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  • 1. What the future holds for few-mode fiber transmission? William Shieh Centre for Energy-Efficient Telecommunications National ICT AustraliaDepartment of Electrical and Electronic EngineeringThe University of Melbourne, Melbourne, Australia
  • 2. PhD Program at Melbourne Uni澳大利亚墨尔本大学电机与电子工程系教授Prof. Shieh目前正在招收优秀博士生(本科或者研究生平均分达到80分及以上, 以及排名30% 以上),可以选择的研究方向包括(1)正交频分复用技术(OFDM)在光或者无线网络中的应用(2)相干光通信,光信号处理以及信道均衡(3)波导设计和非线性的特性描述(4)全光数据包交换,波长转换以及新型光网络结构(5)射频(RF)光子技术,包括RF信号的产生、特性描述、传输以及处理William Shieh教授是澳洲杰出青年(Australian Future fellow)获得者以及美国光学协会院士(Fellowof Optical Society of America)。他的个人主页是http://people.eng.unimelb.edu.au/shiehw/如果您对以上研究方向感兴趣,请与2012年1月15日之前递交申请。欢迎您访问如下网页了解申请信息以及步骤http://www.ee.unimelb.edu.au/future_students/PhD_in_Engineering.html或者直接通过电子邮件跟Shieh教授联系,他的邮箱地址是shiehw@unimelb.edu.au。
  • 3. Centre for Energy Efficient Telecommunications (CEET)
  • 4. Outline• Capacity limit in the current SMF fiber  Analytical expression of fiber capacity• Two-mode fiber (TMF) based transmission  LP10 / LP11 transmission  Two degenerate LP11 modes transmission• Challenges in FMF fiber based systems• Conclusion
  • 5. Motivation  Capacity crunch H. Kogelnik•A. R. Chraplyvy, “The coming capacity crunch,” ECOC’09 Plenary Talk, 2009•R. W. Tkach, “Scaling optical communications for the next decade and beyond,” BellLabs Tech. J. vol. 14, pp. 3-10, 2010•M. Nakazawa, “Hardware paradigm shifts in the optical communication infrastructurewith three “M technologies” OECC’2010
  • 6. Degrees of Freedom for Multiplexing and Modulation (1) Time (2) Frequency (3) Complex constellation or I/Q modulation (4) Polarization (5) Space
  • 7. Nonlinearity Noise Density in SMF FibersThe maximum capacity can be achieved by filling the spectrum with signals f BAssume that the input signal density is I, what is the INL?  I  2 1 8πα β 2 α =  I Ic ≡ , B0 = I NL  Ic  γ 3N s he ln ( B / B0 ) 2π 2 β B 2( N s − 1 + e −αζ LN s − N s e −αζ L )e −αζ L he ≡ −αζ L +1 N s (e − 1) 2 •X. Chen, and W. Shieh, Opt. Express 18, 19039-19054 (2010). •W. Shieh and X. Chen, IEEE Photon. Journal, vol. 3, 158 – 173, (2011).
  • 8. Information Spectral Efficiency in Presence of Fiber Nonlinearity Shannon Capacity Theory:  I  S = log 2 (1 + SNR ) ≅ log 2 1 +   n + I (I / I )  2  0 c   1 2/3  = log 2 1 + ( I c / n0 )  S  3   Ns  (8πα β 2 ) ( 3γ N h ln ( B / B0 ) ) 1/3 −1/3 ≅ log 2  2 2 0 e   3  Spectral Efficiency Parameter Dependence: To increase 1 bit/s/Hz: (i) Reach is to be reduced to half (ii) Nonlinearity coefficient is to be reduced by 2.8 (iii) Chromatic dispersion is to be increased by 8 •X. Chen, and W. Shieh, Opt. Express 18, 19039-19054 (2010). •W. Shieh and X. Chen, IEEE Photon. Journal, vol. 3, 158 – 173, (2011).
  • 9. Why Few-mode Fiber or Two-mode Fiber? [Hij] Tx1 Rx1 In theory, for N-mode fibers, if we have N transmitters and N receivers, we only need Tx2 Rx2 compute NxN H matrix and perform the matrix inversion H-1. Complexity of H-1 TxN RxN scales faster than N2Complex optical design & electronic DSP design.It is sensible to start with the two mode fiber (TMF).TMF fiber contains three spatial modes including LP01 mode and twodegenerate LP11 modes.B.Y. Kim, et al., Opt. Lett., 11, 389-391 (1986).B.Y. Kim, et al., Opt. Lett., 12, 729 (1987)Nevertheless, this still leads to 3 times bandwidth of a SMF fiber.
