In this paper

an alternative way is used as equivalent current method by which a nonideal fiber can be viewed as an ideal fiber in the presence of polarization currents giving rise to electromagnetic fields

and the loss of a fiber is considered to be caused by quasi-guided modes excited by polarization currents. The concept of quasi-guided mode is applied in equivalent current method to calculate the loss of a single mode fiber caused by mode conversion. It is an alterna tive method of coupled mode theory without any concerning of mode coupling

but is mathematically simpler and physically more intuitive than the coupled mode theory.