1. Introduction

With the rapid growth of high performance applications such as ultra-high definition video on demand, cloud computing for the big data [1], extremely coarse-granular yet flexible and spectral/energy efficient datacenter network technologies are receiving increasing importance. Indeed, high-performance computing (HPC), Internet dataceters (IDC), and carrier central office (CCO) high-end applications have already started using 100 Gigabit Ethernet (GbE), which is expected to scale to 400G, 1T, and beyond in the near future [2]. In general, scaling to Terabit/s leads each channel to occupy wider spectral width, resulting in a network with less number of channels and hence less flexibility. Therefore, the coarser-granular the network becomes, the more the elasticity [3] becomes important. Such coarse granularity also poses a serious issue regarding the energy consumption of the network, and all-optical options will have more opportunities rather than relying on high-speed electronic logic devices [4, 5]. Therefore, to realize future datacenter networks, all-optical elastic network technology is a promising candidate since it can provide both energy efficiency and capacity within limited network resources.

Recently, optical Nyquist pulse [6], which is easily generated in optical domain by a Wave-shaper, has been applied to time-division multiplexing (OTDM), offering ultrahigh baudrate beyond electronic limitation with high spectral efficiency as Nyquist WDM or optical OFDM [7, 8]. It is thus possible for long distance transmission of OTDM signal thanks to the improved dispersion and polarization mode dispersion (PMD) tolerances [9–12]. Furthermore, in perspective of network in which the optical signals are bandwidth limited by filtering effect while they are passed and switched through many cascaded add-drop nodes, the networks based on Nyquist WDM techniques basically require wide guard-bands to reduce crosstalks between neighboring channels. The use of ultra-coarse granular Nyquist OTDM signals in this case actually improves spectral efficiency by eliminating such guard-bands. Another good feature of Nyquist OTDM is that once a good short pulse source is given, the baudrate can be changed relatively easily by changing the number of tributaries. These aforementioned properties imply that Nyquist OTDM should be capable of forming an ultra-coarse granular but spectrally efficient elastic networks in which FlexGrid-compatible WSSs (Waveshaper based WSSs) handle such baudrate variable Nyquist OTDMs as well as efficiently add-drop WDM channels of Nyquist OTDM with almost no guard band. Such network aspects have, in fact, been shown in our demonstrations on add-drop operations of Nyquist OTDM-WDM channels over cascaded Waveshapers [13, 14].

Because our previous work did not include the demonstration of transmission over long fiber distance or spectral elasticity using Nyquist OTDM-WDM signals, this paper will demonstrate the full functions for the first time, i.e. the transmission and pass-drop operations in the all-optical elastic Nyquist OTDM-WDM network. OTDM-WDM channels with various baudrates from 43 Gbaud to dual-polarization 344 Gbaud are generated, transmitted over four spans of a 80 km fiber link and a WSS with pass-drop operations in a fully elastic manner. We also implement an in-band baudrate-flexible clock recovery technique by inserting a null-header to Nyquist OTDM channels.

2. Experimental setup

Figure 1 shows experimental setup for generation, network transmission, and reception of baudrate variable Nyquist OTDM-WDM channels. A 43 GHz, 400 fs optical pulse generator (OPG) operating at 1557.768 nm is used as an optical comb source for generation of Nyquist OTDMWDM channels. OPG is based on optical pulse compression of a 43 GHz externally modulated optical pulse train on a comb-like profiled fiber [15]. The pulses from OPG are intensity modulated in a LiNbO₃ modulator (LNM) by 43 Gb/s data with several lengths of pseudorandom binary sequence (PRBS) from a pulse pattern generator (PPG). The modulated signal is then multiplexed in a fiber delay 1:N MUX with multiplexed factor N varied from 1 to 8 to generate flexible baudrate OTDM channels from 43 Gbaud to 344 Gbaud. We then use a programmable optical filter, the so-called Waveshaper [16], for Nyquist filtering at different wavelengths of the same conventional OTDM spectrum to emulate the generation of Nyquist OTDM-WDM channels at variable baudrates. By changing the bandwidths of WDM channels, it is possible to achieve at the same time the spectral elasticity of Nyquist OTDM signals on different wavelengths. Thanks to the wide spectrum of the OPG, many WDM channels of Nyquist OTDM signal with mixed baudrates from 43 Gbaud to 344 Gbaud can be generated as can be seen in Fig. 2.

