Young Kwon - Academia.edu (original) (raw)

Papers by Young Kwon

Eye, 2008

Purpose To explain the origin of changes in corneal asphericity and induced spherical aberration ... more Purpose To explain the origin of changes in corneal asphericity and induced spherical aberration after laser refractive surgery. Methods A rigorous model, CASIM (corneal ablation simulator), has been developed to model the ablation profile design, the shotby-shot ablation process and the corneal remodelling that occurs through healing. The dependence of corneal asphericity, induced spherical aberration, and achieved refractive correction on corneal remodelling, and the ablation efficiency reduction caused by the angle of incidence of the excimer beam on the curved cornea are compared to the clinical outcomes reported in the literature. Results When the exact Munnerlyn formula is used, the CASIM modelling and the clinical data exhibit a high degree of correlation. The modelling predicts that the postoperative cornea will be oblate, with substantial induced spherical aberration. A 6-month postsurgery asphericity is predicted by CASIM with a correlation of R 2 ¼ 0.94. The corneal remodelling included in CASIM accounts, on the average, for 45 and 69% of the increase in asphericity and spherical aberration, respectively, with the remainder due to the ablation efficiency. Conclusions The modelling shows that clinically observed increases in corneal asphericity and induction of spherical aberration can be explained by the effects of corneal remodelling due to healing and by the ablation efficiency reduction due to laser angle of incidence. The model is capable of predicting clinical outcomes for procedures performed with flying spot laser systems and could be used to design compensated ablation profiles to improve the clinical outcomes for custom as well as conventional laser refractive procedures.

SPIE Proceedings, 2002

ABSTRACT A new design of the laser resonator using dual phase conjugation mirrors is discussed. I... more ABSTRACT A new design of the laser resonator using dual phase conjugation mirrors is discussed. It is shown that using the modified Gerchberg-Saxton algorithm, phase conjugation criteria at the both mirrors with finite apertures can be met for the given shape of desired mode. The flexibility of the design method allows the method can be applied to any other resonator design using either transmissive or reflective element(s) inside the cavity to satisfy the given amplitude/phase requirement of desired mode at any location. Unlike previous studies on the phase conjugation resonators, the new dual phase conjugation resonator is shown to provide extremely low loss for the desired mode while providing higher discrimination against the higher order modes. This approach to designing a resonator has been applied to the design of a large Fresnel number cavity for a ideal diode pumped laser with the objective achieving a large filling factor with a single mode and a design example will be presented.

Optics Communications, 2001

A uniform pumping distribution is very important in a side pumped rod geometry. Based on space-de... more A uniform pumping distribution is very important in a side pumped rod geometry. Based on space-dependent rate equations, the threshold pump power and the TEM00 mode output power of a solid state laser with a uniform pump are obtained. The diffraction losses arising from thermally induced spherical aberration have been estimated by the Strehl intensity ratio. The diffraction losses may

Optics & Laser Technology, 2001

Optical Engineering, 2003

The pump beam for an end-pumped solid state laser can likely be fit by a super-Gaussian beam. We ... more The pump beam for an end-pumped solid state laser can likely be fit by a super-Gaussian beam. We normalize the super-Gaussian beam for different orders ( m ). Numerical temperature solutions are obtained for super-Gaussian beams using the finite element method. We find that these numerical temperature solutions can be fit by a Gaussian function. This is important as all of the subsequent thermal radial and tangential stresses, refractive index changes, optical path differences (OPDs), birefringences and depolarization losses can be solved analytically and they follow Gaussian profiles. Different m have different thermal effects as well as different overlap efficiencies. We analyze the effects of different m on different pump powers, different pump beam sizes, and different rod radii. This analysis could provide valuable data for correcting phase distortions of high-power operation.

Applied Optics, 2004

Apertures have been used to select the low-order transverse modes in resonators. The additional d... more Apertures have been used to select the low-order transverse modes in resonators. The additional diffraction losses result in a change in the transverse-mode structure, and the presence of apertures inside a resonator generally distorts the mode shape. The optimization of a multiple-aperture resonator demands an approach that differs from the conventional method in which the mode theory is used. We demonstrate an iterative design method to find optimal phase profiles for the reflector surfaces to build a resonator with multiple apertures to produce a lowest-order mode with much smaller diffraction loss and to satisfy the phase-conjugation conditions at the mirrors. The results are compared with conventional stable resonators, and we show that substantial improvement in round-trip loss and beam quality can also be obtained.

Optics Express, 2008

We developed a rigorous simulation model to evaluate ablation algorithms and surgery outcomes in ... more We developed a rigorous simulation model to evaluate ablation algorithms and surgery outcomes in laser refractive surgery. The model (CASIM: Corneal Ablation SIMulator) simulates an entire surgical process, which includes calculating an ablation profile from measured wavefront errors, generating a shot pattern for a flying spot laser beam, simulation of the shot-by-shot ablation process based on a measured or modeled beam profile, and healing of the cornea after surgery. Using simulated post-surgery corneal shapes for various ablation parameters and beam fluences, we calculated angular dependence of ablation efficiency and the amount of increase in corneal asphericity. Without considering the effect of corneal healing, our result shows the following; 1) ablation efficiency reduction in the periphery depends on the peak fluence of the laser beam, 2) corneal asphericity increases even in the surgery using an ablation profile based on the exact Munnerlyn formula, contrary to previous reports, and 3) post-surgery corneal asphericity increases by a smaller amount in high fluence small Gaussian beam surgery than in low fluence truncated Gaussian beam. Our model can provide improved ablation profiles that compensate for the change of corneal asphericity and induction of spherical aberration in a flying spot laser system, resulting in better surgery outcomes in laser refractive surgeries.

