Long period chiral fiber grating apparatus

Abstract

A long period chiral fiber grating ('LPCFG') that has a number of advantageous properties that can be readily utilized in a number of different applications is provided. The inventive LPCFG is a fiber grating having a pitch that exceeds the wavelength of light propagating therethrough. The LPCFG includes a number of dips in its transmission spectrum, but does not reflect any portion of the signal passing therethrough. The LPCFG is sensitive to changes in the refractive index of its external environment (or in the refractive index of a coating covering the LPCFG cladding). In response to changes in the external refractive index, the transmission dips shift proportionally to changes in the index, thus enabling the use of LPCFG as a fiber sensor element. In addition, the LPCFG is polarization sensitive—one circular polarized wave of one handedness is coupled to the cladding mode stronger than the wave of the other polarization handedness. This enables the LPCFG to be readily utilized in polarizers and for shaping the polarization spectra of signals passing therethrough (in one embodiment configured as multiple LPCFG elements having different pitches). The polarization sensitivity of the inventive LPCFG can also be used for verifying system integrity in chiral fiber sensor systems. In other embodiments of the present invention, the novel LPCFG can be used for modulating amplitude of the light signal propagating therethrough, or as a resonant active structure for add/drop filters.