What is the physical limit of the edge steepness of a flat top beam Profile?
We image the output (IR ns pulses) of a multimode fiber to our target in order to drill holes. But the low steepness of the flat top causes flaking on the edges of the hole. The material is a mulilayer structure.
I like to know the physical relation between the steepness of the beam profile edges with respect to the fiber core diameter, NA and wavelength.
The field in a fiber (single- or multimode) does not drop to zero at the core diameter, it gradually decreases from the core center but somehow protrudes into cladding. Another possible reason for flaking are thermal effects that I think cannot be neglected in nanoseconds..... Though some practical formula that may help to minimize flaking may exist....
Thanks for your prompt answers - it helped!
We added a phase plate into the TEM00 beam and got a soft flat top. That was coupled into a multimode fiber whitch is some 10m long and coiled in order to mix to modes. At the output we use a 4f transformation with a spatial filter (pin hole) and a high performance optic to focus to the target. All this together gives us a nice and sharp flat top – and as you mentioned – it correlates with the number of modes one can create in the fiber. Hence, as bigger the fiber core as better the result. Off cores all is limited by the aperture of the following optics.
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