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Photonics, Volume 11, Issue 3 (March 2024) – 94 articles

Cover Story (view full-size image): Introducing a novel method, we create parallel Bessel-like vortical optical needles with customizable axial intensity distributions by superposing various cone-angle Bessel vortices. Our study explores the interplay between needle separation and length, presenting a super-Gaussian axial profile. Through experimentation, we validate our approach using spatial light modulation to manipulate laser beam spectra. This versatile technique holds promise for applications like microfabrication and laser material processing, including nanochannel machining and glass dicing. View this paper
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17 pages, 4926 KiB  
Article
Synergy of Nd:YAG Picosecond Pulsed Laser Irradiation and Electrochemical Anodization in the Formation of TiO2 Nanostructures for the Photocatalytic Degradation of Pesticide Carbofuran
by Miloš Tošić, Vladimir Rajić, Dejan Pjević, Stevan Stojadinović, Nikša Krstulović, Suzana Dimitrijević-Branković and Miloš Momčilović
Photonics 2024, 11(3), 284; https://doi.org/10.3390/photonics11030284 - 21 Mar 2024
Viewed by 664
Abstract
This study proposes a simple and controlled method for producing TiO2 with phase junction, oxygen vacancies, and Ti3+ by combining picosecond pulsed laser irradiation and electrochemical anodization. Ti mesh was pretreated by irradiating with a picosecond pulsed laser technique using an [...] Read more.
This study proposes a simple and controlled method for producing TiO2 with phase junction, oxygen vacancies, and Ti3+ by combining picosecond pulsed laser irradiation and electrochemical anodization. Ti mesh was pretreated by irradiating with a picosecond pulsed laser technique using an Nd:YAG laser (1064 nm) at two fluencies, 15 J/cm2 and 30 J/cm2. The samples were then subjected to electrochemical anodization to form TiO2 nanotube arrays on the previously laser-treated surface. This study will investigate the possibility of forming TiO2 nanotube arrays on a pre-laser-treated Ti substrate and determine their physicochemical and photocatalytic properties. The samples were characterized by FESEM, XRD, Raman, XPS, and UV-Vis DRS. UV-Vis spectroscopy was used to observe the progress of photocatalytic degradation for all samples, and degradation products were determined using GC-MS. With the synergistic effects of phase junction, oxygen vacancies, and Ti3+, the laser-treated TiO2 with 30 J/cm2 showed a higher photocatalytic degradation rate (85.1%) of the pesticide carbofuran compared to non-laser-treated TiO2 (54.8%), remaining stable during successive degradation cycles, which has promising practical applications. Full article
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17 pages, 1592 KiB  
Article
Pseudo-Stilbene- and Azobenzene-Type Systems for Optical Frequency Conversion: Estimating the First-Order Molecular Hyperpolarizability
by Raiane S. Araújo, José J. Rodrigues, Jr., Márcio A. R. C. Alencar, Jamal Rafique, Sumbal Saba and Luis M. G. Abegão
Photonics 2024, 11(3), 283; https://doi.org/10.3390/photonics11030283 - 21 Mar 2024
Viewed by 668
Abstract
This study investigates the potential of a set of pseudo-stilbene and azobenzene molecular structures to become optical frequency converters for optical communications based on a detailed exploration of the first-order molecular hyperpolarizability (βHRS), which is the microscopic counterpart [...] Read more.
