A single cladding ytterbium doped fibre amplifier pumped at 980 nm that exhibits negligible amount of photodarkening over a long period of time is demonstrated. The output power as a function of time decreased by a very small factor compared to standard single mode ytterbium fibres. To achieve this photodarkening resistant amplifier, a special ytterbium doped fibre has been developed. Codoping with aluminium or other rare-earth such as erbium is shown to decrease the multi-excitation of ytterbium clusters and thus lower photodarkening. Photodarkening was characterized by comparing the amount of excess loss created by core pumping single cladding fibres at high intensity at 980 nm. Photodarkening was found to be directly proportional to the excitation of the ytterbium ions by comparing different pumping scheme and pump wavelength. Core pumping of a single cladding ytterbium doped fibre amplifier at 980 nm represents the worst case scenario for photodarkening. Engineering ytterbium fibres for low photodarkening is therefore critical in pulsed amplification where short length of fibre with high doping level is required as demonstrated with 6 µm core ytterbium fibre amplifier pumped in the core or in the cladding. Photodarkening was correlated to clustering from cooperative luminescence measurement at 500 nm produced by ytterbium clusters that would emit UV radiation under strong pumping.
A mid-infrared supercontinuum source spanning from 3 to 8 μm is demonstrated using a low-loss As2Se3 commercial step-index fiber. A maximum average output power of 1.5 mW is obtained at a low repetition rate of 2 kHz. Thanks to the low NA step-index fiber, the output is single mode for wavelengths above ∼5 μm. The pump source consists of an erbium-doped ZrF4-based in-amplifier supercontinuum source spanning from 3 to 4.2 μm. The effects of both the pump power and As2Se3 fiber length on the output characteristics are studied. To the best of our knowledge, this is the first compact supercontinuum source ever reported to reach 8 μm in a standard step-index fiber.
A procedure for the time-domain optical characterization of an inclusion in a scattering slab is investigated theoretically and experimentally. The method relies on the measurement of a contrast function, which is defined as the time-dependent relative change in the transmitted signal resulting from the presence of the inclusion. Analytical expressions for the contrast functions of absorptive and diffusive inclusions are obtained through a perturbation solution of the diffusion equation. This procedure is used successfully to determine the optical properties of absorptive, diffusive, and mixed inclusions located at midplane in a scattering slab by use of time-resolved transmittance measurements.
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