We have analyzed theoretically and experimentally the characteristics of an apodized LPG with almost zero insertion loss in the filter pass band. The insertion loss of ϳ1 dB due to side-lobes is eliminated through the apodization technique. The wavelength and peak loss of apodized LPG can be effectively controlled by changing the value of dc and maximum ac index change of LPG. We also characterize the low insertion loss gain-flattening filter based on apodization technique. The experimental result matches well with the simulation result in the apodized LPG gain flattening filter.

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We propose a fiber structure based on a single-mode-multimode-single-mode one. A theoretical analysis model based on leaky mode expansion along with a new concept of leaky mode interference is presented to facilitate and enlighten the analysis for the leaky structure. Then we focus on transmission characteristics of the structure in response to surrounding refractive indices (RI) that are higher than that of the silica fiber. In our simulation, output intensity of the structure increases monotonically for RI from 1.46 to 1.55, and changes over 2.0 dB for a change of 0.01 in RI range from 1.46 to 1.48, which is well verified by the following experiments.

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