Capillary optical fibers have a broad spectrum of uses, and are distinguished from conventional fibers by the presence of a hollow central void in the core. This work investigates the modal properties of capillary based optical fibers with an emphasis on their application to attain large effective mode area, and to realize tunable optical fiber couplers. It starts with a comprehensive review of the field, followed by analytical results and full-vectorial numerical simulations of the mode area of various fiber geometries. It is shown that novel designs such as the trench-assisted capillary...