Japanese engineers have reduced the signal losses of a new class of optical fibers to levels comparable to conventional telecommunication fibers.Called “holey” or “photonic crystal” fibers, these novel fibers use internal microstructures to guide light through them (see The Next Generation of Optical Fibers). In contrast, conventional optical fibers depend on total internal reflection of light in a glass central core surrounded by a cladding with a lower refractive index.
At last month’s Optical Fiber Communications Conference in Anaheim, CA, a team from Sumitomo Electric and Hokkaido University reported a dramatic improvement in signal attenuation for photonic crystal fibers-to a mere 0.8 decibels per kilometer, only about four times as much as in the best conventional optical fibers.
Photonic Band Gaps
When very thin parallel layers of two materials with different refractive indexes are stacked on top of each other, reflections between the layers can block transmission of light at certain wavelengths.
A decade ago, Eli Yablonovitch of the University of California at Los Angeles developed this concept into the idea of a “photonic band gap”-a range of wavelengths that cannot travel through the material. (This is an optical analog of the electronic band gap found in semiconductors-a range of electron energies at which currents can’t flow through semiconductors. In principle, photonic band gaps can control the flow of light in an optical device just as electronic band gaps control current in a semiconductor.)
Researchers have found that arrays of regularly spaced parallel holes can create internal microstructures that make photonic band gaps.
Optical Fiber Fabrication
Several groups have taken this approach to make optical fibers. The teams stack together glass rods or tubes with internal holes and fuse them together, then stretch them into long, thin fibers with parallel holes running along their lengths.
To guide light, microstructured fibers must have a central hole or solid region that can transmit light that is blocked by the surrounding photonic-band-gap material. This structure traps the light so it can only travel through the center of the fiber.
True photonic-band-gap guidance can trap light within a hollow core, where the refractive index is lower than in the surrounding material. This is impossible in conventional fibers, says Philip Russell of the University of Bath in Britain.
Russell has shown that removing a group of seven closely packed tubes from the core can produce a holey fiber that guides light through air-something otherwise impossible.
Guiding light through air could avoid certain characteristics of glass that limit data rates. Clear air also is more transparent than glass, so in theory, signals could travel farther through hollow fibers before requiring amplification.
But previous holey fibers have suffered very high signal losses. The best of them had attenuation of 240 decibels per kilometer, more than a thousand times the 0.2 decibels per kilometer of the most transparent conventional optical fibers.
Takemi Hasegawa and five colleagues at Sumitomo Electric and Hokkaido University blamed losses in previous holey fibers on absorption by bonds between hydrogen and oxygen atoms along the holes, probably due to traces of moisture.
To limit this loss, they designed fibers that would keep most light in the glass and tried to keep the holes as clean as possible. Their “hole-assisted lightguide fiber” uses a small array of only four holes to confine light in a central core with a higher refractive index than the surrounding glass.
Although this design retains the high-index core of conventional fibers, the holes change how light pulses spread as they travel through the fiber, giving it characteristics which Hasegawa says can’t be achieved with conventional fibers.