New functionalities for advanced optical interfaces ( Dispersion compensation)

1. New functionalities for advanced optical interfaces (Dispersion compensation) Kazuo Yamane Photonic systems development dept. Fujitsu

2. Outline • Chromatic dispersion effect • Dispersion compensating techniques • Optimization of residual dispersion or its map • PMD compensation • Conclusions Fujitsu

3. Signal distortion due to chromatic dispersion Spectrum broadening Optical spectrum Δλ Difference in group velocity Wavelength Pulse broadening (Waveform distortion) Transmitter output Receiver input Optical fiber Time Time Group velocity Original signal Regenerated signal 1 1 1 1 0 1 Wavelength Δλ Time Time Fujitsu

4. Waveform distortion due to fiber non-linearity High power intensity Refractive index change Frequency chirp Spectrum broadening Waveform distortion due to chromatic dispersion Optical fiber Low optical power High optical power Received waveform Transmitter out Fujitsu

5. 25 ps Dispersion compensation example Transmission fiber Dispersion compensating fiber (DCF) + Positive dispersion (Negative dispersion) Negative dispersion (Positive dispersion) Longer wavelength Slow(Fast) Longer wavelength Fast (Slow) Shorter wavelength Fast(Slow) Slow (Fast) Shorter wavelength 40 Gb/s optical signal Transmitter output After fiber transmission After dispersion comp. Fujitsu

6. DC DC DC DC DC DC DC DC allocations and dispersion maps Post-comp. + Fiber#1 Fiber#2 0 R.D. [ps/nm] Distance [km] - Pre-comp. + Fiber#1 Fiber#2 0 R.D. [ps/nm] Distance [km] - Post- & Pre- comp. + Fiber#1 Fiber#2 0 R.D. [ps/nm] Distance [km] - Fujitsu

7. Residual dispersion and tolerance of receiver Allowable penalty + + Longer wavelength Dispersion tolerance of receiver Center wavelength 0 R.D. [ps/nm] R.D. [ps/nm] Shorter wavelength - - Distance [km] Penalty [dB] Need to consider the variation of tolerance due to characteristics of transmitter, fibre non-linear effects and dispersion map. Even if residual dispersion values are same, the received waveforms are different, affected by these parameters. Parameters affecting to the tolerance - Signal bit rate - Channel counts and spacing - Distance or number of spans - Fibre type - Fibre input power - Pre-chirping of transmitter - Modulation scheme of transmitter - DC allocation / value Fujitsu

8. 0 0 0 0 -20 -20 -20 -20 Optical power (dBm) -40 -40 -40 -40 1545 1545 1545 1545 1542 1542 1542 1542 1548 1548 1548 1548 Wavelength (nm) Wavelength (nm) Wavelength (nm) Wavelength (nm) Comparison of 40Gbit/s modulation schemes NRZ RZ CS-RZ Optical duobinary 108 GHz 180 GHz 165 GHz 70 GHz Now evaluating transmission performance Chromatic dispersion tolerance Fibre non-linear tolerance (Maximum input power) Spectral tolerance (Degradation due to filter narrowing) Fujitsu

9. A past field experiment example • 10Gbit/s 750km WDM field trial between Berlin and Darmstadt (Ref.: OFC/IOOC’99, Technical Digest TuQ2, A. Ehrhardt, et.al.) Link for field trial Berlin Darmstadt Before Optimization E/O O/E Post-amplifier Pre-amplifier After optimization +900 ps/nm -400 ps/nm O/E E/O Post-amplifier Pre-amplifier Fujitsu

10. 2000 2000 1500 1500 1000 1000 500 500 0 0 -500 -500 -1000 -1000 -1500 -1500 -2000 -2000 Dispersion maps and waveforms in the trial Before optimization Dispersion (ps/nm) Channel 1 Channel 3 Channel 2 Channel 4 0 800 400 600 200 Distance (km) After optimization Dispersion (ps/nm) Channel 1 (Before) Channel 1 (After) 0 800 400 600 200 Distance (km) Fujitsu

11. Automatic dispersion compensation example Provisioning & Tracking Provisioning Rx #1 Tx #1 l1 Tx #2 l2 Rx #2 VDC VDC l40 Tx #40 Rx #40 DC DC li Dispersion compensator (fixed or variable) Dispersion Monitor VIPA variable dispersion compensator DC > 0 Line-focusing lens Variable x-axis DC < 0 Optical circulator Focusing lens Collimating lens Glass plate 3-Dimensional Mirror VIPA : Virtually Imaged Phased Array Fujitsu

12. NE NE NE NE NE Dispersion compensation trend Photonic network Manage dispersion or residual dispersion (dispersion map) !! Transmitter / Receiver Adjust parameters including residual dispersion to optimum!! Fujitsu

13. Polarization Mode Dispersion (PMD) Cross-section of optical fiber Practical Ideal Cladding Fast axis Core Slow axis 1st-order PMD Fast Dt Dt Slow D t : Differential Group Delay (DGD) - Well defined, frequency independent eigenstates - Deterministic, frequency independent Differential Group Delay (DGD) - DGD scales linearity with fiber length Fujitsu

14. Higher-order PMD … D tn D t4 D t1 D t2 D t3 Mode-coupling at random locations with random strength Maxwellian distribution of the instantaneous DGD -Frequency dependence of DGD -Statistically varying due to environmental fluctuations Frequency of occurrence Prob.(DGD>3xPMD) = 4x10-5 = 21 min/year -Fiber PMD unit: ps/ km Prob.(DGD>3.5xPMD) =10-6 = 32 sec/year PMD 3.5PMD Instantaneous DGD (ps) Fujitsu

15. Automatic PMD compensation PMD compensation scheme in receiver 40Gb/s waveforms Before PMD comp. PMD comp. device #1 PMD comp. device #2 PMD comp. device #3 O/E module Control algorithm Distortion analyzer After PMD comp. PMD characteristic changes slowly due to “normal” environmental fluctuations (e.g. temperature) But, fast change due to e.g. fiber touching High-speed PMD compensation device & Intelligent control algorithm Fujitsu

16. Conclusions • In fibre optical high bit rate (such as 10G or 40G bit/s) long-haul transmission systems, dispersion compensation is one of the most important items to be considered for design. • Management or optimization of residual dispersion are required for photonic networks, i.e., for fibres, repeaters and optical interfaces. • PMD compensation is also required especially for 40Gbit/s or higher bit rate long-haul systems. Fujitsu