Narrow line diode laser stacks for DPAL pumping

David Irwin, Dean Stapleton, Rajiv Pandey, Tina Guiney, Steve Patterson

DILAS Diode Laser Inc.

Tobias Koenning

Joerg Neukum

2SPIE LASE 8962-14

Outline

• Company overview

• Standard products

• Motivation and applications

• VBG principles

• Design

– Stack design

– Temperature tuning

• Test results

– Power (LIV)

– Beam quality

– Spectral line width

– Wavelength tuning

– Stability

• Summary

• Outlook

– Scalability

– Different wavelength

– Next steps

DILAS Company Data

COMPANY

• Founded 1994

•

• 340 employeesworldwide

• ISO 9001-2008 certified

• SitesMainz (HQ)

Nanjing (Sub.)

Tucson/AZsubsidiary of Rofin Sinar Technologies Inc (Nasdaq: RSTI)

3DILAS Information

PRODUCT RANGE

• Wavelength450nm – 2.3µm

• Single Bars

• Stacks (vertical& horizontal)

• Fiber coupledmodules

• Turn keysystems

APPLICATIONS

• Pumping of rod-, disk- and fiberlaser in OEM andScientific

• Materials Processing

• Medical

• Printing Industry

• Defence

• Projection andDisplay

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Dilas products

Single Emitters (m2k)

Conduction-Cooled Diode Laser Bars

Water-Cooled Vertical Stacks

Water-Cooled Horizontal Stacks

Conduction-CooledQCW Stacks

Fiber-Coupled Modules

Components

Systems

COMPACT

λ = 450nm to 2300nm

P = 5W up to multiple kilowatts

Operating mode = CW, QCW

Fiber core diameter = 100µm, 200µm, 300µm, 400µm, 600µm, 800µm

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Dilas’ fiber-coupled modules

Fiber core diameter

Maximum power

Commercial Not fully qualified

Demonstrated

100µm 50W 120W 180W

200µm 650W 850W 1KW

400µm 1KW 1.2KW 1.5KW

1mm 4KW 5KW N/A

• Fiber NA 0.2 for all modules shown

• All power levels shown for single wavelength

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Dilas’ QCW products

Available wavelength

All Dilas’ wavelength

(635nm to 2300nm)

Typical pulse duration

100µs to milliseconds

Peak powerUp to 500W per bar

(depending on fill factor and wavelength)

Duty cycle <2-3% (typical)

Fiber coupling ≥ 800um fiber core

• bars optimized for high peak power

• bars not suited for high brightness applications due to beam quality

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Dilas’ high power stacks

Available wavelength

All Dilas’ wavelength

(450nm to 3200nm)

Average power

Up to 200W per bar (depending on fill factor

and wavelength)

Cooling Micro channels

Operating mode

CW or pulsed

• Highest power per package size

• Lowest thermal resistance

• Maximum CW power per bar

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Motivation / Applications

Narrow line Applications

High Energy Lasers (DPAL)

Medical diagnostics

Requirements

Extremely narrow spectral line width (≤ 0.1nm)

Tight tolerance of center wavelength (± 0.05nm)

High optical power (≥100W/bar)

9

Motivation – Why DPALs?

• High power conversion efficiency

• High output power

• Poor beam quality

• No stress birefringence

• No beam distortions by thermal effects

• No stress fractures

• No optical damages (i.e. in laser fibers)

• Reduced thermal focussing

• High average output power

• High beam quality

• Single aperture power scaling

Laser diodearray

Gas (vapor) laser gainmedium

Laser outputbeam

Potential to achieve very high output powers in NIR spectral region

10

Motivation – Why DPALs?

Further advantages of DPALs:

• Scalability to high power (cell size, pump power)

• Low quantum defect (reduction of waste heat)

• Excellent thermal management

• Lightweight packaging

• Closed cells (no vacuum pumping or discharge of chemicals)

Directed energy & power beaming applications:

• Processing of photovoltaic cells

• Underwater communication

• Power supply for space stations or propulsion systems

• Laser weapons

11

Physical basics of alkali lasers

Three-level laser with small quantum defect ∆E/E2

Neutral alkali atom (Cs, Rb, K, Na, Li): single valence s-electron

∆EE2

E1

E0

n2P3/2

n2P1/2

n2S1/2

D1 lineλlaser

D2 lineλpump

Fast quenching (collisionalrelaxation, buffergas: He, CH4, ethane etc.)

n: principal quantumnumber for theground configu-ration of alkaliatoms

12

Physical basics of alkali lasers

Summary of transition energies and wavelengths

Laser n λpump E2-E0 E1-E0 λlaser ∆E ∆E/E2

entity (nm) (eV) (eV) (nm) (meV)

Nd3+ 808 1.5344 1.1652 1064 369.2 0.24

Yb3+ 941 1.3176 1.2038 1030 113.8 0.086

Cs 6 852 1.4546 1.3859 894.3 68.7 0.047

Rb 5 780 1.5890 1.5596 794.8 29.4 0.0185

K 4 766 1.6171 1.6099 770.1 7.2 0.0044

Na 3 589 2.1044 2.1023 589.8 2.1 0.0010

Li 2 670 1.8479 1.8479 670.1 0.04 0.00002

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Wavelengths locking with VBGs

