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 a n 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 pm for the straight fiber wedge shape to 0.7 pm for the uptapered wedge shape. Using this technique, a single 980-nm, 30-pm stripe width, broad-area laser provided enough power to pump an erbium-doped fiber amplifier to obtain 24-dB gain. Richard S. Vodhanel (M'89) received the B.S. degree from the University of California, Irvine, in 1974, and the M.S. and Ph.D. degrees from the University of Illinois, Urbana, in 1976 and 1981, respectively, all in physics. His doctoral dissertation was on nuclear gamma ray spectroscopy.In 1980 he joined Bell Laboratories as a member of the Technical Staff, where he conducted research on the topic of single-mode fiber transmission systems. In 1984 he joined Bellcore, Red Bank, NJ, where he is presently engaged in research on coherent-optical fiber transmission systems. He has over 70 publications in physics and optical fiber communications.Dr.

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The mode-field diameter (MFD) of a single-mode fiber can be measured using several different techniques. The extent to which these techniques are consistent depends upon the definition used to compute the MFD and upon the fiber design. In this paper, three measurement techniques (far-field scan, offset joint, and knife-edge scan) are applied to the measurement of both conventional step-index fibers and to non-step-index dispersion-shifted fibers. Using the far-field second moment definition, which is often called the "Petermann 2" definition, measurements made using these three techniques agree to within 1.6 percent.

Abstruct-The curvature dependence of the effective cutoff wavelength is investigated for both matched-cladding and depressed-cladding fibers. Contrary to the depressed-cladding fiber, the cutoff wavelength of the matched-cladding fiber is strongly curvature dependent. A formula is developed for the bending loss of the first higher-order mode group in a depressed-cladding fiber. By taking into account the effect of the stress-induced changes in the refractive index on the loss the first higher-order mode group, the effective cutoff wavelength as a function of curvature is calculated. The theoretical results show good agreement with measured values, which verify the curvature insensitivity of the effective cutoff wavelength of depressed cladding fibers.

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