Search Results (33)

This study analyzed the impact of corneal epithelial refraction on the correction of hyperopic and mixed astigmatism eyes treated with transepithelial photorefractive keratectomy. From the epithelial refraction provided by the diagnostic device, OCT correlations were evaluated with respect to manifest refraction. The postoperative outcomes showed a mean sphere of −0.03 D and a mean cylinder of −0.33 D, with 93% and 98% having 0.5 D, 1 D, or less spherical equivalent refractive error. The epithelium showed preoperative toricity: at 6 mm, the epithelium showed a compensational effect of ~15% for the refractive astigmatism, whereas at 3 mm, the compensation accounted for ~25% of the refractive astigmatism. No correlation was found between preoperative epithelial refraction and refractive deviation after hyperopic or mixed astigmatic transepithelial photorefractive treatment. This work provides insight into the refractive compensatory impact of the epithelium, suggests how one can benefit from that in transepithelial corrections, and sets a framework for the potential induction of errors in non-transepithelial corrections. Full article

With the development of the internet, the incidence of myopia is showing a trend towards younger ages, making routine vision screening increasingly essential. This paper designs an online refractive error screening solution centered on the CFGN (Comparative Feature-Guided Network), a refractive error screening network based on the eccentric photorefraction method. Additionally, a training strategy incorporating an objective model-eye pretraining model is introduced to enhance screening accuracy. Specifically, we obtain six-channel infrared eccentric photorefraction pupil images to enrich image information and design a comparative feature-guided module and a multi-channel information fusion module based on the characteristics of each channel image to enhance network performance. Experimental results show that CFGN achieves an accuracy exceeding 92% within a ±1.00 D refractive error range across datasets from two regions, with mean absolute errors (MAEs) of 0.168 D and 0.108 D, outperforming traditional models and meeting vision screening requirements. The pretrained model helps achieve better performance with small samples. The vision screening scheme proposed in this study is more efficient and accurate than existing networks, and the cost-effectiveness of the pretrained model with transfer learning provides a technical foundation for subsequent rapid online screening and routine tracking via networking. Full article

Background: Managing post-keratoplasty astigmatism remains challenging. Even though graft viability is the main concern in keratoplasty, astigmatism might hinder vision recovery following a successful corneal transplant. Photorefractive keratectomy (PRK) and topography-guided PRK may be options for correcting refractive errors in patients who underwent keratoplasty. The aim of the study was to compare the results of PRK and topography-guided PRK in patients who had undergone corneal keratoplasty. Methods: This study was conducted at the Chair and Ophthalmology Clinic of the Medical University of Silesia, at the Railway Hospital Katowice, from 2023 to 2024. Patients who underwent penetrating keratoplasty due to keratoconus or corneal scar (post-traumatic and post-inflammatory) with a residual spherical myopic or mixed myopic astigmatic refractive defect were included in this study. The studied patients were divided into two groups: 15 patients underwent PRK and 15 patients underwent topography-guided PRK. Each participant underwent a preoperative examination, including uncorrected visual acuity (UCVA) and best spectacle-corrected visual acuity (BSCVA) measured on the Snellen chart (LCD panel for visual acuity testing Frey CP-400, Optotech Medical, Niepołomnice, Niepołomice, Poland), cycloplegic refraction, corneal pachymetry and topography (Schwind Sirius+, Pentacam), wavefront aberrations (Schwind Peramis, Pentacam), applanation tonometry, and anterior and posterior segment examinations, conducted at baseline and 1, 3, 6, and 12 months. Results: Keratoconus was the most common reason for keratoplasty (80% vs. 60%). Following PRK, the mean KI in group (1) decreased significantly to 43.88 ± 3.64 (p < 0.001), and in the group (2), the mean diopters decreased significantly after the intervention to 46.46 ± 2.80 (p < 0.001). The mean spherical refractive error in group (2) changed significantly after the intervention, reaching −2.72 ± 1.28 D (p < 0.001). The mean cylinder in group (1) changed significantly after PRK to have a mean of −2.75 ± 1.44 D (p < 0.001). Also, in group (2), it changed significantly after the intervention to have a mean of −2.95 ± 1.99 D (p < 0.001). There was a significant increase in both uncorrected visual acuity (UCVA) and best-corrected visual acuity (BCVA) after topography-guided PRK at all the follow-up points of 1, 3, 6, and 12 months (p < 0.001). The mean higher-order corneal aberrations (HOAs) after topography-guided PRK were 1.33 ± 0.50, ranging from 0.22 to 2.34 (p < 0.001). Conclusions: Topography-guided PRK is safer and more effective in correcting aberrations and refractive errors after corneal keratoplasty than regular PRK. Additionally, topography-guided PRK reduces high-order aberrations by utilizing advanced topographic mapping of the cornea, enabling precise customization of the treatment to address individual corneal surface irregularities. Full article

