JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 31, NO. 13, JULY 1, 2013 2177 Misalignment Losses in Step-Index Multicore Plastic Optical Fibers Amaia Berganza, Gotzon Aldabaldetreku, Joseba Zubia, Gaizka Durana, and Jon Arrue Abstract—We analyze the extrinsic coupling losses due to misalignments in three different SI-MCPOFs. Four kinds of misalignments are analyzed: longitudinal separation, transversal offset angular misalignment and rotational misalignment. We have performed experimental measurements and complementary computational simulations to assess the effect of misalignments and the inﬂuence of crosstalk and ﬁber characteristics on coupling losses. Index Terms—Geometric optics, loss measurements, optical ﬁbers, optical losses, ray tracing. I. INTRODUCTION M ULTICORE plastic optical ﬁbers (MCPOFs) constitute a good alternative to conventional single-core-POFs due to their smaller sensitivity to bending losses and their large size. These ﬁbers are composed of small cores embedded in a cladding material. As a whole, the cores cover the same large diameter as a conventional POF and, consequently, the advantages of POFs are maintained, whereas bending losses are reduced [1], [2]. Due to such properties, MCPOFs in com- bination with VCSEL are becoming more and more attractive for Fiber To The Home (FTTH) and Fiber To The Building (FTTB) and short-reach optical interconnection applications where they are used as conventional single-core SI POFs, with a higher bandwidth and lower losses[3]–[5]. But they can also be used to transmit different channels using a different core for each channel. In this case, single model ﬁbers (SMFs) and lenses could be used as light source, as well as VCSELs [6], [7]. In all these applications misalignments can occur when two MCPOFs are connected, which causes additional coupling losses. In standard single-core optical ﬁbers, three kinds of misalign- ments may occur: longitudinal separation, transversal offset and angular misalignment [8]. However, when dealing with MCPOFs a new kind of misalignment appears: the rotational misalignment. That is to say, if the cores in the cross sections of Manuscript received January 31, 2013; revised April 19, 2013; accepted May 09, 2013. Date of publication May 16, 2013; date of current version June 05, 2013. This work has been sponsored by the institutions Ministerio de Economía y Competitividad under project TEC2012-37983-C03-01, Go- bierno Vasco/Eusko Jaurlaritza under projects AIRHEM-II, S-PE12CA001, GIC07/156-IT-343-07, and by the University of the Basque Country (UPV/EHU) through program UFI11/16. The authors are with the Department of Communications Engineering, University of the Basque Country (UPV/EHU), ETSI, Bilbao 48014, Spain (e-mail: aldabaldetreku@ehu.es; joseba.zubia@ehu.es; jon.arrue@ehu.es; gaizka.durana@ehu.es). Color versions of one or more of the ﬁgures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identiﬁer 10.1109/JLT.2013.2263634 the transmitting and receiving ﬁbers are not properly aligned, part of the power is lost in the junction. In a previous work, we measured coupling losses in perﬂuo- rinated graded-index (GI) multicore ﬁbers [9]. In this paper, our purpose is to complete that study by analyzing three different step-index (SI) MCPOFs, to explore the behavior of these ﬁbers and the inﬂuence of the number of cores on coupling losses. Furthermore, measurements of rotational misalignments are in- cluded which, as far as we know, have never been carried out. Finally, we discuss the inﬂuence of crosstalk (denoted by the ab- breviation XT) and ﬁber characteristics on coupling losses using the results obtained by accurate experimental measurements and completed by additional computational simulations. The structure of the paper is as follows. First, we introduce the SI-MCPOFs studied in this work. Then, the experimental set-up used to perform the measurements is explained. Afterwards, we describe the simulation conditions. Next, the results obtained by the experimental measurements and simulations are analyzed and discussed. Finally, we summarize the main conclusions. II. CHARACTERISTICS OF THE DIFFERENT SI-MCPOFS ANALYZED In order to observe the inﬂuence of ﬁber characteristics on coupling losses, we have studied three different SI-MCPOFs developed by Asahi Kasei [10]. In these ﬁbers, the cores are arranged in concentric rings around a central core and the nu- merical aperture and the cladding radius are 0.6 and , respectively. We have employed a 19-core-MCPOF, a 37-core- MCPOF and a 217-core-MCPOF with mean core radii of , and , respectively. Fig. 1 shows the cross sections of the investigated ﬁbers. It must be pointed out that the cores in these ﬁbers are not circular, but rather polygonal, due to some unknown effect during the manufacturing process [2]. This ef- fect makes the total area covered by cores larger than in the ideal case of perfectly circular cores, which leads in most cases to more favorable conditions with regard to misalignment, as will be seen later in this paper. Because of these irregularities, the minimum separation dis- tance between cores is different in the real case and the ideal one as can be seen in Table I. These discrepancies lead to dif- ferent ﬁll-factors (area covered by cores/total area), as is shown in Table II. III. MEASUREMENTS A. Experimental Measurements Fig. 2 depicts the experimental set-up used to measure cou- pling losses in MCPOFs due to misalignments. 0733-8724/$31.00 © 2013 IEEE