Photonic Crystal Fibre

Introduction

Photonic crystal fibre (PCF) refers to a new category of optical fibre that is based on the characteristics of photonic crystals. Due to its confinement characteristics or capability of confining light in hollow cores, which is not achievable with usual optical fibre, photonic crystal fibre is being used in fibre lasers, communications involving fibre optics, high-power transmission, nonlinear devices, and highly sensitive gas sensors among others (Bjarklev, Bjarklev, & Broeng, 2003). Specific types of photonic crystal fibre include hole fibre, photonic-bandgap fibre and Bragg fibre. Photonic-bandgap fibre refers to PCFs, which use band gap effects to confine light. Holey fibres refer to PCFs that utilize air holes in their cross sections. They are also referred to as hole-assisted fibres. According to Bjarklev, Bjarklev & Broeng (2003), holey fibres are PCFs that guide light by a usual higher-index core that is adapted to the presence of air holes. Bragg fibre is photonic-bandgap that is formed by circular multilayered film. PCFs might be considered a subcategory of a general category of microstructured optical fibres, which uses not only a refractive index difference but also structural modifications to guide light (Jiang, 2008). In this regard, this paper describes PCFs and discusses its construction and modes of operation.

Description of Phonic Crystal Fibres

With the highly structured cross-section of air spaces and glass, PCFs or microstructured fibres are the ultimate specialty fibre. Despite PCFs recently becoming commercially available, various types are already available to support the fast growing range of applications (Khan, 2008). According to Bjarklev, Bjarklev & Broeng (2003), optical fibres seem to have undergone evolution into various forms since the sensible breakthroughs that resulted in the wider introduction in the 1970 as usual step index fibres and eventually as solitary material fibres. In solitary material fibres, propagation was achieved by efficient air cladding structure. Regular structured fibres, like photonic crystal fibres, generally have a normally uniform cross-section that is microstructured from more one or more than one material (Kiliç, 2008). According to Kiliç (2008), the materials are usually arranged periodically over the cross-section of the fibre as a cladding that surrounds the core in which light is confined. For instance, the first demonstrated photonic crystals comprised of hexagonal pattern of air holes in silica fibre (Jiang, 2008). In 1996, the fibre had a solid core, though it was changed to a hollow core in 1998, at the center where light was confined. Other descriptions included concentric rings made of more than one material (Khan, 2008).

Bragg fibres and fibre Bragg gratings should not be confused. This is because they are different (Bjarklev, Bjarklev & Broeng, 2003). Fibre Bragg gratings comprise of structural variation or intervallic refractive index along the axis of the fibre, as opposed to the difference in the oblique directions, in the photonic crystal fibre. Both fibre gratings and PCFs use the phenomena of Bragg diffraction, although in varying directions. The lowest recorded attenuation of solid core PCF is 0.37 dB/km (Bjarklev, Bjarklev & Broeng, 2003).

Construction of Photonic Crystal Fibres

Photonic fibres are usually constructed by similar methods to those used in the construction of other optical fibres. This method first includes constructing a preform on a scale of centimetres in size (Kiliç, 2008). The constructed preform is then heated and drawn down to a smaller thickness. The thickness is frequently as smaller as that of human hair. However, the features of the fibre are still maintained during the shrinking process (Jiang, 2008). As such, hundreds of kilometers can be manufactured from a single preform. The most popular method of constructing photogenic fibre is stacking, though milling or drilling was also used to construct the oldest design. According to Bjarklev, Bjarklev & Broeng (2003), this formed the consequent basis for constructing the first polymer and soft glass structured fibres.

Jiang (2008) pointed out that most of the photonic crystal fibres have been manufactured with silica. Nevertheless, other glasses have also been utilized in obtaining certain optical properties, including the high optical non-linearity (Bjarklev, Bjarklev, & Broeng, 2003). There seems to be an increasing interest in the construction of photonic crystal fibre from polymer (Kiliç, 2008). This interest has resulted in the wide exploration of various structures, such as ring structured fibres, graded index structures, and hollow core fibres. The three structures have been referred to as microstructured polymer optical fibres (MPOF) (Bjarklev, Bjarklev & Broeng, 2003). An amalgamation of chalcogenide glass and polymer has also been used for wavelengths of 10.6 micrometer, especially where the silica is not transparent (Bjarklev, Bjarklev & Broeng, 2003).

Modes of Operation

According to Bjarklev, Bjarklev & Broeng (2003), the modes of operation of photonic crystal fibre can be categorized into two distinct operational modes. Bjarklev, Bjarklev & Broeng (2003) pointed out that these modes are based on the confinement mechanism used by the photonic crystal fibre. PCFs with a core having a higher average index than the microstructured shell operate using the index guiding principle, which is also used in conventional optical fibre. Such PCFs that operate based on this principle can exhibit a higher effective refractive index between the two layers of the core and the cladding (Jiang, 2008). As a result, PCFs operating based on this principle can exhibit much robust confinement for applications in polarization-maintaining fibres and nonlinear optical devices.

The second mode of operation can be based on the creation of photonic bandgap. In this mode of operation, light is guided by photonic bandgap that is fabricated by the microstructured shell or cladding. According to Khan (2008), such a bandgap can confine light in a hollow air core or even a lower-index core if effectively designed. Khan (2008) cited that bandgap fibres having hollow cores can possibly avoid the limits imposed by the present materials in order to create fibres, which confine light in wavelengths for which materials are not available. According to Bjarklev, Bjarklev & Broeng (2003), this is because the light is majorly in the air, but not in the solid material. The hollow core has another advantage of enabling one to introduce dynamically materials into the core (Bjarklev, Bjarklev & Broeng, 2003). Materials that can be introduced into the hollow core include gases that are to be tested for the availability of any substance (Bjarklev, Bjarklev, & Broeng, 2003). Photonic crystal fibre can also be modified by covering the holes with sol-gels of different or similar index material in order to increase the light transmittance (Jiang, 2008).

Conclusion

Photonic crystal fibre (PCF) refers to a new category of optical fibre that is based on the characteristics of photonic crystals. The three types of photonic crystal fibre include hole fibre, photonic-bandgap fibre and Bragg fibre. Despite PCFs recently becoming commercially available, various types are already available to support the fast growing range of applications. Regular structured fibres, like photonic crystal fibres, usually have a normally uniform cross-section that is microstructured from more one or more than one material. Photonic fibres are usually generally constructed by similar methods to those deployed in the manufacture of other optical fibres. Most of the photonic crystal fibres have been manufactured with silica. The modes of operation of photonic crystal fibre can be categorized into two distinct operational modes.

 

 

 

 

 

 

 

 

 

References

Bjarklev, A., Bjarklev, A., & Broeng, J. (2003). Photonic crystal fibres. London: Springer.

Jiang, R. (2008). Parametric band translation using highly-nonlinear and photonic crystal fibers. California: ProQuest.

Khan, K. (2008). Numerical modeling of wave propagation in nonlinear photonic crystal fiber. Florida: ProQuest.

Kiliç, O. (2008). Fiber based photonic-crystal acoustic sensor. California: ProQuest.