FIBEROPTICS
Honeycomb of holes guides lightA group of researchers at the University of Bath (Bath, England) has constructed an optical fiber made up of a two-dimensional (2-D) photonic-bandgap (PBG) structure that guides light in a fundamentally different way from that of ordinary fiber. Rather than guiding by total internal reflection, the fiber takes advantage of a property of 2-D PBG materials in which propagation of light in directions normal to the periodic plane can be forbidden for certain wavelength ranges.
The body of the fiber consists of a honeycomb array of holes forming air-filled channels that run the length of the fiber (see figure). A single honeycomb in the array has a hole in its center that defines the axis of the waveguide. Contrary to standard fiber, the effective refractive index of the waveguide is lower than the surrounding PBG material. The fiber is made by stacking hundreds of fused-silica rods and capillary tubes together and then drawing the resultant preform into a fiber 36 µm in diameter. Two sorts of holes are created: large holes of 0.8 µm in size that are the shrunken centers of the original capillary tubes and small holes of 75 nm in diameter, called interstitial holes, that are remnants of the gaps between the rods and capillary tubes.
Although the group has not yet accurately modeled the behavior of the PBG fiber, it has determined that both the air holes and the interstitial holes play a part in the fiber`s properties. The out-of-round shape of the large holes, as well as the tiny size of the interstitial holes, contributes to the difficulty of creating a good quantitative model. Small variations in the size of the holes have a large effect on the fiber`s properties, in some cases even causing the waveguiding to disappear. Although drawing a fiber with consistent properties presented a challenge, "we now have the problem licked," says Jonathan Knight, one of the researchers, adding that tests have been done on fiber sections of up to 1 m in length.
The fiber accepts light within a half-angle of 25 and produces a near-field intensity pattern consisting of six lobes. The far-field intensity pattern, also six-lobed, indicates that the fields in opposite lobes of the near-field pattern have opposite signs, contrary to what calculations show for a low-order bandgap. Only calculations that assume a higher-order bandgap produce a far-field pattern that agrees with the experiment. When illuminated with light of different wavelengths, the fiber`s bandgap properties become apparent: it guides light with wavelengths of 458 through 528 nm, but not 633 nm. With a beat length of a few millimeters, the fiber is very birefringent. Dispersion of the PBG material, though not yet measured, is predicted to be very large.
"It`s a bit early to talk about practical uses," says Knight. "The development of this sort of fiber is at the stage where conventional fiber was in the 1960s."