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TRANSCRIPT
Tunable Semiconductor Lasers
a tutorial
Larry A. ColdrenUniversity of California, Santa Barbara, CA
Agility Communications, Santa Barbara, [email protected]
Abstract: Tunable semiconductor lasers continue to be in just about everyone’s list of important components for future fiber optic networks. Various designs will be overviewed with particular emphasis on the widely
tunable (>32nm) types.
2
Contents■ Why Tunable Lasers?
■ Basic Tuning Mechanisms
■ Examples of Tunable Lasers
■ Control of the Wavelength
■ Reliability Issues
3
Optical Network Architecture
Core
EdgeMore bandwidth and services/$Low-cost components and agile architectures
4
Introduction■ Tunable lasers have been of great interest for some time
− Dynamic networks with wavelength reconfigurability− Networking flexibility− Reduced cost
− One time provisioning (OTP) and sparing seen as side benefits■ Current market conditions….
− More cautious approach from carriers and system vendors− OTP and sparing are now the leading applications
■ Tunable lasers are compared with DFB or EML− Important to do “apples to apples” comparison
− Functionality− Performance− Total Cost of Ownership
5
Why Tunable Lasers?■ One time provisioning—inventory and sparing
■ Field re-provisioning—new services without hardware change or truck roll
■ Reconfigurable Optical Add/Drop Multiplexers (ROADM)—Drop and add any channel without demux/mux
■ Wavelength conversion—Eliminates wavelength blocking without OEO line cards
■ Photonic Switching—Eliminates many OEO line cards
■ Wavelength Routing—Use passive optical core
6
Applications –One time provisioning—the universal source
■ Laser is provisioned once only
■ Simplifies manufacturing
■ Drastically reduces inventory
■ Minimizes sparing to a manageable level
■ Simplifies forecasting
7
Applications – Re-provisioning
■ Laser is provisioned many times remotely to set up new services − Seconds timeframe− Point and click or ultimately controlled automatically by software
■ Can only be addressed using a widely tunable laser− Without severe constraints
■ Drastically reduces inventory
■ Simplifies forecasting
8
Tunable filter elements
in outthru
drop add
Applications – Re-configurable OADM
Rx TunableTx
■ Drop and Add without Demux and Mux of all channels■ Must be “hitless” filter tuning■ Eliminates mux/demux and OEO■ Tunable lasers are a key enabler
9
Applications – Photonic Switching 1
■ Photonic switches require O-E-O on I/O to prevent blocking■ Tunability reduces O-E-O requirements in half■ Requires moderately fast switching (ms)
1 2 3 4
5
6
78Tunable
TxRx
1 2 3
4
5
6
78
Grey Optics
SRTx
LRRx
LR TxSR Rx OR
10
Applications – Wavelength Conversion■ Intersection of metro rings■ Wavelengths transition between rings
− in optical domain■ Tunable lasers used to resolve wavelength blocking
− Alternative is a bank of fixed wavelength lasers
Rx Tx
Node2B
Node4B
Node3B
Node1B
Node1R
Node3R
Node2R
Node4R
11
Line cards withTunable lasers
Line cards
N x NLambdaRouter(AWG)
■ High capacity, high density router function—need wide tuning■ Wavelength used to route traffic through passive device■ For Packets requires very fast switching
Tx
Applications – Wavelength Routing(Optical Packet Switching)
12
Contents■ Why Tunable Lasers?
