wdm system design optiwave

WDM COMMUNICATION SYSTEMSWDM COMMUNICATION SYSTEMS

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Simulation and Design using OptiSystem

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OutlineOutline

• A basic WDM system

• Design parameters to consider

• Loss and gain

• Signal to noise ratio

• Dispersion and dispersion compensation schemes

• Fiber nonlinearities

� types

� interplay with dispersion / advantageous nonlinearities

� strategies for their control

• Summary

A Basic WDM SystemA Basic WDM System

Design ParametersDesign Parameters

• OSNR limited

• Eye distortion limited

Loss, chromatic

dispersion, OSNR, FWM,

SPM, SPM, PMD

10 Gbps

(Optical link with optical

amplifiers + WDM)

• OSNR with dispersion

compensation

Loss, chromatic

dispersion, OSNR, four

wave mixing (FWM)

2.5 Gbps


amplifiers + WDM)

• Chromatic dispersion

without dispersion

compensation

• OSNR with dispersion

compensation

Loss, chromatic

dispersion, optical signal

to noise ration (OSNR)

2.5 Gbps


amplifiers)

• Loss limited without

transmit amplifier

• Dispersion limited with

transmit amplifiers

Loss, chromatic

dispersion

2.5 Gbps

(Point to Point)

System limitationParametersBit rate

Parameters to ConsiderParameters to Consider

• Loss/gain

• Optical signal to noise ratio

• Dispersion

• Fiber nonlinearities

– Self phase modulation (SPM)

– Cross phase modulation (XPM)

– Four wave mixing (FWM)

– Stimulated Brillouin scattering (SBS)

– Stimulated Raman scattering (SRS)

• Polarization effects

Loss and Loss CompensationLoss and Loss Compensation

• Fiber loss

– Constant

• Splice loss

• Connector loss

• Component loss

– Well defined

• Optical amplifiers

– Gain depends on input power and �

– Output power depends on input power and �

• Fiber loss

– � dependent

• Splice loss

• Connector loss

• Component loss

– May have � dependence

• Optical amplifiers

– Gain is highly � dependent

– Gain tilt depends on input power

– Gain at a certain � depends on input powers of other channels

Single channel Multi channel

Fiber lossFiber loss

0.1

1

1 1.2 1.4 1.6 1.8

Wavelength (μμμμm)

Loss

(dB/

km)

Old

AllWave

Standard

Low loss windows:0.8 micron1.3 micron1.55 micron (~0.25 dB/km)

Δλ∼80 nm at 1.3 μm

~180 nm at 1.55 μm (Δf~35 THz)

Δf ~ (c/λ2)Δλ

Typical ValuesTypical ValuesDescription Loss value Loss of a connector 0.25 dB Loss of a splice 0.15 dB Loss of the fiber span 0.25 dB/km Loss of a multiplexer 4 dB Loss of a demultiplexer 4 dB

Description Receiver Sensitivity 2.5 Gbps pin diode receiver sensitivity -23 dBm 10 Gbps pin diode receiver sensitivity -16 dBm 2.5 Gbps APD receiver sensitivity -38 dBm 10 Gbps APD receiver sensitivity -30 dBm

State of the art bandwidths 75 nm, 18 dB gain +/- 1.5 dB EDFA and

Raman 76 nm, 20 dB gain, +/- 1 dB Tellurite

EDFA 92 nm transparent bandwidth Raman

EDFA GainEDFA Gain

• Gain and gain tilt depend on

input power

• Effect of cascading

amplifiers

one amplifier

two cascaded

amplifiers

8 channels

Pin

Pin = -13 dBm

Pin = 0 dBm

Pin = 8 dBm

For all cases NF ≈ 4 dB

System Performance, 8 ChannelsSystem Performance, 8 Channels

signal power 3 dBm/ch

Gai

nva

riatio

n~

4dB

Loss = 18 dB

BR = 2.5 Gbps

Signal to Noise RatioSignal to Noise Ratio• Accumulates

• Different sources

– Thermal

– Shot noise

– Optical amplifier noise

• Signal-spontaneous

– Dominant effect

• Spontaneous-

spontaneous

• For multi-channel

system consider

– � dependence of NF

– X-talk as noise source

outo

amp

PSNRF G h f f N

≈× × × × Δ ×

hf: photon energy

F: noise figure

Δf: bandwidth

Namp: number of amplifiers

G: amplifier gain, assumed

equal to span loss

Values at the receiver:

