Central Dark Spot Research Articles

Laguerre-Gaussian Beams with an Increased Dark Area and Autofocusing

We introduce and investigate a novel Laguerre-Gaussian (LG) beam, different from the conventional modal LG beams, which conserve the transverse intensity structure (up to scale) on propagation. The proposed beam does not conserve its structure on free space propagation but possesses some interesting properties. This beam is Fourier-invariant, and it has an increased dark area both in the initial (waist) plane and in the far field. Thus, without changing the topological charge of the beam, varying the radial (lower) index of the associated Laguerre polynomial allows increasing or decreasing the effective diameter of the central dark spot in the intensity pattern. In addition, the beam is autofocusing, i.e., the intensity distribution at the Rayleigh distance from the waist has a shape of the light ring (at any value of the radial index) with the minimal diameter and with the maximal on-ring intensity. Such a beam can be adopted for microparticle manipulation. Increasing the dark area in the focus of a high-aperture spherical lens allows the simultaneous trapping of several absorbing microparticles into this dark area.

Compact dark-field confocal microscopy based on an annular beam with orbital angular momentum.

This study proposes a dark-field confocal microscopy (DFCM) based on fiber-mode excitation-assisted orbital angular momentum annular beam generation with a two-mode fiber to avoid diffraction distortion. The application of optical fibers compresses the DFCM volume, providing new ideas for system miniaturization. System adjustment difficulty is reduced and feasibility is verified by imaging 2D and 3D samples. High imaging contrast is achieved by fully blocking the reflected light and annular illumination with a steady central dark-spot within a propagation distance of 3 m. The application of our scheme can be further extended to detect subsurface defects in optical components and high-contrast biological imaging.

Clinical characteristics and prognostic factors analysis of ethambutol-induced optic neuropathy

Objective To observe the clinical features and visual prognostic factors of ethambutol-induced optic neuropathy (EON). Methods A cohort study. Twenty-four inpatients (46 eyes) identified as EON in Neuro-Ophthalmology Department of Chinese PLA General Hospital from January 2014 to December 2017 were enrolled, including 14 males (26 eyes) and 10 females (20 eyes) with a ratio of 1.4/1 male/female. The average age was 42.79±15.12 years and the average weight was 62.46±12.31 kg. The average time duration between oral administration of ethambutol and occurrence of EON was 9.94±16.49 months. The average time of ethambutol duration was 7.06±11.68 months, with an average accumulative dose of 156.7±1 779.0 g and the average daily dose of 15.07±8.95 mg/(kg·d). All patients were tested with visual acuity, fundus photos, colour vision, OCT, visual field, VEP, orbital MRI and the gene of OPA1 and mitochondrial deoxyribonucleic acid(mtDNA). All the patients accepted drug withdrawal immediately after diagnosis, and were given the treatment of systemic nerve nutrition and improvement of microcirculation for 2 weeks. The time of follow-up was more than 12 months. According to whether the visual acuity (VA) in any of eyes was over than 0.1 at the last follow- up, all the patients were divided into two groups: the bad VA group (VA less than or equal to 0.1) and the better VA group (VA over than 0.1) group. The χ2 test and Fisher's exact probabilistic method test were used to compare the counting data between groups, and the Wlincox rank sum test was used to compare the measurement data. Multiple factors of VA outcome between the patients with bad or better va were analyzed by logistic regression. Results Thirty eyes (65.2%) had VA less than or equal to 0.1 and 5 eyes (10.9%) had VA over than 0.5 at EON onset. The VA of the rest 11 eyes (23.9%) was higher than 0.1 and lower than 0.5. At the last follow- up, 20 eyes (43.5%) had VA less than or equal to 0.1 and 9 eyes (19.6%) had VA over than 0.5, the VA of the rest 17 eyes (36.9%) was higher than 0.1 and lower than 0.5. Fundus examination revealed 7 eyes (15.3%) with optic disc edema. OCT revealed significant loss of the retinal nerve fiber layer (RNFL) in the affected eyes, mainly in the temporal RNFL of the optic disc. All patients had dyschromasia, mainly in distinguishing the color of red and green. The types of visual field defect was as following: central dark spot (52.2%), diffuse visual acuity decreased (30.4%), temporal hemianopsia (17.4%). Orbital MRI revealed that 12/24 (50.0%) patients had T2 lesions with T1 enhancement in 6/24 patients (25.0%). Genetic test showed that 4 patients (16.7%) had gene mutation. Among them, there were 2 patients with OPA1 mutation, 1 with mtDNA 14340 point mutation and 1 with the mtDNA 11778 point mutation. Thirteen patients showed better VA outcomes (over than 0.1) while 11 showed bad VA outcomes after discontinuation of ethambutol. Between the better VA group and the bad VA group, there were statistically significant differences in the daily dose of ethambutol and gene mutation(P=0.031, 0.023). The daily dose was related to visual prognosis of EON while only the daily dose of more than 18 mg/(kg·d) may lead to bad VA outcomes according to the logistic analysis (95% CI 0.007-0.736, OR=0.069, P=0.027). Conclusions EON may have OPA1 and mtDNA mutation with more bilateral eyes involved and less optic edema, which about 43.5% of the patients showed irreversible visual impact. The daily dose of ethambutol is related to the vision recovery. Key words: Ethambutol/adverse effects; Optic nerve diseases; Disease attributes; Prognosis; Root cause analysis

