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Elastic optical networks (EON) based on optical superchannel enables higher spectral flexibility, in which the network nodes should provide multiple all-optical functionalities to manipulate bandwidth-variable data traffic. In this paper, we propose and demonstrate an EON node structure supporting reconfigurable optical superchannel multicasting. The node structure incorporates a shared multicasting module, which performs reconfigurable selection of target incoming/outgoing superchannels/replicas and leverages a group of nonlinear devices to satisfy multiple multicast requests. Moreover, an optical comb is utilized to efficiently provide and manage all pump resources for multicasting with potential cost reduction and phase noise inhibition. Based on the node structure, we experimentally demonstrate polarization division multiplexing (PDM) superchannel multicasting scenarios with different replica amount, input/output locations, and modulation formats. Less than 0.7 dB optical signal-to-noise ratio (OSNR) penalties are demonstrated in multiple multicasting scenarios.
© 2015 Optical Society of America
1. Introduction
In recent years, with the rapid growth of Internet traffic which challenges the optical backbone infrastructure, elastic optical networks (EON) based on optical superchannel has been proposed as a spectrally-efficient solution to adaptively deliver data traffic [1,2 ]. In order to provide greater spectral flexibility, the node technologies in EON should support multiple physical-layer operations on both superchannel itself and arbitrary subband combinations within it [3]. Individual subband switching and reconfigurable optical add-drop multiplexing (ROADM) functionalities have been proved feasible on optical superchannel [3–6 ].
Optical superchannel multicasting could be another useful technology in EON nodes, which evolves from wavelength multicasting in wavelength division multiplexing (WDM) networks [7–10 ]. By generating superchannel replicas at different frequencies, a super-wavelength traffic can be distributed to multiple destinations, which could satisfy network demands with point-to-multipoint feature such as video conference. Different from multicasting/broadcasting based on light splitter, multicasting in spectrum domain can relieve the wavelength continuity constraint to some extent, so that the efficiency of link spectrum utilization can be enhanced and traffic blocking due to spectrum contention on downstream links can be potentially mitigated [11]. Moreover, multicasting at intermediate nodes also helps combat spectral fragmentation problem [12,13 ]. Compared to traditional WDM multicasting, superchannel multicasting in EON should further support bandwidth-variable (BV) polarization division multiplexing (PDM) signals with advanced modulation formats (e. g., quadrature amplitude modulation, QAM) and more flexible input and output spectral range.
Recently we have implemented PDM superchannel multicasting by multiple-pump four-wave mixing (FWM) in highly nonlinear fiber (HNLF) [14–16 ]. But previous works only concentrated on demonstrating the feasibility, and how to efficiently deploy such functional block in the network nodes remains to be investigated. From the network perspective, developing EON node structures that support multicasting of PDM superchannels needs to be addressed as the next step. The desired features of such nodes may include: wide input spectral range, tunable spectral positions of output replicas, capability of simultaneous multicasting for multiple superchannels from the same or different input ports, and low performance penalty after multicasting. Meanwhile, both the amount and locations of required replicas could vary with practical network requirement, so the amount and wavelengths of all pumps should be reconfigurable. Therefore, an efficient solution for pump supply and management in the node would be critical.
The major objective and contribution of this work is the proposal and experimental validation of an EON node structure that supports reconfigurable superchannel multicasting. Both spatial and spectral multicasting of superchannels are enabled by incorporating basic cross-connect part and a new multicasting module shared across the node. In particular, the subbands to be multicast in all incoming superchannels are separated and routed to a group of nonlinear devices for replica generation. After multicasting, desired replicas are switched to corresponding node outputs, thereby satisfying multiple multicasting requirements at one time. On the other hand, all the pumps for multiple multicasting processes are provided by a single optical comb instead of an array of independent tunable lasers, which may reduce the overall cost and managing complexity. By reprogramming a polarization-maintaining (PM) BV wavelength selective switch (WSS) whose outputs are different wavelength combinations from the optical comb, reconfigurable pump supply and efficient pump management are enabled. Based on the proposed node structure, we experimentally demonstrate PDM superchannel multicasting scenarios with different replica amount, input and output location, and modulation formats. Moreover, optical signal-to-noise ratio (OSNR) penalties of less than 0.7 dB are validated in both quadrature phase shift keying (QPSK) and 8-QAM superchannel multicasting.
This paper is organized as follows. The technique principle of proposed EON node structure is detailed in Section 2. Section 3 describes the experiment setup. Experiment results are reported with discussion in Section 4. Finally, the paper is concluded in Section 5.
