One strength of photonic crystal fiber is its flexibility to be designed with desirable optical characteristics. A photonic crystal fiber’s dispersion, in particular, can be adjusted by modifying its geometrical design; therefore, the fibers seem to be appropriate for compensating the dispersion of conventional telecom fiber optic links. Recently, scientists at Tsinghua University in Beijing proposed a design that overcomes the narrow-bandwidth limitation of earlier photonic crystal fibers intended for dispersion compensation in telecom applications.Dispersion is problematic in fiber optic transmission lines because it causes the individual pulses to broaden temporally and, eventually, to overlap. The wavelength dependence of the fiber’s refractive index causes frequency components at one end of a pulse’s spectrum to propagate faster and those at the other end to propagate more slowly, so that the pulse spreads out during propagation over many kilometers of fiber. The common solution to this problem is to insert a length of dispersion-compensating fiber — fiber whose dispersion has the opposite sign from that of the transmission fiber. The dispersion-compensating fiber allows the trailing edge of the pulse to catch up with the leading edge, in effect compressing the pulse to an approximation of its original shape.Ideally, the dispersion-compensating fiber will exactly mirror the dispersion of the transmission fiber, but designing the ideal fiber is difficult. Photonic crystal fibers whose effective refractive index can be tailored by altering their microstructure allow greater design flexibility than conventional, solid fibers.Researchers at many laboratories have explored this flexibility and found that the best design is a symmetrical, dual-core photonic crystal fiber. Such a fiber has an inner core surrounded by an inner cladding. Concentric to this inner structure is a doughnut-shaped outer core, which is, in turn, surrounded by its own doughnut-shaped outer cladding.Controlling dispersionGuidance takes place within both the inner and outer cores, and at the phase-matching wavelength between their respective modes, energy can be coupled back and forth between the two. It is this coupling that produces the desirable dispersion characteristic for the mode traveling in the inner core. But, until now, this dispersion had a very narrow bandwidth because light was tightly contained within the inner core. Only at wavelengths very close to the phase-matching wavelength could energy be coupled between the cores.Figure 1. To weaken the strength of guidance in the inner core, scientists designed their proposed fiber with elongated airholes, resembling grapefruit sections, in the inner cladding (top). The profile of the fiber’s refractive index indicates that light is guided in both the inner and outer cores (bottom). Images reprinted with permission of Optics LettersTo overcome this, the scientists in Beijing weakened the strength of the inner core’s guidance with a novel fiber design (Figure 1) that allowed the light over a broad spectral range to couple from the inner core into the outer core, broadening the spectral range of the desirable negative dispersion.When they calculated the dispersion of their proposed fiber, they saw that it was broadband and almost linear with wavelength (Figure 2). Moreover, this calculation and others not shown here indicate that the shape of the curve is not particularly dependent on geometry; that is, the diameter of the airholes. That means that the fibers can be fabricated relatively easily because small variations in airhole diameter will not be detrimental to their effectiveness.Figure 2. The calculated dispersion (D) of the proposed fiber was broadband and almost linear with wavelength, and only its magnitude, not its shape, depended on the fiber’s geometry. These data assume d2 = 2 μm, and d1 varies as indicated. (As indicated in Figure 1, d1 and d2 are the diameters of the airholes.)Another factor pointing to easy fabrication is the size of the airholes, all of which are more than 1 μm in diameter. This avoids the inherent difficulty of drawing fiber with submicron holes. The usual requirement for “endlessly single mode” propagation in a hexagonal photonic crystal fiber whose core is a single missing airhole is d/Λ Optics Letters, Oct.1, 2006, pp. 2830-2832.