Final Activity Report Summary - FVFECPCF (Full vectorial Finite Element Characterisation of Photonic Crystal Fibres) Photonic crystal fibre (PCF) is a microstructured fibre where an array of holes runs along the length of the waveguide. This special class of optical silica optical waveguide was first introduced by the research group of Bath University, United Kingdom. In simple optical silica fibre a silica core (high index region) is surrounded by a cladding (low index material). In the photonic crystal fibres, the cross section of fibre has a core, where the cladding is made up of an array of air-holes embedded in the same silica material. Depending upon whether the core is made up of a high index region or low index air, the waveguide is called a 'holey fibre' and a photonic band-gap PCF respectively. For the guidance of light, the arrangement of holes was initially thought to be necessarily in a periodic nature. However, in holey fibres where the air-holes are arranged around a silica core, it has been proved that the holes can be of different sizes and can be arranged in an aperiodic manner. Today a wide range of numerical methods are available to study the modal characteristics of a PCF with perfect periodic cladding. Instead, any aperiodic / periodic structures with different holes, shapes and different materials can be accurately determined by using the finite element method. City University has been the pioneer group in introducing the finite element method for characterisation of optical waveguide. The research group led by Prof. Azizur Rahman has developed a full vectorial finite element modal solution approach to understand the light guidance in photonic crystals fibres under the project entitled FVFECPCF. The full vectorial finite element method can be run in powerful computer work stations as well as in the more widely available compact laptop, and hence a convenient research and industrial modelling tool. Its run time and computational memory efficiency arises by the method of handling sparcity of finite element matrices in a numerical calculation. Single mode operation in optical cable is preferable in the industrial telecom sector. Parameters such as modal spot-size, modal loss and hybridness are some important optical parameters that are determining the quality of transmission. Under the project FVFECPCF, the single mode operation of photonic crystal fibre has been fully investigated and hence the corresponding geometrical parameters for different wavelength has determined. Single polarisation single mode PCF with higher modal birefringence can be regarded as preserving the nature (polarization state) of the data. The parameters relating to the quality of the light guidance and correspondingly the geometric design of the PCF parameters were discussed in detail and results presented in conferences. Project FVFECPCF has also investigated PCF applications in the Terahertz region. The Terahertz region has recently found major application in the processes of security scanning and drug identification replacing the need for carcigenous X-ray devices. Targeting the compact THz scanning devices is a future promise in this technology. Presently the hurdle in this technology is that though THz sources and detectors are available, there is a lack of connectors / cables / waveguides. Project FVFECPCF has examined PCF serving similar role to optical cables but in terahertz region. Under the project FVFECPCF, a sophisticated boundary condition -the perfectly matched layer (PML) has been incorporated around the PCF structure. The modal loss of a guided mode in a PCF is due to the combination of the material loss and the leakage loss. The material loss arises due to the complex refractive indices of the material of the PCF, and the leakage loss arises due to the modal index being lower than the surrounding high index cladding regions. With the inclusion of PML layer it has been possible to determine the leakage and bending losses of the PCF in the optical and Terahertz region. In conclusion, a rigorous full vectorial finite element method has been developed and various applications of photonic crystal fibres as single mode and single polarisation fibres and THz-PCF has been investigated, with useful outcomes for telecommunication, securities and scanning technologies.
