Passively Q-Switched Nd:YAG Laser for Spaceborne Laser Altimetry –

Bepi Colombo (Mercury) Altimeter

J. Neumann, S. Hahn, R. Huß,R. Wilhelm, M. Frede, D. Kracht

Overview

• BELA laser requirements & conceptual laser design

• Testbed setup for feasibility study– Laser performance– Temperature dependencies

• Optical & mechanical design of prototype models (PM)

BELA - BepiColombo Laser Altimeter

Transmitter Optics/Detector

DLR Berlin (D)

Receiver Optics/Detector

University of Bern (CH)

Electronic Unit (DPU)

DLR Berlin (D)

Solid-State Laser

Max Planck Institute for Solar System Research (D)

BELA: 11kg, 40W

BELA Diode Pumped Solid-State Laser

Laser Electronic Unit

Von Hoerner & Sulger GmbH

Pump Diode Unit

DILAS Diodenlaser GmbH

Oscillator Amplifier Box

Laser Zentrum Hannover e.V.

for

BELA MPS Laser Technology Preparation Program:

BELA Laser Requirements

– Pulse energy >50mJ– Pulse duration <10ns– Beam quality M²<1.6– Repetition rate 10Hz – Wavelength 1064nm– Radiation 100krad– Op. temperature 20-45°C (laser head)

18-33°C (pump diodes)

BELA Laser System Concept

Pump diode oscillator:

150W peak

Pump diode oscillator:

150W peak

Pump diode amplifier:

1000W peak

Pump diode amplifier:

1000W peak

Laser oscillator:

~ 2mJ

Laser oscillator:

~ 2mJ

Amplifier 2nd stage:

50mJ

Amplifier 2nd stage:

50mJ

Amplifier 1st stage:

~ 22mJ

Amplifier 1st stage:

~ 22mJ

BELA Laser Design Concept

• Fiber coupled pump diodes– Separation of pump source and laser head

• Longitudinal pumping scheme– Optimized overlap pump beam / laser mode– Higher efficiency– Long absorption path in laser crystal

• q-cw pumping– 200µs pump pulse duration as a trade off between

efficiency and output energy

• Passive Q-switching with saturable absorber Cr4+:YAG– Simplicity– Low mass– Low power consumption

• 2-stage amplifier– Splitting of energy into 2 stages to avoid self-lasing

Dic

hroi

c co

atin

gD

ichr

oic

coat

ing

Cr

:Y4+

AG

Nd:

YAG

Nd:

YAG

Out

put c

oupl

er

Beam expander

Master Oscillator Power Amplifier (MOPA)

Miniaturized Testbed

Mass of optical components ca. 40g

Outputcoupler

Ø = 800 mNA = 0.22

µFiber

Nd:YAG Cr :YAG4+

120W

Laser Oscillator

• Pulse energy: 2.4 mJ• Pulse duration: 2.8 ns• Beam quality: M2=1.2

• Peak-pump power: 104 W• Pump duration: 200 µs• Opt.-opt. efficiency: 12 %

Dichroic coatingAR808nm, HR1064nm

Amplifier: 1st Stage

0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0

15

20

25

30

35

40

45

50

55

Out

put E

nerg

y [m

J]

Input Energy [mJ]

Pump Power 505 W 1000 W

Saturated effective amplification @ > 2 mJ seed energy

0 200 400 600 800 1000 12000

10

20

30

40

50

60

70

Out

put p

ulse

ene

rgy

[mJ]

Total amplifier pump power [W]

2 amplifier stages 1 amplifier stage

Input pulse energy 2.4 mJPulse width 2.8 nsPump pulse duration 200 µs

Additional lossesdue to ASE ofthe amplifiers

Power Amplifier

>50mJ @ 2 x 550 W, M²<1.5

Optical-to-optical efficiency: ≈23% @ 200µs

140 160 180 200 220 24010

20

30

40

50

60

Out

put E

nerg

y [m

J]

Pump Duration [µs]

0,20

0,22

0,24

0,26

0,28

0,30

Opt

ical

to O

ptic

al E

ffici

eny

Optical-to-optical Laser Efficiency

Pump Light Absorption

800 802 804 806 808 810

50

60

70

80

90

100

1 % dopant concentration

Crystal length 10 mm 15 mm 25 mm 30 mm

Pum

p po

wer

dep

osit

(%)

Pump wavelength (nm)

