1 Confidential © 1999, Cisco Systems, Inc. Design of the physical layer in Metro DWDM networks Design of the physical layer in Metro DWDM networks Alessandro Barbieri bock-bock.cisco.com/~abarbier

2 © 1999, Cisco Systems, Inc. Cisco Systems Confidential Agenda WDM system overview Loss Management: The problem The solution The limitation Dispersion Management: The problem The solution The limitation The role of PMD and nonlinear effects in Metro Optical Networks design: The problem The solutions The limitations

3 © 1999, Cisco Systems, Inc. Cisco Systems Confidential Basic Elements of a WDM system

4 © 1999, Cisco Systems, Inc. Cisco Systems Confidential Wavelength Division Multiplexing Systems λ1λ1 λ3λ3 λ3λ3 1310nm 850nm Mux/DeMux OEO Pump Transponder-Based WDM System Client Equipment EDFA

5 © 1999, Cisco Systems, Inc. Cisco Systems Confidential Managing Optical Power Loss

6 © 1999, Cisco Systems, Inc. Cisco Systems Confidential UV Absorption OH - Absorption Peaks in Actual Fiber Attenuation Curve Rayleigh Scattering IR Absorption Wavelength in Nanometers (nm) 0.2 dB/Km 0.5 dB/Km 2.0 dB/Km Loss (dB)/km vs. Wavelength S-Band:1460–1530nm L-Band:1565–1625nm C-Band:1530–1565nm Loss Management: Problem Fiber Attenuation

7 © 1999, Cisco Systems, Inc. Cisco Systems Confidential METASTABLE STATE Pump Photon 980 or 1480 nm SIGNAL PHOTON 1550 nm SIGNAL PHOTON 1550 nm Loss Management: Solution Erbium Doped Fiber Amplifier FUNDAMENTAL STATE EXCITED STATE TRANSITION Amplified Signal 1550 nm Amplified Signal 1550 nm

8 © 1999, Cisco Systems, Inc. Cisco Systems Confidential Erbium Doped Fiber Amplifier “Simple” device consisting of four parts: Erbium-doped fiber An optical pump (to invert the population). A coupler An isolator to cut off backpropagating noise IsolatorCouplerIsolatorCoupler Erbium-Doped Fiber (10–50m) Pump Laser Pump Laser Pump Laser Pump Laser

9 © 1999, Cisco Systems, Inc. Cisco Systems Confidential Loss Management: Limitations Erbium Doped Fiber Amplifier Each amplifier adds noise, thus the optical SNR decreases gradually along the chain; we can have only have a finite number of amplifiers and spans and eventually electrical regeneration will be necessary Gain flatness is another key parameter mainly for long amplifier chains Each EDFA at the Output Cuts at Least in a Half (3dB) the OSNR Received at the Input Noise Figure > 3 dB Typically between 4 and 6 Noise Figure > 3 dB Typically between 4 and 6

10 © 1999, Cisco Systems, Inc. Cisco Systems Confidential Loss Management: Limitations EDFA (Cont.) In an ADD/DROP WDM Ring Topology Noise Can Build up (Positive Feedback) Until It Overcomes All the Signals If the Overall Attenuation Is Not Bigger Than the Gain Provided by the Amplifier Chain San Francisco Phoenix San Diego = EDFA = DWDM Equipment Constraint: Ge -αL < 1

11 © 1999, Cisco Systems, Inc. Cisco Systems Confidential Loss Management: Solution Photodetectors A photodetector is a device that measure optical power by converting the energy of the absorbed photons into electrical current Photodetectors for optical communication are basically semiconductor diodes (pn junctions) The link budget can be tuned by choosing the appropriate type of photoreceiver:P-I-N or Avalanche Photo Diode

12 © 1999, Cisco Systems, Inc. Cisco Systems Confidential Photodiode Basic Principle E=hc/λ Photon ΔE<hc/λ Absorption Hole Electron O-E Converter The Electron-Hole Pair Give Rise to an Electrical Current Valence Band Conduction Band

13 © 1999, Cisco Systems, Inc. Cisco Systems Confidential Photodetectors Types There are two type of photodiodes: P-I-N photodiodes: This type employs an intrinsic (not doped) layer of semiconductor between the p-doped and n-doped side in order to extend the usable area to receive photons Avalanche Photo Diode (APD): This is a strongly biased (reverse biasing) pn diode that creates many electron-hole pairs per each photon received; an APD amplifies the signal, therefore it has improved sensitivity (+8/10dBm over a PIN), but even higher noise and saturates with less input power than the PIN diode

14 © 1999, Cisco Systems, Inc. Cisco Systems Confidential PIN Photodiode p p i n n InP InGaAs Transparent Absorptive VRVR Optical Input

