Abstract
Optical gain of 8.3 km−1 is obtained in a neodymium‐doped silica fiber under simulated sunlight (intensity = 2.1 kW m−2). The fiber gain medium (length = 50 m) is coiled and attached to a solid‐state luminescent solar concentrator (LSC) disk (diameter = 180 mm). The disk/fiber unit is placed in a cavity comprising highly reflective (HRM) and dichroic (DM) mirrors to increase the photon confinement by a factor of 2.2 compared with a liquid LSC (also employing HRM/DM). The enhancement is mainly due to the air gap between HRM/DM and LSC, which affords total internal reflection (TIR) in the solid‐state LSC and an ideal boundary condition for the multilayer mirrors. The numerical calculations indicate that an additional 2.2‐times enhancement is achievable with a side wall exhibiting 90% diffuse reflection. This has been experimentally confirmed (1.9‐times optical‐gain enhancement is realized by adding a polytetrafluoroethylene side‐wall reflector). Overall, the solid‐state LSC‐based solar‐pumped laser (SPL) comprising reflective side walls exhibits a gain factor of 4.2, which is superior to its liquid‐state predecessor. Further improvements employing a solid‐state LSC design and fiber optimization will facilitate the market penetration of SPLs.
Highlights
The direct conversion of broadband solar radiation into highly coherent laser radiation has attracted scientific and practical interest in recent years.[1]
Since the luminescent solar concentrator (LSC) was designed to function with highly reflective dielectric mirror (HRM), the sufficient absorption of the incident photons from the lamp was achieved by a round trip through the luminescent layer
The theoretical calculations revealed that the optical gain was significantly enhanced by substituting the previous liquid-based LSC design with a solid-state LSC one owing to two reasons: 1) the total internal reflection (TIR) on the solid-state LSC surface could be explored to improve light confinement and 2) dichroic mirror (DM)/HRM functioned well when a small air gap was maintained between LSC and the mirrors
Summary
The smaller diameter (180 vs 300 mm) and the shorter fiber length (50 vs 190 m) of our LSC compared with those of the liquid-state LSC-based SPL were due to the limitations of the fabrication equipment.[27] DM and HRM were added to the front and rear sides of the LSC, respectively, to enhance the confinement of the downshifted photons for pumping the active fiber. The DM exhibited a cutoff wavelength of 579 nm, which allowed the transmission of a broad range of short-wavelength sunlight into the LSC while preventing the long-wavelength-downshifted photons, which were produced by the luminescent layer, from escaping via the front escape cone of the device
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