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Optics, Volume 2, Issue 1 (March 2021) – 6 articles

Cover Story (view full-size image): The present study aims to design the optimal anti-reflective (AR) engraving on the input-output apertures of a dielectric light-guide. We estimate AR efficiency by the transmittance level in the required wavelength range (Δλ/λ0 = 0.7). We consider dielectric light-guides possessing a high refractive index (Si), which reduces, on the one hand, the requirement to the angular characteristic of the source, but, on the other hand, complicates the reduction of the reflection level. Using nano-pins as engraving, we analyze such factors as packing density on the nano-elements on the aperture, the elements shape, and the possible effect of randomizing their parameters. The predicted level of transmittance is close to 99%, and the flatness (the standard deviation) in the required wavelength range is 0.2%. View this paper
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Article
Prophylactic Corneal Cross-Linking in Myopic Femtosecond Laser-Assisted In Situ Keratomileusis: Long-Term Visual and Refractive Outcomes
Optics 2021, 2(1), 55-62; https://0-doi-org.brum.beds.ac.uk/10.3390/opt2010006 - 15 Mar 2021
Viewed by 554
Abstract
The purpose of our study was to evaluate the safety, effectiveness, predictability, and stability of myopic and astigmatic laser-assisted in situ keratomileusis (LASIK) with simultaneous prophylactic corneal cross-linking (CXL) in thin corneas. In total, 100 eyes from 50 patients who were subjected to [...] Read more.
The purpose of our study was to evaluate the safety, effectiveness, predictability, and stability of myopic and astigmatic laser-assisted in situ keratomileusis (LASIK) with simultaneous prophylactic corneal cross-linking (CXL) in thin corneas. In total, 100 eyes from 50 patients who were subjected to myopic and astigmatism femtosecond LASIK with simultaneous prophylactic CXL were included. The design of the study was retrospective, longitudinal, and observational. All patients had a 48-month follow-up. The MEL 80 excimer laser was utilized with the Aberration Smart Ablation platform. CXL treatment was applied when the predicted stromal thickness was less than 330 µm. Patients’ mean age was 30.22 ± 5.97 years. Previous mean spherical equivalent was −5.50 ± 1.65 (−9.50 to −1.13) diopters (D). Postoperative mean spherical equivalent was −0.24 ± 0.29 (−0.85 to +0.50) D. Visual acuity (VA) of 20/20 or better was observed in 87% of the eyes and no eyes experienced VA loss. Spherical equivalent within ±0.50 D was observed in 93% of eyes, and 4% of eyes varied by 0.50 D or more between 3 and 48 months. Prophylactic corneal cross-linking with simultaneous femtosecond laser-assisted in situ keratomileusis in thin corneas proved to be effective, safe, and predictable. The results remained stable after 48 months of follow-up. Full article
(This article belongs to the Special Issue Novel Research on Optometry and Vision Sciences)
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Editorial
Acknowledgment to Reviewers of Optics in 2020
Optics 2021, 2(1), 54; https://0-doi-org.brum.beds.ac.uk/10.3390/opt2010005 - 01 Feb 2021
Viewed by 510
Abstract
Peer review is the driving force of journal development, and reviewers are gatekeepers who ensure that Optics maintains its standards for the high quality of its published papers [...] Full article
Article
Nature of Photoelectric Effect in a Ge-on-Si SPAD at Ultralow Energy in Incident Pulsed Laser Radiation
Optics 2021, 2(1), 45-53; https://0-doi-org.brum.beds.ac.uk/10.3390/opt2010004 - 31 Jan 2021
Viewed by 529
Abstract
The photoelectric effect in a Ge-on-Si single-photon avalanche detector (SPAD) at an ultralow energy in incident pulsed laser radiation is considered in the frame of the classical theory of the electrodynamics of continuous media. It is shown that the energy of [...] Read more.
