**.**Since the core and cladding have different indices of refraction hence they have different attenuation coefficients α

_{1}and α

_{2}respectively.

**.**For step index fiber, the loss for a mode order (v,m) is given by,

For low-order modes, the expression reduced to

**.**For graded index fiber, loss at radial distance r is expressed as,

The loss for a given mode is expressed by,

Where, P(r) is power density of that mode at radial distance r.

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