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transmission) to achieve a higher CNR after transmission. Fig. 7(a) shows the OSNR tolerance in back-to-back. The dashed line is the theoretical limit and is shown for comparison. The required OSNR for a BER of 1e-3 is approximately 33.8 dB due to the use of 10 dB CSPR. Compared with the theoretical limit, the implementation penalty is approximately 3.4 dB. This implementation penalty can be attributed to imperfections in the components, power ripples (see section 4), residual phase noise, and linear crosstalk between bands. With 10 dB CSPR, the required OSNR for the superchannel alone could be approximately 23.4 dB. This OSNR is similar to that for a coherent system [23], where the required OSNR at BER = 1e-3 is approximately 23.4 dB with 224 Gb/s and would become approximately 23.2 dB after scaled to 214 Gb/s. The signal quality in our DDO-OFDM-S system is almost dominated by the sideband because of the high applied CSPR. Fig. 7(b) shows the measured Q 2 as a function of the fiber launch power after 720 km transmission. The presented curves correspond to the first, fifth, and ninth bands, respectively. The optimum power is approximately 8 dBm, which provides the best performance under the limitations of ASE and nonlinear distortion. If we consider the high CSPR of approximately 10 dB, the launch power for the superchannel alone is only about -2.4 dBm. Fig. 7(c) shows measured Q 2 as a function of the band index in back-to-back and after 720 km SSMF. The optimum launch power of 8 dBm is applied. In back-to-back, the fifth band shows the worst performance (Q 2 ≈ 12 dB) caused by insufficient bandwidth of the receiver’s components; the other bands have Q 2 factors greater than 12 dB. After transmission, the worst performance occurs on the first band, which still yields a Q 2 higher than the 7% FEC threshold of 8.53 dB [24], and the Q 2 factor variation between the different bands is reduced because it is the noise and nonlinear distortion, not the receiver’s bandwidth, that dominate performance. 6.2.1 Discussion Using our proposals, in the 16-QAM experiment, we generated and demodulated the record capacity per wavelength (214 Gb/s) for DDO-OFDM systems. We also achieved a record distance of 720 km SSMF for greater than 100 Gb/s DDO-OFDM systems. Because of the strong nonlinear phase noise at the beginning of the link, a longer distance might be possible by using uncorrelated data bands, for example, approximately 2 dB Q 2 difference, as shown in Fig. 6(c). To reach 720 km transmission, we used a high CSPR of approximately 10 dB, that is, high carrier power, to provide better CNR after transmission. Such a high carrier power would induce strong FWM terms at both sides out of the carriers, which in our experiment, could be removed through moderate optical filtering. In a WDM system, these FWM terms would possibly overlap with the adjacent channels and become in-band nonlinear noise that results in some penalty. Therefore, in WDM systems, CSPR should be carefully controlled and optimized so that strong FWM terms are not involved. Another simple way to avoid FWM is to use S pecial Topic Greater than 200 Gb/s Transmission Using Direct-Detection Optical OFDM Superchannel Wei-Ren Peng, Itsuro Morita, Hidenori Takahashi, and Takehiro Tsuritani polarization interleaving for WDM neighboring channels. However, this would eliminate the possibility of using PDM. 7 Conclusion We have proposed DDO-OFDM-S and OMBR, a high-capacity long-reach solution for DDO-OFDM systems. We have discussed optimum CSPR in back-to-back and after transmission and found that a higher carrier power is needed to improve performance after transmission. We have also derived the analytical form of back-to-back CSPR and verified its correctness using numerical simulations. To maintain the system’s high throughput, we have proposed a low-overhead I/Q imbalance estimation method. Our experiment