A micro lens array, or MLA, is a series of small lenses arranged in a regular grid. Each individual lens also referred to as a lenslet, can be round, square or hexagonal in shape. The micro lens array’s uses include: to either homogenize the radiance from an input beam, generate an array of focused spots from a single source, or focus an array of multiple sources to an array of multiple spots (typically used in fiber coupling).
For homogenization, the optical homogenization of the output beam is due to the refraction of light and hence the chromatic dispersion is much smaller than diffractive optical homogenizers. For the same reason, they do not require that the input light beam possess a certain degree of spatial coherence. Considering these two important facts, micro lens arrays can be used in polychromatic applications in which the light source is not a laser and it has a wide spectrum.
Micro lens arrays have found applications in a variety of industrial sectors. They can be used as a multi focal spot generator, for instance. Such application is already used in most adaptive optics systems being deployed in astronomical sites. In this application, a micro lens array is used to sample the wavefront from a star as it passes through the earth’s atmosphere. The information obtained in this way is used as a feedback in a control system that is used to compensate for the wavefront error. This type of wavefront sampling is referred to as the Shack Hartmann wavefront sensor. Another interesting application is field or plenoptic cameras whose aim is to acquire depth information from a scene by performing multiple imaging on a detector device. This is accomplished with the use of the micro lens array.
More conventional applications for micro lens arrays are those in which they are used as optical homogenizers. When de-focused, the beams generated by every individual lens start overlapping among themselves, and after some distance averaging out any non-uniformity present in the input beam. The size of the final envelope beam is dependent on the curvature on each lens. Thus, the focal length as well as the conic constant are design parameters for the micro lens array. Micro lens arrays are also used to reduce the hot spots that may arise in certain laser materials processing. They can also be used in aesthetic treatments that require the usage of two different operating laser wavelengths at the same time and with the same performance. To achieve best homogenization and shaping performance for such applications it is often preferred to use a broadband diffuser which is a sub type of micro-lens arrays.
