Tunable Semiconductor Lasers

Tunable Semiconductor Lasers

a tutorial

Larry A. ColdrenUniversity of California, Santa Barbara, CA

[email protected]

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






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






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






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






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

Documents

Tunable Semiconductor Lasers