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Optical Communication TechnologyOptical Communication Technology
Lecture 4 WDM Concepts and ComponentsLecture 4 WDM Concepts and ComponentsDr. Huug de Waardt (sheets © prof. Ton Koonen)Textbook Keiser: Chapter 10
TTE-ECO group, COBRA Institute, Eindhoven University of Technologye-mail: [email protected]
Feb 15, 2004
COBRA
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ContentContent
Ch. 10WDM Concepts and Components
operational principlespassive WDM componentstunable sourcestunable filters
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Typical WDM networkTypical WDM network
Transporting multiple wavelength channels through a single fibre
SpectralSelectivityto avoid X-talk
wavelength multiplexer: low-loss combining
wavelength demultiplexer: low-loss separation and good isolation (typ. > 30 dB)
passive devices, no external control; split/combine/tap signals
active devices, external control; tunable sources/filters, amplifiers
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Key system features of WDMKey system features of WDM
Capacity upgradeN times capacity of a single wavelength channelTransparencyany transmission format on each channelWavelength routinguse wavelength as another dimension (in addition to time and space) for networking, by wavelength-sensitive static routingWavelength switchingreconfiguration of the optical layer, with optical add/drop multiplexers, optical cross connects, wavelength converters
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Transmission windows of Transmission windows of fibre fibre
( 3.4 THz)
O-H peak 1310 nm window:1270 - 1350 nm
1550 nm window:1480 - 1600 nm1 nm ↔ 125 GHz
DFB laser:CW linewidth 10 to 50 MHz
ITU-T G.692:ref. 193.100 THzequiv. to 1552.524 nm
channel spacing 100 GHz(alternatives 50 and 200 GHz)
λλ
ν ∆⋅=∆ 2
c1 ppm O-H causes 65 dB/km, at λ=1.39 µm
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AllWaveAllWave® ® fibrefibre **
Removal “water peak”required for Raman pumping!
Water-free→window 1200 - 1650 nmequiv. to 68 THz
* from Lucent Technologies (/Optical Fiber Solutions)
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Wavelength multiplexing Wavelength multiplexing is the multiplier for capacity …is the multiplier for capacity …
0.1
1
10
100
1000
0.01 0.1 1 10 100
Data rate per channel (Gb/s)
Num
ber o
f cha
nnel
s
'80 '83 '86 '87
'89
'91
'93
'95
'95
'96
'98
'98
'00
'00
10Gb/s 100 Gb/s
1 Tb/s
10 Tb/s
TotalCapacity
'00
OFC’00
OFC’00320 Gb/s
3.28 Tb/s
7 Tb/s
ECOC’00
1.28 Tb/s
ECOC’00
OFC’0110.92 Tb/s
OFC’993 Tb/s
1000
RETINA4.48 Tb/s
100 Tb/s
40 Gb/sTDM
160 Gb/sOTDM
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… and Optical Amplifiers for distance… and Optical Amplifiers for distance
Conventional High Speed Transport - 40 Gb/sConventional High Speed Transport - 40 Gb/s
1310RPTR
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TERM TERM40km 40km 40km 40km 40km 40km 40km 40km 40km
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TERM TERM
Fibre Amplifier Based Optical Transport - 40 Gb/sFibre Amplifier Based Optical Transport - 40 Gb/s
Less Fibre
Fewer mid-span sites
One OAfor all 16 λ
OC-48OC-48
OC-48OC-48
OC-48OC-48
OC-48OC-48
OC-48
OLSTERM
OLSRPTR
OLSRPTR
OLSTERM
120 km 120 kmOC-48OC-48
OC-48
OC-48OC-48
OC-48OC-48
OC-48OC-48
OC-48OC-48
OC-48OC-48
OC-48OC-48
OC-48OC-48
OC-48OC-48
OC-48OC-48
OC-48OC-48
120 km
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Passive optical devices Passive optical devices
to split and combine signalswhen dependent on the light wavelength: useful for wavelength (de-)multiplexing
e.g.- N x N coupler (N ≥ 2 )- power splitter (1 x N)- power tap (non-uniform 2 x 2 coupler)- star coupler (N x M)
made from- optical fibres- planar optical waveguides (LiNbO3, InP)- bulk micro-optics (with lenses, filters, ….)
