FB1 (Invited) 10:15 - 10:45
Ultra-Fast Optical Signal Processing in Nonlinear Silicon Waveguides (invited paper) L.K. Oxenløwe, M. Galili, M. Pu, H. Ji, H. Hu, K. Yvind, J.M. Hvam, H.C.H. Mulvad, E. Palushani, J.L. Areal, A.T. Clausen and P. Jeppesen DTU Fotonik, Technical University of Denmark, Ørsteds Plads Building 343 2800 Kgs. Lyngby, Denmark,
[email protected])
Abstract: We describe recent demonstrations of exploiting highly nonlinear silicon nanowires for processing Tbit/s optical data signals. We perform demultiplexing and optical waveform sampling of 1.28 Tbit/s and wavelength conversion of 640 Gbit/s data signals. OCIS codes: (060.4510) Optical communications; (190.4360) Nonlinear optics, devices
1. Introduction The trend of generating higher serial line rates has historically led to cost and power reductions, but to reach Tbit/s per channel bit rates will require optical solutions, as it seems unattainable for electronics. The overall goal is to design ultra-high symbol rate systems with reduced component counts, so as to ease management and reduce power consumption. The challenge is to find the right solutions that will allow this. For instance, in a transmission link, there could be an advantage in reduced number of regenerators, if these are exchanged with ultra-fast all-optical ones [1]. Overall, the high baud rates require fewer components for in-line functionalities, but rely on ultra-fast and hence nonlinear switches with their inherent need for high optical power, and there is a need for optimising these solutions. In this paper we review some promising technologies based on silicon photonics. In particular we use nano-engineered silicon waveguides enabling efficient phase-matched four wave mixing for ultra-high-speed optical signal processing of ultra-high bit rate serial data signals. We show that silicon can indeed be used to control Tbit/s serial data signals, perhaps paving the way for future ultra-fast optical chips. 2. Silicon photonics for optical signal processing – background Silicon is a promising material platform for future optical devices, due to its high refractive index, enabling compact devices, and its ability to perform ultra-fast optical switching by the nonlinear optical Kerr effect. Using nanoengineered pure silicon waveguides (on the order of 250-500 nm in cross-section), often termed nanowires, which are very nonlinear, enables optical control of data signals by a pump pulse [2]. This all-optical approach is ultra-fast and has resulted in 40 Gbit/s [3] and 160 Gbit/s all-optical signal processing [4] and signal processing of a 160 Gbit/s data signal [5], and very recently the switching of a 1.28 Tbit/s data signal [6]. Adding other nonlinear materials to Si slot waveguides can induce a high nonlinearity but avoid the detrimental nonlinear absorption of silicon. Such materials could be organic molecules [9-10], recently used to enable the making of a 40 Gbit/s data modulator [11] and promising attempts at demultiplexing a 170 Gbit/s data signal [12]. The nonlinear response in silicon is ultra-fast and is therefore suitable for processing ultra-high-speed serial data signals [6], which may be interesting for future ultra-fast serial data links, e.g. in data centres, between servers, in super-computers or even for niche applications in the core transport network. To characterise the high-speed potential, the silicon chips are tested in the DTU Fotonik Tbit/s test-bed, shown schematically in figure 1. 1.28 Tbit/s transmitter
300 fs Error-free
Figure 1. The DTU Fotonik Tbit/s test-bed (schematic). Lower left: Generated 1.28 Tbit/s data signal eye diagram. Lower right: demultiplexed 10 Gbit/s data channel.