This study investigates the potential of a set of pseudo-stilbene and azobenzene molecular structures to become optical frequency converters for optical communications based on a detailed exploration of the first-order molecular hyperpolarizability (βHRS), which is the microscopic counterpart of second harmonic generation (SHG). βHRS values were obtained via quantum chemical calculations using the Gaussian 16 software package in solvent and gas-phase media at different wavelengths, i.e., 1064 nm, 1310 nm, and 1510 nm. The latter two wavelengths are of particular interest for optical communications. Our study focused on discerning how the molecular structure influences the βHRS response, explicitly highlighting the influence of the azomethine group (CH=N). The results revealed that the molecular planarity, affected by this group, plays a crucial role in modulating the optical properties. The highest βHRS value in a solvent medium using the CAM-B3LYP/6-311+G(2d,p) level of theory achieved in this work was around 1400 ×1030cm4startvolt1, four orders of magnitude higher than KDP (0.2 ×1030cm4startvolt1), which is a reference in SHG experiments at 1064 nm. The highest calculated βHRS value at the same level of theory and solvent at 1310 nm and 1550 nm was 631 × 1030cm4startvolt1 and 456 × 1030cm4startvolt1, respectively. All these values belong to molecular structures with azo-coupling with donor (4-NMe2) and acceptor (4′-NO2) peripheral groups, designated as AB-3. Full article
(This article belongs to the Special Issue Nonlinear Optics and Hyperspectral Polarization Imaging)
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11 pages, 2518 KiB  
Article
Line-Spacing-Multiplied Optical Frequency Comb Generation Using an Electro-Optic Talbot Laser and Cross-Phase Modulation in a Fiber
by Juanjuan Yan, Haiyan Dong and Yu Wang
Photonics 2024, 11(3), 282; https://doi.org/10.3390/photonics11030282 - 21 Mar 2024
Viewed by 586
Abstract
An optical frequency comb (OFC) generator based on an electro-optic Talbot laser and cross-phase modulation (XPM) in a high nonlinear fiber (HNLF) is designed and demonstrated. The Talbot laser is an electro-optic frequency shifting loop that is used to produce repetition rate-multiplied pulses, [...] Read more.
An optical frequency comb (OFC) generator based on an electro-optic Talbot laser and cross-phase modulation (XPM) in a high nonlinear fiber (HNLF) is designed and demonstrated. The Talbot laser is an electro-optic frequency shifting loop that is used to produce repetition rate-multiplied pulses, and these pulses work as a pump signal that induces the XPM process in the HNLF to modulate the phase of a probe signal. At the output of the HNLF, OFCs with a multiplied line spacing can be generated. The effects of the pump power and the HNLF length on the performance of the generated OFCs are theoretically analyzed. In the experiments, the line spacing of the generated OFCs is multiplied to be 10 GHz, 15 GHz, and 20 GHz with a factor of 2, 3, and 4, respectively. The center of the OFCs is tuned in a 4 nm range by adjusting the wavelength of the probe signal. Full article
(This article belongs to the Special Issue The Emerging Science and Applications of Microwave Photonics)
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21 pages, 947 KiB  
Review
Barium Chalcogenide Crystals: A Review
by Nadezhda Kostyukova, Evgenii Erushin, Andrey Boyko, Galina Shevyrdyaeva and Dmitry Badikov
Photonics 2024, 11(3), 281; https://doi.org/10.3390/photonics11030281 - 21 Mar 2024
Viewed by 675
Abstract
In recent decades, new nonlinear optical materials have been actively developed to create coherent tunable light sources in the mid-infrared (mid-IR) part of the spectrum used in a variety of scientific fields. In the present review, the main attention is focused on barium [...] Read more.
In recent decades, new nonlinear optical materials have been actively developed to create coherent tunable light sources in the mid-infrared (mid-IR) part of the spectrum used in a variety of scientific fields. In the present review, the main attention is focused on barium chalcogenide crystals, including their linear and nonlinear optical properties, laser-induced damage threshold (LIDT), and frequency down-conversion. Full article
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18 pages, 5384 KiB  
Article
Numerical MCRT Simulation and Inter-Symbol Interference Analysis of Laser Multipath Transmission in Airborne Air-to-Air Channels
by Shiqi Wu, Yanan Li, Jun Wang, Haijun Zhou, Chengmao Tang, Yijie Guan, Fei Guo and Xiaofeng Li
Photonics 2024, 11(3), 280; https://doi.org/10.3390/photonics11030280 - 21 Mar 2024
Viewed by 561
Abstract
To address the issues of signal waveform distortion and inter-symbol interference, both of which lead to performance degradation in airborne laser communication due to multipath transmission in airborne air-to-air channels, theoretical analysis and Monte Carlo ray tracing (MCRT) simulation methods were employed. Based [...] Read more.