■ Basic Tuning Mechanisms
■ Examples of Tunable Lasers
■ Control of the Wavelength
■ Reliability Issues
13
Generic Single-Frequency Laser
mλ/2 = nLMirror-1 Mirror-2
GainMode-selection
filter Output
Gain
Mode-selectionfilter Possible modes
Lasing mode
λ
14
Examples of Single-Frequency Lasers■ DFB
− All-elements combined and distributed along length
■ DBR − Elements separated with
individual biases
■ External Cavity− Gain block + external lens &
grating
■ VCSEL− Short cavity for mode selection
Light Out
DFB Gain region
Grating (mode-selection& distributed mirror)
AR HR
gain
Light OutGain Rear Mirror
DBR
Light OutGain Ext. grating mirrorAR
Collimating lens
DBR mirrors Active
Light Out
15
How Tunable Lasers Tune
mλ/2 = nLMode number
WavelengthEffective index
Effective Cavity lengthMode wavelength:
Relative change in wavelength:
∆λ ∆n ∆L ∆mλ n L m= + -
Tuned by mode-selection filter(via index or grating angle)
Tuned by physical length changeTuned by net cavity index change
16
Generic Tunable Single-Frequency Laser
mλ/2 = nL
Mirror-1 Mirror-2
Gain Mode-selection
filter(∆m)
Tunable output
Cavityphase(∆n) (∆L)
Gain
Mode-selectionfilter (∆m)
Possible modes(∆n, ∆L)
Lasing mode
λ
17
Solutions for Tunable LasersLight Out
Gain Phase Rear Mirror
DBR
Light Out
Light Out
S - b e n tw a v e g u id e sW in d o w
8 M ic ro a r r a y D F B - L D s
W in d o w
C h ip s iz e : 0 .4 x 2 .1 5 m m 2
S O A
M M I
■ DBR Lasers− Conventional DBR (<8 nm)− Extended Tuning DBR’s (≥ 32 nm)
■ External Cavity Lasers (≥ 32 nm)− Littman-Metcalf/MEMs− Thermally tuned etalon
■ MEMS Tunable VCSEL (< 32 nm)− Optically or electrically pumped
■ DFB Array (3-4 nm X #DFBs)− On-chip combiner + SOA− Or, off-chip MEMs combiner− Thermally tuned
NEC
18
Contents■ Why Tunable Lasers?
■ Basic Tuning Mechanisms
■ Examples of Tunable Lasers
■ Control of the Wavelength
■ Reliability Issues
19
Examples of Tunable Lasers■ Narrowly tunable (not discussed further)
− Temperature tuned DFBs ~ 3nm − Narrowly tunable 2 or 3 section DBR lasers ~ 8nm
■ DFB selectable arrays− Select DFB array element for coarse tuning + temperature tune for fine
cavity mode tuning− Integrated on-chip combiners + SOAs or off-chip MEMs deflectors
■ External-cavity lasers− External grating reflector for mode-selection filter− Angle-tune mirror for mode selection—coarse tuning− Change length and/or phase section for fine tuning
■ MEMS Tunable VCSELs− Move suspended top mirror by electrostatic or thermal tuning− Single knob tuning for both coarse and fine
■ Widely tunable DBR lasers− Coarse tuning by index tuning of compound mirrors/couplers − Fine tuning by index tuning of phase section− Dual SGDBR or vertical-coupler + SGDBR mode selection filters
20
Wavelength-selectable light sources (WSLs)for wide-band DWDM applications
S-bentwaveguidesWindow
8 Microarray DFB-LDs
Window
Chip size: 0.4 x 2.15 mm2
SOA
MMI
DFB-LD-array-based structure Wide-band tunabilityCompact & stable Multi-λ locker module
Multi λ−locker integrated Wide-band WSL module
Schematic of wide-band WSL
WSLs for S-, C-, L- bands (OFC’02)
8 array, ∆λ ~ 16 nm (∆T = 25K) x 6 devicesMulti λ-locker integrated Wide-band WSL module (OFC’02)∆λ ~ 40 nm (∆T = 45K)
Feature
Performance
21
WSLs for S-, C-, L- bands applications- Lasing spectra -
∆λ ~ 16 nm (∆T 25K) @15 - 40 ℃6 devices → 135 channels @100-GHz ITU-T gridSMSR > 42 dBPf > ~ 10 mW @ IDFB= 100 mA, ISOA= 200 mA
-7 0-6 0-5 0-4 0-3 0-2 0-1 0
01 02 0
1 4 7 0 1 4 9 0 1 5 1 0 1 5 3 0 1 5 5 0 1 5 7 0 1 5 9 0 1 6 1 0W a ve le n g th (n m )
Inte
nsity
(dB
m)
S 1 S 2 C 1 C 2 L 1 L 2
S -b a n d C -b a n d L -b a n d
W a fe r 1 W a fe r 2
22
Fujitsu DFB Array Integrated Tunable Laser
Fujitsu Laboratories Ltd.
