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Since the invention of the first ophthalmoscope in 1851, the quest for quantitative in vivo imaging of ocular surfaces for diagnostic purposes was initiated. Confocal microscopy has been widely implemented in clinics providing excellent spatial resolution and enabling optical sectioning. With the introduction of ultrashort pulsed laser sources, multiphoton microscopy became feasible. Multiphoton microscopy allows 3D tissue imaging. Collagen, elastin, NAD(P)H etc., can be imaged by multiphoton microscopy, providing structural and functional information of the cornea. In this article, we will discuss the principles and potential use and limitations of confocal and multiphoton microscopy for corneal imaging.

Safe use of retinal imaging with two-photon excitation in human eyes is crucial, as the effects of ultrashort pulsed lasers on the retina are relatively unknown. At the time of the study, the laser safety standards were inadequate due to the lack of biological data. This article addresses the feasibility of two-photon retinal imaging with respect to laser safety. In this study, rat retinas were evaluated at various laser exposure levels and with different laser parameters to determine the effects of laser-induced optical damage. The results were experimentally verified using confocal reflectance imaging, two-photon fluorescein angiography, immunohistochemistry, and correlated to the IEC 60825-1 laser safety standard.

Imaging of non-linear optical (NLO) signals generated from the eye using ultrafast pulsed lasers has been limited to the study of ex vivo tissues because of the use of conventional microscopes with slow scan speeds. The purpose of this study was to evaluate the ability of a novel, high scan rate ophthalmoscope to generate NLO signals using an attached femtosecond laser. NLO signals were generated and imaged in live, anesthetized albino rabbits using a newly designed Heidelberg Two-Photon Laser Ophthalmoscope with attached 25 mW femtosecond laser having a central wavelength of 780 nm, pulsewidth of 75 fs, and a repetition rate of 50 MHz. To assess two-photon excited fluorescent (TPEF) signal generation, cultured rabbit corneal fibroblasts (RCF) were first labeled by Blue-green fluorescent FluoSpheres (1 μm diameter) and then cells were micro-injected into the central cornea. Clumps of RCF cells could be detected by both reflectance and TPEF imaging at 6 hours after injection. By 6 days, RCF containing fluorescent microspheres confirmed by TPEF showed a more spread morphology and had migrated from the original injection site. Overall, this study demonstrates the potential of using NLO microscopy to sequentially detect TPEF signals from live, intact corneas. We conclude that further refinement of the Two-photon laser Ophthalmoscope should lead to the development of an important, new clinical instrument capable of detecting NLO signals from patient corneas.

Purpose To assess the efficiency and effectiveness of the technology that creates a hydrophilicity-based refractive index change within a standard intraocular lens (IOL) to alter the refractive characteristics of the IOL. Setting Perfect Lens LLC, Irvine, California, USA. Design Experimental study. Methods The IOL used in this experiment was a standard hydrophobic model (EC-1Y). The refractive index of the material was changed by exposure of the material to a femtosecond laser and the subsequent absorption of water by the material. An experimental system using a femtosecond laser, an acoustic-optic modulator, beam-shaping optics, a scan system, and an objective lens was used to create the refractive index change within the IOL. Experiments were performed to determine the optimum wavelength, energy per pulse, and line spacing to produce the refractive index shaping lens. A power and modulation transfer function (MTF) measurement device for refractive and diffractive IOLs was used to measure the diopter and MTF before and after the creation of the refractive index shaping lens. Results The technology successfully altered the refractive characteristics of the IOL. The refractive index change altered the diopter (D) of the IOL (to within ±0.1 D of the targeted change) without significant diminution in the MTF ( Conclusion The refractive properties of an IOL can be altered by building a refractive index shaping lens within an IOL using a femtosecond laser with minimal diminution in MTF. Financial Disclosure All authors are employed by Perfect Lens, LLC and have a financial interest in the products of Perfect Lens, LLC.

