Advanced Polarization Control for Optimizing Ultrafast Laser Micro-Processing
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School of Engineering PhD Thesis Advanced polarization control for optimizing ultrafast laser micro-processing Thesis submitted in accordance with the requirements of the University of Liverpool for the degree of Doctor in Philosophy by Olivier Allègre February 2013 Advanced polarization control for optimizing ultrafast laser micro-processing 2 Declaration I hereby declare that all the work contained within this dissertation has not been submitted for any other qualification. Signed: Date: 3 Declaration Advanced polarization control for optimizing ultrafast laser micro-processing 4 Summary The ability to control and manipulate the state of polarization of a laser beam is becoming an increasingly desirable feature in a number of industrial laser micro-processing applications. Being able to control polarization would enable the improvement of the efficiency and quality of processes such as the drilling of holes for fuel-injection nozzles, the processing of silicon wafers or the machining of medical stent devices. This thesis presents novel, liquid-crystal-based optical setups for controlling the polarization of ultrafast laser beams, and demonstrates how such optical setups can be used to improve laser micro- processing efficiency and quality. Two experimental strategies were followed: the first used dynamic control of the polarization direction of a linearly polarized beam; the second generated beams with complex polarization structures. Novel optical analysis methods were used to map the polarization structures in the focal region of these laser micro-processing setups, using Laser Induced Periodic Surface Structures (LIPSS) produced on stainless steel sample surfaces at low laser fluence (around 1.5J/cm²), close to the ablation threshold of steel (i.e. 0.16J/cm²). This helped to characterize and calibrate the optical setups used in this thesis. The first experimental method used a fast-response, analogue, liquid-crystal polarization rotation device to dynamically control the direction of linear polarization of a laser beam during micro- processing. Thanks to its flexibility, the polarization rotator could be set-up in various synchronized configurations, for example keeping the polarization direction constantly perpendicular to the beam scanning motion. Drilling and cutting tests were performed on thin (~0.4mm thick) stainless steel sheets using a 775nm femtosecond laser at 24J/cm². The experimental results showed a consistent improvement in the micro-processing quality when the polarization direction was synchronized with the beam scanning motion. The sidewall surface roughness and edge quality of the machined structures were improved significantly, with the dimensions of ripples and distortions divided by a factor of two. The overall processing efficiency was also increased compared to that produced by linear or circular polarizations. The second experimental method used a digital, Liquid-Crystal On Silicon (LCOS) Spatial Light Modulator (SLM) to generate polarization structures with a cylindrical geometry, or Cylindrical Vector Beams (CVBs). A Jones matrix analysis was used to model the optical setup and predict the ability to produce CVBs in this way. The setup was implemented in a 775nm femtosecond laser micro-processing bench and the resulting polarization analyzed with a polarizing filter, demonstrating a polarization purity better than 84%. The amplitude and polarization properties in the focal region of the setup were studied using LIPSS produced on the surface of stainless steel samples at low fluence (1.5J/cm²), to check that the expected state of polarization had been achieved. An analytical model of the experimental setup was developed to explain the experimental results. The model predictions were in agreement with the 5 experimental results and clarified how the polarization and phase structures affect the focal properties of the produced laser beams. Various types of CVBs were used with a high laser beam fluence (24J/cm²) for micro-machining 0.2- 0.4mm thick stainless steel plates. A comparative analysis of micro-machining with radially, azimuthally, circularly and linearly polarized beams was carried out. It was shown that a radially polarized beam was more efficient at drilling and cutting high-aspect-ratio features when the plate thickness was above 0.2mm. The gain in processing speed was better than 5% compared with a circularly polarized beam and better than 10% compared with an azimuthally polarized beam, under the chosen processing parameters. However the processing speed was similar for all these polarization states (radial, azimuthal and circular) when machining 0.2mm thick plates. It was also shown that a radially polarized beam improved the processing quality, reducing the distortions affecting the edge quality of the machined structures. Advanced polarization control for optimizing ultrafast laser micro-processing 6 List of publications to date by author Allegre O. J., Perrie W., Edwardson S. P., Dearden G., Watkins K. G., 2012, Laser microprocessing of steel with radially and azimuthally polarized femtosecond vortex pulses, J. Optics 14 (8): 085601 Allegre O. J., Perrie W., Bauchert K., Liu D., Edwardson S. P., Dearden G., Watkins K. G., 2012, Real-time control of polarisation in ultra-short-pulse laser micro-machining, Appl. Phys. A 107 (2): 445-454 Allegre O. J., Perrie W., Edwardson S. P., Dearden G., Watkins K. G., 2011, Ultra-short pulse laser micro- machining of metals with radial and azimuthal polarization, Proc. ICALEO 2011: 917-925 Allegre O. J., Perrie W., Bauchert K., Dearden G., Watkins K. G., 2010, Real-time control of polarisation in high-aspect-ratio ultra-short-pulse laser micro-machining, Proc. ICALEO 2010: 1426-1433 Allegre O. J., Perrie W., Bauchert K., Liu D., Edwardson S. P., Dearden G., Watkins K. G., 2010, Real-time control of polarization in ultra-short pulse laser micro-processing, Proc. MATADOR 2010: 553-556 Croft J., Edwardson S. P., Williams C. J., Allegre O. J., Dearden G., Watkins K. G., 2010, Embedding arrays of microspheres with optical trapping for micro scale device manufacture, Proc. ICALEO 2010: 1450 7 List of publications to date by author Advanced polarization control for optimizing ultrafast laser micro-processing 8 Acknowledgements I would like to thank Ken Watkins (my primary supervisor), Geoff Dearden, Stuart Edwardson and Walter Perrie, who have an inexhaustible supply of scientific insight and enthusiasm, which has meant that my Ph.D. has been not only interesting, but also enjoyable. It is impossible to spend four years at the Laser Group without appreciating Doug Eckford, who has persistently made sure that the computers are running smoothly, as well as Eamonn Fearon who has tirelessly ensured the smooth running of the Lairdside Laser Engineering Centre, where most of the experimental work has been done. It has been lovely to meet and work alongside so many members of the Laser Group. On a more personal note, I would like to thank Mary, as well as my family and friends for their unwavering support and acknowledge my proof-readers: Mary, my father and Walter Perrie (who all carefully red this thesis). 9 Contents Contents Advanced polarization control for optimizing ultrafast laser micro-processing 10 Contents Summary ...........................................................................................................................................4 Acknowledgements............................................................................................................................8 1 Introduction ................................................................................................................................. 16 1.1 Background to laser technology ................................................................................................. 16 1.1.1 Principle of the laser ........................................................................................................... 18 1.1.2 Industrial laser manufacturing ............................................................................................ 18 1.2 Introduction to ultrafast laser micro-processing ........................................................................ 20 1.2.1 Mode locking ....................................................................................................................... 20 1.2.2 Chirped pulse amplification ................................................................................................ 22 1.2.3 Laser-material interactions mechanisms ............................................................................ 23 1.2.4 Techniques for laser micro-machining ................................................................................ 26 1.3 Introduction to phase and polarization ...................................................................................... 29 1.3.1 Definition of phase and polarization ................................................................................... 29 1.3.2 Mathematical representation of polarized light: Jones vectors ......................................... 31 1.3.3 Mathematical representation of waveplates: Jones matrixes ............................................ 34 1.3.4 Birefringence ....................................................................................................................... 35 1.3.5 Fresnel’s coefficients and Brewster’s angle ........................................................................ 35 1.3.6 Cylindrical