Eye, 2008

Purpose To explain the origin of changes in corneal asphericity and induced spherical aberration ... more Purpose To explain the origin of changes in corneal asphericity and induced spherical aberration after laser refractive surgery. Methods A rigorous model, CASIM (corneal ablation simulator), has been developed to model the ablation profile design, the shotby-shot ablation process and the corneal remodelling that occurs through healing. The dependence of corneal asphericity, induced spherical aberration, and achieved refractive correction on corneal remodelling, and the ablation efficiency reduction caused by the angle of incidence of the excimer beam on the curved cornea are compared to the clinical outcomes reported in the literature. Results When the exact Munnerlyn formula is used, the CASIM modelling and the clinical data exhibit a high degree of correlation. The modelling predicts that the postoperative cornea will be oblate, with substantial induced spherical aberration. A 6-month postsurgery asphericity is predicted by CASIM with a correlation of R 2 ¼ 0.94. The corneal remodelling included in CASIM accounts, on the average, for 45 and 69% of the increase in asphericity and spherical aberration, respectively, with the remainder due to the ablation efficiency. Conclusions The modelling shows that clinically observed increases in corneal asphericity and induction of spherical aberration can be explained by the effects of corneal remodelling due to healing and by the ablation efficiency reduction due to laser angle of incidence. The model is capable of predicting clinical outcomes for procedures performed with flying spot laser systems and could be used to design compensated ablation profiles to improve the clinical outcomes for custom as well as conventional laser refractive procedures.

SPIE Proceedings, 2002

ABSTRACT A new design of the laser resonator using dual phase conjugation mirrors is discussed. I... more ABSTRACT A new design of the laser resonator using dual phase conjugation mirrors is discussed. It is shown that using the modified Gerchberg-Saxton algorithm, phase conjugation criteria at the both mirrors with finite apertures can be met for the given shape of desired mode. The flexibility of the design method allows the method can be applied to any other resonator design using either transmissive or reflective element(s) inside the cavity to satisfy the given amplitude/phase requirement of desired mode at any location. Unlike previous studies on the phase conjugation resonators, the new dual phase conjugation resonator is shown to provide extremely low loss for the desired mode while providing higher discrimination against the higher order modes. This approach to designing a resonator has been applied to the design of a large Fresnel number cavity for a ideal diode pumped laser with the objective achieving a large filling factor with a single mode and a design example will be presented.

Optics Communications, 2001

A uniform pumping distribution is very important in a side pumped rod geometry. Based on space-de... more A uniform pumping distribution is very important in a side pumped rod geometry. Based on space-dependent rate equations, the threshold pump power and the TEM00 mode output power of a solid state laser with a uniform pump are obtained. The diffraction losses arising from thermally induced spherical aberration have been estimated by the Strehl intensity ratio. The diffraction losses may

Optics & Laser Technology, 2001

Optical Engineering, 2003

The pump beam for an end-pumped solid state laser can likely be fit by a super-Gaussian beam. We ... more The pump beam for an end-pumped solid state laser can likely be fit by a super-Gaussian beam. We normalize the super-Gaussian beam for different orders ( m ). Numerical temperature solutions are obtained for super-Gaussian beams using the finite element method. We find that these numerical temperature solutions can be fit by a Gaussian function. This is important as all of the subsequent thermal radial and tangential stresses, refractive index changes, optical path differences (OPDs), birefringences and depolarization losses can be solved analytically and they follow Gaussian profiles. Different m have different thermal effects as well as different overlap efficiencies. We analyze the effects of different m on different pump powers, different pump beam sizes, and different rod radii. This analysis could provide valuable data for correcting phase distortions of high-power operation.

Applied Optics, 2004

Apertures have been used to select the low-order transverse modes in resonators. The additional d... more Apertures have been used to select the low-order transverse modes in resonators. The additional diffraction losses result in a change in the transverse-mode structure, and the presence of apertures inside a resonator generally distorts the mode shape. The optimization of a multiple-aperture resonator demands an approach that differs from the conventional method in which the mode theory is used. We demonstrate an iterative design method to find optimal phase profiles for the reflector surfaces to build a resonator with multiple apertures to produce a lowest-order mode with much smaller diffraction loss and to satisfy the phase-conjugation conditions at the mirrors. The results are compared with conventional stable resonators, and we show that substantial improvement in round-trip loss and beam quality can also be obtained.

Optics Express, 2008

We developed a rigorous simulation model to evaluate ablation algorithms and surgery outcomes in ... more We developed a rigorous simulation model to evaluate ablation algorithms and surgery outcomes in laser refractive surgery. The model (CASIM: Corneal Ablation SIMulator) simulates an entire surgical process, which includes calculating an ablation profile from measured wavefront errors, generating a shot pattern for a flying spot laser beam, simulation of the shot-by-shot ablation process based on a measured or modeled beam profile, and healing of the cornea after surgery. Using simulated post-surgery corneal shapes for various ablation parameters and beam fluences, we calculated angular dependence of ablation efficiency and the amount of increase in corneal asphericity. Without considering the effect of corneal healing, our result shows the following; 1) ablation efficiency reduction in the periphery depends on the peak fluence of the laser beam, 2) corneal asphericity increases even in the surgery using an ablation profile based on the exact Munnerlyn formula, contrary to previous reports, and 3) post-surgery corneal asphericity increases by a smaller amount in high fluence small Gaussian beam surgery than in low fluence truncated Gaussian beam. Our model can provide improved ablation profiles that compensate for the change of corneal asphericity and induction of spherical aberration in a flying spot laser system, resulting in better surgery outcomes in laser refractive surgeries.