Applications of adaptive optics in femtosecond laser material processing

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1 Applications of adaptive optics in femtosecond laser material processing STFC / Photonics KTN - Laser Applications of Adaptive Optics Professor Derryck T. Reid Ultrafast Optics Group School of Engineering and Physical Sciences

2 Ultrafast Optics Group femtosecond optical frequency combs nanoscale microscopy (of ICs) fs laser machining / waveguide writing Fundamental research Applications ultrafast pulse diagnostics and shaping optical coherence tomography (of ICs) ultrafast laser source development

3 Ultrafast Optics Group Femtosecond laser material processing & waveguide writing Adaptive optics - closed-loop control for metal film ablation femtosecond optical frequency combs Fundamental research Applications nanoscale microscopy (of ICs) ultrafast pulse diagnostics and shaping Adaptive optics - open-loop control for waveguide inscription optical coherence tomography (of ICs) ultrafast laser source development

4 Femtosecond machining a typical Ti:sapphire CPA Quantity λ p λ p E p τ p f P peak P av M 2 Value 800 nm >10 nm (FWHM) 0.2 mj <150 fs 5 khz 1.33 GW 1 W 1.3

5 Femtosecond machining why use femtosecond pulses? ns laser machining: heat affected zone (HAZ) created HAZ depends on: pulse duration material heat capacity material thermal conductivity fs laser machining multi-photon absorption (dielectrics) E-field ionizes material, forming a plasma material removed by energetic plasma no heat affected zone Femtosecond machining offers deterministic machining with repeatable machining outcomes no smoothing of edges by melting

Femtosecond machining what can I machine?

free electrons, and this is accompanied by local heating and ionization

fibre) laser radiation causes multi-photon ionization, liberating electrons

absorbed by these free electrons, leading to plasma formation and ablation

6 Femtosecond machining what can I machine? Metals (image, Cr on glass film) laser radiation is absorbed by bound and free electrons, and this is accompanied by local heating and ionization leading to plasma formation and ablation Dielectrics (image, SMF28 telecomm fibre) laser radiation causes multi-photon ionization, liberating electrons which create more free electrons by avalanche ionization radiation is absorbed by these free electrons, leading to plasma formation and ablation Semiconductors (image, GaAs wafer) absorption depends on wavelength / bandgap absorption by free electrons 28µm 200µm

AO for metal film ablation closed-loop scheme Femtosecond laser Pulse shaping adaptive optics Beam shaping adaptive optics Pulse shaping only to minimise pulse durations Full pulse shaping:

7 AO for metal film ablation closed-loop scheme Femtosecond laser Pulse shaping adaptive optics Beam shaping adaptive optics Pulse shaping only to minimise pulse durations Full pulse shaping: Garduno-Mejia et al, "Designer femtosecond pulses using adaptive optics," Opt. Express 11, 2030 (2003) + other papers from control pulse measurements workpiece / inspection wavefront sensing

8 AO for metal film ablation closed-loop scheme Femtosecond laser Pulse shaping adaptive optics Beam shaping adaptive optics Concentrate on beam-shaping for controlling the material-processing outcome in the material Familiar with closed-loop schemes that optimise for a desired beam profile, but here we optimise on the machining outcome itself control pulse measurements workpiece / inspection wavefront sensing

AO for metal film ablation control interface GUI in MATLAB for controlling 1D and 2D mirrors simultaneously Manual control used to vary pulse duration /

9 AO for metal film ablation control interface GUI in MATLAB for controlling 1D and 2D mirrors simultaneously Manual control used to vary pulse duration / intensity Home-built 12-bit 64 channel high-voltage driver Interfaced via RS232 serial port Effect of 2D mirror on machined feature size investigated manually

10 AO for metal film ablation features vs 2D mirror profile Uniform and / or cylindrical (de)focusing can be used to change the size and aspect-ratio of holes produced by a single laser pulse circular irradiance distribution (not at exact focus) opposite elliptical irradiance distributions

