June 28, 2007

High Energy 2-micron Laser Developments

Jirong Yua, Bo C. Trieua, Mulugeta Petrosb, Yingxin Baic, Paul J. Petzarc, Grady J. Kocha, Upendra N. Singha,

Michael J Kavayaa

aNASA Langley Research Center, MS 468, Hampton, VA 23681bScience and Technology Corporation, Hampton, VA 23666

cSAIC, Hampton, VA 23666

2007 Solid State Diode Laser Technology Review

https://ntrs.nasa.gov/search.jsp?R=20070031110 2018-06-20T00:47:20+00:00Z

June 28, 2007

Outline

• Overview 2-micron solid state lasers• Modeling and population inversion

measurement• Side pump oscillator• One Joule 2-µm Laser• Conclusion

June 28, 2007

Solid State 2-micron Lasers

– Tm Lasers (pump diodes 780-805nm)• YAG, YLF, YAlO3, YVO4

– Ho:Tm Lasers (pump diodes 780-805nm)• LuLF, YLF, GdLF, YAG, YVO4

– Tm pumped Ho lasers (pump diodes 780nm)• Tm solid state laser pumped Ho Laser• Tm fiber laser pumped Ho Laser

– Ho Lasers (pump diodes 1900nm)• YAG

– Tm Fiber Lasers

June 28, 2007

Energy transfers between HoEnergy transfers between Ho3+3+ and Tmand Tm3+ 3+ ions ions and Pumpand Pump--probe experimentprobe experiment

0

5

10

153F23F3

3H4

3H5

3H6

3F4

Tm3+

Ener

gy(1

03 cm-1)

SQSQ

5I 8

Ho3+

5I 7

5I 6

probe

pump

5S 2

Pump at 792nm ( 3H6 → 3H4 transition)

Energy transfers :- Self-quenching (PSQ)

two Tm3+ ions in the 3F4 manifold

- (3F4 ⇒ 5I7 ) energy transfer (P(Tm → Ho) )

- (5I7 ⇒ 3F4 ) energy transfer (P(Ho → Tm) )

- Ho , Tm upconversion (PUC )

Probe at 543nm (5I8 → 5S2 transition)Probe the 5I8 ground-state depletion

5I 4

5I 5

Laser

June 28, 2007

Evolution of the probe beam transmission and the corresponding population of the Ho 5I7 manifold

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.60.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.80

Rela

tive

popu

latio

n of

Ho

5 I 7 man

ifold

Time (ms)

Probe beam transmission

0.0

0.1

0.2

0.3

0.4

0.5

Tran

smiss

ion

of th

e pr

obe

beam

(%)

Ho 5I7 relative population

Pump pulse

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.40.0

0.1

0.2

0.3

0.4

0.5

Output pulse

Rela

tive

popu

latio

n of

Ho

5 I 7 leve

l

Time (ms)

Normal Mode Without lasing

Pump pulse

Ho 5I7 population at lasing and without lasing condition

Probe beam transmission and the population of the Ho 5I7 manifold

June 28, 2007

Oscillator features

• Injection seeded• Cavity length >2m Ring• Output coupler Reflectivity ~70%• Diode pump lasers: 36 bars 100W/bar

conductive cooled • crystal doped material length 21mm• undoped LuLF length 15 mm• Laser crystal cooling : H2O• Tube size: 6mm OD 5mm ID AR

coated for 792nm• Laser rod ends wedged 0.5° along c-axis

AR coated for 2.053µm

June 28, 2007

Laser Oscillator Ring Cavity

O.C.

