Low-loss coupling between single-core and multi-core fibers

A coupler comprising:

• a first end configured to optically couple with a plurality of single-core optical fibers;
• a second end configured to optically couple with a multi-core optical fiber;
• a plurality of cores that each extends from the first end to the second end,
• the plurality of cores comprising a first core such that an outer diameter of the first core at the first end is larger than an outer diameter of the first core at the second end,
• the first core comprising a relative refractive index Δ01maxA at the first end from 0.6% Δ to 0.8% Δ and the first core comprising a relative refractive index Δ1maxB at the second end from 0.5% Δ to 1.0% Δ relative to the refractive index of pure silica, and
• the first core comprising a mode field diameter, at 1550 nm wavelength, of 9.5 microns or greater;
• and
• an outer cladding surrounding the plurality of cores and extending from the first end to the second end such that an outer diameter of the outer cladding at the first end is larger than an outer diameter of the outer cladding at the second end, the outer cladding comprising a relative refractive index Δ0E from −0.20% Δ to 0.20% Δ at the first end and at the second end relative to the refractive index of pure silica,
• wherein the coupler comprises a single, contiguous, conical glass member that tapers from the first end to the second end.