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Photonics, Volume 7, Issue 4 (December 2020) – 56 articles

Cover Story (view full-size image): Optical phased arrays (OPAs) are a promising technology that can be used to manipulate the distribution of optical power in the far field by controlling the phase of light emitted by multiple spatially separated transmit apertures. By precisely controlling the relative optical phase at the output of the array, OPAs can be used to deliver a large amount of optical power to a specific location in the far field and enable high-bandwidth mechanism-free beam steering and arbitrary beam shaping. This article introduces an OPA architecture in which multiple follower lasers are phase locked to a leader laser at an arbitrary frequency offset, allowing it to support both coherent and spectral beam combining. The system also supports agile solid-state beam steering at speeds limited by the actuation bandwidth of the lasers. View this paper.
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Open AccessArticle
Electromagnetic Scattering Analysis of SHDB Objects Using Surface Integral Equation Method
Photonics 2020, 7(4), 134; https://doi.org/10.3390/photonics7040134 - 18 Dec 2020
Viewed by 457
Abstract
A surface integral equation (SIE) method is applied in order to analyze electromagnetic scattering by bounded arbitrarily shaped three-dimensional objects with the SHDB boundary condition. SHDB is a generalization of SH (Soft-and-Hard) and DB boundary conditions (at the DB boundary, the normal components [...] Read more.
A surface integral equation (SIE) method is applied in order to analyze electromagnetic scattering by bounded arbitrarily shaped three-dimensional objects with the SHDB boundary condition. SHDB is a generalization of SH (Soft-and-Hard) and DB boundary conditions (at the DB boundary, the normal components of the D and B flux densities vanish). The SHDB boundary condition is a general linear boundary condition that contains two scalar equations that involve both the tangential and normal components of the electromagnetic fields. The multiplication of these scalar equations with two orthogonal vectors transforms them into a vector form that can be combined with the tangential field integral equations. The resulting equations are discretized and converted to a matrix equation with standard method of moments (MoM). As an example of use of the method, we investigate scattering by an SHDB circular disk and demonstrate that the SHDB boundary allows for an efficient way to control the polarization of the wave that is reflected from the surface. We also discuss perspectives into different levels of materialization and realization of SHDB boundaries. Full article
(This article belongs to the Special Issue Advances in Complex Media Electromagnetics)
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Open AccessArticle
Function of Porous Carbon Electrode during the Fabrication of Multiporous-Layered-Electrode Perovskite Solar Cells
Photonics 2020, 7(4), 133; https://doi.org/10.3390/photonics7040133 - 18 Dec 2020
Viewed by 499
Abstract
We demonstrate the effect of sheet conductivity and infiltration using the example of two graphite types, showing that, in general, the graphite type is very important. Amorphous and pyrolytic graphite were applied to carbon electrodes in fully printable carbon-based multiporous-layered-electrode perovskite solar cells [...] Read more.
We demonstrate the effect of sheet conductivity and infiltration using the example of two graphite types, showing that, in general, the graphite type is very important. Amorphous and pyrolytic graphite were applied to carbon electrodes in fully printable carbon-based multiporous-layered-electrode perovskite solar cells (MPLE-PSCs): <glass/F-doped SnO2/compact-TiO2/porous-TiO2+perovskite/porous-ZrO2+perovskite/porous-carbon+perovskite>. The power conversion efficiency (PCE) using amorphous graphite-based carbon (AGC) electrode was only 5.97% due to the low short-circuit photocurrent density (Jsc) value, which was due to the low incident photon-to-current efficiency (IPCE) in the short wavelength region caused by the poor perovskite filling into the porous TiO2-ZrO2 layers. Conversely, using pyrolytic graphite-based carbon (PGC) electrode, Jsc, open-circuit photovoltage (Voc), fill factors (FF), and PCE values of 21.09 mA cm−2, 0.952 V, 0.670, and 13.45%, respectively, were achieved in the champion device. PGC had poorer wettability and a small specific surface area as compared with AGC, but it had better permeability of the perovskite precursor solution into the porous TiO2/ZrO2 layers, and therefore a denser filling and crystallization of the perovskite within the porous TiO2/ZrO2 layers than AGC. It is confirmed that the permeability of the precursor solution depends on the morphology and structure of the graphite employed in the carbon electrode. Full article
(This article belongs to the Special Issue Photovoltaic Materials and Devices)
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Open AccessArticle
Performance Analysis and Enhancement of Free Space Optical Links for Developing State-of-the-Art Smart City Framework
Photonics 2020, 7(4), 132; https://doi.org/10.3390/photonics7040132 - 16 Dec 2020
Viewed by 445
Abstract
In this paper, we have investigated and reported the performance of free-space optical (FSO) links operating in adverse atmospheric conditions. Since FSO links share operational similarity with fiber communication; hence, we believe that a cost-effective FSO framework can play a significant role in [...] Read more.
In this paper, we have investigated and reported the performance of free-space optical (FSO) links operating in adverse atmospheric conditions. Since FSO links share operational similarity with fiber communication; hence, we believe that a cost-effective FSO framework can play a significant role in the transparent integration of high-speed network access backbones with the end-users. Different modulation formats, complemented with spatial diversity techniques, are discussed in this paper to strategize performance optimization of FSO links. Using bit error rate (BER) and signal-to-noise ratio (SNR) as performance metrics, it was found that binary phase-shift keying (BPSK) qualifies as the best technique modulation technique delivering SNR gain of 10 dB over on–off keying (OOK) operating link under similar channel conditions. Further performance optimization was achieved using space diversity reception wherein SNR witnessed a gain of 3 dB gain over the single-channel FSO link. In terms of application, the proposed model can help in developing a citizen-centric smart city ecosystem that can support seamless communication between heterogeneous smart devices. Full article
(This article belongs to the Special Issue Optical Network and Access Technologies)
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Open AccessArticle
Patent Technology Network Analysis of Machine-Learning Technologies and Applications in Optical Communications
Photonics 2020, 7(4), 131; https://doi.org/10.3390/photonics7040131 - 15 Dec 2020
Viewed by 347
Abstract
As the Internet of Things (IoT) develops, applying machine learning on optical communications has become a prospective field of research. Scholars have mostly concentrated on algorithmic techniques or specific applications but have been unable to address the distribution of machine-learning technologies and the [...] Read more.