Flat HR (PZT)

Curved HR

Q-Switch

Curved HR

Injection seeding

Laser Head

Scope

Elec. Servo

June 28, 2007

Cavity Mode Simulation (Ring Cavity with two curved high reflectors)

M1 M2 M3 M4M4

X-axis

Y-axis Laser head Q-switch

June 28, 2007

Laser Output Energy

2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4

40

60

80

100

120

140

160

Sing

le Q

-Sw

itch

Puls

e En

ergy

(mJ)

Diode Pump Energy (J)

% (70% output coupler 8oC rod temperature) % (66% output coupler 8oC rod temperature)

2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2

25

50

75

100

125

150

Sing

le Q

-Sw

itch

Puls

e En

ergy

(mJ)

Diode Pump Energy (J)

70% output coupler 6oC rod temperature 70% output coupler 8oC rod temperature 70% output coupler 12oC rod temperature

June 28, 2007

Laser beam profile

0 20 40 60 80 1000.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6M2x: 1.142M2y: 1.060wx: 0.685wy: 0.655Dx: 2.511Dy: 2.400

Beam

Dia

met

er (m

m)

Distance from lens (cm)2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6

100

150

200

250

300

350

400

450

500

550

Q s

witc

hing

pul

se le

ngth

(nS)

Pump pulse energy of diode (J)

Pulse width

June 28, 2007

Seeding verification

2.4MHz

June 28, 2007

MOPA Experimental Diagram

O.C.Curved HR

PZT controlled Flat HR Flat HR

Q-Switch

Diode-pumped LuLiF4

Diode-pmped LuLiF4 Amp1

Diode-pmped LuLiF4 Amp2

Curved HR

Curved HR

Energy Meter

λ/2 Wave plate

λ/2 Wave plate

HR Prism

HR Prism

45° HR

45° HR

45° HR

45° HR

CWSeederLaser

FaradayIsolator

PZT Controller

June 28, 2007

Amplifier features

• Pump energy 7.2Joules12x6 bar arrays with 100w/bar• Diode laser conductive cooled ‘A’Pkg• Laser crystal Ho:Tm:LuLF 0.5% Ho 6%Tm• Doped Crystal length 41mm• Ends diffusion bonded 15 mm undoped LuLF crystals• Laser crystal cooling H2O • Flow tube size 7mm OD 6mm ID AR coated• Rod ends AR coated for 2.053µm flat• Path configuration double pass

June 28, 2007

Absorbed pump power distribution

4.0 mm Diameter Enclosure:

50.8 Absorbed Watts or 70.6%

3.5 mm Diameter Enclosure:

44.5 Absorbed Watts or 61.8%

3.0 mm Diameter Enclosure:

35.1 Absorbed Watts or 48.8%

2.5 mm Diameter Enclosure:

28.6 Absorbed Watts or 39.7%

2.0 mm Diameter Enclosure:

20.7 Absorbed Watts or 28.8%

5.0 mm Diameter Enclosure:

66.0 Absorbed Watts or 91.7%

4.5 mm Diameter Enclosure:

59.2 Absorbed Watts or 82.2%

June 28, 2007

Single and Double Pass Amplification

4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.01.0

1.5

2.0

2.5

3.0

3.5

4.0

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Out

put/I

nput

Pump pulse energy of diode (J)

Double-pass amplificationSingle-pass amplification

The probe pulse: 180nS, 133mJ Rod length: 4.25cmRod temperature: 8oCDiode temperature:15oC

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.00.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

Pul

se e

nerg

y (J

)

Position in the laser crystal (cm)

Single pass amplificationFirst pass amplificationSecond pass amplification

Probe energy (133 mJ)

June 28, 2007

Amplifier Performances

0.1

0.2

0.3

0.4

0.5

0.6

6 7 8 9 10 11 12

Am

plifi

er O

ne O

utpu

t Ene

rgy

(J)

Pump Energy (J)

Probe 150 mJ

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

8 10 12 14 16 18 20Sy

stem

Out

put E

nerg

y (J

)

System Pump Energy (J)

Probe 511 mJ

June 28, 2007

Objective

• Develop a technology that enables the production of a high-energy and a high- efficiency 2 mm LIDAR transmitter capable of measuring global wind from various platforms.

• Enhance the understanding atmospheric phenomena and improve weather prediction accuracy.

• Reduce risks associated with Doppler Lidar transmitter.

• Identify lifetime sensitive components and initiate early testing.

