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Photonics, Volume 8, Issue 2 (February 2021) – 37 articles

Cover Story (view full-size image): Topological insulators (TIs) yield superior plasmonic properties in the ultraviolet-to-visible wavelength range for both near- and far-field applications. However, previous reports have demonstrated only inefficient wavefront control and non-directional surface–plasmon–polariton (SPP) excitation. This paper numerically investigates both near- and far-field wavefront manipulation capabilities of plasmonic TI Bi2Te3-based gap–surface plasmon (GSP) metasurfaces. By employing the principle of the geometric phase, a beam steering metasurface is presented with a cross-polarization efficiency of 34% at 500 nm. Additionally, near-field functionalities are demonstrated by designing a polarization-dependent GSP metasurface for directional SPP coupling. Our work forms a framework for TI-based GSP metasurfaces with near- and far-field functionalities in the visible wavelength range. View this paper.
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Open AccessArticle
Determination of Optic Axes by Corneal Topography among Italian, Brazilian, and Chinese Populations
Photonics 2021, 8(2), 61; https://doi.org/10.3390/photonics8020061 - 23 Feb 2021
Viewed by 154
Abstract
This study aims to describe a new universal method to identify the relative three-dimensional directions of visual, pupillary, and optical axes of the eye and the angles between them using topography elevation data. The method was validated in a large clinical cohort, and [...] Read more.
This study aims to describe a new universal method to identify the relative three-dimensional directions of visual, pupillary, and optical axes of the eye and the angles between them using topography elevation data. The method was validated in a large clinical cohort, and ethnical differences were recorded. Topography elevation data were collected from 1992 normal eyes of 966 healthy participants in Italy, Brazil, and China. The three main axes were defined as follows: optical axis (OA) was defined as the optimal path of light that passes through the ocular system without refraction. The pupillary axis (PA) line was defined using X and Y coordinates of the pupil centre with the chamber depth, in addition to the centre of a sphere fitted to the central 3 mm diameter of the cornea. The visual axis (VA) was taken by its best approximation, the coaxially sighted corneal light reflex. The alpha angle was measured between the VA and OA, and the kappa angle between the VA and PA. The average values of kappa and alpha angles were 3.41 ± 2.84 and 6.04 ± 2.43 in the Italian population, 2.6 ± 1.53 and 5.87 ± 2.3 in the Brazilian population, and 2.09 ± 1.22 and 3.85 ± 1.48 in the Chinese population. Full article
(This article belongs to the Special Issue Visual Optics and Ophthalmology)
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Open AccessArticle
Independently Controlling Stochastic Field Realization Magnitude and Phase Statistics for the Construction of Novel Partially Coherent Sources
Photonics 2021, 8(2), 60; https://doi.org/10.3390/photonics8020060 - 22 Feb 2021
Viewed by 221
Abstract
In this paper, we present a method to independently control the field and irradiance statistics of a partially coherent beam. Prior techniques focus on generating optical field realizations whose ensemble-averaged autocorrelation matches a specified second-order field moment known as the cross-spectral density (CSD) [...] Read more.
In this paper, we present a method to independently control the field and irradiance statistics of a partially coherent beam. Prior techniques focus on generating optical field realizations whose ensemble-averaged autocorrelation matches a specified second-order field moment known as the cross-spectral density (CSD) function. Since optical field realizations are assumed to obey Gaussian statistics, these methods do not consider the irradiance moments, as they, by the Gaussian moment theorem, are completely determined by the field’s first and second moments. Our work, by including control over the irradiance statistics (in addition to the CSD function), expands existing synthesis approaches and allows for the design, modeling, and simulation of new partially coherent beams, whose underlying field realizations are not Gaussian distributed. We start with our model for a random optical field realization and then derive expressions relating the ensemble moments of our fields to those of the desired partially coherent beam. We describe in detail how to generate random optical field realizations with the proper statistics. We lastly generate two example partially coherent beams using our method and compare the simulated field and irradiance moments theory to validate our technique. Full article
(This article belongs to the Special Issue Structured Light Coherence)
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Open AccessCommunication
Illumination Calibration for Computational Ghost Imaging
Photonics 2021, 8(2), 59; https://doi.org/10.3390/photonics8020059 - 22 Feb 2021
Viewed by 181
Abstract
We propose a fast calibration method to compensate the non-uniform illumination in computational ghost imaging. Inspired by a similar procedure to calibrate pixel response differences for detector arrays in conventional digital cameras, the proposed method acquires one image of an all-white paper to [...] Read more.
We propose a fast calibration method to compensate the non-uniform illumination in computational ghost imaging. Inspired by a similar procedure to calibrate pixel response differences for detector arrays in conventional digital cameras, the proposed method acquires one image of an all-white paper to determine the non-uniformity of the illumination, and uses the information to calibrate any further reconstructed images under the same illumination. The numerical and experimental results are in a good agreement, and the experimental results showed that the root mean square error of the reconstructed image was reduced by 79.94% after the calibration. Full article
(This article belongs to the Special Issue Smart Pixels and Imaging)
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Open AccessArticle
Joint Estimation of Symbol Rate and Chromatic Dispersion Using Delayed Multiplier for Optical Performance Monitoring
Photonics 2021, 8(2), 58; https://doi.org/10.3390/photonics8020058 - 20 Feb 2021
Viewed by 233
Abstract
Symbol rate and chromatic dispersion (CD) are very important for optical performance monitoring. The CD, however, hinders the symbol rate detection. In this paper, we proposed a joint estimation of symbol rate and chromatic dispersion. We show that, when the signal conjugates and [...] Read more.
