A microstructured optical fiber is structured with a core region and a plurality
of cladding regions surrounding the core region. Chromatic dispersion is relatively
flat over a broad wavelength range with a small effective area, without need for
air holes that are unacceptably small. An enlarged effective core area is obtained
by selectively closing the holes in the innermost cladding, which holes are smaller
and easier to close than the holes in other regions.