- 40-50 dB is good

- 30 dB is acceptable

Group Velocity (chromatic) DispersionGroup Velocity (chromatic) Dispersion

• Transmitter and receiver dispersion tolerance

• Placement of dispersion compensator

– Pre

– Post

– Symmetrical

• Accumulated net dispersion

• Nonlinear effects

• Fiber dispersion slope

– Net dispersion for different channels

• Wavelength dependence of compensation element

Multi-channel

• GVD leads to pulse broadening

Single-channel

Fiber Dispersion ValuesFiber Dispersion Values

• In the Erbium window, fibers have different dispersion values and

slopes, which heightens the dispersion-compensation challenge

•J. Lively, “Dealing with the critical problem of chromatic dispersion”, Lightwave, September 1998.

GVD limited Tx DistanceGVD limited Tx Distance• Direct modulated DFB lasers 1

4L

B D λσ<

• Externally modulated source

rms spectral width, a typical

value is about 0.15 nmFor D=16 ps/(km-nm)at 2.5 Gbps, L~ 42 km

For D=16 ps/(km-nm)at 2.5 Gbps, L~ 500 kmat 10 Gbps, L~ 30 km

•G. P. Agrawal, Applications of nonlinear fiber optics, Academic Press, 2001.•R. Ramaswami and K. N. Sivarajan, Optical Networks: A practical Perspective, Morgan Kaufmann, 1998.

2 2

216

cLD B

πλ

<

Dispersion CompensatorsDispersion Compensators• Dispersion compensating fiber

(DCF)– Uses large negative dispersion in 1.5

micrometer window

– Small effective area leading low nonlinear power threshold

– Dispersion slope does not match with that of transmission fiber

• Gratings– Uses wavelength dependent reflective

delay

– Low insertion loss

– Dispersion slope can be written on grating

– Nonlinear power threshold is same as transmission fiber

– Phase response is not smooth

•J. Lively, “Dealing with the critical problem of chromatic dispersion”, Lightwave, September 1998.

Nonlinear Dispersion CompensationNonlinear Dispersion Compensation

•Illustrated later ( see “SPM + Dispersion…”, slides # 32-34

Dispersion Compensation ExampleDispersion Compensation Example

SMF90 km

D = 17 ps/nm/km at 1545 nm,

S = 0.09 ps/nm2/km

DCF19.6 km

D = -80 ps/nm/km at 1545 nm

S = -0.15 ps/nm2/km

8 channels

100 GHz (0.8 nm) separation

10 Gbps bit rate

6 span

DCF CalculationDCF Calculation

-94 ps/nm080 ps/nmResidual

D = -80.64 ps/nm/km

TD/span = -1580.6

ps/nm

D = -81.12 ps/nm/km

TD/span = -1590.9

ps/nm

D = -81.49 ps/nm/km

TD/span = -1597.22 ps/nm

DCF19.6 km

D = -80 ps/nm/km at

1545 nm

S = -0.15 ps/nm2/km

D = 17.38 ps/nm/km

TD/span = 1564.9

ps/nm

D = 17.67 ps/nm/km

TD/span = 1590.9

ps/nm

D = 17.89 ps/nm/km

TD/span = 1610.5 ps/nm

SMF90 km

D = 17 ps/nm/km at

1545 nm

S = 0.09 ps/nm2/km

Ch 8: 193.5 THzCh 4: 193.1 THzCh 1: 192.8 THz

• OC-48 direct mod CD tolerance: ~1500 ps/nm• OC-192 external mod without pre-chirp: ~600 ps/nm• OC-192 external mod with pre-chirp: from 0 to 1500 ps/nm

Simulation results for 8 channel systemSimulation results for 8 channel system

-14 -12 -10 -8 -6 -4 -2

0.5

1

1.5

2

x 10-4

ch 1ch 4ch 8

Received signal power (dBm)

Eye

heig

ht (a

.u.)

ch 1

ch 4

ch 8

-4 dBm -3 dBm

Without dispersion compensation

Dispersion compensation schemesDispersion compensation schemes

•M. I. Hayee and A. E. Willner, PTL 9, pp. 1271, 1997.