Based on the analysis of the IR transmission spectra in the region of stretching vibrations of hydrogen atoms of OH⁻-groups, it was established that the oxygen-octahedral MeO₆ clusters (Me-Li⁺, Nb⁵⁺, vacant octahedron V, impurity ion) of the structure of the compositionally homogeneous crystal LiNbO₃:Er³⁺(3.1 wt%) and the gradient crystal LiNbO₃:Er³⁺(congruent composition by the main components, Er gradient of 0.55 at%/cm) have a shape close to the regular one. In this case, the value of R = [Li]/[Nb] ≈ 1, and in the structure of both crystals, there are practically no point defects in Nb_Li responsible for the photorefraction effect. By using the IR transmission spectra and Klauer’s method, it was found that the volume concentration of OH⁻-groups in the gradient crystal LiNbO₃:Er³⁺ is almost an order of magnitude lower than in the compositionally homogeneous LiNbO₃:Er³⁺(3.1 wt%) crystal. This fact explains the lower hydrogen conductivity of the gradient crystal LiNbO₃:Er³⁺ and the lower photorefraction effect compared to the compositionally homogeneous LiNbO₃:Er³⁺(3.1 wt%) crystal. The results obtained are important for the development of materials for active nonlinear laser media and for the conversion of laser radiation. Full article

This paper presents an open-source time-dependent three-dimensional scalar photorefractive beam propagation model (PRProp3D) based on the well-known split-step method. The angular spectrum method is used for the diffractive steps, and the nonlinearities accumulated at the end of each diffractive step are applied using spatially varying phase screens. Comparisons with previously published experimental results are given for image amplification, photorefractive amplified scattering (fanning) and photorefractive screening solitons. Artifacts can be mitigated by use of step sizes less than 5~10 micrometers and by careful choice of the transverse computation grid size to ensure adequate sampling. Wraparound effects associated with the use of discrete Fourier transforms are mitigated by apodization and beam centering. Full article

The transient dynamics of photocurrents for poly((4-diphenylamino)benzyl acrylate) (PDAA)-based photorefractive (PR) polymers sensitized with perylene bisimide derivative N,N′-diisopropylphenyl-1,6,7,12-tetrachloroperylene-3,4,9,10-tetracarboxyl bisimide (PBI) at various composition ratios were studied. The PR polymer included (4-(diphenylamino)phenyl)methanol (TPAOH) photoconductive plasticizer and (4-(azepan-1-yl)-benzylidene) malononitrile nonlinear optical dye as well, which are needed for inducing PR effects. All the photocurrents measured at 640 nm were well simulated by a two-trapping site model considering photocarrier generation and recombination processes of the charge transfer (CT) complex between PBI and PDAA. The process of photocurrent simulation allowed for analyses of the dependences of hole mobility, quantum efficiency (QE) of photocarrier generation, trapping parameters, and recombination coefficient on the PDAA/TPAOH content. Finally, the PDAA content dependences of the trapping and recombination properties were compared with those of the PR parameters of the optical diffraction efficiency, optical gain, and response time. Full article

