Martelli and Canning realised that the crystal structures that have identical interstitial regions are actually not the most ideal structure for practical applications and pointed out aperiodic structured fibres, such as Fractal fibres, are a better option for low bend losses. Aperiodic fibres are a subclass of Fresnel fibres which describe optical propagation in analogous terms to diffraction free beams.These too can be made by using air channels appropriately positioned on the virtual zones of the optical fibre
DescriptionOptical fibres have evolved into many forms since the practical breakthroughs that saw their wider introduction in the 70s as conventional step index fibres and later as single material fibres 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 fibres such as Bragg fibres. This necessarily resulted in fibre design centred on controlling the near field of an optical mode along this distance in order to control its propagation and attempt to, in most cases, retain its properties after travel.
Photonic crystal fibres are a variant of the microstructured fibres 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 fibre form. The first such fibres did not propagate by such a bandgap but rather by an effective step index - however, the name has for historical reasons, remain unchanged although some researchers prefer to call these fibres "holey" fibres 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" fibres. An extremely important variant was the air-clad fibre 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 fibre design to date based on structuring the fibre 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. Fibres 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 fibre.These Fresnel fibres 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 fibres with propagation losses below that of step-index fibres.
ConstructionGenerally, such fibers are constructed by the same methods as other optical fibers: first, one constructs a "preform" on the scale of centimeters in size, and then heats the preform and draws it down to a much smaller diameter (often nearly as small as a human hair), shrinking the preform cross section but (usually) maintaining the same features. In this way, kilometers of fiber can be produced from a single preform. The most common method involves stacking although drilling/milling was used to produce the first aperiodic designs. This formed the subsequent basis for producing the first soft glass and polymer structured fibres.
Most photonic crystal fiber has been fabricated in silica glass, but other glasses have also been used to obtain particular optical properties (such has high optical non-linearity). There is also a growing interest in making them from polymer, where a wide variety of structures have been explored, including graded index structures, ring structured fibres and hollow core fibers. These polymer fibers have been termed "MPOF", short for microstructured polymer optical fibers (van Eijkelenborg, 2001). A combination of a polymer and a chalcogenide glass was used by Temelkuran et al. (2002) for 10.6 µm wavelengths (where silica is not transparent).
Modes of OperationPhotonic crystal fibers can be divided into two modes of operation, according to their mechanism for confinement. Those with a solid core, or a core with a higher average index than the microstructured cladding, can operate on the same index-guiding principle as conventional optical fiber — however, they can have a much higher effective- refractive index contrast between core and cladding, and therefore can have much stronger confinement for applications in nonlinear optical devices, polarization-maintaining fibers, (or they can also be made with much lower effective index contrast). Alternatively, one can create a "photonic bandgap" fiber, in which the light is confined by a photonic bandgap created by the microstructured cladding — such a bandgap, properly designed, can confine light in a lower-index core and even a hollow (air) core. Bandgap fibers with hollow cores can potentially circumvent limits imposed by available materials, for example to create fibers that guide light in wavelengths for which transparent materials are not available (because the light is primarily in the air, not in the solid materials). Another potential advantage of a hollow core is that one can dynamically introduce materials into the core, such as a gas that is to be analyzed for the presence of some substance. PCF can also be modified by coating the holes with sol-gels of similar or different index material to enhance its transmittance of light.
Ricardo Monroy C.I.17646658