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Abstmct-The power-series expansion method has been used to compute the exact cutoff frequencies of TM modes in step-index fibers for various values of relative refractive index difference (A), ranging from 0.005 to 0.1. The difference between the propagation constants of TM and TE modes, intermodal delay between TM and TE modes, and their variation with respect to the relative refractive-index difference has also been calculated. Numerical results show that for a fiber with A = 0.005 the intermodal delay between TMsl and TEo, modes is of the order of 1.5 pslkm at frequencies far away from cutoff, whereas it is of the order of 0.62 ns/km for A = 0.1, W I. INTRODUCTION AVE propagation in weakly guiding step-index fibers has been analyzed by Gloge [ 11, [2]. Various methods have been presented for the analysis of nonuniform core fibers [3]-[7]. The scaler wave equation has been solved for a-power profile index fibers using the power-series expansion method [3].The purpose of this paper is to show that the powerseries expansion method can be used to solve the vectorial wave equation to compute the dispersion relation and the group-delay characteristics of TE and TM modes in stepindex fibers having any value of relative refractiye-index difference. In this paper, exact values of propagatlon constants and normalized delay and cutoff frequencies of TM modes have been computed even for fibers with large relative refractive-index difference. This information cannot be obtained from the method outlined in [3]. The scalar wave equation can be exactly solved only for TE modes, and not for TM modes [ S I . Numerical data giving the contribution of the nondegeneracy in TE and TM modes to the waveguide dispersion has been arrived at using this method. Thus the present method could be employed to determine the contribution of TE and TM modes to waveguide dispersion in dual-mode fibers where the TEol and TMol modes can propagate along with the HEll mode.

Neutron irradiation induced degradation in the responsivity of Si and Ge p-i-n photodetectors has been studied. The responsivity of an irradiated photodetector normalized to its preirradiation responsivity was found to have a spectral dependence in the case of the Si photodetector. In the case of the Ge photodetector, the relative responsivity was found to be independent of wavelength in the most part of the spectrum covered in the experiment.