why input Gaussian beam attenuate along the index fiber

Hello:
I'm using comsol 4.3b "3D" to simulate a Gaussian beam propagating along a step-index fiber.The refractive index of the core and cladding are both real number. If I use the "port", I could find the fundamental mode,but I want to use a Gaussian beam as the input field. Thus, I used Scattering Boundary Condition to add the input Gaussian beam by giving values to the "incident electric field". There is a problem I can not understand,which is why the Gaussian beam attenuates gradually along the fiber, so I wonder whether I should add some more conditions,such as other boundary conditions.
I hope I could state the problem clearly. I'd appreciate it if anyone could guide me.

Thanks,
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Jueling Li