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The curvature dependence of the effective cutoff wavelength is investigated for both matched-cladding and depressed-cladding fibers. Contrary to the depressed-cladding fiber, the cutoff wavelength of the matched-cladding fiber is strongly curvature dependent. A formula is developed for the bending loss of the first higher-order mode group in a depressed-cladding fiber. By taking into account the effect of the stress-induced changes in the refractive index on the loss the first higher-order mode group, the effective cutoff wavelength as a function of curvature is calculated. The theoretical results show good agreement with measured values, which verify the curvature insensitivity of the effective cutoff wavelength of depressed cladding fibers.