Optical Fibre Mode Structures Measured by Collection Mode Near-Field Scanning Optical Microscopy

High order fibre mode structures are measured by a near-field scanning optical microscope (NSOM) and are determined to be consistent with the standard fibre LP patterns. The mode-field distortions of high-order mode LP₁₁ and LP₂₁ structures by far-field measurement are analysed based on scalar diffraction theory, and the max distortions may reach as high as 5.1% and 6.2%. This shows that the method of using the NSOM is more accuracy than the far-field measurement method, especially for high-order mode structures of optical fibres.