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This paper documents the development of a laser beam homogenizer using a large, square fiber
(channel integrator, kaleidoscope). The main effort addresses the beam injection issues for a
three fiber system. The purpose of using three fibers is to split the input beam into three beams
for use in an industrial laser process. It has been observed by the researchers in the laser beam
shaping community, that strong patterns can be observed in the output irradiance of the output
irradiance of optical fibers when the laser source is focused onto the entrance face of the fiber.
This is due to the fact that although the mode volume of the fiber uniformly filled, the phase is
not suitably randomized. This is analogous to the fact that summing a large number of Fourier
series terms with uniform amplitude and phase will approximate a delta function. If the phases
are randomized, the result will be a uniform speckle pattern. The Fourier components
correspond to the modes of the fiber. Further, in the case of uniform phase input, the phase of
the different modes will change as the light propagates down the fiber due to the difference in
mode propagation velocities. But, the phase will not assume a random distribution if the fiber is
not perturbed. This problem is frequently alleviated using mode scramblers, which usually
employ bends or micro-perturbations of the fiber path. The problem can also exist if the mode
volume of the fiber is not uniformly filled. This will happen if the source NA does not match the
NA of the fiber. This paper will compare and contrast two methods of fiber injection; a
traditional focused beam approach and a lenslet array beam integrator approach. Each design
will be outlined and the performance results will be discussed to determine how well each
method filled the modes of the square fiber.
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