•Sebastian Biga et. al., PTL 11, pp. 605, 1999.

•Giovanni Bellotti et. al., PTL 11, pp.824, 1999.

pre post

symmetrical

Simulation resultsSimulation resultspost pre symmetrical

pre

post/symmetrical

Bit rate = 2.5 Gbps

Bit rate = 10 Gbps

Symmetrical compensation is the best

Simulation results with post-compensationHigher powers

Simulation results with post-compensationHigher powers

Bit rate = 2.5 Gbps11

22

33

22

D = 0D = 16 and -80

33

11

Simulation results with post-compensationHigher powers and higher bit rate

Simulation results with post-compensationHigher powers and higher bit rate

Bit rate = 10 Gbps11

22

33

11

22

33

Dispersion compensation with FBGDispersion compensation with FBG

SMF100 kmL = 0.2 dB/kmD = 16 ps/nm/kmAeff = 72 micron-square

Dispersion compensation with FBGDispersion compensation with FBG

Bit rate = 10 Gbps

11

22

33

11

22 33

Fiber nonlinearitiesFiber nonlinearities

Stimulated Raman

scattering (SRS)

Stimulated Brillouin

scattering (SBS)

Related to the imaginary

part of the refractive

index

Cross phase modulation

(XPM)

Four wave mixing (FWM)

Self phase modulation

(SPM)

Related to the real part of

the refractive index

Multi channelSingle channel

Q factor verses launch power SNR verses launch power

linear

nonlinear

Self phase modulationSelf phase modulation• SPM effects are negligible when 0P α γ<

•

1 12 0 [ ]eff

n W kmcAωγ − −=

• For the fiber we used 1 11.5W kmγ − −≈

• SPM effects can be negligible when the pick power is below 166 mW or 18 dBm average power

• If you use AN amplifiers along the link, the criteria

becomes ( )0 AP Nα γ< . If you use two amplifiers along

the link, the maximum allowable power before the nonlinearity becomes effective decreases by 3 dB

• Dispersion management using DCF can reduce SPM

Self phase modulationSelf phase modulation•Quite different scenarios if acting alone

•…or coupled with dispersion.

•The combination of SPM+Dispersion causes two

interesting phenomena with many consequences for real

transmission systems:

� Modulation instability

� Solitons

• Even when the system operates far from these “pure”

extreme cases, the presence of nonlinearity alters

strongly the dispersive signal propagation and vice versa.

SPM, no Dispersion, L=15 kmSPM, no Dispersion, L=15 km

Input spectrum P = 20 dBm Output spectrum P = 20 dBm Output spectrum P = 23 dBm

Output spectrum P = 26 dBm Output spectrum P = 29 dBm

SPM + Dispersion + CW Input = Modulation Instability


“Optical Fiber Telecommunications IIIa”, ed. by I. Kaminov and T. Koch, chapter

“Fiber Nonlinearities and Their Impact on Transmission Systems” by F. Forghieri, R.

Tkach and A. Chraplyvy




•L= 15+15 km

•DSF, D=0.16 ps/nm/km

•Input CW power P = 19 dBm

•Inline amplifier after the first 15 km span



L=15 km

L=15 km

L=30 km

L=30 kmL=0 km

Spectra

Time DomainWaveforms

SPM + Dispersion + Sech Input = Solitons

SPM + Dispersion + Sech Input = Solitons

•Multi-color solitons

• Δν = 1 THz, ΔτFWHM = 10 ps

•25 km SMF

SPM + Dispersion + Arbitrary Input SPM + Dispersion + Arbitrary Input







•Nonlinear Dispersion Compensation ( nonl. pulse compression)•Negative power penalties

D > 0

D < 0•Additional pulse broadening•Positive power penalties

SPM + Dispersion + (D>0) = Nonlinear Dispersion Compensation

SPM + Dispersion + (D>0) = Nonlinear Dispersion Compensation







D > 0D < 0

|D| = 2.27 ps/km/km(NZDSF)

L = 145 km

B = 10 Gb/s, NRZ

P0 = 14.5 dBm

Dependence of nonl. compensation on the fiber dispersion:150 km, NRZ

Nonlinear Compensation – continued• Single channel transmission at 10 Gb/s in +/- NZDSF

