In the 1970’s researchers demonstrated air-glass fiber profiles as a possible approach to low-loss optical fiber. A few years ago, interest in air-glass fiber profiles was rekindled by the idea of creating two-dimensional photonic band gaps in optical fibers. Typically, these optical fibers have a collection of air holes that run along the length of the fiber. The discovery of a number of interesting physical effects, some not necessarily associated with photonic band gaps, has blossomed into the field of Photonic Crystal Fiber. Along with that, a potentially confusing series of monikers has developed to distinguish between the various embodiments. Fortunately, the physics describing the fibers conveniently separates them into two distinct classes, those employing photonic band gaps for guidance and those that use a type of total internal reflection for guidance. The terms holey fiber, hole-assisted fiber, microstructured fiber, and effective-index fiber refer to fibers that employ total internal reflection as the guidance mechanism. Photonic band-gap fiber, Bragg fiber, and omnidirectional waveguide refer to fibers that use photonic band gaps as the guidance mechanism. We have used the term photonic crystal fiber (PCF), the highlight of this Focus Issue, to represent all these types of fibers. Though not all of the fiber profiles contain extended periodic structures normally associated with crystal structures, most all do possess a degree of regularity in the fiber profile that imparts mechanical and optical advantage.

The authors of this Focus Issue were selected from some of the leading research groups in the field to present their latest results in the area of PCF in order to capture a snapshot of the state-of-the-art in the field. Submissions include a mix of theoretical and experimental presentations, as well as a combination of the two classes of PCF.

PCF technology could yield some advantage over conventional fibers in the areas of polarization, dispersion, nonlinearity, and attenuation of the optically guided fields. The first two papers show how polarization and dispersion properties different from conventional fiber can be achieved in PCF, without sacrificing optical loss. Suzuki et al. and Hasegawa et al. present some of the lowest PCF losses reported to date.

Calculating the dispersive and other optical properties of these fibers has represented a new challenge for optical fiber designers, due to the lack of rotational invariance and the high refractive-index contrast between the air and glass. Ferrando et al. reports approaches to calculating and designing the dispersive properties of PCF.

Highlighting the nonlinear capabilities of PCF are two papers by Eggleton et al. and Furusawa et al., which demonstrate how this new technology is finding its way into device applications.

In photonic band-gap PCF, photonic band gaps are used to confine light, with no requirement on the refractive index of the region the light is guided in. This could potentially lead to fibers with dramatically lower loss than the best conventional fibers of today. The last three papers examine some of the aspects of low-index-core PCF. White et al. discusses the application of a multipole scattering method to analyzing air-core PCF. Ouyang et al. and Johnson et al. discuss the rotationally invariant incarnations of band-gap fiber.

Karl W. Koch

Corning Incorporated, Corning, NY

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