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Flat-top band-rejection filter based on two successively-cascaded helical fiber gratings
Abstract:
We propose and demonstrate a novel method to produce a flat-top band-rejection filter, which is realized by successively cascading two helical long-period fiber gratings (HLPG) with opposite helicities. Unlike most of the other LPG-based flat-top filters obtained to date, the proposed HLPGs have no complex apodization in the grating's amplitude, and only a relatively shorter length (less than 4.6 cm) is needed, thus making this kind of HLPGs easily and particularly suitable to be fabricated by using the CO2 laser technique. As an example, a flat-top filter with a bandwidth of ~13 nm@0.5dB and ~15 nm@1dB has been successfully obtained.
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