[SCI] Transmission-Type Polarization-Insensitive Polarization Plane Excitation
작성일 : 2021-02-22
작성자 : 김종오
In optical communications, light polarization of optical signals is randomly varied during the signal transmission, and system performances can be deteriorated by polarization-dependent loss and gain or polarization-mode dispersion. Polarization scrambling that actively changes the state of polarization (SOP) using a polarization modulation method is often utilized to reduce polarization-related impairments. To make use of polarization-dependent routing devices during the transmission, however, scrambled or depolarized light should be polarized and then scrambled again after passing through the routing devices. Here, we suggest a novel polarization-insensitive polarization plane excitation scheme that enables transmission-type polarization-dependent routing elements to exploit their polarization-dependent routing functions irrespective of the SOP and degree of polarization of an input beam. The proposed scheme is composed of a polarization beam splitter (PBS) for a polarization-diversified loop, two half-wave plates (HWPs) and two 45° Faraday rotators (FRs) for irreciprocal polarization transformation. We could make unpolarized light selectively excite one of the two principal axes of a polarization-dependent element using the polarization decomposition/superposition property of the polarization-diversified loop and the irreciprocal nature of optical Faraday rotation. The operation principle of the proposed scheme is explained using the polarization evolution of light passing through the PBS, FRs, and HWPs that comprise the scheme. The theoretically predicted operation was experimentally verified by utilizing a long-period fiber grating (LPFG) inscribed on high birefringence fiber (HBF) as a polarization-dependent element. The LPFG written on HBF has polarization-dependent loss bands at two resonance wavelengths. By controlling the four waveplates, one of the two loss bands of the LPFG could be selected even for unpolarized input light.