This structural arrangement creates an internal microstructure, which offers another
dimension of light control in thefiber compared to a conventional solid-corefiber.
The arrangement, size, and spacing (known as thepitch) of the holes in the
microstructure and the refractive index of its constituent material determine the
light-guiding characteristics of photonic crystalfibers. Depending on the PCF
structure, light is guided along thefiber by either total internal reflection or by a
photonic bandgap effect.
The fact that the core can be made of pure silica, gives the PCF a number of
operational advantages over conventional fibers, which typically have a
germanium-doped silica core [ 5 ]. These include very low losses, the ability to
transmit high optical power levels, and a strong resistance to darkening effects from
nuclear radiation. Thefibers can support single-mode operation over wavelengths
ranging from 300 nm to more than 2000 nm. The modefield area of a PCF can be
greater than 300μm^2 compared to the 80-μm^2 area of conventional single-mode
fibers. This allows the PCF to transmit high optical power levels without
encountering the nonlinear effects exhibited by conventionalfibers.
For biophotonics applications, some hollow-core PCFs are being used in place
of the coated hollow-corefiber mentioned in Sect.3.8for delivering mid-infrared
light with a broadened transmission window and reduced transmission or bending
loss. Photonic crystalfibers also are being used in a wide variety of optical
fiber-based biosensors. Chap. 7 discusses several of these biosensor applications.
3.10 Plastic Fibers.
Aplastic opticalfiberorpolymer opticalfiber(POF) is an alternative to glass
opticalfibers in areas such as biomedical sensors [ 36 – 40 ]. Most POFs are made of
polymethylmethacrylate (PMMA) with a refractive index of around 1.492, which is
slightly higher than the index of silica [ 5 ]. The size of a POF is normally larger than
typical silicafibers, with diameters ranging up to 0.5 mm. In addition to POF with
large core diameters, currently both multimode and single-mode (SM) plastic
opticalfibers are commercially available. The standard core sizes of multimode
POF include 50-μm and 62.5-μm diameters, which are compatible with the core
diameters of standard multimode glass telecomfibers.
The development of SM POF structures enables the creation of FBGs inside of
plasticfibers, which therefore provides more possibilities for POF-based biosensing
[ 5 ]. Together with the advantages of low cost, inherent fracture resistance, low
Young’s modulus, and biocompatibility, POF has become a viable alternative for
silicafibers in the areas of biomedical applications. For example, some specialfiber
sensor designs, such as the exposed-core technique now being used in PCF, were
first realized using polymer opticalfibers. Although most POFs have a higher
refractive index than silica, single-mode perfluorinated POF with a refractive index
of 1.34 has been fabricated. This low-index fiber allows the potential for
3.9 Photonic Crystal Fibers 79