Formally, the diffraction efficiency (DE) associated with an order m is the ratio of the optical power P m that propagates away from the grating in order m to the optical power P inc incident on the grating, orįor larger-period or lower-frequency gratings in which many orders exist, the choice of groove profile to direct light into a particular order is intuitive. In other words, the profile of the grating grooves dictates the efficiency with which light diffracts into each of the orders. How much light diffracts into each direction is determined by the principle of diffraction at a microscopic level. The PGL Technical Note “The Grating Equation” describes how to predict the different directions light propagates from a grating using optical interference. Diffraction from the groove profile determines efficiency in each order: If the surface irregularity is periodic, such as a series of grooves etched into a surface, light diffracted from many periods in certain special directions constructively interferes, yielding replicas of the incident beam propagating in those directions. When light is incident on a surface with a profile that is irregular at length scales comparable to the wavelength of the light, it is reflected and refracted at a microscopic level in many different directions as described by the laws of diffraction. Gratings are based on diffraction and interference:ĭiffraction gratings can be understood using the optical principles of diffraction and interference.
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