The chemical basis for the alteration of the refractive properties of an intraocular lens with a femtosecond laser was investigated. Three different microscope setups have been used for the study: Laser Induced Fluorescence (LIF) microscopy, Raman microscopy and coherent anti-Stokes Raman Scattering (CARS) microscopy. Photo-induced hydrolysis of polymeric material in aqueous media produces two hydrophilic functional groups: acid group and alcohol group. The spectral signatures identify two of the hydrophilic polar molecules as N-phenyl-4-(phenylazo)-benzenamine (C18H15N3) and phenazine-1-carboxylic acid (C13H8N2O2). The change in hydrophilicity results in a negative refractive index change in the laser-treated areas.

The goal of the project “stereotactic laser-neurosurgery” is the development of a system for careful and minimal-invasive resection of brain tumors with ultrashort laser pulses through a thin probe. A confocal laser-scanning-microscope is integrated in the probe. In this paper, the simulation of its optical properties by a laboratory setup and the expansion by the ability for fluorescence microscopy are reported. For a valuation of the imaging properties, the point-spread-function in three dimensions and the axial depth-transfer-function were measured and thus, among other things, the resolving power and the capacity for depth discrimination were analysed. The microscope will enable intra-operative detection of tumor cells by the method of immunofluorescence. As a first model of the application in the brain, cell cultures, that fluorescein-labelled antibodies were bound to specifically, were used in this work. Due to the fluorescence signal, it was possible to detect and identify clearly the areas that had been marked in this manner, proving the suitability of the setup for minimal-invasive tumor diagnosis.

We investigated the use of ultrashort pulsed (femtosecond) laser technology in corneal refractive surgery. When compared to longer pulsewidth nanosecond or picosecond laser pulses, femtosecond laser-tissue interactions are characterized by significantly smaller and more deterministic photodisruptive energy thresholds, as well as reduced shock waves and smaller cavitation bubbles. We utilized a highly reliable all-solid-state femtosecond laser system for all studies to demonstrate practicality in real-world operating conditions. Contiguous tissue effects were achieved by scanning a 5-/spl mu/m focused laser spot below the corneal surface at pulse energies of approximately 2-4 /spl mu/J. A variety of scanning patterns was used to perform three prototype procedures in animal eyes; corneal flap cutting, keratomileusis, and intrastromal vision correction. Superior dissection and surface quality results were obtained for lamellar procedures (corneal flap cutting and keratornileusis). Preliminary in vivo studies of intrastromal vision correction suggest that consistent refractive changes can also be achieved with this method. We conclude that femtosecond laser technology may be able to perform a variety of corneal refractive procedures with high precision, offering advantages over current mechanical and laser devices and techniques.

A computer-guided laser probe has been developed for the application of ultrashort-pulsed lasers in neurosurgery. It is part of a novel operation concept for the treatment of deep-seated brain tumours. The system combines the positioning accuracy of stereotactic or neuronavigated instruments with the precise and therefore gentle characteristics of surgical lasers. The probe has an outer diameter of 5.5 mm and is directly inserted into the target volume. By a synchronized movement of three coaxial tubes, which guide the embedded optics, the laser radiation is focused at any time onto the current tissue surface. Since every single laser pulse has only a small effective volume of about 8 x 10(5) microm3, the application of a large number of succeeding pulses can be adapted to required geometries. Tissue fragments are removed from the growing operation cavity by continuous irrigation and suction through the laser probe. Blood vessels are detected by a confocal laser-scanning microscope, which is integrated into the probe, and can be closed by an additional coagulating laser. In this paper, the design and technical properties of the laser probe as well as its use in ablation and coagulation experiments are presented. A description of the overall operation system is given.

Nonthermal in vitro ablation of bovine neural tis- sue by using laser-induced optical breakdown generated by ultrashort laser pulses, with durations from 100 fs to 35 ps and pulse energies of up to 165mJ, has been investigated. The experiments were performed at wavelengths ranging from 630 to 1053 nm by using a femtosecond Ti:Sapphire laser, a femtosecond dye laser, and a picosecond Nd:YLF laser sys- tem. Tissue ablations have been achieved by focusing the laser beam on the surface of the tissue, to a spot diameter of 5- 20mm, resulting in the generation of a microplasma. Laser pulses from the Ti:Sapphire laser with 140 fs duration showed a two times higher efficiency of ablation than the longer 30 ps pulses from a Nd:YLF laser with an identical pulse energy. At pulse energies of 140mJ, single pass excisions deeper than 200mm were generated by the 140 fs pulses. In addition, the fluence at threshold of the ablation was found to be reduced for shorter pulse durations. For 3p slaser pulses at 630 nm, we measured the fluence at threshold to be about 5: 3J =cm 2 ; for 100 fs pulses from the same laser the experimental thresh- old was at 1: 5J =cm 2 . Histopathological examinations and scanning electron micrographs confirm the high quality of the excisions. No sign of significant thermal damage was ob- served.