As the Internet of Things (IoT) develops, applying machine learning on optical communications has become a prospective field of research. Scholars have mostly concentrated on algorithmic techniques or specific applications but have been unable to address the distribution of machine-learning technologies and the development of its applications in optical communications from a macro perspective. Therefore, in this paper, machine-learning patents in optical communications are taken as the analytical basis for constructing a patent technology network. The study results revealed that key technologies were primarily in data input and output devices, data-processing methods, wireless communication networks, and the transmission of digital information in optical communications. Such technologies were also applied to perform measurement for diagnostic purposes and medical diagnoses. The technology network model proposed in this paper explores the technological development trends of machine learning in optical communications and serves as a reference for allocating research and development resources. Full article
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Open AccessArticle
Imaging Feature Analysis-Based Intelligent Laser Cleaning Using Metal Color Difference and Dynamic Weight Dispatch Corrosion Texture
Photonics 2020, 7(4), 130; https://doi.org/10.3390/photonics7040130 - 11 Dec 2020
Viewed by 377
Abstract
To improve the laser cleaning efficiency of Q235 carbon steel, an imaging analysis-based intelligent technique is proposed. Both offline and online computations are designed. Regarding the offline procedure, first, the corrosion images are accumulated to compute the gray-level co-occurrence matrix (GLCM) and the [...] Read more.
To improve the laser cleaning efficiency of Q235 carbon steel, an imaging analysis-based intelligent technique is proposed. Both offline and online computations are designed. Regarding the offline procedure, first, the corrosion images are accumulated to compute the gray-level co-occurrence matrix (GLCM) and the concave-convex region features. Second, different laser cleanings are performed to obtain various cleaned images. Third, a new cleaning performance evaluation method is developed: a metal color difference feature and a dynamic weight dispatch (DWD) corrosion texture are computed. Finally, a particle swarm optimization (PSO)-support vector machine (SVM) is utilized to forecast the laser process parameters. The corresponding laser parameters include power, linear velocity, and line spacing. For the online computation, after the GLCM and the concave-convex region features are computed, an iterative computation is used to tune the process parameters: the random laser parameters are generated constantly, and the iteration is performed and terminated only if the PSO-SVM output is positive. The experimental results have shown that the cleaning efficiency of this method can be improved, and the qualified rate is 92.5%. Full article
(This article belongs to the Special Issue Laser Ablation: From Fundamental Science to Applications)
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Open AccessArticle
Wavelength-Dependent Features of Photoelectron Spectra from Nanotip Photoemission
Photonics 2020, 7(4), 129; https://doi.org/10.3390/photonics7040129 - 11 Dec 2020
Viewed by 506
Abstract
If a metal nanotip is irradiated with the light of a wavelength much larger than the nanotip’s radius of curvature, optical near-fields become excited. These fields are responsible for distinct strong-field electron dynamics, due to both the field enhancement and spatial localization. By [...] Read more.
If a metal nanotip is irradiated with the light of a wavelength much larger than the nanotip’s radius of curvature, optical near-fields become excited. These fields are responsible for distinct strong-field electron dynamics, due to both the field enhancement and spatial localization. By classical trajectory, Monte Carlo (CTMC) simulation, and the integration of the time-dependent Schrödinger equation (TDSE), we find that the photoelectron spectra for nanotip strong-field photoemission, irradiated by mid-infrared laser pulses, present distinctive wavelength-dependent features, especially in the mid- to high-electron energy regions, which are different from the well known ones. By extracting the electron trajectories from the CTMC simulation, we investigate these particular wavelength-dependent features. Our theoretical results contribute to understanding the photoemission and electron dynamics at nanostructures, and pave new pathways for designing high-energy nanometer-sized ultrafast electron sources. Full article
(This article belongs to the Special Issue Strong Light Fields Coupled with Plasmonic Nano-Structures)
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Open AccessLetter
Operation of a Single-Frequency Bismuth-Doped Fiber Power Amplifier near 1.65 µm
Photonics 2020, 7(4), 128; https://doi.org/10.3390/photonics7040128 - 09 Dec 2020
Viewed by 457
Abstract
The spectral range between 1650 and 1700 nm is an interesting region due to its potential applications in optical telecommunication and optical-based methane sensing. Unfortunately, the availability of compact and simple optical amplifiers with output powers exceeding tens of milliwatts in this spectral [...] Read more.
The spectral range between 1650 and 1700 nm is an interesting region due to its potential applications in optical telecommunication and optical-based methane sensing. Unfortunately, the availability of compact and simple optical amplifiers with output powers exceeding tens of milliwatts in this spectral region is still limited. In this paper, a single-frequency continuous-wave bismuth-doped fiber amplifier (BDFA) operating at 1651 and 1687 nm is presented. With the improved signal/pump coupling and modified pump source design, the output powers of 163 mW (at 1651 nm) and 197 mW (at 1687 nm) were obtained. Application of the BDFA to the optical spectroscopy of methane near 1651 nm is also described. We demonstrate that the BDFA can be effectively used for signal amplitude enhancement in photothermal interferometry. Full article
(This article belongs to the Special Issue Novel Specialty Optical Fibers and Applications)
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Open AccessCommunication
Programmable High-Resolution Spectral Processor in C-band Enabled by Low-Cost Compact Light Paths
Photonics 2020, 7(4), 127; https://doi.org/10.3390/photonics7040127 - 07 Dec 2020
Viewed by 516
Abstract
The flexible photonics spectral processor (PSP) is an indispensable element for elastic optical transmission networks that adopt wavelength division multiplexing (WDM) technology. The resolution and system cost are two vital metrics when developing a PSP. In this paper, a high-resolution 1 × 6 [...] Read more.
