Femtosecond Excimer Laser Research Articles

Innovations in Femtosecond Laser Technology – The Use of the WaveLight® FS200 Laser for Flap Cutting during LASIK Surgery

Laser-assistedin situkeratomileusis (LASIK) involves the creation of a corneal flap and remodelling of the corneal stroma. This conventionally used a mechanical microkeratome but these devices are associated with variations in the flaps produced. The development of femtosecond excimer laser technology over the past 10 years has markedly improved the safety and efficiency of flap cutting during LASIK and provides superior performance. The Wavelight® FS200 laser is a recent development in femtosecond laser systems. This instrument is calibrated when switched on and prior to application of the applanation cone to the cornea surface, ensuring consistency in flap cutting. The Wavelight FS200 laser has a rapid firing speed that reduces the energy requirement, makes flap lifting easier, reduces procedure time and improves patient comfort. The system also provides consistency of flap diameter and thickness and allows for individualisation of flap morphology based on the patient’s condition. This article discusses clinical experience with the Wavelight FS200 laser for flap cutting in LASIK surgery.

Processing of Dielectric Optical Coatings by Nanosecond and Femtosecond UV Laser Ablation

Microprocessing of dielectric optical coatings by UV laser ablation is demonstrated. Excimer laser ablation at deep UV wavelengths (248 nm, 193 nm) is used for the patterning of thin oxide films or layer stacks. The layer removal over extended areas as well as sub-μm-structuring is possible. The ablation of SiO2, Al2O3, HfO2, and Ta2O5 layers and layer systems has been investigated. Due to their optical, chemical, and thermal stability, these inorganic film materials are well suited for optical applications, even if UV-transparency is required. Transparent patterned films of SiO2 are produced by patterning a UV-absorbing precursor SiOx suboxide layer and oxidizing it afterwards to SiO2. In contrast to laser ablation of bulk material, in the case of thin films, the layer-layer or layer-substrate boundaries act as predetermined end points, so that precise depth control and a very smooth surface can be achieved. For large area ablation, nanosecond lasers are well suited; for patterning with submicron resolution, femtosecond excimer lasers are applied. Thus the fabrication of optical elements like dielectric masks, pixelated diffractive elements, and gratings can be accomplished.

Near-infrared laser ablation of poly tetrafluoroethylene (Teflon) sensitized by nanoenergetic materials

Laser ablation of Teflon doped with size-selected (30–250nm) Al nanoparticles is studied. Unlike pure Teflon, which requires a vacuum-ultraviolet or femtosecond excimer laser for ablation, this sensitized Teflon can be ablated with a near-infrared laser. Using 100ps duration pulses, near-infrared ablation thresholds are lower by about a factor of 10 from excimer ablation of pure Teflon. A mechanism is discussed that involves Teflon decomposition by spherical shock fronts originating at each irradiated nanoparticle. Studies of the distance dependence of this process as a function of particle diameter and oxide layer thickness suggest ways of optimizing the ablation process.

Etching and printing of diffractive optical microstructures by a femtosecond excimer laser

Diffractive optics fabrication is performed by two complementary processing methods that rely on the photoablation of materials by ultrashort UV laser pulses. The spatially selective ablation of materials permits the direct microetching of high-quality surface-relief patterns. In addition, the direct, spatially selective transfer of the ablated material onto planar and nonplanar receiving substrates provides a complementary microprinting operation. The radiation from the ultrashort pulsed excimer laser results in superior quality at relatively low-energy density levels, owing to the short absorption length and minimal thermal-diffusion effects. Computer-generated holographic structures are produced by both modes of operation. Submicrometer features, including Bragg-type structures, are microprinted onto planar and high-curvature optical-fiber surfaces, demonstrating the unique ability of the schemes for complex microstructure and potentially nanostructure development.

Microprinting and microetching of diffractive structures using ultrashort laser pulses

Two complementary methods, the microetching and the microdeposition, of materials using ultrashort pulses of ultraviolet (UV) laser radiation for the fabrication of diffractive structures are presented. Microetching of various solid materials (polymers, metals) using femtosecond excimer lasers produces high definition micron diffractive patterns. The microdeposition method utilizes the selective microablation and transfer of thin metallic films for achieving microstructures of dots, lines and complicated multilevel patterns with micron and even submicron resolution. The short pulse length and the consequent limited thermal diffusion, lowers the ablation threshold and enables the machining and the deposition of high definition features. The superior quality of the results allows the direct, in-one-step process, fabrication of binary amplitude and multilevel optical diffractive structures on planar and cylindrical substrates such as optical fibres.

Microdeposition of metals by femtosecond excimer laser

Microablation and transfer of thin metal films using ultrashort, ultraviolet laser radiation has been studied. A KrF excimer laser ( λ=248 nm) having 500-fs pulse duration is coupled to a high-power image projection micromachining workstation. The laser irradiation is focused onto thin Cr films through the supporting transparent quartz substrates. Single pulses are used to completely remove the metal film. The ablated material is transferred onto a receiving target glass substrate placed parallel to the source film. Experiments were conducted in a miniature vacuum cell under a pressure of 10 −1 Torr. The distance between the source and target surfaces is variable from near-contact to several hundreds of microns. Serial writing of well-defined metal lines and isolated dots, is accomplished using the x– y sample micropositioning system. Optical microscopy and surface profilometry showed deposition of highly reproducible and well-adhering features of a few microns in width for a source–target distance in the neighborhood of 10 μm. The short pulse length limits thermal diffusion, thereby enabling superior definition of the deposited features. Metal patterns were also directly deposited via a parallel-mode mask projection scheme. In a first demonstration of this method, deposited diffractive structures were shown to produce high-quality computer-generated holograms.

X-ray emission from plasmas generated by 450 femtosecond excimer laser pulses

A plasma x-ray source is generated using a 10 mJ, 450 fs KrF excimer laser pulse focused onto a copper target at intensities up to 1.6 × 1015 W/cm2. The energy conversion efficiency from 248 nm laser light to x-rays in the range from 1 keV to 1.4 keV is measured as a function of the laser energy onto the target. The maximum observed efficiency is 0.5% for 5 mJ incident laser energy. The obtained results are compared with efficiencies observed using longer laser pulses of the same wavelength and intensity onto the target.

Nanosecond and femtosecond excimer laser ablation of fused silica

Ablation of fused silica using standard excimer lasers (20–30 ns pulse duration at 193, 248, and 308 nm) and a short pulse laser system (500 fs at 248 nm) is reported. Ablation rates range from several hundred nm/pulse (193 nm or fs-laser) up to about 6 μm/pulse (308 nm). The performance of the ablation is found to depend not only on wavelength and pulse duration but also on the existing or laser induced surface quality (e.g., roughness) of the material. Special ablation phenomena are observed. At 193 nm and moderate fluence (3 J/cm2) ablation takes place at the rear side of a plate without affecting the front side, whereas at higher fluence normal ablation at the front side occurs. At 248 nm (standard excimer) the existence of two consecutive ablation phases is observed: smooth ablation at low rate is followed by explosive ablation at high rate. Using fs-pulses smooth shaped holes are formed during the first pulses, whereas high pulse numbers cause the development of a ripple structure in the ablation craters.

Interferometric method for the characterization of the phase and pulse fronts of femtosecond pulses

An interferometric method is presented to characterize the most important parameters of beams from large aperture femtosecond excimer laser systems. With the use of a special interferometer the shape of both the pulse and the phase fronts can simultaneously be studied. With minor changes, conventional autocorrelation measurements can also be performed.