Although effective, a portion of photorefractive keratectomy (PRK) patients will suffer residual myopia or relapse to myopic regression. This retrospective, non-randomized, comparative study, aimed to compare the efficacy of primary PRK versus PRK performed as retreatment after previous surgery for myopia. Data regarding the right eye of 220 consecutive myopic patients undergoing repeat or primary PRK in 2013–2017 were extracted. Groups were matched for demographics and preoperative spherical equivalent, sphere, astigmatism, uncorrected and corrected distance visual acuity (UDVA and CDVA). Primary outcomes were an efficacy index (ratio between the postoperative UDVA and the preoperative CDVA), a safety index (ratio between the postoperative and the preoperative CDVA), postoperative UDVA and CDVA, and deviation from target refraction. Primary PRK showed significant superiority in logMAR UDVA (0.01 ± 0.05 versus 0.05 ± 0.10, p = 0.001), logMAR CDVA (0.01 ± 0.05 versus 0.04 ± 0.08, p = 0.01), efficacy index (1.00 ± 0.05 versus 0.97 ± 0.09, p = 0.003) and safety index (1.00 ± 0.06 versus 0.98 ± 0.08, p = 0.04) compared to repeat PRK, but had a significantly higher share of patients with postoperative spherical equivalent (74.5% versus 67.3%) and cylinder (74.5% versus 68.2%) in the range of ±0.5 D. To conclude, enhancement PRK leads to inferior efficacy and safety with greater deviation from target refraction. Adjusted nomograms for repeat PRK may be warranted. Full article

Background/Objectives: This study aims to evaluate the efficacy, safety, and predictability of Transepithelial Photorefractive Keratectomy (TPRK) using the SmartPulse^® technology excimer laser for the correction of myopia and myopic astigmatism, assessing visual and refractive outcomes over a one-year follow-up period. Methods: This retrospective cohort study analyzed data from patients who underwent TPRK at the Ophthalmology Department—“Victor Babes” University of Medicine and Pharmacy in Timisoara (Romania), between January 2019 and June 2023. The procedure was performed using the SmartPulse^® Technology of the SmartSurf^ACE AMARIS 750S excimer laser (SCHWIND eye-tech-solutions, Kleinostheim, Germany). Preoperative assessments included visual acuity, refraction, and corneal measurements, with postoperative evaluations conducted for up to 12 months. Results: This study included 92 eyes from 46 patients (mean age 29.02 years, 63% male). At 12 months post-op, 100% achieved UDVA 20/25 or better, with an efficacy index of 1.01. Refractive accuracy was 96% within ±0.50 D of the target and astigmatism ≤ 0.50 D in 99% of eyes. The safety index was 1.01. Corneal haze occurred in 8.70% of eyes and was effectively managed with dexamethasone drops. Conclusions: TPRK with the SmartPulse^® technology excimer laser demonstrated high efficacy and safety in correcting myopia and myopic astigmatism, achieving stable visual outcomes over one year. The procedure also showed excellent predictability with a low incidence of complications, supporting its use as a reliable refractive surgery option. Full article

Background: Single-step transepithelial photorefractive keratectomy (TPRK) is a modern refractive surgery technique that offers a no-touch approach for correcting myopia and astigmatism. This study aims to evaluate the short-term visual and refractive outcomes of TPRK in patients with myopia and astigmatism over a 6-month follow-up period. Methods: This retrospective cohort study included patients who underwent single-step TPRK using the Amaris 750S laser platform with SmartSurf^ACE and SmartPulse^® technologies, targeting a plano refraction. Procedures were performed with aspheric, non-wavefront-guided profiles, and outcomes were assessed postoperatively. Results: 96% of eyes achieved 20/20 or better uncorrected distance visual acuity (UDVA), with 98% reaching 20/25 or better, and 100% achieving 20/32 or better. UDVA was the same or better than preoperative corrected distance visual acuity (CDVA) in 96% of eyes, and no eyes lost two or more Snellen lines. Refractive outcomes showed strong precision, with 93% of eyes within ±1.00 D of the target. Astigmatism correction was accurate, with 100% of eyes having ≤1.00 D of astigmatism, and 80% achieving a precise astigmatism angle of error between −5° and 5°. Conclusion: Single-step TPRK with SmartSurf^ACE and SmartPulse^® technologies provides highly effective and predictable visual and refractive outcomes for myopia and astigmatism. The procedure consistently delivers precise corrections with minimal complications, making it a reliable option for refractive surgery. Full article

