Tapered holey fibers for spot size and numerical aperture conversion

Summary form only given. A major problem with standard step index fibers is their inefficiency in coupling to integrated optical devices and waveguides, semiconductor devices such as laser diodes, other fiber waveguides with different properties, etc. Coupling losses are mainly caused by mismatch of the modal field distributions, and partly by changes in wave impedance. It is known that holey fiber may be designed to remain single mode over an extremely wide range of wavelengths. It follows that a fixed wavelength the size of the waveguide may be scaled by an appreciable factor and remain single mode. Consequently, adiabatically tapered holey fibers may be used to perform significant scaling of guided mode-field distributions with low loss. The effective index of the guided mode may also vary significantly. We have derived general guidelines for adiabatic taper design, and calculated the mode-field distributions at both ends of an optimally shaped taper.