal power using a volume Bragg grating with variable reectivity.
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