PHOTONICS CENTER

6/3/2003 1External Cavity Laser Diode Arrays - Thermal Effects

Gregory Blasche

Bennett GoldbergBoston University

Physics Department

M. Selim ÜnlüBoston University

Electrical & Computer

Engineering

http://ultra.bu.edu/

Thermal profile of high power laser diode arrays and implications in line-narrowing

using external cavities

PHOTONICS CENTER

6/3/2003 2External Cavity Laser Diode Arrays - Thermal Effects

Outline

Motivation Thermal Profile of Laser Diode Arrays

• Origin of thermal profile• Linewidth due to thermal profile

External Cavity Laser Diode Array• Implications of thermal profile• Power and linewidth of external cavity

Conclusions

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6/3/2003 3External Cavity Laser Diode Arrays - Thermal Effects

Motivation

HP-MRIHuman lung images courtesy of Mitch AlbertBrigham and Womens Radiology Dept. “Distal Airways in Humans: Dynamic Hyperpolarized 3He MR Imaging—Feasibility” Radiology 2003; 227:575–579

Hyperpolarized Noble GasMagnetic Resonance Imaging

Gases are generated via a spin-exchange with optically pumped Rubidium.

Optical absorption linewidth of Rb is 0.2 nm/atm. (Helium3 can be generated at high pressure, Xenon129 is generated at 1 atmosphere)

High power laser diode arrays have linewidths greater than 2nm.

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6/3/2003 4External Cavity Laser Diode Arrays - Thermal Effects

Thermal Profile of Laser Diode Arrays

FWHM linewidth of entire array is >2nm (At 50% operating current)

Linewidth of each diode is <2nm

Broadening is due to inhomogeneous heating/cooling of the laser bar

790 795 800Wavelength (nm)

Po

we

r/n

m (

arb

. Un

its)

Spectra for water-cooledLDA (different coolant

directions)

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6/3/2003 5External Cavity Laser Diode Arrays - Thermal Effects

712

714

716

718

720

722

724

726

0 5 10 15 20 25

Air Cooling

Have constant cooling along bar, but edges have less

heating than middle.

Inferred from 15W Fiber-coupled LDA

(Optopower)

Diode

Pe

ak W

ave

len

gth

(n

m)

5 10 15 20 25

795.5

795

794.5

796

796.5

797

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6/3/2003 6External Cavity Laser Diode Arrays - Thermal Effects

Water Cooling

For feedback, operate at much less current than free-running

Diodes in the middle have a lower thermal coefficientthan diodes on the edges.

As well, the water heats upas it flows through the heat sink.

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6/3/2003 8External Cavity Laser Diode Arrays - Thermal Effects

Water Cooling

1

11

22

33

44

Results for a 44-emitter 10WLaser Diode Array (similar for 50W)

0 10 20 30 40 50-2.0-1.5-1.0-0.50.00.51.01.52.02.5

Diode

Pe

ak S

hift

(nm

)

PHOTONICS CENTER

6/3/2003 9External Cavity Laser Diode Arrays - Thermal Effects

Additional Problems: Non-uniform Thresholds and IV Characteristics

7A

8A

9A

10A

Current

Laser Diode Array Imaging

Cooling right sideCooling left side

PHOTONICS CENTER

6/3/2003 10External Cavity Laser Diode Arrays - Thermal Effects

External Cavity Laser Diode Array

Use a grating to form a Littrow external cavity.

The thermal profile causes an effective detuning of the feedback.

Detuning causes a loss of power in the desired feedback mode.

PHOTONICS CENTER

6/3/2003 11External Cavity Laser Diode Arrays - Thermal Effects

External Cavity Laser Diode Array – Littrow Cavity Scheme

fc

f1 f1 f2 f2

Top view

Side view

LDA

CollimatingLens

Imaging Lenses

PBS λ/2 Waveplate

Grating

Monitor

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6/3/2003 12External Cavity Laser Diode Arrays - Thermal Effects

PHOTONICS CENTER

6/3/2003 13External Cavity Laser Diode Arrays - Thermal Effects

Off-resonance Feedback(Single Diode)

15°25°35°

Detuning by 2nm gives ~30% lossof power in feedback mode.

790 792 794 796 798 800

0

1

2

3

4

5

Po

wer

/nm

(ar

b. u

nit

s)

Wavelength790 792 794 796 798 800

05

101520253035404550

Po

wer

/nm

(ar

b. u

nit

s)

Wavelength (nm)

PHOTONICS CENTER

6/3/2003 14External Cavity Laser Diode Arrays - Thermal Effects

Off-Resonance Feedback

0 10 20 30 40 500.5

0.6

0.7

0.8

0.9

1.0

0.0 0.5 1.0 1.5 2.0 2.50.5

0.6

0.7

0.8

0.9

1.0

λ Shift (nm)

Rel

ativ

e M

axim

um

Diode

Rel

ativ

e E

ffic

ienc

y

Net result is a 20%+/-10% loss of power in feedback mode due to thermal profile

PHOTONICS CENTER

6/3/2003 16External Cavity Laser Diode Arrays - Thermal Effects

Power

Current (A)

Absorbed P

ower (W

)

Tot

al P

ower

(W

)

0.4nm FB (tot)

0.4nm FB (abs)

Free (tot)

Free (abs)

(note: laser goes up to 50A)

10 12 14 16 18 200

2

4

6

8

10

12

14

16

18

20

0

1

2

3

4

5

790 795 800

0.0

0.1

0.2

0.3

0.4

0.5

0.2nm fitted peak, 0.4nm fitted entire feedback. Note, not to scale.

PHOTONICS CENTER

6/3/2003 17External Cavity Laser Diode Arrays - Thermal Effects

Aspheric Aberrations

If use simple lenses,diodes off axis will have non-optimal

coupling

Bar Curvature

Some diodesoperate at incorrectwavelength due to

being off-axis.

Heat Expansion

Laser must warmup before operating

at correct temperature

Additional Issues

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6/3/2003 18External Cavity Laser Diode Arrays - Thermal Effects

ConclusionsHave developed a >10W, 0.2nm wide

line-narrowed laser diode array which should be capable of >20W, with 12W absorbed.

Thermal effect limit the total power of the laser due to effective detuning

Can tune the laser in frequency and linewidth