Efficiency analysis of periodic diffractive optical elements - Suitability of various approaches

Improved manufacturing techniques make diffractive optics a very common element in modern optical systems. Exact estimates of their operating efficiency is of utmost importance. Thus the diffraction efficiency of a multi-level blazed grating is evaluated two ways: analytic computation using the rigorous coupled wave analysis and approximation based on the thin element model. According to the obtained results, the computational error for a periodic diffractive optic element is calculated as a function of the period-length to wavelength ratio as well as the dependence on the phase modulation depth, and an error map is developed. An approximated model based on the thin element model, in conjunction with "shadow blocking", whereby some of the rays are blocked by the surface corrugation, is suggested. Using the above approach, an expression for the diffraction efficiency is derived, yielding very good results for moderate period-length to wavelength ratios with minimal computational complexity. The suggested approach is thus advantageous in such regimes, whereas the exact rigorous calculations requires massive computation effort and the classical thin element approximation is not accurate enough.