'Load-to-Fracture within a laser-head' - A new
approach to determine the fracture parameters of
Nd:YAG lasing elements
R. Feldman, S. Jackel, I. Moshe, A. Meir , S. Shimony
R. Feldman; [email protected]
Soreq Research Center, Yavne 81800, Israel
Optical Engineering 2014 ‫הכנס להנדסה אופטית‬
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INTRODUCTION
Laser gain components may fracture upon exposing to high pump
power.
Fracture starts at surface defects arise from manufacturing processes.
To decrease fracture tendency
 Improve material quality.
 Increase component size ( decrease thermal loading).
 Make a tougher material (by surface treatment)
 Use special (composite) components.
Traditional strength measurement is performed by the 4-point
flexture strength (bending) technique.
In the present study, however, a new technique was introduced, based
on optical loading within a laser head (pumping).
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The problem: Fractured of large size
laser rods
a
b
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Fracture mechanics of brittle materials
Theoretical tensile strength
 thf 
E SE
E

a0
10
EYAG= 310 GPa,
theoretical = 31,000 MPa,
actual= 133-315 MPa (single crystal)
σactual=229-378 MPa (ceramic)
Figure-of-Merit for thermal stress resistance:
Fracture mechanics relates the tensile
strength to fracture toughness, KIc using :
It is possible to improve the tensile strength of
brittle materials by reducing the flaw size
RT 
 f  ( 1  )
E
f 
K Ic
Y a
γ – surface energy,
k – thermal conductivity,
 – Poisson's ratio,
α – thermal expansion coefficient,
E – elastic modulus
a – depth of the flaw which leads to failure
Y – geometry factor (about unity)
KIc – fracture toughness - measure the
material’s resistance to crack
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Thermal stress profiles within a laser rod
800
TENSILE STRENGTH, (MPa)
Tensile strength distribution
3 components of the normal stress:
 r2
 E
1
 r ( r )   Trod   2  1  
4
 Rrod
 1 
 ( r ) 
 3r
 E
1
 Trod   2  1  
4
 Rrod
 1 
2
 2  r2
 E
1
 z ( r )   Trod   2  1  
2
 Rrod
 1 
Temp. difference between the rod center and surface:
Trod 
1
4 

Ph
,

Max. tensile stress on the rod surface:
1
E
 max   Trod 
2
1 
600
within 10 mm dia. Nd:YAG rod
400

z
200
0
r
-200
-400
radial (r)
tangential ()
axial (z)
-600
-800
0
1
2
3
4
5
DISTANCE FROM ROD CENTER , r (mm)
 max 
E
P
 h
8 ( 1   ) 
The max. stress theory
(W.J.M. Rankine)
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Tensile-Strength measurement of optical
materials by “load-to-fracture”
Thermal Shock: rapid water cooling of preheated specimen.
Static Temperature Gradient: by simultaneous heating and
cooling.
Transient thermal shock: using a CW CO2 laser.
Mechanical bending: 3- or 4-point bending.
Thermal loading within a laser-head.
First attempt to use the technique of thermal loading
First
attempt
to use
the latter
technique
of a
laser rod
within
a laser-head
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Tensile-Strength measurement:
Mechanical bending vs. thermal loading
Four-point fluxure test
F
Thermal loading within a laser-head
Cooling water
Loading member
y
Test specimen
Cylindrical
bearing
Support
member
x
h
Laser - Head
b
L