Symbol rate and chromatic dispersion (CD) are very important for optical performance monitoring. The CD, however, hinders the symbol rate detection. In this paper, we proposed a joint estimation of symbol rate and chromatic dispersion. We show that, when the signal conjugates and multiplies with the delayed replica, the spectral line can be restored. The proposed method provides a fast and simple solution for joint estimation as traditional tentative CD scanning is time consuming. The simulation shows that the root-mean-squared error (RMSE) for CD was 39.5 ps/nm and the symbol rate was 2.4 MHz. For the squared-root-raised-cosine (SRRC) pulse shape with a roll-off factor of 0.1, the experimental results show that 25,000 input samples were needed for an error-free estimation. The RMSE is 105.6 ps/nm and 63.5 kHz for CD and symbol rate, respectively. Full article
(This article belongs to the Section Optical Communication and Network)
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Open AccessCommunication
Switchable Metasurface with VO2 Thin Film at Visible Light by Changing Temperature
Photonics 2021, 8(2), 57; https://doi.org/10.3390/photonics8020057 - 18 Feb 2021
Viewed by 272
Abstract
We numerically demonstrated switchable metasurfaces using a phase change material, VO2 by temperature change. The Pancharatnam–Berry metasurface was realized by using an array of Au nanorods on top of a thin VO2 film above an Au film, where the optical property [...] Read more.
We numerically demonstrated switchable metasurfaces using a phase change material, VO2 by temperature change. The Pancharatnam–Berry metasurface was realized by using an array of Au nanorods on top of a thin VO2 film above an Au film, where the optical property of the VO2 film is switched from the insulator phase at low temperature to the metal phase at high temperature. At the optimal structure, polarization conversion efficiency of the normal incident light is about 75% at low temperature while that is less than 0.5% at high temperature in the visible region (λ 700 nm). Various functionalities of switchable metasurfaces were demonstrated such as polarization conversion, beam steering, Fourier hologram, and Fresnel hologram. The thin-VO2-film-based switchable metasurface can be a good candidate for various switchable metasurface devices, for example, temperature dependent optical sensors, beamforming antennas, and display. Full article
(This article belongs to the Special Issue Active/Reconfigurable Metasurfaces)
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Open AccessArticle
Stability Constraints on Practical Implementation of Parity-Time-Symmetric Electromagnetic Systems
Photonics 2021, 8(2), 56; https://doi.org/10.3390/photonics8020056 - 18 Feb 2021
Viewed by 225
Abstract
Recently, several applications leveraging unconventional manipulation of electromagnetic radiation based on parity-time symmetry have been proposed in the literature. Typical examples include systems with unidirectional invisibility and asymmetric refraction. Such applications assume an inherent system stability and no occurrence of unbounded signal growth [...] Read more.
Recently, several applications leveraging unconventional manipulation of electromagnetic radiation based on parity-time symmetry have been proposed in the literature. Typical examples include systems with unidirectional invisibility and asymmetric refraction. Such applications assume an inherent system stability and no occurrence of unbounded signal growth or unwanted self-oscillations. Here, a general instability issue of parity-time-symmetric systems is investigated, with particular emphasis on a recently proposed system based on resistive metasurfaces. Explicit closed-form stability criterion is derived, crosschecked and verified by both time-domain transient simulations and the measurements on an experimental demonstrator operating in a lower radiofrequency range. Results of this study lead to the conclusion that any parity-time-symmetric system is necessarily marginally stable. Finally, it is shown that such a marginally stable system may easily become unstable if not designed carefully. Full article
(This article belongs to the Special Issue Advances in Complex Media Electromagnetics)
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Open AccessArticle
Computational Modeling and Simulation to Increase Laser Shooting Accuracy of Autonomous LEO Trackers
Photonics 2021, 8(2), 55; https://doi.org/10.3390/photonics8020055 - 18 Feb 2021
Viewed by 205
Abstract
In this paper, we introduce a computational procedure that enables autonomous LEO laser trackers endowed with INSs to increase the current accuracy when shooting at middle distant medium-size LEO debris targets. The code is designed for the trackers to throw the targets into [...] Read more.
In this paper, we introduce a computational procedure that enables autonomous LEO laser trackers endowed with INSs to increase the current accuracy when shooting at middle distant medium-size LEO debris targets. The code is designed for the trackers to throw the targets into the atmosphere by means of ablations. In case that the targets are eclipsed to the trackers by the Earth, the motions of the trackers and targets are modeled by equations that contain post-Newtonian terms accounting for the curvature of space. Otherwise, when the approaching targets become visible for the trackers, we additionally use more accurate equations, which allow to account for the local bending of the laser beams aimed at the targets. We observe that under certain circumstances the correct shooting configurations that allow to safely and efficiently shoot down the targets, differ from the current estimations by distances that may be larger than the size of many targets. In short, this procedure enables to estimate the optimal shooting instants for any middle distant medium-size LEO debris target. Full article
(This article belongs to the Special Issue Laser Interaction with Materials)
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Open AccessCommunication
A Closer Look at Photonic Nanojets in Reflection Mode: Control of Standing Wave Modulation
Photonics 2021, 8(2), 54; https://doi.org/10.3390/photonics8020054 - 17 Feb 2021
Viewed by 198
Abstract
The photonic nanojet phenomenon is commonly used both to increase the resolution of optical microscopes and to trap nanoparticles. However, such photonic nanojets are not applicable to an entire class of objects. Here we present a new type of photonic nanojet in reflection [...] Read more.
