Grating-waveguide structures and their applications in high-power laser systems

Transcription

1 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 th June 2016, Buchs

2 What is a Grating Waveguide Structure? Answer: Combination of a sub-wavelength grating and planar waveguide diffraction grating (<) + + field accumulation i.e waveguide air Waveguide substrate Single layer GWS Multilayer GWS Slide 2

3 Outline Grating Waveguide Structure (GWS): Introduction Applications in high-power lasers Polarization selective GWS Polarization and wavelength selective GWS Summary Slide 3

4 Grating Waveguide Structure: Introduction diffraction grating (<) + field accumulation i.e waveguide grating waveguide substrate 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 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 Slide 4 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

5 Grating Waveguide Structure: Introduction 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: 1.0 n=+10-2 C 0 Pow 1.0 = C 0 Pow + 2 nm Slide Wavelength nm Wavelength nm 10 3

6 Grating Waveguide Structure: Introduction 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) Slide 6 Depend on choice of production technique (lithography + etching) and its precision Often costly process calibration required for each new fabrication run

7 Grating Waveguide Structure: Introduction Fabrication resist substrate Resist coating e - or Photo-resist substrate Exposure e - or Laser beam substrate Development Residual resist substrate Cleaning Plasma/chemicals substrate Etching Dry or Wet substrate or Coating Sub-sequent coating, etc substrate Lift-off substrate Slide 7

8 Outline Grating Waveguide Structure (GWS): Introduction Applications in high-power lasers Polarization selective GWS Polarization and wavelength selective GWS Summary Slide 8

9 Polarization state and gratings Linear polarization: linear gratings TM polarization TE polarization grating lines Radial and azimuthal polarization: circular gratings Slide 9 radial polarization azimuthal polarization

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 Slide 10 * Weber et al., Phys. Procedia 12, 21 (2011)

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 TE (azimuthal) polarization TM (radial) polarization (~ 100%) Coupling to a leaky mode Slide 11 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

12 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 Ta 2 O 5 /SiO 2 TE polarization TM polarization (~ 100%) Coupling to a leaky mode R radial = 99.92% (design) R azimuthal =88.2% (design) Generation of beams with radial polarization Slide 12

13 Polarization selective GWS: generation of beams with radial/azimuthal polarization Reflectivity measurement & laser test Slide 13 R azim = 99.8%+/- 0.2% (measured) R radial = 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%

14 Outline Grating Waveguide Structure (GWS): Introduction Applications in high-power lasers Polarization selective GWS Polarization and wavelength selective GWS Summary Slide 14

15 Polarization and wavelength selective GWS: narrow bandwidth and linearly polarized thin-disk laser (beneficial for SHG) The resonant reflection effect* At resonance. 100 % Reflectivity grating waveguide substrate Coupling n air n g n s Coupling condition = k inc + K g i.e. N eff =sin + m*/ phase shift Destructive interference Slide 15 *A. Avrutsky and V.A. Sychugov, Journal of Modern Optics, 36(11), (1989)

16 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 Ta 2 O 5 film (Ta 2 O 5 ) on fused silica substrate 50 nm binary grating etched from top 300 nm Ta O 2 5 Fused silica substrate Measured reflectivity at 1030 nm: 99% Maximum power extracted: 70 W, Optical efficiency: 24.3% (M 2 ~ 1.1) Laser emission bandwidth (FWHM): 25 pm (~ 9 GHz) Degree of linear polarization: > 99% Slide 16 Loss still high M. Vogel, M. Rumpel, et al., Optics Express, 20(4), (2012)

17 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 AOI~10 9L-30nm: R TE = 99.9% 7L-30nm: R TE = 99.7% 5L-30nm: R TE = 99.6% Ta 2 O 5 SiO 2 Fused Silica (substrate) nm 171 nm 120 nm 171 nm 120 nm 171 nm 120 nm 538 nm 236 nm 9-layer design, 30 nm grating depth nm 171 nm 120 nm 171 nm 120 nm 538 nm 236 nm 7-layer design, 30 nm grating depth nm 171 nm 120 nm 538 nm 236 nm 5-layer design, 30 nm grating depth 1,0 1,0 0,9 0,9 Reflectivity 0,8 0,7 0,6 9L-30nm 7L-30nm Reflectivity 0,8 0,7 0,6 9L-30nm 7L-30nm Measurement accuracy 0.2% Slide 17 0,5 5L-30nm 0,5 5L-30nm Simulation Measurement 0,4 0, Wavelength [nm] Wavelength [nm] a) b)

