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Generation of Higher Order Aberrations with Soft Contact Lens by Excimer Laser Ablation Geunyoung Yoon*, Keiu Tokumura, Takahisa Jitsuno, and Masahiro Nakatsuka INTRODUCTION METHOD Although the human eye has a relatively simple optical system, it is not perfect. It has been well known that the human eye suffers from the wavefront aberration that includes both lower (defocus and astigmatism) and higher order aberrations. The lower order aberration has been effectively corrected with conventional ophthalmic lenses such as spectacles and contact lenses. However, it is more complicated to correct for the higher order aberrations. Ocular wavefront sensing technique [1] allows us to precisely understand optical quality of the eye by measuring the wave aberrations. It also makes it possible to correct the higher order aberration using advanced methods such as adaptive optics [2], laser refractive surgery [3] and customized optics [4]. Optical and psychophysical tests have demonstrated that correcting these aberrations significantly improves visual performance even in normal eyes. The visual benefit is even more substantial when correcting the aberration in eyes having abnormal corneal conditions such as keratoconus (abnormal cone shape cornea) and penetrating keratoplasty (corneal transplant). Adaptive optics is a powerful and noninvasive tool to achieve higher order correction in real time. However, it is an impractical method simply because of the size of the entire system. Although refractive surgery has been proven to be practical and effective to correct the aberration, it is a non-reversible surgical method and its availability is restricted by factors such as corneal thickness and the amount of the aberration. The interest in using customized optics to correct the aberration has been increased. This special optics includes phase plates, customized contact lenses and intraocular lenses (IOLs). Phase plate (spectacles capable of correcting the higher order aberration) has been proven to be an effective way to compensate for the static aberrations in the eye. This special optical component has an arbitrary surface profile created to correct the eye’s aberrations on a plate made of optical plastic. The successful correction of the higher order aberration resulted in significant visual improvement in both normal and abnormal eyes. However, we also found that decentration of the eye’s pupil due to changes in a gaze angle results in poor correction performance causing reduction of visual benefit. To overcome this limitation, customized soft contact lenses which follow most of the eyes’ movements was proposed as an alternative way to correct the aberration. This report presents the feasibility of generating the higher order aberration with soft contact lenses of which surface was ablated by an excimer laser. A laser ablation algorithm was developed to calculate the laser pulse distribution to create an arbitrary pattern of the higher order aberration. Convolution theory was used to design effective overlapping of laser spots. The algorithm allowed for minimizing both the residual RMS error and surface roughness by determining laser ablation parameters such as beam size, ablation rate and transition zone. With this algorithm, an ArF2 pulse laser (Xantos S-200 ArF, Coherent) was used to ablate the surface of soft contact lenses. The laser spot diameter was 500 µm and the laser pulse fluence was 0.2 mJ that produced an estimated ablation depth per pulse of 0.05 µm on hydrated soft contact lens material. Figure 1 shows a schematic diagram of the apparatus for the laser ablation with the excimer laser. A telescope with convex and concave lenses was used to demagnify the original laser beam. A two-axes steering mirror was used to scan laser spots onto targeted locations on the surface. Pulse repetition rate was 40 Hz limited by scanning speed of the steering mirror. After the ablation, the higher order aberrations of the soft contact lenses immersed in saline solution were measured with a Shack-Hartmann wavefront sensor to evaluate the accuracy of the laser ablation [5]. Fig. 1. Optical layout to ablate a surface of soft contact lens with an ArF excimer laser. RESULT Theoretical calculation with the laser ablation algorithm indicated that a proper transition zone (extrapolated from the ablation profile of the optical zone) of at least half of the laser spot diameter can significantly reduce residual ablation error along the edge of the optical zone. The calculations for different amounts of overlapping spots also showed the tradeoffs between the amplitudes of the residual error and surface roughness. Both the residual error and surface roughness were decreased with the increased amount of spot overlapping. However, with too much overlapping, the residual RMS error was increased again. This is because of the fact that an ablation rate per pulse can not be infinitely small. Therefore, it is important to precisely determine the ablation parameters that minimize the error and roughness. In our experiment of ablating the surface of soft contact lenses, we created different individual Zernike’s higher order aberrations, coma (3rd order), trefoil (3rd order) and spherical aberration (4th order) with 1-µm RMS error. These aberrations were chosen since they have been measured as the most dominant higher order aberrations in eyes with abnormal conditions. The measured RMS values of the ablated soft contact lenses for coma, trefoil and spherical aberration were 1.02 µm, 0.85 µm and 0.97 µm, respectively. Although those targeted aberrations were accurately generated with the laser ablation, small but, significant amounts of other undesirable higher order aberrations were also observed. The residual higher order RMS wavefront errors of these samples were 0.26 µm, 0.31 µm, and 0.24 µm, respectively. differences in wavefront shapes are caused by the induced other higher order aberrations. CONCLUSION We successfully demonstrated the feasibility of using an ArF excimer laser to produce the higher order aberration on the surface of soft contact lenses. The same methodology can be used to ablate the back surface of soft contact lenses. Back surface customized soft contact lenses can have the potential to significantly reduce the ocular higher order aberration and to improve the stability of lens position after blinking, which provides stable correction of vision especially in eyes with abnormal corneal conditions. ACKNOWLEGEMENT(S) This research was supported by NIH Grant No. 5R01EY14999 and a RPB Grant. REFERENCE(S) [1] Liang J, Grimm B, Goelz S, Bille J, “Objective measurement of wave aberrations of the human eye with the use of a Hartmann-Shack wave-front sensor”, J. Opt. Soc. Am. A 11, 1949-1957 (1994). [2] Yoon G, Williams DR “Visual performance after correcting the monochromatic and chromatic aberrations of the eye”, J. Opt. Soc. Am. A 19, 266-275 (2002). [3] Yoon G, Jeong TM, Cox IG, Williams DR. “Vision improvement by correcting higher-order aberrations with phase plates in normal eyes” J. Refract. Surg.; 20 (5), S523-527 (2004). [4] MacRae SM, Applegate RA, Krueger RR “The future of customization”, Customized Corneal Ablation; The Quest for Supervision, Slack Inc., Chapter 30, 339-346 (2001). [5] Jeong TM, Menon M, Yoon G. “Measurement of wave-front aberration in soft contact lenses by use of a Shack-Hartmann wave-front sensor” Appl. Opt.; 44 (21) 4523-4527 (2005). *Department of Ophthalmology and Institute of Optics, University of Rochester Fig. 2. Wavefront maps for target and ablated aberrations, (a) spherical aberration, (b) trefoil, and (c) vertical coma. Wavefront maps for targeted and ablated aberration were also shown in Fig. 2. There was a good agreement in both amplitudes and shape between the maps. Slight