An optical fiber has a core with a diameter d.sub.c and a reflective index
n.sub.c ; a primary cladding concentrically surrounding the core and
having an outer diameter d.sub.pc and a refractive index n.sub.pc less
than the core refractive index n.sub.c ; and a secondary cladding
concentrically surrounding the primary cladding and having an outer
diameter d.sub.sc and a refractive index n.sub.sc less than the primary
cladding refractive index n.sub.pc.