2. Technique principles
2.1 Superchannel multicasting-capable EON node structure
Figure 1(a) illustrates the N × N EON node structure supporting reconfigurable superchannel multicasting, which consists of an N × N cross-connect part and a multicasting module. The broadcast-and-select cross-connect part consists of optical couplers (OC) and BV-WSS [6]. For the multicasting part, deploying a set of tunable pump lasers might bring challenges in implementation cost as well as managing complexity. We propose to use a monolithic optical comb [17] followed by a multi-port PM BV-WSS as a centralized pump resource pool for all multicast processes. Optical comb offers a set of stable and flat frequency tones in a compact way, while the PM BV-WSS is reconfigurable to output different wavelength combinations for pump supply and management. When multiple multicasting requests arrive, all the subbands to be multicast in incoming superchannels are separated by an N × M BV-WSS and forwarded to appropriate nonlinear devices (NLD, equipped M in total). If HNLF is employed as nonlinear device, PDM superchannel multicasting can be achieved using multiple-pump FWM [14–16 ] with multiple co-polarized pumps provided by optical comb. The generated replicas are switched by an M × N BV-WSS to corresponding output ports of the node. In this way, multiple multicasting requests can be simultaneously met, and each input superchannel can be either spectrally or spatially (i. e., light-split [18]) multicast to multiple output ports. The spectral position of each replica can be either unchanged or shifted from the original superchannel according to practical network conditions, which enhances the flexibility of the network. All the BV-WSSs can be reprogrammed to create lightpaths with or without multicasting for different traffic requests by exploiting proper control plane technologies (e.g. software-defined networking, SDN [19]).
Fig. 1 Proposed EON node structures supporting reconfigurable superchannel multicasting. NLD: nonlinear device.
Figure 1(b) shows a modified N × N node structure, in which the BV-WSSs are placed at node inputs instead of node outputs to perform superchannel selection. This way, the N × M BV-WSS in the multicasting module can be saved and substituted by an optical switch, which can be more cost-effective. In order to support c concurrent multicasting requests from the same node input, N + c ports would be needed for each BV-WSS to separate the c signals to be multicast.
2.2 Optical comb-based multicasting module for PDM superchannels
The working principle of the multicasting module with optical comb is described as follows. Let Δf denote the tone spacing of the optical comb, and the center frequency and bandwidth of a superchannel S to be multicast are denoted by fS and BS respectively. A single multicasting request is considered here for clarity. The spectral locations of generated superchannel replicas {fR1, fR2, …, fRM} are determined based on both the spectrum availability on the downstream links (to avoid traffic collision) and relative physical-layer constraints. When PDM superchannel multicasting is implemented based on multiple-pump FWM in HNLF and optical comb, the following physical-layer constraints should be satisfied.
FWM constraint (fPj and fPk are pump frequencies):
Each pump spacing should be an integer multiple of Δf:
The frequency relationship among different pumps should be carefully configured. Exponentially growing spaced (EGS) scheme [14] or recursive pump-adding scheme [16] can be adopted depending on specific multicast request. In addition, the frequency relationship between pumps and original superchannel should follow:
In (3), p is a non-negative integer, and the polarization sensitivity of PDM superchannel multicasting reduces as p increases [15]. fP 1 denotes the center frequency of pump P 1 that is spectrally nearest to S. BG is bandwidth of a guard-band introduced to prevent replicas from being contaminated by pump idlers. R is the total amount of newly-generated superchannel replicas. In view of the constraints above due to FWM principle, the spectrum availability on all the multicast downstream links should be jointly considered to decide the frequencies of pumps and passband of BV-WSSs, so that replicas can be generated and forwarded without spectrum collision. Then the PM BV-WSS is configured to output tone combinations of optical comb as pumps.
Another potential benefit of the optical comb-based pump scheme here is the inhibition of phase noise (PN) from pumps. It has been shown that single-polarization WDM multicasting by FWM with coherent pumps can be almost free from pump PN [20]. In the PDM superchannel multicasting supported by the EON node, the output tones/pumps of the optical comb are also coherent. So the phase noise from pumps is cancelled out and the superchannel replicas do not suffer from this impairment in principle. As such, a seed laser with larger linewidth can be deployed to reduce the cost of optical comb. We verify this benefit by experiment, as will be shown in Section 4.
