Gautam DasS. N. SarkarUniversity of CalcuttaDepartment of Electronic ScienceCalcutta 700 009, IndiaE-mail: sns@cucc.ernet.inAbstract. A novel and extremely accurate near Gaussian approxima-tion for the fundamental mode in single-mode fibers having arbitrary in-dex profiles is proposed and verified by the variational method. The spotsize calculated from our approximation matches closely the analyticaland exact numerical results for step and parabolic index fibers over alarge range of values of the normalized frequency V. For the low Vregion where the Gaussian approximation is not accurate, our results aresuitable to accurately predict the propagation characteristics of devicessuch as directional couplers involving evanescent field coupling.