0.5 × 1.8 mm
23
Fujitsu Wavelength Tuning Characteristics
Temperature tuning Spectra at 32 wavelengths
Fujitsu Laboratories Ltd.
24
Santur Switched DFB Array1 mm
12 element DFB array, each temperature tuned 3nm for 36nm total tuning range -only one laser on at a time
MEMS mirror couples the selected laser to fiber
Advantages:DFB characteristics (optical quality, reliability, wavelength stability)No SOA, tuning sections, phase-sensitive mechanicsHigh yield, low cost passive alignment (MEMS does the rest)Built-in shutter/VOA
25
Santur 20 mW Module Performance
Full band tunability (36nm C-band, 42nm L-band)Built-in wavelength locker (25GHz channel spacing)>50dB SMSR, 2MHz linewidthTypical tuning time ~ 2secResistant to shock and vibe with no servo (10G causes < 0.2dB fluctuation in power)
26
Intel External-Cavity Approach (acquired from New Focus)
•• Double sided external cavity laser design, well known in test anDouble sided external cavity laser design, well known in test and d measurement applicationsmeasurement applications
•• Temperature tuned etalonTemperature tuned etalon replaces mechanical tuning devicereplaces mechanical tuning device
•• No moving partsNo moving parts, , but challenging packaging requirementsbut challenging packaging requirements
27
Littman-Metcalf Cavity (after New Focus)
HR Coating
Collimating Lens
AR Coating
Laser DiodeChip
PivotPoint
WavelengthTuning
DiffractionGrating
LaserOutput
Retroreflector
28
Iolon External-Cavity Laser with MEMs Mirror Movement
29
Tunable VCSELs (optically pumped)
■ Cortek-Nortel-Bookham?
■ Component technologies− MEMS− Thin Film− InP Laser− Packaging
■ Advantages:− High Power− Wide Tuning Range− Continuously Tunable
30
31
Many more 7 – 10 nm designs
Agere “Narrowly” Tunable DBR/SOA/EAM
Gain Tuning EA Modulator
Bragg mirror select FP modeTuning current moves Bragg mirror
EA-DBR Operation
L
High Low
λ
Tuning Current
A Five Stage Bell Labs Design
GainSection
DBRMirror
OpticalAmplifier
Detector“Power Monitor”
Modulator
32
Extended tuning range:SSGDBR--NELPhase modulated gratings
33
Gain Coupler Phase Reflector S-DBR400µm 600µm 150µm 900µm
p-InP
n-InP
QWs structures λg = 1.3 µm λg = 1.38 µm
0
1
2
3
4
5
6
7
8
1515 1525 1535 1545 1555 1565
Wavelength [nm]Co
upler c
urre
nt [m
A]
0
5
10
15
20
25
30
1515 1525 1535 1545 1555 1565
Refle
ctor
curre
nt [m
A]
Extended tuning range: GCSR--ADC-Altitun
SGDBR + GACC
λg = 1.55 µm
Agility’s Extended Tuning Range Technology:Widely Tunable SGDBR Lasers
35
Front Mirror Gain Phase
Rear MirrorAmplifier
Sampled Grating Tunable Lasers
-50
-40
-30
-20
-10
0
10
Lase
r Em
issi
on, d
Bm
0
0 .2
0 .4
0 .6
0 .8
1
1530 1540 1550 1560 1570
B ackFront
Mirr
or R
efle
ctiv
ity
W avelength (nm )
12 4 3
■ 5-10X Tuning Range of DBR
■ Reliable, Manufacturable InP Technology
■ Can Cover C band, L band or C + L
Light Out
Rel
ativ
e Po
wer
(dB
)
36
Advantages of Monolithic IntegrationWidely Tunable SG-DBR Laser with integrated SOA and EAM
Light Out
Front Mirror Gain Phase
Rear Mirror
SG-DBR Laser
AmplifierModulator
EA Modulator SOA
Advantages:smaller space (fewer packages)lower cost (fewer package components)lower power consumption (lower coupling losses)high reliability (fewer parts)
37
Fast Wavelength Switching of SGDBR Lasers
0
20
40
60
80
100
0 5 10 15 20 25 30 35 40 45
Optical signal at final ITU +/- ~10 GHz
Cou
nt
SwitchingTime (ns)
Current source rise time can be designed for application.Inherent laser limit is in ~ 2-10 ns range.Thermal transients can complicate rapid switching.
