Ultrashort pulse lasers enable reliable and versatile high precision ablation and surface processing of various materials such as metals, polymers and semiconductors. However, when modifications deep inside the bulk of transparent media are required, nonlinear pulse material interactions can decrease the precision, since weak focusing and the long propagation of the intense pulses within the nonlinear media may induce Kerr self-focusing, filamentation and white light generation. In order to improve the precision of those modifications, simultaneous spatial and temporal focusing (SSTF) allows to reduce detrimental nonlinear interactions, because the ultrashort pulse duration is only obtained at the focus, while outside of the focal region the continuously increasing pulse duration strongly reduces the pulse intensity.<p> </p> In this paper, we review the fundamental concepts of this technology and provide an overview of its applications for purposes of multiphoton microscopy and laser materials processing. Moreover, numerical simulations on the nonlinear pulse propagation within transparent media illustrate the linear and nonlinear pulse propagation, highlighting the differences between conventional focusing and SSTF. Finally, fs-laser induced modifications in gelatine are presented to compare nonlinear side-effects caused by conventional focusing and SSTF. With conventional focusing the complex interplay of self-focusing and filamentation induces strongly inhomogeneous, elongated disruptions. In contrast, disruptions induced by SSTF are homogeneously located at the focal plane and reduced in length by a factor >2, which is in excellent agreement with the numerical simulations of the nonlinear pulse propagation and might favor SSTF for demanding applications such as intraocular fs-laser surgery.
The spatial and temporal behavior of ultrashort pulses has drawn more and more attention. Especially in laser
material processing, such spatio-temporal behavior has significant influences. In this paper we present a brief
analysis on the pulse front tilt (PFT) in simultaneous spatial and temporal focusing (SSTF) mathematically and
in simulations. We apply paraxial field tracing based on the Collins integral for modeling the spatio-temporal
focusing process. Using the shift theorem of the Fourier transformation, we present an explanation of the PFT
in focus for general input pulses. Next, by assuming a Gaussian lateral pulse shape, an analytical solution for
the field distribution at any position in the region is obtained. In this work we take the influence of an initial
PFT before focusing into considerations as well and find potential way to control the PFT during the focusing
process. Finally with the optical modeling software VirtualLab<sup>TM</sup> we present rigorous simulations of the SSTF
to verify our mathematical conclusions.
In this paper we analyze the pulse front tilt (PFT) in simultaneous spatial and temporal focusing mathematically
and in simulations. We apply paraxial field tracing based on the Collins integral for modeling the spatio-temporal
focusing process. Using the shift theorem of the Fourier transformation, we can explain the PFT in focus for
general input pulses. Next, by assuming a Gaussian lateral pulse shape, an analytical solution for the field
distribution at any position in the region is obtained. Compared with previous works, we take the influence
of an initial PFT into considerations as well. The theoretical calculation is valid for incoming pulses with
temporally chirp and/or initial PFT. Finally with the optical modeling software VirtualLab™ we presented
rigorous simulations of the SSTF to verify our mathematical conclusions.
Femtosecond lasers are a versatile tool to process transparent materials like glasses, polymers or ophthalmic tissue.
However, when focusing pulses of several μJ into the material, the high intensity near the laser focus leads to undesired nonlinear side effects like self-focusing and filamentation, resulting in an increased length of the induced plasma or the fragmentation of the breakdown volume. To overcome this limitation, we studied the influence of simultaneous spatial and temporal focusing (SSTF) on the laser induced optical breakdown (LIOB) in water. For this purpose, the incoming laser pulse is spectrally separated by a grating stretcher setup and recompressed by the focusing optics. Due to the increased pulse duration outside of the laser focus, the nonlinear laser-material interaction is confined to the focal region. We investigated the formation of the plasma and the resulting disruption in water by shadow imaging. With conventional focusing (τ = 70 fs, NA = 0.1) self-focusing, filamentation and breakup of the disruption volume was observed for pulse energies > 2 μJ, leading to a breakdown length of ~ 800 μm at a pulse energy of 8 μJ. With SSTF the axial length of the breakdown is significantly reduced by a factor of ~ 2. Plasma formation and the resulting disruption stay within the focal region. No self-focusing could be observed for pulse energies up to 8 μJ. Therefore, SSTF appears to be a promising tool to induce photodisruptions in transparent materials even with low numerical aperture, e.g. for precise fs-laser surgery within the posterior segment of the eye.
We demonstrate a femtosecond fiber laser system delivering >5-μJ, sub-400-fs pulses at a pulse repetition rate of
200 kHz. At constant average power the pulse repetition rate of this Watt-level femtosecond laser can be adjusted up to
several MHz. The laser is monolithically integrated from the oscillator to the booster amplifier stage. The system was
applied for structuring metallic as well as transparent media as e.g. biological tissues in ophthalmology.