New functionalities for advanced optical interfaces Dispersion compensation

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

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

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

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 Received waveform Transmitter out 4 High optical power Fujitsu

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

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

Residual dispersion and tolerance of receiver Allowable penalty R. D. [ps/nm] Longer wavelength Center wavelength 0 Shorter wavelength - Distance [km] Penalty [d. B] - 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 7 Dispersion tolerance of receiver R. D. [ps/nm] + + Fujitsu

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

A past field experiment example n 10 Gbit/s 750 km WDM field trial between Berlin and Darmstadt (Ref. : OFC/IOOC’ 99, Technical Digest Tu. Q 2, A. Ehrhardt, et. al. ) Berlin Link for field trial 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 9 Pre-amplifier Fujitsu

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

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

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

Polarization Mode Dispersion (PMD) Cross-section of optical fiber Cladding Practical Ideal Fast axis Core Slow axis 1 st-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 13 Fujitsu

Higher-order PMD D t 1 D t 2 D t 3 D t 4 D tn … -Frequency dependence of DGD -Statistically varying due to environmental fluctuations -Fiber PMD unit: ps/ km Frequency of occurrence Mode-coupling at random locations with random strength Maxwellian distribution of the instantaneous DGD Prob. (DGD>3 x. PMD) = 4 x 10 -5 = 21 min/year Prob. (DGD>3. 5 x. PMD) =10 -6 = 32 sec/year PMD 3. 5 PMD Instantaneous DGD (ps) 14 Fujitsu

Automatic PMD compensation scheme in receiver 40 Gb/s waveforms Before PMD comp. device #1 PMD comp. device #2 PMD comp. device #3 Control algorithm O/E module Distortion analyzer 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 15 Fujitsu After PMD comp.

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