Repetition Rate Excimer Laser Research Articles

Femtosecond-assisted laser in situ keratomileusis for high myopia correction: Long-term follow-up outcomes.

Report the outcomes of laser in situ keratomileusis (LASIK) for high myopia correction after long-term follow-up. Retrospective, consecutive, clinical study including 70 eyes that underwent LASIK using a 500 Hertz (Hz) repetition rate excimer laser and femtosecond technology for flap creation. Visual, refractive, corneal aberrations, and correlation among the variables were evaluated during 3 years. In addition, 34 eyes were followed during 5 years. Significant improvement of 17 logMAR lines was observed in uncorrected vision at 5 years (p < 0.01). This was consistent with spherical equivalent reduction that came from mean preoperative -7.79 diopters (D) to 5 years postoperative -0.24 D (p < 0.01). Significant induction of primary spherical aberration and coma aberration was also found (p < 0.01) at 3 months with levels of 0.61 µm and 0.47 µm, respectively, with no further changes at 5 years (p > 0.05). Pearson correlation showed that the amount of high-order aberrations at 3 months postoperativeley was significantly correlated with the changes in the keratometry throughout the 5 years (delta K) (R2 0.242 p = 0.05). Finally, preoperative corneal asphericity showed an inverse correlation with the delta K (R2 -308 p = 0.01). Results from this study suggests that LASIK for high myopia correction using 500 Hz repetition rate excimer laser provides stable visual, refractive and aberrometric results after 5 years of follow-up. A more prolate cornea and the amount of higher-order aberrations induced after LASIK are factors that negatively impact in the long-term stability of the procedure.

Laser-Assisted in Situ Keratomileusis with Optimized, Fast-Repetition, and Cyclotorsion Control Excimer Laser to Treat Hyperopic Astigmatism with High Cylinder.

To evaluate the visual outcomes after femtosecond laser-assisted laser in situ keratomileusis (LASIK) surgery to correct primary compound hyperopic astigmatism with high cylinder using a fast repetition rate excimer laser platform with optimized aspheric profiles and cyclotorsion control. Eyes with primary simple or compound hyperopic astigmatism and a cylinder power ≥3.00 D had uneventful femtosecond laser-assisted LASIK with a fast repetition rate excimer laser ablation, aspheric profiles, and cyclotorsion control. Visual, refractive, and aberrometric results were evaluated at the 3- and 6-month follow-up. The astigmatic outcome was evaluated using the Alpins method and ASSORT software. This study enrolled 80 eyes at 3 months and 50 eyes at 6 months. The significant reduction in refractive sphere and cylinder 3 and 6 months postoperatively (p<0.01) was associated with an improved uncorrected distance visual acuity (p<0.01). A total of 23.75% required retreatment 3 months after surgery. Efficacy and safety indices at 6 months were 0.90 and 1.00, respectively. At 6 months, 80% of eyes had an SE within ±0.50 D and 96% within ±1.00 D. No significant differences were detected between the third and the sixth postoperative months in refractive parameters. A significant increase in the spherical aberration was detected, but not in coma. The correction index was 0.94 at 3 months. Laser in situ keratomileusis for primary compound hyperopic astigmatism with high cylinder (>3.00 D) using the latest excimer platforms with cyclotorsion control, fast repetition rate, and optimized aspheric profiles is safe, moderately effective, and predictable.

Corneal Higher Order Aberrations After LASIK for High Myopia With a Fast Repetition Rate Excimer Laser, Optimized Ablation Profile, and Femtosecond Laser–assisted Flap

To evaluate corneal higher order aberrations (HOAs) after LASIK for the correction of high myopia using a new generation of excimer laser (500-Hz repetition rate) and optimized ablation profiles. Retrospective consecutive study including 29 eyes from 17 patients (age range: 24 to 61 years) with high levels of myopia (spherical equivalent refraction ≥8.50 diopters). All cases underwent LASIK using the sixth-generation Amaris (SCHWIND eye-tech-solutions) excimer laser and flap creation with the IntraLase (Abbott Medical Optics) femtosecond laser. Postoperative changes in corneal HOAs and corneal asphericity were analyzed during 6-month follow-up. Significant improvement in uncorrected distance visual acuity and spherical equivalent was observed (P<.01). Corrected distance visual acuity did not change after surgery (P=.37). Significant increases in corneal root-mean-square (RMS) HOA, RMS spherical aberration (SA), and RMS coma were observed 6 months after surgery (P<.01). Corneal asphericity for the 4.5-mm (Q45) and 8-mm (Q8) corneal diameter also increased significantly during the postoperative period (P<.01). Significant correlations were found between postoperative RMS HOA and RMS SA and RMS coma (r(2)=0.375, P=0.000 and r(2)=0.596, P=.000, respectively). Linear regression analysis showed a significant relationship between postoperative Q45 and preoperative pachymetry and treatment optical zone (R(2)=0.24, P<.05). A correlation was also found between postoperative Q8 and preoperative sphere and treatment ablation zone (R(2)=0.459, P<.006). A significant induction of corneal HOAs still exists with the latest generation Schwind excimer laser. Linear regression analysis will help predict postoperative asphericity obtained when using LASIK for the correction of high myopia.