Example layoutExample layout

DCF20 kmL = 0.5 dB/kmD = -80 or -72 ps/nm/kmAeff = 30 micron-square


Bit rate = 10 Gbps

EDFAG = 35 dBNF= 0 dB

Simulation results with single channelSimulation results with single channel

0 dBm

10 dBm

13 dBm

Signalpower Total dispersion = 0

Residual dispersion = 800 ps/nmdistance

accumulated dispersion

distance


•G. Bellotti et. al., “Dependence of self-phase modulation impairments on residual dispersion in 10 Gb/s based terrestrial transmission using standard fiber”, IEEE Photon. Tech. Lett. 11, pp. 824, 1999.

Cross phase modulationCross phase modulation• Refractive index modulation due to one signal causes

phase modulation in other co-directional channels

• As a rough estimate, the channel power is restricted with

( )2 1ch chP Nα γ< −⎡ ⎤⎣ ⎦

where, chN is the number of channels

• For a two channel system, limiting power is approximately 56 mW (17.5 dBm). For a 10 wavelength system, the limit is about 10 mW (10 dBm)

• Under ideal conditions (group velocities matched) XPM is two times more effective than SPM

• Both similar and very different from SPM…

Cross phase modulationCross phase modulation• The main difference is that the two (or more) channels have

different group velocities.

• That fact leads to averaging and possibly to complete elimination of the XPM perturbation. An increase in the separation decreases the penalty which originates from the XPM

• Separation between channels also affects the XPM (negligible for > 1 nm channel spacing for SMF, NZDSF, but not DSF )

L= LwL= 0

Cross phase modulation example 1Cross phase modulation example 1

• 2 channels at 2.5 Gb/s

• channel spacing 1 nm (1550 nm)

• Initial pulse separation 800 ps

• Conventional SMF, D=16 ps/nm/km

• Signal power Ps = 2 mW,

• "Pump" power Pp = 20 mW,

Results: The calculated results show that the disperion can lessen the efects of XPM It can also induce:

• pulse jitter

• parasitic frequency shifts







Cross phase modulation example 1Cross phase modulation example 1

Signal spectra

Signal spectra zoomed

Example 2: layoutExample 2: layout

DCF20 kmL = 0.5 dB/kmD = -80 or 72 ps/nm/kmAeff = 30 micron-square


Bit rate = 10 Gbps

EDFAG = 35 dBNF= 0 dB

Simulation results with 8 channelsSimulation results with 8 channels

0 dBm/ch

10 dBm/ch

13 dBm/ch

Signalpower

t.d = 0 ps/nm inputspectrum

distance


distance


t.d = 1000 ps/nm

outputspectrum

distance


outputspectrum

inputspectrum

•S. Bigo et. al., “Investigation of cross-phase modulation limitation over various types of fiber infrastructures”, IEEE Photon. Tech. Lett. 11, pp. 605, 1999.•M. I. Hayee and A. E. Willner, “Pre- and post-compensation of dispersion and nonlinearities in 10-Gb/s WDM systems”, IEEE Photon. Tech. Lett. 9, pp. 1271, 1997.

Four Wave MixingFour Wave Mixing

• FWM causes noisy artifacts on the channel grid, nonlinear crosstalk

• Beating between two signals generates harmonics at the

difference frequencies

Four Wave MixingFour Wave Mixing

• FWM efficiency depends on signal power, channel spacing, and dispersion

• If the GVD of the fiber is relatively high 2

2 5 /ps kmβ > ,

the FWM efficiency factor almost vanishes for a typical

channel spacing of 50 GHz or higher

• If the channel is close to zero dispersion wavelength of the fiber, considerably high power can be transferred to FWM

components.

• To reduce the effect of FWM to the system performance, you can use either uneven channel spacing or the dispersion-management technique or both

Four Wave MixingFour Wave MixingHow does it depend on dispersion and channel spacing?How does it depend on dispersion and channel spacing?

“Optical Fiber Telecommunications IIIa”, ed. by I. Kaminov and T. Koch“Optical Fiber Telecommunications IIIa”, ed. by I. Kaminov and T. Koch

10 Gbps, WDM transmission over 1500 km10 Gbps, WDM transmission over 1500 km

•H. Taga, “Long distance transmission experiments using the WDM technology”, J. Lightwave Tech. 14, pp. 1287, 1996.

