EFFICIENT POWER COUPLING FROM A 980-NM, BROAD-AREA LASER TO A SINGLE-MODE FIBER USING A WEDGE-SHAPED FIBER ENDFACE

A theoretical and experimental study of a new, efficient technique to couple a broad-area laser, emitting a highly elliptical beam, to a single-mode fiber without the use of bulk optical components is presented. The technique involves butt coupling the laser to a wedge-shaped fiber endface. Such an endface approximates a cylindrical lens which corrects for the phase front mismatch between the curved laser beam wavefront and the planar fiber beam. The fabrication process uses a wedge-shaped polishing tool and a simple polishing procedure. A theoretical formula for the coupling efficiency in the absence of both angular and transverse misalignments is derived. Based on the estimated mode field radii of the two-dimensional laser beam and assumed mode field radius of the fiber beam, a maximum coupling efficiency of 46% is predicted by the theory compared to the measured value of 47% (15.2-mW power coupled to the single-mode fiber) obtained by using a well-designed wedge-shaped fiber endface. For the square endface, the measured coupling efficiency was 20%. The technique was further refined by incorporating an uptapered, wedge-shaped endface to decrease the transverse misalignment sensitivity. The transverse misalignment tolerance for 3-dB reduction in maximum coupled power increases from 0.4 µm for the straight fiber wedge shape to 0.7 µm for the uptapered wedge shape. Using this technique, a single 980-nm, 30-µm stripe width, broad-area laser provided enough power to pump an erbium-doped fiber amplifier to obtain 24-dB gain. © 1990 IEEE