Solid State Laser (DPSSL) Amplifier

exhaustedcrumMechanics

Oct 24, 2013 (3 years and 9 months ago)

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A Scalable Design for a High Energy,
High Repetition Rate, Diode
-
Pumped
Solid State Laser (DPSSL) Amplifier

Paul Mason,

Klaus
Ertel, Saumyabrata Banerjee,
Jonathan Phillips,
Cristina Hernandez
-
Gomez, John
Collier



Workshop on
Petawatt

Lasers at Hard X
-
Ray Light Sources

5
-
9
th
September 2011
,
Dresden, Germany


paul.mason@stfc.ac.uk

STFC Rutherford Appleton Laboratory
,

Centre for Advanced Laser Technology and Applications

R1 2.62 Central Laser Facility, OX11 0QX, UK

+44 (0)1235 778301

Motivation


Next generation of high
-
energy PW
-
class lasers


Multi
-
J to kJ pulse energy


Multi
-
Hz repetition rate


Multi
-
% wall
-
plug efficiency



Exploitation


Ultra
-
intense light
-
matter interactions


Particle acceleration


Inertial confinement fusion



High
-
energy DPSSL amplifiers needed


Pumping
fs
-
OPCPA or
Ti:S

amplifiers


Drive laser for ICF


Pump technology for HELMHOLTZ
-
BEAMLINE

Beamline

Facility

HELMHOLTZ
-

BEAMLINE

Amplifier Design Considerations



Requirements


Pulses from 10’s J to 1 kJ, 1 to 10 Hz, few ns duration,
efficiency 1 to 10%



Gain Medium









Ceramic
Yb:YAG

down
-
selected as medium of choice



Amplifier Geometry

Long fluorescence lifetime

Higher energy storage potential

Minimise number

of diodes (cost)

Available

in large size

Handle high energies

Good

thermo
-
mechanical properties

Handle

high average power

Sufficient

gain cross section

Efficient energy extraction

Low quantum defect

Increased

efficiency & reduced heat load

High surface
-
to
-
volume ratio

Efficient cooling

Low (overall) aspect ratio

Minimise ASE

Heat flow parallel to beam

Minimise

thermal lens

STFC Amplifier
Concept


Diode
-
pumped multi
-
slab amplifier


Ceramic Yb:YAG gain medium


Co
-
sintered absorber cladding for ASE suppression


Distributed face
-
cooling by stream of
cold

He gas


Heat flow along beam direction


Low overall aspect ratio & high surface area


Operation at cryogenic temperatures


Higher o
-
o efficiency


reduction of re
-
absorption


Increased gain cross
-
section


Better thermo
-
optical & thermo
-
mechanical properties


Graded doping profile


Equalised heat load in each slab


Reduces overall thickness (up to factor of ~2)

