The past decade has brought about significant advances in Ultrashort pulse lasers technology. The development of broad—band solid state gain media opened up new possibilities for ultrashort pulse generation. In particular, the development of all—solid--state ultrashort pulse devices promise to make such devices rugged and reduce their cost.
Over the last few years a number of microscopical techniques have been developed that take advantage of ultrashort optical pulses. All these techniques rely on temporal pulse integrity at the focal point of a high-numerical aperture (NA) focusing system. We have investigated the dispersion induced broadening for pulses on the optical axis, using the two-photon absorption autocorrelation (TPAA) technique. We demonstrate that the induced broadening can be pre- compensated for by a properly designed dispersion pre- compensation unit for pulses as short as 15 femtosecond. Another source of pulse broadening in high-NA focusing systems is due to radial variations in the dispersion over the pupil of the objective. This may cause differences in the group delay between on-axis and outer ray wave packets, as well as differences in the broadening of the wave packets themselves. In this paper we present experimental results on the measurement of these radial variations in the dispersion characteristics over the aperture of high-NA microscope objectives, using a slightly modified TPAA technique.
Pulse broadening of ultrashort optical pulses, as short as 15 femtoseconds, due to the propagation through high- numerical-aperture microscope objectives can be pre- compensated to ensure temporal pulse integrity at the focal point. The predictions from dispersive ray-tracing calculations show excellent agreement with the experimental results from two-photon absorption autocorrelation for the Zeiss CP-Achromat 100X/1,25 oil microscope objective. From this, general predictions can be inferred for dispersion in most types of microscope objectives. Key element to the work is a carefully designed dispersion pre- compensation configuration, which minimizes pulse broadening due to residual third order dispersion. The capability to focus these ultrashort pulses with control of the pulse definition at the focal point is important for two-photon absorption and time-resolved microscopy.
Until recently, experiments in ultrashort pulse science have involved measuring the spectrum and autocorrelation of the input pulse(s) and only measuring the integrated energy or perhaps time-resolved energy of the output signal. These experiments ignored the information contained in the input and output pulse phases and intensity profiles. New pulse measurement techniques such as frequency-resolved optical grating, when combined with older techniques such as spectral interferometry, now allow the complete characterization of the pulses. These techniques allow measurements of the intensity, phase, and polarization state of ultrashort pulses as functions of time (or frequency) and space. These techniques work for wavelengths from the UV to the IR and for extremely weak pulses and very high power pulses. They also allow entirely new classes of experiments for measuring ultrafast phenomena. Now the phases and temporal profiles of the input pulses may be measured and controlled, and the intensity and phase of the output pulses can also be measured. These new measurement techniques have thus greatly increased the obtainable information in ultrafast experiments. This paper reviews current pulse measurement methods including frequency-resolved optical grating and spectral interferometry and describes how they are changing the way that ultrashort pulse experiments are performed.
Nonlinear absorption through laser-induced breakdown (LIB) offers the possibility of localized energy deposition in linearly transparent media and thus of non-invasive surgery inside the eye. The general sequence of events--plasma formation, stress wave emission, cavitation--is always the same, but the detailed characteristics of these processes depend strongly on the laser pulse duration. The various aspects of LIB are reviewed for pulse durations between 80 ns and 100 fs, and it is discussed, how their dependence on pulse duration can be used to control the efficacy of surgical procedures and the amount of collateral effects.
Extensive research of ultrashort ocular damage mechanisms has shown that less energy is required for retinal damage for pulses shorter than one nanosecond. Laser minimum visible lesion thresholds for retinal damage from ultrashort (i.e. < 1 ns) laser pulses occur at lower energies than in the nanosecond to microsecond laser pulse regime. WE review the progress made in determining the trends in retinal damage from laser pulses of one nanosecond to one hundred femtoseconds in the visible and near-infrared wavelength regimes. We discuss the most likely damage mechanism(s) operative in this pulse width regime and discuss implications on laser safety standards.
We characterized the effects of pulse duration, pulse energy, and spot separation on intrastromal corneal photodisruption to determine parameters that achieve optimal surface quality and tissue plane separation. Experiments utilized two laser systems, a 60 picosecond Nd:YLF laser and a 450 femtosecond Nd:Glass laser, both operating at 1.06 micrometers wavelength. Photodisruption was performed by tightly focusing the laser beam 150 microns below the tissue surface and scanning it in a spiral pattern to create a plane. A cut to the surface was made with the laser and the two surfaces separated to form a flap. Tissue plane separation was graded according to the additional mechanical dissection required. Internal surfaces were analyzed with standard histologic methods and scanning electron microscopy. We found that the Nd:YLF laser required approximately three times the pulse energy to achieve intrastromal cuts. Picosecond parameters also required more mechanical dissection and produced lower surface quality than optimal femtosecond parameters. We conclude that femtosecond laser pulses offer significant advantages that make them ideal candidate tools for high precision intrastromal corneal surgery. The flexibility in laser pulse delivery opens up a number of potential surgical applications not possible with current mechanical or laser devices.