 

Fig. 1 Experimental setup for transmission and pass-drop operations of elastic Nyquist OTDM-WDM signals.

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Fig. 2 Spectra of dual-polarization Nyquist OTDM-WDM signal with different spectral arrangements: 3×688 Gbaud (344 Gbaud/pol) (a), 5×344 Gbaud (172 Gbaud/pol) (b), 9×172 Gbaud (86 Gbaud/pol) (c), and mixed baudrate Nyquist OTDM-WDM signals (d). Passed (e) and dropped (f) spectra of the Nyquist OTDM-WDM signal in (d) at WSS node.

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Depending on the signals’ baudrate (N×43 Gbaud), a raised cosine (RC) function with suitable bandwidth (Bo) and roll-off factor (α) is used for Nyquist pulse generation. Here, we set Bo larger than the baudrate by 43 GHz for the purpose of clock recovery (CR). The extra 43 GHz bandwidth is used to insert an optical null-header which taking up one tributary into the Nyquist OTDM. The optical null-header results in a strong 43 GHz clock tone distribution. The CR technique has been proposed in [17], but only for conventional OTDM in which the time space of the null-header can be varied. It is important to note that the CR for Nyquist OTDM is hardly possible by other state-of-the-art ultrafast CR techniques [18, 19] since the raised cosine waveform causes interference between neighboring tributaries. Detailed results about the generation of Nyquist OTDMs and clock recovery will be presented in the next section in Fig. 3 and Fig. 4. After the WDM Nyquist filtering, the signals are polarization multiplexed to double the total system capacity by a polarization multiplexing emulator.

 

Fig. 3 Eye patterns of back-to-back signal at different baudrates: 43 Gbaud (a), 86 Gbaud (b), 172 Gbaud (c), and 344 Gbaud (d). Nyquist pulse waveforms (dot) and RC function fitting (solid line) for 86 Gbaud, 172 Gbaud, and 344 Gbaud Nyquist OTDM signals (e). The signals are captured by high resolution (1 ps) optical sampling oscilloscope (OSO).

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Fig. 4 RMS timing jitter of 43 GHz RF clock recovered from different baudrate signals as a function of transmission distance (a). Insets in (a) are the recovered clocks for 172 Gbaud signal after 4 spans, and for 344 Gbaud signal after 2 spans. RF spectrum of 172 Gbaud Nyquist OTDM over 4 spans after optical-electric conversion (b).

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The transmission network consists of four spans of 80 km dispersion managed fiber, a FlexGrid-compatible WSS, and an EDFA to compensate for the fiber and WSS losses. The 80 km dispersion managed fiber link includes a 56 km super large area fiber (SLA, D = 19.4 ps/nm/km at 1550 nm) followed by a 24 km inverse dispersion fiber (IDF, D = –44.5 ps/nm/km at 1550 nm). The link is fully compensated for both dispersion and dispersion slope. The launched power into each span is 7.6 dBm per polarization. Spectrum elasticity of the network is carried out at WSS nodes through pass-drop operations of Nyquist OTDM-WDM channels.

At each WSS node, the dropped Nyquist OTDM channel is polarization demultiplexed by a polarization controller and a polarizer before sent to a 100 m highly nonlinear fiber for nonlinear sampling down to 43 Gbaud base lines based on four-wave mixing (FWM). The sampling pulses operating at wavelength of 1545 nm, repetition rate of 43 GHz, and pulsewidth of 700 fs are generated from a mode-locked fiber laser (MLFL) followed by a comb-like profiled fiber for pulse compression. The MLFL is driven by a 43 GHz RF clock that is extracted from the dropped Nyquist OTDM with the clock distribution technique mentioned earlier. A commercial clock recovery scheme based on a phase lock loop (PLL) operating at electronic bandwidth (43 GHz) is used for all baudrate OTDM signals. After filtered by a 4.3 nm BPF, the demultiplexed OTDM tributaries are sent to signal analyses equipment such as optical sampling oscilloscope and BER tester.

