High-Power Laser Diodes

Christoph S. Harder

Harder&Partner

charder@swissonline.ch

+41 79 219 9051

LEOS Annual Meeting, Montreal November 1, 2006

2

High Power Laser Diodes

1. Single Mode Pump Lasers

EDFA: Killer application

Drove Technology to Maturity in 10 years

2. Beam Machining Tool

Fiber laser: Pump lasers

Direct Diode

Acknowledgement

Dr. Berthold Schmidt, Bookham

Dr. Toby Strite, JDSU

Prof. Ursula Keller, ETH

Prof. Amos Hardy, Technion

Dr. Martin Achtenhagen, EPFL

3

High Power Laser Diodes: 20 years ago

Narrow stripe laser: COMD: Mirror blows up at high powers

Investigate COMD in 80‘s for InGaAlAs lasers for MO storage Computer Interconnect

In the 80‘

Never achieved reliablebeamstability for highvolume applications

-> Spread beam to decrease powerdensity at facet:

Coherent arrays

Surface grating lasers

MOPA

Taper Laser

Alpha DFB

VCSEL

4

Time to COMD: 20 years ago

Chemist fixed problem

Solved in 1987 (E2)

www.bookham.com

E2 passivated 980nm pumps

In 1990 started again 9 lasers (Corning had approached us for 980nm EFApumps).

EDFA: Killer application drove technology quickly to maturity

8 of 9 are still alive after 16 years

190

210

230

250

270

290

310

330

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

Time (years)

Curr

ent(m

A)

150mW, 50C and 75C case

$920 / Km / Gbit/sRRRRRRRRRRRR RRRR RRRRRR RR RRRR

One Regenerator Every 40 KmOne Regenerator Every 40 Km

1310nm DirectModulation

One Fiber Amplifier Every 120 KmOne Fiber Amplifier Every 120 Km

4 Channel WDM/External

Modulation

$150 / Km / Gbit/s

External Modulation Enables Cost ReductionExternal Modulation Enables Cost Reduction

7

Single Lateral Mode Laser:Ridge Waveguide

Ridge Waveguide Index guided mode

Excellent coupling to fiber

Temperature insensitive current confinement

High linear power

Low loss waveguide Increase power by making chip longer

Low temperature process Reliable

No regrowth

Material InGaAlAs for best material properties

Widespread ~2‘000‘000 pumps based on this technology shipped, 50% terrestrial and

50% of intercontinental (submerged) internet is powered up by thesepump technology

Submerged systems: No fail of consequence

8

‘Shift’ Kink: Observation

Observation (1991)

Sudden kinks in fiber coupled power

Standard countermeasure:

Increasing loss for higher order modes (to keep them below threshold): Doesnot work

Farfield observation

Still single ‘humped’, but shifted during kink

9

Shift Kink: Waveguide Dispersion

Waveguide

Index increases with local heating

Waveguide becomes multimode

Dispersion characteristics of waveguide

Phase lasing condition (integer number of wavelengths in one roundtrip) can be meet forone frequency (0 = 1 ) for fundamental and higher order modes at the same time

Index moving up withtemperatureprofile (drive current)

Round

trip

conditio

n

For

Coherent coupling

10

Shift Kink: Coherent Coupling

Small asymmetry (e.g. at front mirror) couples power fromfundamental to higher order mode

Phasematch condition given at special dispersion point(temperatureprofile, i.e. current) ‘lateral mode locking’ at this current

1st resonantcoupling coupling

Farfield

F R F R F R F0th asymmtric

element

0th and 1st: Same frequency one roundtrip0th. N wavelengths1st: N-1 wavelengths

f

h

f

h

11

Shift Kink: Lateral Mode Locking

Coherent coupling, lateral mode locking

Power gets coherently coupled (only one lasing frequency) away from fundamental modeinto higher order

Locking range

Interference within waveguideAchtenhagen, Hardy and Harder, JQE Vol24 pp2225

www.bookham.com

Length Scaling

Improve performance by making laser chip longer

1. Low loss waveguide

2. Need facets which can sustain high powers

0 500 1000 1500 2000 2500-200

0

200

400

600

800

1000

1200

1400

1600 G08 (2004): 3.6mm

G07 (2002)

G03 (1996)

G05 (1999)

G06 (2000)

ex-

facet

light

outp

ut

po

wer

(mW

)

injected current (mA)

P-side up mountedCW-operation @ 25 °C

G01 (1990): 0.6mm

www.bookham.com

0

0.5

1

1.5

2

0 0.5 1 1.5 2 2.5 3

Injection current (A)

Lig

ht

ou

tpu

tp

ow

er

(W)