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Basic star coupler (Basic star coupler (losslesslossless))
E.g., by fusing together cores of N single-mode fibres over a few millimeters
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2x2 fused 2x2 fused biconical biconical taperedtapered fibrefibre couplercoupler
Coupler draw length 2L + WEvanescent field couplingDirectional coupler (P3 and P4 are -50 to -70 dB below P0)
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FibreFibre coupler characteristicscoupler characteristics
P2 / P0
Coupler draw length (mm)
P1 / P0
P2 / P0
P1 / P0
wavelength (nm)
Nor
mal
ised
pow
erN
orm
alis
ed p
ower
( )
( )κ
κ
κ
t coefficien couplingwith cos
:lossless) (ifenergy ofon conservatisin
20201
202
zPPPP
zPP
⋅⋅=−=
⋅⋅=
Variation of κ with λ
(15 mm long coupler)
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Performance specificationsPerformance specifications
dBlog10Crosstalk
dBlog10 loss Insertion
0
3
⋅=
⋅=
PP
PP
j
i
dBlog10 loss Excess
%100 ratio Splitting
21
0
21
2
+
⋅=
⋅
+
=
PPP
PPP
e.g.3 dB coupler:P1 = P2
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Scattering matrix (or Propagation matrix)Scattering matrix (or Propagation matrix)
Po1 = (1-ε) Pi1Po2 = ε Pi1
=
=
=⋅=
2221
1211
2
1
2
1 matrix scattering and and with ssss
aa
bb
SabaSb
Reciprocity → s12 = s21Conservation of energy → b1
* b1 + b2* b2 = a1
* a1 + a2* a2
with s11 = √1-ε follows
−−
=εε
εε1
1j
jSε = 0.5 for3dB coupler
} Φ11 = 0, Φ12 = π/2
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2x2 2x2 Waveguide Waveguide couplerscouplers
Uniformly symmetric coupler
Uniformly asymmetric coupler(i.e., one waveguide wider)
Interaction between guides:dep. on width w, gap s, index n1 between guides
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Symmetric Symmetric waveguidewaveguide coupler characteristics coupler characteristics (1/2)(1/2)
( )
( )
21
22
22
2
202
direction -yin t coefficien extinction and
2
t coefficien couplingwith
sin: theorymode Coupled
kq
q
qweq
ezPP
y
yz
qsy
z
−=
+=
⋅⋅=
−
−
β
βββ
κ
κ
κ α
Complete power transfer at guide length L = (m+1)·π/2κ with m=0, 1, 2, ..
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SymmetricSymmetric waveguidewaveguide coupler characteristics coupler characteristics (2/2)(2/2)
( ) zezPP ακ −⋅⋅= 202 sin
andκ is roughly proportional to λ
→ periodic wavelength dependence
Guide loss considered negigible, α = 0
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Asymmetric Asymmetric waveguidewaveguide coupler characteristicscoupler characteristics
Amplitude of coupled power dep. on λ
( )
direction-in guidesbetween difference phase with
2
where
sin
222
22
2
0
2
z
g
egzgP
P z
β
βκ
κ α
∆
∆
+=
⋅⋅= −
Flattened response at lower λ with <100% coupling ratio
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N x M star couplersN x M star couplers
Combine powers from N inputs, divide equally among M outputsN x N : e.g. by fusing fibres
+=
+=
∑ =
N
i iout
in
PPN1 ,
log10log10
lossexcess losssplittinglosstotal
E.g., excess loss 0.4 dB for 7x7 couplercoupling ratios difficult to control in fabrication
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Star coupler by cascading 3 dB Star coupler by cascading 3 dB -- couplerscouplers
N = 2n with n integernumber of 3 dB - couplers needed is (N/2) 2log Nexcess loss = - 10 log ( FT
^ 2log N) dB, where 1-FT is fraction of power lost per 3 dB - coupler
8x8star coupler
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MachMach--Zehnder Zehnder Interferometer Interferometer -- layoutlayout