Threshold values of laser-induced optical breakdown (LIOB) on the surface of human corneal tissues, human enamel, and bovine brain tissues are presented. The data are obtained by using a regeneratively amplified Nd:YLF laser and a multistage dye laser system, respectively. The measured decrease in threshold fluence at shorter pulse durations is in good agreement with the authors' theoretical model.

Plasma-mediated ablations of brain tissue have been performed using picosecond laser pulses obtained from a Nd:YLF oscillator/regenerative amplifier system. The laser pulses had a pulse duration of 35 ps at a wavelength of 1.053 µm. The pulse energy varied from 90 µJ to 550 µJ at a repetition rate of 400 Hz. The energy density at the ablation threshold was measured to be 20 J/cm2. Comparisons have been made to 19 ps laser pulses at 1.68 µm and 2.92 µm from an OPG/OPA system and to microsecond pulse trains at 2.94 µm from a free running Er:YAG laser. Light microscopy and scanning electron microscopy were performed to judge the depth and the quality of the ablated cavities. No thermal damage was induced by either of the picosecond laser systems. The Er:YAG laser, on the other hand, showed 20 µm wide lateral damage zones due to the longer pulse durations and the higher pulse energies.

Laser scanning tomography can be used to assess retinal nerve fiber layer thickness and optic disc topography of the human eye. A pinhole is located at a plane conjugate to the focal plane of the scanning laser beam. This so-called confocal configuration assures that only the light originating from the illuminated focal plane on the retina passes through the pinhole and is detected by the photomultiplier. Consequently, images with high spatial resolution in all directions are obtained. An active optical system (active mirror) further improves the lateral/depth resolution of the laser tomographic scanner. By partially compensating for the optical aberrations introduced by the cornea and lens, the active optical system allows the illuminating beam to be enlarged to 6 mm, thus improving depth resolution twofold.

Collagen, as the most abundant protein in the human body, determines the unique physiological and optical properties of the connective tissues including cornea and sclera. The ultrastructure of collagen, which conventionally can only be resolved by electron microscopy, now can be probed by optical second harmonic generation (SHG) imaging. SHG imaging revealed that corneal collagen fibrils are regularly packed as a polycrystalline lattice, accounting for the transparency of cornea. In contrast, scleral fibrils possess inhomogeneous, tubelike structures with thin hard shells, maintaining the high stiffness and elasticity of the sclera.

Plasma-mediated ablations were performed on human donor corneas with a short pulsed Nd:YLF laser system at 1053 nm. The pulses were 60 psec in duration at a repetition rate of 1.0 kHz. The laser beam was oriented perpendicular to the cornea surface. The threshold energy densities for ablation of epithelium, Bowman's membrane and stroma were measured. They were 6.1 +/- 1.8 J/cm2, 21.0 +/- 5.1 J/cm2 and 10.4 +/- 1.8 J/cm2, respectively. The mean rate of tissue removal at the stromal energy density threshold was about 1 micron per pulse. The walls of the laser excisions were smooth with distortions of less than 1 micron. A new quantitative model of plasma-mediated ablation is introduced and found to closely predict the observed results. Based on the promising nature of the experimental data further investigations are planned in the use of a mode locked Nd:YLF laser as an alternative to excimer lasers for refractive corneal surgery.

