In recent years it has become clear that a nonlinear formulation of test-particle theories has to be preferred over the traditional quasilinear approach. Nonlinear transport theories such as the nonlinear guiding-centre theory or the weakly nonlinear theory are based on certain assumptions which cannot be derived systematically. One of the key inputs into these theories is the velocity correlation function. In the current paper the Taylor-Green-Kubo formulation is used to deduce a general relation between the mean square displacement of the particle, the running diffusion coefficient and the velocity correlation function. This relation can be used to extract velocity correlation function from test-particle simulations and from transport theories. The latter possibility is the subject of the current paper. These results, which are essential for the improvement of nonlinear transport theories, are compared with standard models applied previously. An additional result of this paper is that for realistic wave spectra, perpendicular diffusion is recovered even within the magnetostatic slab model.

We demonstrate a watt-level mid-infrared supercontinuum source, with the spectrum covering the infrared region from 2 to 6.5 µm, in an all-fiber structured laser transmission system. To further improve the SC spectral bandwidth, power and system compactness in the follow-up As2S3 fiber, we theoretically and experimentally explored some knotty problems that would potentially result in the As2S3 fiber end-facet failure and low SC output power during the high-power butt-coupling process and proposed an optimal coupling distance on the premise of the safety of As2S3 fiber end face. In addition, we also built a multi-pulse pumping model for the first time to more precisely estimate the SC spectral evolution in As2S3 fiber. This work will give an important reference to someone who is working on the all-fiber structured, high-power mid- and far-infrared supercontinuum source.

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We present the development of an As2S3 fiber-based reusable inline optical connector; and, for the first time to the best of our knowledge, integrated it into a 2 μm chirped pulse amplification system as the pulse stretcher. By this mechanism, a near 20 times pulse stretching ratio was achieved in 0.5 m-long this As2S3 fiber and the introduction of insertion loss as low as 2.84 dB while it was connected with silica fiber. After the follow-up fiber amplification and self-compression mechanism, the laser average output power was boosted to 1.97 W and pulse duration was optimized to 2.53 ps. Moreover, to further inspect the laser pulse property, the pulses were then delivered into a 5.3 m-long ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) fiber which was fusion spliced to SM1950 fiber. Thanks to the 0.4-dB low fusion splice loss between these fibers, a 1.56-W supercontinuum source with bandwidth of 1.93-3.50 μm is obtained. The measured two hours of root-mean-square value of power fluctuation of this laser source is 0.13%, indicating an ultra-high long-term power stability. This work will lay a solid foundation in the realization of the all-fiber structured high-power thulium-doped fiber amplifier with high compactness.