Recursive design for an efficient HOE with different recording and readout wavelengths.

The design of a holographic optical element for focusing a collimated off-axis beam to an on-axis point is described. It is designed according to a novel recursive technique in which the recording is done at a wavelength which differs from the readout. In this recursive technique the final holographic optical element is recorded by using other holograms to provide the aspheric recording wavefronts necessary for reducing the aberrations and maximizing the diffraction efficiency. The design is illustrated with an example where an f/3.0 focusing element is recorded at 514.5 nm and read out at 1064 nm. A spot size of 15microm and a diffraction efficiency of ~60% were measured.