Grating-waveguide structures and their
applications in high-power laser systems
Marwan Abdou Ahmed*, Martin Rumpel, Tom Dietrich, Stefan Piehler, Benjamin
Dannecker, Michael Eckerle, and Thomas Graf
Institut für Strahlwerkzeuge (IFSW), Universität Stuttgart, Pfaffenwaldring 43, Stuttgart,
Germany
Workshop: Optical Coatings for Laser Applications 2016
09th June 2016, Buchs
Slide 2
What is a Grating Waveguide Structure?
Answer: Combination of a sub-wavelength grating and planar waveguide
air
Waveguide
substrate
diffraction grating (<)
field accumulation i.e
waveguide
+ +
Single layer GWS Multilayer GWS
Slide 3
Outline
Grating Waveguide Structure (GWS): Introduction
Applications in high-power lasers
Polarization selective GWS
Polarization and wavelength selective GWS
Summary
Slide 4
Grating Waveguide Structure: Introduction
A GWS is characterized by unique resonances thanks to the excitation of
„true“ guided modes or leaky modes
Resonances can be in
reflection,
transmission or
diffraction
grating
waveguide
substrate
By a proper design of the GWS
parameters it is possible to modulate
the reflected, transmistted, or
diffracted beam from 0 to 100% for a
given polarization, wavelength and
angle of incidence (AOI) due to
interferences or coupling phenomena
diffraction grating (<)
field accumulation i.e
waveguide
+
These phenomena are very sensitive to GWS opto-geometrical
parameters. A precise control of the manufacturing is required
to successfully transform a design to the actual GWS
Slide 5
Opto-geometrical parameters of a GWS are:
Refractive indices (cover medium, substrate and coated layers)
Thicknesses of coated layers
Grating parameters (period, duty-cycle, groove depth, shape)
Deviation of these parameters will lead to detrimental deviation of the
function of the GWS (e.g. spectral shift, reduced polarization selectivity,
reduced diffraction efficiency, etc…)
Examples:
Grating Waveguide Structure: Introduction
1.020 1.030 1.040
0.2
0.4
0.6
0.8
1.0
C 0 Pow
Wavelength nm 103
n=+10-2
1.020 1.030 1.040
0.2
0.4
0.6
0.8
1.0
C 0 Pow
Wavelength nm 103
= + 2 nm
Slide 6
Refractive indices and thicknesses of waveguide (coated layers)
Usually specified by suppliers but not always precisely enough known for
requirements in GWS design
Better to measure them
e.g. by “M-lines spectroscopy”
Accuracy refractive index <10-3
Accuracy layer thickness <5 nm
Grating parameters (period, duty-cycle, groove depth, shape)
Depend on choice of production technique (lithography + etching) and its
precision
Often costly process calibration required for each new fabrication run
Grating Waveguide Structure: Introduction
Slide 7
Fabrication
Grating Waveguide Structure: Introduction
substrate substrate
substrate
substrate
substrate substrate
Resist coating
e- or Photo-resist
Exposure
e- or Laser beam
Development
Etching
Dry or Wet
Residual resist
Cleaning
Plasma/chemicals
Sub-sequent
coating, etc…substrate
Coating
substrateLift-off
or
resist
Slide 8
Outline
Grating Waveguide Structure (GWS): Introduction
Applications in high-power lasers
Polarization selective GWS
Polarization and wavelength selective GWS
Summary
Slide 9
Applications in high-power lasers
Polarization state and gratings
Linear polarization: linear gratings
Radial and azimuthal polarization: circular gratings
