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Spectral Slicing of a Supercontinuum Source for WDM/DS-OCDMA Application Cedric Ware1*, Steevy Cordette1, Catherine Lepers2, Ihsan Fsaifes2, Bertrand Kibler3 Christophe Finot3, and GuyMillot3 1 Institut TELECOM, TELECOM ParisTech, CNRSLTCI, Communications and Electronics department 46 rue Barrault, 75634 Paris CEDEX13, France 2 Laboratoire PhLAM, CNRS, UMR 8523, Universite des Sciences et Technologies de Lille 59655 Villeneuve d’Ascq, France 3 Institut Carnot de Bourgogne, UMR 5209 CNRS-Universite´ de Bourgogne 9 av. Alain Savary, BP 47 870, 21078 Dijon CEDEX, France *
[email protected] ABSTRACT WDM and optical CDMA are leading contenders to easily upgrade access network performances in terms of multiple access technique. Both methods can be used at once, using a single multiwavelength optical source. We show, numerically and experimentally, that spectral slicing of a 10-GHz pulse train broadened to a supercontinuum yields pulses suitable for use in a direct sequence optical CDMA system. Simulations with optical CDMA encoders and decoders based on superstructured fiber Bragg gratings indicate good performance can be expected. Keywords: optical access networks, coherent continuum generation, spectral slicing, WDM, direct-sequence OCDMA, superstructured fiber Bragg gratings. 1. INTRODUCTION The continued development of innovative high-bandwidth services aimed at the general public, such as online social networking, video on demand, “triple-play” packages, keeps the expectations of data communication network users on the rise. An ever-renewed need for speed must be satisfied, and the bottleneck to be lifted is current in the “last mile”, namely in access networks. The physical medium of choice for (fixed) broadband is undoubtedly optical fiber, upon which systems with enormous capacities have been demonstrated, up to 25 Tbps on a single fiber [1, 2]. This technology is now available not only for core networks, but all the way to the end-users’ premises. However, in the context of access networks, while a single fiber’s theoretical capacity is more than sufficient for any foreseeable need of numerous users simultaneously, sharing this huge bandwidth among many people, especially with the aforementioned flexibility requirements, remains an active research topic. Currently-deployed passive optical networks (PONs) such as EPON and GPON typically shares up to 2.5 Gbps among 16 to 64 users, for a single-user bandwidth on the order of 100 Mbps, using time-division multiplexing (TDM) [3]. To further increase PONs’ multiplexing performances, upgraded systems are being developed. First, wavelength-division multiplexing (WDM), then code-division multiple access (CDMA), are prime candidates for tomorrow’s and longer-term PONs [4, 5]. However, in the access network context, managing multiple laser sources for different wavelengths is even more cumbersome than in core networks, due to the large number of end-users and optical network units (ONUs). An interesting technique to alleviate this problem is to use a single multiwavelength source, which would then be kept in the central office (CO), to supply the right wavelength through the same fiber as for data transmission [6]. Such a source should have a broad spectrum, spurring an interest in supercontinuum (SC) generation to be spectrally sliced into multiple channels [5, 7]. On top of each such channel, multiplexing performances can be further increased using optical CDMA (OCDMA). Since optical systems routinely operate at bit rates which render electronic processing expensive if at all possible, all-optical techniques are being sought for OCDMA, such as: direct-sequence [8, 9]; fast-frequency hopping [10]; and frequency encoding OCDMA [11]. In the framework of the French National Agency for Research (ANR) project Supercode, we are evaluating optical pulses obtained by SC spectral slicing, for use in OCDMA encoders and decoders based on superstructured fiber Bragg gratings (S-FBG), numerically and experimentally.
⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ This work was supported by the French National Agency for Research (ANR) through the nonthematic (“blanc”) project Supercode (ANR-06-BLAN-0401).
978-1-4244-2626-3/08/$25.00 ©2008 IEEE
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2. SYSTEM ARCHITECTURE Our proposed OCDMA system architecture is shown in Fig. 1. We focus on the CO-to-ONUs transmission.
Figure 1. Proposed architecture for WDM/DS-OCDMA transmission system using a spectrally-sliced SC light source. Each slice is further split by passive couplers, modulated with a user’s data, and encoded. The OCDMA encoders (resp. decoders) are S-FBGs, which split (resp. recompose) each SC pulse into several.
