Chapter 4 – Circulator

Publisher Summary This chapter discusses the operating principles, analysis methods, and design considerations of optical circulators together with their typical applications in wavelength-division multiplexing (WDM) systems. Optical circulators can be divided into two groups based on their functionality. One is the full circulator, in which light passes through all ports in a complete circle. The other is the quasicirculator, in which light passes through all ports sequentially but light from the last port is lost and cannot be transmitted back to the first port. The operation of optical circulators is based on principles that include polarization splitting and recombining together with nonreciprocal polarization rotation, and asymmetric field conversion with nonreciprocal phase shift. Dielectric coatings-based polarization beam splitters were used to construct optical circulators in the early stage of circulator development. A schematic diagram of a 4-port circulator, where two dielectric coating-based polarization beam splitter cubes were used to split the incoming beam into two beams with orthogonal polarization, is analyzed. An optical circulator can be constructed using two-beam interference with nonreciprocal phase shifting without the need for polarization beam splitting. It is found that all nonreciprocal polarization rotation-based designs share a common structure with a minimum of three functional elements, which includes polarization splitting and recombining elements, nonreciprocal polarization rotation elements, and polarization-dependent beam steering elements.