The performance of step index up-tapers subjected to certain index perturbations in investigated by computer simulation. Concentric symmetric and asymmetric index profile perturbations as well as localized spherical, called regions bubbles, having a different index are studied. It is found that some low-valued perturbations of small scale size (less than a wavelength) do not alter the optical fiber performance. However, the amount of beam expansion in the up-taper may be ultimately limited by such perturbations. Excessive magnification in the presence of perturbations can cause a distortion of the quasi-Gaussian shape and symmetry of the pure step-index fundamental mode as well as increase the excess loss and degrade the lateral and axial displacement tolerance improvements of the up-tapers. For example, an index variation of Delta n approximately=10/sup -5/ in conjunction with a perturbation scale size of 40 mu m can be detrimental. Bubble-type perturbations could also cause similar problems. A single 12.8- mu m-diameter bubble with a relative index of -0.02, embedded in a 128- mu m-diameter step index core with relative index of 0.5%, produced a 0.3-dB excess loss.
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