Transformation of double-half inverse Gaussian hollow beams into superposition of finite Airy beams using an optical Airy transform

In the present work, we investigate theoretically the transformation of a double-half inverse Gaussian hollow (DHIGH) beam into a superposition of finite Airy beams by using an optical Airy transform system. The analytical expression of the generated finite Airy beam is derived by means of the generalized Huygens–Fresnel integral diffraction. Numerical calculations show that the optical Airy transform system converts the central dark spot of the hollow incident beam into peak intensity. It is demonstrated that the characteristics of the transverse intensity distribution of the output finite Airy beam are determined by the phase pattern’s constants of the Airy transform system and the radius size of the incident DHIGH beam. Moreover, it is shown that the profile of the central peak intensity of the generated beam can be controlled by means of the radius size of the incident hollow DHIGH beam.

Research on taper zone coupling from single-core fiber to annular-core hollow beam fiber

We designed and manufactured a novel annular-core hollow beam fiber which could directly yield ring light with a central dark spot inside the beam employing MCVD technique and a custom-made fiber drawing tower. The tapered fiber zone geometric shapes at varied stretching speed between the single-core fiber and the annular-core hollow beam fiber were studied theoretically. According to the beam propagation method, the bi-tapered coupling energy transmission between these two fibers was simulated and analyzed. Moreover, by adopting a fusion splicing and stretching technique at the fiber-linked point, an effective coupling approach had been fulfilled.

Anisotropic Swirling Surface Acoustic Waves from Inverse Filtering for On-Chip Generation of Acoustic Vortices

From radio-electronics signal analysis to biological samples actuation, surface acoustic waves (SAW) are involved in a multitude of modern devices. Despite this versatility, SAW transducers developed up to date only authorize the synthesis of the most simple standing or progressive waves such as plane and focused waves. In particular, acoustical integrated sources able to generate acoustical vortices (the analogue of optical vortices) are missing. In this work, we propose a flexible tool based on inverse filter technique and arrays of SAW transducers enabling the synthesis of prescribed complex wave patterns at the surface of anisotropic media. The potential of this setup is illustrated by the synthesis of a 2D analog of 3D acoustical vortices, namely "swirling surface acoustic waves". Similarly to their 3D counterpart, they appear as concentric structures of bright rings with a phase singularity in their center resulting in a central dark spot. Swirling SAW can be useful in fragile sensors whose neighborhood needs vigorous actuation, and may also serve as integrated transducers for acoustical vortices. Since these waves are essential to fine acoustical tweezing, swirling SAW may become the cornerstone of future micrometric devices for contactless manipulation.

Clad photon sieve for generating localized hollow beams

A novel photon sieve structure called clad photon sieve is proposed to generate localized hollow beams and its design principle and focusing properties are studied. The clad photon sieve is composed of the internal zone and external zone with pinholes being positioned on the dark zones. Pinholes in the internal zone and in the external zone give destructive interference to the focus, leading to localized hollow beams being generated on the focal plane. Focusing properties of clad photon sieve with different focal lengths, zone numbers and modulation factors are also studied by theoretical calculations, numerical simulations and experiments, showing that the central dark spot size can be controlled by the focal length and rings number, and the intensity of the central dark spot varies with different modulation factors related with the internal zone and the external zone. This photon sieve can be useful for trapping and manipulating of particles and cooling of atoms.

Herpes Labialis-Induced Erythema Multiforme along Blaschko's Lines.