Development of key components and technologies for a high repetition rate and high-power excimer laser

Key components and technologies have been developed for an ultrahigh repetition rate XeCl excimer laser of 5 kHz to be used for industrial applications. A compact axial blower having a revolution rate of up to 10 000 min−1 and a maximum pressure of 16.2 kPa in air was made with a canned magnetic coupler to circulate a laser gas at a flow velocity of over 150 m/s in a discharge region. Materials constituting a laser chamber were tested to prevent discharge instability by gas contamination to enable long time operation. The dominant cause of the instability was investigated by a simple simulation. For a preionization, a novel sealed-off x-ray tube was developed to compare the suitability in a high repetition rate operation with that of conventional UV preionization. The gas due to the shock and acoustic waves generated by discharge pulses was measured to design the damper, which enabled the suppression of the gas turbulence by around a tenth. To simplify cumbersome laser maintenance, a new power supply provided by a novel fast switching semiconductor device was evaluated by operating the laser. The experimental laser apparatus integrating these key components and technologies was operated to confirm the practical availability for high repetition of up to 5 kHz. Many kinds of basic experiments have been performed to increase repetition rate, average power, and reliability. Though these experimental results have not yet been integrated and performed simultaneously, we have achieved operation up to an average output power of 0.56 kW at 5 kHz. Based on the results and empirical knowledge, the prospect for a practical high repetition rate excimer laser was discussed.

Development of All Solid State Exciter for High Repetition Rate Excimer Laser.

We developed an all solid state exciter composed of a semiconductor switch with MAGTs (MOS Assisted Gate-triggered Thyristors) and a single-stage MPC (Magnetic Pulse Compression circuit) for high repetition rate XeCl excimer laser. Thirty two (32) MAGTs connected in series were used as the semiconductor switch for a 25kV-900A-0.6μs pulse generation. For MPC, cobalt-based amorphous alloy treated by annealing was selected as core materials for the purpose of reducing magnetic losses.The continuous operation with an average laser power of 43W at 3kHz was verified with this exciter. The efficiency of the switch of 75% was obtained at 3kHz operation. The experimental results verify that replacing thyratrons with the all solid state exciter is possible.

5 kHz high repetition rate and high power XeCl excimer laser

A 5 kHz high repetition rate excimer laser has been developed. An average laser power of 0.56 kW at 5 kHz operation, which remains stable for 1 s, is obtained. This duty time is limited only by overheating of the switching thyratrons. The 5 kHz operation is attained by some improvements to the gas conditions, the UV-preionization scheme, and the gas flow speed in the discharge region. The gas is circulated at an effective flow velocity of 137 m/s by a newly developed two-stage axial blower. The gas conditions are optimized experimentally to achieve 5 kHz operation. To obtain uniform preionization, the locations of pin electrodes for the preionization are determined on the basis of numerical results.

New KrF and ArF Excimer Lasers for Advanced Deep Ultraviolet Optical Lithography

Advanced deep ultraviolet (DUV) optical lithography requires reliable high repetition rate excimer lasers at low cost of ownership. We optimized all components of the laser cavity. The result is a new metal/high-density ceramic laser tube with long life discharge components. It meets the specifications of the next generation of DUV laser sources. The new laser cavity was tested in several test runs at 193 nm (ArF) and 248 nm (KrF) and demonstrated excellent gas lifetime data when compared to conventionally designed lasers. For the first time a KrF laser was successfully operated in a quasi sealed-off mode for more than 1 billion pulses. The lifetime data of narrow bandwidth lasers necessary for DUV optical lithography tools also increase significantly. A 500 Hz 6 W laser with a bandwidth of 0.9 µ m passed in a marathon test 1.1 billion pulses and the gas lifetime exceeded 200 million pulses routinely. Additionally, we operated this laser at 8 W average power and 600 Hz.

Laser beam quality of high pulse repetition frequency excimer lasers

The influence of acoustic perturbations of an active disturbing medium on the far-field diffraction pattern is both theoretically and experimentally investigated. The theoretical results are in good agreement with experiments performed with a high repetition rate excimer laser (λ=308 nm, PRF=500 Hz). This theoretical approach explains the decrease of laser energy and fluence and the distortion of the beam shape in the far-field occuring in high pulse repetition frequency excimer lasers.

High Repetition-Rate Excimer Laser (II). 4kHz, 200W XeCl laser operation in low Xe contens.