We describe a novel scanning laser ophthalmoscope (SLO)-based on a video-rate second harmonic generation imaging microscope. A titanium:sapphire femtosecond laser was coupled to a modified SLO. The laser beam was scanned over the sample, and the light produced by second harmonic generation (SHG) was collected for imaging at video-speed. The device was used for imaging the lamina cribrosa (LC) of enucleated pig eyes. A resolution comparable to that of commercial multiphoton microscopes was reached. The SHG images were used for determining the average pore size of the LC determined from the images; the pressure dependence of the pore size was studied by the artificial increasing of the hydrostatic pressure in the eye. A pressure increase of 44.3 mmHg enlarged the average pore size of 62 analyzed pores by a statistically significant amount. The relative pore growth was measured at four different pressure levels in 25 pores. The pressure was increased in 15 mmHg steps. A general tendency for monothonic growth was observed, although single pores grew by no means linearly.

Background and Objective Diode pumped, all-solid-state ultrafast lasers are now widely used to perform minimally invasive refractive surgery and keratoplasty procedures. Despite such use, a systematic study concerning ultrafast laser–tissue interactions is lacking. We determined the corneal ablation threshold as a function of the laser pulse width and stromal depth by simultaneous monitoring of the intensity of the laser-induced plasma and the second harmonic generation signals (SHG) from the collagen. Study Design/Materials and Methods Ablation thresholds in porcine cornea samples were determined using three diode pumped all-solid-state ultrafast lasers (a Nd:glass femtosecond laser, a Yb:KYW femtosecond laser, and a Nd: YAG picosecond laser) over a range from 800 femtoseconds to 20 picoseconds. Results Corneal ablation threshold remained nearly constant within the first 200 µm of stroma and was consistent with previous findings with the threshold proportional to the square root of the laser pulse width. Conclusion Corneal ablation thresholds can be precisely determined by simultaneous monitoring of the intensity of the laser-induced plasma and the SHG from the cornea. Lasers Surg. Med. 39:654–658, 2007. © 2007 Wiley-Liss, Inc.

PURPOSE: To record the distribution and spectrum of human retinal pigment epithelial cell lipofuscin (LF) by two-photon-excited fluorescence (TPEF) and confocal laser scanning microscopy. METHODS: Ex vivo TPEF imaging of the human retinal pigment epithelium (RPE) of human donor eyes was conducted with a multiphoton laser scanning microscope that employs a femtosecond Ti:sapphire laser as an excitation laser source. The spectrum of autofluorescence of LF granules was analyzed with a confocal laser scanning microscope coupled to a UV argon laser. RESULTS: TPEF examination allowed for imaging of RPE cell morphology and intracellular distribution of LF granules with high-contrast and submicrometer resolution. Variations in cell size and shape as well as in autofluorescence spectra of individual LF granules were recorded. The typical diameter of LF granules was found to be below 1 mum, with some RPE cells possessing larger granules. Remarkably, enhanced blue-green autofluorescence was observed from these larger LF granules. CONCLUSIONS: TPEF imaging represents a novel tool for the investigation of morphologic and spectral characteristics of human RPE cells. Spectral variations of individual LF granules may indicate differences in the complex molecular composition. Compared to conventional single-photon excited autofluorescence, TPEF with a tunable laser source allows for reduced photo damage and deeper sensing depth. It may help to elucidate further the pathophysiological role of LF accumulation as a common downstream pathogenetic pathway in retinal diseases. With the proof of principle from this ex vivo study, further work is now planned to evaluate the safety of TPEF RPE imaging in RPE cultures and animal models.

Nd:glass femtosecond laser is promising as next generation mini-invasive eye surgical laser, with the advantages of excellent beam quality, high surgical precision and minimized side effects. However, there are still many open questions concerning the precision, efficiency and collateral effects of femtosecond laser refractive surgery. By non-invasive microscopic imaging methods including confocal, multiphoton, second harmonic and atomic force microscopy, we successfully characterized the three dimensional corneal ultrastructure without applying fixation and slicing. Based on the intrinsic properties of collagen, second harmonic cornea imaging proved to be outstanding to analyze the outcome of femtosecond laser intrastromal ablations. Strong contrast and large sensing depth second harmonic image was obtained without fixation, sectioning or labelling. The three dimensional ultrastructure of porcine cornea after Nd:glass femtosecond laser intrastromal surgery was examined to evaluate the concepts of minimum-invasive all-optical refractive eye surgery. No thermal damages were recognized and the surgical outcome appeared highly predictable. Due to the similarities between the physical principals of nonlinear laser scanning microscopy and femtosecond laser ablations, a setup of the Nd:glass femtosecond laser system integrating both the surgery and probing functions was proposed.