in 1
in 2
out 1
out 2
in planar integrated opticsMany important applications: external modulators, wavelength convertors
wavelength (de)multiplexers
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MachMach--ZehnderZehnder Interferometer Interferometer -- analysis (1/3)analysis (1/3)
( )( )
[ ][ ]
( ) ( )( ) ( ) )(2/sin)(2/cos
)(2/cos)(2/sin
02cos2sin using and With
2sin)(2cos)(2cos)(2sin)(
/2 and ,at and at with ,
2sin2cos2cos2sin
2/exp002/exp
1
12
1matrix n propagatio
22,22
11,12*
2,2,2,
22,22
11,12*
1,1,1,
21,,2
,,
222,111,2,
222,111,1,
x22,11,
2,
1,
2,
1,
λλ
λλ
λλλλ
λπλλ
ϕ
ϕ
ininoutoutout
ininoutoutout
xoutxoutxoutxout
ininout
ininout
xinin
in
incouplercoupler
out
out
coupler
PLkPLkEEP
PLkPLkEEP
)/∆Lk()/∆Lk(EEEP
)/∆Lk(E)/∆Lk(EjE)/∆Lk(E)/∆Lk(EjE
kEE
)/∆Lk(-)/∆Lk()/∆Lk()/∆Lk(
jEE
EE
LjkLjk
jj
⋅∆⋅+⋅∆⋅=⋅=
⋅∆⋅+⋅∆⋅=⋅=
=⋅⋅⋅⋅==
⋅−⋅=
⋅+⋅=
=
⋅⋅⋅⋅
=
⋅⋅⋅=
∆⋅−
∆⋅=
⋅=
∗
∆
∆
poweroutput
ngmultiplexi Wavelength
MMM
M:shifter phase
M:coupler dB 3
Introduces frequency beyond detector bandwidth
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MachMach--ZehnderZehnder Interferometer Interferometer -- analysis (2/3)analysis (2/3)
( )
hs. wavelengt two theof spacing freq. is where
2112
112 i.e. ,integer with
)12(2/2/ and )1(2/
0
1
21
2121
21
1
ν
νλλ
πλλ
π
ππ
∆
∆⋅=
−=∆
→
=∆⋅
−=∆⋅−
+⋅=∆⋅+⋅=∆⋅
=
−
effeff
eff
out,
ncnL
LnLkkm
mLkmLk
∆LP
arms MZI in difference length
that such need we then , thus and 2port output to inputs bothmultiplex to designed If
( ) ( )( ) ( ) )(2/sin)(2/cos
)(2/cos)(2/sin
22,22
11,12*
2,2,2,
22,22
11,12*
1,1,1,
λλ
λλ
ininoutoutout
ininoutoutout
PLkPLkEEP
PLkPLkEEP
⋅∆⋅+⋅∆⋅=⋅=
⋅∆⋅+⋅∆⋅=⋅=
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MachMach--ZehnderZehnder Interferometer Interferometer -- analysis (3/3)analysis (3/3)
0
0.2
0.4
0.6
0.8
1
0 0.5 1 1.5 2 2.5
trans
mis
sion
(T12)^2
(T11)^2
in 1 → out 1( in 2 → out 2 )
in 1 → out 2( in 2 → out 1 )
m+2m m+1m+½
∆L / λ
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FourFour--channel wavelengthchannel wavelength multiplexermultiplexer
( )
ν
νν
∆⋅=∆=
∆⋅=∆⋅
=∆∆⋅
=∆=∆
−eff
jnjstagen
effeff
ncLNN
Ln
cLn
cLL
2 : 2r with multiplexe 1-to-For
2222 1321
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Reflection gratingReflection grating
Differentwavelength ⇒Different angleof diffraction
Grating equationΛ ( sin θi + sin θd ) = m λ with integer order mSeparation of many wavelengths by a single element
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Grating Grating demultiplexerdemultiplexer
lens
normal to grating surface
θdθi
focal length f
λ1 λ2
λ1, λ2
lens
grating
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Practical grating Practical grating demultiplexersdemultiplexers
From: Fibre Optic Communication Devices, N. Grote et al., Springer Verlag, 2001,Ch. 7 Wavelength selective devices, M. Smit, T. Koonen, H. Herrmann, W. Sohler
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Bragg grating formation within a Bragg grating formation within a fibrefibre
All-fibre →low cost, low loss (ca. 0.1 dB),easy coupling,pol. insensitive, low temperature coeff., simple packagingNarrow-band reflection filter, reflecting atλ Bragg
Local increase in n
Grating writing with UV beams
fibre
( )
gratings strong ..for weak 1..5.0s with 2
bandwidth FWHM
modulationindex sinusoidaluniformfor )1( coeff. coupling with tanhty reflectiviPeak
2h wavelengtreflection Bragg
22
Bragg22
max
Bragg
≈
Λ
+
⋅⋅≈∆
−⋅⋅≈=
Λ=−
Lnns
VnLR
n
coreBraggFWHM
eff
δλλ
λδπκκ
λ