shows that in greater than 1 Gb/s DDO-OFDM systems, the proposed DDO-OFDM-S enables unprecedented capacity of 214 Gb/s per wavelength over an unprecedented distance of 720 km SSMF. References [1] P. J. Winzer, G. Raybon, S. Haoyu, A. Adamiecki, S. Corteselli, A. H. Gnauck, D. A. Fishman, C. R. Doerr, S. Chandrasekhar, L. L. Buhl, T. J. Xia, G. Wellbrock, W. Lee, B. Basch, T. Kawanishi, K. Higuma, and Y. Painchaud,“100 Gbit/s DQPSK transmission: from laboratory experiments to field trials,”IEEE J. Lightwav. Technol., vol. 26, no. 20, pp. 3388-3402, Oct. 2008. [2] M. Daikoku, I. Morita, H. Taga, H. Tanaka, T. Kawanishi, T. Sakamoto, T. Miyazaki, T. Fujita,“100-Gbit/s DQPSK transmission experiment without OTDM for 100G Ethernet transport,”, IEEE J. Lightwav. Technol., vol. 25, no. 1, pp. 139-145, Jan. 2007. [3] R. I. Killey, P. M. Watts, M. Glick, and P. Bayvel,“Electronic dispersion compensation by signal pre-distortion,”in Optical Fiber Commun. Conf. (OFC’06), Anaheim, CA, Paper OWB3. [4] S. L. Jansen, I. Morita, K. Forzesh, S. Randel, D van den Borne, H. Tankak, “Optical OFDM, a hype or is it for real?”Proc. European Conf. Optical Cummun. (ECOC ’08), Brussels, Paper Mo3E3. [5] A. J. Lowery, L. Du, and J. Armstrong,“Orthogonal frequency division multiplexing for adaptive dispersion compensation in long haul WDM systems,”Proc. Optical Fiber Commun. Conf. (OFC ’06), Anaheim, CA, paper PDP39, 2006. [6] S. Seno, E. Horiuchi, T. Sugihara, T. Uesugi, T. Ichikawa, Y. Baba, and T. Mizuochi, “A GMPLS-based path establishment experiment with tunable dispersion compensation over in-field fibers,”Proc. European Conf. Optical Commun. (ECOC 2010), Turin, Italy, paper Tu.4.B.3. [7] M. Yagi, S. Tanaka, S. Satomi, S. Ryu, K. Okamura, M. Aoyagi and S. Asano,“Field trial of GMPLS triple plane integration for 40 Gbit/s dynamically reconfigurable wavelength path network,”Electron. Lett., vol. 41, pp. 492-494, Apr. 2005. [8] A. Amin, H. Takahashi, I. Morita, and H. Tanaka,“100-Gbps direct-detection OFDM transmission on independent polarization tributaries,”IEEE Photon. Technol. Lett., vol. 22, no. 7, pp. 468-470, 2010. [9] D. Qian, N. Cvijetic, J. Hu, and T. Wang,“108 Gbit/s OFDMA-PON with polarization multiplexing and direct detection,”IEEE J. Lightwave Technol., vol. 28, no. 4, pp. 484-493, Feb. 2010. [10] B. J. C. Schmidt, Z. Zan, L. B. Du and A.J. Lowery,“120 Gbit/s over 500-km using single-band polarization-multiplexed self-coherent optical OFDM,”IEEE J. Lightwav. Technol., vol. 28, no. 4, pp. 328-335, Feb. 2010. [11] W.-R. Peng, H. Takahashi, I. Morita, and H. Tanaka,“Transmission of a 214-Gbit/s single-polarization direct-detection optical OFDM superchannel over 720-km standard single mode fiber with EDFA-only amplification,”Proc. Optical Fiber Commun. Conf. (ECOC 2010), Torino, Italy, Paper PDP 2.5. [12] W.-R Peng, H. Takahashi, I. Morita, and H. Tanaka, "117-Gbit/s optical OFDM super-channel transmission over 1200-km SSMF using direct detection and EDFA-only amplification," presented at Optical Fiber Commun. Conf. (OFC 2011), Los Angeles, CA, Paper OThX1. [13] Q. Yang, Y. Tang, Y. Ma, W. Shieh,“Experimental demonstration and numerical simulation of 107-Gbit/s high spectral efficiency coherent optical OFDM,”J. Lightwav. Technol., vol. 27, no. 3, pp. 168-176, Feb. 2009. [14] A. J. Lowery, L. B. Du, and J. Armstrong,“Performance of optical OFDM in ultra-long haul WDM lightwave systems,”J. Lightwav.Technol., vol. 25, no. 1, pp. 131-138, Jan. 2007. [15] S. L. Jansen, I. Morita, and H. Tanaka,“10 × 121.9 Gbit/s PDM-OFDM transmission with 2-b/s/Hz spectral efficiency over 1,000 km of SSMF,”Proc. Optical Fiber Commun. Conf. (OFC 2008), San Diego, CA, paper PDP2, 2008. ➡To P. 29 March 2012 Vol.10 No.1 ZTE COMMUNICATIONS 17
