IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 22, NOVEMBER 15, 2007 1855 Demultiplexing of 320-Gb/s OTDM Data Using Ultrashort Flat-Top Pulses R. Slavík, L. K. Oxenløwe, M. Galili, H. C. H. Mulvad, Y. Park, Member, IEEE, J. Azaña, Member, IEEE, and P. Jeppesen Abstract—We report on the use of a novel all-fiber flat-top pulse shaping technique for improving performance and timing jitter tol- erance of a switch made for 320 and 640 Gb/s to 10 Gb/s signal demultiplexing. Index Terms—Optical fiber communication, optical fiber filters, optical switches. I. INTRODUCTION F OR high-bit-rate single-channel data signals (80 Gb/s and above), timing jitter becomes an important detrimental factor [1] that complicates signal processing operations like switching, demultiplexing, and add–drop. Tolerance to the timing jitter can be increased, e.g., by employing square-like (flat-top) optical pulses for a switch gating [2], [3]. The flat-top waveforms are usually obtained by reshaping of Gaussian or Sech shape pulses generated by mode-locked lasers. Previ- ously reported techniques used for optical switching include a reshaper based on a piece of highly birefringent fiber and a polarizer [2], spectral sinc-function shaping filter (e.g., using fiber Bragg gratings, FBGs [3]), or more complicated filtering schemes (e.g., based on optical Fourier transform with an electro-optical time-lens [4], [5]). Although these techniques proved to be capable of a considerable increase of the system tolerance to the timing jitter with a maximum measured timing tolerance of almost 50% of the time slot [2]–[4] (i.e., 3 ps at 160 Gb/s), none of them was demonstrated at speeds over 160 Gb/s. This is caused mainly by technological challenges connected with scaling the above-mentioned 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. Manuscript received May 25, 2007; revised July 25, 2007. This work was supported by the Danish Research Council funded project Ultra-Net, by the Eu- ropean COST-291 program, and by the Grant Agency of the Czech Republic under Contract KJB200670601. R. Slavík is with the Institute of Photonics and Electronics, 182 51 Prague, Czech Republic (e-mail: slavik@ufe.cz). L. K. Oxenløwe, M. Galili, H. C. H. Mulvad, and P. Jeppesen are with COM- DTU, Technical University of Denmark, DK-2800 Lyngby, Denmark (e-mail: lo@com.dtu.dk). Y. Park and J. Azaña are with the Institut National de la Recherche Scien- tifique (INRS), Montréal, QC H5A 1K6, Canada (e-mail: azana@emt.inrs.ca). 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.2007.907638 Recently, there appeared a new technique for flat-top pulse generation [6] using a uniform long period fiber grating (LPG). This pulse shaper is based on a novel filtering scheme based on optical differentiation. In this scheme, the “double-pulse” generated as a differential of a transform-limited symmetric input pulse (Gaussian, Sech) is incoherently superimposed with the original waveform that fills in the valley in the double-pulse leading to the formation of a flat-top pulse [6]. Due to the large bandwidth inherently provided by LPGs, this technique is well suited for pulse reshaping operations down to the subpicosecond regime and, as a result, it is easily scalable to ultrahigh bit rates (e.g., 640 Gb/s). Moreover, unlike the previously reported techniques, this scheme does not require precise control of the relation between the input pulse and the filter characteristics in terms of bandwidth, provided the differentiator bandwidth is larger than that of the input pulse. This gives an additional flex- ibility, as a single LPG pulse shaper can be used with different short-pulse sources emitting pulses of various durations and shapes. The rise/fall edges of the flat-top pulse are given by the rise/fall edges of the input pulse, while the duration is scaled with the duration of the input pulse. Furthermore, we have shown that a considerable dispersion-induced flat-top distortion can be compensated through a strain applied to the LPG [7]. This technique, however, has not yet been reported to be used in any ultrahigh-bit-rate application, except for our preliminary work that focused on 320 Gb/s [8]. In this report, we use the LPG-based flat-top pulse shaper to improve the performance and to increase the timing jitter toler- ance of a 320 and 640 Gb/s to 10 Gb/s demultiplexer. In these experiments, we were able to generate flat-top pulses of dif- ferent durations (suitable for both 320 and 640 Gb/s) using a single LPG-based filter sample. We show that for the 320-Gb/s system, the demultiplexer increases the jitter tolerance from 150 to 500 fs, while for the 640-Gb/s system, error-free operation (corresponding to bit-error ratio (BER) 10 ) could not be reached without the use of the flat-top pulses. II. PRINCIPLE AND SETUP Fig. 1 shows a schematic of the experimental setup used for the 640-Gb/s experiment. The flat-top pulse generator consists of an Erbium glass oscillating (ERGO) pulse source emitting 1.2-ps full-width at half-maximum (FWHM) pulses at 10 GHz at 1543 nm with rms timing jitter of 150 fs. These pulses are further soliton compressed using an optical amplifier in satura- tion to obtain 500-fs pulses (measured with an autocorrelator). Subsequently, the pulses are propagated through the LPG filter. At the output, flat-top pulses of slightly various durations can 1041-1135/$25.00 © 2007 IEEE