advancements in high efficiency semiconductor lasers for ......dpal: diode-pumped alkali laser...
TRANSCRIPT
-
LLNL-PRES-741702This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344. Lawrence Livermore National Security, LLC
AdvancementsinHighEfficiencySemiconductorLasersforHighPowerApplications
InstituteforEnergyEfficiencySeminarSeries
UniversityofCalifornia,SantaBarbara
PaulLeisherandBobDeri
November16,2017
-
2LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
§ DiodelasersatLawrenceLivermoreNationalLaboratory
§ Powerscalingindiodelasers
§ Designforhighefficiencyandthermalmanagement
§ Othercausesofpowersaturation
§ Reliabilityconsiderations
§ Brightnessconsiderations
Contents
-
3LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
Highperformancelasersystemsareneededforcriticallaboratorymissionsandalignedwithseveralcompetencyareas
High-energydensityscience
Directedenergy
Additivemanufacturing
Photo by Kate Hunts/LLNLM. Matthews et al., Optics Express, (2017).
Spacesecurity
-
4LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
AVLISWorld’s highest average power tunable laser
T-REXWorld’s brightest laser gamma-ray source
Heat Capacity LaserWorld’s highest average power solid state laser
Nova PetawattWorld’s highest peak power laser
NIFWorld’s most energetic laser
ARCWorld’s highest energy petawatt system
DPALHigh average power diode pumped alkali laser
MercuryWorld’s highest average power 10Hz laser
HAPLSWorld’s highest average power petawatt-class laser
Compton Source High energy photon Compton source test bed
LLNLhasbeendeliveringleadingedgelaserandopticssolutionsforover40years
-
5LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
Comparedtoflashlamppumping,diodesoffer:
§ Higheraveragepower
§ Greatertotalsystemefficiency
§ Increasedpeakpower
§ Improvedbeamquality
§ Betterreliability
§ Reducedcoolingrequirements
§ Improvedstability
§ Morecompactsystems
§ Reducedoperationalcosts
Diodepumpingistheprimaryenablingtechnologyforhighperformancelasersystems
HAPLS– Worlds’s highestaveragepowerpetawatt-classlaser
HAPLS– World’shighestaveragepowerpetawatt-classlaser
Diodepumpingisagamechanger
-
6LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
DiodelasersareacriticalenablingtechnologyforhighenergylasersystemswhichspantheNIF&PSdirectorate
CoreNIF
DMPA:Diode-pumpingwillincreasesNIFstabilityandenablesgreaterreliableshotenergy
APT
HAPLS:Diode-pumpingenableslowcoststructure($/W)forcommercialapplications
DODT
DPAL:Diode-pumpedalkalilaserenableslowSWaP fordirectedenergyweapons
-
7LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
LaserDiodeTechnologyatLLNL– ManykWtoMW
GOLD– 120Hz,256kW HAPLS– 10Hz,4x800kW
-
8LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
800kW(peak)QCWdiodearrayinaction
-
9LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1994 1998 2002 2006 2010 2014 2018
Rela
tive
Norm
aliz
ed S
emic
onuc
tor
Diod
e L
aser
Pro
cure
men
ts
Year
HighpowersemiconductorlasersarebecomingincreasinglyimportantatLLNL
HAPLS– 10Hz,3.2MWGOLD– 120Hz,256kW
1994 1998 2002 2006 2010 2014 2018
HAPLS
GOLD
DPAL
LIFE
Fiber
TACL
Mercury
Year
-
10LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
Whatarethecriticalperformanceparametersforcommercially-availablehighpowerdiodelasers?
Averagepower
Irradiance
E/Oefficiency SpectralwidthCenter
wavelength
Peakpower
Reliability
Cost
Robustness(shock/vibe)
Divergence
www.dilas.comwww.lasertel.comwww.lasertel.com
www.nlight.com
-
11LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
S&Tchallenge– LLNL/DOEdiodepumprequirementsarenotthesameasthosefortypicalindustrialapplications
Commercialrequirements LLNLrequirements
§ Focusonfiber-coupledpumpsfortheindustrialfiberlasermarket
§ Laserheadpower>10Wto10,000hours
§ Temperaturerange:15°Cto35°C
§ Cost istheprimaryconcern
§ Primarywavelengths808and9XXnm
§ CW isimportant
§ LowSWaP isdesirable
§ FocusonbarsandstacksforhighestpeakandCWpowerdensity
§ Laserheadpower>10kWto1,000’shours
§ Temperaturerange:wider
§ Performance istheprimaryconcern
§ Primarywavelengths7XXand88Xnm
§ CWandQCWarebothimportant
§ LowSWaP iscritical
COTSdiodepumpedsourcesareusuallynotsuitableforLLNLprogramsThismeansourdiode-pumpedlaserprogramsoftenrequiredevelopmentand/orNRE
-
12LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
HowdowepromotediodedevelopmenttomeetLLNLneeds?