T = 5 °C

T = 25 °C

T = 50 °C

T = 75 °C

P-side up mountedCW - operation

980nm single mode pump chip: 2004

T0 ~ 165 KIth ~ 45 mA @ 25°C

Reliability Better than 500FIT (0.5%/year) at Pop=850mW

Wallplug Efficiency >60% peak, >50% up to 800mW

Beam Single lateral mode beyond 1200mW, shift kink: solved

Emission spot: 0.7um*2um

150MW/cm2

www.bookham.com

Low Cost High Performance Packaging: 2004

Performance (Commercial Products) 200mW uncooled MiniDIL (-5C to 75C) with telcom reliability

600mW Peltier stabilized BTF with telcom reliability

12’000h at 75C

+2/-4%, afterwards stable

Stability of coupled Power

15

EDFA Pump Power Development

EDFA pumps: Matured-> Power increase stopped-> Cost reduction done-> Spectral stability and noise done

Evolution of 980nm Single Mode Power

0

250

500

750

1000

1250

1500

1750

2000

1985 1990 1995 2000 2005 2010Year

Po

we

r[m

W] ,

Roll-over Power

Power in SM Fiber with High Reliability (Fr<500FIT)

Price

Factor of 10 reductionto 2$/mW

16

Classic Laser: Beam Machining Tool

Hybrid Laser Systems

Hybrid systems (Diodes, Lamps, Crystals, Dielectric Mirrors), Non hermetic cavity, Free spacepropagation

Beam limited by heat in active material

Applications

Academia: Ideal for manipulation of beam

Industry: Delicate operation

Diode Pumped disk

10um

Diffraction Limit

1um

Diffraction Limit

Diode Pumped rod

Lamped Pumped rod

CO2

17

Power Photonics: Fiber LaserFiber Delivered Beam Machining Tool

Solid State Hermetically sealed Diodes coupled to Fibers Fiber delivery

Technology Apply telecom technology to power photonics

‘SM’ fiber Fiber laserBundleof ‘SM’ fib

er

18

Fiber Laser: MOPA

Seed laser Fiber laser: Good spectral control

Need external modulators (Pockels Cell)

Diode laser: Excellent dynamic control FP laser have poor spectral control, of no concern

DFB have excellent spectral and dynamic control

Pumplaser Single emitter broad area MM diode

Seed laserPump Diode

Pump Diode

Dual Clad Yb

MMPMFPMF

19

Yb fiber wavelength: 9xx bands

Yb: Glass fiber absorption and emission spectrum

Wide pump band: 870nm to 980nm

Blue band (915nm): Good absorption, wideband Preferred for lower power, high gain stage

Green band (940nm..960nm): Lowest absorption, wideband, high optical conversion Preferred for very high power stage

Red band (976nm): Highest absorption, narrow width Preferred for high gain amplifiers and q-switched lasers with short fiber (SBS) Pump diode challenge: Diode wavelength control (+/-2nm) necessary

20

Fiber combinerFused and Proximity

Fused: (6+1)*1 Proximity: (2+1)*1

Fused can be extended to beyond 20 inputs

Proximity needs high brightness pumps

0.06/0.4620um/400umOutput

0.226*105umPump Ports

HI 1060Signal input

NAFiber

www.bookham.com

90 µm emission spot

0 2 4 6 8 10 120

2

4

6

8

10

12

CW m easurem entp-side down m ounted on C-m ount

lig

ht

ou

tpu

tp

ow

er

(W)

in jection current (A)

15°C25°C55°C75°C

SES8-9xx-01 performance

• Electro-Optical

– Power: 8 W @ 8 A

– Reliability: <5% fail in 10‘000h

CW= 960 nm

30

25

20

15

10

5

0

po

wer

(W)

3020100

current (A)

Q-CW(0.3ms)

www.bookham.com

0

1

2

3

4

5

6

7

8

9

10

11

12

ligh

to

utp

ut

po

we

r(a

.u.)

0 100 200 300 400 500 600 700 800 900 1100

time (h)

Broad Area 960nm (SES8-960-01)

• W=90um, P=13A (11W), Tj=130C

• Multi Cell Testing ongoing

www.bookham.com

MM Uncooled Module with >14W

• Record Performance:

– >14W @ 18A and 10°C Ths

– Standard MU package

• Module fully qualified

• MSA with EM4

0

2

4

6

8

10

12

14

16

0 2 4 6 8 10 12 14 16 18 20

injection current (A)

Ex

-Fib

reP

ow

er

(W)

10°C

25°C

45°C

© 2006 JDSU. All rights reserved.24

New generation 9xx broad area chip

19.0W maximum CW power from ~ 100um aperture

T=15C

0

5

10

15

20

25

30

0 5 10 15 20 25 30

Current, A

Po

wer,

W

pulsed

CW

0

2

4

6

8

10

12

14

16

0 5 10 15

Current, A

Po

we

r,W

25C

35C

45C

55C

70C

90C

© 2005 JDSU. All rights reserved.25

6396 Chip Reliability Improvement

MLE Results:

Revised reliability:– P=8.0W

– Th=35C

– Median time to failure=1,500,000 hrs (60% C.L.)