The flexible photonics spectral processor (PSP) is an indispensable element for elastic optical transmission networks that adopt wavelength division multiplexing (WDM) technology. The resolution and system cost are two vital metrics when developing a PSP. In this paper, a high-resolution 1 × 6 programmable PSP is investigated and experimentally demonstrated by using low-cost compact spatial light paths, which is enabled by a 2 K (1080p) liquid crystal on silicon (LCoS) and two cascaded transmission gratings with a 1000 line/mm resolution. For each wavelength channel, the filtering bandwidth and power attenuation can be manipulated independently. The total insertion loss (IL) for six ports is in the range of 5.9~9.4 dB over the full C-band. The achieved 3-dB bandwidths are able to adjust from 6.2 GHz to 5 THz. Furthermore, multiple system experiments utilizing the proposed PSP, such as flexible spectral shaping and optical frequency comb generation, are carried out to validate the feasibility for the WDM systems. Full article
(This article belongs to the Special Issue Photonics for Emerging Applications in Communication and Sensing)
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Open AccessArticle
Study of the Optical Properties of Multi-Particle Phosphors by the FDTD and Ray Tracing Combined Method
Photonics 2020, 7(4), 126; https://doi.org/10.3390/photonics7040126 - 06 Dec 2020
Viewed by 444
Abstract
It is well known that the optical properties of multi-particle phosphor are crucial to the light performance of white light-emitting diodes (LEDs). Note that the optical properties including scattering or absorption properties for a single particle are easy to be calculated. However, due [...] Read more.
It is well known that the optical properties of multi-particle phosphor are crucial to the light performance of white light-emitting diodes (LEDs). Note that the optical properties including scattering or absorption properties for a single particle are easy to be calculated. However, due to the large computation considering the complicated re-scattering and re-absorption, it is difficult to calculate the scattering behaviors of the multi-particles. A common method to reduce the computation, which can cause unknown deviations, is to replace the multi-particle scattering properties by using the average scattering data of single particles. In this work, a cluster of multi-phosphor particles are directly simulated by the finite-difference time-domain (FDTD) method. The total scattering data of the cluster was processed as a bulk scattering parameter and imported to the Monte-Carlo ray-tracing (RT) method to realize a large-scale multi-particle scattering calculation. A polynomial mathematical model was built according to the multi-particle scattering data. An experiment was carried out for verifying the accuracy of the method in this work. The mean absolute percentages of the previous method are 1.68, 2.06, and 1.22 times larger than the multi-particle method compared with the experimental curves, respectively. Full article
(This article belongs to the Section Optoelectronics and Optical Materials)
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Open AccessArticle
Scaling of Beam Collective Effects with Bunch Charge in the CompactLight Free-Electron Laser
Photonics 2020, 7(4), 125; https://doi.org/10.3390/photonics7040125 - 04 Dec 2020
Viewed by 505
Abstract
The CompactLight European consortium is designing a state-of-the-art X-ray free-electron laser driven by radiofrequency X-band technology. Rooted in experimental data on photo-injector performance in the recent literature, this study estimates analytically and numerically the performance of the CompactLight delivery system for bunch charges [...] Read more.
The CompactLight European consortium is designing a state-of-the-art X-ray free-electron laser driven by radiofrequency X-band technology. Rooted in experimental data on photo-injector performance in the recent literature, this study estimates analytically and numerically the performance of the CompactLight delivery system for bunch charges in the range 75–300 pC. Space-charge forces in the injector, linac transverse wakefield, and coherent synchrotron radiation in bunch compressors are all taken into account. The study confirms efficient lasing in the soft X-rays regime with pulse energies up to hundreds of microjoules at repetition rates as high as 1 kHz. Full article
(This article belongs to the Special Issue Photonics, Optics and Laser Technology)
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Open AccessArticle
Abnormal Fano Profile in Graphene-Wrapped Dielectric Particle Dimer
Photonics 2020, 7(4), 124; https://doi.org/10.3390/photonics7040124 - 03 Dec 2020
Viewed by 477
Abstract
We give a theoretical study on the near field enhancement and far field spectrum of an adjacent graphene-wrapped sphere dimer with different radii. The Fano profile is found in the near field enhancement spectrum of such a symmetry-broken dimer system, which is, however, [...] Read more.
We give a theoretical study on the near field enhancement and far field spectrum of an adjacent graphene-wrapped sphere dimer with different radii. The Fano profile is found in the near field enhancement spectrum of such a symmetry-broken dimer system, which is, however, hidden in the far field spectrum. We demonstrate that this kind of Fano profile is rising from the coupling of dimer’s plasmon hybridization modes by analyzing the dipole moments of each sphere. Moreover, different orientation of incident wave polarization will lead to the different plasmon hybridization coupling, thus giving rise to a different Fano profile. By changing the Fermi energy level, we could achieve tunable Fano profile in near field enhancement. Full article
(This article belongs to the Special Issue Recent Advances in the Study of Light Propagation in Optical Fibers)
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Open AccessArticle
Surface Plasmon Resonance Sensor Based on Polymer Liquid-Core Fiber for Refractive Index Detection
Photonics 2020, 7(4), 123; https://doi.org/10.3390/photonics7040123 - 03 Dec 2020
Viewed by 483
Abstract
In this work, a surface plasmon resonance (SPR) sensor based on a novel liquid-core polymer optical fiber (POF) is proposed and numerically analyzed for refractive index (RI) detection. The polytetrafluoroethylene (PTFE) fiber is selected as the platform for SPR sensing. We combine the [...] Read more.