11 AO for metal film ablation closed loop feature shaping simulated annealing algorithm Adaptive 2D control changes beam to tailor machined feature to match a pre-determined target shape

12 AO for metal film ablation closed loop feature shaping feature extraction cropping / centration thresholding sum all bright pixels to calculate error value XOR with target shape

13 AO for metal film ablation closed loop feature shaping Convergence towards the target as the simulated annealing algorithm proceeded. L to R: starting condition, intermediate result and final annealed hole Final mirror voltages:

$refractive index increase in certain$ create a waveguide Transverse scanning

14 AO for waveguide inscription open-loop control Focused femtosecond pulses cause refractive index increase in certain glasses and crystals, sufficient to create a waveguide Transverse scanning method used, but this leads to highly astigmatic guides

regions Slit method 2 : place a slit before the machining lens to reduce NA along one axis only,

15 AO for waveguide inscription open-loop control Astigmatism can be solved in 2 different ways Multi-scan process 1 : build up a square-profiled waveguide from many parallel index-modification regions Slit method 2 : place a slit before the machining lens to reduce NA along one axis only, maintaining intensity but expanding beam 1. Nasu et al Opt. Lett. 30, 723 (2005); 2. Ams et al, Opt. Express 13, 5676 (2005)

16 AO for waveguide inscription index profiles vs mirror shapes Actuator voltage patterns Beam profiles before objective, and resulting waveguide facet images Transillumination images

17 AO for waveguide inscription waveguide characterisation 980 nm and 1550 nm fibre pigtailed diode lasers launched into waveguides to analyse their guiding properties Some guides written at higher powers show multi-mode behaviour:

18 AO for waveguide inscription waveguide characterisation Single-mode (a, b) and multi-mode (c) operation at 1550 nm

19 Future directions Curved waveguides written using adaptive beam control Neither multiple-pass nor slit-assisted writing is well-suited to inscribing waveguides that turn through 90 Smaller feature sizes Manipulating the ablation threshold using adaptive pulse shaping could be used (in some materials) to obtain smaller feature sizes Alternative target profiles Could optimise machining on other metrics such as hole symmetry Feature shape control using SLM A spatial light modulator (SLM) would offer greater control over the beam profile

20 Acknowledgements Colleagues at HWU Prof. Ajoy Kar Prof. Alan Greenaway Past and present post-docs and PhD students Dr Robert Thomson Dr Stuart Campbell Dr Reda-El Agmy Mr Graeme Craik Dr Jesus Garduno Dr Ian Blewett Past and present Master students Mr Simon Triphan Ms Helene Bulte Mr Alexander Bockelt The Leverhulme Trust

21 Publication summary full articles are downloadable from HWU adaptive femtosecond pulse shaping work Jesus Garduno-Meja, Alan H. Greenaway and Derryck T. Reid Programmable spectral phase control of femtosecond pulses by use of adaptive optics and real-time pulse measurement JOURNAL OF THE OPTICAL SOCIETY OF AMERICA B 21, (2004) J. Garduno-Meja, A. H. Greenaway and D. T. Reid Designer femtosecond pulses using adaptive optics OPTICS EXPRESS 11, (2003) HWU adaptive femtosecond machining work S. Campbell, S. M. F. Triphan, R. El-Agmy, A. H. Greenaway and D. T. Reid Direct optimization of femtosecond laser ablation using adaptive wavefront shaping JOURNAL OF OPTICS A - PURE AND APPL. OPT. 9, (2007) HWU adaptive femtosecond waveguide inscription work R.R. Thomson, A.S. Bockelt, E. Ramsay, S. Beecher, A.H. Greenaway, A.K. Kar & D.T. Reid Shaping ultrafast laser inscribed optical waveguides using a deformable mirror OPTICS EXPRESS 16, (2008)

Recent Advances in Ultrafast Laser Subtractive and Additive Manufacturing

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