Radiation from graded-index single-mode fibres

Radiation from the fundamental mode due to sources in graded-index single-mode optical fibres can be examined by determining the effect of the presence of the perturbing core on the free-space (i.e. uniform cladding) radiation field. We look at this effect for fibres with power-law-profile cores. Using these examples we discuss the following two approximation schemes: (i) a new equivalent step approximation where the step has the same `degree of guidance¿¿ as the graded core and (ii) a previously proposed WKB asymptotic approximation.