In this work, a surface plasmon resonance (SPR) sensor based on a novel liquid-core polymer optical fiber (POF) is proposed and numerically analyzed for refractive index (RI) detection. The polytetrafluoroethylene (PTFE) fiber is selected as the platform for SPR sensing. We combine the PTFE-based POF with the liquid-core structure by introducing a hole filled with analyte into the fiber center. The hole also acts as the fiber core to guide the incident light. This design helps to realize the detection of solutions with low RI values (around 1.33), while keeping the distinguished sensing characteristics of the liquid-core structure. Two side air holes are introduced into the cladding and a thin silver film protected by a titanium dioxide layer is plated on the wall of one air hole, which helps to control the mode coupling. In order to optimize the design of this sensor, the impacts of parameters such as metal layer thicknesses and the central hole radius are investigated using the full-vector finite element method (FEM). After optimization, our design shows a wavelength interrogation sensitivity reaching up to 16,750 nm/RIU and an average full-width at half-maximum (FWHM) of 42.86 nm in the RI range of 1.325–1.35. Full article
(This article belongs to the Special Issue Optical Fiber Grating Sensing Technology and Application)
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Open AccessArticle
Optical Bistability in an Optomechanical System with N-Type Atoms under Nonresonant Conditions
Photonics 2020, 7(4), 122; https://doi.org/10.3390/photonics7040122 - 01 Dec 2020
Viewed by 356
Abstract
In this paper, the phenomenon of the optical bistability of a cavity field is theoretically investigated in an optomechanical system containing an N-type atomic ensemble. In this hybrid optomechanical system, the atoms are coupled with two controlling light fields besides coupling with [...] Read more.
In this paper, the phenomenon of the optical bistability of a cavity field is theoretically investigated in an optomechanical system containing an N-type atomic ensemble. In this hybrid optomechanical system, the atoms are coupled with two controlling light fields besides coupling with the cavity field. Under the nonresonant condition, we analyze the influences of the coupling strength between cavity and atoms, Rabi frequencies of the controlling light field, the detuning between the controlling light field and atoms, and pump field power on the optical bistable behavior of mean intracavity photon number. The nonlinear distribution of the mean intracavity photon number has a potential application in field optical switches and optical bistable devices. Full article
(This article belongs to the Section Quantum Photonics and Technologies)
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Open AccessReview
A Review of Silent Substitution Devices for Melanopsin Stimulation in Humans
Photonics 2020, 7(4), 121; https://doi.org/10.3390/photonics7040121 - 30 Nov 2020
Viewed by 464
Abstract
One way to study the specific response of the non-visual melanopsin photoreceptors of the human eye is to silence the response of cones and rods. Melanopsin photoreceptors (ipRGC), highlighted in the early 2000s, are intimately linked to the circadian rhythm and therefore to [...] Read more.
One way to study the specific response of the non-visual melanopsin photoreceptors of the human eye is to silence the response of cones and rods. Melanopsin photoreceptors (ipRGC), highlighted in the early 2000s, are intimately linked to the circadian rhythm and therefore to our sleep and wakefulness. Rest and sleep regulation, health and cognitive functions are all linked to ipRGC and play an important role in work and human relationships. Thus, we believe that the study of ipRGC responses is important.We searched and reviewed scientific articles describing instrumentation dedicated to these studies. PubMed lists more than 90,000 articles created since the year 2000 that contain the word circadian but only 252 with silent substitution. In relation to melanopsin, we found 39 relevant articles from which only 11 give a device description for humans, which is incomplete in most cases. We did not find any consensus for light intensity description, melanopsin contrast, sequences of melanopsin light stimulation and optical setup to expose the retina to the light. Full article
(This article belongs to the Special Issue Optical Instrumentation)
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Open AccessArticle
Slit Diffraction Spectrum Manipulation in Non-Paraxial Regions via the Spatial-Spectral Correspondence Relationship
Photonics 2020, 7(4), 120; https://doi.org/10.3390/photonics7040120 - 30 Nov 2020
Viewed by 329
Abstract
In the past, a two-dimensional aperture diffraction of light in the non-paraxial region could only be studied using the Huygens integral without functional forms. This work presents a special case—a one dimension slit where the functional form can be obtained. The monochromatic light [...] Read more.
In the past, a two-dimensional aperture diffraction of light in the non-paraxial region could only be studied using the Huygens integral without functional forms. This work presents a special case—a one dimension slit where the functional form can be obtained. The monochromatic light intensity distributions are investigated in detail. Using the correspondence relationship, the diffracted spectra of polychromatic light in that region can be readily found. Three interesting spectral effects are described: spectral switches, multi-level data transmission, and optical wavelength ruler. Since the functional form is derived without approximation, it is applicable to a region very near to the slit, including the wavelength region or even sub-wavelength scale. Thus, for light with micron-order wavelength (visible to near infrared (NIR) band), these results are valuable to micro- or nano-optics, especially for studies of the spatial intensities or spectral characteristics in the non-paraxial region. Full article
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Open AccessArticle
Automated Nuclear Lamina Network Recognition and Quantitative Analysis in Structured Illumination Super-Resolution Microscope Images Using a Gaussian Mixture Model and Morphological Processing
Photonics 2020, 7(4), 119; https://doi.org/10.3390/photonics7040119 - 30 Nov 2020
Viewed by 397
Abstract
Studying the architecture of nuclear lamina networks is significantly important in biomedicine owing not only to their influence on the genome, but also because they are associated with several diseases. To save labor and time, an automated method for nuclear lamina network recognition [...] Read more.
