wdm system design optiwave
TRANSCRIPT
WDM COMMUNICATION SYSTEMSWDM COMMUNICATION SYSTEMS
ste
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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
(Optical link with optical
amplifiers + WDM)
• Chromatic dispersion
without dispersion
compensation
• OSNR with dispersion
compensation
Loss, chromatic
dispersion, optical signal
to noise ration (OSNR)
2.5 Gbps
(Optical link with optical
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
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
“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
SPM + Dispersion + CW Input = Modulation Instability
SPM + Dispersion + CW Input = Modulation Instability
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
“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
“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
•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
“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
“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
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
SMF100 kmL = 0.25 dB/kmD = 16 ps/nm/kmAeff = 72 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
accumulated dispersion
•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
“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
“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
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
SMF100 kmL = 0.25 dB/kmD = 16 ps/nm/kmAeff = 72 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
accumulated dispersion
distance
accumulated dispersion
t.d = 1000 ps/nm
outputspectrum
distance
accumulated dispersion
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
•H. Taga, “Long distance transmission experiments using the WDM technology”, J. Lightwave Tech. 14, pp. 1287, 1996.
•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
•H. Taga, “Long distance transmission experiments using the WDM technology”, J. Lightwave Tech. 14, pp. 1287, 1996.
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