The propagation of light beams in photovoltaic pyroelectric photorefractive crystals is modelled by a specific generalization of the nonlinear Schrödinger equation (GNLSE). We use a variational approximation (VA) to predict the propagation of solitary-wave inputs in the crystals, finding that the VA equations involve a dilogarithm special function. The VA predicts that solitons and breathers exist, and the Vakhitov–Kolokolov criterion predicts that the solitons are stable solutions. Direct simulations of the underlying GNLSE corroborates the existence of such stable modes. The numerical solutions produce both regular breathers and ones featuring beats (long-period modulations of fast oscillations). In the latter case, the Fourier transform of amplitude oscillations reveals a nearly discrete spectrum characterizing the beats dynamics. Numerical solutions of another type demonstrate the spontaneous splitting of the input pulse in two or several secondary ones. Full article

The type and nature of refractive surgery procedures has greatly increased over the past few decades, allowing for almost all patient populations to be treated to extremely high satisfaction. Conventional photorefractive keratectomy involves the removal of the corneal epithelium through mechanical debridement or dilute alcohol instillation. An improvement to this method utilises laser epithelial removal in a single-step process termed transepithelial photorefractive keratectomy (transPRK). We explore the history of transPRK from its early adoption as a two-step process, identify different transPRK platforms from major manufacturers, and describe the role of transPRK in the refractive surgery armamentarium. This is a narrative review of the literature. This review finds that TransPRK is a safe and effective procedure that works across a variety of patient populations. Though often not seen as a primary treatment option when compared to other corneal-based procedures that offer a faster and more comfortable recovery, there are many scenarios in which these procedures are not possible. These include, but are not limited to, cases of corneal instability, previous refractive surgery, or transplant where higher-order aberrations can impair vision in a manner not amenable to spectacle or contact lens correction. We discuss refinements to the procedure that would help improve outcomes, including optimising patient discomfort after surgery as well as reducing corneal haze and refractive regression. Full article

A nonlinear optical double-doped single-crystal LiNbO₃:Gd:Mg (Gd concentration is 0.003, Mg—0.65 wt % in the crystal) has been researched by several optical methods: laser conoscopy, photoinduced light scattering (PILS), optical spectroscopy, and Raman scattering. The crystal has been shown to have no photorefraction effect and a high optical uniformity. Fine features of the crystal structure have been studied via Raman spectra. Spectra have been registered in the first and second orders, they have been excited by visible (532 nm) and near-IR (785 nm) lasers. Registered Raman spectra have the fundamental vibrations of the crystal lattice of the A₁(TO,LO)- and E(TO,LO)-type symmetry located in the range of 150–900 cm⁻¹. A number of low-intensity Raman bands in the 900–2000 cm⁻¹ region have been determined to correspond to the second-order Raman spectrum. These bands are polarized and appear only in certain polarization-scattering geometries. They appear in the spectrum excited by visible radiation, but their number and intensity are much lower than those excited by near-IR lasers. Oxygen-octahedral MeO₆ clusters in our case can contain Li, Nb, Gd, or Mg in the Me site. The clusters in the LiNbO₃:Gd:Mg crystal structure are slightly distorted compared with similar clusters in the nominally pure LiNbO₃ crystal. It has been established that the value R = [Li]/[Nb] in the studied crystal is ≈ 1. Such a ratio usually characterizes a nominally pure stoichiometric crystal. Full article

