POLARIZATION-SELECTIVE MODE-COUPLING IN 2-MODE HI-BI FIBERS

We propose and demonstrate a mode coupler which converts either of the LP(01) polarization states in a two-mode high-birefringence (Hi-Bi) f iber to the LP(11) mode with the same polarization. We use coupled-mod e theory to develop design rules based on the polarization splitting o f the beat length between the two lower-order modes. The device can be operated either as a narrow-band device in a region with large interm odal group delay difference or as a broadband device in a region with zero group delay difference. We use this novel device as a key compone nt in a two-mode Hi-Bi fiber polarizer. In this configuration either o f the polarization eigenstates can be selected and transmitted with an adjustable extinction ratio which can be as large as 30 dB. The coupl ing loss in the transmitted state of polarization can be less than 0.2 dB. In the broadband polarizer we demonstrate -20 dB extinction over 42 nm with a potential for considerable improvement, We also propose a nd experimentally investigate an increased differential group delay ob tained by propagating one polarization state in the LP(11) mode instea d of in the LP(01) mode. The largest differential group delay measured in this configuration is 14.5 ps/m which is seven times larger than t he differential group delay between the polarization modes. We discuss several possible uses in fiber sensors and measure the transmission o f a proposed two-coupler configuration.