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Thermal profile of high power laser diode arrays and implications in line-narrowing using external cavities. Gregory Blasche Bennett Goldberg Boston University Physics Department M. Selim Ü nl ü Boston University Electrical & Computer Engineering http://ultra.bu.edu/. Motivation

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

Gregory Blasche

Bennett Goldberg

Boston University

Physics Department

M. Selim Ünlü

Boston University

Electrical & Computer

Engineering

http://ultra.bu.edu/

External 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

Outline

External Cavity Laser Diode Arrays - Thermal Effects

motivation
Motivation

Hyperpolarized Noble Gas

Magnetic 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.

HP-MRI

Human lung images courtesy of Mitch Albert

Brigham and Womens Radiology Dept.

“Distal Airways in Humans: Dynamic

Hyperpolarized 3He MR Imaging—

Feasibility” Radiology 2003; 227:575–579

External Cavity Laser Diode Arrays - Thermal Effects

thermal profile of laser diode arrays
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

Power/nm (arb. Units)

790

795

800

Wavelength (nm)

Spectra for water-cooled

LDA (different coolant directions)

External Cavity Laser Diode Arrays - Thermal Effects

air cooling
Air Cooling

797

796.5

Peak Wavelength (nm)

796

795.5

Have constant cooling along

bar, but edges have less

heating than middle.

Inferred from 15W

Fiber-coupled LDA

(Optopower)

795

794.5

5 10 15 20 25

Diode

External Cavity Laser Diode Arrays - Thermal Effects

water cooling
Water Cooling

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

Diodes in the middle have a lower thermal coefficient

than diodes on the edges.

As well, the water heats up

as it flows through the heat sink.

External Cavity Laser Diode Arrays - Thermal Effects

water cooling7
Water Cooling

2.5

2.0

1.5

1.0

44

0.5

Peak Shift (nm)

0.0

-0.5

33

-1.0

-1.5

22

-2.0

0

10

20

30

40

50

11

Diode

1

Results for a 44-emitter 10W

Laser Diode Array

(similar for 50W)

External Cavity Laser Diode Arrays - Thermal Effects

additional problems non uniform thresholds and iv characteristics
Additional Problems: Non-uniform Thresholds and IV Characteristics

7A

8A

Cooling right side

Cooling left side

9A

Laser Diode Array Imaging

10A

Current

External 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.

External Cavity Laser Diode Array

External Cavity Laser Diode Arrays - Thermal Effects

external cavity laser diode array littrow cavity scheme
External Cavity Laser Diode Array – Littrow Cavity Scheme

fc

Top view

LDA

PBS

Grating

λ/2

Waveplate

Collimating

Lens

Side view

f1

f1

f2

f2

Imaging Lenses

Monitor

External Cavity Laser Diode Arrays - Thermal Effects

off resonance feedback single diode
Off-resonance Feedback(Single Diode)

5

50

45

4

40

35

3

30

25

Power/nm (arb. units)

Power/nm (arb. units)

2

20

15

1

10

5

0

0

790

792

794

796

798

800

790

792

794

796

798

800

Wavelength

Wavelength (nm)

Detuning by 2nm gives ~30% loss

of power in feedback mode.

15°

25°

35°

External Cavity Laser Diode Arrays - Thermal Effects

off resonance feedback
Off-Resonance Feedback

1.0

1.0

0.9

0.9

0.8

0.8

Relative Maximum

Relative Efficiency

0.7

0.7

0.6

0.6

0.5

0.5

0.0

0.5

1.0

1.5

2.0

2.5

0

10

20

30

40

50

Diode

λ Shift (nm)

Net result is a 20%+/-10% loss of power

in feedback mode due to thermal profile

External Cavity Laser Diode Arrays - Thermal Effects

power
Power

20

5

18

0.5

16

4

0.4

14

12

3

0.3

10

0.2

Absorbed Power (W)

Total Power (W)

8

2

0.1

6

0.0

4

1

790

795

800

2

0

0

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

10

12

14

16

18

20

Current (A)

0.4nm FB (tot)

0.4nm FB (abs)

Free (tot)

Free (abs)

(note: laser goes up to 50A)

External Cavity Laser Diode Arrays - Thermal Effects

additional issues
Additional Issues

Heat Expansion

Laser must warm

up before operating

at correct temperature

Bar Curvature

Some diodes

operate at incorrect

wavelength due to

being off-axis.

Aspheric Aberrations

If use simple lenses,

diodes off axis will

have non-optimal

coupling

External Cavity Laser Diode Arrays - Thermal Effects

conclusions
Have 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

Conclusions

External Cavity Laser Diode Arrays - Thermal Effects