The photoelectric effect in a Ge-on-Si single-photon avalanche detector (SPAD) at an ultralow energy in incident pulsed laser radiation is considered in the frame of the classical theory of the electrodynamics of continuous media. It is shown that the energy of incident laser radiation which is shared among a huge number of electrons in a Ge matrix can concentrate on only one of these through the effect of the constructive interference of the fields re-emitted by surrounding electrons. Conservation of energy in this case is upheld because of a substantial narrowing of the effective bandgap in heavily doped p-Ge, which is used in the design of the SPAD considered. Full article
(This article belongs to the Special Issue Feature Papers in Optics)
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Editorial
Towards Laser-Based Calibration-Free Quantification of Trace Elements
Optics 2021, 2(1), 43-44; https://0-doi-org.brum.beds.ac.uk/10.3390/opt2010003 - 04 Jan 2021
Viewed by 549
Abstract
It is told that Theodore Maiman called the first operating laser source he and his co-worker Charles Asawa realized 60 years ago [...] Full article
(This article belongs to the Special Issue 60 Years of Lasers—Laser–Matter Interaction)
Article
Optimising the AR Engraved Structure on Light-Guide Facets for a Wide Range of Wavelengths
Optics 2021, 2(1), 25-42; https://0-doi-org.brum.beds.ac.uk/10.3390/opt2010002 - 31 Dec 2020
Viewed by 611
Abstract
The present study is aimed at designing anti-reflective (AR) engraving on the input–output surfaces of a rectangular light-guide. We estimate AR efficiency, by the transmittance level in the angular range, determined by the light-guide. Using nano-engraving, we achieve a uniform high transmission over [...] Read more.
The present study is aimed at designing anti-reflective (AR) engraving on the input–output surfaces of a rectangular light-guide. We estimate AR efficiency, by the transmittance level in the angular range, determined by the light-guide. Using nano-engraving, we achieve a uniform high transmission over a wide range of wavelengths. In the past, we used smoothed conical pins or indentations on the faces of light-guide crystal as the engraved structure. Here, we widen the class of pins under consideration, following the physical model developed in the previous paper. We analyze the smoothed pyramidal pins with different base shapes. The possible effect of randomization of the pins parameters is also examined. The results obtained demonstrate optimized engraved structure with parameters depending on the required spectral range and facet format. The predicted level of transmittance is close to 99%, and its flatness (estimated by the standard deviation) in the required wavelengths range is 0.2%. The theoretical analysis and numerical calculations indicate that the obtained results demonstrate the best transmission (reflection) we can expect for a facet with the given shape and size for the required spectral band. The approach is equally useful for any other form and of the facet. We also discuss a simple way of comparing experimental and theoretical results for a light-guide with the designed input and output features. In this study, as well as in our previous work, we restrict ourselves to rectangular facets. We also consider the limitations on maximal transmission produced by the size and shape of the light-guide facets. The theoretical analysis is performed for an infinite structure and serves as an upper bound on the transmittance for smaller-size apertures. Full article
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Review
Signal and Image Processing in Biomedical Photoacoustic Imaging: A Review
Optics 2021, 2(1), 1-24; https://0-doi-org.brum.beds.ac.uk/10.3390/opt2010001 - 31 Dec 2020
Cited by 1 | Viewed by 1050
Abstract
Photoacoustic imaging (PAI) is a powerful imaging modality that relies on the PA effect. PAI works on the principle of electromagnetic energy absorption by the exogenous contrast agents and/or endogenous molecules present in the biological tissue, consequently generating ultrasound waves. PAI combines a [...] Read more.
Photoacoustic imaging (PAI) is a powerful imaging modality that relies on the PA effect. PAI works on the principle of electromagnetic energy absorption by the exogenous contrast agents and/or endogenous molecules present in the biological tissue, consequently generating ultrasound waves. PAI combines a high optical contrast with a high acoustic spatiotemporal resolution, allowing the non-invasive visualization of absorbers in deep structures. However, due to the optical diffusion and ultrasound attenuation in heterogeneous turbid biological tissue, the quality of the PA images deteriorates. Therefore, signal and image-processing techniques are imperative in PAI to provide high-quality images with detailed structural and functional information in deep tissues. Here, we review various signal and image processing techniques that have been developed/implemented in PAI. Our goal is to highlight the importance of image computing in photoacoustic imaging. Full article
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