0 10 20 30 40 50 60 70 80 90 100
Switching time = 10 ns
Time (ns)
Lig
ht P
ower
Lig
ht P
ower
Channel 50 on
Channel 50 off
Channel 10 off
Channel 10 on
ElectronicTriggerV
olta
ge
0 10 20 30 40 50 60 70 80 90 1000 10 20 30 40 50 60 70 80 90 100
Switching time = 10 ns
Time (ns)
Lig
ht P
ower
Lig
ht P
ower
Channel 50 on
Channel 50 off
Channel 10 off
Channel 10 on
ElectronicTriggerV
olta
ge
Packet Switching Applications
38
SG-DBR Laser with Integrated SOA
>100 50 GHz ITU ChannelsFiber coupled power = 13dBm = 20mWSMSR > 40 dB SOA: Power leveling, blanking, and VOA w/o degradation of SMSRChannel switching time (software command verified channel) < 10 ms
-40
-30
-20
-10
0
10
20
1570 1580 1590 1600 1610
Fibe
r Cou
pled
Pow
er (d
Bm)
Wavelength (nm)
40 dB
High Power Widely Tunable Laser:
13dBm
L-band
39
RIN & Linewidth Dependence on Power
-150
-145
-140
-135
-130
0 2000 4000 6000 8000 10000
40 mA, 5.7 dBm60 mA, 7.5 dBm80 mA, 8.6 dBm100 mA, 9.2 dBm120 mA, 9.8 dBm150 mA, 10.5 dBm
RIN
(dB
/Hz)
Frequency (MHz)
SOA Current
0
1
2
3
4
5
7
8
9
10
11
12
13
1525 1530 1535 1540 1545 1550 1555 1560 1565
Line
wid
th (M
Hz)
Output Pow
er (dBm
)
Wavelength (nm)
White FM Noise Density vs. λRIN vs. SOA Current
■ RIN is only weakly dependent on output power (SOA current).■ Linewidth is less than 2.5 MHz across all wavelengths
− Scales with Laser Power as expected.
40
0
1
2
3
4
5
1.53 1.54 1.55 1.56 1.57
Dis
pers
ion
Pen
alty
@ 1
0-10 e
rror
rate
, dB
Wavelength, µm
350 km
275 km
200 km
OC-48Std. SMF
Dispersion penalty at 10-10 errors/s error rate for 200, 275, and 350 km of standard SMF for 38 ITU channels sampled across C-band.
SGDBR-SOA-EAMTransmission Characteristics
1528 nm
1540 nm
1550 nm
1560 nm
PRBS 231-1 at 2.5 Gb/s4th order Bessel-Thomson filterSONET mask with 25% margin
Unfiltered 2.7 Gb/s
41
SGDBR-SOA-EAMRF-ER, Pave, & VOA Operation
Ave. power >5 dBm and RF ER > 10 dB across C-bandOutput power dynamic range of ~10 dB w/ small change in SMSR and Wavelength (open loop operation)
4
4.5
5
5.5
6
10
11
12
13
14
15
192 193 194 195 196
Tim
e A
vera
ged
Pow
er (d
Bm
)
RF Extinction R
atio (dB)
Optical Frequency (THz)
3 dBm
2 dB
0 dB
-3 dB
~1550 nm
42
OC-192 Operation of EAM
Integration technology compatible with higher bit rates> 10 dB RF ER across C-bandNot optimized, improvements to come
PRBS 231-1, Vp-p = 3V
43
MZ-SGDBR (UCSB)
Light Out
Curved waveguides200µm
MMI Length:96µm
Taper:20µm
Width: 9µm
44
Chirp parameter as function of DC extinction curve for 550µm
-8
-6
-4
-2
0
2
-35
-30
-25
-20
-15
-10
-5
-4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 0
Alpha 1525nmAlpha 1545nm
(Power (dBm) 1525nmPower (dBm) 1545nm
Chi
rp P
aram
eter
Insertion Loss (dB)
Arm #1 DC Bias (V)
Measured by the Devaux method
Limagnrealn
eff
effchirp α
φα∆
∆=
∆
∆=
2)()(
• Negative chirp when increasing reverse bias ‘turns on’ modulator
• > 20 dB extinction with 2V drive
Extinction & Chirp: MZ-SGDBR (UCSB)
45
-8
-6
-4
-2
0
0 5 10 15 20
Nor
mal
ized
S21
Frequency (GHz)
10Gbit/s Eye 1015-1
PRBS
-4V BiasBCB
• BCB for low capacitance
• Lumped drive– can improve with traveling wave electrodes
MZ-SGDBR RF Performance: Lumped (UCSB)
46
Contents■ Why Tunable Lasers?
