An efficiency optimization approach for two-wavelength diffractive optical elements

In this paper, we present a design of a phase-only diffractive optical element (DOE), which is capable of producing two distinct diffractive patterns for two wavelengths with high and equal diffraction efficiencies. The DOE is constructed by two one-sided surface profiles that are stacked together and in contact with a thin focusing lens. One is designed for a wavelength of the incident light to generate a pattern on the back focal plane, and the other is for the second wavelength to generate a different diffractive pattern without affecting the first pattern. The second profile simply provides a multiple of 2-pi phase delay for the first wavelength and thus has no effect on the first pattern. In the proposed algorithm, the iterative Fourier transform algorithm with the stepwise quantization method is modified to calculate the two phase profiles simultaneously in the iterations. High and equal diffraction efficiency (74.5%) for the two wavelengths was achieved with 2 phase profiles of 4 uniform phase levels. In addition, a method of multiple bounding controls was proposed to increase the SNR in the regions of interest so as to improve the chromatic performance of the double wavelength design. Different SNR's (5.7 dB and 1.7 dB) were achieved at signal and noise regions.