978-1-4244-8340-2/11/$26.00 ©2011 IEEE
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In the Tbit/s test-bed, an up-to 1.28 Tbit/s data signal is generated by optical time division multiplexing (OTDM) of a 10 Gbit/s base rate data signal based on very short optical pulses (300 fs FWHM). The pulses are data modulated (amplitude or phase) and compressed in a highly nonlinear fibre based compression stage and multiplexed in a fibre delay line multiplexer up to 1.28 Tbit/s. This signal may then be used to test a variety of components and functionalities, including transmission [20] and as described here signal processing using silicon. 3. Ultra-fast optical signal processing: recent demonstrations at DTU Fotonik We have recently shown that we can process Tbit/s data signals using FWM in an all-Si nanowire. We have demonstrated demultiplexing and optical sampling of a 1.28 Tbit/s data signal [6], and moreover, very recently shown that the pump repetition rate can be increased to 320 and 640 GHz, when we did 320 and 640 Gbit/s wavelength conversion [7-8]. Figure 2 shows results on this wavelength conversion of 320 Gbit/s OOK and 320 and 640 Gbit/s DPSK data signals. This is the highest signal processing speed ever using silicon, and figure 2 shows full BER characterization. This functionality is also promising for low energy consumption: we used as low as 190 fJ/bit for the 640 Gbit/s case. We have previously shown 95 fJ/bit for 640 Gbit/s wavelength conversion using highly nonlinear fiber (HNLF) [13]. Wavelength conversion is more challenging than demultiplexing, as all time channels need to be switched simultaneously, whereas a demultiplexer only switches one channel at a time. It is still not well established which effect the free carriers generated through two-photon absorption have on the FWM processing speed, so the demonstration of ultra-high speed wavelength conversion indicates that it may be possible to isolate
Figure 2. Silicon photonics for ultra-fast signal processing. Examples here: 320 and 640 Gbit/s wavelength conversion using fourwave mixing (FWM) in a silicon nanowire. Left: Basic principle of FWM in a Si nanowire with short pulses, e.g. for demultiplexing. Middle: Eye diagrams for 320 Gbit/s wavelength conversion using on-off keying (OOK) data [7]. Right: Wavelength conversion using differential phase shift keying (DPSK) data at 320 [8] and 640 Gbit/s [7], including autocorrelation traces and bit error rate (BER) measurements for all data channels.
the FWM effect from the TPA/FCA effects and allow for ultra-fast processing. Note that the BER limitations in the 640 Gbit/s wavelength conversion result is due to OSNR limitations, stemming from the relatively poor conversion efficiency of the used device and the insertion loss, resulting in a very modest FWM output power. Figure 3 shows results on using a low-rate control pulse. For demultiplexing, a 10 GHz rep-rate control pulse train is locked to one of the OTDM channels, creating a FWM idler with the data content of that channel. If instead the control pulse is scanned across the data signal very slowly, the resulting FWM idler sample points may be collected by a slow photo-detector and stored directly on a 1 Gsample/sec oscilloscope, i.e. the full data waveform can be sampled, and eye diagrams may be constructed. Figure 3 (left) shows sampling results when measuring on 320 Gbit/s, 640 Gbit/s and 1.28 Tbit/s data signals. In all cases, clearly resolved data pulses are observed with clear and open eye diagrams. Comparing the sampled measured pulse width to an auto-correlation of the data pulse, the timing resolution may be derived [21] and in all cases a timing resolution of ~365 fs is obtained. This is one of the highest timing resolutions ever demonstrated, and certainly1.28 Tbit/s is the highest bit rate of a data signal which has ever been demonstrated. Comparing the 1.28 Tbit/s sampled waveform in figure 3 (bottom) to the eye diagram of figure 1 (lower left), it is clearly observed that the silicon based sampling system offers a considerably higher timing resolution, where the individual data pulses are seen not to overlap, as figure 1 would otherwise suggest. The sampling pulse source is a mode-locked fibre ring laser with carbon nanotubes as saturable absorber running at
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16 MHz rep-rate. Figure 3 middle and right show demultiplexing results from 1.28 Tbit/s OOK and DPSK data. In all cases BER of 10E-9 or close to it can be obtained. The fact that both amplitude and phase modulated data can be processed shows that Si-nanowires are indeed very versatile as well as offering very high timing resolution. 1.28 Tbit/s DPSK at BER 10E-4
780 fs
1.28 Tbit/s DPSK
510 fs
Figure 3. Tbit/s all-optical sampling in a silicon waveguide (left) and Tbit/s demultiplexing of OOK data (middle) and DPSK data (right). Left: top – 320 Gbit/s eye diagram, middle – 640 Gbit/s eye diagram, bottom – 1.28 Tbit/s eye diagram. Middle: top – autocorrelation trace of 1.28 Tbit/s OOK data signal and 470 fs FWHM demultiplexing control pulse source, bottom – bit error rate measurement of 1.28 Tbit/s OOK demultiplexing showing BER < 10E-9. Right: top – all 128 x 10 Gbit/s DPSK channels demultiplexed and receiver power measured at BER 10E-4 showing a 4 dB spread, bottom – full BER curves for 8 channels demultiplexed showing near 10E-9 BER performance is possible. Error floor due to OSNR limitations, caused by low FWM efficiency in this device.
