DWDM Transmission Technology and Photonic Layer Network Chao-Xiang Shi Sprint Transmission Network Development Group Advanced Technology Laboratories 1 Adrian , Burlingame CA 94010 1

Outline DWDM Technology in terrestrial network - DWDM capacity and transmission distance: technology review - DWDM transmission system - Span design in DWDM transmission - Optical transmitter in DWDM system: DFB laser with external modulator - Wavelength multiplex/de-multiplex technology in DWDM: AWG, Dielectric filter, and Fiber grating type - Two-stage optical fiber amplifier - Optical amplification, bandwidth , and capacity - Optical fiber nonlinearity: SPM, XPM, SBS, and FWM - Polarization mode dispersion (PMD) limitation for 10 Gbit/s and beyond

Continue DWDM technology in Submarine network - PMD compensation technology DWDM technology in Submarine network - capacity and transmission distance : technology review - uniquely designed LCF fiber and non-zero dispersion shift fiber - chromatic dispersion compensation in Submarine transmission - PMD concern in submarine transmission - one stage Er. Doped fiber amplifier - comparison of WDM transmission between terrestrial and submarine network Photonic layer network - Optical network architecture - Protection and restoration mechanism for IP/ATM directly over WDM optical network

Continue for all-optical networks - Key issue in Metro WDM network and - Issues of protocols and interfaces requirements for all-optical networks - Key issue in Metro WDM network and possible solutions - Application of Metro WDM equipment in transparent transport network: Experimental Verification Emerging Technology of Optical Network - Optical CDM (CDMA) - Optical Packet Switching Network

Today Technology Tomorrow Technology After… DWDM Capacity and transmission: Technology review Today Technology 1530 - 1560 nm window (used to call C-band) 80 ~ 100 channels of 2.5 Gb/s (50 GHz spacing) 32 ~ 40 channels of 10 Gb/s (100 GHz spacing) 70 ~ 90 km span length 4 in-line optical amplifiers and 5 spans total 400 km transmission for 10 Gbit/s total 600 km transmission for 2.5 Gbit/s Tomorrow Technology 1530 - 1600 nm window (used to call L-band) 100 ~ 200 channels of 2.5 Gb/s 64 ~ 100 channels of 10 Gb/s After… 1480 - 1530 nm window by Raman amplification

DWDM transmission system OC-48/ OC-192 70-90km Tx OC-48/ OC-192 Uni-directional transmission OSC 1510 nm or 1480 nm OC-48/ OC-192 OC-48/ OC-192 70-90km Bi-directional transmission 1510 nm or 1480 nm

3 span: span distance 90 km, total 270 km Span design in DWDM transmission OC192 (10 Git/s) +6~8 dBm/ch 3 span: span distance 90 km, total 270 km 4 span: span distance 80 km, total 320 km 5 span: span distance 70 km, total 350 km OC 48 (2.5 Gbit/s) 3 span: span distance 120 km, total 360 km 5 span: span distance 100 km, total 500 km 8 span: span distance 80 km, total 640 km

DFB laser with External modulation (for backbone long distance) Optical transmitter in DWDM system: DFB laser with external modulator DFB laser with External modulation (for backbone long distance) Wavelength stable, narrow band DFB laser - DFB laser spectrum width : ~ 20 mHz - wavelength stability: +/- 0.01 nm DFB laser integrated with EA modulator - Low chirping effect - polarization stability - low driving power required DFB laser with external LN modulator - polarization problem - high driving power required - chirping problem DFB laser with Direct modulation (for local area short distance) - spectrum broaden - wavelength stability

AWG (array waveguide grating) Wavelength multiplex/demultiplex technology in DWDM: AWG, Dielectric filter, Fiber grating l WDM Mux/Demux AWG (array waveguide grating) - Insertion loss : 6 ~ 8 dB (insertion loss is almost - channel crosstalk ~ 25 db - application for higher channel number Dielectric filter WDM Mux/Demux -insertion loss: increases when channel number increases -channel crosstalk: 25 ~ 30 dB -application for lower channel number WDM Mux/Demux Fiber Bragg grating - need optical circulator - cascade multipile grating to form a WDM Mux/Demux