Externalcommercialdevelopmentandprocurement
www.lasertel.com
3.2MWdiodebackplaneproducedbyLasertelfortheHAPLSprogram
Examplehighpowerdiodeteststationforassessingeffectsofback-irradiance
Internalresearch,development,andcapabilityinvestments
-
13LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
§ DiodelasersatLawrenceLivermoreNationalLaboratory
§ Powerscalingindiodelasers
§ Designforhighefficiencyandthermalmanagement
§ Othercausesofpowersaturation
§ Reliabilityconsiderations
§ Brightnessconsiderations
Contents
-
14LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
Howdoyoumakealasermorepowerful/energetic?
Makeitbigger!
Why?
-
15LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
§ Increasingthecross-sectionalareareducesthermalresistanceandhencethethermalgradient
§ Maximumoutputpowerisstronglydependentonthermalresistance
§ Powerscalingcanbeachieved(tofirstorder)bymakingthelaserlarger
Thermalresistance inhighpowerdiodelasers
https://tinyurl.com/ybvsq77h
0
100
200
300
400
500
0 100 200 300 400
Powe
r (W
)
Current (A)
0.8 K/W
0.4 K/W
0.2 K/W
0.1 K/W
0 K/W
808 nm50% FF cm barCW, 25°C
-
16LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
Wheredidthemissingpowergo?
0
2
4
6
8
10
12
0 2 4 6 8
Powe
r (W
)
Current (A)
Actual
Perfect
808 nmCW, 25°C1.5mm x 0.2mm
0.0
0.5
1.0
1.5
2.0
2.5
0 2 4 6 8
Volta
ge (V
)
Current (A)
Actual
Perfect
808 nmCW, 25°C1.5mm x 0.2mm
0%
20%
40%
60%
80%
0 2 4 6 8
Dist
ribut
ion
of In
put P
ower
Current (A)
Useful Output
Voltage Loss
Threshold Loss
Slope Loss
Actual Voltage Loss21%
Threshold Loss5%
Slope Loss15%
Useful Output59%
I = 8 Amps
-
17LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
Voltageloss– diodelaservoltagedefect
Voltagedefectinadiodelaserisanalogoustoquantumdefectinasolidstatelaser
Epump ElaserVdiodeVphoton
laser
laserpump
EEE -
= defect Quantum
Solidstatelaser Diodelaser
)V()(
241 where
defect Voltage
mλ.
qhc
V
VVV
photon
photon
photondiode
µl»=
-=
heat
P.LeisherandS.Patterson,Proc.ofDEPSSSDLTR,(2013).
-
18LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
Theinternaldifferentialquantumefficiencyηi
representsthefractionofinjectedcarrierswhich(wheninjectedabovethreshold)produceaphotoninsidethelaser
Slopeloss– externaldifferentialquantumefficiency
Theexternal differentialquantumefficiencyηd representsthe
fractionofinjectedcarrierswhich(wheninjectedabovethreshold)produceaphotonthatsuccessfully
leavesthelasercavity
Lowabsorptionlossiskey 0.00.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
0
2
4
6
8
10
12
0 5 10 15
Volta
ge (V
)
Out
put p
ower
(W)
Current (A)P.LeisherandS.Patterson,Proc.ofDEPSSSDLTR,(2013).
Electrons per second in
Current in, ACoulombs per second
Photons per second out
Power out, WJoules per second
Photons out per electron in
Scale by charge of electron Scale by energy of photon
LaserElectrons
P.Crump,R.Martinsen,P.Leisher,CLEO,(2007).
-
19LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
Sowhathappensasyoutrytodriveadiodelaserharder?