June 28, 2007

Compact Laser Design Goal

• Pulse energy: >250mJ• Repetition rate: 10Hz• Wavelength: 2.053 µm• Laser material: LuLF 0.5% Ho, 6%Tm• Pulse length: > 100ns• Line width: < 2.5 MHz• Heterodyne frequency offset: 105 MHz• Beam quality: <1.3 diffraction limit• Beam size: 6 mm at the amplifier

output

June 28, 2007

Environment Requirements

• Platform: ground-based (Airborne qualify-able)

• Operational Temperature 0°C -30°C• Operating Altitude Range Sea level to 30,000 ft• Vibration 2.0 g-rms• Coolant Temperature 5 °C and 15 °C • Coolant Flow

– Laser rod .5 GPM– Diode Laser 2 GPM– Bench 2 GPM

• Coolant Pressure 50 psi at 6 GPM

June 28, 2007

Mechanical Design Guidelines

• Laser enclosure compact, sealed, and dry air purged

• Optical benchpopulated on both sidestemperature controlled

• Optical mountshardened- space laser inherited Optical height 1 inch

June 28, 2007

Enclosure & Optical Bench

11.5”

6.5”

June 28, 2007

Optical Layout Side 1

26.5”

11.5

June 28, 2007

Optical Layout Side 2

June 28, 2007

3m Long Ring Resonator Design

• The rod is placed between two curved mirrors.

• Angles between the folding mirrors minimized.

• In the final configuration a 4m radius of curvature mirror is selected.

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

0 500 1000 1500 2000 2500 3000

TEMoo Mode size for various curved mirrors

3m ROC4m ROC5m ROC3.5m ROC

mod

e ra

dius

(mm

)

distance from Output mirror (mm)

QS

ROD

June 28, 2007

10Hz Oscillator Performance

June 28, 2007

Seeding Verification

Seeded pulse has no mode-beating Unseeded pulse

June 28, 2007

Oscillator Line Width

2.4MHz

June 28, 2007

Amplifier Architecture Considerations

Osc.2

Osc.3

Option 3 selected - Minimum loss, No optical damage, and Optical distortion corrected

Osc.

Mirror at rod end

1

Half waveplate

FaradayIsolator

Half waveplate Amplifier

June 28, 2007

Amplifier rod size selection

907045Probe E (mJ)

2151901701581191084mm rod E. (mJ)

1801721341368296895mm rod E. (mJ)

432432432Probe dia. (mm)

Single pass gain for 4mm rod ~ 2.3 4mm rod with a 3mm probe performs better.

• 4 and 5 mm diameter rods were compared• Probe energy and size were varied

June 28, 2007

4.0 mm Diameter Laser Rod Absorption

4.0 mm Diameter Enclosure:

63.1 Absorbed Watts or 87.6%

3.5 mm Diameter Enclosure:

53.7 Absorbed Watts or 74.6%

3.0 mm Diameter Enclosure:

44.3 Absorbed Watts or 61.5%

2.5 mm Diameter Enclosure:

35.3 Absorbed Watts or 49.0%

2.0 mm Diameter Enclosure:

25.7 Absorbed Watts or 35.7%

June 28, 2007

Amplifier Thermal Lensing

• Amplifier thermal lensing is -1.1m in the x-axis and -1.8m in y-axis.

• To reduce this effect the c-axis of the amplifier rod is oriented orthogonal to the oscillator rod.

• Once the thermal lensing was measured, the parameter is used in an optical model and a cylindrical correction lens was chosen and implemented that circularized the beam.

June 28, 2007

Double pass amplifier performance

50

100

150

200

250

300

60 70 80 90 100 110 120 130

10Hz Amplifier Double-pass Performance

out

put(m

J)

current(A)

probe=95 mJ

Rod temperature=8°C

50

100

150

200

250

300

350

60 70 80 90 100 110 120 130

Double pass amplifier performance

double pass output @5°C

out

put (

mJ)

current(A)

probe=101mJ

Amplifier gain: double pass ~3

June 28, 2007

Conclusion

• A diode-laser-side-pumped 2 µm Ho:Tm:LuLF laser oscillator and two amplifiers (MOPA) have been developed

Master OscillatorMaster Oscillator MOPAMOPAOutput energy 142 mJ (SP) 1.2 J(SP)Optical efficiency 4.3 % 6.5 %