Patent ID: 7590319

Claim:
An optical coupling method comprising: providing a first graded-index with no optical converging elements connected thereto; confronting an exit end of the first graded-index optical fiber to an incident end of a second graded-index optical fiber or a light-receiving device; and forming a gap between said exit end and said incident end, said gap being at least 200 μm; and further comprising a light propagating in an inner core of the optical fiber, said inner core having a refractive index distribution; and a light propagating in an outer core of the optical fiber surrounding said inner core and having a substantially constant refractive index, wherein the light propagates in said outer core which has an outer diameter in the range of 250 μm to 1000 μm, and wherein the light propagates in the outer and the inner cores with a ratio of a diameter of said inner core to said outer diameter being in the range of 0.67 to 0.87.