Diode pumped Nd:glass all-solid-state femtosecond laser is promising for next generation refractive surgery, with the advantages of excellent surgical precision, minimal tissue damage and improved system stability and compactness. The microscopic evaluation of the outcome of femtosecond laser surgery is crucial before clinical applications. By two-photon laser scanning microscopy and non-invasive second harmonic imaging, the three dimensional ultrastructure of the porcine cornea is visualized without requiring slicing or staining. The minimal-invasive corneal flap cutting and non-invasive intrastromal surgery are investigated. Femtosecond laser intrastromal surgery demonstrated high ablation precision and mimimal side effects. However, there are elongated filaments/streaks observed in the cornea stroma, most likely due to the focusing optics and self-focusing.

Refractive surgery in the pursuit of perfect vision (e.g. 20/10) requires firstly an exact measurement of abberations induced by the eye and then a sophisticated surgical approach. A recent extension of wavefront measurement techniques and adaptive optics to ophthalmology has quantitatively characterized the quality of the human eye. The next milestone towards perfect vision is developing a more efficient and precise laser scalpel and evaluating minimal-invasive laser surgery strategies. Femtosecond all-solid-state MOPA lasers based on passive modelocking and chirped pulse amplification are excellent candidates for eye surgery due to their stability, ultra-high intensity and compact tabletop size. Furthermore, taking into account the peak emission in the near IR and diffraction limited focusing abilities, surgical laser systems performing precise intrastromal incisions for corneal flap resection and intrastromal corneal reshaping promise significant improvement over today's Photorefractive Keratectomy (PRK) and Laser Assisted In Situ Keratomileusis (LASIK) techniques which utilize UV excimer lasers. Through dispersion control and optimized regenerative amplification, a compact femtosecond all-solid-state laser with pulsed energy well above LIOB threshold and kHz repetition rate is constructed. After applying a pulse sequence to the eye, the modified corneal morphology is investigated by high resolution microscopy (Multi Photon/SHG Confocal Microscope).

The application of lasers in neurosurgery is at present limited to a few special techniques like laser-induced interstitial thermotherapy (LITT) or percutaneous laser disc decompression (PLDD). Some neurosurgeons use high-power cw lasers for tissue coagulation to avoid bleeding. One reason for the small spreading of laser applications is the missing control of heat transfer into healthy tissue. As heat diffusion takes place symmetrically in all directions, exact coagulation is restricted to tumours with simple geometries.

We investigated refractive corneal surgery in vivo and in vitro by intrastromal photodisruption using a compact ultrafast femtosecond laser system. Ultrashort-pulsed lasers operating in the femtosecond time regime are associated with significantly smaller and deterministic threshold energies for photodisruption, as well as reduced shock waves and smaller cavitation bubbles than the nanosecond or picosecond lasers. Our reliable all-solid-state laser system was specifically designed for real world medical applications. By scanning the 5 micron focus spot of the laser below the corneal surface, the overlapping small ablation volumes of single pulses resulted in contiguous tissue cutting and vaporization. Pulse energies were typically in the order of a few microjoules. Combination of different scanning patterns enabled us to perform corneal flap cutting, femtosecond-LASIK, and femtosecond intrastromal keratectomy in porcine, rabbit, and primate eyes. The cuts proved to be highly precise and possessed superior dissection and surface quality. Preliminary studies show consistent refractive changes in the in vivo studies. We conclude that the technology is capable to perform a variety of corneal refractive procedures at high precision, offering advantages over current mechanical and laser devices and enabling entirely new approaches for refractive surgery.