To address the issues of signal waveform distortion and inter-symbol interference, both of which lead to performance degradation in airborne laser communication due to multipath transmission in airborne air-to-air channels, theoretical analysis and Monte Carlo ray tracing (MCRT) simulation methods were employed. Based on airborne application conditions, this research conducted numerical simulations of the Gaussian beam multipath transmission in an air-to-air channel, with a focus on analyzing the impact of meteorological conditions, communication distance, transmitter power, and initial pulse width on the pulse time spreading characteristics of received optical signals. In addition, an analysis of these parameters’ impact on the inter-symbol interference (ISI) and bit error rate (BER) was conducted. The research findings can serve as a reference for the design of anti-interference techniques in airborne laser communication links. Full article
(This article belongs to the Special Issue Free-Space Optical Communication: Physics and Applications)
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12 pages, 2378 KiB  
Article
Comparison of Optogalvanic and Laser-Induced Fluorescence Spectroscopy
by Laurentius Windholz
Photonics 2024, 11(3), 279; https://doi.org/10.3390/photonics11030279 - 21 Mar 2024
Viewed by 630
Abstract
When investigating complex atomic spectra, it may happen accidentally that two or even several transitions between different pairs of combining energy levels have nearly the same wavenumber, and the observed spectral lines are overlapping (blend situations). In such cases, investigations of hyperfine structures [...] Read more.
When investigating complex atomic spectra, it may happen accidentally that two or even several transitions between different pairs of combining energy levels have nearly the same wavenumber, and the observed spectral lines are overlapping (blend situations). In such cases, investigations of hyperfine structures can be very helpful in the identification of the involved transitions. In this paper, two complicated blend situations within the spectra of lanthanide atoms (Praseodymium and Lanthanum) are discussed as examples. The experimental methods applied are optogalvanic and laser-induced fluorescence spectroscopy, combined with emission spectra gained via Fourier transform spectroscopy. It is shown that, in such cases, a combination of optogalvanic and laser-induced fluorescence detection is necessary to find all transitions contributing to the observed spectral signatures. Full article
(This article belongs to the Special Issue Sensitive Laser Spectroscopy)
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11 pages, 2251 KiB  
Article
Excitation and Tuning of Optical Tamm States in a Hybrid Structure with a Metal Film Adjacent to a Four-Layer Polymer–Liquid Crystal Stack
by Victor Y. Reshetnyak, Igor P. Pinkevych, Timothy J. Bunning and Dean R. Evans
Photonics 2024, 11(3), 278; https://doi.org/10.3390/photonics11030278 - 21 Mar 2024
Viewed by 555
Abstract
Absorption, reflection, and transmission coefficients of the hybrid structure formed by a metal film and a holographic polymer–liquid crystal grating (HPLCG) are theoretically studied in the spectral region of the HPLCG band gap. HPLCG cells consist of four alternating layers, two layers of [...] Read more.
Absorption, reflection, and transmission coefficients of the hybrid structure formed by a metal film and a holographic polymer–liquid crystal grating (HPLCG) are theoretically studied in the spectral region of the HPLCG band gap. HPLCG cells consist of four alternating layers, two layers of polymer and two layers of the same liquid crystal (LC), but with different orientations of the LC director. The appearance of reflection, transmission, and absorption peaks in the HPLCG band gap due to the excitation of optical Tamm states (OTSs) at the metal film–HPLCG interface is investigated. The dependence of the spectral manifestation of OTSs on the parameters of the hybrid structure is also studied. A comparison is made with the corresponding results for the case when HPLCG cells of a hybrid structure consist of one polymer layer and one LC layer (two-layer HPLCG). Full article
(This article belongs to the Special Issue The State of the Art of Photonics in Europe)
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12 pages, 10197 KiB  
Article
Photoreflectance Analysis of InAsPSb/InGaAs Multi-Quantum Well LED Structures with Different Well/Barrier Numbers
by Behnam Zeinalvand Farzin, S. Bahareh Seyedein Ardebili, Tae In Kang, Jong Su Kim, Phuc Dinh Nguyen and Sang Jun Lee
Photonics 2024, 11(3), 277; https://doi.org/10.3390/photonics11030277 - 21 Mar 2024
Viewed by 581
Abstract
InAsPSb is an emerging material used as an efficient barrier in quantum well structures, and the resulting devices can be employed in the mid-infrared region of the electromagnetic spectrum. This study investigates the photoreflectance spectra of two InAsPSb/InGaAs multi-quantum well light-emitting diodes with [...] Read more.
