Patent ID: 8218928

Claim:
An optical spatial mode filter for use with multimode input signals including a fundamental mode and at least one higher order mode, the spatial mode filter comprising: a graded index (GRIN) optical lens including an input and an output for receiving at the input a multimode input signal and exhibiting a radial refractive index profile for spatially separating the at least one higher order mode from the fundamental mode of an optical signal propagating along the optical axis thereof, wherein the GRIN optical lens comprises a quarter-pitch length L defined as L=π(2g), where g is a focusing parameter of the GRIN optical lens; 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, the small core defined as an aperture of the pinhole element, with the aperture disposed to align with the optical axis and the predetermined length of the small core optical fiber selected to permit further propagation of substantially only the fundamental mode of the multimode input signal along the optical axis.