Photonic-crystal Fiber - Description

Description

Optical fibers have evolved into many forms since the practical breakthroughs that saw their wider introduction in the 1970s as conventional step index fibers and later as single material fibers where propagation was defined by an effective air cladding structure. However, no matter how exotic the method of propagation, and no matter the material system, the sole driver for most that time has been the transportation of light from one point to another, whether by step-index confinement determined by simple Fresnel reflections or by coherent Fresnel reflections in bandgap fibers such as Bragg fibers. This necessarily resulted in fiber design centered on controlling the near field of an optical mode along this distance to control its propagation and attempt to, in most cases, retain its properties after travel.

Photonic crystal fibers are a variant of the microstructured fibers reported by Kaiser et al. They are an attempt to incorporate the bandgap ideas of Yeh et al. in a simple way by stacking periodically a regular array of channels and drawing into fiber form. The first such fibers did not propagate by such a bandgap but rather by an effective step index – however, the name has, for historical reasons, remained unchanged although some researchers prefer to call these fibers "holey" fibers or "microstructured" optical fibers in reference to the pre-existing work from Bell Labs. The shift into the nanoscale was pre-empted by the more recent label "structured" fibers. An extremely important variant was the air-clad fiber invented by DiGiovanni at Bell Labs in 1986/87 based on work by Marcatili et al. in 1984. This is perhaps the single most successful fiber design to date based on structuring the fiber design using air holes and has important applications regarding high numerical aperture and light collection especially when implemented in laser form, but with great promise in areas as diverse as biophotonics and astrophotonics.

In general, regular structured fibers such as photonic crystal fibers, have a cross-section (normally uniform along the fiber length) microstructured from one, two or more materials, most commonly (because its easier to make) arranged periodically over much of the cross-section, usually as a "cladding" surrounding a core (or several cores) where light is confined. For example, the fibers first demonstrated by Russell consisted of a hexagonal lattice of air holes in a silica fiber, with a solid (1996) or hollow (1998) core at the center where light is guided. Other arrangements include concentric rings of two or more materials, first proposed as "Bragg fibers" by Yeh and Yariv (1978), a variant of which was recently fabricated by Temelkuran et al. (2002) and others.

More interestingly, has been the recognition that the periodic structure is actually not the best solution for many applications. Fibers that go well beyond shaping the near field now can be deliberately designed to shape the far-field for the first time, including focusing light beyond the end of the fiber. These Fresnel fibers use well known Fresnel optics which has long been applied to lens design, including more advanced forms used in aperiodic, fractal, and irregular adaptive optics, or Fresnel/fractal zones. Many other practical design benefits include broader photonic bandgaps in diffraction based propagating waveguides and reduced bend losses, important for achieving structured optical fibers with propagation losses below that of step-index fibers.

(Note: PCFs and, in particular, Bragg fibers, should not be confused with fiber Bragg gratings, which consist of a periodic refractive index or structural variation along the fiber axis, as opposed to variations in the transverse directions as in PCF. Both PCFs and fiber Bragg gratings employ Bragg diffraction phenomena, albeit in different directions.)