3. Experiment setup
We conduct experiments with different network scenarios to demonstrate the EON node supporting reconfigurable superchannel multicasting. The network setup is depicted in Fig. 2(a) , which consists of one source node in which optical PDM superchannel is generated (Node 1), an intermediate node with the structure presented in Fig. 1(a) (Node 2) and two destination nodes where the multicast superchannels are received (Node 3&4).
Fig. 2 Experiment setup. (a) Network overview. (b) Node 1. (c) Node 2. (d) Node 3&4. (e) Spectrum of amplified comb lines with total power of 15 dBm.
Node 1 generates a superchannel with four subbands and total bandwidth of 50 GHz, the detailed setup of which is shown in Fig. 2(b). Each subband is modulated with QPSK or 8-QAM single-carrier frequency division multiplexing (SCFDM) utilizing discrete Fourier transform spread orthogonal frequency division multiplexing (DFT-S-OFDM) [5,21 ]. For digital SCFDM data generation, 200-point DFT, 256-point inverse DFT, 4 pilots, and 16 samples of cyclic extension are adopted. An arbitrary waveform generator (AWG) with sampling rate of 12.5 GS/s generates baseband SCFDM signal. The coupled output of four lasers is fed into an IQ modulator and modulated by the baseband SCFDM signal to generate a superchannel before passing a PM Erbium-doped fiber amplifier (EDFA). PDM is emulated with a polarization controller (PC), a polarization beam splitter (PBS), an optical delay line with delay of exactly one SCFDM symbol period, and a polarization beam combiner (PBC).
Figure 2(c) shows detailed setup of Node 2. The cross-connect part is composed of an optical coupler and two BV-WSSs (Finisar Waveshaper 4000S) with EDFAs for loss compensation. The superchannel from Node 1 is power-split and fed into the multicasting module, which is amplified and coupled with the comb-generated co-polarized pumps before being sent into a nonlinear device for multicasting. A tunable seed laser at 193.725 THz is fed into a phase-modulation-based optical frequency comb generator (OFCG) together with 25 GHz RF signal to produce comb lines equally spaced at 25 GHz [see Fig. 2(e)]. The linewidth of the seed laser (and thus each comb tone) is about 1 MHz. A PM BV-WSS (Finisar Waveshaper 1000SP) is used to select different combinations of amplified comb tones before power boosting and a polarization controller (PC). To emulate practical nodes, two alternate nonlinear devices are equipped, namely HNLF1 and HNLF2. The attenuation coefficient (AC), zero-dispersion wavelength (ZDW) and dispersion slope (DS) of HNLF1 are ~0.75 dB/km, ~1550 nm and ~0.02 ps/nm2/km respectively. The AC, ZDW and DS of HNLF2 are ~0.9 dB/km, ~1567 nm and ~0.03 ps/nm2/km respectively. The length and nominal nonlinear coefficient of HNLF1&2 are both 1 km and 10 W−1/km. Multiple-pump FWM scheme [14] is adopted for PDM superchannel multicasting. Afterwards, the produced replicas are selected and forwarded to destination nodes. The bandwidth of BV-WSS for the 50 GHz superchannels is 62.5 GHz.
At Node 3&4 [see Fig. 2(d)], a target superchannel or replica after pre-amplification and 50 GHz tunable optical band-pass filter (OBPF) is sent for coherent detection. The linewidth of local oscillator (LO) is about 100 kHz. One subband is detected at a time, and offline processing algorithms are similar with [5].
4. Experimental results and discussion
In the first experiment, we conduct five network scenarios to investigate the input spectral range supported by the multicasting-capable EON node. Both 1-to-3 and 1-to-7 QPSK superchannel multicasting with replica spacing of 100 GHz are studied. HNLF1 is employed. The spectra of pumps selected by PM BV-WSS and HNLF output in scenario A1 to A5 together with Q-factor performance of each newly-generated superchannel replica are shown in Figs. 3(a)-3(e) , respectively. The Q factor (blue squares in the figures) of each replica is derived from error vector magnitude (EVM) [22] and is an average of four subbands inside. For reference, the Q factor of original superchannel after having passed through Node 2 is measured to be 17 dB. The Q factors of all replicas are better than the 7% forward error correction (FEC) threshold of 8.53 dB [15] (dash lines in the figures). The two orthogonal polarizations in all cases have similar performance. 1-to-3 multicasting with input superchannel at 195.7 THz (near 1530 nm), 191.7 THz (near 1565 nm) and 193.4 THz (near 1550 nm) are demonstrated, while 1-to-7 multicasting guarantees post-FEC error-free performance when the input is at 195.05 THz (~1537 nm) and 191.9 THz (near 1565 nm). In this experiment, the total power on the signal branch is 8.5 dBm and the total power on the pump branch is 20.5 dBm. The total power is below the stimulated Brillouin scattering (SBS) threshold of employed HNLF. The Q factor performance of replicas can be further enhanced by optimizing these powers for FWM interaction [14], optimizing spectral spacing between pumps and signal, and by filtering out noise from EDFAs [20]. Meanwhile, when the input superchannel and selected pumps are located near ZDW of HNLF2, the node can reconfigure the N × M BV-WSS to use HNLF2 for potential conversion efficiency improvement [23]. Also, optical combs with improved tone-to-noise ratio can be used to achieve better multicast performance. To conclude, the experimental results indicate that the proposed node structure has a strong potential to support 1-to-3 and 1-to-7 PDM superchannel multicasting with input wavelength range of full C-band.