radial polarization azimuthal polarization
TM polarization TE polarization
grating lines
Slide 10
Applications in high-power lasers
Polarization selective GWS: Generation of beams with radial/azimuthal polarization (beneficial for material processing*: cutting, welding, drilling)
Common state of the art polarizations are linear or circular (elliptical): homogeneous polarization state over the beam cross-section
Radial or azimuthal polarization = inhomogeneous polarization state over the beam cross-section
* Weber et al., Phys. Procedia 12, 21 (2011)
Slide 11
Applications in high-power lasers
Polarization selective GWS: generation of beams with radial/azimuthal polarization
Structure: circular sub-wavelength grating + fully dielectric multilayer mirror
Principle of leaky-mode grating mirror
Reduction of the reflectivity of the undesired polarization
The orthogonal polarization does not „see“ the grating and exhibits a
reflectivity close to that of the HR mirror without grating
Only the polarization with the lowest losses (highest Reflectivity) will
oscillate in the laser
TM (radial)
polarization
(~ 100%)
TE (azimuthal)
polarization
Coupling to
a leaky mode
Slide 12
Applications in high-power lasers
Polarization selective GWS: generation of beams with radial/azimuthal polarization
Design & Fabrication method: SBIL (Scanning beam Interference Lithography) + RIE
Grating: Period=930 nm, Depth=20-25 nm
Multilayer: 29 (/4) alternating Ta2O5/SiO2
Rradial = 99.92% (design)
Razimuthal =88.2% (design)
TM
polarization
(~ 100%)
TE
polarization
Coupling to
a leaky mode
Generation of beams with
radial polarization
Slide 13
Polarization selective GWS: generation of beams with radial/azimuthal polarization
Reflectivity measurement & laser test
Razim = 99.8%+/- 0.2% (measured)
Rradial = 90% +/- 0.2% (measured)
Demonstration of up to 660 W output power (Opt. Eff. ~ 45-50%),
M²<2.3
DORP (degree of radial polarization): 98.5% +/-0.5%
Applications in high-power lasers
Slide 14
Outline
Grating Waveguide Structure (GWS): Introduction
Applications in high-power lasers
Polarization selective GWS
Polarization and wavelength selective GWS
Summary
Slide 15
Polarization and wavelength selective GWS: narrow bandwidth and linearly
polarized thin-disk laser (beneficial for SHG)
The resonant reflection effect*
At resonance….
Coupling condition
= kinc + Kg i.e.
Neff=sin + m*/
Applications in high-power lasers
grating
waveguide
substrate
nair
ng
ns
phase shift
Destructive interference
100 % Reflectivity
Coupling
*A. Avrutsky and V.A. Sychugov, Journal of Modern Optics, 36(11), 1527-1539 (1989)
Slide 16
Applications in high-power lasers
Polarization and wavelength selective GWS: narrow bandwidth and linearly
polarized thin-disk laser (beneficial for SHG)
Resonant grating mirror: Single-layer corrugated waveguide
300 nm Ta2O5 film (Ta2O5) on fused silica substrate
50 nm binary grating etched from top
Measured reflectivity at 1030 nm: 99%
Maximum power extracted: 70 W, Optical efficiency: 24.3% (M2 ~ 1.1)
Laser emission bandwidth (FWHM): 25 pm (~ 9 GHz)
Degree of linear polarization: > 99%
300 nm Ta O2 5
Fused silica substrate
M. Vogel, M. Rumpel, et al., Optics
Express, 20(4), 4024-4031 (2012)Loss still high
Slide 17
Applications in high-power lasers
Polarization and wavelength selective GWS: narrow bandwidth and linearly
