1. “Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece “Recent Trends in Optical Transmission Systems” Thomas Sphicopoulos (thomas@di.uoa.gr) Optical Communications Laboratory National and Kapodistrian University of Athens, Greece

2. “Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece 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. “Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece The Optical Value Chain

4. “Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece Evolution of Transmission Rates/Channel

5. “Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece Wavelength Division Multiplexing (WDM) λ2λ2 λ1λ1 λ3λ3 λ4λ4 λ2λ2 λ1λ1 λ3λ3 λ4λ4 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. “Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece Transmission Impairments Linear Impairments: Optical Losses Chromatic Dispersion Polarization Mode Dispersion Non-linear Impairments: Self Phase Modulation Cross Phase Modulation Four Wave Mixing Stimulated Raman Scattering Stimulated Brillouin Scattering

7. “Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece 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. “Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece Linear Impairments: Dispersion 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) 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!

9. “Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece Linear Impairments: Polarization Mode Dispersion (PMD) The principal polarization axes of the fiber may change randomly along the cable due to temperature / size variations. This causes Polarization Mode Dispersion The fiber is not completely circular and hence supports two degenerate modes with slightly different group velocities (birefrigence) PMD can also cause pulse broadening at high bit rates

10. “Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece Nonlinear Impairments due to the non- linearity of the refractive index 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) Intensity of the Electric Field

11. “Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece 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. “Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece 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/ 0 C) 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: LiNbO 3 Mach Zehnder Modulator (electroptical effect) Electroabsorption Modulator (electroabsorption effect)

13. “Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece 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. “Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece 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. “Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece 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. “Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece 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. “Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece System Design: Dispersion Management (2) Dispersion Management for multi-channel 10Gb/s To further residual dispersion at edge channels we use pre/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. “Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece System Design: Dispersion Management (2) Expanding the Bandwidth from ~20nm to ~40nm EE-PDF: A eff Enlarged Positive Dispersion Fiber SC-DCF: Slope Compensating Dispersion-Compensation Fiber

19. “Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece System Design: Dispersion Management (3) Moving to 40Gb/s… It is preferable to lower accumulated dispersion

20. “Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece 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. “Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece 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. “Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece System Design: Mitigation of Nonlinearities Methods for Reducing Nonlinearities: FWM Use unequal channel spacings Use optical prechirped pulses

23. “Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece 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. “Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece How to Model and Design? (1) Use Numerical Tools: Numerically Solve the Propagation Equation A=A(z,t): Envelope of the Electric Field β 2 (z): Second order dispersion γ(z): Nonlinear Kerr Coefficient You can add amplifier gain and noise in each span

25. “Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece How to Model and Design? (2) Use Numerical Tools: Calculate Q-factor from Receiver Eye-Diagram For Gaussian Noise:

26. “Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece 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 Δf ch =100GHz, L span =80Km, N span =4

27. “Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece 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. “Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece 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 (~10 6 ) for probabilities of the order of 10 -14 Single span system, N ch =8, Δf ch =100GHz

29. “Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece 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. “Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece 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. “Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece 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. “Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece THANK YOU FOR YOUR ATTENTION!