S pecial Topic Spatial Mode-Division Multiplexing for High-Speed Optical Coherent Detection Systems William Shieh, An Li, Abdullah Al Amin, Xi Chen, Simin Chen, and Guanjun Gao Spatial Mode-Division Multiplexing for High-Speed Optical Coherent Detection Systems William William Shieh Shieh 1 1 1 1 1 1, 2 , , An An Li Li , , Abdullah Abdullah Al Al Amin Amin , , Xi Xi Chen Chen , , Simin Simin Chen Chen , , and and Guanjun Guanjun Gao Gao (1. Department of Electrical and Electronic Engineering, The University of Melbourne, 3010 Parkville, VIC, Australia; 2. State Key Laboratory of Information Photonics and Optical Communications (Beijing), Beijing University of Posts and Telecommunications, 100876, China) Abstract Spatial mode-division multiplexing is emerging as a potential solution to further increasing optical fiber capacity and spectral efficiency. We report a dual-mode, dual-polarization transmission method based on mode-selective excitation and detection over a two-mode fiber. In particular, we present 107 Gbit/s coherent optical OFDM (CO-OFDM) transmission over a 4.5 km two-mode fiber using LP01 and LP11 modes in which mode separation is performed optically. Keywords coherent communications; few-mode fiber; mode converter; fiber optic components T1 Introduction he fast advance of high-speed optical transport systems has spurred the use of dense wavelength division multiplexing (DWDM), polarization multiplexing, coherent detection, and multilevel modulation schemes in optical fiber transmission. However, higher required optical signal-to-noise ratio (OSNR) causes higher transmission power per channel, and this may give rise to impairments caused by fiber nonlinearity [1], [2]. Recently, there has been increasing interest in few-mode fiber (FMF) as the next-generation fiber for achieving capacity beyond that of standard single-mode fiber (SSMF) [3]-[7]. One of the advantages of FMF, for example, two-mode fiber (TMF), is that spacial modes are multiplexed, and this doubles or triples fiber capacity without exponentially increasing SNR (as is the case with SSMF fiber) [4]-[7]. By adding another degree of freedom (here, the spatial modes with respect to the wavelength, polarization, and higher-order modulation), the information data coding can also be made more efficient [8]. Compared with previous approaches based on conventional multimode fiber (MMF) [9]-[12], FMF has better mode selectivity and can more easily manage mode impairments. Recently, a number of groups have mode-division multiplexed (MDM) LP01 and LP11 modes [4], [5], two degenerate LP11 modes (LP11a + LP11b) [6], and even all three modes (LP01 + LP11a + LP11b) [7] over FMFs or TMFs. The 18 ZTE COMMUNICATIONS March 2012 Vol.10 No.1 advance to FMF transmission requires new research on topics ranging from device to system level, including TMF design, TMF-compatible component design, and TMF transmission. In mode-multiplexed transmission systems, mode-selective components are critical. All the mode-selective devices proposed in [4] and [5] fall into two main categories: free-space based and fiber based. The former uses phase masks that are based on liquid crystal on silicon (LCoS) spatial light modulator (SLM) [6] or specially fabricated glass plate [7]. Free-space components are often bulky whereas a fiber-based one is compact and can be easily integrated. The coupler proposed in [13] could be a promising solution for future MDM systems even though fabrication of the coupler involves sophisticated fiber etching, fusion, and tapering. A simple, tunable mechanical pressure-induced long-period fiber grating (LPFG) is an efficient mode converter [14]. An LPFG mode converter has been applied in a 2 × 10 Gb/s non-return-to-zero (NRZ) MDM system [5] and a 107 Gb/s mode-multiplexed OFDM transmission system [4]. We propose 107 Gbit/s coherent optical OFDM (CO-OFDM) transmission over a 4.5 km TMF using LP01 and LP11 modes in which mode separation is performed optically. 2 Mode-Selective Component Design 2.1 LPFG-Based Mode Converter The main purpose of the mode converter is to convert
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