The phase retardation of light induced by the birefringent parts of the human retina in vivo is measured with an electro-optical ellipsometer using the principle of confocal imaging. A scanning unit allows to examine an area of 25 degrees by 12.5 degrees on the retina with a resolution of 256 by 128 points with both, incident and exit beam transmitting the cornea at a fixed position. Due to this fixed beam position the Muellermatrix of the cornea can be calculated using light which is specularly reflected on blood vessels lying above the nerve fiber layer of the retina. The obtained images show a homogenous radial distribution of the retinal retardation around the fovea without the appearance of the so called Haidinger brushes. The areas with the thickest nerve fiber layers, the arcuate bundles, appear in their typical arc and were measured quantitatively. In addition, an alternative method for the compensation of corneal birefringence is evaluated by focusing the light beam onto the surface of the lens. Hereby, the measured area in the center of the cornea is 3 X 0.75 mm2.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The dependence of the fluence at the threshold of laser- induced optical breakdown on the laser pulse duration has been investigated experimentally and theoretically for human cornea, human enamel, and bovine brain tissue. For the experiments in the range from 100 fs to 200 ps, we used a femtosecond dye laser system and a picosecond Nd:YLF laser system. We observed a significant decrease of the fluence at the threshold when reducing the pulse duration. The measured dependence on the pulse duration is in good agrement with our model.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Three nonlinear (beta) -barium borate (BBO) crystals were used to generate the fourth and fifth harmonic of a Nd:YLF picosecond laser with a fundamental wavelength of 1053 nm. The UV laser pulses were used to study the surface ablation of bovine soft tissue. Whereas the general study was concentrated on brain tissue, first results were also obtained with cornea tissue. The ablation efficiency as a function of pulse energy and the ablation threshold were determined for both UV wavelengths. Histology and SEM- microscopy of the performed laser lesions are presented. Furthermore, time-resolved flash photographs we recorded by illuminating the ablation area with green picosecond laser pulses and detecting the probe beam with a CCD chip. The photographs yielded information about the physical properties of the ablation process. The emission of a shock wave into the air environment as well as the ejection of dissected ablation fragments was observed for pulse energies down to 10 (mu) J. Both UV wavelengths can be combined with an ophthalmic application system, which comprises an xyz- scanning unit for realizing precalculated ablation patterns and a z-tracker for detecting the current surface position of the cornea. The information of the z-tracker can be used to compensate the axial eye-movement with the xyz-scanning unit.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In Ophthalmology the application of ultrashort laserpulses is already well established for special intraocular surgeries like posterior capsulotomy or periphere iridotomy (see e. g. [SP85]). Also the application of picosecond pulses for intrastromal ablation in refractive surgery is under investigation. Recently the process of plasma-induced or photodisruptive laser-tissue interaction is also discussed with respect to other medical applications, e. g. lithotripsy, angioplasty and the resection of brain tumours in neurosurgery [Fis+94]. As the pulse duration is much shorter than the thermal diffusion time, tissue ablation without thermal side-effects (carbonisation and coagulation zones) is possible. On the other hand, the expansion of the microplasma leads to the generation of a shock wave, a high pressure transient moving radially outword with an initial velocity exceeding the sound velocity. Histological examinations do not reveal any dammage of adjacent tissue, but with these methods possible effects on a subcellular level cannot be examined.