Figure 2. Simulated and experimental SC pulses at the output of the HNLF. (a) Simulated power and chirp in the time domain. (b) Simulated spectrum. (c) Experimental spectrum, linear and log scale (d) Spectral width evolution, simulated (solid line) and measured (dots). A SC pulsed source is realized by spectrally enlarging the output of a mode-locked laser (MLL) in a highly nonlinear fiber (HNLF). This configuration lends itself to flat-spectrum SC generation [12], which allows us to carve N WDM channels using a wavelength demultiplexer. Passive optical splitters further divide each wavelength into J beams, each of which will bear one user’s data. To this use, they are modulated using external modulators, then encoded for DS-OCDMA by S-FBG encoders. These encoders, also figured in Fig. 1, are similar in design to those in [9, 13]: pulses from the split source undergo partial reflection at different times along the S-FBG. Each sub-pulse thus represents a “1” chip in a direct-sequence OCDMA code. An optical circulator allows to extract the reflected beam into the output fiber. The resulting channels are recombined by optical power combiners, then launched into the PON towards the ONUs. At the receiver end, the ONUs separate their intended signals from the others by means of a S-FBG OCDMA decoder similar to the S-FBG encoder, except that the positions of the individual Bragg gratings in the decoder are complementary to those in the encoder. 3. SIMULATION AND EXPERIMENTAL RESULTS 3.1 Supercontinuum generation We have generated a SC from spectrally broadened pulses, and performed spectral slicing on it, both numerically and experimentally. A 10 GHz pulse train from a Pritel MLL is boosted to an average power up to 30 dBm by a fiber amplifier, which also entails some solitonic compression. The resulting amplified pulses were characterized through their spectrum and autocorrelation, and found to be 1.06 ps long sech pulses. These pulses are then injected into the 500 m long OFS HNLF, which yields a SC signal, still pulsed. Figure 2 shows a good agreement between our simulations and measurements, both in the shape of the spectrum and the evolution of its width as a function of the average power at the input of the HNLF. The 35 nm wide, spectrally flat region centered at 1550 nm appears well-suited to spectral slicing. 3.2 Spectral slicing We simulated the effect of a WDM demultiplexer on these SC pulses. Various demultiplexer profiles and bandwidths ere tested with several SC profiles corresponding to different propagation lengths in the HNLF. Figure 3 shows that the resulting sliced pulse shapes and lengths depend primarily on the demultiplexer’s profile rather than the original pulses’. This was
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expected, as the original SC pulses are strongly chirped. Their spectral components in a single WDM channel are present only for a very short time, on the order of a picosecond; the bandwidth of each channel not being large enough to accommodate ultrashort pulses, the demultiplexer acts as a filter, broadening the pulses to 4 – 8 ps, depending on the actual demultiplexer profile.
Figure 3. Simulated autocorrelation of SC pulses sliced by Figure 4. Simulated and experimental SC pulses after two WDM demultiplexers. Left: 50 GHz Gaussian. Right: slicing. The pulses from the different output ports are 200 GHz super Gaussian. The SCs correspond to different similar, both in spectrum (a) and autocorrelation traces (b). propagation lengths in the HNLF. The resulting pulse shape The simulated autocorrelation (dots in (b)) matches the is determined by the demultiplexer profile, rather than the experimental traces. original pulse. The corresponding experiment was performed with a WDM demultiplexer, with a 100 GHz channel spacing and a Gaussian 50 GHz channel bandwidth. Figure 4 shows that the pulse profiles on all output channels are quite similar to the simulated one and to each other. The estimated pulse lengths vary between 8.9 and 11.4 ps (simulated: 8.1), yielding a ∆t ·∆ν product between 0.44 and 0.57 (Gaussian pulses having 0.44). 3.3 OCDMA coding We have simulated the propagation of pulses from the sliced SC source along the OCDMA encoders and decoders. Figure 5(a) shows the encoder output; the direct-sequence code is embodied by a pulse sequence resulting from the partial reflections in the S-FBG. The sequence is clean, except for some noise starting at 400 ps, due to multiple reflections in the device, and not linked to the use of a SC source. Figure 5(b) shows the output of two possible decoders: one matched to the encoder, corresponding to the same code, which yields the code’s autocorrelation; the other corresponding to another code, resulting in the two codes’ intercorrelation. In the matched case, the initial pulse, correctly reconstituted, stands out against the multiple-access interference (MAI) which is represented by the mismatched case. To check this against experiment, the actual devices’ fabrication is currently underway, and we intend to test them against the source shortly thereafter.
Figure 5. Simulated OCDMA coding, using the sliced pulses in S-FBG encoders and decoders: optical power as a function of time. (a) Output from an encoder. (b) Output from an encoder-decoder cascade, with matched codes (autocorrelation) and mismatched codes (intercorrelation). 3.4 System design The simulated encoder and decoder performances show a promising result for use in a full system configuration. However, some issues remain to be investigated. First, it is important to minimize the crosstalk between wavelengths at the receivers. Although it was proposed that the SFBG encoders and decoders, being optical filters in their own right, could perform this on the fly, our simulations indicate that our particular devices do not have a sufficient rejection level. Interchannel crosstalk would then result in high-frequency oscillations in the sub-pulses, hampering reception. We therefore believe we cannot forgo either a WDM demultiplexer within the PON, or optical filters integrated with each S-FBG decoder.
