Fiber is built of one material (usually silica) and light guiding is obtained through the presence of air holes in the area surrounding the solid core.
There are two main classes of MOF Structured optical fibers, those based on channels running along their entire length go back to Kaiser and Co in 1974.
They are an attempt to incorporate the bandgap ideas of Yeh et al. in a simple way by stacking periodically a regular array of channels and drawing into fiber form.
An extremely important variant was the air-clad fiber invented by DiGiovanni at Bell Labs in 1986/87 based on work by Marcatili et al. in 1984.
[5] This is perhaps the single most successful fiber design to date based on structuring the fiber design using air holes and has important applications regarding high numerical aperture and light collection especially when implemented in laser form, but with great promise in areas as diverse as biophotonics and astrophotonics.