These array apertures are used for spatial filters for tuning bandwidth.
The beam emitted from a laser rod is clean and coherent in the immediate vicinity of the laser assembly. As the beam passes through air, it becomes contaminated by dust particles. As the beam passes through a transmissive optic device such as a lens or prism, it becomes additionally contaminated by lens material and surface optical defects.
This contamination produces random fluctuations in the intensity profile of the laser beam and degrades the spatial coherence. This laser beam contamination is called spatial noise. A spatial filter is basically a lens-pinhole assembly used to remove spatial noise.
Our spatial filter assembly is composed of a microscope objective lens, a pinhole, and a means of mounting and focusing these components.
An objective lens should be chosen according to the amount of beam expansion required.
The size of the pinhole required for a spatial filter assembly may be approximated using the following expression:
P = pinhole diameter
λ = wavelength of light
f = focal length of the lens
d = diameter of laser beam