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“Recent Trends in Optical Transmission Systems”

“Recent Trends in Optical Transmission Systems”. Thomas Sphicopoulos (thomas@di.uoa.gr) Optical Communications Laboratory National and Kapodistrian University of Athens, Greece. Advantages of Optical Technology. Optical Technology Provides: Ultra Low Transmission Losses Ultra Wide Band

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“Recent Trends in Optical Transmission Systems”

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  1. “Recent Trends in Optical Transmission Systems” Thomas Sphicopoulos (thomas@di.uoa.gr) Optical Communications Laboratory National and Kapodistrian University of Athens, Greece

  2. Advantages of Optical Technology Optical Technology Provides: • Ultra Low Transmission Losses • Ultra Wide Band • Very High Bitrates • (Mostly) Linear Behavior • Very Low Crosstalk • But: • Optics are not smart • InP / Si / Polymer platforms do not yet provided increased scale of integration • No means of storage

  3. The Optical Value Chain

  4. Evolution of Transmission Rates/Channel

  5. λ1 λ2 λ1 λ3 λ2 λ4 λ3 λ4 Wavelength Division Multiplexing (WDM) The aggregate bit rate can be drastically increased by using Wavelength Division Multiplexing (>1Tb/s exhibited ) In optical transmission systems, the available bandwidth can exceed 40nm To efficiently utilize this enormous bandwidth one can assign each channel a different wavelength and lead all the wavelengths inside the fiber Channel spacing as narrow as 10GHz(!) can be achieved!

  6. Transmission Impairments • Linear Impairments: • Optical Losses • Chromatic Dispersion • Polarization Mode Dispersion • Non-linear Impairments: • Self Phase Modulation • Cross Phase Modulation • Four Wave Mixing • StimulatedRaman Scattering • StimulatedBrillouin Scattering

  7. The Fiber: A Nearly Lossless Channel Typical Losses can be as low as 0.2dB/Km Poses no problem if optical amplification is used

  8. Linear Impairments: Dispersion As in most types of waveguides the different spectral parts of the pulse travel with slightly different phase velocities (chromatic dispersion) This causes pulse broadening! • Types of Cables according to dispersion: • G652: D~15-20ps/nm/Km (λ=1.55μm) • G653: D~0ps/nm/Km (λ=1.55μm) • G655: D~2-6ps/nm/Km (λ=1.55μm)

  9. Linear Impairments: Polarization Mode Dispersion (PMD) The fiber is not completely circular and hence supports two degenerate modes with slightly different group velocities (birefrigence) The principal polarization axes of the fiber may change randomly along the cable due to temperature / size variations. This causes Polarization Mode Dispersion PMD can also cause pulse broadening at high bit rates

  10. Nonlinear Impairments due to the non-linearity of the refractive index Intensity of the Electric Field Self Phase Modulation: Phase modulation due to the intensity modulation of the Signal (introduces chirp) Cross Phase Modulation: Phase modulation due to the intensity modulation of other interfering wavelength channels (pulse broadening and time jitter) Four Wave Mixing: Crosstalk with other nearby channels due to frequency mixing (three photon interaction)

  11. Nonlinear Impairments: Stimulated Scatterings • Brillouin Scattering: Energy is transferred from a photon to an acoustic phonon (molecular vibration) and to a photon of smaller frequency (≈-10GHz) (unwanted reflections at the source). • Raman Scattering: Energy is transferred from a photon to an optical phonon (molecular vibration) and to a photon of smaller frequency (optical crosstalk from higher to lower frequency channels) • Current WDM systems avoid problems with both type of scatterings by limiting the optical power and increasing the channel spacing

  12. Technological Landmarks: Optical Sources • Distributed Feed Back Lasers (DFB) are ideal Optical Sources for ~40Gb/s providing: • High Launch Power (>20mW) • Wavelength Stability (~0.001nm/0C) • Very Low RIN (>-145dB/Hz) • High Side Mode Suppression Ratio (<-45dB when MQW is used) • Narrow Linewidths (~2MHz) At ~40Gb/s only external modulation can be used: • LiNbO3 Mach Zehnder Modulator (electroptical effect) • Electroabsorption Modulator (electroabsorption effect)

  13. Technological Landmarks: Amplifiers • Two types of Amplification is used: • Erbium Doped Fiber Amplifier (EDFA): • High Gain (~40dB) • High Output Power (~400mW) • Very Low Noise • Very Linear • Wide Band (~40nm) Raman Amplifier • Higher Power than EDFA (~700mW) • Can offer distributed and/or lumped amplification • Ultra wide band (~100nm)

