A micro lens array is an assembly of very small lenses, referred to as lenslets, whose most common purpose is to homogenize the radiance from an input beam.
In the most basic form of a micro lens array, or MLA, every lenslet possesses the same radius of curvature. A beam traversing the MLA will be subjected to a local and periodic modulation that in turn will give rise to a series of small spots at the focal plane of the lenslets. Immediately after, the multiple sub beams will overlap with each other creating a new, more uniform, beam envelope. Any irregularity present in the original beam will be effectively averaged out by this process. Furthermore, even if the input beam has a Gaussian profile, which is characterised by a peak in the centre with slowly decaying tails, the resulting output beam will tend to have a uniform radiance plateau surrounded by hard edges. This important feature stems from the fact that the area of the beam hitting an individual lens can be assumed to be uniform.
A micro lens array works solely by following the rules dictated by geometrical optics. In other words, refraction of light is the dominant effect. Hence, it presents a very low, if any, chromatic dispersion. For this reason, it can be safely assumed that micro lens arrays can exhibit broadband operation. A more advanced form of micro lens arrays is the Broadband Diffuser. This type of diffuser, unlike the traditional micro lens array, has a pseudo-random formation with varying lenslets heights and diameters, providing better uniformity and shaping abilities than the standard MLA while still enjoying its inherent benefits. Moreover, as a result of the random homogenization, the input beam does not have to be a Gaussian single mode with high coherence. The input beam can in fact be multimode and with low coherence. Thus, when any of these conditions exist, a broadband diffuser micro lens array is the best option for beam homogenization.
One widespread application of micro lens arrays is in wavefront sensors for adaptive optics applications. One of the most versatile types of wavefront sensing instruments is the so-called Shack-Hartman sensor. In this instrument the wavefront from a distant object is made to pass through a micro lens array. A 2D detector is positioned at the focal plane of the MLA and thus an array of focal spots is obtained. When the wavefront is flat, that is, it has no optical aberrations, the detected spots in the array are basically equidistant. On the other hand, when the wavefront has some degree of aberration, each spot will move some distance from their nominal positions. The amount of lateral shift, as well as the directions, is a direct indicator of the local tilt of the wavefront at that beam portion covered by the lenslet. With all these spots’ shift information, the entire wavefront can be reconstructed.