■ Basic Tuning Mechanisms
■ Examples of Tunable Lasers
■ Control of the Wavelength
■ Reliability Issues
47
Control Issues
■ Finding the desired channel − Look-up tables vs. channel counting?− Is global wavelength monitor required?− Must look-up tables be updated over life?
■ Staying on the desired channel− Is locker required to meet spec?− Is single knob control from locker sufficient over life?
48
Generic Tunable Single-Frequency Laser
mλ/2 = nL
Mirror-1 Mirror-2
Gain Mode-selection
filter(∆m)
Tunable output
Cavityphase(∆n) (∆L)
Gain
Mode-selectionfilter (∆m)
Possible modes(∆n, ∆L)
Lasing mode
λ
49
Control comparison across types
DFB Array/SOA Varray(j) T Igain(j) ∆ISOA
DFBs/MEMs VM1, VM2(j) T Igain(j) VM1, VM2(j)
SGDBR/SOA Im1, Im2 Iφ ISOA ∆ISOA
Ext. Cavity VMθ VML, Iφ Igain VMshutter
VCSEL/MEMs VM1 V*M1 Igain --------
Laser λcoarse λfine Amplitude VOA
50
Iolon Control Scheme for Ext. Cavity Laser
51
Agility Control of SG-DBR Lasers
Control Circuitry
■ DWDM DBR− Power Control− Temperature Control(fixed)− Wavelength Locking− Mirror Control (Locking?)
■ DWDM DFB comparison− Power Control− Temperature Control− Wavelength Locking
Power Control
Wavelength LockingMirror Control
Light Out
Front Mirror Gain Phase Rear Mirror
SG-DBR
SOA
52
Contents■ Why Tunable Lasers?
■ Basic Tuning Mechanisms
■ Examples of Tunable Lasers
■ Control of the Wavelength
■ Reliability Issues
53
Wavelength Reliability
■ It’s not enough to just put out the right power in a single mode for a long time (old criterion)
■ Prior to end-of-life of a multi-channel DWDM source, power & wavelength must be in spec.
■ Intimately linked to wavelength control (or lack of it)− Finding the desired channel
− Look-up tables vs. channel counting?− Is global wavelength monitor required?− Must look-up tables be updated over life?
− Staying on the desired channel− Is locker required to meet spec?− Is single knob control from locker sufficient over life?
■ If look-up tables must be updated, how can this be done reliably?