Plasma mediated ablations of calf brain tissue and corneal tissue have been performed with ultrashort pulses provided by different laser systems: A 30 ps Nd:YLF amplified laser with up to 1.5 mJ of pulse energy and a powerful 180 fs Ti:Sapphire oscillator/regenerative amplifier laser system capable of pulse energies of up to 300 (mu) J. With the femtosecond pulses the threshold in energy density needed for initiating the ablation process was found to be significantly lower than with longer pulse durations. Furthermore, the amount of ablated material is by a factor of two greater with the femtosecond pulses compared to the results obtained with the picosecond pulses. Histological examinations did not show any thermal or structural damage in adjacent tissue with pulses in the pico- and femtosecond range as it could be found after irradiation with longer pulses. The morphology of the excisions is of a very high quality.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In-vitro ablation experiments of calf brain tissue with ultrashort lasers are reported. Excisions have been performed by the mechanism of plasma-mediated ablation with focused picosecond laser pulses. Results achieved with a 35 ps oscillator/regenerative amplifier Nd:YLF laser at 1053 nm and its second and fourth harmonic at 526 and 263 nm, respectively, are presented. Preliminary experiments with 19 ps laser pulses at wavelengths near 3 micrometers generated by optical parametric amplification are reported. Here, during the ablation with low photon energies no plasma was induced. Furthermore, the spectrum of laser-induced plasma sparks on the surface of fresh calf brain tissue was recorded. Atomic line widths were measured and used to determine the electron density of the plasma. With energy densities of about 100 J/cm2 values between 2 (DOT) 1017 and 5 (DOT) 1018 cm-3 were found.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In order to predict corneal curvature change after intrastromal surgery three finite element models are developed and compared. The models differ essentially in the underlying assumptions about the mechanical behavior of the cornea. The first model assumes an isotropic corneal structure with a nonlinear stress/strain behavior, thus providing a lower limit for the corneal curvature change. Contrary model two assumes transverse isotropy combined with a linear stress/strain behavior, in order to provide an upper limit for corneal curvature change. Although differing in their structural assumptions, model one and two consider the cornea as being a solid structure. A third model, based on a multilayered thin shell theory, shows that this might not be sufficient. In addition the calculations are run for two different ablation patterns. The performed analyses are extremely useful in choosing ablation patterns and dimensions for intrastromal refractive surgery. But as the model predictions of corneal curvature change differ in about 4 dpt for a given pattern, more subtle modifications and a better knowledge of the mechanical parameters of the cornea are necessary in order to obtain applicability of model calculations to predict the actual outcome of a performed surgery.

A new method for the fast measurement of the Z—transferfunction in the Fovea Centralis of the human eyeusing a confocal laser scanning device is presented. The tested eye is illuminated by a collimated helium—neonlaser beam and therefore focuses the light onto the retina. Reflected light is detected using confocal techniques, i.e. a pinhole in the detection unit assures that only light originating from the focal plane is sampled. Thefocus of the laser beam is scanned in one dimension along the visual axis of the eye through the retina by adding a slide defocus to the incoming beam. The Z—transferfunction can be obtained directly by measuringthe intensity of the reflected light for different focal planes. A fixation target is offered to the eye at infinity tostabilize the accommodation as well as the direct viewing. Different pinholes in the detection unit are testedto investigate signal to noise ratio and depth resolution. Furthermore, the wave aberrations of the tested eyesand the laser scanning system itself are measured using a Hartmann—Shack wavefront sensor. Once the waveaberrations are known the intensity distribution in the plane of the detection pinhole can be calculated usingdiffraction theory. Thus, the intensity which passes through a pinhole of a certain diameter on the optical axisof the system can be obtained and the Z—transferfunction can be simulated. A comparison of the measuredand simulated Z—transferfunctions for different sizes of the detection pinhole has been performed and showssubstantial similarity.

The effect of different wavelengths on corneal tissue ablation has been studied using the 1st, 2nd, and 4th harmonics of a picosecond Nd:YLF laser system. This laser system consists of an actively mode locked Nd:YLF oscillator generating 30 ps pulses and a regenerative amplifier. The laser beam was focussed onto fresh human donor or calf corneas. The corneas could be moved in 3D patterns using computer controlled stepping motors. The amount of the ablated tissue and the quality of the resulting surfaces were judged by light and scanning electron microscopy. The histology showed that the ablation depths of the IR and green laser pulses are very similar at a given energy density. SEMs revealed that UV light causes a denser packaging of the underlying collagen fibers, similar to the pseudo Bowman's membrane generated with excimer lasers. Further studies are planned concerning the investigation of the 5th harmonic at 211 nm.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In vitro ablation experiments of calf brain tissue using a picosecond Nd:YLF laser system are reported. The laser lesions were studied with special regard to the ablation rate and to possible injuries of tissue adjacent to the excisions. Histological examinations prove the excellent quality of the lesions, showing no signs of thermal damages or carbonization. Furthermore, the influence of higher photon energies on the ablation threshold was studied.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Ultrashort laser pulses enable a process of tissue ablation that is of great interest for medical therapy. In contrast to long pulsed or cw lasers, where the applied energy leads to heating and vaporization of irradiated tissue, the ablation process in the ultrashort pulse range is initiated by a plasma formation in the laser focus, resulting in tissue removal.1 Using picosecond pulses this ablation mechanism is already well established in ophthalmology. The accuracy and the absence of undesired thermal side effects made the plasma-mediated ablation a promising technique in other medical fields, e.g., neurosurgery.