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Second, the reconstituted pulse at the decoder output (Fig. 5(b)) has a length of the same order of magnitude as the original sliced pulses, a few picoseconds. Barring the use of an ultrafast photoreceiver, which might be a bit expensive to be incorporated into each ONU, this will be temporally broadened. This will impose the chip time to be at least the final pulse width, and therefore restrict the bit rate to a few Gbps per user, considering the final pulse width and the code length. This is still quite respectable for an access network. Finally, this also may affect resistance to MAI: a time-broadened pulse will necessarily have a lower peak power, making its separation from MAI noise harder in the worst case, where MAI is synchronous with the received signal. In the event, possible solutions are the use of optical gating, or saturable absorbers fast enough to react at the shorter pulse’s time scale. 4. CONCLUSIONS We have realized a multiwavelength source using spectrally-sliced supercontinuum. It delivers pulses which exhibit good agreement between simulations and measurements, and appear suitable for use in a direct-sequence OCDMA system. The OCDMA encoders and decoders have been tested numerically, and experimental confirmation is expected shortly. After some design issues are settled, the next step is a full-up BER test and measurement. This concept could be adapted to other methods of SC generation, e.g. in photonic crystal fibers. ACKNOWLEDGMENT We thank our colleague Stefan Wabnitz for initiating this project as well as leading it throughout its first year. REFERENCES [1] A. H. Gnauck, G. Charlet, P. Tran, P. J. Winzer, C. R. Doerr, J. C. Centanni, E. C. Burrows, T. Kawan-ishi, T. Sakamoto, and K. Higuma, “25.6-Tb/s WDM transmission of polarization-multiplexed RZ-DQPSK signals,” J. Lightwave Technol., vol. 26, no. 1, pp. 79-84, Jan. 2008. [2] G. Charlet, J. Renaudier, H. Mardoyan, P. Tran, O. B. Pardo, F. Verluise, M. Achouche, A. Boutin, F. Blache, J.-Y. Dupuy, and S. Bigo, “Transmission of 16.4Tbit/s capacity over 2,550km using PDM QPSK modulation format and coherent receiver,” in Proc. OFC, Postdeadline paper PDP3, San Diego, Feb. 2008. [3] T. Koonen, “Fiber to the home/fiber to the premises: What, where, and when?” Proceedings of the IEEE, vol. 94, no. 5, pp. 911-934, May 2006. [4] L. G. Kazovsky, W.-T. Shaw, D. Gutierrez, N. Cheng, and S.-W. Wong, “Next-generation optical access networks,” J. Lightwave Technol., vol. 25, no. 11, pp. 3428-3442, Nov. 2007. [5] C.-S. Bres, I. Glesk, and P. R. Prucnal, “Demonstration of an eight-user 115-Gchip/s incoherent OCDMA system using supercontinuum generation and optical time gating,” IEEE Photon. Technol. Lett., vol. 18, no. 7, pp. 889-891, Apr. 2006. [6] K. Iwatsuki, J. ichi Kani, H. Suzuki, and M. Fujiwara, “Access and metro networks based on WDM technologies,” J. Lightwave Technol., vol. 22, no. 11, pp. 2623-2630, Nov. 2004. [7] H. Takara, E. Yamada, T. Ohara, K. Sato, K. Jinguji, Y. Inoue, T. Shibata, and T. Morioka, “106×10 Gb/s, 25 GHz-spaced, 640 km DWDM transmission employing a single supercontinuum multi-carrier source,” in Proc. OFC, Anaheim, Mar. 2002. [8] X. Wang and K. Kitayama, “Analysis of beat noise in coherent and incoherent time-spreading OCDMA,” J. Lightwave Technol., vol. 22, no. 10, pp. 2226-2235, Oct. 2004. [9] I. Fsaifes, C. Lepers, M. Lourdiane, P. Gallion, V. Beugin, and P. Guignard, “Source coherence impairments in a direct detection direct sequence optical code-division multiple-access system,” Appl. Opt., vol. 46, no. 4, pp. 456-462, Feb. 2007. [10] S. Ayotte and L. A. Rusch, “Experimental comparison of coherent versus incoherent sources in a four-user λ-t OCDMA system at 1.25 Gb/s,” IEEE Photon. Technol. Lett., vol. 17, no. 11, pp. 2493-2495, Nov. 2005. [11] S. Ayotte, M. Rochette, J. Magne´, L. A. Rusch, and S. LaRochelle, “Experimental verification and capacity prediction of FE-OCDMA using superimposed FBG,” J. Lightwave Technol., vol. 23, no. 2, pp. 724–731, Feb. 2005. [12] F. Parmigiani, C. Finot, K. Mukasa, M. Ibsen, M. A. Roelens, P. Petropoulos, and D. J. Richardson, “Ultraflat SPM-broadened spectra in a highly nonlinear fiber using parabolic pulses formed in a fiber Bragg grating,” Opt. Express, vol. 14, no. 17, pp. 7617-7622, Aug. 2006. [13] I. Fsaifes, C. Lepers, A.-F. Obaton, and P. Gallion, “DS-OCDMA encoder/decoder performance analysis using optical low-coherence reflectometry,” J. Lightwave Technol., vol. 24, no. 8, pp. 3121-3128, Aug. 2006.
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