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Comprehensive Analysis for the Consecutively-Cascaded Single-Helix Long-Period Fiber Gratings With Opposite Helicities

Characteristics- and polarization-dependences of the transmission spectrum for a consecutively cascaded single-helix long-period fiber gratings with opposite helicities (CC-SHLPGs-OH) have been comprehensively analyzed for the first time. It has been found that the TE/TM cladding mode produced in the first single-helix long-period fiber grating (SHLPG) will largely affect the mode couplings in the later SHLPG, and as a result, a tunable flat-top band-rejection filter could be obtained once if the resonant wavelengths and the coupling strengths of the two SHLPGs are suitably selected. As a concept-proof example, measurements for a flat-top band-rejection filter have been experimentally presented. Moreover, it has been found that the proposed CC-SHLPGs-OH will exhibit a strong polarization-dependence spectrum as well as an enhanced circular dichroism (CD) especially at the vicinity of the dip wavelength. By making use of this kind of CD characteristics, one can easily produce a polarization converter which enables to turn the left-circular-polarization light to right-circular-polarization one and vice versa.