International Journal of Optics and Applications 2017, 7(2): 31-36 DOI: 10.5923/j.optics.20170702.01 Eye-Diagram-Based Evaluation of RZ and NRZ Modulation Methods in a 10-Gb/s Single-Channel and a 160-Gb/s WDM Optical Networks Faramarz E. Seraji 1,* , Marzieh Sadat Kiaee 2 1 Optical Communication Group, Iran Telecom Research Center, Tehran, Iran 2 Electrical Engineering Department, Ghiasodin Jamshid Kashani Inst. of Higher Edu, Abyek, Qazvin, Iran Abstract In this evaluation, we have compared two popular return to zero (RZ) and non-return to zero (NRZ) modulation formats in a 100-km and 400-km single-channel and wavelength division multiplexing (WDM) optical networks for bit rate of 10-Gb/s and 160-Gb/s, respectively, by using Optisystem software. The results showed the better performance of the RZ technique in tolerating the non-linear effects and noisy environments. It’s shown that NRZ technique is a good option for single-channel networks and long-haul WDM links. The obtained evaluation results show that the RZ technique is a better choice for short-distance WDM links. Keywords External Modulation, Direct Modulation, Optical Link, RZ, NRZ, WDM System 1. Introduction Return to zero (RZ) and non-return to zero (NRZ) are the popular techniques, which are used to encode optical pulses in optical networks. In a simple comparison, the NRZ technique requires less bandwidth for transmission than the RZ and it is not sensitive to laser phase noise. Also, while the NRZ is more economical, the RZ, on the other hand, is more tolerant to nonlinearity than the NRZ [1]. In a study, the RZ and the NRZ techniques have been compared in an optical duobinary system. The results showed that NRZ pulse shape was superior compared with RZ for duobinary transmission in all the cases that were studied including systems that are limited by amplified-spontaneous noise, fiber chromatic dispersion and self-phase modulation [2]. Another study has been done to compare standard forward error code (FEC) in 40 Gb/s optical transmission systems with NRZ, RZ, and carrier-suppressed RZ (CS-RZ) modulation formats. The results showed the NRZ format had a similar tolerance to intersymbol interference due to the interaction of dispersion and nonlinearities as that of RZ and CS-RZ formats without FEC [3]. A comparison between the NRZ and the RZ data formats with respect to polarization mode dispersion (PMD)-induced system degradation showed the RZ performance better than the NRZ [4]. * Corresponding author: feseraji@itrc.ac.ir (Faramarz E. Seraji) Published online at http://journal.sapub.org/optics Copyright © 2017 Scientific & Academic Publishing. All Rights Reserved Another comparison between the RZ and the NRZ modulation formats for 40-Gb/s time division multiplexing (TDM) system, showed that for upgrading the existing network to 40-Gb/s, the RZ-modulation format is superior compared to conventional NRZ-modulation. They reported that transmission distances of 400 km and 1200 km with bit rate of 40 Gb/s are feasible within the RZ- and the NRZ-modulation formats [5]. The effects of amplified spontaneous emission in the networks with the RZ, the NRZ, and the CS-RZ modulation formats were evaluated by using an Erbium-doped fiber amplifier (EDFA). The results showed the smaller bandwidth was required by using NRZ format, while better performance of the RZ format was obtained to withstand fiber non-linearity effects [6]. A comparison of the RZ and the NRZ modulations in laser intra-satellite communication systems showed that by using an amplifier, the RZ format had a better performance for a long transmission distance; and without an amplifier, the RZ- and the NRZ-modulation formats had the same performance [7]. In a 2.5-Gb/s optical network, the RZ- and the NRZ-modulation formats have been compared, where the BER for the network with the NRZ- and the RZ-modulation techniques were 10 -14 and 10 -12 , respectively, with 30-km single-mode fiber link. The results of this study showed the better performance of the NRZ format [8]. Another comparison between the RZ- and the NRZ-techniques has been reported in a 4-users WDM system with bit rate of 2.5-Gb/s per channel. The results for transmission distance of 400 km showed the BER of 10 -22