Laser beam transformation utilizing the effect of multimode interference in multimode (MM) optical fiber is
thoroughly investigated. When a Gaussian beam is launched to an MM fiber, multiple eigenmodes of the MM fiber are
excited. Due to interference of the excited modes, optical fields that vary with the MM fiber length and the signal
wavelength are generated at the output facet of the MM fiber. Diffractive propagation of these confined fields can yield
various desired intensity profiles in free space. Our calculations show that, an input fundamental Gaussian beam can be
transformed to frequently desired beams including top-hat, donut-shaped, taper-shaped, and low-divergence Bessel-like
within either the Fresnel or the Fraunhofer diffraction range, or even in both ranges. Experiments on a monothic fiber
beam transformers consisting of a short piece of MM fiber (~ 10 mm long) and a single-mode signal delivery fiber were
carried out. The experimental results indicate the functionality and high versatility of this simple fiber device. The
performance of this fiber device can be easily and widely manipulated through parameters including the ratio between
the core diameters of the SM and MM fiber segments and the length of the MM fiber segment. In addition, the intensity
profile of the output beam can be controlled by tuning the signal wavelength even after the fiber device is fabricated.
Most importantly, this technique is highly compatible with the technology of high power fiber lasers and amplifiers and
fiber delivery systems.