JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 28, NO. 5, MARCH 1, 2010 707 Analysis of an Ultrashort PCF-Based Polarization Splitter Jung-Sheng Chiang, Member, IEEE, Member, OSA, Nai-Hsiang Sun, Senior Member, IEEE, Shih-Chiang Lin, and Wen-Fung Liu Abstract—We propose a new structure of polarization splitter based on a dual-elliptical-core photonic crystal fiber. The polariza- tion splitter has a simple and symmetric directional coupler config- uration. The vector boundary element method is used to investigate the propagation characteristics of the device under various condi- tions. Results of the numerical calculation relevant to the design conditions are presented. A 0.3-mm-long splitter with an extinc- tion ratio of 23 dB is obtained. Index Terms—Birefringence, photonic crystal fiber (PCF), po- larization splitter, vector boundary element method. I. INTRODUCTION P OLARIZATION splitters, which can split one light into two orthogonal polarization states, are one of essential components in optical fiber communication and optical fiber sensing systems. They can be applied as polarization division multiplexer, polarization mode dispersion compensator, pump combiner in optical amplifier, and so on. Various types of optical polarization splitters have been reported in literature [1]–[16]. There are fabricated polarization splitters based on Y-branched or directional-coupler type semiconductor waveguides [1]–[8]. Most of them have relatively complex structure and materials. The polarization splitters based on conventional optical fiber di- rectional couplers [9]–[16] are also proposed. They include the polarization splitter employing the birefringent-fiber coupler, fused fiber coupler, or polished fiber coupler. One major dis- advantage of conventional fiber-based polarization splitters is that a long coupler length is required because the birefringence of conventional glass fiber is small. For example, the lengths of splitter reported on [14] and [15] are 262 mm and 22.5 mm, respectively. In recent years, photonic crystal fibers (PCFs) have aroused great attention, because of their unique advantages of control of light. PCFs are composed of a single material (generally silica) with multiple air-holes periodically arranged around the core Manuscript received June 26, 2009; revised September 28, 2009. First pub- lished November 24, 2009; current version published February 19, 2010. This work was supported in part by the National Science Council of the Republic of China under Grant NSC 97-2221-E-214-056. J.-S. Chiang and N.-H. Sun are with the Department of Electrical En- gineering, I-Shou University, Kaohsiung 840, Taiwan, R.O.C. (e-mail: cjs@isu.edu.tw; snh@isu.edu.tw). S.-C. Lin is with the Department of Communication Engineering, I-Shou Uni- versity, Kaohsiung 840, Taiwan, R.O.C. (e-mail: sclin@isu.edu.tw). Wen-Fung Liu is with the Department of Electrical Engineering, Feng Chia University, Taichung, 407, Taiwan, R.O.C. (e-mail: wfliu@fcu.edu.tw). Digital Object Identifier 10.1109/JLT.2009.2036945 that run along the length of the cladding. In general, PCFs can be divided into two classes: index-guiding holey fibers [17]–[20] and photonic bandgap fibers [21]–[23]. The PCFs possess nu- merous unique properties such as wide single-mode wavelength range, anomalous dispersion at visible and near-infrared wave- lengths, highly adjustable effective mode area and nonlinearity, flexible design, and high birefringence. In index-guiding PCFs, a high air-silica index contrast is intrinsically favorable to en- hance the modal birefringence of the fiber core [24], which provides new possibilities to design high-quality polarization- dependent components. For PCF-based multicore-coupled de- vices, the PCF stacking procedure makes it straightforward to produce multicore structure with great geometrical precision and without additional fabrication efforts [25]. The PCF-based polarization splitter can achieve good extinction ratios and short lengths in contrast with conventional dual-core fiber- or wave- guide-based polarization splitters [26]. Lately, several groups of scientists investigated PCF-based directional couplers, man- ufactured directly as a twin-core PCF, by the fused biconical tapered method or joining two side-polished single-core PCFs [27]–[29]. A highly birefringent dual-core PCF was proposed to re- alize a compact polarization splitter [30]–[33]. Although these PCF structures achieve good performance (the best results: splitter length is 0.995 mm and extinction ratio is 21.359 dB) of polarization splitter, several different air-hole sizes need to be carefully adjusted to optimize the performance. Currently, designing the splitters with a simple air-hole pattern is a key issue to make them practical. It can be overcome by introducing a design with a large polarization-dependent birefringence. Consequently, there still exists no compact fiber-based polar- ization splitter that is simultaneously characterized by a high extinction ratio ( dB), a short length ( mm), and easy fabrication technology. In this paper, a new structure of ultrashort polarization splitter is proposed based on high birefrigent dual-elliptical-core PCF [34]. The vector boundary element method (VBEM) [35]–[38] is used to investigate characteristics of the device including splitter length and extinction ratio. The simulation results demonstrate that a 0.3 mm-long polarization splitter with the extinction ratio of 23 dB can be obtained. Besides the great performance, another advantage of the proposed structure is simplified that only one air-hole size needs to be adjusted to achieve the polarization splitter behavior. Therefore, the proposed polarization splitter possesses easy fabrication merit. 0733-8724/$26.00 © 2010 IEEE