15 © 1999, Cisco Systems, Inc. Cisco Systems Confidential Managing Chromatic Dispersion

16 © 1999, Cisco Systems, Inc. Cisco Systems Confidential Dispersion Management: Problem Chromatic Dispersion (CD) The optical pulse tend to spread as it propagates down the fiber generating Inter-Symbol-Interference (ISI) and therefore limiting either the bit rate or the maximum achievable distance at a specific bit rate Physics behind the effect The refractive index has a wavelength dependent factor, so the different frequency-components of the optical pulses are traveling at different speeds Bit 1Bit 2 Bit 1Bit 2Bit 1Bit 2 Bit 1Bit 2 Bit 1Bit 2

17 © 1999, Cisco Systems, Inc. Cisco Systems Confidential Dispersion Management: Problem Fiber Dispersion Characteristic Dispersion Coefficient ps/nm-km nm 1550nm Normal Single Mode Fiber (SMF) >95% of Deployed Plant Dispersion Shifted Fiber (DSF)

18 © 1999, Cisco Systems, Inc. Cisco Systems Confidential Dispersion Management: Problem Increasing the Bit Rate Higher Bit Rates experience higher signal degradation due to Chromatic Dispersion: OA 10Gb/s Dispersion 16 Times Greater Dispersion 16 Times Greater Dispersion Scales as (Bit Rate) 2 Time Slot OA 2.5Gb/s Dispersion 1)

19 © 1999, Cisco Systems, Inc. Cisco Systems Confidential Dispersion Management: Solution Direct vs. External Modulation Laser diode’s bias current is modulated with signal input to produce modulated optical output Approach is straightforward and low cost, but is susceptible to chirp (spectral broadening) thus exposing the signal to higher dispersion The laser diode’s bias current is stable Approach yields low chirp and better dispersion performance, but it is a more expensive approach Electrical Signal in Direct Modulation External Modulation I in Optical Signal out Electrical Signal in DC I in Mod. Optical Signal Unmodulated Optical Signal External Modulator

20 © 1999, Cisco Systems, Inc. Cisco Systems Confidential Dispersion Management: Solution Dispersion Compensation In the Normal (1) Dispersion Regime Shorter Wavelengths Travel Slower (BLUE Is Slower Than RED) (1)In the Normal Dispersion Regime the Dispersion Coefficient Is D > 0 While in the Anomalous Regime It Is D < 0 Note: f = c/

21 © 1999, Cisco Systems, Inc. Cisco Systems Confidential Dispersion Management: Solution Dispersion Compensation (Cont.) Dispersion Compensating Fiber : By joining fibers with CD of opposite signs and suitable lengths an average dispersion close to zero can be obtained; the compensating fiber can be several kilometers and the reel can be inserted at any point in the link, at the receiver or at the transmitter Note: Although the Total Dispersion Is Close to Zero, This Technique Can Also Be Employed to Manage FWM and CPM Since at Every Point We Have Dispersion Which Translates in Decoupling the Different Channels Limiting the Mutual Interaction

22 © 1999, Cisco Systems, Inc. Cisco Systems Confidential Dispersion Management: Limitation Chromatic Dispersion CD places a limit on the maximum distance a signal can be transmitted without electrical regeneration: For directly modulated (high chirp laser) L D = 1/ B  D  (1) D dispersion coefficient (ps/km-nm):  source line width or optical bandwidth (nm): 0.5nm B bit rate (1/T where T is the bit period): 2.5Gb/s L D ~ 47 km (*) For externally modulated (very low chirp laser  f ~ 1.2B ) L D ~ Gb/s (*) L D ~ 61 10Gb/s and 17ps/nm*km (*) Source: Optical Fiber Communication IIIA, Chap. 7

23 © 1999, Cisco Systems, Inc. Cisco Systems Confidential The role of Polarization Mode Dispersin and Nonlinear effects in WDM systems

24 © 1999, Cisco Systems, Inc. Cisco Systems Confidential Polarization Mode Dispersion (PMD) The optical pulse tends to broaden as it travels down the fiber; this is a much weaker phenomenon than chromatic dispersion and it is of some relevance at bit rates of 10Gb/s or more Physics behind the effect If the core of the fiber lacks a perfect circular symmetry, the two components (along the x and y axis) of the electric field of the light pulse travel with different speeds nxnx nyny Ex Ey Pulse As It Enters the Fiber Spreaded Pulse As It Leaves the Fiber