The photonic nanojet phenomenon is commonly used both to increase the resolution of optical microscopes and to trap nanoparticles. However, such photonic nanojets are not applicable to an entire class of objects. Here we present a new type of photonic nanojet in reflection mode with the possibility to control the modulation of the photonic nanojet by a standing wave. In contrast to the known kinds of reflective photonic nanojets, the reported one occurs when the aluminum oxide hemisphere is located at a certain distance from the substrate. Under illumination, the hemisphere generates a primary photonic nanojet directed to the substrate. After reflection, the primary nanojet acts as an illumination source for the hemisphere, leading to the formation of a new reflective photonic nanojet. We show that the distance between the hemisphere and substrate affects the phase of both incident and reflected radiation, and due to constructive interference, the modulation of the reflective photonic nanojet by a standing wave can be significantly reduced. The results obtained contribute to the understanding of the processes of photonic nanojet formation in reflection mode and open new pathways for designing functional optical devices. Full article
(This article belongs to the Special Issue Photonic Jet: Science and Application)
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Open AccessArticle
Experimental Investigation on the Sputtering Process for Tantalum Oxynitride Thin Films
Photonics 2021, 8(2), 53; https://doi.org/10.3390/photonics8020053 - 15 Feb 2021
Viewed by 202
Abstract
Metal oxynitrides are compounds between nitrides and oxides with a certain level of photocatalytic functions. The purpose of this study is to investigate an appropriate range of oxygen flow rate during sputtering for depositing tantalum oxynitride films. The sputtering process was carried out [...] Read more.
Metal oxynitrides are compounds between nitrides and oxides with a certain level of photocatalytic functions. The purpose of this study is to investigate an appropriate range of oxygen flow rate during sputtering for depositing tantalum oxynitride films. The sputtering process was carried out under fixed nitrogen but variable oxygen flow rates. Post rapid thermal annealing was conducted at 800 °C for 5 min to transform the as-deposited amorphous films into crystalline phases. The material characterizations of annealed films include X-ray diffraction and Raman spectroscopy for identifying crystal structures; scanning electron microscope for examining surface morphology; energy-dispersive X-ray spectroscopy to determine surface elemental compositions; four-point probe and Hall effect analysis to evaluate electrical resistivity; UV-visible-NIR spectroscopy for quantifying optical properties and optical bandgaps. To assess the photocatalytic function of oxynitride films, the degradation of methyl orange in de-ionized water was examined under continuous irradiation by a simulated solar light source for six hours. Results indicate that crystalline tantalum oxynitride films can be obtained if the O2 flow rate is chosen to be 0.25–1.5 sccm along with 10 sccm of N2 and 20 sccm of Ar. In particular, films deposited between 0.25 and 1.5 sccm O2 flow have higher efficiency in photodegradation on methyl orange due to a more comprehensive formation of oxynitrides. Full article
(This article belongs to the Special Issue Photonic Devices and Systems)
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Open AccessArticle
An Optical Analog-to-Digital Converter with Enhanced ENOB Based on MMI-Based Phase-Shift Quantization
Photonics 2021, 8(2), 52; https://doi.org/10.3390/photonics8020052 - 14 Feb 2021
Viewed by 205
Abstract
An optical analog-to-digital converter (OADC) scheme with enhanced bit resolution by using a multimode interference (MMI) coupler as optical quantization is proposed. The mathematical simulation model was established to verify the feasibility and to investigate the robustness of the scheme. Simulation results show [...] Read more.
An optical analog-to-digital converter (OADC) scheme with enhanced bit resolution by using a multimode interference (MMI) coupler as optical quantization is proposed. The mathematical simulation model was established to verify the feasibility and to investigate the robustness of the scheme. Simulation results show that 20 quantization levels (corresponding to 4.32 of effective number of bits (ENOB)) are realized by using only 6 channels, which indicates that the scheme requires much fewer quantization channels or modulators to realize the same amount of ENOB. The scheme is robust and potential for integration. Full article
(This article belongs to the Special Issue Photonics for Emerging Applications in Communication and Sensing)
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Open AccessArticle
Comparing Performance of Deep Convolution Networks in Reconstructing Soliton Molecules Dynamics from Real-Time Spectral Interference
Photonics 2021, 8(2), 51; https://doi.org/10.3390/photonics8020051 - 13 Feb 2021
Viewed by 318
Abstract
Deep neural networks have enabled the reconstruction of optical soliton molecules with more complex structures using the real-time spectral interferences obtained by photonic time-stretch dispersive Fourier transformation (TS-DFT) technology. In this paper, we propose to use three kinds of deep convolution networks (DCNs), [...] Read more.
Deep neural networks have enabled the reconstruction of optical soliton molecules with more complex structures using the real-time spectral interferences obtained by photonic time-stretch dispersive Fourier transformation (TS-DFT) technology. In this paper, we propose to use three kinds of deep convolution networks (DCNs), including VGG, ResNets, and DenseNets, for revealing internal dynamics evolution of soliton molecules based on the real-time spectral interferences. When analyzing soliton molecules with equidistant composite structures, all three models are effective. The DenseNets with layers of 48 perform the best for extracting the dynamic information of complex five-soliton molecules from TS-DFT data. The mean Pearson correlation coefficient (MPCC) between the predicted results and the real results is about 0.9975. Further, the ResNets in which the MPCC achieves 0.9906 also has the better ability of phase extraction than VGG which the MPCC is about 0.9739. The general applicability is demonstrated for extracting internal information from complex soliton molecule structures with high accuracy. The presented DCNs-based techniques can be employed to explore undiscovered mechanisms underlying the distribution and evolution of large numbers of solitons in dissipative systems in experimental research. Full article
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Open AccessArticle
Enhanced Pulse Compression within Sign-Alternating Dispersion Waveguides
Photonics 2021, 8(2), 50; https://doi.org/10.3390/photonics8020050 - 13 Feb 2021
Viewed by 298
Abstract
We show theoretically and numerically how to optimize sign-alternating dispersion waveguides for maximum nonlinear pulse compression, while leveraging the substantial increase in bandwidth-to-input peak power advantage of these structures. We find that the spectral phase can converge to a parabolic profile independent of [...] Read more.