We determined the wavelength dependence of the minimum spot size of a laser beam focused through human sclera to evaluate the potential for transcleral glaucoma surgical techniques using ultrashort-pulsed lasers. The spectrum of the forward scattered light was measured by collimating the incident and transmitted beam in a spectrophotometer. This spectrum shows that sclera is highly scattering until 1100 nm, after which, the transmission spectrum is similar to water. To measure the minimal spot size, a laser beam was focused on the back surface of sclera of differing thickness. The minimum spot at 800 nm, 1060 nm, 1301 nm, and 1557 nm was imaged. At 800 nm, the spot size was invariant upon focal lens position, being a thousand fold larger than the incident beam spot size. As the wavelength increased, the area of the spot decreased, so that at 1557 nm, the minimal spot size was on the order of the incident beam spot size.
Several laser systems are currently under investigation for the purpose of removing hard dental tissues. However, either undesired thermal side effects or the lack of efficiency have already been demonstrated in most cases. In this paper, advantages and limitations of using ultrashort laser pulses with either picosecond or femtosecond durations are discussed. The major advantages associated with these pulse durations is the ability to produce very precise cavities without significant thermal side effects. Even disruptive effects due to shock wave generation seem to be negligible at moderate pulse energies close to the ablation threshold. The quality of these cavities is found to be superior to the quality achievable with other laser systems. Moreover, a spectroscopical analysis of the laser-induced plasma sparks enables an on-line health diagnosis of the irradiated volume. Limitations arise from the development of a suitable delivery system and from the cost of generating ultrashort laser pulses.
Plasma luminescence spectroscopy was used for precise ablation of bone tissue without damaging nearby soft tissue using an ultrashort pulse laser. Strong contrast of the luminescence spectra between bone marrow and spinal cord provided the real time feedback control so bone tissue is selectively ablated while preserving the spinal cord.
Three enzymes differing in their structural composition were irradiated by UV lasers to study the effect of temperature, protein concentration and addition of small molecules on their sensitivity to radiation exposure. The laser-induced effects were due to the structural complexity of the protein molecules and depended on the dose applied, the wavelength and the density of irradiation. The multi-enzyme 2- oxoglutarate dehydrogenase complex was subjected to pronounced irradiation-induced changes whereas the response of the two other enzymes was less significant. Reduction of the protein levels in irradiated samples was important under the XeCl laser coercion and the effects depended on the doses applied. The laser irradiation effects are suggested to be realized by means of conformational changes in the protein molecules and intermolecular association- dissociation processes.
Because of low operating speed and excessive collateral damage, lasers have not succeeded in replacing conventional tools in many surgical and dental applications. Recent developments now allow the new generation of amplified ultrashort pulse lasers to operate at high repetition rates and high single pulse energies. A Titanium:sapphire Chirped Pulse Regenerative Amplifier system operating at 1 KHz and 50 fs pulse duration, was used to demonstrate ultrashort pulse ablation of hard and soft tissue. Maximum ablation rates for enamel and dentin were approximately 0.650 micrometers /pulse and 1.2 micrometers /pulse respectively. Temperature measurements at both front and rear surface of a 1 mm dentin and enamel slices showed minimal increases. Scanning electron micrographs clearly show that little thermal damage is generate by the laser system. If an effective delivery system is developed, ultrashort pulse system may offer a viable alternative as a safe, low noise dental tool.
Some medical applications involving the interaction between light and biological tissues require both the knowledge of optical characteristics of tissues and a realistic treatment of light transport into them. In this work we describe a transillumination technique testing water solutions of Intralipid by the transmitted radiation intensity. The experimental apparatus includes a diode laser ((lambda) emiss equals 820 nm), a detection fiber, a PMT and a Digital Signal Analyzer. The performances of this very simple and cheap system are comparable to those ones obtained with more sophisticated apparatus; the results show that this technique could represent a preliminary step toward the realization of an user-friendly and cheap laser system for measuring optical parameters of tissues.
Retinal lesions produced by ultrashort laser pulses in the pico- and femtosecond range were examined by electron microscopy. Retinal pigment epithelial (RPE) cells that contained fractured and striated melanosomes typically exhibited severe damage to the other components of the cell. However, having observed RPE cell damage without coincident fractured melanosomes, it is thought that melanosome fracture itself is not responsible for the damage that occurs within the RPE cell. Nevertheless, the percentage of melanosomes fractured per lesion seems to parallel the severity of damage within that lesion site. No trend existed between percentage of melanosomes fractured and the peak power of laser delivery. However, with decreasing laser pulsewidth, there was a decrease in the percentage of melanosomes showing fracture.