Two-stage Optical fiber amplifier DCF optical filter OSC 980 nm pump EDFA1 1480 nm EDFA2 WDM 980 nm low noise pump laser for first stage EDFA 1480 nm high power pump laser for second EDFA DCF (dispersion compensation fiber) is required for 10 Gbit/s Attenuater is needed for 2.5 Gbit/s Optical isolator is used to reduce back ASE noise impact Optical filter is used for gain equalization Total gain of fiber amplifier is from 25 dB to 30 dB N.F. (noise figure): 5 ~ 7dB Output power : +17 ~ +23 dBm Flatten gain : +/- 1 dB with 30 nm ~ 40 nm over Er. gain range Dynamic input range: 15 dB

Optical amplification, bandwidth , and capacity Fiber loss 0.4 db 0.25 db 1310 nm 1550 nm Wavelength (l) C band: 1530 ~ 1560 nm (100 Ghz channel space for 10 Gbit/s, total 40 channels, 50 Ghz channel space for 2.5 Gbit/s, total 96 channels ) L band: 1560 ~ 1600 nm (40 channel available for 10 Gbit/s, i.e. 40 gbit/s, , and 100 channels available for 2.5 gbit/s) S band: 1480 ~ 1520 nm (40 channel available for 10 Gbit/s, i.e. 40 gbit/s, , and 100 channels available for 2.5 gbit/s) Total 1.2 Tbit/s capacity S Band: Raman amplification L Band: EDFFA, Ti-EDFA C Band: EDFA

Fiber nonlinearity: SPM, XPM, SBS, and FWM SPM: Self-phase modulation - Create positive chirping, which cause pulse distortion due to fiber dispersion - Result in the optical spectrum broaden which limits the channel space XPM: Cross phase modulation - Phase modulation between two channels due to fiber Kerr effect - Convert phase noise (due to ASE) to intensity noise via fiber dispersion - Limit channel space (for 10 Gbit/s channel space is 100 Ghz , 0.8nm) SBS: Stimulated Brillouin Scattering - Creating a new wave in backward direction through interaction between light wave and acoustic wave - SBS threshold can be reduced by decreasing the power level and increasing optical spectrum. - For 10 Gbit/s, FM modulation (~100 Mhz) of DFB laser can reduce the SBS threshold from +5 dBm to +10 dBm. FWM: Four wave mixing - Optical parametric process through 3 or 4 light wave. - Cause nonlinear channel crosstalk when transmission near zero dispersion wavelength (a critical problem for dispersion-shift fiber) - Standard SMF-28 is good to suppress FWM, but has too much chromatic dispersion - True wave fiber has larger enough dispersion to suppress FWM, and small enough chromatic dispersion, but still has dispersion slope problem.

Polarization mode dispersion limitation for beyond 10 Gbit/s Y-polarization Y-polarization X-polarization t X-polarization X t ~ c, (nx-ny) and L PMD is caused by differential group delay (DGD) between two - polarization modes PMD is a statistic process satisfying Maxwellian distribution PMD becomes serious issue for 10 Gbit/s and beyond PMD design - Instantaneous PMD should be smaller than 25% pulse width - Assuming fiber PMD is 0.3 ps/km^1/2, 400 km fiber gives mean PMD 6 ps. If we use safety number 4 for Maxwellian distribution, the instantaneous PMD is 24 ps. Which means 0.3 ps/km^1/2 PMD gives 400 km distance limitation for 10 Gbit/s.

PMD compensation technology Y-polarization X-polarization Long distance SM fiber Polarization controller (PC) X PM fiber Receiver Transmitter feedback control signal Electronic process PM fiber: with high PMD due to strong fiber birefringence PMD induced by long distance single mode fiber can be canceled by using a short PM fiber with a greater PMD Feedback control signal to adjust input polarization of PM fiber, so that the fast polarization axis of single mode fiber matches to the slow axis of PM fiber and vice versa.