803 nm

807 nm

∆λ(FWHM) = 2.5 nm

6 m m

0m m 5m m 10m m 15m m 20m m 25m m

0m m 5m m 10m m 15m m 20m m 25m m

6 m m

Absorption > 95% between 804nm and 809nm for 25mm crystal

Pump diode shift: 0.25nm/K-> 3.75nm for ∆T=15K

Temperature Range:Oscillator Pump Diodes

170

180

190

200

210

220

230

240

20 22 24 26 28 30 32 34 36 38 40 422,30

2,35

2,40

2,45

804 805 806 807 808 809

pum

p tim

e to

gen

erat

e pu

lse

[µs]

oscillator pump diode temperature [°C]

outp

ut p

ulse

ener

gy [m

J]

oscillator pump diode wavelength [nm]

Temperature acceptance range: ∆T > 15K

Temperature Range:Amplifier Pump Diodes

Temperature acceptance range: ∆T > 15K

804 805 806 807 808

49

50

51

52

53

54

5514 16 18 20 22 24 26 28 30 32

Lase

r Pul

se E

nerg

y (m

J)

Pump Wavelength (nm)

Diode Laser Temperature (Amp 1)

Testbed Laser Performance

Pulse energy 54mJ Pump energy 245mJ Pump power 2x 550W +100WPump duration 200µsBeam quality M²<1.5Pulse duration 2.8ns

Temperature acceptance range for pump diodes ∆T > 15K

Requirements for Flight Model• Mass (laser system) 4kg

Laser head <1.3kg• Total ionizing radiation dose 100krad• Vibration level (Sojuz) 26grms

• Operating temperature 20-45°C• Non-operating temperature -40-60°C

• Sealed pressurized box to avoid contamination of optics

Most of the mass due to housing in sealed box

Mass of optical components approx. 40g

2 Prototype Models• Prototype I

Optical functionality for integration in BELA instrument prototype

• Prototype II• Laser operation during thermal cycling

• Verification of thermal model

• Vibration tests of subcomponents

Modular concept: reversible screw joints / O-ring sealed -> easy replacement of non-adequate parts

Prototype Model Optical Materials

• Mirrors, lenses, windows fused silica• Aspheric lenses in pump optics Co550 • Laser crystals Nd3+:YAG

• Passive Q-switch Cr4+:YAG

Materials already used for other space missions, i.e. can be made radiation hard

Planar Testbed

Oscillator

Amplifier 2

Amplifier 1Beamexpander

Pump Optics

Planar design not suitable for pressurized box within mass budget due to pressure induced misalignment

Optical Design for Prototype

Oscillator

Amplifier 2

Oscillator

Amplifier 1

Amplifier 2

Amplifier 1

• 3D Optical Design (Zemax)• Only reflection angles at

bending mirrors have changed compared to testbed

Mechanical Mounts for Optics

1. Pump optics2. Laser crystal

3. Q-switch4. Output mirror5. Beam expander6. Bending mirrors

1

1

1

2

2 2

3

456

6

6

6

Optical Bench

Pump fiberreceptacles

Laser output window

Electrical feedthroughsfor housekeeping

Mounting screws

Mounting

Laser output window

Mounting by M4 screws

Stabilizing Jacket

for stability againsttorsional misalignment

Aluminum alloy

Thermal interface

Sealing Jacket

• Height 25cm• Diameter 12cm (14cm)• Mass ca. 1500g

for pressure tightness

Mechanical Design

• Good volume to surface ratio -> low mass for pressurized box(Prototype Model ~1.5kg, Flight Model <1.0kg)

• Rugged optical bench due to highly symmetrical design (mainly symmetrical radial forces induced by pressure difference)

• Low thermally induced optical misalignment due to almost symmetrical thermal load

• Transport of dissipated heat via massive optical bench

Future Work

• Thermal cycling test end of 2006• Vibration tests of critical subcomponents• Radiation hard optical components

• Mechanical redesign after thermal cycling / vibration tests

• Replacement of screw joints by irreversible joining techniques (welding, soldering, etc.)

Qualification procedure

Contact:

Laser Zentrum Hannover e.V. Jörg Neumann Hollerithallee 8 D-30419 HannoverGermany

E-mail: j.neumann@lzh.de

This work was funded by

• German Aerospace Center (DLR)

• Max Planck Institute for Solar System Research (MPS)

and performed within the framework of the BELA Laser Industrial Team

for