18 Opt-opt efficiency [%] Slide 18 Implementation in high-power CW fundamental mode thin-disk laser GWS as folding mirror HR, convex 3000 mm mm Resonator 60 mm 500 mm 380 mm HR, plane ~ 10 OC, T = 5% Laser Pumping power [W] HR mirror 5L-30nm 7L-30nm 9L-30nm Yb:YAG disk on diamond heat sink, concave 2300 mm Output power [W] Radius [mm] Normalized intensity [arb. units] Opt-opt efficiency [%] 0,8 1.6 RWG Disk 0, ,0 0,4 0,2 5L- 30nm HR, plane Beam radius HR 7L- 30nm HR, convex 0 0, Length Wavelength [mm] [nm] a) Tunability L- OC 30nm HR mirror 5L-30nm 5L-40nm 5L-50nm Pumping power [W] Output power [W]

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 R Fresnel R Leaky Grating: -1 st diffraction order in reflection All power directed to -1 st diffraction order Slide 19 *N. Destouches, M. Abdou Ahmed, et al., Opt. Express 13, (2005)

20 The resonant diffraction effect: Design and spectroscopic characterization (meas. diffraction efficiency) High efficiency (99.8% measured) in the -1 st order under Littrow angle Laser Diffraction order Littrow- Angle L GWS Slide 20 M. Rumpel et al., Optics letters 37(20), , 2012

21 Implementation in high-power CW fundamental mode thin-disk laser (IR) R = 96%, cav 500mm Plane, HR R Disk = 3.85m D = 15 mm d = 130µm Plane, HR 1030 λ = 969 nm 1. Plane, HR passes module θ P Out = 620 L = 56.4 W 2. GWM in Littrow condition Pump spot diameter = 5.5 mm Total resonator length = 2.1m Pumping wavelength: 969 nm Slide 21

22 High-power CW fundamental mode thin-disk laser (IR) Grating: 620W 1.2kW Pump, η opt ~ 51.6 %, M² x = 1.33 ; M² y = 1.22 Output power (1030 nm) in W Output power FM with HR, 4% OC, unpolarized Output power FM with GWM, 4% OC, polarized P Out = 620 W Opt. efficiency in % Slide Pump power (969 nm) in W

23 High-power CW fundamental mode thin-disk laser (IR) Laser emission spectra (HR/ GWM: M 2 < 1.3) > 200 kw/cm 2 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% Slide 23 Normalized counts in a.u. 1,0 0,8 0,6 0,4 0,2 Normalized counts in a.u. 1,0 GWM 0,5 Δλ = 0.02 nm 0,0 1030, , ,14 Wavelength in nm 0, Wavelength in nm GWM HR

24 High-power CW fundamental mode thin-disk laser (SHG Green) 515 nm Plane, HR 1030, HT 515 LBO crystal Concave 500mm, HR 1030, HR nm 2w Beam = 460 µm Plane, HR 1030 θ L = 56.4 Pump: λ = 969 nm Pump spot diameter = 5.5 mm TDL module, 24 passes GWM in Littrow configuration Total resonator length = 2.1 m Pumping wavelength: 969 nm LBO: Type I (CPM), (4x4x15) mm³ Beam diameter in the LBO: 460 µm Slide 24

25 High-power CW fundamental mode thin-disk laser (SHG Green) M² x = 1.34 ; M² y = Output power 515 nm Optical efficiency 515 nm 40 Output power (515nm) in W M x,y ² < 1.3 P green = 403 P pump = 990 W, η opt (P green /P pump ) ~ 40.7 % Opt. efficiency in % Slide 25 Pump power (969nm) in W

26 Implementation in high-power CW multimode thin-disk laser (IR) Qualification tests at very high-power Output beam OC Output power [W] P Out = 620 W Pump power [W] P out HR P out GWS out HR out GWS Efficiency [%] Littrow 400 mm Yb:YAG disk, concave 3640 mm on water cooled diamond heat sink 1550 mm Slide 26 Up to 1788W (>125kW/cm²) reached without damage of the grating!

27 Laser emission spectra for HR and GWS HR GWS Wavelength selection + stabilization with intra-cavity GWS Slide 27

28 Outline Grating Waveguide Structure (GWS): Introduction Applications in high-power lasers Polarization selective GWS Polarization and wavelength selective GWS Summary Slide 28

29 Summary Conclusion GWS enables the generation of high-power beams with radial polarization High-power fundamental mode and multimode SHG in thindisk laser demonstrated using a GWS as polarization and wavelength selective device GWS enables efficiency increase when compared to standard approaches (etalon, TFP) TEM 00 : P 515nm = 403 W 40.7% opt. efficiency MM: P 515nm = 1080 W 39.5% opt. efficiency Outlook LIDT experiments Further power scaling (green) TEM 00 > 1 kw & > 2 kw in MM operation Slide 29

30 Acknowledgment. GA n Thank you for your attention Slide 30