Capacity and transmission distance Current Transmission Technology 1530 ~1560 nm window of EDFA - 10 Gbit/s X 16 ch transmission (channel space 0.6 nm) - 45 ~ 50 km span length - ~ 150 in-line optical amplifiers - total 7500 km transmission without electronic regenerter for 10 Gbit/s Future Transmission Technology - 10 Gbit/s x N (N=32~50) transmission - 20 Gbit/s WDM technologies - 40 Gbit/s WDM technologies

…. …. Uniquely designed LCF fiber and non-zero dispersion shift fiber (NZ-DSF) EDFA LCF fiber NZ-DSF fiber EDFA …. …. 25 km 25 km LCF (Large core fiber) - chromatic fiber dispersion -2 ps/km.nm - large effective area 75 ~ 80 um^2 - bigger dispersion slope - suppression of nonlinear effect - used in first half span distance for higher channel power NZ-DSF fiber - chromatic fiber dispersion -2 ps/km.nm - smaller dispersion slope - used in second half span for smaller power - to reduce accumulation of chromatic dispersion

..…. …. Chromatic dispersion compensation in Submarine transmission EDFA LCF fiber NZ-DSF fiber EDFA EDFA Standard SMF fiber EDFA ..…. …. 25 km 25 km 50 km 10 span 500 km Standard single mode fiber (SMF) is used for chromatic dispersion compensation Dispersion compensation is performed at every 10 span (500 km) In order to resolve dispersion slope problem, pre-dispersion and post-dispersion compensation are needed at transmitter and receiver ends

how is PMD impact for ultra- long distance such as PMD concern in submarine transmission how is PMD impact for ultra- long distance such as Submarine transmission (7500 km)? - PMD is accumulated through the long distance transmission by both fiber cable and every optical component. - define a low PMD fiber (PMD as low as 0.008 ps/km^1/2). Over 7500 km, mean fiber PMD =6.9 ps . - define each optical component with a small PMD, e.g, EDFA with 0.1 ps, WDM with 0.1ps.

One stage Er. Doped fiber amplifier Opt. isolator ASE filter Gain equalization filter Er. fiber 980 nm pump laser module 980 nm low noise pump laser module for first stage EDFA Optical isolator is used to reduce back ASE impact Optical filter is used for gain equalization ASE filter (FBG) is used to get off ASE and its accumulation Total gain of fiber amplifier is from 10 dB to 12 dB small N.F. (noise figure): ~4 dB Output power : ~ +11 dBm

more than 100 span and fiber amplifiers at 10 Gbit/s, but Comparison of WDM transmission between terrestrial and submarine network Why submarine network can transmit over 7500 km with more than 100 span and fiber amplifiers at 10 Gbit/s, but terrestrial network can only handle 5 span over 400 km? 7500 km vs/ 400 km is a big difference! - Submarine transmission network is a pre-defined system, which is more like a well controlled experimental system in Lab. - In terrestrial network, the characteristic of fiber in underground is unknown. The system designer should build equipment to cover a lot of statistic cases.

Next Generation Network l R IP/WDM R R R l l R R R Router Non-IP Data Source ATM Switch R IP/SONET SONET DCS or ADM R Optical XC or ADM IP/ATM l Optical line System

All Optical Network: WDM Long Haul, Metro Backbone, and Local Collecting Ring Central Node 1 2 6 WDM Metro Backbone ring Hub 3 Hub 5 WDM local collecting ring 4 WDM local collecting ring

The ring size of metro backbone WDM network is Description of Metro WDM Ring The ring size of metro backbone WDM network is defined to be from 100 km to 200 km, and WDM local collecting ring is defined from 20 km to 50 km. In order to have a transparent (protocol independent) transport optical network also for the low cost reason, no electronic regenerators should be allowed in Metro WDM rings. Optical amplifiers might be needed in WDM metro backbone ring network, but not in WDM local collecting ring. Metro WDM ring should be self-healing optical ring. network protection and restoration should be at photonic layer .

