diffraction gratings and does - Breault Research Organization, Inc.

DIFFRACTION GRATINGS AND DOESFundamentals of Diffractive Optical Elements. . . . .dimensional diffraction grating. It can also be regarded as a three-dimensionalinterference filter.A volume hologram has a thickness that is much greater than the thickness of itsthin holographic counterpart. Even then, it is still very thin, perhaps on the order oftens of microns. The larger thickness is needed to record the three-dimensionalfringe pattern. The thicker the volume hologram, the more light is diffracted intothe first order. In fact, efficiencies of 100% are achievable.Volume holograms de-couple wavelength and the diffraction angle. In other words,only a specific wavelength is diffracted at a specific angle in the hologram.Because of this property, you can store many holograms at one time. Just changethe incident wavelength. Similarly, you can change the angle of incidence toachieve the same effect.Volume holograms are highly susceptible to shrinkage. Therefore, if shrinkage isnot accounted for, the replay wavelength must be shorter than the recordingwavelength. Only at special wavelengths is light diffracted at a particular angle. Inthis sense, the volume hologram acts like a spectral filter.ASAP can simulate volume holograms, but it does so using the grating equation.However, we must set up appropriate diffraction efficiency models with theCOATING MODELS option in ASAP, and specifically use the interface diffractoption with this coating model.ASAP simulates volume holograms as thin holograms with appropriately adjusteddiffraction efficiencies. Most other lens design codes also use this technique.However, they sometimes include approximated diffraction efficiency calculationsfrom, for example, Kogelnik’s theory (Bell Systems’ Technical Journal, 48, 2909,1969). This simulation technique assumes that the primary difference betweenvolume and surface holograms is the behavior of the diffraction efficiencies.ASAP Technical Guide 33