Acousto-optic interaction in optical fiber is examined from the perspective of copropagating optical and acoustic vortex modes. Calculation of the acousto-optic coupling coefficient between different optical modes leads to independent conservation of spin and orbital angular momentum of the interacting photons and phonons. We show that the orbital angular momentum of the acoustic vortex can be transferred to a circularly polarized fundamental optical mode to form a stable optical vortex in the fiber carrying orbital angular momentum. The technique provides a useful way of generating stable optical vortices in the fiber medium.
The acousto-optic mode coupling from core to LP11(cl) mode of a dispersion-compensating fiber excited by two acoustic waves with orthogonal perturbations is studied by use of a novel composite piezoelectric transducer. Highly polarization-dependent mode-selective spectra are observed that are due to coherent acousto-optic coupling between the core and various constituent cladding modes. Potential applications for implementing an all-fiber wavelength-tunable polarization analyzer are also discussed.
We analyze the transmission characteristics of a Sagnac loop interferometer containing a polarization-dependent loss element and lossless polarization-converting elements by use of Jones matrices. We show that polarization independence in the transmission mode can be achieved in such a configuration and that maximum transmittance occurs when a half-wave plate is used for the polarization-converting element. The result is verified experimentally for a fiber acousto-optic tunable filter and cascaded long-period fiber gratings with either intrinsic or process-induced polarization-dependent filtering characteristics.
An all-fiber, narrowband, tunable polarization controller is proposed and demonstrated. The device is based on coherent acousto-optic mode coupling induced by two orthogonal acoustic waves on a dispersion-compensating fiber. The cooperative coupling between the two polarizations of the core mode and the TE01 cladding mode through the two gratings permits indirect coupling between the two polarizations of the core mode with nearly 100% efficiency, which makes the polarization-controlling function possible. Experimental results verify the operation of the polarization controller with an insertion loss of <1 dB.
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