High Power Semiconductor Lasers - BostonPhotonics.org

High Power Semiconductor Lasers

Gary M. Smith

IEEE Photonics Society, Boston Section Seminar

14 Nov 2012

Distribution Statement "A" (Approved for Public Release, Distribution Unlimited). The views expressed are those of the authors

and do not reflect the official policy or position of the Department of Defense or the U.S. Government.

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Semiconductor Lasers

• Semiconductor lasers (also called diode lasers) are the smallest family of lasers - can be about the size of a sesame seed

• Technology to make them is similar to that used to make computer chips (integrated circuit fabrication)

• Easy to provide power with low voltages and currents

0.5 mm

0.1 mm

0.3 mm

Bare Laser Chip

6 mm diameter package

Optical fiber coupled

laser diodes for

telecommunication

(vol. of 2 cm3)

Distribution Statement "A" (Approved for Public Release, Distribution Unlimited). The views

expressed are those of the authors and do not reflect the official policy or position of the

Department of Defense or the U.S. Government.

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High Power Semiconductor Lasers

Single-Mode Emitter

e.g. Slab-Coupled

Optical-Waveguide Laser

(SCOWLs)

Multi-Mode Emitter

e.g. Broad-Area Laser

SCOWL

M2 ~ 1.2

Near diffraction-limited

Multiple modes

5000 µm

5 µm Both

Large mode areas (spread heat)

Power ~ 1-20 W




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Why Semiconductor Lasers?

• Most efficient laser systems

available use semiconductor

lasers directly or indirectly

• Indirect: brightness

conversion using laser diode

pumps for solid-state lasers

(DPSS), semiconductor

discs, fiber lasers, or alkali

lasers (DPAL)

Semiconductor Laser

Pumped

Schröder, D., et al., “Roadmap to low cost, high

brightness diode laser power out of the fiber”,

Proc. SPIE 7583, 758309-1 (2010).

Transforming Watts to kiloWatts




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Outline

• Introduction

• 5 key high-power laser attributes

• Survey of state-of-the-art semiconductor laser systems

• Alternative: surface emitting lasers

• Beam combining for higher brightness

• Summary




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5 Key High Power Laser Attributes

1. Power

– How high?

Power




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Courtesy Erik Zucker, JDSU

Bro

ad

-Are

a L

asers




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1. Power

– How high?

2. Efficiency

– (Output optical power)

(Input electrical power)

Efficiency

Power




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Laser Power and Efficiency

0%

10%

20%

30%

40%

50%

0

0.5

1

1.5

2

2.5

3

3.5

0 1 2 3 4 5 6

PC

E (%

)

Op

tica

l Po

we

r (W

)

Current (A)

MIT-LL SCOWL

λ = 1060 nm

L = 10 mm

T = 15°C

Coherent

λ ~ 980 nm

Broad-Area Lasers

100 µm Stripes, 2-4 mm cavity lengths

Pmax ~ 5-20 W

Efficiency ~ 50-70%

M2 ~ 5-20

Courtesy Rajiv Pathak, Coherent

Single-Mode Lasers

5 µm Waveguide, 2-10 mm cavity lengths

Pmax ~ 1-3 W

Efficiency ~ 35-50%

M2 ~ 1-2




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1. Power

– How high?

2. Efficiency



3. Reliability

– How long will it last?

Reliability Efficiency

Power

After Erik Zucker, JDSU




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Reliability

10

100

1000

0 100 200 300 400 500 600

BGS Fiber Power (mW)

Su

dd

en

Failu

re R

ate

(F

ITs) CLT2.1

CLT2.2

CLT3

0

500

1000

1500

2000

2500

3000

3500

0 1000 2000 3000 4000 5000 6000 7000

Po

we

r(m

W)

Current(A)

5 mm

1 cm

Front facet SEM after COD

MIT-LL SCOWL

CW, 15°C

Corning Lasertron 980 nm Chip Generations

Circa 2003

MIT-LL SCOWL




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1. Power

– How high?

2. Efficiency



3. Reliability


4. Étendue (Beam Quality)

– How tightly can it be focused?


Étendue Power


A 2q

W = πq2 Étendue = A W




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Brightness Plot

Broad-area Lasers

Beam Parameter Product =

BPP = M2 x λ / π

= NA x (fiber radius)

Brightness = Power / Étendue




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1. Power

– How high?

2. Efficiency



3. Reliability


4. Étendue (Beam Quality)

– How small can it be focused?

5. Cost

– Is it competitively priced?

– Necessary for commercial success


Étendue Power

Cost





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JD

SU

Sellin

g P

rice




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Survey of State-of-the-Art: Fiber-Coupled Diode Laser Systems




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JDSU Building Block

Multiple 10 W emitters





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JDSU 2 kW System


Using 2 kW JDSU Stingrays to

pump Yb-doped fiber laser




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JDSU Brightness Plot





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Laserline Bars and Stacks

www.laserline-inc.com




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Laserline Diode Systems

www.laserline-inc.com




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Trumpf Direct Diode System

www.us.trumpf.com




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IPG Direct Diode System

www.ipgphotonics.com




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Brightness Comparison





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Outline

• Introduction





• Summary




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Courtesy Chad Wang, FLIR




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Courtesy Toby Garrod, Alfalight




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Courtesy Toby Garrod, Alfalight




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Outline

• Introduction





• Summary




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Laser Beam Combination Approaches

• Conventional side-by-side

– Overlapping far fields

– Beam quality proportional to N1/2

– Brightness no greater than single element

• Wavelength beam combining (WBC)

– Inherently multi-wavelength

– No far-field sidelobes

– Similar to wavelength-division-multiplexing in fiber optic

communications

– Brightness scales as fgN (fg <1)

