Grating-waveguide structures and their applications in ... · Implementation in high-power CW...

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

*[email protected]

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:


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


Slide 7

Fabrication


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





Summary

Slide 9


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


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


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



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


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%


Slide 14

Outline





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*/


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




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




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


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



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


*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


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


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

%


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


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


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


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


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


Laser emission spectra for HR and GWS

HR

Wavelength selection + stabilization with intra-cavity GWS

GWS

Slide 28

Outline





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

Date post:	25-Oct-2020
Category:	Documents
Upload:	others
View:	2 times
Download:	0 times

Grating-waveguide structures and their applications in ... · Implementation in high-power CW...

Documents