polarized thin-disk laser (beneficial for SHG)
Combination of partial reflector and GWS
(PR=uarter-wave layers sequence)
GWS was designed to operate at an AOI~10°
Measurement of reflectivity @ AOI~10°
9L-30nm: RTE = 99.9%
7L-30nm: RTE = 99.7%
5L-30nm: RTE = 99.6%
Measurement accuracy 0.2%a) b)
Simulation Measurement
1026 1028 1030 1032 10340,4
0,5
0,6
0,7
0,8
0,9
1,0
7L-30nm
9L-30nm
5L-30nm
Re
fle
ctivity
Wavelength [nm]
1026 1028 1030 1032 10340,4
0,5
0,6
0,7
0,8
0,9
1,0
7L-30nm
9L-30nm
5L-30nm
Re
fle
ctivity
Wavelength [nm]
9-layer design,30 nm grating depth
7-layer design, 30 nm grating depth
236 nm
538 nm
236 nm
538 nm
5
1
2
3
4
7
65
1
2
3
4
7
6
9
8
120 nm
171 nm
120 nm
120 nm
171 nm
120 nm
171 nm
120 nm
120 nm
171 nm
120 nm
171 nm
5-layer design, 30 nm grating depth
236 nm
538 nm
5
1
2
3
4120 nm
171 nm
120 nm
SiO2
Ta O2 5
Fused Silica(substrate)
Slide 18
Implementation in high-power CW fundamental mode thin-disk laser
Applications in high-power lasers
Beam radiusResonator
380 mm
650 mm
HR, convex 3000 mm
HR, plane
60 mm
~ 10°
OC, T = 5%
Laser500 mm
Yb:YAG disk on diamond heat sink,concave2300 mm
1.2
0.8
0.4
00 400 800 1200
1.6 DiskRWG HR, convex
OC
HR,plane
Length [mm]
Radiu
s [m
m]GWS
as
folding
mirror
1029 1030 1031 10320,0
0,2
0,4
0,6
0,8
1,0
HR
9L-
30nm
No
rma
lize
din
ten
sity
[arb
.u
nits]
Wavelength [nm]
5L-
30nm
7L-
30nm
1029 1030 1031 10320,0
0,2
0,4
0,6
0,8
1,0
No
rma
lize
din
ten
sity
[arb
.u
nits]
Wavelength [nm]
HR
5L-
50nm
5L-
40nm
5L-
30nm
a) b)Tunability
50 100 150 200 2500
10
20
30
40
50
Op
t-o
pte
ffic
ien
cy
[%]
Pumping power [W]
HR mirror
5L-30nm
7L-30nm
9L-30nm
0
20
40
60
80
100
120
140O
utp
utp
ow
er
[W]
50 100 150 200 2500
10
20
30
40
50
HR mirror
5L-30nm
5L-40nm
5L-50nmOp
t-o
pte
ffic
ien
cy
[%]
Pumping power [W]
0
20
40
60
80
100
120
140
Ou
tpu
tp
ow
er
[W]
a) b)
Slide 19
Polarization and wavelength selective GWS: narrow bandwidth and linearly
polarized thin-disk laser (beneficial for SHG)
The resonant diffraction effect*
Grazing incidence: Coupling of leaky modes
Grating: phase-shift RFresnel RLeaky
Grating: -1st diffraction order in reflection
All power directed to -1st diffraction order
Applications in high-power lasers
*N. Destouches, M. Abdou Ahmed, et al., Opt. Express 13, 3230-3235 (2005)
Slide 20
The resonant diffraction effect:
Design and spectroscopic characterization (meas. diffraction efficiency)
High efficiency (99.8% measured) in the -1st order under Littrow angle
Applications in high-power lasers
GWSLittrow-
Angle L
Laser
Diffraction order
GWSIncidence
Angle L
The same laser
Diffraction order
M. Rumpel et al., Optics letters 37(20), 4188-4190, 2012
Slide 21
Applications in high-power lasers
Implementation in high-power CW fundamental mode thin-disk laser (IR)
POut = 620 W
14°
14°
24-passes module θL = 56.4°
2. GWM in Littrow condition
Plane,
HR 1030
R = 96%, cav 500mm
RDisk = 3.85m
D = 15 mm
d = 130µm
λ = 969 nm
14°
Plane,
HR 1030
1. Plane, HR 1030
Pump spot diameter = 5.5 mm
Total resonator length = 2.1m
Pumping wavelength: 969 nm
Slide 22
0 200 400 600 800 1000 1200 14000
100
200
300
400
500
600
700
800
Ou
tpu
t p
ow
er
(103
0 n
m)
in W
Pump power (969 nm) in W
Output power FM with HR, 4% OC, unpolarized
Output power FM with GWM, 4% OC, polarized
0
10
20
30
40
50
60
70
Opt.