We show theoretically and numerically how to optimize sign-alternating dispersion waveguides for maximum nonlinear pulse compression, while leveraging the substantial increase in bandwidth-to-input peak power advantage of these structures. We find that the spectral phase can converge to a parabolic profile independent of uncompensated higher-order dispersion. The combination of an easy to compress phase spectrum, with low input power requirements, then makes sign-alternating dispersion a scheme for high-quality nonlinear pulse compression that does not require high powered lasers, which is beneficial for instance in integrated photonic circuits. We also show a new nonlinear compression regime and soliton shaping dynamic only seen in sign-alternating dispersion waveguides. Through an example SiN-based integrated waveguide, we show that the dynamic enables the attainment of compression to two optical cycles at a pulse energy of 100 pJ which surpasses the compression achieved using similar parameters for a current state-of-the-art SiN system. Full article
(This article belongs to the Section Lasers, Light Sources and Sensors)
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Open AccessArticle
Mie Resonance Engineering in Two Disks
Photonics 2021, 8(2), 49; https://doi.org/10.3390/photonics8020049 - 13 Feb 2021
Viewed by 239
Abstract
Recently the recipes to achieve the high-Q subwavelength resonances in an isolated dielectric disk have been reported based on avoided crossing (anticrossing) of the TE resonances under variation of the aspect ratio of the disk. In a silicon disk that recipe gives an [...] Read more.
Recently the recipes to achieve the high-Q subwavelength resonances in an isolated dielectric disk have been reported based on avoided crossing (anticrossing) of the TE resonances under variation of the aspect ratio of the disk. In a silicon disk that recipe gives an enhancement of the Q factor by one order of magnitude. In the present paper we present the approach based on engineering of the spherical Mie resonances with high orbital index in two coaxial disks by two-fold avoided crossing of the resonant modes of the disks. At the first step we select the resonant modes of single disk which are degenerate because of the opposite symmetry. Approaching of the second disk removes this degeneracy because of interaction between the disks. As a result at certain distances we realize the hybridized anti-bonding resonant modes whose morphology becomes close to the spherical Mie resonant mode with high orbital index. Respectively the Q factor of the anti-bonding resonant mode can be enhanced by three orders of magnitude compared to the case of single disk. Full article
(This article belongs to the Section Optoelectronics and Optical Materials)
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Open AccessArticle
Enhancing Multi-Distance Phase Retrieval via Unequal Interval Measurements
Photonics 2021, 8(2), 48; https://doi.org/10.3390/photonics8020048 - 12 Feb 2021
Viewed by 309
Abstract
In the conventional methods of multi-distance phase retrieval, the diffraction intensity patterns are recorded at equal intervals, which can induce slow convergence or stagnation in the subsequent reconstruction process. To solve this problem, a measurement method with unequal intervals is proposed in this [...] Read more.
In the conventional methods of multi-distance phase retrieval, the diffraction intensity patterns are recorded at equal intervals, which can induce slow convergence or stagnation in the subsequent reconstruction process. To solve this problem, a measurement method with unequal intervals is proposed in this paper. The interval spacings between adjacent measurement planes are decreased gradually. A large gap accelerates retrieval progress, and a short span helps to recover detailed information. The proposed approach makes full use of the available measured dataset and simultaneously generates variations in diversity amplitude, which is a crucial issue for the techniques of multi-image phase retrieval. Both computational simulations and experiments are performed. The results demonstrate that this method can improve the convergence speed by 2 to 3 times and enhance the quality of reconstruction results in comparison to that of the conventional methods. Full article
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Open AccessCommunication
Monochromatic Conical IR Emission from Decaying KrF Laser Filaments in Xenon as Coherent Stimulated Four-Wave Mixing Process
Photonics 2021, 8(2), 47; https://doi.org/10.3390/photonics8020047 - 12 Feb 2021
Viewed by 246
Abstract
We develop theoretical background for the new nonlinear optical phenomenon of narrowly directed monochromatic IR conical emission which has been recently observed when 248-nm UV filaments propagate in xenon (V. D. Zvorykin, et al., Laser Phys. Lett. 13, 125404 (2016)). We treat it [...] Read more.
We develop theoretical background for the new nonlinear optical phenomenon of narrowly directed monochromatic IR conical emission which has been recently observed when 248-nm UV filaments propagate in xenon (V. D. Zvorykin, et al., Laser Phys. Lett. 13, 125404 (2016)). We treat it as coherent stimulated four-wave mixing process in which two pump KrF laser photons are converted into the coupled pair of resonance IR(828 nm) and VUV (147 nm) photons through 5p5(2P3/2)6p[1/2]05p5(2P3/2)6s[3/2]1o and 5p5(2P3/2)6s[3/2]1o1S0 transitions. We explore the coherent interaction regime which proceeds at a time scale shorter than transverse relaxation time T2. The momentum and energy conservation laws determine the characteristic angle of conical emission. We find that the threshold of this coherent process is determined by the KrF laser pump pulse area. Full article
(This article belongs to the Special Issue Novel Nonlinear Phenomena with Strong Laser Fields)
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Open AccessReview
Applications of Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy
Photonics 2021, 8(2), 46; https://doi.org/10.3390/photonics8020046 - 12 Feb 2021
Viewed by 230
Abstract
The surface-enhanced Raman scattering (SERS) is mainly used as an analysis or detection tool of biological and chemical molecules. Since the last decade, an alternative branch of the SERS effect has been explored, and named shell-isolated nanoparticle Raman spectroscopy (SHINERS) which was discovered [...] Read more.