0
5
10
15
20
25
30
35
40
0 5 10 15 20 25
Powe
r (W
)
Current (A)
Actual
Perfect
808 nmCW, 25°C1.5mm x 0.2mm
0%
20%
40%
60%
80%
0 5 10 15 20 25
Dist
ribut
ion
of In
put P
ower
Current (A)
Useful Output
Voltage Loss
Threshold Loss
Slope Loss
Actual
1. Additionalcurrentincreasesheatduetoimperfectefficiency
2. Thefractionofinputpowergoingtoheatincreases(atfirst)duetohighervoltageloss(electricalseriesresistance)
3. Thisself-heatingeventuallycausestheslopeloss(differentialquantumefficiency)togrowexponentiallywithcurrent
4. Eventuallyapointisreachedwheremorecurrentcausesnonetincreaseinpowerduetorapiddecreaseinefficiency0
25
50
75
100
125
150
175
200
0 5 10 15 20 25
Junc
tion
Tem
pera
ture
(ºC)
Current (A)
-
20LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
Whydoesself-heatinglimittheoutputpowerofdiodelasers?
Peak Gain vs Temperature (at N=1.5000e+012cm-2)
Temperature (K)100 200 300
Peak
Gai
n (c
m-1
)
2000
4000
6000
8000
Legend:
PeakGain
0
0.5
1
0.8 0.9 1 1.1 1.2
Ferm
i-Dira
c Di
strib
utio
n
E/Ef
300K77K
0K
Occupied states below this level can contribute to gain
Occupied states above this level do not contribute to gain
1. Differentialgaindecreaseswithincreasingtemperature,somorecarriersareneededtoreachthreshold.This,inturn,increasestheabsorptionlossinthestructure.
P.O.Leisher,etal.,CLEO,(2010).
-
21LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
Whydoesself-heatinglimittheoutputpowerofdiodelasers?
2. Poorclampingofthegaininsidetheactiveregioncausescarrierdensitieselsewhereinthestructuretoincreaseaswell,furtherincreasingtheabsorptionlosses.
0E+0
1E+17
2E+17
3E+17
4E+17
5E+17
6E+17
7E+17
2.5 3.0 3.5 4.0 4.5 5.0 5.5
Inte
nsity
(ar
bitra
ry u
nits
)
Carr
ier
Dens
ity (c
m-3
)
Position (μm)
375K350K325K300K
-
22LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
From T=300K à 77K, n(T)3 is reduced by ~35X
RAuger ~ C(T)*n(T)3
From T=300K à 77K, C(T) is reduced by ~40X
From T=300K à 77K, RAuger is reduced by ~1400X
Conduction band
Valence band
-Dk Dk
-DE
DE
Whydoesself-heatinglimittheoutputpowerofdiodelasers?
Electron
Trap
Hole
Ec
EtEi
Ev
Hole
1 2
3or Et – Ei
0.0
0.2
0.4
0.6
0.8
1.0
1.2
50 100 150 200 250 300
Rela
tive
Reco
mbi
natio
n Ra
te
Temperature (K)
Et-Ei = 10 meV
3. Increasingtemperaturecausesnon-radiativerecombinationtoincrease,reducingthedifferentialquantumefficiencyofthedevice.
Shockley-Hall-Reed(trap-assisted)Recombination
P.O.Leisher,etal.,CLEO,(2010).
AugerRecombination
-
23LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
§ DiodelasersatLawrenceLivermoreNationalLaboratory
§ Powerscalingindiodelasers
§ Designforhighefficiencyandthermalmanagement
§ Othercausesofpowersaturation
§ Reliabilityconsiderations
§ Brightnessconsiderations
Contents
-
24LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
Howcandiodelaserpowerbescaled?
Option2
Improvethermalmanagement
Option1
Increaseefficiency
0
100
200
300
400
500
600
0 200 400 600
Powe
r (W
)
Current (A)
1.0 K/W
0.6 K/W
0.4 K/W
0.2 K/W
0.1 K/W
880 nmCW, 25C50% FF cm bar
0
100
200
300
400
500
600
0 200 400 600
Powe
r (W
)
Current (A)
45% E/O
53% E/O
60% E/O
68% E/O
76% E/O
880 nmCW, 25C50% FF cm bar
-
25LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
Whydoeslaserdiodeefficiencymatter?
