566 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 14, NO. 3, MAY/JUNE 2008 640 Gb/s Timing Jitter-Tolerant Data Processing Using a Long-Period Fiber-Grating-Based Flat-Top Pulse Shaper Leif Katsuo Oxenløwe, Radan Slav´ ık, Senior Member, IEEE, Michael Galili, Hans Christian H. Mulvad, Anders Thomas Clausen, Yongwoo Park, Member, IEEE, Jos´ e Aza ˜ na, Member, IEEE, and Palle Jeppesen, Member, IEEE Abstract—We report on the use of a novel all-fiber flat-top pulse shaping technique for improving performance and timing jitter tolerance of a switch made for 640–10 Gb/s signal demultiplex- ing. The jitter tolerance is increased to almost 30% of the one-bit time window, and an increase of the receiver sensitivity by 13 dB compared to a nonflat-top pulse is reported. Index Terms—Optical fiber communication, optical fiber filters, optical switches. I. INTRODUCTION F OR HIGH-SPEED serial data transmission that operates at rates of 160 Gb/s and above, management of the timing jitter becomes increasingly challenging. A few groups around the world have demonstrated error-free 640 Gb/s demultiplexing using various ultrafast techniques: a nonlinear optical loop mir- ror (NOLM) [1]–[3], a parametric fiber switch [4], and lately, by filtering the chirp from a semiconductor optical amplifier (SOA) [5]. Common to all demonstrations is the use of ultralow phase noise, and hence, low timing jitter pulse sources (rms tim- ing jitter <100 fs). The tolerance to timing jitter in these systems is very low, but can be increased by the use of system compo- nents (e.g., switches, demultiplexers, add-drop, etc.) with a high tolerance to the timing jitter. High-speed, jitter-tolerant optical components may be obtained, e.g., by generating a square-like, i.e., flat-top gating window in ultrafast (fs-response) Kerr-effect- based nonlinear optical components, e.g., a NOLM as in [6]. For optimum performance, the gating pulse has to be shorter than the one-bit time window, and at the same time, it should have constant intensity over a time interval as long as possible. Both these requirements can be fulfilled when using flat-top pulses. Manuscript received November 1, 2007; revised December 14, 2007. This work was supported in part by the Danish Research Council Funded Project Ultra-Net, in part by the European COST-291 Program, and in part by the Grant Agency of AV CR, Czech Republic, under Contract KJB200670601. L. K. Oxenløwe, M. Galili, H. C. H. Mulvad, A. T. Clausen, and P. Jeppesen are with the Department of Communications, Optics and Ma- terials (COM), Technical University of Denmark (DTU), Lyngby DK- 2800, Denmark (e-mail: lo@com.dtu.dk; mg@com.dtu.dk; hchm@com.dtu.dk; atc@com.dtu.dk; park@emt.inrs.ca; pj@com.dtu.dk). R. Slav´ ık is with the Institute of Photonics and Electronics, Academy of Sciences of the Czech Republic (AS CR), Prague 182 51, Czech Republic (e-mail: slavik@ufe.cz). Y. Park and J. Aza˜ na are with the Institut National de la Recherche Sci- entifique (INRS), Montr´ eal, QC H5A 1K6, Canada (e-mail: park@emt.inrs.ca; azana@emt.inrs.ca). 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/JSTQE.2007.915395 The flat-top waveforms are usually obtained by reshaping of Gaussian or sech 2 shaped pulses generated by mode-locked lasers. Previously reported techniques used for nonlinear optical switching include a reshaper based on a piece of highly bire- fringent fiber and a polarizer [7], [8], a spectral sinc-function shaping filter (e.g., using fiber Bragg gratings (FBGs) [6]), or more complicated filtering schemes (e.g., based on the optical Fourier transform with an electrooptical time lens [9], [10]). Although these techniques proved to be capable of a consider- able increase of the system tolerance to the timing jitter with a maximum measured timing tolerance of almost 50% of the time slot [6]–[10] (i.e., ∼3 ps at 160 Gb/s), none of them was demon- strated at data rates above 160 Gb/s. This is caused mainly by technological challenges related to scaling the aforementioned pulse reshaping techniques to shorter optical pulses (i.e., faster waveforms). This is, for instance, the case of FBG-based flat-top pulse reshapers; the scaling of this technique down to the subpi- cosecond regime would be very challenging as it would require the fabrication of complex sinc-apodized FBGs with submil- limeter lengths. Above 160 Gb/s, a more promising scheme seems to be the flat-top pulse shaper based on optical differen- tiation in a long-period fiber grating (LPG) that was reported recently [11], [12]. It has an inherent advantage of operation with pulses of arbitrary durations, and hence, does not require a precise matching of the pulse shaper parameters with those of the used pulse source. Recently, we reported on the use of this technique to increase the timing jitter tolerance of a system operated at 320 Gb/s [13]. The flat-top pulse shaper allowed for an increase of the tolerance to the timing jitter up to 500 fs (cor- responding to about 18% of the bit time slot). In addition, the power penalty was about 5 dB lower when using flat-top rather than Gaussian-like gating pulses. Within the same report, we also showed some very preliminary results for a 640 Gb/s sys- tem. However, the relatively low-quality system performance obtained at that time did not allow us to demonstrate the full po- tential of this scheme for operation at a 640 Gb/s data rate. This was, however, considerably improved by our recent effort [14]. Here, we present that this new switch configuration allows for jitter-tolerant operation even at 640 Gb/s. We show that jitter of 500 fs (corresponding to almost 30% of the bit period) can be tolerated with a power penalty of less than 5 dB. This represents an improvement of the system performance even with respect to the measured 320 Gb/s system evaluation [13]. Moreover, we present the results on the demultiplexing of all 64 tributary channels, and also show a substantial improvement in receiver 1077-260X/$25.00 © 2008 IEEE