Studying the architecture of nuclear lamina networks is significantly important in biomedicine owing not only to their influence on the genome, but also because they are associated with several diseases. To save labor and time, an automated method for nuclear lamina network recognition and quantitative analysis is proposed for use with lattice structured illumination super-resolution microscope images in this study. This method is based on a Gaussian mixture model and morphological processing. It includes steps for target region generation, bias field correction, image segmentation, network connection, meshwork generation, and meshwork analysis. The effectiveness of the proposed method was confirmed by recognizing and quantitatively analyzing nuclear lamina networks in five images that are presented to show the method’s performance. The experimental results show that our algorithm achieved high accuracy in nuclear lamina network recognition and quantitative analysis, and the median face areas size of lamina networks from U2OS osteosarcoma cells are 0.3184 μm2. Full article
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Open AccessArticle
Coherent Beam Combining Using an Internally Sensed Optical Phased Array of Frequency-Offset Phase Locked Lasers
Photonics 2020, 7(4), 118; https://doi.org/10.3390/photonics7040118 - 28 Nov 2020
Viewed by 495
Abstract
Coherent beam combining can be used to scale optical power and enable mechanism-free beam steering using an optical phased array. Coherently combining multiple free-running lasers in a leader-follower laser configuration is challenging due to the need to measure and stabilize large and highly [...] Read more.
Coherent beam combining can be used to scale optical power and enable mechanism-free beam steering using an optical phased array. Coherently combining multiple free-running lasers in a leader-follower laser configuration is challenging due to the need to measure and stabilize large and highly dynamic phase differences between them. We present a scalable technique based on frequency-offset phase locking and digitally enhanced interferometry to clone the coherence of multiple lasers without the use of external sampling optics, which has the potential to support both coherent and spectral beam combining, and alleviates issues of voltage wrapping associated with actuating feedback control using electro-optic modulators. This technique was demonstrated experimentally using a tiled-aperture optical phased array in which the relative output phase of three free-running lasers was stabilized with an RMS output phase stability of λ/104. Full article
(This article belongs to the Special Issue Photonics, Optics and Laser Technology)
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Open AccessArticle
A Super-Fast Free-Electron Laser Simulation Code for Online Optimization
Photonics 2020, 7(4), 117; https://doi.org/10.3390/photonics7040117 - 27 Nov 2020
Viewed by 418
Abstract
The X-ray free-electron lasers (FELs) have stimulated the growing interest of researchers in different fields. This gives rise to an increasing simulation work of design and optimization of FEL facilities and demonstrations of novel FEL ideas. Most of the multi-dimensional simulation codes in [...] Read more.
The X-ray free-electron lasers (FELs) have stimulated the growing interest of researchers in different fields. This gives rise to an increasing simulation work of design and optimization of FEL facilities and demonstrations of novel FEL ideas. Most of the multi-dimensional simulation codes in use require large computational resources, while one-dimensional simulation codes can merely give an acceptable description of the FEL amplification process. This paper presents the development of a super-fast time-dependent FEL simulation code, which is mainly designed for seeded FEL and its application on a seed FEL facility. The diffraction factors have been introduced in order to model the effects of radiation field diffraction and drastically simplified the working equations. Meanwhile, a specially designed module for seeded FEL has been added in order to study the laser-electron beam interactions with arbitrarily parameters. The code can also provide a fairly accurate, quasi-real-time assistant tool for online optimization. Full article
(This article belongs to the Section Lasers, Light Sources and Sensors)
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Open AccessArticle
Theoretical Effect of Coma and Spherical Aberrations Translation on Refractive Error and Higher Order Aberrations
Photonics 2020, 7(4), 116; https://doi.org/10.3390/photonics7040116 - 27 Nov 2020
Viewed by 405
Abstract
(1) Background: The purpose of the study is to present a simple theoretical account of the effect of translation of coma and spherical aberrations (SA) on refractive error and higher order aberrations. (2) Methods: A computer software algorithm was implemented based on previously [...] Read more.
(1) Background: The purpose of the study is to present a simple theoretical account of the effect of translation of coma and spherical aberrations (SA) on refractive error and higher order aberrations. (2) Methods: A computer software algorithm was implemented based on previously published methods. The effect of translation (0 to +1 mm) was analyzed for SA (0 to +2 µm) and coma (0 to +2 µm) for a circular pupil of 6 mm, without any rotation or scaling effect. The relationship amongst Zernike representations of various aberrations was analyzed under the influence of translation. (3) Results: The translation of +0.40 µm of SA (C[4,0]) by +0.25 mm with a pupil diameter of 6mm resulted in induction of tilt (C[1,1]), −0.03D defocus (C[2,0]), +0.03D astigmatism (C[2,2]) and +0.21 µm coma (C[3,1]). The translation of +0.4 µm of coma (C[3,1]) by +0.25 mm with a pupil diameter of 6 mm resulted in induction of tilt (C[1,1]), −0.13D defocus (C[2,0]) and +0.13D astigmatism (C[2,2]). A theoretical quantitative relationship between SA, coma, astigmatism and defocus is presented under the influence of translation. (4) Conclusion: The results can act as a guide for the clinician, in order to readily assess theoretical impact of wavefront map translation from pupil center to the visual axis. The resultant refractive coupling has to be taken into consideration especially when treating eyes with an abnormal corneal shape and/or large pupil center to corneal vertex chord. Full article
(This article belongs to the Special Issue Visual Optics and Ophthalmology)
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Open AccessCorrection
Correction: Pereyra, P. Photonic Transmittance in Metallic and Left Handed Superlattices. Photonics 2020, 7, 29
Photonics 2020, 7(4), 115; https://doi.org/10.3390/photonics7040115 - 23 Nov 2020
Viewed by 354
Abstract
The author wishes to make the following corrections to this paper [...] Full article
Open AccessArticle
Highest Cost First-Based QoS Mapping Scheme for Fiber Wireless Architecture
Photonics 2020, 7(4), 114; https://doi.org/10.3390/photonics7040114 - 22 Nov 2020
Viewed by 550
Abstract
The combination of a high-speed wireless network with passive optical network technologies has led to the evolution of a modern integrated fiber wireless (FiWi) access network. Compared to broadband wireless networks, the FiWi network offers higher bandwidth with improved reliability and reduced maintenance [...] Read more.
