Engineered diffusers are optical diffusers that have better control over the homogeneity and divergence angles than a normal or conventional diffuser. Thus, making an engineered diffuser involves a design stage in which the characteristics of the desired output beam are considered and play an important role.
We can distinguish two types of engineered diffusers: the broadband diffuser and the diffractive diffuser. The first type of engineered diffuser is very efficient across many different wavelengths. This stems from the fact that a broadband diffuser shares the same structure as a micro lens array but with the remarkable difference that every lenslet in the array can have a different radius of curvature, or some other type of high frequency modification . Using this extra design parameter, the divergence angles as well as the degree of homogeneity on the output beam can be more accurately controlled. The output beam shape and divergence angles will vary with wavelength but they will do so according to ray optics dispersion formulas whose effect is considerably less than the dispersion due to diffractive effects.
The second type of engineered diffusers are the diffractive diffusers. As per the reason just mentioned above, a diffractive diffuser works best with quasi-monochromatic light like laser beams. The great advantage that a diffractive diffuser can offer is that the output beam can have any desired shape. This means that a diffractive diffuser can adapt the shape of the output beam to the geometrical requirements of the system application. For instance, a diffractive diffuser can be used with the high-power multimode lasers that are currently used in aesthetic treatments to not only homogenise the beam that goes into a patient’s skin, but also to project a specific form or array of forms that could be beneficial to the treatment in question. The same logic applies to other high power laser applications such as laser welding or laser ablation. Another advantage of an engineered diffractive diffuser is that the edges of the beam can be steeper than those from a beam coming out from a broadband diffuser. This is due to the fact the diffractive diffuser does not clip the beam with micro-lens apertures with their aperture diffraction, but rather shapes the entire input beam.
Engineered diffusers of either type, broadband and diffractive diffusers, are also used to reduce the speckle contrast which can be very noticeable and, in some cases, degrading to the system application. This useful property of speckle contrast reduction arises from the suppression of spatial coherence resulting from the scrambling of the beam as it passes through the engineered diffuser.
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