Fig. 3 (a)-(e) Spectra of scenario A1-A5. Blue squares inside show measured Q factors of newly-generated replicas.
Then, we demonstrate spectral tunability of replicas with the proposed node structure. The pump frequencies are adjusted by simply reconfiguring the PM BV-WSS, so that the frequencies of replicas can be accordingly tuned. Both 1-to-3 and 1-to-7 PDM-QPSK superchannel multicasting scenarios are investigated. In 1-to-3 multicasting scenarios, the original superchannel is centered at 192.1 THz. In 1-to-7 multicasting scenarios, the original superchannel is centered at 192.625 THz. According to the input frequencies in these cases, the pumps are placed near ZDW of HNLF2 and the node switches to use HNLF2 [23]. The seed light of the optical comb is placed at about 195.5 THz in this measurement for better tone/pump-to-noise ratio. Figures 4(a)-4(g) plot spectra of HNLF output in scenario B1 to B7, including 1-to-3 multicasting with copy spacing of 75, 100, 125 and 150 GHz, and 1-to-7 multicasting with copy spacing of 75, 100, and 125 GHz. In each scenario, the frequencies of pumps are determined according to criteria (1)-(3) in Subsection 2.2. As two examples, Figs. 4(d) and 4(g) also shows measured Q factors of newly-generated superchannels in case B4 and B7 by blue squares, which are all well above the FEC limit. In this experiment, the total power on the signal branch is 8.5 dBm and the total power on the pump branch is 20.5 dBm. If larger replica spacing is required, the multicasting module can continuingly enlarge the pump spacing by reconfiguring the PM BV-WSS. The power of pumps and signal for FWM interaction should be further optimized to guarantee the signal quality in these cases. Meanwhile, optical comb with smaller tone spacing can be employed for finer tunability. The tunability may be improved by using other spectrum conversion schemes, which can also be implemented by the node structure. For example, multicasting based on cascaded sum frequency generation (SFG) and difference frequency generation (DFG) in periodically poled Lithium Niobate (PPLN) waveguide can be introduced, in which each signal replica can be tuned independently by tuning its corresponding DFG pump [12,19 ].
Fig. 4 (a)-(g) Spectra of HNLF output in scenario B1-B7. Blue squares inside show measured example Q factors of newly-generated replicas in scenario B4 and B7.
Figures 5(a) and 5(b) show spectra of each superchannel replica at the node output in scenario B4 and B7. Figures 5(c) and 5(d) show OSNR values of replicas in scenario B4 and B7, respectively. The original superchannel is routed through the cross-connect part to the output directly, and its OSNR remains ~40 dB. The newly-generated replicas have OSNRs of around 20 dB. In routing, modulation and spectrum assignment (RMSA) for a lightpath with superchannel multicasting, we should fully consider not just the general loss/impairments of transmission and switching, but also degradation of OSNR and filtering effects induced by the multicasting module. Moreover, advanced configuration of BV-WSSs along the route can be considered to alleviate the adverse effects of additional filtering, such as differentiated passband scheme [24] and superfilter technique [25].
Fig. 5 (a)-(b) Spectra of each superchannel replica at the output of the EON node in scenario B4 and B7. (c)-(d) OSNR of each superchannel replica at the output of the EON node in scenario B4 and B7.