A high repetition-rate excimer laser has been developed, in which a 4-stage axial blower circulates the laser gas up to 120 m/s. The discharge region of 10 × 20 × 610 mm3 is UV-preio-nized by pin sparks located along both sides of main electrodes. The output characteristics of XeCl laser were optimized for high repetition rate operations, varying conditions such as total gas pressure, gas mixture ratio, and applied voltage. High Xe contents were found to deterio-rate the laser stability. The residual Xe2+ and Cl ions localized on the cathode surface are sup-posed to generate filamentary channels, which cause laser energy reduction. Stable 4 kHz op-eration was achieved at a clearance ratio CR =3 with 200 W average laser power in a gas mix-ture of Xe/HCl/He=1.06/0.27/177 kPa.

Microstructure of YBaCuO superconducting thin films deposited by high power and high repetition rate excimer lasers

The deposition parameters used for the preparatiion of thin YBaCuO films on MgO substrates by the pulsed laser ablation method are of the utmost importance in controlling the microstructure. The film quality is mainly controlled by the deposition rate, the laser fluence and the substrate temperature. The best quality films are produced for low enough values of the deposition parameters where well-oriented grain growth occurs with the c axis perpendicular to the substrate surface. Higher values of the deposition parameters result in higher nucleation rates leading to a continuous loss of epitaxy up to the formation of randomly oriented nanocrystalline films and hence to a strong current leakage at grain boundaries which lowers the critical temperature.

High repetition rate KrF laser plasma x-ray source for microlithography

As part of a development programme for a high-intensity laser-plasma X-ray source, experiments have been carried out using a high repetition rate excimer laser (up to 100 Hz; 249 nm, 300 mJ). Remedies are given to problems inherent to operating this type of source at high repetition rates. The combined remedies eliminate the target debris problem. X-ray mask structures down to 0.5 μm have been imaged. After implementation of tested upgrades, this source will enable exposures within ten seconds per stepfield.

High Repetition-Rate Eximer Laser. 3kHz, 135W Operation.

A high repetition-rate excimer laser has been developed. A 4-stage axial blower circulating las-er gas at more than 110m/s and a high-speed command charging power supply with a thyratron were employed for the high repetition-rate operation. The discharge region of 10×20×610mm3 is UV-preionized by pin sparkes along both sides of main electrodes. The output characteristics of XeCl were investigated both for Ne and for He buffer gas mixtures. The average power decreased abruptly at more than 2kHz in the Ne buffer gas. On the other hand, stable operation up to 3kHz was attained at a gas flow rate of 110m/s with 135W average laser power in the He buffure gas. The power decrease over at more than 2kHz in Ne is supposed to be due partially to the rise in the gas temperature caused by the blower.

Design and characteristics of high pulse repetition rate and high average power excimer laser systems

The authors describe the design and the performance characteristics of an X-ray preionized discharge pumped XeCl laser system operated at very high repetition rate frequency and high average power (200 W). The laser system is mainly composed of a fast flow subsonic closed cycle wind tunnel, a laser head, a high average power electrical excitation system, and an X-ray preionization source. The laser head design and the damping of acoustic waves induced by fast electrical energy deposition of 50 J/L bar allows this device to be operated at repetition rate frequencies of up to approximately=1700 Hz. >

Discharge excited high-repetition-rate excimer lasers.

Discharge excited high repetition-rate excimer lasers will be useful to the material process of a next generation. In this paper, the technologies for the discharge excited high repetition-rate excimer lasers including gas circulation system, magnetic pulse compressor, pre-ionizer and gas recycling system, are discussed. We wish to report a high power KrF excimer laser with a new capacitor-transfer-type configuration, which delivered an average power of 150W at a high efficiency of-3.8% and a high repetition-rate of 700Hz.

Narrow Band, Near UV, High Repetition Rate Laser Induced Fluorescence System for use as an Edge Diagnostic in Fusion Machines*

A laser system for impurity diagnostics in the edge region of plasma devices is described. It consists of a scanning, single mode, cw dye laser followed by a 3-stage, fast flow dye cell amplifier, pumped by a high repetition rate excimer laser (60 mJ/pulse at 130 Hz, for 308 nm output). Substantial improvements are achieved over previous systems in scan speed (30 GHz/100 ms) and velocity resolution (now small relative to the widths of thermal distributions). The usefulness of high resolution is demonstrated by a model calculation for Fe velocity spectra involving the presence of thermal and sputtered flux, and spatial averaging. The high output pulse power (0.8 MW at 604 nm, 80 kW at 302 nm) allows efficient frequency doubling and can be used to vary the effective bandwidth by power broadening. Broadband operation (50 GHz FWHM) is also possible, for saturated measurements of atomic density. Laboratory velocity spectra for Fe atoms sputtered in the ground state demonstrate the capability for such measurements in a single Tokamak discharge at estimated densities of 108 atoms/cm3.