Two series of LiNbO₃:Mg:B crystals have been grown and studied. Two doping methods—have been used. The crystals—have been co-doped with Mg and a non-metallic dopant, B. The physicochemical features of the growth—have been considered for LiNbO₃:Mg:B crystals obtained from a boron-doped melt. The charge—has been prepared using different technologies: homogeneous (HG) and solid-phase (SP) doping. The same two methods have been used to grow single-doped LiNbO₃:Mg crystals. A control near-stoichiometric (NSLN) crystal—has been grown via the HTTSSG (high-temperature top-seeded solution growth) method from a congruent melt (Li/Nb ≈ 0.946) with 5.5 wt% K₂O. The characteristics of the LiNbO₃:Mg:B crystals—have been compared with those of the LiNbO₃:Mg and NSLN crystals. Physicochemical and structural reasons have been established for the differences in the distribution coefficients of magnesium (K_D) during the growth of the HG- and SP-doped LiNbO₃:B:Mg and LiNbO₃:Mg crystals. The optical characteristics of the LiNbO₃:B:Mg crystals—have been studied via optical spectroscopy, laser conoscopy and photoinduced light scattering (PILS). The influence of boron on the microstructure, compositional and optical uniformities and optical damage resistance of the LiNbO₃:Mg:B crystals—has been estimated. Optimal technological approaches to growing optically uniform LiNbO₃:B:Mg crystals have been determined. LiNbO₃:Mg:B crystals have been shown to have a significant advantage over the commercially used LiNbO₃:Mg crystals since large LiNbO₃:Mg:B crystals can be grown without stripes. Such stripes usually appear perpendicular to the growth axis. In addition, the photorefractive effect is suppressed in LiNbO₃:Mg:B crystals at lower magnesium concentrations ([Mg] ≈ 2.5 mol%) than in LiNbO₃:Mg ([Mg] ≈ 5.5 mol%). Full article

Bowman’s layer is an acellular layer in the anterior stroma found in the corneas of humans, most other primates, chickens, and some other species. Many other species, however, including the rabbit, dog, wolf, cat, tiger, and lion, do not have a Bowman’s layer. Millions of humans who have had photorefractive keratectomy over the past thirty plus years have had Bowman’s layer removed by excimer laser ablation over their central cornea without apparent sequelae. A prior study showed that Bowman’s layer does not contribute significantly to mechanical stability within the cornea. Bowman’s layer does not have a barrier function, as many cytokines and growth factors, as well as other molecules, such as EBM component perlecan, pass bidirectionally through Bowman’s layer in normal corneal functions, and during the response to epithelial scrape injury. We hypothesized that Bowman’s layer represents a visible indicator of ongoing cytokine and growth factor-mediated interactions that occur between corneal epithelial cells (and corneal endothelial cells) and stromal keratocytes that maintain the normal corneal tissue organization via negative chemotactic and apoptotic effects of modulators produced by the epithelium on stromal keratocytes. Interleukin-1 alpha, produced constitutively by corneal epithelial cells and endothelial cells, is thought to be one of these cytokines. Bowman’s layer is destroyed in corneas with advanced Fuchs’ dystrophy or pseudophakic bullous keratopathy when the epithelium becomes edematous and dysfunctional, and fibrovascular tissue commonly develops beneath and/or within the epithelium in these corneas. Bowman’s-like layers have been noted to develop surrounding epithelial plugs within the stromal incisions years after radial keratotomy. Although there are species-related differences in corneal wound healing, and even between strains within a species, these differences are not related to the presence or absence of Bowman’s layer. Full article

Various crystalline silicon carbide (SiC) polytypes are emerging as promising photonic materials due to their wide bandgap energies and nonlinear optical properties. However, their wafer forms cannot readily provide a refractive index contrast for optical confinement in the SiC layer, which makes it difficult to realize a SiC-based integrated photonic platform. In this paper, we demonstrate a 3C-SiC-on-insulator (3C-SiCoI)-based integrated photonic platform by transferring the epitaxial 3C-SiC layer from a silicon die to a borosilicate glass substrate using anodic bonding. By fine-tuning the fabrication process, we demonstrated nearly 100% area transferring die-to-wafer bonding. We fabricated waveguide-coupled microring resonators using sulfur hexafluoride (SF₆)-based dry etching and demonstrated a moderate loaded quality (Q) factor of 1.4 × 10⁵. We experimentally excluded the existence of the photorefractive effect in this platform at sub-milliwatt on-chip input optical power levels. This 3C-SiCoI platform is promising for applications, including large-scale integration of linear, nonlinear and quantum photonics. Full article