CORE-MODE CUTOFF FOR FINITE-CLADDING LIGHTGUIDES

Fundamental core-mode 'cutoff' is a useful concept for tapered 'single-mode' lightguides with either a finite cladding surrounded by a lower-index external medium ( equals 'second cladding') or, equivalently, a raised inner cladding. This cutoff is not acutely sensitive to the external index for reasonable cladding/core diameter ratios. The dependence on core grading is given via universal step core curves and simple moment formulas for both planar guides and fibers. Arbitary core profiles can be treated; generalization to higher-order modes is given, and polarization effects may be included.