4. Conclusion We have described recent advances in the use of silicon nanowires for all-optical signal processing. We have described signal processing of 1.28 Tbit/s data signals and shown wavelength conversion of 640 Gbit/s data signals. All in all, we believe there are promising materials and optical techniques to enable future ultra-high-speed data rate communications. 5. References [1] K. Hinton, et al, IEEE JSTQE, vol. 14, no. 3, (2008) [2] S. F. Preble et al, “Changing the colour of light in a silicon resonator,” Nature Photonics 1 293–296, (2007) [3] R. Salem et al, “Signal regeneration using four-wave mixing on silicon chip,” Nature Photonics 2, 35–38 (2008) [4] B. G. Lee et al, “160-Gb/s Wavelength Conversion Using Dispersion-Engineered Silicon,” CLEO 2009, CThBB1 [5] F.Li et al, Optics Express, Vol.18, No.4 (2010) [6] H. Ji et al, OFC 2010, paper PDC7 [7] H. Hu et al, “Silicon Chip based Wavelength Conversion of Ultra‐High Repetition Rate Data Signals”, OFC 2011, postdeadline paper PDPA8 [8] H. Hu et al,” 320 Gb/s Phase-Transparent Wavelength Conversion in a Silicon Nanowire”, OFC 2011, paper OWG6 [9] J. Leuthold et al, Nonlinear silicon photonics, Nature Photonics, Vol.4, 535-544 (2010) [10] C. Koos et al,” Highly-Nonlinear Silicon Photonics Slot Waveguide”, Proc. OFC2008, paper PDP25 [11] L. Alloatti et al, “40 Gbit/s Silicon-Organic Hybrid (SOH) Phase Modulator”, Proc. ECOC 2010, paper Tu.5.C.4 [12] C. Koos et al, “All-optical high-speed signal processing with silicon-organic hybrid slot waveguides” Nature Photon. vol 3, 216-219 (2009) [13] H. Hu et al., “640 Gbit/s and 1.28 Tbit/s polarisation insensitive all-optical wavelength conversion,” Optics Express, vol. 18, (2010) [14] B.H. Kolner J.Quant. Electron. vol. 30 (1996) [15] L.F. Mollenauer et al, CLEO 2002, CPDB1-1 [16] T. Hirooka et al, JLT 24(7), (2006) [17] J.L. Areal et al, OFC 2011, paper OThN5 [18] E. Palushani et al, J.Quantum Electron., vol. 45, no. 11, pp. 1317-1324, (2009) [19] H.C.H. Mulvad et al, OFC 2011, paper OThN2 [20] H. Hu et al,” Error-free transmission of serial 1.28 Tbaud RZ-DPSK signal”. ECOC 2010, Paper P4.18. [21] P. A. Andrekson. Nonlinear optical BER based high resolution all-optical waveform sampling. Laser and photonics reviews, (3),231, 2007
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