High Power Dense Spectral Combination Using … · Available Lasers and VHGs Christophe Moser, CEO ... (DWM) Volume Holographic Gratings for High Power Lasers DWM ... AHPSL_presentation_Ondax

Ondax,Inc. Page 1 AHPSL conference Sept 17-19 2007

High Power Dense Spectral Combination Using Commercially

Available Lasers and VHGsChristophe Moser, CEO

[email protected]

Contributors:Gregory Steckman, Frank Havermeyer, Wenhai Liu: Ondax Inc.

Christian Wessling, Martin Traub, Dieter Hoffman Fraunhofer Institute for Laser Technology, Aachen, Germany

Stefan HeinemannFraunhofer USA Center for Laser Technology, Plymouth, MI, U.S.A


Content:

Introduction: Coarse vs. Dense Wavelength Multiplexing (DWM)

Volume Holographic Gratings for High Power Lasers DWM

DWM Architecture – 1kW Fiber Coupled Unit

Fabrication and Reliability of Volume Holographic Gratings

Applications

Conclusions


Coarse vs. Dense λ Multiplexing

80X 91X 94X 97X

970

971

980DENSE λ

multiplexing

Commercially AvailableHigh Power Laser DiodeWavelengths

COARSE λmultiplexing


Challenges for Dense λ Muxing

Broad Linewidth:2.5 – 4nm

High TemperatureDependence:~0.3nm/oC

MUXING GRID


Requirement for Dense λ Muxing

Laser Diodes

Combiner

10 X improvement over state of the art laser diodesi.e ~ <0.03nm/oC

10 X improvement over state of the art laser diodesi.e ~ 0.2nm

Temperature Dependence

Linewidth

20 X improvementOver State of The Art dichroicsi.e < 0.5nm

Bandpass


VHG-FAC for Laser Linewidth Reduction

Volume Holographic Gratings (VHGs) in glass are critical for

narrowing the spectral linewidth of High Power Laser Diodes

VHG-FACHPSL

1,00E+06

3,00E+06

5,00E+06

7,00E+06

9,00E+06

1,10E+07

1,30E+07

940 950 960 970 980 990 1000

l [nm]

Inte

nsity

[a.u

.]

I = 90AI = 90A locked P = 70,1 W („free running“* )

P = 67,4 W (locked)VBG with 90% efficiency

* without FAC lensVHGs in FAC lenses improve off-the-shelf diode lasers:

10 X in linewidth : 4nm 0.2nm30 X in temperature dependence: 0.3nm/oC 0.01nm/oC


VHG-FAC Working Principle

Commercially AvailableVHG-FAC CombinesCollimationAnd Wavelength NarrowingInto One


972 973 974 975 976 977 9780

2000

4000

6000

8000

10000

12000

14000

16000

15A 25A 35A 45A 55A

Inte

nsity

[a.u

.]

wavelength [nm]972 973 974 977 978

Wavelength [nm]975 976

960 965 970 975 980 985 9900

2000

4000

6000

8000

10000

12000

14000

16000

15A 25A 35A 45A 55A

Inte

nsity

[a.u

.]

wavelength [nm]

960 965 970 975 980 985 9900

2000

4000

6000

8000

10000

12000

14000

16000

15A 25A 35A 45A 55A

Inte

nsity

[a.u

.]

wavelength [nm]960 965 970 985 990975 980Wavelength [nm]

6nm30nm

Linewidth Reduction Results

Lineshape without VHG-FAC Lineshape with VHG-FAC

“Wavelength Stabilization of HPDL Array - Fast Axis Collimation Optic with Integrated VHG”:C. Schnitzler, S. Hambuecker, O. Ruebenach, V. Sinhoff, G. Steckman, L. West, C. Wessling, D. Hoffman,Ingeneric GmbH, Ondax, Inc., Fraunhofer ILT, Photonics West, Jan. 2007


Effects of Bar Smile and VHG Alignment

972 973 974 975 976 977 9780

2000

4000

6000

8000

10000

12000

14000

Optimum 1° 2° 3°

int

wavelength [nm]

Linewidth narrowing independent of bar smile and up to 3o FAC misalignment

Rolling 1o

Rolling 3o 972 973 974 977 978Wavelength [nm]

975 976

1nm

Off-the-shelf High Power Laser Diodes are collimated andlinewidth-narrowed to 0.2nm and collimated with a single VHG-FAC. These sources meet the requirement for dense wavelength multiplexing.


