Efficiency of a holographic wave-front converter

The conversion efficiency of a holographic spherical-to-spherical wave-front converter of the phase-volume zone-plate type is discussed. Calculation has been made by applying an extended Kogelnik unidimensional coupled-wave theory, taking into account a situation when the converter is an off-axis type and also another situation when optical waves of different wavelengths are used for forming and illuminating. It has been found that the maximum efficiencies decrease with the angle subtended by the converter but hardly change with the off-axis angle, and efficiencies larger than 90% can be expected at the coupling parameter k(0)d of nearly 1.8 even for an illuminating-to-forming wavelength ratio >2, if the illuminating source is set at the optimum position.