InAsPSb is an emerging material used as an efficient barrier in quantum well structures, and the resulting devices can be employed in the mid-infrared region of the electromagnetic spectrum. This study investigates the photoreflectance spectra of two InAsPSb/InGaAs multi-quantum well light-emitting diodes with 6 and 15 quantum well periods. The photoreflectance of the samples was analyzed at various temperatures and excitation powers. By examining the Franz-Keldysh oscillations in the spectra, we explored the influence of the number of well layers on the electric field strength in the junction. The results showed that the number of quantum wells can influence the electric field at the junction, potentially impacting the overall performance of the devices. The simulation of the electric field strength aligns with the results of the photoreflectance analysis. This suggests that the field extracted from Franz-Keldysh oscillations characterizes the field inside the multi-quantum wells, offering potential reasons for the observed effects on the number of multi-quantum wells in the field. Full article
(This article belongs to the Special Issue Recent Advances and Future Perspectives in LED Technology)
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10 pages, 2111 KiB  
Article
Analysis of Plasma Dynamics in He-Ne Lasers with Different Gas Ratios
by Yuanhao Mao, Jing Hu, Hongteng Ji, Shiyu Guan, Dingbo Chen, Qiucheng Gong, Wei Liu, Xingwu Long and Zhongqi Tan
Photonics 2024, 11(3), 276; https://doi.org/10.3390/photonics11030276 - 21 Mar 2024
Viewed by 537
Abstract
He-Ne lasers play a crucial role in ultra-precision measurement and optical sensing across various fields. For many applications based on He-Ne lasers, a higher output power is required to enhance the accuracy and signal-to-noise ratios of the associated optical measurements. However, conventional methods [...] Read more.
He-Ne lasers play a crucial role in ultra-precision measurement and optical sensing across various fields. For many applications based on He-Ne lasers, a higher output power is required to enhance the accuracy and signal-to-noise ratios of the associated optical measurements. However, conventional methods to increase the output power by reducing the diameter of the He-Ne laser discharge capillary inevitably result in higher diffraction losses and constrain the lasing performance. Here, we propose an approach to enhance laser pumping efficiency and output power through optimizing the ratios of He and Ne gasses. The validity of our proposal has been confirmed by both numerical simulations of He-Ne laser plasma discharge processes and experimental demonstrations, showing that the optimal gas ratio increases with the capillary diameter and total gas pressure. Full article
(This article belongs to the Special Issue Laser Technology and Applications)
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14 pages, 2478 KiB  
Article
Classifying Raman Spectra of Colon Cells Based on Machine Learning Algorithms
by Maria Lasalvia, Crescenzio Gallo, Vito Capozzi and Giuseppe Perna
Photonics 2024, 11(3), 275; https://doi.org/10.3390/photonics11030275 - 21 Mar 2024
Viewed by 666
Abstract
Colorectal cancer is very widespread in developed countries. Its diagnosis partly depends on pathologists’ experience and their laboratories’ instrumentation, producing uncertainty in diagnosis. The use of spectroscopic techniques sensitive to the cellular biochemical environment could aid in achieving a reliable diagnosis. So, we [...] Read more.
