THEORETICAL-ANALYSIS OF CUTOFF CONDITION OF DOUBLY-CLAD OPTICAL-FIBERWITH HELICALLY CONDUCTING OUTER BOUNDARY

An analysis of the cut-off conditions of a doubly-clad helical optical fiber is made, making use of boundary conditions relevant to a helica lly conducting boundary. The fabrication of optical fibers is now so a dvanced that the construction of a helical optical fiber, although dif ficult is not unfeasible. The cut-off equation is obtained from the ch aracteristic equation for both the W-profile and step-profile. These m odes get merged at cut-off. It is found that a cut-off value is obtain ed when the dimensionless V-parameter is equal to 2.405 below which th e fiber acts as a monomode guide. Subsequent zero crossing of the cut- off equation shift towards higher V-values as the width of the interme diate layer decreases, finally tending to the value V = 3.83 when the width is zero. Also these crossings are obtained for higher V-values w hen one increases the refractive index ratio of the inner cladding and that of the core. Finally no effect of the pitch angle of the helix i s found on the cut-off condition, although such a dependence is expect ed for guided modes away from cut-off.