A new method has been developed to create coherent laser light efficiently with direct optical coupling of the Sun’s energy into the gain medium for multiple uses. New advances in solar cell photovoltaic (PV) technologies have greatly improved their efficiencies, mostly by improving their ability to convert many wavelengths or wider bands of the solar spectrum to electricity. New advances in actively doped fibers and optical glasses have been shown to produce very broad, multi-line absorption bands as well as stimulated emission lines, or laser lines. By designing the optical cavity system to feed back all emission bands into the gain media for amplification, a multi-wavelength source can be generated requiring no electronics.

Photovoltaics are capable of incorporating higher intensities to convert to electrical power with the use of solar concentrators, but the total solar array size including these collectors, as well as increased heating of the system, remain the same or get worse, respectively. Ultimately, the laser source(s) can be installed on a dedicated orbiting power station whose sole purpose is to deliver power periodically to assorted targets, even on a lunar base or lunar lander. Solar pumped lasers have been studied in the past, but not with new photonic crystalline fiber (PCF) technology with enhanced multi-element dopant concentrations to incorporate wider spectral bands of the Sun, and produce discrete multiple wavelengths for the PVs as well.

PCF-based lasers can readily produce intensities in the near and far field many orders of magnitude higher at continuous operation and at single or multiple discrete wavelengths. Fiber lasers have demonstrated high (>50%) optical efficiency with diffraction-limited beam quality, and have outstanding thermal management properties since the pump radiation can be distributed along extended fiber lengths.

PCF doped fiber technology was used for high power generation for very efficient, broad-spectrum pump absorption and conversion, as well as a PCF bundling technique on the pump coupling end to increase efficiency and to relax requirements on the collection optics (telescope) pointing at the Sun. The use of a passive PCF fiber bundle at the telescope’s focal plane will provide for a wide-angle field of view for reduced pointing sensitivity for pump light collection, as well as higher coupling efficiency. Furthermore, the use of multiple rare earth dopants in the PCF active portion of the fiber laser will widely broaden the absorption spectrum of the solar radiation, all coupling to the specific laser line, selected by the incorporated fiber Bragg gratings used as end mirrors. Solar radiation that is not absorbed into the laser transition levels will simply pass through the system and out the other end, and thus not contribute to heat or negative optical nonlinear effects.

This work was done by Barry Coyle and Paul Stysley of Goddard Space Flight Center, and Demetrios Poulios of American University. GSC-16820-1

Photonics Tech Briefs Magazine

This article first appeared in the September, 2014 issue of Photonics Tech Briefs Magazine.

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