Effective index imaginary part doesn't appear.

Hi


I am trying to measure the confinement loss in a photonic crystal fiber using the fundamental mode effective index. for a wavelength 1.55um and upwards, the fundamental mode has an inedx of 1.270578-1.825e-10i , which means I could calculate the confinement loss from the imaginary part. But for any smaller wavelength, the effective index of fundamental mode becomes real. It seems like COMSOL isn't calculating imaginary part below e-10i range.

Any solution to this problem ?