Dear Editor: Erythema multiforme (EM), first described by Von Hebra in 1862, is an acute, self-limiting mucocutaneous condition induced by some immune reaction to infections or pharmacologic antigens1. The distribution of EM along Blaschko's lines (BL) is extremely unusual. A 28-year-old, non-consanguineous Hindu male patient presented with an asymptomatic linear lesion of 3 days' duration along with multiple vesicular eruptions over the right side of his upper lip for 5 days. The lesion consisted of multiple erythematous annular eruptions. He had a history of recurrent vesicular eruptions on the lip during the last 5 years. There was no other systemic illness or history of similar skin rash. Physical examination revealed the presence of multiple, discrete, small target-like lesions, most having a central dark spot, on the right forearm along BL. Some lesions were confluent, forming a bigger and irregularly shaped lesion. A group of tiny vesicular eruptions were present on the upper lip (Fig. 1A, B). Fig. 1 Erythema multiforme on the right forearm along Blaschko's line. Insets: (A) grouped vesicles with crusting due to herpes labialis on the upper lip, (B) close-up of the target lesions. (C) Low-power view (H&E, ×100) showing an intact epidermis ... It was diagnosed clinically as EM along BL with recurrent herpes labialis. Routine tests of blood, urine, and stool samples showed no abnormality. Biopsy and histopathological examination from a lesion on forearm revealed exocytosis, extensive interface dermatitis, necrotic keratinocytes, dermal edema, and mononuclear cell infiltration in the upper dermis (Fig. 1C). Proposed mechanisms of EM include type IV hypersensitivity immune response mediated by T lymphocytes2, genetics and infections. The most possible etiologic factor is herpes simplex virus. Infection due to Mycoplasma pneumoniae, vulvovaginal candidiasis, hepatitis C virus; drugs like acetaminophen, penicillin, and sulfonamides; and diseases like systemic lupus erythematosus (SLE), small vessel vasculitis, urticaria, and granuloma annulare are also implicated. The lesions of EM usually start in a symmetric fashion over the acral areas and extensor surfaces of the limbs, and progress centripetally. Koebnerization may influence the distribution. BL are known to represent mosaicism3. Besides nevi, autosomal and X-linked dominant diseases; polygenic diseases such as lichen planus, psoriasis, eczema, SLE, pemphigus vulgaris, granuloma annulare, linear drug eruption, and graft-versus-host disease may also be distributed along BL. In polygenic diseases, distribution along BL as an isolated manifestation or as a superimposed manifestation on classic generalized disease may occur owing to postzygotic mutation involving one of the predisposing genes. This is also known as loss of heterozygosity (LOH), which gives rise to a patch of homozygous or hemizygous tissue. The possible mechanisms for the development of LOH are mitotic recombination, gene conversion, point mutation, deletion, or mitotic nondisjunction. Blaschko-linear EM is extremely rare and has been described in only two cases (Table 1)4,5. None of these previous cases involved active herpes labialis at the time of presentation. One of them, however, involved a history of recurrent herpes labialis5. The case reported by Micalizzi and Farris5 was linear; however, morphological assessment of the lesions was difficult with the clinical photograph provided. Our case possibly is the first case of EM associated with active herpes labialis and with a distribution along BL. This case provides strong evidence in support of a genetic mechanism in the pathogenesis of EM. Table 1 Comparing all published cases of linear erythema multiforme4,5

Esophageal Inlet Patch: A Rare Cause of Melena

Purpose: Esophageal inlet patches are congenital anomalies of heterotopic gastric mucosa occurring in the proximal esophagus. They are infrequently seen, generally asymptomatic and typically do not warrant intervention. While rare complications may occur, there is only one documented case of bleeding from an inlet patch. We report an unusual case of melena resulting from an esophageal inlet patch. A 41-year-old woman with a past medical history of gastric bypass surgery was referred for evaluation of melena. She had a history of gastroesophageal reflux which was self-managed with intermittent antacids or over-the-counter lansoprazole. She denied nausea, vomiting, dysphagia, odynophagia, melena, and hematochezia. Her exam revealed stable vital signs, a benign abdomen with healed surgical scars and hemoccult positive stool. A colonoscopy revealed no abnormalities. An UGI with small bowel follow-through was normal. An EGD showed a 1-cm, irregular area of salmon-colored mucosa in the proximal esophagus with a central dark spot consistent with recent bleeding. No hemostatic intervention was required. Biopsies of the area revealed gastric mucosa with chronic inflammation. The gastric pouch, anastomosis, and small bowel mucosa was without abnormality. She was treated with omeprazole with resolution of her melena. There is limited available information in the literature on esophageal inlet patches. Endoscopy reveals salmon-colored mucosa distinct from the surrounding esophageal mucosa. Definitive diagnosis is made by histologic evaluation revealing gastric mucosa. Clinical manifestations are unusual, ranging from acid-related manifestations to dysplastic changes. The relationship with Helicobacter pylori is unclear. However, there has been only one previously reported case of bleeding associated with an inlet patch. This case illustrates the importance of careful endoscopic assessment and high level of suspicion in the appropriate clinical circumstances.