3. Experimental results

To show the spectral elasticity of the network based on Nyquist OTDM-WDM signals, we generate different cases of spectral arrangement as shown in Fig. 2. For the ease of observation that raised cosine functions are successfully generated for different baudrate signals, here we bypass the 1:N OTDM multiplexer. By this way, the spectra are still coherent and stable for observation. Figs. 2(a)–2(c) show the generation of WDM channels of the same baudrate: 3×688 Gbaud (344 Gbaud/pol), 5×344 Gbaud (172 Gbaud/pol), and 9×172 Gbaud(86 Gbaud/pol) Nyquist OTDM-WDM signals, respectively. Fig. 2(d), on the other hand, is the case of mixed baudrate channels from 86 Gbaud (43 Gbaud/pol) to 688 Gbaud (344 Gbaud/pol). These spectral arrangements are easily done in optics by designing suitable Nyquist filters in the Waveshaper. As presented in the previous section, we make the bandwidth of Nyquist OTDM channels Bo larger than the data signal’s baudrate by 43 GHz for the purpose of clock recovery. The spectral efficiency of the Nyquist OTDM in this case equals to $\frac{\text{N}}{\text{N}+1}$ symbol/s/Hz where N is the number of OTDM tributaries. For instance, SE of 344 Gbaud (N=8) Nyquist OTDM is about 0.9 symbol/s/Hz while that of 172 Gbaud one (N=4) is 0.8 symbol/s/Hz.

For network transmission investigations throughout the rest of the paper, we focus on the case in Fig. 2(d) with mixed baudrate Nyquist OTDM-WDM channels including 2×86 Gbaud (43 Gbaud/pol), 2×172 Gbaud (86 Gbaud/pol), 2×344 Gbaud (172 Gbaud/pol), and one 688 Gbaud (344 Gbaud/pol) channels. Precisely, total network capacity is 1.892 Tbit/s dual-polarization Nyquist OTDM-WDM (946 Gbit/s/pol) with ultra-wide range of granularity from 43 Gbaud to 2×344 Gbaud. In Figs. 2(e) and 2(f), we split neighboring WDM channels of Fig. 2(d) into different ports of a WSS to perform pass-drop functions at a node. These results indicate that WSSs could well handle such ultra-coarse granular, flexible baudrate Nyquist OTDMs as well as efficiently add-drop WDM channels of Nyquist OTDM with almost no guard band. Looking at pass-drop spectra in Figs. 2(e) and 2(f), even though it is out of the scope of this paper, it suggests that the all-optical elastic nodes should also include the capability of rearranging the spectral resources to cope with the dynamic add-drop traffic demands, the so-called spectral defragmentation. Such a functionality is required to operate in a seamless manner to maximize the spectral efficiency in the network as in our recent demonstration [14].

Figures 3(a)–3(d) shows eye patterns of different baudrate signals at 0 km transmission. Although there are strong overlaps between neighboring pulses due to the severe bandwidth limitation by the filtering, a clear eye opening is obtained thanks to the waveform of the raised cosine function having orthogonality condition. Null-headers for clock distribution are also clearly observed for 86 Gbaud, 172 Gbaud, and 344 Gbaud OTDM channels. To confirm the formation of the Nyquist OTDM signal as well as characteristics of the Nyquist filtering, we measure Nyquist pulse waveforms as shown in Fig. 3(e). Note that the eye patterns and waveforms are captured by high resolution (1 ps) optical sampling oscilloscope (OSO). As designed, the measured waveforms for 86 Gbaud, 172 Gbaud, and 344 Gbaud signals are well fit with RC functions with (Bo = 129 GHz, α = 0.4), (Bo = 215 GHz, α = 0.25), and (Bo = 386 GHz, α = 0.15), respectively. Definition of RC function can be found in [20]. Although the choice of α equal to 0 is desirable for densely spectral packing, the prominent side lobes and infinite extent of the sinc function would cause interferences among pulses. Besides, the optical generation of roll-off factor in practice is limited even with the state-of-the-art Waveshaper employed in the experiment. Since the steepness of the filter shape of the Waveshaper does not change with the bandwidth as can be seen in the spectra of different baudrate signals in Figs. 2(e) and 2(f), the roll-off factor of the applied raised cosine function becomes smaller when the bandwidth increases.

