JOURNAL OF LIGHTWAVE TECHNOLOGY,VOL. 23, NO. 3,MARCH 2005 1145 Orthogonal Wavelength-Division-Multiplexing Technique Feasibility Evaluation R. Llorente, J. H. Lee, R. Clavero, Member, OSA, M. Ibsen, Member, OSA, and J. Martí Abstract—In this paper, a novel high-spectral-efficiency modula- tion scheme using square time pulses in orthogonal wavelength-di- vision multiplexing is proposed. Experimental results demonstrate the significant reduction of the interchannel linear crosstalk-in- duced penalty compared with Gaussian return-to-zero (RZ) mod- ulation. The proposed technique allows a maximum spectral effi- ciency of 1 b/s/Hz. Simulation studies and experimental work con- firming the expected performance are presented. Index Terms—Optical modulation, optical pulse shaping, or- thogonal functions, wavelength-division multiplexing (WDM). I. INTRODUCTION T HE emerging demand of increased transmission ca- pacity makes it particularly interesting to introduce new modulation and multiplexing techniques targeting high-spec- tral-efficiency (bits per second per hertz) dense-wavelength-di- vision-multiplexed (DWDM) systems. In the last years, several novel approaches to augment the spectral efficiency have been proposed, such as vestigial sideband multiplexing [1], bandwidth limitation [2], and orthogonal polarization multiplexing [3]. In the meantime, the orthogonal spectrum packaging of electrical carriers has proven its high spectral efficiency and resistance to system and transmission impairments [4] in its electrical counter- part modulation—orthogonal frequency-division multiplexing (OFDM). The electrical OFDM modulation is implemented using digital inverse Fourier transform (FT) operation in the transmitter and straight FT operation in the receiver. This ap- proach cannot be applied in the optical domain as digital signal processors are not available at core network line rates. Optical orthogonal techniques have been proposed to en- hance detector performance on heavily crosstalked channels in conventional DWDM channels [5]. In this paper, a novel orthogonal wavelength-division multiplexing (OWDM) modu- lation using superstructured fiber Bragg gratings (SSFBGs) as a square pulse shaper followed by the orthogonal interleaving of the sinc-shaped spectrum from each channel is proposed and demonstrated at 10 Gb/s. The results show a significant reduction of the interchannel linear crosstalk penalty compared with conventional Gaussian return-to-zero (RZ) modulation. Manuscript received May 11, 2004; revised October 5, 2004. This work has been supported in part by the European Commission through the IST-28657 TOPRATE project. R. Llorente, R. Clavero, and J. Martí are with the Fiber-Radio Group, Nano- Photonics Technology Centre, Universidad Politecnica de Valencia, 46022 Va- lencia, Spain. J. H. Lee is with the Photonics Research Center, Korea Institute of Science and Technology, Seoul 136-791, Korea. M. Ibsen is with the Optoelectronics Research Centre, University of Southampton, Southampton SO17 1BJ, U.K. Digital Object Identifier 10.1109/JLT.2005.843526 II. PRINCIPLE OF OPERATION Let us consider the conventional intensity detection of one DWDM channel (channel ) with optical spectrum envelope in the presence of an adjacent channel (linear crosstalk) with frequency separation . The channel is cen- tered at circular frequency , and the channel is cen- tered at circular frequency . At the recep- tion stage, the photodetected intensity is proportional to the power spectrum at the photodetector and can be calculated as (1) After photodetection, we have signal components (channel ) and noise from the linear crosstalk presence (channel ). Signal of interest (2) Noise contribution (3) To increase the system spectral efficiency, it is necessary to reduce the noise contribution (3). This noise can be mitigated using time/spectrum shaping techniques, recently enabled by the use of SSFBGs applied to transmission systems [6]. Noise reduction by spectrum shaping requires the interference from a given DWDM channel over the adjacent channels to be moved from the central frequency of the interfered channels (where most optical power is confined) to the side areas of these chan- nels, where it will be removed by the detection filter (conven- tional intensity-detection scheme). This spectrum-shaping tech- nique can be accomplished using optical square pulses for trans- mission: let us consider now a DWDM system employing op- tical square pulses of width for data transmission. In this par- ticular case, the optical spectrum envelope (relative to as ref- erence) is the sinc function (4) Both sinc-shaped spectrum envelopes (4) present power nulls at frequencies relative to , integer. If the channel separation is precisely set to , then the in- terference from over exhibits a power null at fre- quency , i.e., the noise contribution (3) disappears at the center of (the channel under study). This is also true in the case of DWDM channels employing the same line signaling, pro- vided the channel separation is . This reduc- tion of the linear crosstalk at the central wavelength of each 0733-8724/$20.00 © 2005 IEEE