Optical Protection Efficiency 1+1 OSNCP (Path Switch) vs. OSPRING (Optical Line) l5 1+1 OSNCP l5 OSPRING Interconnections between routers requires 4 protection wavelengths with path switch Same interconnections between routers requires 1 protection wavelength with OSPRING

2-Fiber OMS/SPRING (conventional switching) No Wavelength Conversion Required Working Protection Protection Working li - lN/2 lN/2 - lN li - lN/2 lN/2 - lN (li) (lk) (li) (lk) fiber 1 fiber 2 B fiber cut fiber 2 fiber 1 AÔC A Ring Switch AÔC C CÔA CÔA D

2-Fiber OMS/SPRING (w/G.841 undersea protocol) No Wavelength Conversion Required Working Protection Protection Working li - lN/2 lN/2 - lN li - lN/2 lN/2 - lN (li) (lk) (li) (lk) fiber 1 fiber 2 B fiber cut fiber 1 fiber 2 AÔC A Ring Switch AÔC C CÔA CÔA D

Optical Network Evolution Issues How to transport large pipes (OC-48c & above) reliably? Should OC-192 be deployed in an existing OC-48 based network? Should SONET be bypassed for ATM, FR, and IP transport over wavelengths? No standards on optical data interface, multi-vendor interoperability What survivability architecture best balances performance, cost, and flexibility? Is synchronization required for optical network? Mechanisms for providing OCH trail trace, mechanisms to discover fiber topology, performance monitor and management across administrative boundaries. Meeting latency requirements in detecting, reporting, localizing, and reacting to faults (e.g. protection switching).

Survivability Alternative Tradeoffs Service Layer Mesh Every survivability mechanism makes tradeoffs: Speed vs. Facility Cost (Overbuild) is most fundamental Physical Layer (SONET & Optical) Schemes Centralized Mesh Maximum Outage Distributed Mesh Good MS/ SPRING SNCP MSP Good Facility Cost [Restoration Overbuild]

Metro WDM Network’s Key Issue: Limited Number of OADM Nodes and Small Ring Size Central Node OADM OADM OADM

l3 l4 l1 l2 l1 Central Node : l8 l8 l6 l5 l7 Metro WDM Network’s solution: Boost and Pre-Amplifiers l3 l4 l1 l2 Att. OADM OADM OADM OADM Boost- Amp l1 Central Node : l8 Pre- Amp OADM OADM OADM OADM l8 l6 l5 l7

Metro WDM Network’s solution: One Line-amplifier ATT. ATT OADM OADM OADM OADM EDFA Input (after ATT control) EDFA Input (before Tx ATT) l1 l2 l4 l3 Central Office l3 l2 l1 l4 l5 l7 l8 l3 l1 l5 l7 l8 l4 . EDFA l8 l1 l2 l8 ... OADM OADM OADM OADM EDFA Output l8 l7 l6 l5

Metro WDM Network’s solution: Line-Amplifier with Gain Slope OADM OADM OADM OADM EDFA Input l1 l4 l3 l2 l2 l1 l5 l7 l8 l3 Central Office l4 . Gain curve EDFA l8 l l4 l3 l8 ... OADM OADM OADM OADM EDFA Output l8 l7 l6 l5

Metro WDM Network: Experimental Set-up B C D TX RX LR Splitter lA lB 7dB SR Client Combiner Switch OADM filter

Transparent WDM Network: SONET-Less , Photonic Layer Restoration By Metro WDM Equipment Hub OADM OADM A WDM Long Haul Network D Hub Hub B OADM OADM Metro WDM Metro WDM Network 1 Network 2

Hybrid WDM Metro and Long Haul: Experimental Set Up 16 ch. Long Haul WDM Transmission 500 km Fiber Transponder Fiber cut l16 l16 ... : : Metro WDM Network 1 HP Digital Scope l1 l1 B 40 ch. Long Haul WDM Transmission 500km F lC lF G A lA lA D E lE lE LA2 LA3 lC lF Error output C H l1’ l1’ LA1 LA4 : : Tektronix ST2400 SONET testset TA RA l40’ l40’

Protection time when 16 channel long haul WDM fails Error period Error free Error free

Protection time when 40 channel long haul WDM fails Error free Error period

BER results for working and protection path

Emerging Technology: Optical CDM (CDMA) using Fiber Brag Gratings “1” Optical circuit dd dd ... FBG n ... FBG n l1 l2 ln Input l1 l2 ln dt dt “1” “1 0 1 0 0 1 ” “1 0 1 0 0 1 ” Output Fiber Dispersion Compensation EDFA

Optical Packet Switching Network Node 2 1 100 4 5 3 IP/ATM Network Optical packet switching ring network …. l100 l1 l2 l5 l4