The combination of a high-speed wireless network with passive optical network technologies has led to the evolution of a modern integrated fiber wireless (FiWi) access network. Compared to broadband wireless networks, the FiWi network offers higher bandwidth with improved reliability and reduced maintenance costs due to the passive nature of passive optical network (PON). Since the quality of service (QoS) is a baseline to deploy high-speed FiWi broadband access networks, therefore, it is essential to analyze and reduce the typical problems (e.g., bandwidth and delay) in the high-speed next-generation networks (NGANs). This study investigates the performance of a fiber wireless architecture where a 10-Gigabit-capable passive optical network (XGPON) and fifth generation of wireless local area network (WLAN) (i.e., IEEE 802.11ac) are integrated. Both technologies take benefits from each other and have pros and cons concerning the QoS demands of subscribers. The proposed work offers a very flexible QoS scheme for the different types of services of 5G WLAN and XGPON with the help of the highest cost first (HCF) algorithm, which leads to reduced upstream delays for delay-sensitive applications. The simulation results show that the HCF algorithm boosts the performance of the dynamic bandwidth assignment (DBA) scheme and results in up to 96.1%, 90.8%, and 55.5% reduced upstream (US) delays for video: VI(T2), background: BK(T3), and best effort: BE(T4) traffic in enhanced-distributed-channel-access (EDCA) mode. Compared to earlier work, the HCF and immediate allocation with the colorless grant (IACG) DBA combination results in the reduction of up to 54.8% and 53.4% mean US delays. This happens because of 50% to 65% better bandwidth assignment by the IACG DBA process due to efficient mapping by the HCF algorithm. Full article
(This article belongs to the Section Optical Communication and Network)
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Open AccessLetter
Effective Medium Theory for Multi-Component Materials Based on Iterative Method
Photonics 2020, 7(4), 113; https://doi.org/10.3390/photonics7040113 - 20 Nov 2020
Viewed by 458
Abstract
For biomedical applications in the terahertz band, composites such as macromolecule compounds, biotissues and phantoms are studied. A description of dielectric properties of composite materials using mathematical models has its own fundamental and technological importance. In this work, we present an iterative effective [...] Read more.
For biomedical applications in the terahertz band, composites such as macromolecule compounds, biotissues and phantoms are studied. A description of dielectric properties of composite materials using mathematical models has its own fundamental and technological importance. In this work, we present an iterative effective medium theory for multi-component materials. The model has good performance in describing composite materials with more than two components. The theory is evaluated by comparing with the complex permittivity of three different composite materials. A comparison with other commonly used models is given in the form of relative errors. Full article
(This article belongs to the Special Issue Terahertz Biophotonics)
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Open AccessArticle
Synthesis, Crystallography, Microstructure, Crystal Defects, Optical and Optoelectronic Properties of ZnO:CeO2 Mixed Oxide Thin Films
Photonics 2020, 7(4), 112; https://doi.org/10.3390/photonics7040112 - 18 Nov 2020
Viewed by 546
Abstract
We report the synthesis and characterization of pure ZnO, pure CeO2, and ZnO:CeO2 mixed oxide thin films dip-coated on glass substrates using a sol-gel technique. The structural properties of as-prepared thin film are investigated using the XRD technique. In particular, [...] Read more.
We report the synthesis and characterization of pure ZnO, pure CeO2, and ZnO:CeO2 mixed oxide thin films dip-coated on glass substrates using a sol-gel technique. The structural properties of as-prepared thin film are investigated using the XRD technique. In particular, pure ZnO thin film is found to exhibit a hexagonal structure, while pure CeO2 thin film is found to exhibit a fluorite cubic structure. The diffraction patterns also show the formation of mixed oxide materials containing well-dispersed phases of semi-crystalline nature from both constituent oxides. Furthermore, optical properties of thin films are investigated by performing UV–Vis spectrophotometer measurements. In the visible region, transmittance of all investigated thin films attains values as high as 85%. Moreover, refractive index of pure ZnO film was found to exhibit values ranging between 1.57 and 1.85 while for CeO2 thin film, it exhibits values ranging between 1.73 and 2.25 as the wavelength of incident light decreases from 700 nm to 400 nm. Remarkably, refractive index of ZnO:CeO2 mixed oxide-thin films are tuned by controlling the concentration of CeO2 properly. Mixed oxide-thin films of controllable refractive indices constitute an important class of smart functional materials. We have also investigated the optoelectronic and dispersion properties of ZnO:CeO2 mixed oxide-thin films by employing well-established classical models. The melodramatic boost of optical and optoelectronic properties of ZnO:CeO2 mixed oxide thin films establish a strong ground to modify these properties in a skillful manner enabling their use as key potential candidates for the fabrication of scaled optoelectronic devices and thin film transistors. Full article
(This article belongs to the Special Issue Advanced Optical Materials and Devices II)
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Open AccessLetter
Dual-Core Fiber-Based Interferometer for Detection of Gas Refractive Index
Photonics 2020, 7(4), 111; https://doi.org/10.3390/photonics7040111 - 15 Nov 2020
Viewed by 622
Abstract
We demonstrate a dual-core fiber-based Mach–Zehnder interferometer that could be used for precise detection of variations in refractive indices of gaseous samples. The fiber used here have a solid germanium-doped silica core and an air core that allows gases to flow through. Coherent [...] Read more.
