In the present approach (see Figure 2), the beams from multiple fiber-pigtailed single-mode laser diodes are coupled to single-mode optical fibers that have been placed together in a hexagonal-close-packing planar array. An array of diffractive microlenses, custom-designed and fabricated on a glass substrate by electron-beam lithography, is placed in front of the fiber array. The custom design and position of the lens array are chosen, according to the precisely measured actual positions of the fibers, so that the single-mode beams emerging from all the single-mode optical fibers are focused on the same small circular spot centered on the input face of a suitable multimode optical fiber. In use, the beam emerging from the output end of the multimode fiber would be focused onto the front facet of an Nd:YAG NPRO crystal in the usual way. It is anticipated that the AΩ of the pump light thus incident on the crystal would be less than the maximum single-frequency-lasing value.
This work was done by Duncan Liu, Daniel Wilson, Yueming Qiu, and Siamak Forouhar of Caltech for NASA’s Jet Propulsion Laboratory.
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