Pump beam
max

3Fc
4bh2
D
D2
Tensile stress on
lower surface of
bar
Concentrating lens


Diode-laser array

 max 
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r
z
P
E
 h
8(1  ) 
Ring-on-Ring fluxure test
Weibull analysis for the strength of small
Nd:YAG rods within a laser head
Nd:YAG laser rods
500
Un-treated p.-fractured
400
Un-treated s.-fractured
400
Treated_1 p.-fractured
Treated_2 p.-fractured
300
Treated s.- not fractured
5 rods
300
p. = polycrystalline
s. = single crystal
200
200
100
100
PROBABILITY OF FAILURE
Nd:YAG Laser rods
mm2, 1.1 at.% Nd
FRACTURE TENSILE STRENGTH f (MPa)
FRACTURE LONGITUDINAL HEAT POWER DENSITY Phf
~ (W/cm)
Unstrengthened
Strengthened
1.0
500
ceramic
(3.258 mm)
0.8
crystal
crystal
ceramic
Unfractured
0.6
0.4
0.2
0.0
0
100
200
300
400
500
THERMAL LOADING, Ph/ (W/cm)
0
0
0
10
20
30
40
50
DIODE CURRENT I (A)
Thermal loading, Ph/, replaces
the tensile strength, .
Referring to non-fractured
rods under the highest power
limit as if being fractured.
Probability of failure
P
i  12
N
Maximum thermal loading of
strengthened Nd:YAG laser rods:
Ceramic:
320 W/cm
Single crystal: > 434 W/cm
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Scaling factors obtained by Weibull analysis
for poly- and single-crystalline Nd:YAG
Treatment
Single
crystal
Polycrystalline
Untreated
321±16
345±17
Etched
1119±61
566±28
Fracture heat power
density, Phf
(W/cm)
Untreated
170±33
108±6
(within a laser-head)
Etched
434±22
276±21
317±29
Measure technique
Fracture tensile strength,
σf ,
(MPa)
(by bending)
Strengthened poly-crystalline elements exhibit
lower strength compared to Nd:YAG single crystal
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Defects in crystalline Nd:YAG elements
Defects resulted from crystal growth process
Microscopic defects: mainly dislocations such as
# Edge dislocations
# Zigzag dislocations
# Helices dislocations
Macroscopic defects: facets, scattering particles, bubbles,
constitutional supper cooling (striations)
Can be avoided by proper selection of the crystal
Defects induced in laser components
Scratches, voids, foreign particles, and contaminants at the element
outer faces – arise from the manufacturing process
Can be avoided by careful manufacturing process
Fracture of a laser component starts at defects
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After
etching
scratches
(a)
After-etching defects
(b)
0.5 mm
0.5 mm
(c)
Various defects on the
cylindrical surface of the
laser rod could be identified
after the final polishing and
coating process.
(d)
0.2 mm
(e)
0.5 mm
(f)
0.2 mm
0.2 mm
In spite of our efforts,
these defects could not be
avoided.
In addition, an unidentified organic material
penetrated into the rough
surface of the rod.
After etching un-identified
metallic spots
Such defects may lead to catastrophic
damage to the laser element
Non- etched
etched
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Surface absorption
Surface absorption may result from –
• “Beilby layer”
• Remains of fixing / bonding materials
• Micro-scratches
Temperature increase is proportional to the energy absorbed
per unit length, causing hot-spots.
Absorption:
Eabs=k·t·(As+β·l)
As=2·αs·δ
A special cleaning procedure was used to removes surface
contaminations.
Rods have not being fractured after their final cleaning
1. N. Bloembergen, IEEE J. Quantum Electron. QE-10, pp. 375-386 (1974).
2. G. Mann, G. Phillipps, Optical Materials 4 (1995), pp. 811-814.
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Fabrication process for enhanced laser rods
Commercial fabrication
process
Out-drilling of raw rods
from preferred sites
Center-less grinding
Polishing
AR coating
Special strengthening
procedure
Fine grinding of the cylinder
Thermo-chemical etching
Re-polishing & Re-coating
Special cleaning procedure
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Nd:YAG laser rods
240
mm2
250
~
Untreated - fractured
200
1 rod
Treated - not fractured
Treated by supplier - fractured
200
160
2 rods
120
.
9 rods
150
1 rod
100
80
50
40
0
0
0
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2
4
6
8
Pabs (kW)
~
~ A 
PhB  PhS   S 
 AB 
1
mS
FRACTURE TENSILE STRENGTH f (MPa)
FRACTURE LONGITUDINAL HEAT POWER DENSITY Phf (W/cm)
Fracture Heat Power Density vs. Absorbed
pumped power for large rods
High strength of the chemicallyetched Nd:YAG laser rods of full size:
Thermal Loading:
Tensile Strength:
125 - 200 W/cm
125 - 225 MPa
Weibull analysis of full-size Nd:YAG rods
(ϕ8-10 x 205 mm)
2
1.0
m=9
~
Ph0=(1096) W/cm
Nd:YAG Laser rods
PROBABILITY OF FAILURE Pf
ln {ln [1/(1-Pf)]}
m = 11
~
Ph0=(84W/cm
0
m=
~
Ph0=(1407) W/cm
-2
m=
Ph0=(1136) W/cm
~
-4
4.2
4.4
4.6
~
ln Phf
4.8
(b)
(8-10205 mm2)
0.8
Possible failure zone
(a)
0.6
0.4
11 rods
0.2
1 rod
not fractured
not fractured
0.0
5.0
0
50
100
150
~
FRACTURE LONGITUDINAL HEAT POWER DENSITY Phf (W/cm)
Fracture:
 Strengthened rods did not fracture at the
highest pump power of the laser.
 Rods fractured after re-polishing and re-coating
Probability of failure of full-size rod, P:
 Un-treated rods:
P < 45 W/cm
 Strengthened rods: P > 110 W/cm
~
~ A 
PhB  PhS   S 
 AB 
Final cleaning procedure required
1
mS
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Maximum pump-power density applied on
single crystalline Nd:YAG laser rods
Source
Maximum pumppower density
(W/cm)
Reference
115 a
[B2]
R.L. Byer, 1985
(Stanford, USA)
150 mm
W. Koechner, 1996/2006
Not specified
115 a/150 a
[K1]/[K0]
J.J. Chang, 1998
(LLNL, USA)
5 × 150 mm2
1.0 at.% Nd
160
[C1]
W. Schöne et al., 1997
(Hannover, Germany)
6 × 110 mm2
0.5 at.% Nd
250
[S2]
A. Takada et al., 1999
(Toshiba, Japan)
8 × 118 mm2
0.8 at.% Nd
380
[T1]
Present work
10 × 205 mm2
0.9 at.% Nd
500
[F1]
(a)
16
Laser rod
Probably calculated from tensile strength:
 max 
P
E
 h,
8(1  ) 
Summary
Unique method was developed for strength measurement of laser
rods within laser heads.
This method was applied on polycrystalline (ceramics) and single
crystalline laser rods.
Results indicate the reliability of the new technique.
Together with the strengthening process, the new technique enabled
to demonstrate a multi-KW CW laser output from a chain of
amplifiers.
It is more instructive to examine the durability of Nd:YAG laser
elements by their thermal loading within a laser head, as this is the
environment these components operate.
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Thanks for your attention !
R. Feldman; [email protected]