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
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: [email protected]
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