Finally, we investigate bit-error ratio (BER) versus OSNR performance of PDM superchannel multicasting supported by the node structure. The second subband of each replica is measured. As demonstration, QPSK format is used to measure OSNR-BER curve of scenario A1 and B7, while 8-QAM format is used to measure OSNR-BER curve of scenario B1, B4 and B5. The original superchannel after having passed through Node 2 always has negligible OSNR penalty with respect to back-to-back (BtB) case. The results of newly-generated replicas in these cases are shown in Figs. 6(a)-6(e) respectively. Less than 0.7 dB OSNR penalty is observed at 7% FEC threshold [15] (BER = 3.8e-3, dash lines in the figures) in all cases, although pumps with about 1 MHz linewidth are employed. This implies that the proposed EON node can implement superchannel multicasting with low penalty, which is preferable to practical network requirements.
Fig. 6 (a)-(e) OSNR-BER curve of newly-generated superchannels in scenario A1, B7 with QPSK format and B1, B4 and B5 with 8-QAM format.
5. Conclusion
We have proposed EON node structures supporting reconfigurable optical superchannel multicasting, in which multiple multicasting requests can be simultaneously met using a multicasting module shared across the node. All pumps are generated by an optical comb and a reconfigurable WSS is followed to manage pump combinations in the node. Based on the EON node structure, optical superchannel multicasting scenarios with different replica amount, input/output locations, and modulation formats are experimentally demonstrated to validate the feasibility. The OSNR penalties of generated replicas are smaller than 0.7 dB for both QPSK and 8-QAM superchannels.
Acknowledgment
This work was supported by National Basic Research Program of China (973 Program, No. 2014CB340105 and No. 2012CB315606) and NSFC (No. 61377072, No. 61275071 and No. 61205058).
References and links
1. X. Liu and S. Chandrasekhar, “Superchannel for next-generation optical networks,” in Optical Fiber Communication Conference (OSA, 2014), paper W1H.5. [CrossRef]
2. O. Gerstel, M. Jinno, A. Lord, and S. J. B. Yoo, “Elastic optical networking: a new dawn for the optical layer?” IEEE Commun. Mag. 50(2), s12–s20 (2012). [CrossRef]
3. E. Pincemin, M. Song, J. Karaki, O. Zia-Chahabi, T. Guillossou, D. Grot, G. Thouenon, C. Betoule, R. Clavier, A. Poudoulec, M. Van der Keur, Y. Jaouen, R. Le Bidan, T. Le Gall, P. Gravey, M. Morvan, B. Dumas-Feris, M. L. Moulinard, and G. Froc, “Multi-band OFDM transmission at 100 Gb/s with sub-band optical switching,” J. Lightwave Technol. 32(12), 2202–2219 (2014). [CrossRef]
4. R. Dischler, F. Buchali, and A. Klekamp, “Demonstration of bit rate variable ROADM functionality on an optical OFDM superchannel,” in Optical Fiber Communication Conference (OSA, 2010), paper OTuM7. [CrossRef]
5. Y. Chen, J. Li, C. Zhao, L. Zhu, F. Zhang, Y. He, and Z. Chen, “Experimental demonstration of ROADM functionality on an optical SCFDM superchannel,” IEEE Photonics Technol. Lett. 22(17), 1315–1317 (2012).
6. Y. Chen, J. Li, C. Zhao, Y. Zhong, Y. He, and Z. Chen, “Experimental demonstration of subband switching functionality on optical superchannel,” in Optical Fiber Communication Conference (OSA, 2012), paper JW2A.1. [CrossRef]
7. Y. Wang, C. Yu, T. Luo, L. Yan, Z. Pan, and A. E. Willner, “Tunable all-optical wavelength conversion and wavelength multicasting using orthogonally polarized fiber FWM,” J. Lightwave Technol. 23(10), 3331–3338 (2005). [CrossRef]
8. D. Wang, T. H. Cheng, Y. K. Yeo, J. Liu, Z. Xu, Y. Wang, and G. Xiao, “All-optical modulation-transparent wavelength multicasting in a highly nonlinear fiber Sagnac loop mirror,” Opt. Express 18(10), 10343–10353 (2010). [CrossRef] [PubMed]
9. A. Yi, L. Yan, B. Luo, W. Pan, J. Ye, and Z. Chen, “One-to-eleven (11×10-Gbps) all-optical NRZ-to-RZ format conversion and wavelength multicasting using a single pump in a HNLF,” in Optical Fiber Communication Conference (OSA, 2011), OThE5.