+20%efficiency‒30%inputpower
‒60%coolingor
2.5Xpowerperbar
70% Efficiency
1 kW OpticalOutput
1.4 kW Electrical Input
0.4 kW Heat
50% Efficiency
1 kW OpticalOutput
2.0 kW Electrical Input
1.0 kW Heat
“Efficiencyisn’teverything,butit’sdamnclosetoit…”
-
26LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
Maximumpowercorrelateswellwithconversionefficiency
P.Crump,R.Martinsen,P.Leisher,CLEO,(2007).
-
27LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
Voltagedefectreducedbybandgapanddopingoptimization
Primaryoriginsofexcessvoltagedefect:1. Bandgapofwaveguide(turn-onvoltage)2. Bulkconductivityofthep-cladding(resistance)
-7
-6.5
-6
-5.5
-5
-4.5
-4
-3.5
2 2.25 2.5 2.75 3 3.25
Ener
gy B
and
(eV)
Position (um)
Elec FermHole FermCond BandVal Band
0 V
-7
-6.5
-6
-5.5
-5
-4.5
-4
-3.5
2 2.25 2.5 2.75 3 3.25
Ener
gy B
and
(eV)
Position (um)
Elec FermHole FermCond BandVal Band
1.2 V
P.LeisherandS.Patterson,Proc.ofDEPSSSDLTR,(2013).
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 1 2 3 4 5 6
Pote
ntia
l (V)
Position (um)
Dif ference
0 V
1.2 V
PhotonEnergy
ΔEWG-QW
Rp-clad
-
28LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
Remaininglimitationsofefficiencyinlargeopticalcavitylasers
Lossesinatraditionallargeopticalcavitylaseraredominatedbyfreecarrierabsorptioninthequantumwell
P.Crump,R.Martinsen,P.Leisher,CLEO,(2007).
Examplepareto ofthecalculatedabsorptionlossesinalargemodelaserbywherethey
occurinthestructure.
QW73%
p-WG13%
p-Clad7%
n-WG4% n-Clad3%
-
29LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
1. Freecarrierabsorptionlossesinthequantumwell,aswellasdivergenceangle,canbereducedbymakingthewaveguideverylarge
2. Carefulplacementofthequantumwellensuressingle-modeoperation
3. Doesamuchbetterjobofbalancingthelossesinthestructure(nolongerdominatedbythequantumwell)
Superlargeopticalcavityandd/Γ
Ref
ract
ive
inde
x
Inte
nsity
(arb
uni
ts)
PositionFeng,et.al.,J.ofSemiconductors,vol.30,no.6,(2009).
-
30LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
Thermalresistanceminimizedby:
1. Junction-downbonding(maketheinterfaceasthinaspossible)
2. Newermaterials(increasedthermalconductivity)
3. Improvedcoolerarchitectures(higherheattransfercoefficients)
Thermalresistanceofbroadareadiodelasers
PhotocourtesyofCoherent/DILAS
-
31LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
High-efficiency980nmsingle-emittersforfiber-coupledpumps
0
10
20
30
40
50
60
70
0
2
4
6
8
10
12
14
16
18
0 3 6 9 12 15 18Ef
ficie
ncy
(%)
Out
put p
ower
(W)
Current (A)
976 nmCW, 25°C
95 μm stripe3.8 mm cavity
>16WCWfromsingleemitters(980nm,>68%peakE/O)
0
200
400
600
800
1000
1200
60 61 62 63 64 65 66 67 68 69 70
Freq
uenc
y 980-nm Peak Efficiency Bin
10 devices
L.Bao,J.Wang,M.DeVito,Z.Chen,P.Leisher,etal.,Proc.SPIE,(2011).Disclaimer:Thisexampleselectedfromtheliteratureduetoemphasizemanufacturingdistribution.ThisisnotanendorsementofnLightCorp.oritsproducts.
-
32LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
High-efficiency940nmbarsforpumpingofYb thindisklasers
A.Pietrzak,Proc.SPIE,(2015).
~1kWQCWfromsinglebars(940nm,64%peakE/O)
Disclaimer:Thisexampleselectedfromtheliteratureduetoemphasizemanufacturingdistribution.ThisisnotanendorsementofJenoptik Corp.oritsproducts.