10 Gbps, WDM transmission over 1500 km10 Gbps, WDM transmission over 1500 km


•Total power is 11 dBm

•BR = 10 Gbps

•To reduce the interaction due to FWM:

� Managed dispersion

Zero dispersion wavelength of the system is at 1558.2 nm

Residual dispersion: -634.5 ps/nm at 1553.5 nm

-364.5 ps/nm at 1555.5 nm

-27 ps/nm at 1558.0 nm

243 ps/nm at 1560 nm

� Unequal channel spacing

1553.5 nm, 1555.5 nm, 1558.0 nm, and 15560.0 nm

Power spectrumPower spectrum

Experiment Simulation

a) baseline

b) after 1500 km transmission

(a)

(b)

Eye diagramsEye diagrams


Experiment Simulation

ch 1, Q = 16.9 ch 2, Q = 15.9

ch 3, Q = 19.2 ch 4, Q = 17.9

ch 1, Q = 15.8 ch 2, Q = 14.9

ch 3, Q = 19.2 ch 4, Q = 13.5

Stimulated Raman scatteringStimulated Raman scattering

•

Short wavelength channels act as pumps for longer wavelength channels

•

The Raman threshold for a single channel system is given by

16 effth

R eff

AP g L≈ where

1effL α≈ for long fibers

•

SRS is also a function of the number of the channels and the channel power

•

For a single channel system, the Raman threshold is about

500 mW near 1.55 micrometer if 131 10 /Rg m W−= ×

•

For a 20 channel system, thP exceeds 10 mW

• thP is around 1 mW for a 70 channel system

•

SRS has little impact on system performance

Simulation resultsSimulation results

16 CW channel100 GHz separation20 mW/channel

Stimulated Brillouin scatteringStimulated Brillouin scattering•

Lightwave interacts with acoustic wave in fiber, scatters power backwards

•

Threshold level depends on source line-width, effective

core area, and effective fiber length 21 eff

thB eff

AP g L≈

•

Typical value for Bg is about 115 10 /m W−×

•

The threshold value also depends on modulation format and duration of pulse

•

Some values: � 9 dBm for CW light

� 12 dBm for externally modulated transmitter � >18 dBm for externally modulated transmitter with

source wavelength dither

•

SBS has little effect on system performance

Modulation formatsModulation formats• Most common modulation formats are Non-Return-to-Zero

(NRZ) and Return-to-Zero (RZ)

• Due to higher peak power, NRZ may suffer more from

nonlinearities

• Due to shorter pulse width, RZ may suffer more from

dispersion

• Studies show that 10 Gbps WDM systems, in general,

operate better by using RZ modulation in high power regime

• It is hard to go give any specific guideline due to complex

interaction between dispersion and nonlinear effects

J. Yu and P. Jeppesen, “Investigation of cross-phase modulation in WDM systems with NRZ and RZ modulation formats”, Opt. Comm. 184, pp. 367, 2000M. I. Hayee and A. E. Willner, “NRZ versus RZ on 10-40 Gb/s dispersion managed WDM transmission systems”, IEEE Photon. Tech. Lett. 11, pp. 991, 1999

Project layoutProject layout

SMF

D = 17 ps/nm/km

Aeff = 80 micron-square

DCF

D = -85 ps/nm/km

Aeff = 22 micron-squareBit rate = 10 Gbps

Simulation resultsSimulation results

Launch power

-10 dBm

-7 dBm

0 dBm

10 dBm

15 dBm

NRZ RZ

SummarySummary• During the design process consider

– SNR at low powers

– Nonlinear effects at high powers, WDM systems

– GVD at high bit rates

– Modulation format

• Several alternatives to compensate dispersion

• Symmetrical dispersion compensation preferred

• But post compensation can produce similar results

• Managed dispersion can reduce the effects of nonlinearities, but manipulating chromatic dispersion has both positive and negative influence on nonlinearities

• The nonlinearities can result in negative penalties if the system is operated in the proper regime

wdm system design optiwave

Documents

stimulated raman scattering

cross phase modulation

single channel system

chapter fiber nonlinearities

dispersion compensation osnr

phase modulation

channel spacing

channel power