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)