-
33LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
§ DiodelasersatLawrenceLivermoreNationalLaboratory
§ Powerscalingindiodelasers
§ Designforhighefficiencyandthermalmanagement
§ Othercausesofpowersaturation
§ Reliabilityconsiderations
§ Brightnessconsiderations
Contents
-
34LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
0
3
6
9
12
15
18
0 4 8 12 16 20Po
wer
(W)
Current (A)
3.8 mm5.0 mm7.6 mm
12345
0 2 4 6 8
Rth
(K/W
)
Length (mm)
Cavitylengthscalingprovidesefficientpathtothermalresistancereductionwithoutcompromisingbeamquality
0
1
2
3
4
5
6
0
2
4
6
8
10
12
14
16
2003 2006 2009 2012 2015 2018
Sing
le-e
mitt
er c
avity
leng
th (m
m)
Rate
d si
ngle
-em
itter
CW
pow
er (W
)
Year
Single-emitteroutputpowerandcavitylengtharecloselycorrelated
WhydidscalingstallatL=5mm?
BasedonreportedresultsbynLight
Improvementsinsingle-emitteroutputpowerenabledbycavitylengthscaling
Root-causeandsolutionnotyetknown
J.Bai,P.Leisher,etal.,Proc.SPIE,(2011).
-
35LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
Othercausesofpowersaturationinhighpowerdiodelasers
SlideprovidedbyP.Crump
-
36LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
§ Athighbias,powers>20W/100μm,extremeconditionsarise— Highcarrierdensity— Hightemperature— Highphotonflux— Highelectricalfieldandhighcurrentdensity
§ Theseleadtointeractingmixofpowersaturationterms,including:— Current-drivenselfheating(degradedthreshold,slope) →(“T0,T1”- wellknown)— Bias-drivenleakage(BDL) →Avrutin— Gainsaturation(orspectralholeburning,SHB) →Peters— Longitudinalspatialholeburning(LSH) →Peters,Leisher— Twophotonabsorption(2PA) →Garrod,Juodawlkis— Waveguidecollapse(WC) →Crump
§ Manycanbeaddressedbydesignimprovements
Briefsummaryofknownfactorsleadingtopowersaturation
-
37LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
Longitudinalspatialholeburning(LSHB)
T. Hao, J. Song, and P. Leisher, Proc. SPIE, (2014).
Spatialnonuniformityinthelongitudinalgainprofileiscausedbyextremeasymmetryoflongcavity(>2mm)
devicesandresultsinreductioninthelaserdiodeoutputpower
-
38LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
ExperimentalsetuptotestforpresenceofLSHB
10
20
30
40
50
60
70
80
90
0.0 0.4 0.8 1.2 1.6 2.0 2.4
Aver
age
Gre
y Va
lue
Current (A)
MeasuredPerfect
!"# = %&'
808 nm, 100 μm x 1500μm
T.Hao,J.Song,andP.Leisher,Proc.SPIE,(2014).
-
39LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
LSHB– modeledvs.measuredresults
5.0E+36
1.0E+37
1.5E+37
2.0E+37
2.5E+37
3.0E+37
73
78
83
88
93
98
0 300 600 900 1200 1500
N2(1
/cm
6 )
Gre
y Va
lue
Cavity Length (μm)
1.3A measured1.3A calculatedw/o LSHB
PR HR
5.0E+36
1.0E+37
1.5E+37
2.0E+37
2.5E+37
3.0E+37
73
78
83
88
93
98
0 300 600 900 1200 1500
N2(1
/cm
6 )
Gre
y Va
lue
Cavity Length (μm)
2.4A measured2.4A calculatedw/o LSHB
PR HR
T.Hao,J.Song,andP.Leisher,Proc.SPIE,(2014).