eff
icie
ncy
in
%
Applications in high-power lasers
High-power CW fundamental mode thin-disk laser (IR)
Grating: 620W Output @ 1.2kW Pump, ηopt ~ 51.6 %, M²x = 1.33 ; M²y = 1.22
POut = 620 W
Slide 23
Applications in high-power lasers
High-power CW fundamental mode thin-disk laser (IR)
Laser emission spectra (HR/ GWM: M2 < 1.3)
> 200 kW/cm2 CW intra-cavity power density on grating mirror surface at 620 W
output power and 4% OC transmission (15.5 kW intra-cavity power)
Measured DOLP > 99.8%
1028 1029 1030 1031 1032 1033 1034 1035 10360,0
0,2
0,4
0,6
0,8
1,0
No
rma
lize
d c
ou
nts
in
a.u
.
Wavelength in nm
GWM
HR
1030,02 1030,08 1030,140,0
0,5
1,0 GWM
No
rma
lize
d c
ou
nts
in a
.u.
Wavelength in nm
Δλ = 0.02 nm
Slide 24
Applications in high-power lasers
2wBeam = 460 µm
TDL module, 24 passes
θL = 56.4°
GWM in Littrow
configuration
Plane,
HR 1030,
HT 515
Concave 500mm,
HR 1030,
HR 515
Pump: λ = 969 nm
Plane,
HR 1030
LBO crystal515 nm
1030 nm
High-power CW fundamental mode thin-disk laser (SHG – Green)
Pump spot diameter = 5.5 mm
Total resonator length = 2.1 m
Pumping wavelength: 969 nm
LBO: Type I (CPM), (4x4x15) mm³
Beam diameter in the LBO: 460 µm
Slide 25
Applications in high-power lasers
0 100 200 300 400 500 600 700 800 900 1000 11000
50
100
150
200
250
300
350
400
450 Output power 515 nm
0
5
10
15
20
25
30
35
40
45
Optical efficiency 515 nm
Opt.
eff
icie
ncy in %
Pump power (969nm) in W
Outp
ut
pow
er
(515nm
) in
W
Pgreen = 403 W @ Ppump= 990 W,
ηopt (Pgreen/Ppump) ~ 40.7 %
M²x = 1.34 ; M²y = 1.77
Mx,y² < 1.3
High-power CW fundamental mode thin-disk laser (SHG – Green)
Slide 26
Applications in high-power lasers
Implementation in high-power CW multimode thin-disk laser (IR)
Qualification tests at very high-power
Up to 1788W (>125kW/cm²) reached without damage of the grating!
POut = 620 W
0 1000 2000 3000 4000 50000
200
400
600
800
1000
1200
1400
1600
1800
2000
2200
2400
Ou
tpu
t p
ow
er
[W]
Pump power [W]
Pout
HR
Pout
GWS
out
HR
out
GWS
0
10
20
30
40
50
Effic
ien
cy [%
]
OC
400 mm
Output beam
1550 mm
Littrow
Yb:YAG disk,concave 3640 mm
on water cooleddiamond heat sink
Slide 27
Applications in high-power lasers
Laser emission spectra for HR and GWS
HR
Wavelength selection + stabilization with intra-cavity GWS
GWS
Slide 28
Outline
Grating Waveguide Structure (GWS): Introduction
Applications in high-power lasers
Polarization selective GWS
Polarization and wavelength selective GWS
Summary
Slide 29
Summary
Conclusion
GWS enables the generation of high-power beams with radial
polarization
High-power fundamental mode and multimode SHG in thin-
disk laser demonstrated using a GWS as polarization and
wavelength selective device
GWS enables efficiency increase when compared to
standard approaches (etalon, TFP)
TEM00: P515nm = 403 W → 40.7% opt. efficiency
MM: P515nm = 1080 W → 39.5% opt. efficiency
Outlook
LIDT experiments
Further power scaling (green) TEM00 > 1 kW & > 2 kW in MM
operation
Slide 30
Acknowledgment
.
GA n°. 619177
Thank you for your attention
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