Patent ID: 8068705

Claim:
An optical fiber extending along a longitudinal fiber axis and comprising: at least one cladding centered along the fiber axis; and a monolithic elongated multimode (MM) core concentric with and surrounded by the one cladding, the monolithic MM core being configured with a double bottleneck-shaped cross section having an input region which receives a single mode SM input beam that excites a fundamental mode distortionlessly propagating along the MM core, the single and fundamental modes having respective Gaussian intensity profiles with respective mode field diameters which substantially match one another in the input region, the MM core being configured with a refractive step index profile having a centrally located dip which is centered on an axis of symmetry of the MM core and gradually enlarges between the input and central regions of the core so as to transform the Gaussian intensity profile of the fundamental mode into a two-peak shaped profile of the fundamental mode in a central region of the MM core, and further narrows towards an output region of the MM core so as to transform the two-peak shaped intensity profile of the fundamental mode into substantially Gaussian intensity profile thereof.