V number, modes a fibre can support, K 2-27Photo-induced refractiveIndex change
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Add/Drop Add/Drop multiplexermultiplexer with FBGwith FBG
circulator or 3 dB couplerFBG
for λxcirculator λ1 .., λ’x, .. λNλ1 .., λx, .. λN
drop λx add λ’xRx Tx
FBG = Fibre Bragg Grating
notch-type filtering→ no bandpass narrowing when cascadingnot tunable (except with tunable FBG)
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Phased Array (PHASAR), Phased Array (PHASAR), or Arrayed or Arrayed WaveguideWaveguide Grating (AWG)Grating (AWG)
Generalization of MZ interferometer
Lengths of adjacent waveguides differ by ∆L
θns
nc
Multiplexer Min = N, Mout = 1Demultiplexer Min = 1, Mout = NStarcoupler Min = Mout = N
Act as lenses
d
Grating equation (phase matching) nsd sin θ + nc ∆L = m λ (interference order m )Pass wavelength for center input to center output waveguide λc = nc ∆L / mChannel spacing proportional to 1 / ∆L (narrow channel spacing, large AWG !!)Periodic → Free Spectral Range for opposite ports ∆νFSR = c / ng ∆L
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Add/Drop node with PHASARAdd/Drop node with PHASAR
Tx
Tx
Tx
Rx
Rx
Rxdata in
data out
Phased Array EDFA
Add/Drop Switch Matrix
fibreλ1, λ2, λ3 λ1, λ’2, λ3
λ’2λ2
add/drop on channel 2 (i.e. λ2 , λ’2 )
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Tunable sourcesTunable sources
Options:Series of discrete DFB or DBR laser diodes- Expensive- Extensive monitoring and control circuitryWavelength-tunable laser- By changing temperature (ca. 0.1 nm/K) or injection current (1 to 5 GHz/mA)Multi-wavelength laser arraySpectrally sliced source (e.g., LED + wavelength demux + ext. modulators)
Applications:Spare laser for DWDMDynamic wavelength setting (e.g., for wavelength routing)
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Number of channelsNumber of channels
To avoid crosstalk:∆λchannel ≈ 10 ∆λsignal
Max. number of channels in tuning range N ≈ ∆λtune / ∆λchannel
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Tunable 3Tunable 3--section DBR Lasersection DBR Laser
Tuning by changing effective refractive index
typ. index change up to 1%→ tuning range 10 to 15 nm
eff
efftune
nn∆
=∆
λλ
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WavelengthWavelength--switchableswitchable laser diode arraylaser diode array
integrated with EA modulator and amplifier
Ref.: AT&T Bell Labs,Young et al.,ECOC’95
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Spectrally sliced sourceSpectrally sliced source
LED
∆λFWHM
λλ1
modulator
modulator
λdemux
data 1
opticalamplifier
λmuxdata N
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Tunable optical filterTunable optical filter
Mostly based on same principles as static filters
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System parameters for Tunable optical filtersSystem parameters for Tunable optical filters
tuning range
maximum number of resolvable channels=tuning range / minimum channel spacing;channel spacing ≈ 3 to 10 times channel 50%-bandwidth, dep. on modulation scheme + filter char.tuning speedms speed enough for circuit-switched networks, sub-µs for packet-switchedattenuationpolarisation dependencestability : filter characteristics should shift less than a few % of filter bandwidth during the lifetime or during environmental changessizecosts
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Tunable 2x2 directional couplerTunable 2x2 directional coupler
λ1, .., λi−1, λi+1, .., λΜ
..