DirectobservationofLSHBconfirmsitspresenceandaffectondiodelaserperformance
-
40LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
Challenge:samedatareproduciblewithdifferentmodels
-
41LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
§ DiodelasersatLawrenceLivermoreNationalLaboratory
§ Powerscalingindiodelasers
§ Designforhighefficiencyandthermalmanagement
§ Othercausesofpowersaturation
§ Reliabilityconsiderations
§ Brightnessconsiderations
Contents
-
42LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
Semiconductorlasersfailgraduallyorsuddenly
0
1
2
3
4
5
0 2 4 6Current (A)
Out
put p
ower
(W) Time zero
+ 1 week 3.5 A+ 1 week 4.5 A+ 1 week 6.0 A
Dislocationnetworkgrowth→Increasednonradiativerecombination
Defect-initiatedthermalrunaway→catastrophicoptical(mirror)damage
808nm laser diode
Laser
Sub-mount
Laser
Sub-mount
COMD
808nm laser diode
Laser
Sub-mount
Laser
Sub-mount
COMD
SuddenFailure
GradualDegradation
-
43LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
Catastrophicopticalmirrordamage
0 50 100 150
Inte
nsity
(arb
. uni
ts)
Position (μm)
Local intensityspike (filament)
0 50 100 150
Inte
nsity
(arb
. uni
ts)
Position (μm)
COD
0 50 100 150
Inte
nsity
(arb
. uni
ts)
Position (μm)
COD threshold
0 50 100 150
Inte
nsity
(arb
. uni
ts)
Position (μm)
Local defect
COD threshold
0 50 100 150
Inte
nsity
(arb
. uni
ts)
Position (μm)
Long timescales
COD threshold
0 50 100 150
Inte
nsity
(arb
. uni
ts)
Position (μm)
Short timescales?
COD
-
44LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
1. QuantumWellIntermixing
MitigationofCOMD– Facetpassivation
http://www.intenseco.com/technology/default_pdf.asp M.Peters,V.Rossin,andE.Zucker,USPatentEP1903646B1
Scribe Markslaserbars
1. Wafer section preparation (pre-scribe) 2. Load into modified MBE & evacuate
3. Bar cleave & stack in vacuum 4. Deposit ZnSe, both facets
N.Chand,etal.Electron.Lett.,vol.32,pp.1595–1596,1996.R.Lambert,et.al.,IEEEJ.LightwaveTech.,vol.24,no.2,(2006).
2. Etched/RegrownFacets
3. Si/Si3N4 Passivation 4. ZnSeepitaxialmirroronfacet(EMOF)
-
45LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
MitigationofCOMD– Facetpassivation
R.M.Lammert,et.al.,Proc.SPIE,(2005).
FacetpassivationgreatlyincreasestheCOMDlimit
-
46LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
BulkCODvs.COMD
COD SEMCOD SEM
Facetpassivationmethodologieshavebecomesoeffectivethatmanydiodesnowfailatdefectswithinthebulkofthedevice
BulkCatastrophicOpticalDamage CatastrophicOpticalMirrorDamage
-
47LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
§ DiodelasersatLawrenceLivermoreNationalLaboratory
§ Powerscalingindiodelasers
§ Designforhighefficiencyandthermalmanagement
§ Othercausesofpowersaturation
§ Reliabilityconsiderations
§ Brightnessconsiderations
Contents
-
48LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
Definitionoflaserbrightness
Related to the optical invariant
Brightnessisafixedquantityofalasersource.Itcannotbeincreasedthroughtheuseofpassiveopticsalone.
-
49LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
§ Reducedabsorptionlengthat976nmincreasesthresholdfornonlinearities
§ Broadgainbandwidtharound915nmreducestemperaturecontrolrequirements
Applicationofhigh-brightnesspumps– fiberlaserpumping
Highbrightnessenablesnon-linearitysuppressionviahighmodaloverlapwhich
allowsforreducedfiberlength
Highpowerfiberlasersrequirehighbrightnessdiodelaserpumpsources
HighermodalgainShorterfibers
Pump Δλ FWHM QuantumDefect
915nm >50nm 8%
976nm 9nm 5%
-
50LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
Brightpowerislimitedbyearlyrollofchipbrightness
0
0.1
0.2
0.3
0.4
0.5
0.6
0
2
4
6
8
10
12
14
0 3 6 9 12 15
Slow
-Axi
s Li
near
Brig
htne
ss
(BPP
, W/m
m-m
rad)
Pow
er (W
)
Current (A)
LIVrolloverat18WBIVrolloverat10W
60%
65%
70%
75%
80%
85%
90%
0
10
20
30
40
50
60
70
0 3 6 9 12Current (A)
Coup
ling
Effic
ienc
y (%
)
Pow
er (W
)
Fiber-coupled
Collimated
Fiber-couplingefficiencyrollswithdrivecurrent
J.Bai,P.Leisher,etal.,Proc.SPIE,vol.7953,(2011).