We demonstrate a dual-core fiber-based Mach–Zehnder interferometer that could be used for precise detection of variations in refractive indices of gaseous samples. The fiber used here have a solid germanium-doped silica core and an air core that allows gases to flow through. Coherent laser beams are coupled to the two cores, respectively, and thus excite guiding modes thereby. Interferogram would be produced as the light transmitted from the dual cores interferes. Variations in refractive index of the hollow core lead to variations in phase difference between the modes in the two cores, thus shifting the interference fringes. The fringe shifts can be then interrogated by a photodiode together with a narrow slit in front. The resolution of the sensor was found to be ~1 × 10−8 RIU, that is comparable to the highest resolution obtained by other fiber sensors reported in previous literatures. Other advantages of our sensor include very low cost, high sensitivity, straightforward sensing mechanism, and ease of fabrication. Full article
(This article belongs to the Special Issue Optical Fiber Grating Sensing Technology and Application)
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Open AccessArticle
Ellipticity of High-Order Harmonics Generated by Aligned Homonuclear Diatomic Molecules Exposed to an Orthogonal Two-Color Laser Field
Photonics 2020, 7(4), 110; https://doi.org/10.3390/photonics7040110 - 12 Nov 2020
Cited by 1 | Viewed by 527
Abstract
We investigate emission rate and ellipticity of high-order harmonics generated exposing a homonuclear diatomic molecule, aligned in the laser-field polarization plane, to a strong orthogonally polarized two-color (OTC) laser field. The linearly polarized OTC-field components have frequencies rω and sω, [...] Read more.
We investigate emission rate and ellipticity of high-order harmonics generated exposing a homonuclear diatomic molecule, aligned in the laser-field polarization plane, to a strong orthogonally polarized two-color (OTC) laser field. The linearly polarized OTC-field components have frequencies rω and sω, where r and s are integers. Using the molecular strong-field approximation with dressed initial state and undressed final state, we calculate the harmonic emission rate and harmonic ellipticity for frequency ratios 1:2 and 1:3. The obtained quantities depend strongly on the relative phase between the laser-field components. We show that with the OTC field it is possible to generate elliptically polarized high-energy harmonics with high emission rate. To estimate the relative phase for which the emission rate is maximal we use the simple man’s model. In the harmonic spectra as a function of the molecular orientation there are two types of minima, one connected with the symmetry of the molecular orbital and the other one due to destructive interference between different contributions to the recombination matrix element, where we take into account that the electron can be ionized and recombine at the same or different atomic centers. We derive a condition for the interference minima. These minima are blurred in the OTC field except in the cases where the highest occupied molecular orbital is modeled using only s or only p orbitals in the linear combination of the atomic orbitals. This allows us to use the interference minima to assess which atomic orbitals are dominant in a particular molecular orbital. Finally, we show that the harmonic ellipticity, presented in false colors in the molecular-orientation angle vs. harmonic-order plane, can be large in particular regions of this plane. These regions are bounded by the curves determined by the condition that the harmonic ellipticity is approximately zero, which is determined by the minima of the T-matrix contributions parallel and perpendicular to the fundamental component of the OTC field. Full article
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Open AccessArticle
Object Shape Measurement Based on Brox Optical Flow Estimation and Its Correction Method
Photonics 2020, 7(4), 109; https://doi.org/10.3390/photonics7040109 - 11 Nov 2020
Viewed by 324
Abstract
In this work, a new method of measuring surface shape based on Brox optical flow estimation is presented. The measuring system consists of a projector, a measured object and a charge coupled device (CCD) camera. The grating fringes are projected onto the reference [...] Read more.
In this work, a new method of measuring surface shape based on Brox optical flow estimation is presented. The measuring system consists of a projector, a measured object and a charge coupled device (CCD) camera. The grating fringes are projected onto the reference plane at a small angle. Two fringe images—before and after placing the measured object on the reference plane—are captured, respectively. Then, the optical flow field between two images is evaluated by using Brox optical flow algorithm. The theoretical relationship between the optical flow field and the height of the measured surface is established. According to the relationship, the height distribution of the measured object can be retrieved quickly without phase-to-height transformation. However, the calculated height distribution has been found to be deviated from its true value. To solve the problem, a correction scheme suitable for the optical flow method is proposed. By using the correction scheme, the accuracy of the calculated result is greatly improved. Simulations and experiments are completed to verify the feasibility of the proposed method and the accuracy of the correction method. The results show that the proposed method is more accurate than that of the Fourier transform method. Compared with traditional surface shape measurement, the optical flow method has some obvious advantages: (1) Only two frame images are required to recover the height distribution. (2) Relatively simple in measurement process and calculation so less time consuming. (3) Because the optical flow method contains time factor itself, it is more suitable for dynamic measurement. (4) No restrictions on projection pattern. Full article
(This article belongs to the Special Issue Recent Advances in the Study of Light Propagation in Optical Fibers)
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Open AccessArticle
A Computational Study on Performance Improvement of THz Signal from a Grating Photoconductive Antenna
Photonics 2020, 7(4), 108; https://doi.org/10.3390/photonics7040108 - 11 Nov 2020
Viewed by 391
Abstract
A diffractive grating is a well-known optical component and is extensively used in many applications. This research work explores application of the diffractive grating in a photoconductive antenna (PCA) of a terahertz time domain spectroscopy (THz-TDS) system, by utilizing benefits of a sub-wavelength [...] Read more.