Three different laser systems have been investigated according to their possible application in dentistry: a free running and a Q-switched microsecond Ho:YAG laser, a free running microsecond Er:YAG laser and picosecond Nd:YLF laser system consisting of an actively mode locked oscillator and a regenerative amplifier. The experiments focused on the question if lasers can support or maybe replace ordinary drilling machines. For this purpose several cavities were generated with the lasers mentioned above. Their depth and quality were judged by light and electron microscopy. The results of the experiments showed that the picosecond Nd:YLF laser system has advantages compared to other lasers regarding their application in dentistry.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A laser system is described that offers manifold applications in ophthalmic surgery. The Nd:YLF solid-state laser emits at a wavelength of 1.053 micrometers . With pulse durations of 40 psec less energy is required to obtain optical breakdown compared to nanosecond lasers. The lower pulse energy allows a substantial confinement of the laser-tissue interaction. With a highly sophisticated delivery system eye movements can be tracked during surgery and a precise placement of the laser focus can be achieved. The high transmission coefficient of ocular structures for wavelengths around 1 micrometers enables the Nd:YLF laser to perform several internal eye surgeries such as cataract fragmentation, intrastromal refractive surgery, internal sclerostomy, or iridotomy. First clinical results of some of these procedures are summarized.

In this paper an extension of the principle of the Hartmann-Shack wavefront sensor to the neasureirtent of the sphericity of a specular reflection surface is presented. The features of this widely usable system are discussed by means of the application of this approach to the modeling of surface aberrations of the cornea of the eye. The results of measurements performed with a first prototype on a set of stainless steel calibration spheres of different radii are shown.

Many problems in ophthalmology deal with the three-dimensional structure of various parts of the living human eye. A complete confocal laser scanning microscope for diagnosis of eye disorders and first clinical results are described. Geometric measurements of the cornea and topographic measurements of the retinal substructures are presented. In addition, laser scanning tomography can be used to assess quantitatively the morphometry of the retina of the human eye. The confocal imaging mode of the laser tomographic scanner (LTS) is applied to evaluate the thickness of the nerve fiber layer. The contrast of nerve fiber layer images can be enhanced by polarization-dependent imaging modes, such as Fourier ellipsometry (FE) and differential interference contrast (DIC) imaging. To improve the depth resolution, an active-optical focusing system is employed.

Nonlinear laser scanning microscopy is widely used for noninvasive imaging in cell biology and tissue physiology. However, multiphoton fluorescence imaging of dense, transparent connective tissue (e.g., cornea) is challenging since sophisticated labeling or slicing is necessary. High-resolution, high-contrast second harmonic generation (SHG) imaging of corneal tissue based on the intrinsic structure of collagen is discussed. The three-dimensional corneal ultrastructure in depths up to hundreds of microns can be probed noninvasively, without any staining or mechanical slicing. As an important application of second harmonic imaging in ophthalmology, the modification of corneal ultrastructure using femtosecond laser intrastromal ablation is systematically investigated to evaluate next-generation refractive surgical approaches.

In spite of the dramatic changes in feature size, die size and defect density, wafer inspection continues to be done manually. However, fast automatic wafer inspection will be a key element for efficient submicron VLSI-process technologies. A new two-step laser scanning procedure is described, resulting in processing speeds of up to 40 million pixels per second and spatial resolution down to 0.1 μm.

Laser scanning tomography can be used to quantitatively assess the morphometry of the retina of the human eye. In this paper, the confocal imaging mode of the Laser Tomographic Scanner (LTS) is applied to evaluate the thickness of the nerve fiber layer. The contrast of nerve fiber layer images can be enhanced by polarization dependent imaging modes, like Fourier - ellipsometry (F.E.) and differential interference contrast (DIC) imaging. To improve the depth resolution, an active-optical focusing system is employed.