Asymmetric fiber microlenses for efficient coupling to elliptical laser beams

It is demonstrated that it is possible to fabricate asymmetric hyperbolic microlenses directly on the ends of single-mode optical fiber to enhance the fiber´s coupling to elliptical laser beams. The lenses, with controlled eccentricity ratios, are made by micromachining the end of the fiber with a pulsed CO/sub 2/ laser, as the fiber is directed by computer control about the focussed laser beam. Coupling efficiencies of 84% (-0.74 dB) have been realized with single transverse mode lasers at a wavelength of 0.98 mu m having an approximately 3 to 1 beam ellipticity. With multitransverse-mode lasers at 1.48 mu m having similar ellipticities, the asymmetric lenses demonstrate a 2-dB increase in coupling efficiency over symmetric hyperbolic microlenses. The authors have coupled about 120 mW from such a 360 mW laser, used to pump erbium-doped fiber amplifiers, into single-mode fiber.<>