Diffraction gratings are diffractive elements that can be used to split a beam into other sub beams. The new beams are referred to as diffraction orders and have the same beam characteristics as the original beam.
Traditionally, diffraction gratings have had widespread use in spectroscopy applications. When the input beam is polychromatic, there is very strong chromatic dispersion resulting from diffraction. Thus, the input polychromatic beam is split into its many chromatic components in a rainbow-like manner. The amount of dispersion and angular orientation of the output beams can be controlled by tuning the parameters of the diffraction grating like the period distance. Thus, a custom diffraction grating can be produced for a specific type of spectrometer. A more recent application is pulse-compression and expansion (for ultra fast laser construction), where high power levels are often used.
In general, a diffraction grating can be of two types. The first type is the amplitude grating and is characterised by a periodic structure, like a square wave, in which the light is either blocked entirely or let pass. This will result in major light losses, the exact amount depending on the duty cycle of the periodic structure. Also, there is some amount of light that after passing through the amplitude grating is still unaffected in the sense that it does not experience a change on its trajectory.
The other type of custom diffraction grating is the phase grating. In this case the features on the periodic structure do not block any light. Instead, the portions that were blocking light in the amplitude grating, in the phase grating they add a phase, or retardation. The beam then is said to be phase modulated and this causes diffraction as well. In fact, in terms of angular orientation of the diffraction orders, they behave the same as in the amplitude grating. What makes the phase grating to really stand out is that the diffraction efficiency, that is, the percentage of light landing onto the first orders is greatly improved. In addition, the amount of light that goes to the zeroth order is in turn reduced significantly. In principle this should be completely nulled but due to tolerancing and manufacturing errors is never suppressed entirely
Phase gratings that have a duty cycle of 50% are commonly referred to as Damman gratings and there are well known formulas to calculate their diffraction efficiency taking into account the number of phase levels available for the periodic structure. In the simplest of phase gratings, the number of phase levels is two and this is called a binary phase grating.
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