~175K

Modelling


Laser physics


Assumptions


Target output fluence 5 J/cm²


Pump 940 nm, laser 1030 nm


Efficiency & gain


Optimum doping x length product
for maximum

storage

~ 50%


Optimum aspect ratio to minimise
risk of ASE (g
0
D < 3) of ~1.5


Extraction


Extraction efficiency ~ 50%


Thermal & fluid mechanics


Temperature distribution


Stress analysis


Optimised He flow conditions

50%

3.8

Cr
4+
:YAG

Yb:YAG

HiPER

HiLASE

/
ELI

Prototype

DiPOLE

Extractable energy

~ 1 kJ

~ 100 J

~ 10 J

Aperture

14 x 14 cm

200 cm
2

5 x

5 cm

25 cm
2

2 x 2 cm

4 cm
2

Aspect ratio

1.4

1.2

1

No. of slabs

10

6

4

Slab thickness

1 cm

0.7 cm

0.5 cm

No. of doping levels

5

3

2

Average doping
level

0.33 at.%

0.79

at.%

1.65 at.%

HiPER

HiLASE

/
ELI / XFEL

Extractable energy

~ 1 kJ

~ 100 J

Aperture

14 x 14 cm

200 cm
2

5 x

5 cm

25 cm
2

Aspect ratio

1.4

1.2

No. of slabs

10

6

Slab thickness

1 cm

0.7 cm

No. of doping levels

5

3

Average doping
level

0.33 at.%

0.79

at.%

Scalable Design

DiPOLE Prototype Amplifier


Design sized for ~ 10 J @ 10 Hz



Aims


Validate & calibrate numerical models


Quantify ASE losses


Test cryogenic gas
-
cooling technology


Test (other) ceramic gain media


Demonstrate viability of concept



Progress to date


Cryogenic gas
-
cooling system commissioned


Amplifier head, diode pump lasers & front
-
end
installed


Full multi
-
pass relay
-
imaging extraction
architecture under construction


Initial pulse amplification tests underway


Cr
4+

Yb
3+

Ceramic YAG disk with
absorber cladding

Diode pump laser


4 x co
-
sintered ceramic Yb:YAG disks


Circular 55 mm diameter x 5 mm thick


Cr
4+

absorbing cladding


Two doping concentrations (1.1 & 2.0 at.%)

Optical Gain Material

Cr
4+

Yb
3+

35 mm

55 mm

Pump

2 x 2

cm²

PV

0.123

wave

Fresnel limit ~84%

940 nm

1030 nm

Amplifier
Head Design


Schematic


CFD
modelling

Uniform

T
across
pumped region ~ 3K

He flow

40 m
3
/hr ~ 25 m/s @ 10 bar,
175
K

Pump

Pump

Vacuum

Disks

pressure

windows

vacuum

windows

Diode Pump Laser


Built by Consortium


Ingeneric
,
Amtron

&
Jenoptic




Two systems supplied



0
= 939 nm,

FWHM

< 6 nm


Peak power
20
kW, 0.1 to 10 Hz


Pulse duration 0.2 to 1.2
ms


Uniform square intensity profile


Steep well defined edges


~ 80 % spectral power within


3 nm


Good match to Yb:YAG absorption

spectrum @ 175K

Measured

20 mm

20 mm

DiPOLE Laboratory

Cryo
-
cooling
system

2 x 20 kW diode
pump lasers

Amplifier
head

Front
-
end Injection Seed


Free
-
space diode
-
pumped MOPA design


Built by Mathias
Siebold’s

team @ HZDR Germany



Cavity
-
dumped
Yb:glass

oscillator


Tuneable 1020 to 1040 nm





~ 0.2 nm


Fixed temporal profile


Duration 5 to 10 ns


PRF up to 10 Hz


Output energy up to 300 µJ



Multi
-
pass
Yb:YAG

booster

amplifier


6 or 8 pass configuration


Output energy ~ 100
mJ


nsec

oscillator

Booster
pump
diode

Amplifier
crystal

100
mJ

output

Polarisation
switching
waveplate

Initial Pulse Amplification Results


Simple bow tie extraction architecture


1, 2 or 3 passes


Limited by diffraction effects









Injection seed


Gaussian beam expanded to overfill pump region


Energy ~ 60
mJ


Pump

Pump

Seed

Amplified

beam

Spatial Beam Profiles @ 100K, 1 Hz

E = 2.6 J @ 10 Hz

Gain


8

Gain


6

Pulse Energy v. Pump Pulse Duration


3 passes @ 1 Hz












Relay
-
imaging multi (6 to 8) pass extraction architecture is
required to allow >10 J energy extraction at 175K

5.9 J

Onset of ASE loss

Conclusions


Cryogenic gas cooled Yb:YAG amplifier offers potential for
efficient, high energy, high repetition rate operation


At least 25% optical
-
to
-
optical efficiency predicted



Proposed multi
-
slab architecture should be scalable to

at least 1 kJ generating ns pulses at up to 10 Hz


Limit to scaling is acceptable B
-
integral



DiPOLE prototype amplifier shows very promising results


Installation of relay
-
imaging multi
-
pass should deliver 10 J @ 10 Hz



Strong candidate pump technology for generating high
energy, ns pulses at ~ 1 Hz for HELMHOLTZ
-
BEAMLINE



Thank you for your attention!


Any
Questions
?