Making unpolarized light sensitive to polarization-sensitive devices

Here we propose a novel scheme in which depolarized or unpolarized light can respond to polarization- dependent optical elements such as an input-polarization- dependent wavelength-selective filter. An optical apparatus for realizing the proposed scheme consists of a polarization beam splitter, two Faraday rotators, and four waveplates. By harnessing the polarization decomposition/ superposition property of a polarization-diversity loop and the irreciprocity of optical Faraday rotation, we could make unpolarized light selectively excite one of the two principal axes of a polarization-dependent element like polarized one while maintaining the unpolarized nature. The principle of operation of this apparatus was theoretically analyzed through the investigation of the polarization variation of light passing through the optical elements comprising it. Then, its polarization-selective feature was experimentally verified by employing a long-period fiber grating inscribed on polarization-maintaining fiber, which had polarizationdependent spectral dips with different resonance wavelengths, as a polarization-dependent element in the polarization- diversity loop. One of the two spectrally separated resonances of the grating could be freely chosen through the proper control of the waveplates in the apparatus.