Published in IET Optoelectronics Received on 1st May 2009 Revised on 4th September 2009 doi: 10.1049/iet-opt.2009.0033 Special Issue – Selected papers inspired by the Semiconductor and Integrated Optoelectronics (SIOE 2009) Conference ISSN 1751-8768 Coupling characteristics of a soft glass nematic liquid crystal photonic crystal fibre coupler M.F.O. Hameed 1 S.S.A. Obayya 1 K. Al Begain 1 A.M. Nasr 2 M.I. Abo el Maaty 2 1 Faculty of Advanced Technology, University of Glamorgan, Pontypridd CF37 1DL, UK 2 Faculty of Engineering, University of Mansoura, Egypt E-mail: sobayya@brunel.ac.uk Abstract: This study presents the coupling characteristics of a novel soft glass photonic crystal fibre (PCF) coupler infiltrated with a nematic liquid crystal (NLC–PCF). The soft glass provides optical properties that cannot be obtained by silica such as high refractive index, high rare earth solubility and mid-infrared transmission. In addition, the nematic liquid crystal (NLC) offers high tunability with the temperature and external electric field, and increases the birefringence between the two fundamental polarised modes in the proposed coupler. Therefore the NLC–PCF coupler has stronger polarisation dependence than the low birefringence conventional silica PCF coupler. The effects of the coupler geometrical parameters, rotation angle of the director of the NLC and temperature on the coupler performance are investigated. The analysis is carried out using the full vectorial finite difference method and the beam propagation study is performed by the full vectorial finite difference beam propagation method. The numerical results reveal that the polarisation dependence of NLC– PCF coupler is approximately 2.5 times that of the conventional silica PCF coupler with shorter coupling length. In addition, the NLC–PCF coupler has strong potential to offer practical design for short polarisation splitter as well as multiplexer–demultiplexer. 1 Introduction Photonic crystal fibres (PCFs) [1, 2] have been recently under intensive study, since they offer unique and uncommon properties which cannot be achieved in conventional optical fibres. PCFs can be endlessly single mode over a wide wavelength range [3], have a large effective mode area [4] and furthermore can be tailored to achieve nearly zero and flat dispersion over a wide range of wavelengths [5]. PCFs are usually made of pure silica with a periodic distribution of air holes in the cladding region. There are two types of PCF, high and low index core PCFs, which are very promising structures in terms of properties and possible applications. The high index core PCF [1] can be achieved by removing one or more of the central air holes. Therefore the effective index of the silica core is greater than that of the hole cladding which guarantees the index guiding through the PCF. On the other hand, if the central hole is enlarged (low index core) [2], the light can be guided by the photonic bandgap effect, which allows the confinement of the light in the low index core. However, this type of PCFs has a narrow wavelength range in which the light is guided. In addition, a good accuracy of the periodicity of the lattice is required to obtain a clear bandgap effect. Therefore guidance in the high index core PCFs is much easier to achieve than the bandgap guidance. The fibre coupler is one of the most important components in the optical communication systems. Mangan et al. [6] have shown that it is possible to use the PCF as an optical fibre coupler which has some advantages compared with the conventional optical fibre couplers. PCF couplers can be easily realised by simply introducing two adjacent defects in the PCF. In addition, they have short coupling length and more flexibility in design freedom. For these reasons, several studies have been reported for the double core PCF couplers [6–9]. The coupling characteristics of two different dual core PCF couplers were 264 IET Optoelectron., 2009, Vol. 3, Iss. 6, pp. 264–273 & The Institution of Engineering and Technology 2009 doi: 10.1049/iet-opt.2009.0033 www.ietdl.org