Condition for realizing single-mode operation and energy distribution characteristics in far-field of annular core fibers

Based on the weak guidance approximation, the condition for realizing single mode operation of annular core fiber (ACF) and the energy distribution in far-field are discussed. The theoretical results demonstrate that when an ACF realizes single-mode operation, the annular beam emitted from the end of ACF diffracts to a central circular area in the far-field with good energy focusability. Besides, the beam width gets smaller when the thickness of the guiding section and the refractive index difference between the cladding and fiber core are reduced.