In Fig. 4(a) we investigate the performance of the clock recovery technique by measuring root mean square (RMS) timing jitter of the recovered RF clock as a function of network transmission distance. The RMS timing jitter is measured by a 80 GHz bandwidth sampling oscilloscope (Agilent 86100C DCA-J) with a precision time base module (Agilent 86107A) triggered by a 10.75 GHz original reference clock. In this measurement, temporal resolution is limited to around 200 fs by the jitter noise floor of the oscilloscope. An alternative measurement by integrating the single sideband (SSB) phase noise spectrum of the RF clock could be used for more accurate timing jitter evaluation. As seen in Fig. 4(a), the recovered clocks with low timing jitter are obtained for various baudrate signals. Since the quality of the recovered clock depends on the width of the optical null header, higher baudrate signals of narrower null headers gives larger timing jitters [17]. Therefore, at 0 km transmission about 300 fs jitter clock is extracted from 344 Gbaud signal, which is larger than from the lower baudrate signals. The measured timing jitters for 43 Gbaud, 86 Gbaud, and 172 Gbaud are around 200 fs due to the 200 fs jitter noise floor of the oscilloscope. Insets of Fig. 4(a) are waveforms of 43 GHz recovered clocks for 172 Gbaud signal after four spans (320 km, jitter = 315 fs), and 344 Gbaud signal after two spans (160 km, jitter = 365 fs). The good performance clock recovery is due to the suitable insertion of a null-header to Nyquist OTDM, resulting in a stable and strong 43 GHz clock tone distribution as can be seen in Fig. 4(a), RF spectrum of 172 Gbaud Nyquist OTDM over 320 km after optical-electric conversion. In this paper, we do not investigate the transmission performance of 344 Gbaud signal longer than 2 spans because the poor stability of the employed fiber delay OTDM multiplexer makes the experiment system less tolerant to the high timing jitter of the recovered clock. Figure 5 shows eye patterns of different baudrate channels dropped at every WSS nodes. Open eye openings with the maintenance of Nyquist waveforms are achieved in all cases of baudrates and transmission distances.

 

Fig. 5 Eye patterns of different baudrate signals dropped at WSS nodes.

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In order to evaluate the transmission performance, we measure the bit error rate (BER) characteristics of different baudrate signals at all WSS nodes as shown in Fig. 6 for X-polarization signals. Insets in Fig. 6 are eye patterns of demultiplexed signals of 43 Gbaud, 86 Gbaud, 172 Gbaud signals after 4 spans, and 344 Gbaud signal after 2 spans. BER levels smaller than 10^–9 are achieved in all cases. BER curves of the back to back signal are slightly different in Figs. 6(a)–6(d) due to the small differences in the quality of the 43 Gbaud base signals which are generated separately in time for each case in the experiment. However, the power penalties of the transmitted signals with respect to their back to back at 0 km are comparable in all cases of baudrate. We also measure the power penalties at BER of 10^–9 in comparison with the back to back of all tributaries for 172 Gbaud and 344 Gbaud Nyquist OTDMs (X- and Y-polarization signals) after 4-span and 2-span transmissions. The results are shown in Figs. 7(a) and 7(b), respectively. Small power penalty variations were observed among the tributaries as well as between X- and Y-polarizations signals.

 

Fig. 6 BER characteristics of Nyquist signals (X-polarization) at different baudrates: 43 Gbaud (a), 86 Gbaud (b), 172 Gbaud (c), and 344 Gbaud (d), dropped at WSS nodes. Inset in each case is eye pattern of demultiplexed tributary.

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Fig. 7 Power penalty at BER of 10⁻⁹ of all demultiplexed tributaries (X- and Y-polarization) for 172 Gbaud Nyquist OTDM after 4 spans (a), and for 344 Gbaud Nyquist OTDM after 2 spans (b).

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4. Conclusion

Ultrahigh baudrate Nyquist OTDM signal has the advantage over Nyquist WDM by eliminating guard-bands, thus improves the spectral efficiency in the network perspective. We have successfully demonstrated the transmission and pass-drop operations of Nyquist OTDM-WDM signal with baudrate from 43 Gbaud to dual-polarization 344 Gbaud over 320-km SLA-IDF with four WSS nodes. The achieved results suggest that the Nyquist OTDM-WDM network could be a promising candidate for highly efficient, ultra-coarse-granular, elastic optical network in the future big-data centric era.