Photonic Bandgap Fiber Eyed for Telecommunications
Researchers at BlazePhotonics Ltd. and at the University of Bath, both in Bath, UK, have fabricated a new low-loss photonic bandgap fiber. Its attenuation of 1.72 dB/km represents a significant improvement over the lowest loss previously reported for such a fiber, 13 dB/km. The development brings the attenuation of the new fibers a giant step closer to the ~0.15 dB/km of conventional telecom optical fibers and emphasizes their potential for long-haul data transmission.
Unlike conventional optical fibers, photonic bandgap fibers do not guide light by total internal reflection but rely on a photonic bandgap in the fiber's cladding. In a close analogy to the electronic bandgap in a semiconductor, a photonic bandgap occurs in a periodic optical material when the periodicity prohibits solutions to Maxwell's equations for certain wavelengths. The result is that photons whose energy falls within the bandgap cannot exist in the material. Photons impinging on the material cannot enter it, so they must be reflected. A photonic bandgap fiber consists of a hollow core surrounded by a cladding whose periodicity creates a bandgap for the photons guided in the fiber's core (Figure 1).
Figure 1. A scanning electron microscope image of the photonic bandgap fiber shows the hollow core and the periodic structure of the cladding.
The researchers fabricated their fiber by the "stack and draw" technique, in which hollow glass capillaries are painstakingly stacked in a periodic array, and then the capillaries in the center of the array are removed to create a preform. The preform is heat-softened, and a fiber is drawn from it by conventional techniques.
Photonic bandgap fibers are of intense interest for telecom applications because their optical characteristics -- e.g., birefringence and dispersion -- can be tailored readily by fiber design, and because they have the potential to have lower loss than conventional glass fiber. The ~0.15-dB/km loss in conventional fiber cannot get much lower because it is caused by fundamental processes: Rayleigh scattering at short wavelengths and multiphoton absorption at long wavelengths. These losses become trivial in a photonic bandgap fiber because most of the optical power propagates in air, not in glass. The normalized field intensity in the fiber has its first minima at the core-cladding interface (Figure 2), and approximately 98.3 percent of the optical power is in the hollow core. An additional 1 percent of the power is in the cladding's airholes, and only 0.7 percent propagates in glass.
Figure 2. The normalized field intensity at the end of 500 m of fiber shows the first dip at the core-cladding interface.
The dominant loss mechanisms in a photonic bandgap fiber result from scattering and from coupling energy from the fundamental core mode to other core and cladding modes. These effects are caused primarily by geometric irregularities along the fiber's length. Realizing this, the UK researchers took pains to fabricate the fiber extremely uniformly, especially the two rings of capillaries nearest the core. They believe that this is the main reason they were able to measure attenuation as low as 1.72 dB/km at 1565 nm -- the upper edge of the C-band.
They calculate that the attenuation of their fiber should drop to 0.95 dB at 1950 nm, because the dominant loss mechanisms -- mode coupling and scattering -- scale with the inverse cube of the wavelength. They believe that photonic bandgap fibers ultimately will have losses well below those of conventional fibers, but that the lowest losses will occur somewhere between 1800 and 2000 nm -- a spectral region where erbium-doped fiber amplifiers are ineffectual.
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