-
51LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
Brightnessrollcausedbyslowaxisblooming
0
0.1
0.2
0.3
0.4
0.5
0.6
0
2
4
6
8
10
12
14
0 3 6 9 12 15
Slow
-Axi
s Li
near
Brig
htne
ss
(BPP
, W/m
m-m
rad)
Pow
er (W
)
Current (A)0
2
4
6
8
10
12
14
16
18
0 3 6 9 12 15 18 21
FWH
M S
low
-Axi
s An
gle
(º)
Current (A)
ABOLN (EMOF20-10)3.8 mm chip25ºC
ModelledBaseline
Measured
Causedbyincreaseinslow-axis-divergence(andreductioninbeam
quality)withincreasingdrivecurrent
LIVrolloverat18WBIVrolloverat10W
J.Bai,P.Leisher,etal.,Proc.SPIE,vol.7953,(2011).
-
52LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
Shortdescriptionofslowaxisblooming
Diode Stripe (top down view)
Slow Axis Divergence at I1
Slow Axis Divergence at I2 > I1
• Increase in slow axis divergence driven by gradients in the active region temperature • The slow axis divergence in broad area diode lasers increases as the diode drive current
increases• The diode is often capable of providing reliable power well in excess of what can be efficiently
coupled into the fiber.
As the diode drive current increases the spot size increases. overfilling the real space aperture. decreasing fiber coupling and endangering the fiber reliability.Tighter focusing risks overfilling the fiber numerical aperture
Fiber acceptance angle
Diode Stripe
Spot overfills aperture
Ascurrentincreases,slow-axisbloomingoverfillstheacceptanceNAofthefiber
ImagecourtesyofS.Patterson
-
53LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
Time-resolvedindexmodelasafunctionofdrivecurrent
CW, Junction down, 3.8mm, 1.4 etch depth
3.399
3.400
3.401
3.402
3.403
3.404
3.405
3.406
1.18 1.20 1.22 1.24 1.26 1.28 1.30 1.32
Inde
x of
Ref
ract
ion
Position Along Slow Axis (mm)
3W
6W
9W
0W
CW, Junction down, 3.8mm
0.0E+00
2.0E-04
4.0E-04
6.0E-04
8.0E-04
1.0E-03
1.2E-03
1.4E-03
1.6E-03
0 3 6 9 12 15In
dex
Diff
eren
ce (Δ
n)Current (A)
Δn
J.Bai,P.Leisher,etal.,Proc.SPIE,vol.7953,(2011).
Slow-axisbloomingiscausedbythermallensingalongtheslowaxisofthediodelaser
-
54LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
Mitigationofthermallensing:1)Engineertheheatsource
Longercavitylengths→Higherrateablepower
J.Bai,P.Leisher,etal.,Proc.SPIE,vol.7953,(2011).
-
55LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
-20 -15 -10 -5 0 5 10 15 20
Slow
-Axi
s Fa
r Fie
ld In
tens
ity
(arb
. uni
ts)
Angle (º)
CW, 25ºC12 Amps
Controldesign
Experimentaldesign
Mitigationofthermallensing:2)Engineertheheatflow
AuPlating
Chip
J.Bai,P.Leisher,etal.,Proc.SPIE,vol.7953,(2011).
Promote1Dheatflowtoreducelateralgradientbyformingvoidsinthebondlineoneithersideoftheemitter
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 10 20 30 40 50 60 70 80 90 100Gold thermal path thickness h (µm)
Thermal resistance
Temperature gradient
Emitter
Gold pad
Front view after bondingTop view before bonding
X-view after bondingX-view before bonding
Front view after bondingTop view before bonding
X-view after bondingX-view before bonding
-
56LLNL-PRES-741702 – P. Leisher – UCSB IEE Seminar – November 16, 2017
§ AdvancedlaserdevelopmentatLLNLrequiresanddriveshighpowerdiodelaserdevelopment
§ Self-heating theprimarycauseofpowersaturation
§ Efficiency isking– itisbettertonotgeneratetheheatthantrytodealwithit
§ Scalingcavitylengthhasenabledrecentimprovements(5mm,