10. J. Wang, J.-Y. Yang, and A. Willner, “Constellation manipulation for optical multicasted hexadecimal coding/decoding of 10-Gbauds 16-QAM using non-degenerate FWM in HNLFs,” in Optical Fiber Communication Conference (OSA, 2012), OTh3H.3. [CrossRef]
11. T. Rahman, G. Ellinas, and M. Ali, “Lightpath- and light-tree based groupcast routing and wavelength assignment in mesh optical networks,” J. Opt. Commun. Netw. 1(2), A44–A55 (2009). [CrossRef]
12. A. Malacarne, G. Meloni, G. Berrettini, N. Sambo, L. Poti, and A. Bogoni, “Optical multicasting of 16QAM signals in periodically-poled Lithium Niobate waveguide,” J. Lightwave Technol. 31(11), 1797–1803 (2013). [CrossRef]
13. B. Guo, Y. Xu, P. Zhu, Y. Zhong, Y. Chen, J. Li, Z. Chen, and Y. He, “Multicasting based optical inverse multiplexing in elastic optical network,” Opt. Express 22(12), 15133–15142 (2014). [CrossRef] [PubMed]
14. Y. Chen, J. Li, P. Zhu, B. Guo, L. Zhu, Y. He, and Z. Chen, “Experimental demonstration of 400 Gb/s optical PDM-OFDM superchannel multicasting by multiple-pump FWM in HNLF,” Opt. Express 21(8), 9915–9922 (2013). [CrossRef] [PubMed]
15. P. Zhu, J. Li, Y. Chen, Y. Xu, N. Zhang, B. Guo, Z. Chen, and Y. He, “Demonstration of 1-to-13 PDM-8QAM SCFDM superchannel multicasting in HNLF,” in Optical Fiber Communication Conference (OSA, 2014), paper Th1D.3. [CrossRef]
16. P. Zhu, J. Li, Z. Wu, X. Chen, Y. Xu, B. Lin, Z. Chen, and Y. He, “Recursive pump-adding scheme for optical superchannel multicasting based on FWM,” Opt. Commun. 347, 25–30 (2015). [CrossRef]
17. M. Kourogi, T. Enami, and M. Ohtsu, “A monolithic optical frequency comb generator,” IEEE Photonics Technol. Lett. 6(2), 214–217 (1994). [CrossRef]
18. L. Gong, X. Zhou, X. Liu, W. Zhao, W. Lu, and Z. Zhu, “Efficient resource allocation for all-optical multicasting over spectrum-sliced elastic optical networks,” J. Opt. Commun. Netw. 5(8), 836–847 (2013). [CrossRef]
19. N. Sambo, F. Paolucci, G. Meloni, F. Fresi, L. Potì, and P. Castoldi, “Control of frequency conversion and defragmentation for super-channels [invited],” J. Opt. Commun. Netw. 7(1), A126–A134 (2015). [CrossRef]
20. G.-W. Lu, T. Sakamoto, and T. Kawanishi, “Pump-phase-noise-tolerant wavelength multicasting for QAM signals using flexible coherent multi-carrier pump,” in Optical Fiber Communication Conference (OSA, 2015), paper M2E.2. [CrossRef]
21. X. Chen, A. Li, G. Gao, and W. Shieh, “Experimental demonstration of improved fiber nonlinearity tolerance for unique-word DFT-spread OFDM systems,” Opt. Express 19(27), 26198–26207 (2011). [CrossRef] [PubMed]
22. R. Schmogrow, B. Nebendahl, M. Winter, A. Josten, D. Hillerkuss, S. Koenig, J. Meyer, M. Dreschmann, M. Huebner, C. Koos, J. Becker, W. Freude, and J. Leuthold, “Error vector magnitude as a performance measure for advanced modulation formats,” IEEE Photonics Technol. Lett. 24(1), 61–63 (2012). [CrossRef]
23. K. Inoue, “Four-wave mixing in an optical fiber in the zero-dispersion wavelength region,” J. Lightwave Technol. 10(11), 1553–1561 (1992). [CrossRef]
24. N. Sambo, G. Meloni, F. Paolucci, F. Cugini, M. Secondini, F. Fresi, L. Pot, and P. Castoldi, “Programmable transponder, code and differentiated filter configuration in elastic optical networks,” J. Lightwave Technol. 32(11), 2079–2086 (2014). [CrossRef]
25. F. Paolucci, F. Cugini, F. Fresi, G. Meloni, A. Giorgetti, N. Sambo, L. Potí, A. Castro, L. Velasco, and P. Castoldi, “Superfilter technique in SDN-Controlled elastic optical networks [invited],” J. Opt. Commun. Netw. 7(2), A285–A292 (2015). [CrossRef]
