Excess loss in practical depressed-cladding fibres (comment with reply)

The physical model developed by Love and Henry (see ibid., vol.26, no.727-8, 1990) to explain spikes in the leakage loss curve of depressed cladding fibres beyond the fundamental mode cut-off wavelength, is based on the assumption that the field amplitude of the HE₁₁ supermode at the cladding jacket interface is much less than that of the HE₁₂ supermode. The commenter claims that at lambda _x, both the HE₁₁ and HE₁₂ modes have equal high absorption losses and the taper perturbation serves only to equalise power in the two modes. He introduces the ratio of the field amplitudes within the cladding and core and claims that from lambda _x these are in the ratio T^-2 for the HE₁₁ and HE₁₂ modes, where T<<1. He claims that the HE₁₁ mode absorption is a factor T₂ smaller than the HE₁₂ absorption. Since absorption depends on intensity, rather than amplitude, one would expect this factor to be of the order R⁴. The authors of the original article suggest that this approach to the estimation of differential absorption is misleading. A precise determination of modal attenuation requires knowledge of both the real and imaginary parts of the refractive index of the jacket material.