Continuous manipulation of doughnut focal spot in a large scale

We theoretically demonstrate that the doughnut focal spot can continuously be manipulated by synthetically using various beam modulation techniques. Comparatively, a more evident effect can be expected by different orders of phase modulation, while accurate manipulation stems from changing the phase diversity between two arms in an image inverting interferometer (III). The size of central dark spot can thus be continuously adjusted in a theoretically infinite scale, although it may actually be limited by resolution of Spatial Light Modulator (SLM). This approach brings additional flexibility to many applications, such as optical tweezers.

Manipulation of doughnut focal spot by image inverting interferometry

Based on image inverting interference combined with phase modulation, we theoretically demonstrate that the doughnut focal spot can readily be manipulated, and either shrinkage or expansion of size of the central dark spot is possible in a large scale (peak-to-peak value: 0.555λ-0.830λ, or 93.3%-140.8% compared with the standard one). As the interference and phase modulation can both be achieved by a double Porro prism, it is feasible to introduce this approach into optical tweezers to improve their performance. As much as 33.9% intensity of stimulated emission depletion (STED) beam can be reduced if the further optimized configuration is utilized in STED microscopy.

Propagation of confluent hypergeometric beam through uniaxial crystals orthogonal to the optical axis

Based on the theory of paraxial electromagnetic beams in anisotropic medium, propagation properties of a linearly polarized confluent hypergeometric (HyG) beam through uniaxial crystals orthogonal to the optical axis are investigated. Analytical expressions are derived for the resultant optical field components. Effects of separate beam parameters together with the ratio of refractive indices of crystals on transversal intensity distributions are revealed by numerical calculations, respectively. It is shown that the beam profile finally converts into an elliptical shape due to influences of anisotropic crystals. Moreover, it is also indicated that through suitable selection of parameters, types of the optical vortices of resultant field would change from a central dark spot to a slight dark ring or an axial shadow during the propagation. These numerical results may provide alternative advantages to the trapping of tiny particles by utilizing HyG beams in practical experiments.

The predictive value of subjective symptoms and clinical signs for the presence of treatment-requiring exudative age-related macular degeneration

The introduction of vascular endothelial growth factor inhibitors for the treatment of exudative age-related macular degeneration (AMD) has increased the referral rates of AMD patients with visual symptoms to treating centres considerably. However, a large proportion of the referred patients do not qualify for treatment implying that considerable resources could be saved if these patients could be identified on the basis of the clinical data available in the referring nonspecialized setting. A prospective observational study of 1682 consecutive patients referred with suspicion of exudative AMD qualifying for intravitreal angiostatic treatment. On the basis of the structured interviewing about symptoms, ophthalmoscopy, optical coherence tomography scanning, and fluorescein angiography, the patients were divided into two groups: one qualifying for and another not qualifying for treatment. Multiple logistic regression was used to identify independent parameters predicting the need for treatment. The presence of metamorphopsia, dyschromatopsia, retinal haemorrhages and exudates, central retinal thickness, and the absence of micropsia were highly significant individual determinants of treatment-requiring AMD. Sudden onset and worsening of symptoms and the presence of a central dark spot covaried with the occurrence of retinal haemorrhages, whereas reduced visual acuity and blurred vision covaried with the presence of both haemorrhages and exudates. Patients with treatment-requiring AMD can be reliably identified by questioning about the presence of metamorphopsia and dyschromatopsia and the absence of micropsia, combined with ophthalmoscopical detection of retinal haemorrhages and exudates. This information may improve the triage of patients considered for referral.

Propagation and diffraction of optical vortices

We explore the propagation and diffraction of optical vortices (Laguerre–Gaussian beams) of varying azimuthal index past a circular obstacle and Young’s double slits. When the beam and obstacle centers are aligned the famous spot of Arago, which arises for zero azimuthal index, is replaced for non-zero azimuthal indices by a dark spot of Arago, a simple consequence of the conserved phase singularity at the beam center. We explore how for larger azimuthal indices, as the beam and obstacle centers are progressively misaligned, the central dark spot breaks up into several dark spots of Arago. Using Young’s double slits we can easily measure the azimuthal index of the vortex beam, even for polychromatic vortices generated by broadband supercontinuum radiation.

The dark spots of Arago

We explore the diffraction and propagation of Laguerre- Gaussian beams of varying azimuthal index past a circular obstacle both experimentally and numerically. When the beam and obstacle centers are aligned the famous spot of Arago, which arises for zero azimuthal index, is replaced for non-zero azimuthal indices by a dark spot of Arago, a simple consequence of the conserved phase singularity at the beam center. We explore how the dark spot of Arago behaves as the beam and obstacle centers are progressively misaligned, and find that the central dark spot may break into several dark spots of Arago for higher incident azimuthal index beams.