• Coherent beam combining (CBC)

– Phasing to narrow far field and increase intensity

– Requires phase control to much better than

– Brightness goes as ffN (ff <1)

– Highest spatial and spectral brightness possible

Courtesy T.Y. Fan, MIT-LL




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Wavelength Beam Combining (WBC) Basic Architecture

• Beams are overlapped at dispersive element

• Output coupler and grating provide optical feedback for unique wave-

length control of elements and overlap beams in both near and far fields

V. Daneu et al., Optics Letters, 25, 405-407 (2000)

Lens (f) Diffraction

Grating

Output

Coupler

SCOWL

Array

f f

Microlens

Array

d




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Nearly Ideal WBC of Large Laser Arrays

• Nearly ideal beam combining of a large (10’s of elements) laser array

• 100-element slab coupled optical waveguide laser (SCOWL) array at ~980 nm

– Single-mode elements

– 100-µm laser pitch

• Highest brightness diode array demonstrated to date

– 50-W output power with M2 ~ 1.2 Position Along Array

Wa

ve

len

gth

Output Spectrum

M2 = 1.2

Output Power Output Far-Field

10 mm

15

nm

Beam

Qualit

y (

M2)

Chann et al. (2005)

Huang et al. (2007) Distribution Statement "A" (Approved for Public Release, Distribution Unlimited). The views



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Fiber-coupled direct-diode laser

> 2000 W in 50 m/0.15 NA fiber (95% power content)

967 nm center wavelength

TeraDiode’s 2-kW fiber-coupled diode laser

27Courtesy Robin Huang, Teradiode




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• Single pass amplifier to minimize feedback effects [Slab-coupled Optical Waveguide Amplifier (SCOWA)]

• Planar lightwave circuit (PLC) seed distribution

• Transform lens to focus array output

• Diffractive Optical Element (DOE) to combine multiple beams into single output

• Use current modulation on SCOWAs to adjust phase of each element (SPGD hill-climbing algorithm)

Coherent Beam Combining

Seed




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• 47 element Planar Lightwave Circuit (PLC) for seed distribution

– 6 fiber inputs, 1x8 splitters on-chip to achieve 47 seeds to SCOWAs

• AlN multilayer ceramic for individual addressable current distribution

• Microimpingement cooler for heat removal

47 Element SCOWA Array

47 Elements Operated at 1.6 A each

20°C Cooling Water, Unseeded

47 SCOWA Array

12.5 mm x 5 mm




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Schematic of CBC Module Experimental Layout

DFB Master

Oscillator YDFA

1x6 Fiber

Splitter

7x8 PLC

Splitter

SCOWA

Module

Transform

Optics DOE

Raw Power

Meter

Beam

Blocks Combined

Power Meter Relay

Telescope

SPGD

Detector

DOE

Image

DOE

Far

Field

M2 Meter

Diagnostics include simultaneous measurements of combined beam

near-field, far-field, beam quality, power, and SPGD control signal Distribution Statement "A" (Approved for Public Release, Distribution Unlimited). The views



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Combined Beam Power

• Achieved 40.1 W with high combining efficiency (87%)

• Test to failure (not plotted): Achieved 50.1 W combined power with 80%

combining efficiency at 1.85 A / channel average with maximum seed

available (~65 mW / channel)

– Three SCOWAs failed

0 0.2 0.4 0.6 0.8 1 1.2 1.40

10

20

30

40

50

60

Drive Current (A)

Pow

er

(W)

RC4 Array 3 Combining Test

Raw

Combined

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Com

bin

ing E

ffic

iency

47 element

SCOWA array,

50 mW seed per

channel

(per channel average)

87% to 90%




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• Mode evolution consistent with single element SCOWA measurement

– SCOWA mode shrinks in the slow axis with increasing current (thermal)

• Measured M2 values of ~1.2 x 1.3 for all currents

Combined Beam Diagnostics

0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.41

1.05

1.1

1.15

1.2

1.25

1.3

1.35

1.4

Drive Current (A)M

2 (

90/1

0 M

eth

od)

Combined Beam Quality

Array Dimension

Non-Array Dimension

(per channel average) 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.44

4.2

4.4

4.6

4.8

5

5.2

5.4

5.6

5.8

6

Drive Current (A)

Scale

d B

eam

Dia

mete

r (

m)

Combined DOE Far Field

Array Dimension

Non-Array Dimension

(per channel average)




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Single SCOWL

Coherently

Combined

SCOWAs




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Future Fusion Reactor Application

Courtesy Ryan Feeler, NG Cutting Edge Optronics

NIF = National Ignition Facility

LIFE = Laser Inertial Fusion Energy U.S. Dept of Energy

Learn more at life.llnl.gov




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Fusion (cont.)

Courtesy Ryan Feeler, NG Cutting Edge Optronics




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Summary

• High-power semiconductor lasers are most efficient lasers available

– Used directly can provide lab efficiencies of 60+%, fiber-coupled systems of 45+%

– Used indirectly to pump most other state-of-the-art laser systems

• Fiber lasers, disc lasers, diode-pumped solid-state (DPSS) lasers, and diode-pumped alkali lasers (DPALs)

• Efforts continue to increase efficiency and brightness

• Beam combining is key to producing kWatt+ powers from Watt class devices

– Incoherent beam combining: used in most commercial systems

• Brightness not increased

– Path to higher brightness:

• Wavelength beam combining: coarse WDM used in several commercial systems, fine WDM being commercialized by Teradiode (Westford, MA)

• Coherent beam combining




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High Power Semiconductor Lasers - BostonPhotonics.org