25 © 1999, Cisco Systems, Inc. Cisco Systems Confidential Transmission Limitations Due to Polarization Mode Dispersion PMD accumulates with a squared root dependence on the fiber length:   = D PMD  L   differential delay between the x and y component of the electric field D PMD PMD coefficient The distance versus bit rate limit can be determined using: B 2 L ~ 0.02/(D PMD ) 2 (*) D PMD typical values between 0.5 and 2 ps/  km (**) If D PMD = 1.4 ps/  km and B = 10Gb/s L is limited to 100km If D PMD = 0.14 ps/  km and B = 10Gb/s L is limited to 10000km (*) Source: Optical Fiber Communication IIIA, Chap. 6 (**) Source: Optical Networks, Chap. 5

26 © 1999, Cisco Systems, Inc. Cisco Systems Confidential Fiber Nonlinearities As long as optical power within an optical fiber is small, the fiber can be treated as a linear medium; that is the loss and refractive index are independent of the signal power When optical power level gets fairly high, the fiber becomes a nonlinear medium; that is the loss and refractive index depend on the optical power

27 © 1999, Cisco Systems, Inc. Cisco Systems Confidential Cross Phase Modulation In WDM systems intensity fluctuation of one channel can affect the phase of other channels  CPM induced chirp  dispersion induced distortion Chromatic dispersion limit the effect of CPM because the interfering pulses of different channel tend to “walk away” from each other limiting the reciprocal interaction To limit CPM distortion, channel power should be below 10mW for 5 channels and below 1mW for 50 channels(*) Decreasing the number of channels reduces CPM effects Increasing the channel spacing reduces CPM effects Dispersion management can also be used by dispersion management techniques (*) Application of Nonlinear Fiber Optics, Chap. 7

28 © 1999, Cisco Systems, Inc. Cisco Systems Confidential Four Wave Mixing FWM is the dominant source of crosstalk and loss in WDM systems; the beating among many channels generates new tones as sidebands; in the worst case of equally spaced channels most new frequencies coincide with existing channels and generates interference; in the best case the WDM channels experience just a power depletion f ijk - f i = f j - f k (i,j <> k) f 113 f 112 f 123 f 213 f 223 f 132 f 312 f 221 f 332 f 321 f 231 f 331

29 © 1999, Cisco Systems, Inc. Cisco Systems Confidential FWM Performance Impact Like CPM in the presence of dispersion FWM is less efficient because of the “walk away” effect of different channels Using Dispersion Shifted Fiber greatly enhances the FWM process Reducing the channel count and the channel spacing also reduces FWM penalties Adopting an unequal channel spacing limits FWM cross-talk but the channel power depletion is still present

30 © 1999, Cisco Systems, Inc. Cisco Systems Confidential Wavelength Dispersion ps/nm-km nm Normal Single Mode Fiber (SMF) >95% of Deployed Plant Dispersion Shifted Fiber (DSF) Managing CPM and FWM: Non-Zero Dispersion Shifted Fiber Nonzero Dispersion Shifted Fibers (NZDSF) ~+3ps/nmkm ~-3ps/nmkm 1550nm

31 © 1999, Cisco Systems, Inc. Cisco Systems Confidential NZDSF Flavors: Lucent TrueWave vs. Corning LEAF TrueWave fibers: Small amount of chromatic dispersion throughout the EDFA band (~1550nm); this dispersion prevents phase matching among the various signals reducing CPM and FWM Corning LEAF: Similar to TrueWave as far as dispersion goes; however it has a large effective area design that reduced the light intensity and therefore all the nonlinear effects

32 © 1999, Cisco Systems, Inc. Cisco Systems Confidential Appendix

33 © 1999, Cisco Systems, Inc. Cisco Systems Confidential Definitions: refractive index & propagation constant Relationship between frequency and wavelength in electromagnetic radiations: c =  x The refractive index (n) of a material is the ratio of the speed of light in the vacuum to the speed of light in that material: n=c/v. The propagation speed of an electromagnetic radiation depends on the refractive index and the wavelength and it is determined by the propagation constant: β=2 πn/λ

34 © 1999, Cisco Systems, Inc. Cisco Systems Confidential The physics behind Chromatic Dispersion An optical pulse S is composed of a series of monochromatic waves : S(  cos(2 πc λ 1 t - β 1 z) + cos(2 πc λ 2 t - β 2 z) + … Where β 1 != β 2 != …  the different waves composing the pulse propagates at different speed

35 © 1999, Cisco Systems, Inc. Cisco Systems Confidential Calculating Transponders CD distance limitation Transponder Dispersion Tolerance (TDT) is usually expressed in: [ps/nm] The dispersion coefficient D is expressed in [ps/nm*km] To calculate the distance: L MAX = TDT/D [ ps / nm * nm*km / ps = km] Eg. ONS has 1800ps/nm of TDT on regular SMF D = 18 ps/nm*km L MAX = 1800/ 18 = 100km

36 © 1999, Cisco Systems, Inc.