The surface-enhanced Raman scattering (SERS) is mainly used as an analysis or detection tool of biological and chemical molecules. Since the last decade, an alternative branch of the SERS effect has been explored, and named shell-isolated nanoparticle Raman spectroscopy (SHINERS) which was discovered in 2010. In SHINERS, plasmonic cores are used for enhancing the Raman signal of molecules, and a very thin shell of silica is generally employed for improving the thermal and chemical stability of plasmonic cores that is of great interest in the specific case of catalytic reactions under difficult conditions. Moreover, thanks to its great surface sensitivity, SHINERS can enable the investigation at liquid–solid interfaces. In last two years (2019–2020), recent insights in this alternative SERS field were reported. Thus, this mini-review is centered on the applications of shell-isolated nanoparticle Raman spectroscopy to the reactions with CO molecules, other surface catalytic reactions, and the detection of molecules and ions. Full article
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Open AccessArticle
Performance Evaluation of Highly Nonlinear Fiber (HNLF) Based Optical Phase Conjugation (OPC) in Long Haul Transmission of 640 Gbps 16-QAM CO-OFDM
Photonics 2021, 8(2), 45; https://doi.org/10.3390/photonics8020045 - 10 Feb 2021
Viewed by 270
Abstract
This paper presents the quantitative measurement through an experimental test of 640 Gbps 16-QAM coherent-optical orthogonal frequency-division multiplexing (CO-OFDM) over 800 km optical fiber with mid-link optical phase conjugation (OPC) using highly nonlinear fiber (HNLF). The first focus is the OPC parameter optimization, [...] Read more.
This paper presents the quantitative measurement through an experimental test of 640 Gbps 16-QAM coherent-optical orthogonal frequency-division multiplexing (CO-OFDM) over 800 km optical fiber with mid-link optical phase conjugation (OPC) using highly nonlinear fiber (HNLF). The first focus is the OPC parameter optimization, including the optimization of HNLF length and signal/pump power that inputs into OPC. Four different HNLFs, as the illustrative examples, are investigated. The second focus is to investigate the effects of fiber dispersion, nonlinearity, and amplified spontaneous emission (ASE) noise on the long-haul transmission of 16-QAM CO-OFDM signal, and the OPC compensation efficiency. The performance evaluation focuses on the conversion efficiency (CE), received signal constellation, Q-factor improvement, and bit error rate (BER) at the receiver end. Such end-to-end performance evaluation is important because the 16-QAM CO-OFDM signal status is heterogeneous and the mitigation of transmission impairments to the signal is still unclear. The OPC parametric optimization is achieved experimentally using commercially available HNLFs with different scenarios and the numerical results are interpreted in conjunction with simulations. 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 AccessCommunication
Extending the Photon Energy Coverage of a Seeded Free-Electron Laser via Reverse Taper Enhanced Harmonic Cascade
Photonics 2021, 8(2), 44; https://doi.org/10.3390/photonics8020044 - 09 Feb 2021
Viewed by 309
Abstract
External seeded free-electron lasers (FELs) hold promising prospects for producing intense coherent radiation at high harmonics of a conventional laser. The practical harmonic up-conversion efficiencies of current seeding techniques are limited by various three-dimensional effects on the electron beam. In this paper, a [...] Read more.
External seeded free-electron lasers (FELs) hold promising prospects for producing intense coherent radiation at high harmonics of a conventional laser. The practical harmonic up-conversion efficiencies of current seeding techniques are limited by various three-dimensional effects on the electron beam. In this paper, a novel method is proposed to extend the wavelength coverage of a seeded FEL by combining the reverse taper undulator with the echo-enabled harmonic generation. The proposed technique can significantly enhance the bunching at ultra-high harmonics and preserve the electron beam qualities from degradation by deleterious effects. Theoretical analysis and numerical simulation are performed, and the results demonstrate that stable, intense, nearly fully coherent FEL pulses with photon energy up to 1 keV can be generated. The proposed technique may open up new opportunities to obtain laser-like pulses at sub-nanometer wavelength. Full article
(This article belongs to the Section Lasers, Light Sources and Sensors)
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Open AccessCommunication
High-Q-Factor Silica-Based Racetrack Microring Resonators
Photonics 2021, 8(2), 43; https://doi.org/10.3390/photonics8020043 - 06 Feb 2021
Viewed by 381
Abstract
In this paper, ultrahigh-Q factor racetrack microring resonators (MRRs) are demonstrated based on silica planar lightwave circuits (PLCs) platform. A loaded ultrahigh-Q factor Qload of 1.83 × 106 is obtained. The MRRs are packaged with fiber-to-fiber loss of ~5 dB. A [...] Read more.
In this paper, ultrahigh-Q factor racetrack microring resonators (MRRs) are demonstrated based on silica planar lightwave circuits (PLCs) platform. A loaded ultrahigh-Q factor Qload of 1.83 × 106 is obtained. The MRRs are packaged with fiber-to-fiber loss of ~5 dB. A notch depth of 3 dB and ~137 pm FSR are observed. These MRRs show great potential in optical communications as filters. Moreover, the devices are suitable used in monolithic integration and hybrid integration with other devices, especially in external cavity lasers (ECLs) to realize ultranarrow linewidths. Full article
(This article belongs to the Section Optical Communication and Network)
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Open AccessArticle
The Optical Properties of InGaN/GaN Nanorods Fabricated on (-201) β-Ga2O3 Substrate for Vertical Light Emitting Diodes
Photonics 2021, 8(2), 42; https://doi.org/10.3390/photonics8020042 - 06 Feb 2021
Viewed by 338
Abstract
We fabricated InGaN/GaN nanorod light emitting diode (LED) on (-201) β-Ga2O3 substrate via the SiO2 nanosphere lithography and dry-etching techniques. The InGaN/GaN nanorod LED grown on β-Ga2O3 can effectively suppress quantum confined Stark effect (QCSE) compared [...] Read more.
