Polymer optical waveguide platform for the development of mode-controlling devices

In a mode-division-multiplexing (MDM) system, different spatial modes of a few-mode fiber carry different signal channels and many mode-controlling devices, such as mode converters, mode (de)multiplexers, mode filters, and mode switches, are required. Among the various technologies available for the implementation of such devices, the polymer waveguide technology offers many distinct advantages. First, the polymer waveguide fabrication process based on photolithography can produce highly precise waveguide structures, which is essential for the realization of devices that involve phase matching, such as gratings and asymmetric directional couplers. Second, the spin-coating process for forming polymer thin films allows the construction of multilayer and three-dimensional (3D) waveguide structures, such as vertical directional couplers and branches. 3D integrated optics provides a particularly flexible approach for the development of devices to manipulate fiber modes, which are 3D in nature. The process also allows the insertion of special material, such as graphene, in a polymer waveguide to achieve unusual functions. Third, the large thermo-optic coefficient of polymer can be explored for the realization of effective tunable devices and switching devices. Fourth, being low-index-contrast structures, polymer waveguides can be easily designed to match the dimensions of few-mode fibers and thus minimize the fiber-waveguide coupling loss. For the same reason, polymer waveguide devices often show polarization-insensitive performance. This paper presents a review of a wide range of mode-controlling devices developed at City University of Hong Kong, which take advantage of the many nice features of the polymer waveguide platform.