Applications requiring uniform power distribution (i.e. surface treatment, re-cladding, etc) will generally utilize a step index fiber which has a uniform index of refraction within the core and delivers an output beam with a power distribution resembling a "top hat". For deeper material interaction such as cutting or welding, gradient Index fiber with an index of refraction that transitions from highest at the core center, decreasing in a "Gaussian" profile outside toward the cladding, assists in preserving high-brightness input spots. Gradient index fiber produces a spot which has a higher central power density than with step index fiber.

Other considerations play an important role in selection of the fiber to be used. Laser sources vary considerably in beam quality, power density, stability, etc. For any given application to perform reliably, the optical characteristics of the laser source will have to be accommodated. Generally, higher power lasers suffer a trade off in beam quality as power levels are increased. For example, a highly multimode beam will not focus into as small of a fiber as a high-quality beam with low order mode structure. To get around this, designers can resort to fast (short focal length) lenses, but care must be taken because the spot produced by short focal length lenses will be larger relative to slower lenses.

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