  • 10. Some Scenarios of Mode Multiplexed Systems (1) Short-reach or interconnect High differential-mode-delay (DMD) low mode crosstalk, optical demulitpelxing F. Yaman, et al., Opt. Express, 18, 21342 (2010).N. Hanzawa ‘Demonstration of mode-division multiplexing transmission over 10 km two-mode fiber with mode coupler’, OFC’2011, Paper TWA4. (2) Long-reach Low DGD, high mode crosstalk, electronic demulitpelxing (3) Higher-order constellation such as 16-QAM and beyond Electronic demulitpelxing
  • 11. TMF Fiber Parameters Refractive Index for core and cladding n _ core = 1.4518 n _ clad = 1.4440 Refractive Index Difference n _ core - n _ clad= ∆n = 0.0054 n _ core Numerical Aperture NA = 0.1505 n _ core 2 - n _ clad 2 V= ( 2π a/λ ) ⋅ NA= ( 2π × 5.935 /1.55 ) × 0.1505 = 3.6218 For Cornings SMF-28e® SMF, V = 2.0396 For Cornings Infinicor® MMF, V =~ 20.8453.
  • 12. Two-mode or Three-mode Fiber? 1.460 core Modal Index n eff LP01 1.450 LP11 cladding 1.440 1.430 cutoff LP11 LP01 2323nm 1.420 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 Wavelength (μm)2 LP modes: LP01 and LP11 a b3 Spatial modes: LP01 and 2 degenerate LP11: LP11 and LP116 Fiber modes: 2 (polarization) x 3 (spatial )
  • 13. Two-mode LP10 / LP11 Transmission Systems Polarization Polarization Multiplexing Controlled coupling to LP01 mode DemultiplexingTx 1 LP01-LP11 Dual-mode Rx1 amplifiersTx 2 Rx2 SMF SMFTx 3 Rx3 TMFTx 4 Rx4 LP01-LP11 Mode LP01-LP11 Mode Multiplexing Demultiplexing
  • 14. Mode Converter LB = 2π /( β 01 - β11 ) Grating period = Beating length = 500 µ m Deformation Mode converter 1 SMF TMF nominal 50% conversion ratio LP01 LP01+LP11 Mode stripper DeformationMode converter 2 LP11+LP01 SMF nominal 100% TMFconversion ratio. LP +LP LP01 01 11 Mode stripper R. C. Youngquist, J. L. Brooks, and H. J. Shaw, "Two-mode fiber modal coupler," Opt. Lett. 9, 177-179 (1984)
  • 15. CO-OFDM Experiment Setup One-symbol delay Optical OFDM Tx Center 50:50 PBC Splicing MS1 MC1 Band:: Band 1 2 3 λ 4.5 km TMF EDFA 50 % MC LP11 Rx PD ADC Offline MS2 Offline MC2 EDFA Polarization Polarization PD ADC 2x2 MIMO 2x2 MIMO Diversity Diversity OFDM PD ADC OFDM 90° hybrid 90° hybrid detection detection PD ADC 100 % MC LO LO LP01 Rx MS3 PD ADC Offline Offline EDFA Polarization Polarization PD ADC 2x2 MIMO 2x2 MIMO Diversity Diversity OFDM PD ADC OFDMPBC: Polarization-beam-combiner 90° hybrid 90° hybrid detection detectionMS: Mode stripper MC: Mode converter PD ADCLO: Local oscillator TMF: Two-mode fiber LO LOTx/Rx: Transmitter/Receiver107 Gb/s dual-mode dual polarization transmission over 4.5-km TMF fiber.‘X’ indicates controlled coupling between LP01 modes of SMF and TMF by center splicing.