A tourist with tungiasis

A 35-year-old man presented with a slightly painful papule with a central dark spot on his right heel ([Figure 1][1]). It had developed 2 weeks before his return to Canada after a 3-month vacation in Ecuador. The lesion was typical of tungiasis, which is endemic to Ecuador. A skin biopsy revealed

Formation and evolution of far-field diffraction patterns of divergent and convergent Gaussian beams passing through self-focusing and self-defocusing media

The formation and the evolution of the far-field patterns of the Gaussian beams passing through a thin self-focusing medium and a thin self-defocusing medium are studied using the Fresnel–Kirchhoff diffraction theory, and the effects of the different Kerr media and the different Gaussian beams on the far-field pattern formation and evolution are analysed by taking into consideration both the change in the additional phase shift induced by the refractive index and the change in the sign of the radius of wavefront curvature of the laser beam passing through the nonlinear medium. Our results show that, when either the divergent Gaussian beam passes through the self-defocusing medium or the convergent Gaussian beam passes through the self-focusing medium, the far-field intensity distribution pattern is a series of thick diffraction rings with a central dark spot; while, when either the divergent Gaussian beam traverses the self-focusing medium or the convergent Gaussian beam traverses the self-defocusing medium, the far-field intensity distribution pattern is a series of thin diffraction rings with a central bright spot. Our results also show that whether the central dark spot or the central bright spot occurs depends mainly on the sign of the product of the additional phase shift and the radius of the wavefront curvature. When the sign is negative, that is, when the divergent Gaussian beam passes through the self-defocusing media, or the convergent Gaussian beam passes through the self-focusing media, the central dark spot surrounded by the thick diffraction rings will emerge in the far field; while, when the sign is positive, that is, when the divergent Gaussian beam traverses the self-focusing media, or the convergent Gaussian beam passes through the self-defocusing media, the central bright spot with the thin diffraction rings will occur in the far field. The difference between the diffraction patterns is attributed to the interplay of the strong spatial self-phase modulation caused by the refractive index change of the medium and the changes in sign of the radius of the wavefront curvature of the incident Gaussian beam. The results obtained in this paper are of significance to the design of practical nonlinear optical limiters for the eye or sensor protection and many other applications.

Bessel function output from an optical correlator with a phase-only encoded inverse filter

We report on a technique for producing a Bessel function correlation output for an arbitrary input pattern. The central dark spot at the center of the Bessel function correlator output is narrower than the width of the normal correlation spot and can be extremely useful for locating the center of the correlation signal. The Bessel function is produced by convolution of the extremely sharp correlation produced by an inverse filter with the Bessel function and is encoded with a single phase-only liquid-crystal spatial light modulator. To encode amplitude information on the filter, we spatially modulate the phase encoded on the filter. Amplitude modulation is obtained by modulation of the diffraction efficiency of the phase grating. Experimental results are presented.

Analysis of dark spot formation in absorbing liquid media

A theoretical analysis based on coupled field-matter equations is given to describe the recently observed phenomenon of a central dark spot formation of a Gaussian beam transmitted through an absorbing defocusing liquid medium. We find that such a pattern formation, which is accompanied by normal defocusing rings in the far field, originates from interplay between the wave-front curvature of the Gaussian beam and strong spatial self-phase medulation arising from thermally induced refractive index change in the medium. Results of numerical analysis for a thin medium are shown to be in a good quantitative agreement with our experimental findings. Further, the dark spot formation is also predicted by using a focused Gaussian beam and self-focusing medium.

Phase shift microscopes

We report on a new class of scanning optical microscopes with twice the resolution of conventional microscopes. The new microscopes use standard components to produce high intensity images, with large working distances and large depths of focus, without additional mechanical complexity. The microscopes scan a focused laser beam which has passed through a pattern of phase shifts arranged in a spiral staircase. These phase shifts generate a narrow dark spot in the center of the focused beam. The width of this central dark spot is about half the diffraction limit. The signal from this “phase shifted” beam is subtracted from the signal produced by a conventional Gaussian beam which is simultaneously scanned. The difference between the two detected signals corresponds to the signal from a bright beam of width equal to the width of the dark central area. Since only linear operations on observed optical signals are used, there is a useful improvement in resolution of about a factor of 2. This improves both the accuracy with which feature edges may be located, as well as the resolution of small features such as the corners of narrow lines.