We fabricated InGaN/GaN nanorod light emitting diode (LED) on (-201) β-Ga2O3 substrate via the SiO2 nanosphere lithography and dry-etching techniques. The InGaN/GaN nanorod LED grown on β-Ga2O3 can effectively suppress quantum confined Stark effect (QCSE) compared to planar LED on account of the strain relaxation. With the enhancement of excitation power density, the photoluminescence (PL) peak shows a large blue-shift for the planar LED, while for the nanorod LED, the peak position shift is small. Furthermore, the simulations also show that the light extraction efficiency (LEE) of the nanorod LED is approximately seven times as high as that of the planar LED. Obviously, the InGaN/GaN/β-Ga2O3 nanorod LED is conducive to improving the optical performance relative to planar LED, and the present work may lay the groundwork for future development of the GaN-based vertical light emitting diodes (VLEDs) on β-Ga2O3 substrate. Full article
(This article belongs to the Special Issue Wide Bandgap Semiconductor Photonic Devices)
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Open AccessFeature PaperArticle
Optical Characterization of Ultra-Thin Films of Azo-Dye-Doped Polymers Using Ellipsometry and Surface Plasmon Resonance Spectroscopy
Photonics 2021, 8(2), 41; https://doi.org/10.3390/photonics8020041 - 05 Feb 2021
Viewed by 429
Abstract
The determination of optical constants (i.e., real and imaginary parts of the complex refractive index (nc) and thickness (d)) of ultrathin films is often required in photonics. It may be done by using, for example, surface plasmon resonance [...] Read more.
The determination of optical constants (i.e., real and imaginary parts of the complex refractive index (nc) and thickness (d)) of ultrathin films is often required in photonics. It may be done by using, for example, surface plasmon resonance (SPR) spectroscopy combined with either profilometry or atomic force microscopy (AFM). SPR yields the optical thickness (i.e., the product of nc and d) of the film, while profilometry and AFM yield its thickness, thereby allowing for the separate determination of nc and d. In this paper, we use SPR and profilometry to determine the complex refractive index of very thin (i.e., 58 nm) films of dye-doped polymers at different dye/polymer concentrations (a feature which constitutes the originality of this work), and we compare the SPR results with those obtained by using spectroscopic ellipsometry measurements performed on the same samples. To determine the optical properties of our film samples by ellipsometry, we used, for the theoretical fits to experimental data, Bruggeman’s effective medium model for the dye/polymer, assumed as a composite material, and the Lorentz model for dye absorption. We found an excellent agreement between the results obtained by SPR and ellipsometry, confirming that SPR is appropriate for measuring the optical properties of very thin coatings at a single light frequency, given that it is simpler in operation and data analysis than spectroscopic ellipsometry. Full article
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Open AccessArticle
Topological-Insulator-Based Gap-Surface Plasmon Metasurfaces
Photonics 2021, 8(2), 40; https://doi.org/10.3390/photonics8020040 - 04 Feb 2021
Viewed by 488
Abstract
Topological insulators (TIs) have unique highly conducting symmetry-protected surface states while the bulk is insulating, making them attractive for various applications in condensed matter physics. Recently, topological insulator materials have been tentatively applied for both near- and far-field wavefront manipulation of electromagnetic waves, [...] Read more.
Topological insulators (TIs) have unique highly conducting symmetry-protected surface states while the bulk is insulating, making them attractive for various applications in condensed matter physics. Recently, topological insulator materials have been tentatively applied for both near- and far-field wavefront manipulation of electromagnetic waves, yielding superior plasmonic properties in the ultraviolet (UV)-to-visible wavelength range. However, previous reports have only demonstrated inefficient wavefront control based on binary metasurfaces that were digitalized on a TI thin film or non-directional surface plasmon polariton (SPP) excitation. Here, we numerically demonstrated the plasmonic capabilities of the TI Bi2Te3 as a material for gap–surface plasmon (GSP) metasurfaces. By employing the principle of the geometric phase, a far-field beam-steering metasurface was designed for the visible spectrum, yielding a cross-polarization efficiency of 34% at 500 nm while suppressing the co-polarization to 0.08%. Furthermore, a birefringent GSP metasurface design was studied and found to be capable of directionally exciting SPPs depending on the incident polarization. Our work forms the basis for accurately controlling the far- and near-field responses of TI-based GSP metasurfaces in the visible spectral range. Full article
(This article belongs to the Special Issue Plasmonic Metasurfaces)
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Open AccessReview
Inverted p-down Design for High-Speed Photodetectors
Photonics 2021, 8(2), 39; https://doi.org/10.3390/photonics8020039 - 04 Feb 2021
Viewed by 365
Abstract
We discuss the structural consideration of high-speed photodetectors used for optical communications, focusing on vertical illumination photodetectors suitable for device fabrication and optical coupling. We fabricate an avalanche photodiode that can handle 100-Gbit/s four-level pulse-amplitude modulation (50 Gbaud) signals, and pin photodiodes for [...] Read more.
We discuss the structural consideration of high-speed photodetectors used for optical communications, focusing on vertical illumination photodetectors suitable for device fabrication and optical coupling. We fabricate an avalanche photodiode that can handle 100-Gbit/s four-level pulse-amplitude modulation (50 Gbaud) signals, and pin photodiodes for 100-Gbaud operation; both are fabricated with our unique inverted p-side down (p-down) design. Full article
(This article belongs to the Special Issue Advanced Ultra High Speed Optoelectronic Devices)
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Open AccessArticle
Accurate Power-Efficient Format-Scalable Multi-Parallel Optical Digital-to-Analogue Conversion
Photonics 2021, 8(2), 38; https://doi.org/10.3390/photonics8020038 - 04 Feb 2021
Viewed by 421
Abstract
In optical transmitters generating multi-level constellations, optical modulators are preceded by Electronic Digital-to-Analog-Converters (eDAC). It is advantageous to use eDAC-free Optical Analog to Digital Converters (oDAC) to directly convert digital bitstreams into multilevel PAM/QAM optical signals. State-of-the-art oDACs are based on Segmented Mach-Zehnder-Modulators [...] Read more.
