Dielectric tapers with curved axes and no loss

An "exact" theory is developed for the design of single or muitimode dielectric tapers of arbitrary cross sections and straight or curved axes that have the unique property of guiding with no conversion either to other guided modes or to radiation modes. This remarkably simple theory encompasses a very wide gamut of strictly adiabatic tapers: some very simple, such as linear tapers with straight axes, some very sophisticated, such as directional couplers made of noncoplanar guides tapered and curved differently.