λ1, λ2, .., λΜ M electrodes
.. λi
change refractive index of waveguide → select wavelengthtuning range: about 60 nmbandwidth: around 1 nmfast (electro-optic tuning, on LiNbO3)
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Tunable multiTunable multi--stage MZIstage MZI
tuning range: 5 - 10 nmbandwidth: < 0.1 nmslow (thermo-optic tuning, on silica), fast (electro-optic, on LiNbO3)complex multi-stage tuning process
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Tunable Tunable Fibre FabryFibre Fabry--Perot filterPerot filter
fibre
highly reflectivecoating
+ Vpiezo-electriccrystal
housing
tuning range: around 50 nm (dep. on mirror spacing)bandwidth = tuning range / finesse, typ. 0.5 nmslow (piezo-electric crystal)low loss (few dB)
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FP FP -- operating principles (1/3)operating principles (1/3)
inputbeamEi
reflectedbeamsEr
transmittedbeamsEt
parallelreflectiveplates
n
θ
d
lens
multiple beam interference
( )
θνπλθ
πδ
δδ
δ
δδ
cos4cos
22
1
...
...
00
0
222
24422222
⋅⋅⋅⋅
=⋅
⋅⋅⋅=
⋅⋅−⋅
⋅=⋅⋅⋅=
+
+++=
−
∞
=
⋅−
⋅−
⋅−−
∑
cdnnd
eRATaEeARaTE
Eerata
EerataEerataEtaE
iik
kiki
ikikk
ii
ii
it
with
whereδ : phase shift during one cavity roundtripν : light frequencyT=t2 : intensity transmission coeff. of plateR=1-T : intensity reflection coeff. of plateA=a2 : intensity loss coeff. of cavity
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FP FP -- operating principles (2/3)operating principles (2/3)
transmitted intensity
( )( )( )
( )
( )
( )
⋅+−
−⋅⋅=
⋅+−
−⋅⋅=
⋅−+⋅
⋅=
⋅−⋅−⋅
⋅=⋅= −
cdnARAR
RAI
ARAR
RAIARRA
TAI
eAReARTAIEEI
i
ii
iiittt
νπ
δδ
δδ
2sin41
1
2sin41
1cos21
11
22
2
22
2
22
2
2*
FP power transfer function periodic, with period Free Spectral Range
]m[2
[Hz]2
2
2 dncFSRFSR
ndcFSR
⋅⋅=⋅==
λννλν or (note that
dλ / dν = -c / ν2)
E.g., for d=120 µm, λ=1.55 µm, n=1, is FSRλ=10 nm
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FP FP -- operating principles (3/3)operating principles (3/3)
[Hz]12 R
Rdn
cFWHM
−⋅=∆
πν
-3 dB bandwidth(Full Width at Half Maximum; for R≈1)
RRFSRF
FWHM −=
∆=
1π
ν
Finesse
2
min
max
11
⋅−⋅+
==RARA
TT
C
Contrast factor
E.g., for A=1 and R=0.9 is C=361=25.6 dB and F=29.8
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AcoustoAcousto--optic tunable filteroptic tunable filter
input wavelengths matching the grating Bragg condition are TE↔ TM converted, and coupled to other branchgrating set up by SAWmulti-wavelength droppingvery wide tuning (400 nm)slow (several µs needed to fill interaction length with SAW)bandwidth around 1 nm
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Tunable add/drop Tunable add/drop mux mux with FBGwith FBG
tuning range: limited, nm rangebandwidth: narrow (< 1 nm)slow (stress-tuning, with PZT)