In optical transmitters generating multi-level constellations, optical modulators are preceded by Electronic Digital-to-Analog-Converters (eDAC). It is advantageous to use eDAC-free Optical Analog to Digital Converters (oDAC) to directly convert digital bitstreams into multilevel PAM/QAM optical signals. State-of-the-art oDACs are based on Segmented Mach-Zehnder-Modulators (SEMZM) using multiple modulation segments strung along the MZM waveguides to serially accumulate binary-modulated optical phases. Here we aim to assess performance limits of the Serial oDACs (SEMZM) and introduce an alternative improved Multi-Parallel oDAC (MPoDAC) architecture, in particular based on arraying multiple binary-driven MZMs in parallel: Multi-parallel MZM (MPMZM) oDAC. We develop generic methodologies of oDAC specification and optimization encompassing both SEMZM and MPMZM options in Direct-Detection (DD) and Coherent-Detection (COH) implementations. We quantify and compare intrinsic performance limits of the various serial/parallel DD/COH subclasses for general constellation orders, comparing with the scant prior-work on the multi-parallel option. A key finding: COH-MPMZM is the only class synthesizing ‘perfect’ (equi-spaced max-full-scale) constellations while maximizing energy-efficiency-SEMZM/MPMZM for DD are less accurate when maximal energy-efficiency is required. In particular, we introduce multiple variants of PAM4|8 DD and QAM16|64 COH MPMZMs, working out their accuracy vs. energy-efficiency-and-complexity tradeoffs, establishing their format-reconfigurability (format-flexible switching of constellation order and/or DD/COH). Full article
(This article belongs to the Special Issue Reconfigurable Photonic Interconnects)
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Open AccessArticle
Proposal of Highly Efficient Quantum Well Microring Resonator-Loaded Optical Phase Modulator Integrated with Antenna-Coupled Electrodes for Radio-over-Fiber
Photonics 2021, 8(2), 37; https://doi.org/10.3390/photonics8020037 - 03 Feb 2021
Viewed by 365
Abstract
Radio-over-fiber (RoF) technology for low-loss, high-speed millimeter-wave transmission using optical fibers has been attracting attention. We propose a highly efficient microring resonator (MRR)-loaded InGaAs/InAlAs multiple-quantum-well (MQW) phase modulator with an antenna coupled electrode (ACE) for 60 GHz RoF systems, and its modulation characteristics [...] Read more.
Radio-over-fiber (RoF) technology for low-loss, high-speed millimeter-wave transmission using optical fibers has been attracting attention. We propose a highly efficient microring resonator (MRR)-loaded InGaAs/InAlAs multiple-quantum-well (MQW) phase modulator with an antenna coupled electrode (ACE) for 60 GHz RoF systems, and its modulation characteristics are theoretically discussed. This modulator is able to directly convert wireless millimeter-wave (MMW) signals into optical signals without an external power supply. The MRR used in the waveguide structure increases the optical phase change obtained by the unique quantum confinement Stark effect in the MQW through phase enhancement effects, while the ACE based on a coupled microstrip resonant electrode applies a strong standing wave electric field to the waveguide. The proposed modulator is expected to provide tens of times higher phase modulation efficiency than a conventional gap-embedded planar antenna-integrated modulator. Full article
(This article belongs to the Special Issue Radio over Fiber)
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Open AccessArticle
Interband, Surface Plasmon and Fano Resonances in Titanium Carbide (MXene) Nanoparticles in the Visible to Infrared Range
Photonics 2021, 8(2), 36; https://doi.org/10.3390/photonics8020036 - 01 Feb 2021
Viewed by 435
Abstract
Since the discovery of the optical properties of two-dimensional (2D) titanium carbide (MXene) conductive material, an ever increasing interest has been devoted towards understanding it as a plasmonic substrate or nanoparticle. This noble metal-free alternative holds promise not only due to its lower [...] Read more.
Since the discovery of the optical properties of two-dimensional (2D) titanium carbide (MXene) conductive material, an ever increasing interest has been devoted towards understanding it as a plasmonic substrate or nanoparticle. This noble metal-free alternative holds promise not only due to its lower cost but also its 2D nature, hydrophilicity and apparent bio-compatibility. Herein, the optical properties of the most widely studied Ti3C2Tx MXene nanosheets are theoretically analyzed and absorption cross-sections are calculated exploiting available experimental data on its dielectric function. The occurrence of quadrupole surface plasmon mode in the optical absorption spectra of large MXene nanoparticles is demonstrated for the first time. The resonance wavelengths corresponding to interband transitions, longitudinal and transversal dipole oscillations and quadrupole longitudinal surface plasmon mode are identified for single and coupled nanoparticles by modeling their shapes as ellipsoids, disks and cylinders. A new mechanism of excitation of longwave transversal surface plasmon oscillations by an external electric field perpendicular to the direction of charge oscillations is presented. Excitingly enough, a new effect in coupled MXene nanoparticles—Fano resonance—is unveiled. The results of calculations are compared to known experimental data on electron absorption spectroscopy, and good agreement is demonstrated. Full article
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Open AccessArticle
Design and Fabrication of Large-Size Powersphere for Wireless Energy Transmission via Laser
Photonics 2021, 8(2), 35; https://doi.org/10.3390/photonics8020035 - 30 Jan 2021
Viewed by 272
Abstract
The powersphere is a device used for maximizing the conversion of light in wireless energy transmission via laser. It is a spherical structure made up of thousands of photovoltaic cells. Due to the large dimensions and existence of many holes in the spherical [...] Read more.
