An optical fiber has a core, an inner cladding and an outer cladding on. In a
first region the inner cladding refractive index has a first value which differs
from the core refractive index by about 0.0045. The inner cladding refractive index
changes along said fiber from the first value to a second value at a free-space
end. The second value matches the core refractive index, so that the effective
diameter of the core at the free-space end is that of the core plus the inner cladding.
Hence the outer cladding acts to constrain the light in the end region. The spot-size
and the V number (normalized frequency) are increased, thereby increasing offset tolerances.
