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Micro Lens Array

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A Micro lens array contains many small lenses, also referred to as lenslets, with the same curvature and size. They are all resting on the same glass or plastic substrate, so they form a single optical component. Their operating principle is based on conventional ray optics, with each micro lens creating an image at the far field that is the fourier transform of the lens aperture shape. This enables functions such as homogenization and shaping laser light into rectangles, lines or squares, as well as the generation of dot arrays, as we explain below.

How the IFUs work: each IFU consists of a microlens that guides the light from a small sky area, normally centred on a celestial object (e.g., a distant galaxy) and sends it on to the entry of the spectrograph (inside the dotted box).

Micro lens arrays are used in various industrial settings. For instance, they provide a very efficient method to homogenize the radiance from a beam with no concerns about the beam being monochromatic or not. Any irregularity on the beam is completely averaged out. Furthermore, the overall radiance pattern is converted into a Flat Top beam. This interesting property arises from the fact that the beam is sampled at intervals given by the spacing among lenses. Within the lens pitch, the beam can be said to be almost completely uniform.

Given the beam homogenization property, micro lens arrays are also used as transmissive diffusers in applications like small video projectors or in medical instruments, such as those used in aesthetic treatments. Also, given that the absorption is minimal, they are well poised to be used with high power lasers.

Micro lens arrays can also be used to create a multi spot image plane. In this case, each lenslet generates a single beam and if the image plane is positioned at the focal plane of the lens array, each focal spot is clearly discerned from the neighbouring new sub beams. This feature can also be harnessed for other different applications. For instance, when the input beam has some inherent wavefront aberrations, which can simply be seen as a departure from a perfect flat phase beam, a micro lens array can be used to sample those errors. Information about the local tilt of the wavefront can be obtained by looking at the position of the individual focal spots. This in turn can be directly related to the overall wavefront aberrations. This is the principle of the Shack-Hartmann wavefront sensor which is a bespoke instrument used widely in adaptive optical systems, from astronomy to ophthalmoscopy.

Another interesting application for micro lens arrays is their use in plenoptic or field cameras. These devices are capable of retrieving depth information from a single image capture. At the heart of such device is also a micro lens array.

 

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