1254 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 16, AUGUST 15, 2007 Mode Coupling in Plastic Optical Fiber Enables 40-Gb/s Performance Arup Polley and Stephen E. Ralph Abstract—We report 40-Gb/s capability of 50- m core plastic optical fiber using differential modal delay measurements and power penalty due to intersymbol interference computations. The results are explained via a comprehensive multimode fiber model that includes mode coupling (MC) and differential modal attenuation (DMA). We show that strong MC can enable 40-Gb/s transmission for reach in excess of 100 m even in the presence of irregularities in the refractive index profile that prevent 10-Gb/s performance without MC. Furthermore, we show that DMA effects are negligible and that the mode power distributions are not a good indicator of bandwidth. Index Terms—Optical fiber communication. I. INTRODUCTION G RADED index plastic optical fiber (GI-POF) has shown the potential for high-speed data transmission over short- reach links [1]. Strong mode coupling (MC) [2]–[4] and dif- ferential modal attenuation (DMA) [5] have both been reported to be effective in improving POF bandwidth. Indeed, 11 Gb/s has been demonstrated in GI-POF for distances of 100 m [1]. However, the achievable performance of modern GI-POF has not been explored and the sensitivity of the performance bene- fits of MC in the presence of index profile irregularities has not been reported. We show that 40 Gb/s over 100-m GI-POF is achievable [6], [7] using any launch condition. We explain the results using a comprehensive multimode fiber (MMF) model using the previously reported MC strength without the need to invoke DMA. We also show that knowledge of the mode power distri- bution (MPD) is not necessarily a good indicator of bandwidth. We, therefore, focus on the temporal behavior of GI-POF and demonstrate that 40 Gb/s should be readily achievable in any variety of MMF provided that the index profile meets modest accuracy constraints and that the required MC, quantified here, is present. II. EXPERIMENTAL RESULTS The high-resolution impulse response of 200-m 50- m core GI-POF from Chromis Fiberoptics is determined using 16-ps Manuscript received December 15, 2006; revised May 14, 2007. The authors are with the School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0250 USA (e-mail: arup@ece.gatech.edu; stephen.ralph@ece.gatech.edu). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2007.901743 Fig. 1. Experimentally measured DMD of 200-m length of 50- m core GI-POF. The deconvolved DMD is 50 ps and thus supports 40 Gb/s. pulses at 850 nm launched via single-mode fiber at different off- sets from the fiber core center (Fig. 1). The receiver is a commer- cial 50- m MMF photodetector followed by a digital sampling scope with a net bandwidth of 25 GHz. The differential modal delay (DMD), defined as the temporal width at 25% of the maximum intensity across all offset re- sponses, is 66 ps and is limited by the receiver; the maximum difference between the peaks is 2 ps and the deconvolved DMD is 50 ps. The deconvolved response for 0- m offset is 29-ps full-width at half-maximum (FWHM). The variation in FWHM across all offsets is 4 ps thus demonstrating that any launch condition, including overfilled launch, yields equivalent perfor- mance. Using the deconvolved channel responses, we estimate that the optical power penalty for 200-m GI-POF channel is 4 and 10 dBo for 30 and 40 Gb/s, respectively. The corresponding penalty for 40 Gb/s with 100-m reach is 4 dBo, which is com- parable to the 3.6-dBo penalty allotted in the 10-Gb/s Ethernet standard. A 40-Gb/s receiver suitable for MMF may yield lower power penalties than we report here. Eye diagrams and bit-error ratios at 20, 30, and 40 Gb/s confirm the bandwidth performance il- lustrated by the DMD [7]. III. FIBER MODEL A comprehensive MMF model [8] is used to evaluate the tem- poral response and evolution of MPDs. The mode solver deter- mines the transverse electric field profiles and group delays of the propagating modes for arbitrary refractive index profiles. We use profile irregularities known to exist in GI-POF including pure, but not ideal, -profiles and mixed profiles where the alpha varies across the radius. We neglect the small material dispersion; however, we use the exact field profiles, hence the 1041-1135/$25.00 © 2007 IEEE