applications of adaptive optics in femtosecond laser … of adaptive optics in femtosecond laser...
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Applications of adaptive optics in femtosecondlaser material processing
Professor Derryck T. Reid
Ultrafast Optics Group
School of Engineering and Physical Sciences
www.ultrafast.hw.ac.uk
STFC / Photonics KTN - Laser Applications of Adaptive Optics
Ultrafast Optics Group
femtosecond
optical frequency
combs
nanoscalemicroscopy
(of ICs)
ultrafast laser
source development
fs laser machining / waveguide
writing
ultrafast pulse
diagnostics and shaping
optical coherence
tomography (of ICs)
Fundamental research
Applications
Ultrafast Optics Group
Femtosecond laser
material processing &
waveguide writing
Adaptive optics - closed-loop control for metal film ablation
Adaptive optics - open-loop control for waveguide
inscription
femtosecondoptical frequency
combs
nanoscalemicroscopy
(of ICs)
ultrafast laser source
development
ultrafast pulse diagnostics and
shaping
optical coherence
tomography (of ICs)
Fundamental research
Applications
Femtosecond machininga typical Ti:sapphire CPA
Quantity
λp
∆λp
Ep
∆τp
f
Ppeak
Pav
M2
Value
800 nm
>10 nm (FWHM)
0.2 mJ
<150 fs
5 kHz
1.33 GW
1 W
1.3
Femtosecond machiningwhy 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
� no smoothing of edges by melting
Femtosecond machining
offers deterministic
machining with repeatable
machining outcomes
Femtosecond machiningwhat 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
Pulse shapingPulse shaping
adaptive opticsadaptive opticsBeam shapingBeam shaping
adaptive opticsadaptive optics
wavefrontwavefront
sensingsensing
pulsepulse
measurementsmeasurements
workpiece /workpiece /
inspectioninspection
Femtosecond Femtosecond
laserlaser
AO for metal film ablationclosed-loop scheme
control
•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 www.ultrafast.hw.ac.uk
Pulse shapingPulse shaping
adaptive opticsadaptive optics
Pulse shapingPulse shaping
adaptive opticsadaptive opticsBeam shapingBeam shaping
adaptive opticsadaptive optics
wavefrontwavefront
sensingsensing
pulsepulse
measurementsmeasurements
workpiece /workpiece /
inspectioninspection
control
Femtosecond Femtosecond
laserlaser
AO for metal film ablationclosed-loop scheme
• 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
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…
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
AO for metal film ablation closed loop feature shaping
• Adaptive 2D control changes beam to tailor machined feature to match a pre-determined target shape
simulated annealing algorithm
AO for metal film ablation closed loop feature shaping
feature
extraction
cropping /
centrationthresholding
XOR with
target shape
sum all bright pixels
to calculate error
value
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:
� Focused femtosecond pulses cause refractive index increase in certain glasses and crystals, sufficient to create a waveguide
� Transverse scanning method used, but this leads tohighly astigmatic guides
AO for waveguide inscription open-loop control
� Astigmatism can be solved in 2 different ways
� Multi-scan process1: build up a square-profiled waveguide from many parallel index-modification regions
� Slit method2 : place a slit before the machining lens to reduce NA along oneaxis only, maintaining intensity but expanding beam
1. Nasu et al Opt. Lett. 30, 723 (2005); 2. Ams et al, Opt. Express 13, 5676 (2005)
AO for waveguide inscription open-loop control
� Actuator voltage patterns
� Beam profiles
before objective, and resulting waveguide facet images
� Transillumination
images
AO for waveguide inscription index profiles vs mirror shapes
� Some guides written at higher powers show multi-mode behaviour:
AO for waveguide inscription waveguide characterisation
� 980 nm and 1550 nm fibre pigtailed diode lasers launched into waveguides to analyse their guiding properties
� Single-mode (a, b) and multi-mode (c) operation at 1550 nm
AO for waveguide inscription waveguide characterisation
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
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
Publication summaryfull articles are downloadable from www.ultrafast.hw.ac.uk
� HWU adaptive femtosecond pulse shaping workJesus 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, 833-843 (2004)
J. Garduno-Meja, A. H. Greenaway and D. T. Reid
Designer femtosecond pulses using adaptive optics
OPTICS EXPRESS 11, 2030-2040 (2003)
� HWU adaptive femtosecond machining workS. 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, 1100-1104 (2007)
� HWU adaptive femtosecond waveguide inscription workR.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, 12786-12793 (2008)