  • 16. Transmission Parameters Parameters Value for OFDM Transmission Unit Polarization 2 Band 3 Mode 2 (LP01+LP11) Bit Rate (raw) 25 (per pol/band/mode) * 6 Gbit/s Bit Rate (net) 107 Gbit/s Symbol Period 7.2 ns Bandwidth 6.5625 (per band) GHz No. of Subcarriers 64 Total No. of Symbols 500 No. of Training Symbols 20 Cyclic Prefix (CP) 1/8 of observation window Modulation Format QPSK Fiber Length 4.5 km Launch Power 5.5 dBm Receive Power -0.5(LP01) / -5.3(LP11) dBmY. Ma, Y. Tang, and W. Shieh, "107 Gbit/s transmission over multimode fibre …" Electron.Lett. 45, 848-849 (2009).
  • 17. Experiment ResultsOptical SpectraLP01 0.1nm/div LP11 0.1nm/div 10dB 10dB 19.69GHz 19.69GHz 1549.21nm 1549.21nm ConstellationLP01 LP11 Band1 Band2 Band3 Band1 Band2 Band3
  • 18. Q Factors for 12 TributariesLP01 Band1 Band2 Band3 Avg.pol-x 19.5 18.4 18.1 18.7pol-y 18.5 18.3 17.9 18.3Avg. 19.0 18.4 18.0 18.5LP11 Band1 Band2 Band3 Avg.pol-x 15.2 18.6 16.2 16.9pol-y 14.7 17.0 16.5 16.2Avg. 15.0 17.8 16.4 16.5 No error was measured out of 100,590 bits for each band, polarization and mode measured.
  • 19. Two Degenerate LP11 Modes Transmission (a) 0.9 mm jacket Rotating FC connector SMF TMF Core-center Mode stripper Mode converter aligned splice coiling V-groove CL TMF CL CL BS SMF TMF IR beam profiler (b) TMF V-groove LP11a BS CL TMF LP11b
  • 20. ‘Dream’ System of Multimode Fiber LinkNarrow Linewidth (<10 ~ 100 MMF with lowKHz) laser Array loss: ~ 0.2 dB/km MMF OADM MMF MMF MMF MUX Amplifier DeMUX
  • 21. Review of Progress of Few-mode Transmission Low-speed Short-reach Systems •J. Sakai, et. al., Trans. Micro. Theory & Techn. 26, 658-665 (1978). •K. Kitayama, et. Al., IEEE J. Quantum Electron. (Lett.), vol.QE-15, pp. 6-8, 1979. High-speed Long-reach systems using Conventional MMF •Z. Tong, et. al., OECC’2008, paper PDP5. •Z. Tong, et. al., Electronics Letters, vol. 44, pp. 1373-1375, 2008. High-speed Long-reach systems using FMF fiber OFC’2011, Postdeadline papers •A. Li et al, Proc. OFC, 2011, p.PDPB8. •M. Salsi et al., Proc. OFC, 2011, p.PDPB9. •R. Ryf et al, Proc. OFC, 2011, p.PDPB10 ECOC’2011, 8 more postdeadline papers on few-mode/core fibers
  • 22. Exponential Internet Traffic Growth R. W. Tkach, Bell Labs Tech. J., vol. 14, 2010Bandwidth needs to scale up ~ 30 dB for the next two decades!!
  • 23. Implications of 30 dB of More BandwidthTransponders Fiber cables ROADMs Optical Amplifiers System Complexity Implications Power Space
  • 24. Trade off between Spectral and Energy Efficiency 512 QAM Spectral Efficiency [b/s/Hz] n 256 nno Sha 64 64QAM 512QAM 101 36QAM 16 OFDM/64QAM 16QAM QPSK 4 2010 (PDM) 100 0 5 10 15 20 25 30 35 Required SNR per bit (dB)(i) Strive for high spectral efficiency with low energy efficiency(ii) Strive for high energy efficiency with low spectral efficiencyBut few-mode transmission can achieve both high spectral and energy efficiency
  • 25. Spatial Mode Multiplexing: a Promise or a Curse?•Will the system be too complex and never be practical?•Need to be mindful of DSP complexity for MIMO processing• Electronics is getting better, but not apanacea; Could be energy-hog.
  • 26. Conclusion• Spatial mode division multiplexing (SMDM) has recently been demonstrated to be an additional degree of freedom for achieving ultrahigh capacity beyond that of SSMF fiber.• Should be always mindful of complexity involved when proposing devices and subsystems for SMDM based systems.• But FMF or SMDM is a fertile ground for innovation.