A diffractive grating is a well-known optical component and is extensively used in many applications. This research work explores application of the diffractive grating in a photoconductive antenna (PCA) of a terahertz time domain spectroscopy (THz-TDS) system, by utilizing benefits of a sub-wavelength grating structure. The grating PCA structure was modeled and simulated by COMSOL Multiphysics software (COMSOL, Inc., Burlington, MA, USA). Performance of the proposed PCA design is studied in terms of its induced photocurrent. The effects of geometrical parameters of the grating are also investigated and analyzed through its optical and electrical responses. Thanks to the increase in absorption of the incident laser’s electric field, the simulation results show a 63% increment of the induced photocurrent in the grating PCA, compared with the conventional planar PCA. Full article
(This article belongs to the Section Optoelectronics and Optical Materials)
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Open AccessCommunication
Numerical Investigation on a Hyperlens with a Large Radius Inner-Surface for Super-Resolution Imaging
Photonics 2020, 7(4), 107; https://doi.org/10.3390/photonics7040107 - 10 Nov 2020
Viewed by 383
Abstract
Hyperlensing devices have drawn great attention in recent years due to their ability to amplify the subwavelength image of objects with more detail and information. In this work, a hyperlens with a radian inner surface is designed and demonstrated. The proposed hyperlens is [...] Read more.
Hyperlensing devices have drawn great attention in recent years due to their ability to amplify the subwavelength image of objects with more detail and information. In this work, a hyperlens with a radian inner surface is designed and demonstrated. The proposed hyperlens is capable of imaging different types of sub-wavelength objects efficiently. Plasmonic resonant cavity is also employed in order to achieve a super-resolution imaging effect. Different objects are investigated to test the performance of the proposed hyperlens. As expected, our hyperlens shows better tolerance than the conventional hyperlensing designs and can achieve imaging resolution down to 60 nm for different types of objects. Full article
(This article belongs to the Special Issue Advanced Metamaterials and Metadevices)
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Open AccessArticle
Asymmetric Carrier Divider with an Irregular RF Phase on DD-MZ Modulator for Eliminating Dispersion Power Fading in RoF Communication
Photonics 2020, 7(4), 106; https://doi.org/10.3390/photonics7040106 - 09 Nov 2020
Viewed by 468
Abstract
The main problem of intensity modulation (IM) in radio-over-fiber (RoF) communication is dispersion power fading (DPF), which occurs when the signal is transmitted through a dispersive link that causes a sideband cancelation effect. The DPF level of the RoF link is determined by [...] Read more.
The main problem of intensity modulation (IM) in radio-over-fiber (RoF) communication is dispersion power fading (DPF), which occurs when the signal is transmitted through a dispersive link that causes a sideband cancelation effect. The DPF level of the RoF link is determined by the deviation factor (DF). The optical single-sideband (OSSB) modulation scheme, which is generated by driving one of the dual-drive Mach–Zehnder modulators (DD-MZMs), is usually used to overcome DPF. The DF value of OSSB modulation at modulation index m = 0.1 increases from 0.008 to 0.930 at m = 1. It can be said that this method is only effective at reducing DF at low m. However, as well-known information of the DD-MZM system, high-efficiency optic–electric conversions can be obtained at high m values, but DF will increase. Therefore, reducing the DPF value for high m ≥ 0.1 is interesting. It is known that in wireless communication, to reduce the impact of fading, direct signals are amplified and signals with irregular phases are used. Moreover, this paper proposes the DD-MZM with an asymmetric carrier divider as a direct signal and combines it with an irregular radio frequency (RF) phase to reduce the DPF at high m. The carrier that is generated by laser diode (LD) power (PIN) is divided asymmetrically as power modulation (PDD-MZM) and carrier arm (CA) power (PCA). Furthermore, the minimum DF is obtained when the PIN is separated as 75% for PCA and 25% for PDD-MZM with an irregular RF signal of θ = 48° and a bias point value of γ = ¾. As a result, with the same power as OSSB, this proposed structure produces DF at m = 0.1 and m = 1 with values of 0.008 and 0.03, or it can reduce DF of 96.7% at m = 1. The mathematical model and simulation model have very good agreement, which validates the proposed method. Full article
(This article belongs to the Special Issue Radio over Fiber)
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Open AccessReview
Machine Learning Techniques in Radio-over-Fiber Systems and Networks
Photonics 2020, 7(4), 105; https://doi.org/10.3390/photonics7040105 - 07 Nov 2020
Cited by 1 | Viewed by 627
Abstract
The radio-over-fiber (RoF) technology has been widely studied during the past decades to extend the wireless communication coverage by leveraging the low-loss and broad bandwidth advantages of the optical fiber. With the increasing need for wireless communications, using millimeter-waves (mm-wave) in wireless communications [...] Read more.
The radio-over-fiber (RoF) technology has been widely studied during the past decades to extend the wireless communication coverage by leveraging the low-loss and broad bandwidth advantages of the optical fiber. With the increasing need for wireless communications, using millimeter-waves (mm-wave) in wireless communications has become the recent trend and many attempts have been made to build high-throughput and robust mm-wave RoF systems during the past a few years. Whilst the RoF technology provides many benefits, it suffers from several fundamental limitations due to the analog optical link, including the fiber chromatic dispersion and nonlinear impairments. Various approaches have been proposed to address these limitations. In particular, machine learning (ML) algorithms have attracted intensive research attention as a promising candidate for handling the complicated physical layer impairments in RoF systems, especially the nonlinearity during signal modulation, transmission and detection. In this paper, we review recent advancements in ML techniques for RoF systems, especially those which utilize ML models as physical layer signal processors to mitigate various types of impairments and to improve the system performance. In addition, ML algorithms have also been widely adopted for highly efficient RoF network management and resource allocation, such as the dynamic bandwidth allocation and network fault detection. In this paper, we also review the recent works in these research domains. Finally, several key open questions that need to be addressed in the future and possible solutions of these questions are also discussed. Full article
(This article belongs to the Special Issue Radio over Fiber)
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