– 6% F at 20khr (60% C.L.)

hrs

n

eVE

op

A

6101.5

7.2

61.0

54.0

1%

5%

10%

50%

90%

99%

1000.00 1.00E+610000.00 1.00E+5Time, hrs

Unre

liabili

ty

6396 c

6390

6390

6396

26

Power Photonics: Beam Machining Tool

Many applications do not need diffraction limited beam: Direct Diode

200um,0.22

600um,0.22

1500um,0.22

50um.0.22

Direct Diode

27

Number of Lateral Modes in Fiber

With single mode lasers at 0.5W, all material processing needscan be met

‘Just’ challenge of coupling single mode laser diodes to fiber

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

0 200 400 600 800 1000 1200 1400 1600

Diameter (um)

Nu

mb

er

of

Mo

des

,

(bo

thP

ol)

,

www.bookham.com

Integrated Array of Single Mode Lasers

Single Mode Bar:

– 50 single mode lasers from 1 cm bar

– Power: 40W @ 50A

– Wavelength: 975nm0

10

20

30

40

50

0 10 20 30 40 50 60

Current (A)

Optic

alP

ow

er(W

)

0.0

0.5

1.0

1.5

2.0

2.5

Volta

ge

(V)

Product (by LIMO): Single mode laser bar coupled with lens array to 50umfiber:

25 W from 50um NA=0.22

29

Number of Lateral Modes in BA chip

Low NA broad area laser radiance: Closing in on single mode lasers 8W from NA=0.15NA: 400mW per lateral mode

Reduce NA of broad area laser to increase radiance NA dominated by thermal blooming ->Need chip with very high power conversion

100um BA, #of modes

0

5

10

15

20

25

30

35

40

0 0.1 0.2

NA

Nu

mb

er

of

mo

des

,

30

Low NA Broad Area Lasers:Thermal Blooming

Top view of BA laser

NAdn

n

dT

Issues of LowNAlasers

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0 0.05 0.1 0.15 0.2 0.25

NA

dn

dT=10K

Low NA laser Achieved by low dn waveguide

dn Ridge

Parasitic through lateral temperature profile dT=10K -> dn=0.0025 -> NA=0.13

Leads to ,blooming’

31

Diode Power Conversion

Material limits: Even after optimized mirror losses (Sf , Rf , Rb ) and low threshold current. Due to limited mobility and carrier mass there are always trade-offs in

doping levels (series resistance Rs vs free carrier absorption) and

Bandgap discontinuities (leakage losses vs injection barriers)

Today’s approach: InGaAlAs material system

Asymmetric (thin p-region), low aluminum, low confinement LOC, low doping levels Holes have poor conductivity and high free carrier losses.

Relatively low barriers for high mobility and good injection (some thermal and vertical leakage)

PVIout

VI

P

)ln(2

)ln(

)ln(1

1

ff

bf

P

R

L

R

RS

seshFeFhfV RRV

IEE

eVhE

eV

111

I

I

I

I leakthI 1

Mobility:Series resistance

Density of States:Free carrier absorption

Bandgap discontinuities1. Thermal and vertical leakage2. Injection barriers

www.bookham.com

Bar with 425W CW at 980nm

– 425W at 980nm, 1cm, 50% FF

– On standard MCC

– 3.6mm long laser cavity

400

300

200

100

0

CW

pow

er(W

)

6005004003002001000

current (A)

33

Direct Coupled Diodes:

Practical limits to radiance?

© 2006 JDSU. All rights reserved.34

High-efficiency bars

>75% wall plug efficiency from 120W 940nm bar(SHEDS design)

Performance of JDSU/SHEDS 100W Bars

0

25

50

75

100

125

150

0 20 40 60 80 100 120

Drive Current (A)

Ou

tpu

tP

ow

er

(W)

50%

55%

60%

65%

70%

75%

80%

Po

we

rC

on

ve

rsio

nE

fficie

nc

y

35

High Power Laser Diodes

Single Mode Pump Diodes: Matured

Frequency doubling for displays, Printerarrays, Seed lasers (Fiber Lasers)

Broad Area Pump Diodes:

Fiber Lasers

Direct Diode Systems

Power Photonics still at beginning. Enabled by laser diodes with

Power efficiency

Beam matched to fiber coupling