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

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

Signal distortion due to chromatic dispersion (Waveform distortion) Optical spectrum Spectrum broadening 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 1 Wavelength Δλ Time Time Fujitsu

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 Transmitter out Received waveform Fujitsu

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) Shorter wavelength Slow (Fast) 40 Gb/s optical signal 25 ps Transmitter output After fiber transmission After dispersion comp. Fujitsu

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

Residual dispersion and tolerance of receiver Allowable penalty + + Longer wavelength Dispersion tolerance of receiver Center wavelength R.D. [ps/nm] R.D. [ps/nm] Shorter wavelength - - Distance [km] Penalty [dB] 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 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. Fujitsu

Comparison of 40Gbit/s modulation schemes NRZ RZ CS-RZ Optical duobinary Optical power (dBm) Wavelength (nm) -20 -40 1542 1545 1548 108 GHz 180 GHz 165 GHz 70 GHz This shows the optical waveforms and the optical spectra for four 40-Gbit/s optical modulation schemes. Their respective spectral bandwidth at a 20-dB reduction was located here. RZ and CS-RZ has a larger spectral bandwidth than that of NRZ. Optical duobinary has a narrower spectral bandwidth than that of NRZ. CS-RZ has a narrower spectral bandwidth than that of RZ. We compared experimentally the chromatic dispersion tolerance, optical power limit, and the waveform degradation caused by optical MUX/DEMUX filters for these modulation schemes. Next, I will talk about the experiment results. Now evaluating transmission performance Chromatic dispersion tolerance Fibre non-linear tolerance (Maximum input power) Spectral tolerance (Degradation due to filter narrowing) Fujitsu

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 O/E E/O Post-amplifier Pre-amplifier After optimization -400 ps/nm +900 ps/nm O/E E/O Post-amplifier Pre-amplifier Fujitsu

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

Automatic dispersion compensation example Provisioning & Tracking Provisioning l1 Tx #1 Rx #1 l2 Tx #2 VDC VDC Rx #2 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 Optical circulator DC < 0 Focusing lens Collimating lens Glass plate 3-Dimensional Mirror VIPA : Virtually Imaged Phased Array Fujitsu

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

Polarization Mode Dispersion (PMD) Cross-section of optical fiber Ideal Practical 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

Higher-order PMD … D t1 D t2 D t3 D t4 D tn Mode-coupling at random locations with random strength -Frequency dependence of DGD Maxwellian distribution of the instantaneous 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

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

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