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Simultaneous spatial and temporal focusing of femtosecond pulses: a new paradigm for material processing and tissue ablation
Block, Erica K.
Block, Erica K.
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2015
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Femtosecond lasers are now prolific in many disciplines. While the mechanisms of femtosecond-material interactions are widely understood, femtosecond lasers as industrial and medical tools still have shortcomings. Currently conventional state of the art platforms are unable to support low numerical aperture (NA) beams (that provide large focal volumes and long working distances) without sacrificing axial precision. Furthermore inline (refractive) delivery systems that are necessary for industrial and clinical medical applications are currently hindered by nonlinear effects when delivering femtosecond pulses with tens of microJoule pulse energies and greater. In this thesis Simultaneous Space Time Focusing (SSTF) is presented as a new paradigm to move the field of femtosecond micromachining significantly forward. With this system we have delivered microjoule femtosecond pulses with low numerical aperture geometries (<0.05 NA) with characteristics that are significantly improved compared to standard focusing paradigms. Nonlinear effects that would normally result in focal plane shifts and focal spot distortion are mitigated when SSTF is employed. As a result, it is shown that SSTF will enable surgical implementations that are presently inhibited. Initial ablation experiments of ocular tissue show unprecedented performance with this technique. Implementation of SSTF, in the past, has been overly complicated. Multiple compressors and diffraction gratings resulted in low throughput efficiency. In the second part of this thesis we have focused on significantly streamlining the SSTF design into a flexible, single grating, integrated SSTF/chirped-pulse amplification system with an inline (refractive) delivery system to move towards industrial and clinical medical applications. For the first time this design also allows for variation of the beam aspect ratio of an SSTF beam, and thus the degree of pulse-front tilt at focus, while maintaining a net zero-dispersion system. Accessible variation of pulse front tilt gives full spatiotemporal control over the intensity distribution at the focus and another degree of freedom in ablation processes. Finally, real-time visualization of the femtosecond machining process is vital for industrial/medical applications, especially in medical where imaging is through scattering materials. At present a secondary imaging laser system is needed in conjunction with the surgical laser. Using complex, off the shelf, refractive optics we have created a real-time, inline (refractive), delivery system that is robust to scattering and integrated within a single machining system. Notably the design decouples the imaging field of view (FOV) and resolution of the cutting beam despite it being the product of a singular system.
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