The powersphere is a device used for maximizing the conversion of light in wireless energy transmission via laser. It is a spherical structure made up of thousands of photovoltaic cells. Due to the large dimensions and existence of many holes in the spherical surface, there are some drawbacks in machining, such as limited movement space of the machines, long cycle, low precision, and high cost. In this context, with a powersphere irradiated by the laser as the model, the principle of powersphere is deduced theoretically. It is proven that the illuminance value at any position on the inner wall of the powersphere is equal, and the calculation formula of this value is derived. Based on this theory and the comparative analysis of processing methods and the results of processing experiments, the structure of the powersphere is designed. The experimental processing of the powersphere is carried out by selecting the welding method. Finally, two hemispherical powersphere frames are processed, which are connected by screws to form a ball frame for the installation of photovoltaic cells. The results show that the improved design and fabricating method can process the powersphere quickly, accurately, and economically. A comparative experiment of powersphere and photovoltaic panel was carried out. The experimental results show that the powersphere has the function of light uniformity and repeated use of laser. So, the designed and processed powersphere is consistent with the theoretical analysis. Full article
(This article belongs to the Special Issue Optical Instrumentation)
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Open AccessArticle
Compressive Coded Rotating Mirror Camera for High-Speed Imaging
Photonics 2021, 8(2), 34; https://doi.org/10.3390/photonics8020034 - 30 Jan 2021
Viewed by 328
Abstract
We develop a novel compressive coded rotating mirror (CCRM) camera to capture events at high frame rates in passive mode with a compact instrument design at a fraction of the cost compared to other high-speed imaging cameras. Operation of the CCRM camera is [...] Read more.
We develop a novel compressive coded rotating mirror (CCRM) camera to capture events at high frame rates in passive mode with a compact instrument design at a fraction of the cost compared to other high-speed imaging cameras. Operation of the CCRM camera is based on amplitude optical encoding (grey scale) and a continuous frame sweep across a low-cost detector using a motorized rotating mirror system which can achieve single pixel shift between adjacent frames. Amplitude encoding and continuous frame overlapping enable the CCRM camera to achieve a high number of captured frames and high temporal resolution without making sacrifices in the spatial resolution. Two sets of dynamic scenes have been captured at up to a 120 Kfps frame rate in both monochrome and colored scales in the experimental demonstrations. The obtained heavily compressed data from the experiment are reconstructed using the optimization algorithm under the compressive sensing (CS) paradigm and the highest sequence depth of 1400 captured frames in a single exposure has been achieved with the highest compression ratio of 368 compared to other CS-based high-speed imaging technologies. Under similar conditions the CCRM camera is 700× faster than conventional rotating mirror based imaging devices and could reach a frame rate of up to 20 Gfps. Full article
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Open AccessReview
Quantum Reinforcement Learning with Quantum Photonics
Photonics 2021, 8(2), 33; https://doi.org/10.3390/photonics8020033 - 28 Jan 2021
Viewed by 715
Abstract
Quantum machine learning has emerged as a promising paradigm that could accelerate machine learning calculations. Inside this field, quantum reinforcement learning aims at designing and building quantum agents that may exchange information with their environment and adapt to it, with the aim of [...] Read more.
Quantum machine learning has emerged as a promising paradigm that could accelerate machine learning calculations. Inside this field, quantum reinforcement learning aims at designing and building quantum agents that may exchange information with their environment and adapt to it, with the aim of achieving some goal. Different quantum platforms have been considered for quantum machine learning and specifically for quantum reinforcement learning. Here, we review the field of quantum reinforcement learning and its implementation with quantum photonics. This quantum technology may enhance quantum computation and communication, as well as machine learning, via the fruitful marriage between these previously unrelated fields. Full article
(This article belongs to the Special Issue The Interplay between Photonics and Machine Learning)
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Open AccessArticle
Single Headlamp with Low- and High-Beam Light
Photonics 2021, 8(2), 32; https://doi.org/10.3390/photonics8020032 - 27 Jan 2021
Viewed by 311
Abstract
Generally, automobiles are typically equipped with separate headlamp lanterns for generating low- and high-beam light. Compared with separate headlamp lanterns, a single headlamp producing both low- and high-beam light can be more compact and have less mechanical complexity. The single headlamp structure has [...] Read more.
Generally, automobiles are typically equipped with separate headlamp lanterns for generating low- and high-beam light. Compared with separate headlamp lanterns, a single headlamp producing both low- and high-beam light can be more compact and have less mechanical complexity. The single headlamp structure has become a main emphasis of research that manufacturers will continue to focus great efforts on in the future. A novel design of a single headlamp generating both low- and high-beam light is proposed in this study. The proposed headlamp consists of a compound ellipsoidal reflector, a baffle plate, a condenser lens, and LED array devices generating low- and high-beam light. The compound ellipsoidal reflector comprises a primary ellipsoidal reflector for generating low-beam light and a secondary ellipsoidal reflector for generating high-beam light. Monte Carlo ray tracing simulations were performed to confirm the optical characteristics of the proposed design. A prototype of the proposed headlamp was also fabricated and assembled to verify the design’s effectiveness. The simulated and measured illuminance distributions of the low-beam and high-beam light had the desired light patterns. Moreover, all the simulated and measured illuminances of each point and line met the ECE R112 regulation for low-beam and high-beam light. The proposed headlamp in this study is feasible for the application of single headlamp generating both low- and high-beam light. Full article
(This article belongs to the Special Issue Photonics, Optics and Laser Technology)
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