  14. Technological Landmarks: MUX/DEMUX • Arrayed Waveguide Gratings (AWGs): • Can multiplex up to 1000 channels! • Channel spacing can be as small as 10GHz! • Commercial systems multiplex 64 channels x 50GHz • Can be integrated with SOAs and provide an integrated ADD/DROP MUX • Have small polarization sensitivity • Have small insertion loss • Can be designed with “flat-top” transfer function

  15. System Design: Dispersion Management (1) First Generation Dispersion Management System • DSF=Dispersion Shifted Fiber • SMF=Single Mode Fiber • This scheme was used in the past for single channel ~5Gb/s systems but is unsuitable for WDM: • high nonlinearity • Compensates dispersion for one wavelength

  16. System Design: Dispersion Management (2) Dispersion Management for multi-channel 10Gb/s LCF = Large Core Fiber NZDSF= Non-Zero DSF • LCF is used first to reduce non-linearity • SMF is placed in the middle of the period and the accumulated dispersion alternates sign

  17. System Design: Dispersion Management (2) Dispersion Management for multi-channel 10Gb/s • To further residual dispersion at edge channels we usepre/post-compensantion (50:50) on a channel by channel basis: • Less Maximum Dispersion • Less Waveform Distortion Overall: • Less Nonlinearity • Ideal for 16x10Gb/s (~20nm)

  18. System Design: Dispersion Management (2) Expanding the Bandwidth from ~20nm to ~40nm EE-PDF: Aeff Enlarged Positive Dispersion Fiber SC-DCF: Slope Compensating Dispersion-Compensation Fiber

  19. System Design: Dispersion Management (3) • Moving to 40Gb/s… • It is preferable to lower accumulated dispersion

  20. System Design: Integrated Optics Dispersion Compensation • Modifying the Geometry of an Arrayed Waveguide Grating by a Variable Reflecting Membrane introduces Second Order Dispersion that can be used to compensate the accumulated dispersion of a multiwavelength 40Gb/s signal • Tunable: Applying Voltage • 1000ps/nm Tuning Range

  21. System Design: Electronic Dispersion Compensation One idea is to predistort the signal for each channel Use a MZM interferometer to predistort the signal in order to counteract the effects of dispersion Works very well in theory but you need fast electronics and D/A (even if you parallelize!)

  22. System Design: Mitigation of Nonlinearities Methods for Reducing Nonlinearities: FWM • Use unequal channel spacings • Use optical prechirped pulses

  23. System Design: Mitigation of Nonlinearities Methods for Reducing Nonlinearities: XPM • Dispersion Compensation at each span • High channel spacing • Pre-chirped optical pulses • Advanced modulation schemes

  24. How to Model and Design? (1) Use Numerical Tools: • Numerically Solve the Propagation Equation A=A(z,t): Envelope of the Electric Field β2(z): Second order dispersion γ(z): Nonlinear Kerr Coefficient You can add amplifier gain and noise in each span

  25. How to Model and Design? (2) Use Numerical Tools: • Calculate Q-factor from Receiver Eye-Diagram For Gaussian Noise:

  26. How to Model and Design? (2) Example: Estimate Performance of Modulation Formats in G655 fibers: FWM limits the quality of multichannel systems and hence DPSK has superior performance Δfch=100GHz, Lspan=80Km, Nspan=4

  27. How to Model and Design? (3) But: • The Gaussian Assumption is usually not valid! • The Q-factor provides a crude estimate for the error probability • Use Saddle-Point approximation to compute the error probability from the MGF (if it is known!) • Use Monte Carlo methods to estimate the error probability numerically

  28. How to Model and Design? (3) Example: Estimation of FWM probability density function using MCMC simulations Gaussian PDF is inadequate! MCMC requires very few iterations (~106) for probabilities of the order of 10-14 Single span system, Nch=8, Δfch=100GHz

  29. Small Size Components: Photonic Crystals as a Possible Candidate for Nanophotonics Photonic Crystals: Artificial Periodic Structures • Exhibit Bandgaps (no guided modes exist) • “Defects” introduce highly localized modes • Confine light (can implement sharp bends) • Are highly non-linear (signal processing)

  30. Slow Light: Towards Integrated All-Optical Buffers? • Certain waveguiding structures can support pulse propagation with very low group velocities • Coupled Resonator Optical Waveguides (CROWs) • Integrated Optical Delay Lines • Photonic Memories • Signal Processing (Linear + Nonlinear)

  31. In conclusion… Optical Transmission Systems have made significant advances and are operational. But much can be gained by improving optical integration and exploring optical buffering!

  32. THANK YOU FOR YOUR ATTENTION!

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