Capillary optical fibers: design and applications for attaining a large effective mode area

We study the application of capillary optical fibers to attain a large effective mode area for nonlinearity suppression in high-power lasers and amplifiers. The effect of the inclusion of an air capillary on the modal properties of a conventional optical fiber, such as the effective index of the guided modes, is calculated using fully vectorial analytical techniques and verified using the finite element method. It is illustrated that our design transcends the recently predicted limit on the effective mode area obtainable in conventional optical fibers at 1064 nm. The influence of bending on the mode area is investigated in detail. We further propose alternative designs, such as the depressed-clad capillary optical fiber and the trench-assisted capillary optical fiber, and we carry out simulations to show how they can be used to reduce losses in optical fibers. The variation of the fractional power contained within the capillary is calculated to determine the effect on nonlinearity suppression. We also show that by including a nanosized air capillary in the core of a photonic crystal fiber, the effective mode area can be further enhanced even in bent conditions. (C) 2011 Optical Society of America