A micro lens array, or MLA, is an optical component that is used for many purposes in optical system applications. It is composed of a series of small lenses, all packed together with fixed separations, in a square or hexagonal grid. Despite being a refractive component, the MLA can still be considered a thin optical element, similar to a window or diffractive optical element, for example
One of the applications of MLAs is beam radiance averaging. If a beam with some non-uniformities or ripples passes through an MLA, the effect of all the lenses in the array is to average out any radiance features present on the beam. If the beam were to be used for illumination, for example, the intended illuminated area would be more uniform.
This averaging is achieved by transforming the Gaussian radiance of a common laser beam to a radiance pattern that resembles more the desired Top Hat radiance sought after in many optical applications. This occurs because each lens, or lenslet in the micro lens array affects only a small portion of the input Gaussian beam. Thus, within that portion of the beam the radiance is almost constant or uniform and when all the sub beams merge together again at some propagation distance, the result is a flat top profile.
The conversion of a Gaussian profile beam to a Top hat beam is in fact better accomplished with a diffractive optical element diffuser but there are some situations in which a micro lens array will be more suitable compared to a DOE. When the beam is highly multi-mode and is also incoherent, the performance of the MLA will be a better alternative to obtain a Top Hat Beam. Furthermore, in the case when the beam is not even monochromatic or, from a practical sense, the MLA must operate with two different beams of different wavelengths at the same time, the micro lens array is the best option by far.
Another very important application for micro lens arrays is wavefront sensing in imaging application through a distorted medium. The focal plane of the micro lens array can provide useful information about the aberrations of the wavefront on the input beam. Within the zone allocated for each lenslet, the position of the focal spot is an indicator of the tilt of the local sub beam. Combining all these measurements together, the overall shape of the wavefront can be retrieved- this is called a shack-Hartmann sensor.
