Highly GeO 2 Doped Polarization Maintaining PCF for Dispersion Compensation Sk. Nayemuzzaman 1 , Shekh Farhad Uddin Ahmed, Md. Imrul Hasan and Mohammad Faisal Department of Electrical and Electronic Engineering Bangladesh University of Engineering and Technology, Dhaka-1205, Bangladesh 1 nayem.buet@gmail.com Abstract—In this paper a highly GeO 2 doped single mode equiangular spiral photonic crystal fiber (HDES-PCF) is proposed to achieve high birefringence with extremely high negative dispersion, higher nonlinearity with low confinement loss for E+S+C+L+U band of interest. From the simulation results, large negative dispersion of −602.7803 ps/nm/km at 1.33 μm and −903.5316 ps/nm/km at 1.55 μm is achieved. High birefringence 5.8×10 -2 at 1.33 μm, 5.3×10 -2 at 1.55 μm with very low confinement loss is also achieved. Our proposed structure is also suitable for nonlinear applications because of nonlinearity ~133.9298 W −1 km −1 is achieved at 1.33 μm and ~79.1229 W −1 km −1 is achieved at 1.55 μm wavelength. The proposed PCF can be used in dispersion compensation, optical sensor, higher bit rate coherent communication, and polarization maintaining fiber etc. Index Terms— Birefringence, Photonic crystal fiber, Dispersion compensation, Finite element method I. INTRODUCTION Photonic crystal fiber is a new technology that paves the way in optical communication due to its unique properties. In comparison to conventional optical fiber, PCF shows basic properties like birefringence, nonlinearity and chromatic dispersion that can be tailored to achieve extraordinary outputs. For example, birefringence can be produced by adopting asymmetric structures in the PCF design. Many methods can be applied to increase birefringence. The fiber core can be designed to be asymmetrical (e.g., double or triple defects in the fiber core). High birefringence up to the order of 10 -2 is found in recent research works. Birefringence of 0.01544 is reported in [1], 0.0315 is reported in [2] and 0.022 is reported in [3]. Higher value of birefringence is a key concern for coherent communication and sensing application etc. Nonlinearity is also an important property of PCF that was also discussed in the reports mentioned above. Nonlinear coefficient of 58 W −1 km −1 and 68 W −1 km −1 is reported in [2], [3] respectively. However higher nonlinearity is achieved using GeO 2 doped core. Spiral design with higher nonlinearity of is reported for visible range [4]. The ES-PCF is much compact and simple as compared to conventional PCF. So increasing nonlinearity will let us use PCF for nonlinear applications like four-wave mixing, supercontinuum generation, and second harmonic generation. In addition, it is also desired to decrease the complexity of PCF design. Standard single mode fiber (SSMF) used in the long distance transmission channel can face pulse broadening due to its higher value of positive dispersion. It limits the transmission capacity. To mitigate this problem dispersion compensating fiber (DCF) in the transmission link can be used as an effective way to compensate the positive dispersion in the system. This is necessary to neutralize the dispersion as it creates great trouble in higher bit rate transmission. The main purpose of DCF should have large negative with flattened dispersion [5]. Also higher birefringence is necessary to maintain the polarization state which is useful in first order polarization mode dispersion (PMD) compensation. Birefringent PCFs also have potential applications in signal processing systems, electro-optical modulation systems and optical sensor designs [6]. Researchers around the world worked past years on birefringence and negative dispersion. Silva et al. in [7] proposed a Ge doped core PCF having low negative dispersion of −212 ps/nm/km that demands long fiber requirements. The DCFs proposed by Islam et al. based on equiangular spiral photonic crystal fibers (ES-PCFs) in [8] and [9] provide average negative dispersion of −227 and −393 ps/nm/km respectively. Another DCF reported by Tee et al. [10] that exhibits high average negative dispersion of −457.40 ps/nm/km. Moreover, polarization properties were not considered here. Recently, an octagonal PCF has been proposed in [11] to achieve large negative dispersion of −558.96 ps/nm/km. This structure has different sized elliptical air-holes that make the fabrication process too complex. So having large negative dispersion is a challenging goal for dispersion compensating applications. In this paper, an equiangular spiral PCF is designed to achieve suitable dispersion property along with highly GeO 2 doped core for higher nonlinearity. The elliptical core offers higher birefringence. II. DESIGN METHODOLOGY AND FABRICATION Figure 1 shows the cross-section of proposed Highly Doped-Equiangular Spiral PCF (HDES-PCF) with concentrated GeO 2 doped elliptical core in the center. The cladding structure has gradually increasing air-holes C1, C2, C3, C4, C5, C6, C7 and C8 as in Fig. 1. The host material is silica. Two ellipses E0, E1 and two extra air-holes A0, A1 are added to introduce asymmetry in the design. This PCF has 6 arms having 8 rings per arm with equiangular spiral design. The position of the center of each circular air-hole is adopted from the following equations.        (1)        (2) Where α is the constant angle between a radius of the equiangular spiral and the tangent at the end point of the radius and r spiral is the distance at any point along the spiral arm from the centre of the structure. Each air-hole is separated 9th International Conference on Electrical and Computer Engineering 20-22 December, 2016, Dhaka, Bangladesh 978-1-5090-2963-1/16/$31.00 ©2016 IEEE 550