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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.

Experimental evidence is presented demonstrating avalanche ionization as the dominant mechanism for dielectric breakdown in silicon with ultrafast laser pulses at above-gap photon energies. Data are presented for pulses between 80 fs and 9 ns at 786 nm and 1.06 \ensuremath{\mu}m. Associated electric fields range from 0.3 to 40 MV/cm. Avalanche ionization coefficients range from ${10}^{10}$ to ${10}^{14} {\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ and are discussed in relation to semiempirical dc ionization theory and recent ac Monte Carlo calculations. Correlation is obtained between electron collision times and associated ionization rates.

Recently, ultrafast lasers have received increased attention in a variety of biomedical applications, including refractive corneal surgery. The laser-tissue interaction with ultrashort laser pulses is based on laser-induced optical breakdown (LIOB), which results in the generation of a microplasma. Due to the expansion of the hot plasma a shock wave and a cavitation bubble are generated. As a result of this process, termed photodisruption, tissue in the focal volume is destroyed. LIOB can be achieved at significantly smaller threshold energies when using femtosecond laser pulses [1]. Furthermore, shock wave and cavitation bubble effects are reduced for the shorter pulses [2], allowing for minimally invasive and well localized corneal surgical procedures.

We evaluated the efficacy, safety, and stability of femtosecond laser intrastromal refractive procedures in ex vivo and in vivo models. When compared with 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 clinical practicality. Contiguous tissue effects were achieved by scanning a 5 μm focused laser spot below the corneal surface at pulse energies of approximately 2 - 4 microjoules. 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 keratomileusis). Preliminary in vivo evaluation of intrastromal vision correction in a rabbit model revealed consistent and stable pachymetry changes, without significant inflammation or loss of corneal transparency. 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.

We investigated three potential femtosecond laser ophthalmic procedures: intrastromal refractive surgery, transcleral photodisruptive glaucoma surgery and photodisruptive ultrasonic lens surgery. A highly reliable, all-solid-state system was used to investigate tissue effects and demonstrate clinical practicality. Compared with longer duration pulses, femtosecond laser-tissue interactions are characterized by smaller and more deterministic photodisruptive energy thresholds, smaller shock wave and cavitation bubble sizes. Scanning a 5 (mu) spot below the target tissue surface produced contiguous tissue effects. Various scanning patterns were used to evaluate the efficacy, safety, and stability of three intrastromal refractive procedures in animal eyes: corneal flap cutting, keratomileusis, and intrastromal vision correction (IVC). Superior dissection and surface quality results were obtained for the lamellar procedures. IVC in rabbits revealed consistent, stable pachymetric changes, without significant inflammation or corneal transparency degradation. Transcleral photodisruption was evaluated as a noninvasive method for creating partial thickness scleral channels to reduce elevated intraocular pressure associated with glaucoma. Photodisruption at the internal scleral surface was demonstrated by focusing through tissue in vitro without collateral damage. Femtosecond photodisruptions nucleated ultrasonically driven cavitation to demonstrate non-invasive destruction of in vitro lens tissue. We conclude that femtosecond lasers may enable practical novel ophthalmic procedures, offering advantages over current techniques.

To evaluate transscleral glaucoma surgery techniques using ultrashort pulsed lasers, we attempted to produce photodisruption on the inner surface of the sclera without damaging the overlying tissue. We identified two methods, using pulses centered at 1700 nm and a transparency inducing drug, to produce the spatial and temporal confinement of the pulse necessary to produce photodisruption in the highly scattering sclera. When fully developed these concepts may help address the longstanding limitations of current glaucoma surgical techniques.

We investigated refractive corneal surgical procedures utilizing amplified femtosecond laser pulses from a reliable and compact femtosecond laser system suited for medical applications. Procedures such as corneal flap cutting and laser keratomileusis have been demonstrated in vivo and in vitro. Cutting was found to be well confined and highly precise.

We present experiments indicating that it is possible to ablate the sclera without damage to the overlaying conjunctiva. A femtosecond Nd:glass laser with 8.5-/spl mu/J pulse energy was used to ablate the top layers of sclera when the laser was delivered through the conjunctiva. In a separate experiment, the same laser was able to create a drainage channel through the whole thickness of the sclera from the outside inward. We created the channel by scanning the laser along spiral patterns at increasing depths into the sclera. Approximately 100 spiral scans were performed, for a total procedure time of 8 min. The diameter of the hole at the outer and inner surfaces was approximately 0.5 mm and 0.15 mm, respectively. Pictures of a side cut show little or no collateral tissue damage.

Femtosecond lasers emitting in the wavelength range around 1 /spl mu/m are attractive sources for a variety of applications. At 1.06 /spl mu/m, neodymium-doped glass offers both the relatively broad gain bandwidth, which is necessary for femtosecond pulse generation, and a strong absorption band around 800 nm, which allows for direct diode-pumping. Femtosecond pulses from diode-pumped Nd:glass lasers were previously reported, using intracavity semiconductor saturable absorber mirrors (SESAMs) to obtain mode-locking. In these lasers prism pairs were used for compensation of intracavity material dispersion and of self-phase modulation, limiting the reliability and compactness of the systems. Towards more compact and reliable femtosecond lasers we demonstrate a SESAM-mode-locked Nd:silicate laser, which uses Gires-Tournois interferometer mirrors for dispersion compensation and which is pumped by only a single laser diode.