Laser Bar

FAC lens

SAC lens

VBGFeedback

• VBG provides selective feedback• Effective gain increased within

selected wavelength range• Critical design parameters (VBG):

• Index contrast• Reflectivity / Diffraction Eff.• Thickness• Uniformity (in VBG and between

VBGs)

• Critical design parameters (Diode)• Epitaxy design-active region

gain• Cavity length• Facet reflectivity

VBG center wavelength is temperature dependent (3pm/°C)

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Spectral width of ensemble

• Center wavelength of VBG varies in manufacturing

• Power differences between bars cause further differences in central wavelength

• ⇒ Individual bar spectra don’t usually exactly overlap

• ⇒ Spectral line width of ensemble broadens

Design challenge

• Mount resistive heater to VBGs• Adjust temperature of each VBG

individually to overlap bar spectra• Minimize spectral line width• Use heaters to shift center

wavelength of ensemble• Avoid active cooling due to heat

rejection complexities

Solution

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Module design

Standard 15-bar Stack with FAC lenses

Front view of DPAL stack with FACs, SACs, and heated VBGs

Rear view of DPAL stack showing 30 pin connector for heaters

FACs

VBGs

Heaters

SACs

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Tuner design

Heater Controller Tuning software

Temperature controller to heat VBGs and tune wavelength

• Individual set-point for each laser bar (15 channels)

• Master channel to tune entire stack at once

• Stand alone or software controlled operation

• Save settings for various operating points

Final LI curve of 15-bar stack

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Optical Power 1000W

Operating current 76.3A

Operating Voltage 28.5V

Water temperature 23°C

Electrical-to-optical efficiency 46%

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Beam profiles

Far Field - Fast Axis Far Field – Slow Axis

Test Parameter Fast Axis Slow Axis

Divergence FWHM 3.9mrad 22.7mrad

Divergence 90% power content 4.6mrad 21.0mrad

Near field beam size 90% power content 51.5mm ~10mm

Near Field

51.5mm

10mmNote: Near field fill factor << 50%

-> Brightness can be doubled by use of stripe mirrors

3.6mm

Output Spectrum with tuned gratings

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50A (600W), 23°C 80A (1030W), 23°C

Test Parameter 50A 80A

Center wavelength (vacuum) 780.246nm 780.293nm

Bandwidth (3dB or FWHM) 0.072nm (35.5GHz) 0.083nm (40.9GHz)

Bandwidth (95% power enclosed) 0.136nm (67GHz) 0.166nm (81.8GHz)

Wavelength drift of 47pm (23.2GHz) over 400W change in optical power

Wavelength tuning

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Test Parameter Heaters off Optimum Blue shift Red shift

Center wavelength (vacuum) 780.037nm 780.254nm 780.174nm 780.329nm

Bandwidth (3dB or FWHM) 0.11nm (54.2GHz)

0.076nm (37.4GHz)

0.069nm (34.0GHz)

0.080 (39.4GHz)

Bandwidth (95% power enclosed) 0.23nm (113.3GHz)

0.140nm (69.0GHz)

0.134nm (66GHz)

0.146nm (71.9GHz)

• Heaters initially off

• Tuning @ optimum to minimize spectral width

• ‘Master’ setting for blue and red shift (no optimization)

Test procedure

Tuning range: 0.155nm (76.4GHz)

Stability Test: 1hr burn-in @ 50A

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Temperature [°C]

Spectral width [nm]

Temperature [°C]

Spectral width[nm]

• Water temperature fluctuations due to PID controller of chiller

• Optical power varies with water temperature

• Center wavelength stable within 19pm (peak to valley

Stability Test

• Induced change in water temp. (+/-5°C )

• +/-20pm (+/-9.9GHz) change in center wavelength

Temperature change

Summary

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Parameter Heaters off

Power 1030W @ 80A600W @ 50A

Center wavelength (vacuum) 780.29nm @ 80A780.25 @ 50A

Bandwidth (3dB) 0.083nm (40.9GHz) @ 80A0.072nm (35.5GHz) @ 50A

Bandwidth (95% enclosed power) 0.166nm (81.8GHz) @ 80A0.136nm (67.0GHz) @ 50A

Tunable range 0.155nm (76.4GHz)

Center wavelength drift over operating temperature

+/-20pm (+/- 9.9GHz) over +/- 5°C

Fast Axis divergence 3.9mrad FWHM

Slow Axis divergence 22.7mrad FWHM

• High power diode laser stack for Rubidium pumping

• VBGs to narrow spectrum

• Heaters to tune individual bar spectra

– Minimize spectral line width

– Tune center wavelength of ensemble

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Outlook

Increase power per bar to 200W per bar

Improve assembly process to minimize cost

Develop closed loop heater control

Planned Tasks

Transfer technology to different wavelengths

Increase number of total bars to scale power

Contract# FA9451-12-D-0191

Thank you!

Thank you for your attention !

CONTACT:

Dr. Jörg Neukum

DILAS Diodenlaser GmbH

www.dilas.de

Email: j.neukum@dilas.de

Phone: +49 6131 9226 140