Laser beam homogenizing: limitations and constraints

Laser beam homogenizing and beam shaping are key enabling technologies for many applications today. Periodic microlens arrays are widely used to transform Gaussian or non-uniform beam profile into a uniform "flat-top". Each microlens element samples the input beam and spreads it over a given angular distribution. Incoherent beams that are either temporally or spatially incoherent can produce very uniform intensity profiles. However, coherent beams will experience interference effects in the recombination of the beams generated by each individual microlens element. Rotating or moving elements, such as a rotating diffuser or a vibrating optical fiber, are used to average these interference patterns. An integration of several different patterns will smooth out the intensity profile. Unfortunately, this averaging is not always possible. Some applications require a single shot from a pulse laser or work at very high data rates that do not allow an averaging over 10 to 50 frames. We will discuss the concepts of Köhler illumination and Köhler integrators and its limitations and constrains for laser beam homogenizing. We will show how micro-optical elements comprised of a randomly varying component can be used to smooth out interference and speckle effects within the far-field intensity profile.