Spatial Filtering of Higher Order Modes in Multimode Fibers

An optical spatial mode filter for use with multimode input signals including a fundamental mode and at least one higher order mode, the spatial filter comprising

• a graded index (GRIN) optical lens including an input and an output, the GRIN lens for receiving the multimode input signal at the input and exhibiting a refractive index profile for spatially separating the at least one higher order mode from the fundamental mode of an optical signal propagating along an optical axis thereof;
• and
• a pinhole element coupled to the output of the GRIN optical lens, the pinhole element comprising a section of small core optical fiber of a predetermined length and having a core defined as an aperture, with the aperture disposed to align with the GRIN lens optical axis, the predetermined length being a minimum length for stripping away any higher order modes and permitting further propagation of substantially only the fundamental mode of the multimode input signal.