Multicore optical fiber with depressed index common cladding

A multicore optical fiber, comprising:

• a depressed index common-cladding region having a refractive index Δcc;
• and
• a plurality of core portions disposed within the depressed index common-cladding region, wherein each core portion comprises:
• a central axis,
• a core region extending from the central axis to an outer radius r1, the core region comprising a relative refractive index Δ1 relative to pure silica,
• an inner-cladding region encircling and directly contacting the core region and extending from the outer radius r1 to an outer radius r2, the inner cladding region comprising a relative refractive index Δ2 relative to pure silica,
• a trench region encircling and directly contacting the inner cladding region and extending from the outer radius r2 to an outer radius r3, the trench region comprising a relative refractive index Δ3 relative to pure silica and having a trench volume greater than 20% Δmicron2 and less than 75% Δmicron2, and
• an outer-cladding region encircling and directly contacting the trench region and extending from the outer radius r3 to an outer radius r4 and comprising a relative refractive index Δ4 relative to pure silica,
• wherein the refractive index of the depressed index common-cladding region Δcc is less than the refractive index of the outer-cladding region Δ4, and
• wherein a difference between the refractive index of the depressed index common-cladding region Δcc and the refractive index of the first outer-cladding region Δ4 is greater than 0.05% Δ.