54
What causes the wavelength to change
∆λ ∆n ∆L ∆mλ n L m= + -
Tuned by mode-selection filter(via index or grating angle)
Tuned by physical length changeTuned by net cavity index change
Physical Causes, assuming a fixed look-up table:∆n – Changes in internal temperature, Tint, or carrier lifetime, τc∆L – Physical movements—solder relaxation, MEMs charging∆m – ∆n of DBR, ∆θ of ext. grating, or MEMs charging
55
Critical issues for wavelength stability
DFB Array/SOA j, Ig(j), T, ISOA ∆n(Tint) ∆Ig
DFBs/MEMs j, Ig(j), T, VM1(j), VM2(j) ∆n(Tint) ∆Ig
SGDBR/SOA Im1, Im2, Iφ, ISOA ∆nDBR(τc) ∆m
Ext. Cavity VMθ, VML, Iφ, Igain, VMshut ∆L(VM), ∆m(VM), ∆n(Tint) ∆Ig
VCSEL/MEMs VM1, Ig ∆L(VM)
Laser Variables in Table *Critical ∆λ issues
*Requiring table update or global channel locator
56
Estimated Open-loop Wavelength Shifts
DFB Array/SOA ∆n(Tint) ∆Ig 40GHz (10GHz/SOA feedback)
DFBs/MEMs ∆n(Tint) ∆Ig 40GHz
SGDBR/SOA ∆nDBR(τc) ∆m <10GHz
Ext. Cavity ∆L(VM), ∆m(VM), 100GHz (MEMs charging)
∆n(Tint) ∆Ig
VCSEL/MEMs ∆L(VM) 1000GHz
Laser Critical ∆λ issues ∆λ @ EOLgain (No table update)
• Only SGDBR lands on correct mode near EOL Open-loop• Others require global channel monitor or the like
57
0.5
1.5
2.5
3.5
4.5
5.5
6.5
7.5
0.5 1.5 2.5 3.5 4.5 5.5
rtFM
rtB
M
t=96 Hours t=2304 Hours
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0 1 2 3 4 5 6
240 Hours576 Hours1080 Hours1416 Hours2088 Hours2304 Hours2640 Hours
■ Corresponds to > 100 yrs of operation■ Aging gives fixed amount of root current increase to provide a shift
in the “mode map” to higher current .
Effects of SGDBR Mirror Aging: Measurement
58
Very High SGDBR Wavelength Stability and Reliability
~ 106 Device Hours measured.
Very low Bragg Wavelength Aging Rates < 0.5 pm/ year at worse case.
Gain and SOA sections have similar MTTF and failure distribution.
OK for open-loop operationno mode hops or incorrect channels
3 2 1 0 1 2 3
Mirrors - Experimental Mirrors - Least Squares Fit
MTTF ~ 350yrsσ ∼ 0.56
104
103
102
101
MTTF ~ 350 yrsσ ~ 0.56Ea=0.55 eV, n=2λFITS @ 25 yrs < 1
Cumulative Failures, % 0.1 1 5 20 50 80 95 99 99.9
Mirr
or L
ife T
ime
(yrs
)
59
SGDBR Laser/SOA FITs vs. Time
0
50
100
150
200
250
0 5 10 15 20 25
Operating Time (yrs)
Failu
re R
ate
(FIT
s)
2*Mirror Failure Rate Amplifier Only Failure RateGain Only Failure Rate Total Failure Rate
• Open-loop failure rate vs. time
• Gain section determines EOL
• Closed-loop mirror control has also been implemented to monitor any drift
60
0.01
0.1
1
10
100
1000
10000
1 10 100
Lasi
ng W
avel
engt
h Sh
ift (p
m/Y
ear)
DBR Current Density (kA/cm2)
Use Current Density
(~4kA/ cm2)
Agility, 2002
*Mawatari, 1999DFB EOL
SGDBR vs DFB Chip Reliability
■ Historically, DBR Reliability WAS Poor…
■ Defects in the grating area, found to be primary cause of DBR failure.
■ Improvement to re-growth (InP/InP) and minimal grating area of SG-DBR, allow equivalent or better performance vs. DFB’s.
*Mawatari et al, “Lasing Wavelength Changes Due to Degradation in Buried Heterostructure DBR Laser”, Journal of Lightwave Technology, v.17, no.5 1999
61
Summary■ Tunable lasers can reduce operational costs
■ Narrowly tunable versions have some short term inventory/sparing cost advantages but newer full-band types offer many further opportunities
■ Several configurations have emerged for current applications
■ Monolithic integration offers significant potential for reducingsize, weight, power, & cost
■ Wavelength control issues still exist for many configurations. Look-up table updating and/or global channel monitors are necessary in some cases.
■ Reliability has been proven for the SGDBR version without any updating of the look-up tables or need for channel searching