VHG for Dense λ Muxing (1)

Volume Holographic Gratings (VHGs) in glass provide a solution fornarrow wavelength combiners:

High PowerLaser λ1

High PowerLaser λ2

λ2 – λ1 <1nm

Spectral combining of high power linewidth-reduced laser diodes


Properties of VHG Combiners

BC

• VHG combiners work in reflection• VHG combiners have sub-nanometer reflection bandwidth• Diffraction efficiency depends strongly on angle of incidence

for >90% combining efficiency, use FAC with less than 2 mrad rest divergence and 15 degree angle of incidence


Summary: Dense Wavelength MultiplexingOff-the-shelfHigh Power Laser Diodes

VHG-FAC

Transform off-the-shelf High Power Laser Diodes intosuitable sources for DWM

VHG Combiner~90% Efficiency


Architecture with DWM

GeometricalStacking Polarization

MultiplexingBeam

Transformation

λ1

DWM

λn

Fiber Coupling

Repeat DWM with 80Xnm, 90Xnm, 94Xnm, 97Xnmand combine with coarse dichroic filters

CoarseWM

Dense WavelengthMultiplexing (DWM)

Modules

λ1

DWM

λn

λ1

DWM

λn


978nm

975nm

GeometricalStacking

DWM

DWM

DWM

978nm

975nm

978nm

975nm

PolarizationMultiplexing

Beam Transformation

911nm

908nm

GeometricalStacking

DWM

DWM

DWM

911nm

908nm

911nm

908nm

PolarizationMultiplexing

Beam Transformation

Coa

rse

wav

elen

gth

Mul

tiple

xing 952 Watts

Fiber Coupled

D=600umNA=0.175

1kW Fiber Coupled with DWM70W bars

80W bars


Geometry of the Laser Diode Bars

1kW Fiber Coupled with DWM

0.5% AR Coated Front FacetExtended Locking Range (20nm)



VHGCombiner

2 Wavelength Combiner Block

High Power Laserwith VHG



Average combining efficiency for 6 units 908/911: 89.5%

Average combining efficiency for 6 units 975/978: 79.7%





Facility Unit (Water Cooling, Current Drivers): 1000 x 600 x 1200mm3 (LxWxH)

DWM Laser Diodes


2 4 6 8 10 12 14 16 18 200

102030405060708090

100Goal

ηtransmission98.5% ηdiffraction 90% η

λ locking 95%

Based on performance for 1kW demo

ηtransmission97% ηdiffraction 85% η

λ locking 93%

Cum

ulat

ive

com

bini

ng e

ffici

ency

[%]

Number of beam sources

Outlook Multi-kW with DWM

By multiplexing more wavelengths, the power in the fiber can be increased without increasing the fiber core or its numerical aperture.

With the same performance VHGs used in the 1kW demo, up to 10 sources canbe densely multiplexed with a cumulative efficiency of 70%.

Multi-kW fiber coupledpower can thus be achievedfrom a 600 µm coreand 0.175 N.A. fiber


VHG Fabrication and TestWafer Fabrication – and Automated 100% Wafer Testing

Example of a wafer measurement from the database.All optical parameters of VHG measured at high spatial resolution.

45mm

35mm

150 µmspatial

resolution

efficiency

Efficiency [%]


VHG Damage Threshold

• Pulsed 1064nm • 20Hz Rep. Rate• 20ns Pulse Width• 0.5mm Spot Size

High Damage Threshold : 3.85 J/cm2 or 175 MW/cm2

Samples tested with AR coating meeting MIL-C-675


VHG Environmental Reliability

VHGs Kept at 150oC Show No Measurable Degradation Over Time

0 2000 4000 6000 8000 10000 120001.48640

1.48642

1.48644

1.48646

1.48648

1.48650

1.48652

1.48654

1.48656

1.48658

1.48660

Ref

ract

ive

Inde

x

Time (hrs)0 2000 4000 6000 8000 10000 12000

3.10

3.20

3.30

3.40

∆n (1

0-4)

Time (hrs)

Grating strength (∆n)Index variation


Applications of Kilowatt Laser Diode Systems

0.000001

0.00001

0.0001

0.001

0.01

0.1

1

0.1 1 10 100 1000 10000 100000Power (W)

BPF

ADADAD

Printing

Thermal Marking

Soldering

PlasticsWelding

Deep PenetrationWelding

Metal Sheet Cutting

SelectiveLaser PowderRemelting

CleaningMelting

Hardening

Brazing

CO2

Diode Laser

DPSSL

BPF: Beam Parameter FactorBPF: Beam Parameter FactorEfficiency * Beam Quality Efficiency * Beam Quality


Summary

Volume holographic gratings (VHG) are enabling Dense Wavelength Multiplexing withHigh Power Lasers Diodes by:

narrowing the emission spectra of commercially availablehigh power laser diodes and combining them

Commercially available VHGs are wafer produced and 100% tested.

FAC-VHGs incorporate collimation and line narrowing to reduce packaging costs.

952 W coupled in 600µm, NA:0.175 fiber with DWM by Fraunhofer Institute, Aachen:

Power loss due to wavelength narrowing: 5-10%

Combining efficiency at 908/911: 90%

Combining efficiency at 975/978: 80% cause by mismatch of theslow axis divergence (90% achievable)

Multi-kW fiber coupled power by adding Dense and Coarse wavelength combining

Documents

High Power Dense Spectral Combination Using … · Available Lasers and VHGs Christophe Moser, CEO ... (DWM) Volume Holographic Gratings for High Power Lasers DWM ... AHPSL_presentation_Ondax