4. Promise and SlgniRcance of MPOFs Microstructured optical fibres fabricated in glass have generally been made by capillary stacking techniques, which result in a hexagonal arrangement of the holes. The large range of processing options available for polymers, including casting, casting around capillaries, extrusion etc. in addition to the thermal fusing of capillary stacks, means that potentially a much larger range of hole structures is possible. In addition, drawing of polymer fibres causes an alignment of the polymer chains, which protects the hole Structure from collapse. This means that the drawing process is quite robust, which may make it easier to produce non~circular holes, and potentially band-gap fibres, than in the case of glass micrortructured fibres. Polymers can be tailored in terms of composition to a much larger degree than glass fibres. Doping options for glass are limited by the need to avoid phase separation, and use of materials that do not decompose at glass processing temperatures (about 2000'C). In polymers, the low processing temperatures (10&25U0C) mean that organic materials such as dyes or non-linear chromophores can be easilyincluded without decomposition. In addition to doping, the polymer structure can be modified by grahing to allow large mass fractions of the desired material. Surfactant techniques also allow relatively large quantities of inclusions to be added. Examples of the types of materials that could be used in MPOF are: polymers with enhanced non-lineuities, electio or magneto-optic effects, metallic or rare-earth inclusions, birefringent materials such as liquid crystals, photorefractive and photochromic materials, dyes, polymers used in the detection of particular compounds and porous materials. The polymers can be specifically designed to allow the fabrication of particular fibreoptic components based on MPOF. MPOFs can be fabricated from a single polymer, without the need for dopants to modify the refractive index. As a result, a much larger range ofpolymers is available for MPOF, includingcondensation polymers, catalyrically formed polymers, biopolymers, sol-gel polymers and chain addition polymers. In addition, by using closely spaced hales that are small compared to the optical wavelength, virtually any refractive index profile can be obtained as the spatial average of the polymer-air matrix, without the use of any dopants.'