Colorectal cancer is very widespread in developed countries. Its diagnosis partly depends on pathologists’ experience and their laboratories’ instrumentation, producing uncertainty in diagnosis. The use of spectroscopic techniques sensitive to the cellular biochemical environment could aid in achieving a reliable diagnosis. So, we used Raman micro-spectroscopy, combined with a spectral analysis by means of machine learning methods, to build classification models, which allow colon cancer to be diagnosed in cell samples, in order to support such methods as complementary tools for achieving a reliable identification of colon cancer. The Raman spectra were analyzed in the 980–1800 cm−1 range by focusing the laser beam onto the nuclei and the cytoplasm regions of single FHC and CaCo-2 cells (modelling healthy and cancerous samples, respectively) grown onto glass coverslips. The comparison of the Raman intensity of several spectral peaks and the Principal Component Analysis highlighted small biochemical differences between healthy and cancerous cells mainly due to the larger relative lipid content in the former cells with respect to the latter ones and to the larger relative amount of nucleic acid components in cancerous cells compared with healthy ones. We considered four classification algorithms (logistic regression, support vector machine, k nearest neighbors, and a neural network) to associate unknown Raman spectra with the cell type to which they belong. The built machine learning methods achieved median values of classification accuracy ranging from 95.5% to 97.1%, sensitivity values ranging from 95.5% to 100%, and specificity values ranging from 93.9% to 97.1%. The same median values of the classification parameters, which were estimated for a testing set including unknown spectra, ranged between 93.1% and 100% for accuracy and between 92.9% and 100% for sensitivity and specificity. A comparison of the four methods pointed out that k nearest neighbors and neural networks better perform the classification of nucleus and cytoplasm spectra, respectively. These findings are a further step towards the perspective of clinical translation of the Raman technique assisted by multivariate analysis as a support method to the standard cytological and immunohistochemical methods for diagnostic purposes. Full article
(This article belongs to the Special Issue Advanced Photonic Sensing and Measurement II)
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24 pages, 2074 KiB  
Review
A Comprehensive Review of UAV-Assisted FSO Relay Systems
by Nwanze J. Nzekwu, Marco A. Fernandes, Gil M. Fernandes, Paulo P. Monteiro and Fernando P. Guiomar
Photonics 2024, 11(3), 274; https://doi.org/10.3390/photonics11030274 - 20 Mar 2024
Viewed by 684
Abstract
The evolving requirements of next-generation mobile communications networks can be met by leveraging vertically deployed Unmanned Aerial Vehicle (UAV) platforms integrated with Free Space Optical communications (FSO). This integration offers a flexible and scalable architecture capable of delivering high-rate communication without requiring licenses [...] Read more.
The evolving requirements of next-generation mobile communications networks can be met by leveraging vertically deployed Unmanned Aerial Vehicle (UAV) platforms integrated with Free Space Optical communications (FSO). This integration offers a flexible and scalable architecture capable of delivering high-rate communication without requiring licenses while aligning with the multi-gigabit paradigm. In recent times, the increasing availability of commercial aerial platforms has facilitated experimental demonstrations of UAV-enabled FSO systems, which play a crucial role in proposed backhaul networks and point-to-point communications by overcoming Line-of-Sight (LOS) challenges. These systems can be rapidly deployed to meet sudden demand scenarios. This document provides a comprehensive review of relevant field demonstrations of UAV-enabled FSO relay systems, with a particular focus on commercially available, free-flying platforms that are driving advancements in this domain. It categorizes the different platforms by considering the operational altitudes of these systems and their payload actuation capacity, which determines their adaptability to variables. The analysis aims to distill the design considerations that lead to optimal performance regarding communications throughput and other relevant metrics. Moreover, it also attempts to highlight areas where design choices have fallen short, indicating gaps in current research efforts toward the widespread adoption of UAV-enabled FSO relay systems. Finally, this work endeavors to outline effective design considerations, guidelines, and recommendations to bridge these identified gaps. It serves as a valuable reference guide for researchers involved in developing UAV-enabled FSO relay systems, enabling them to make informed decisions and pave the way for the successful implementation of such systems. Full article
(This article belongs to the Special Issue New Perspectives in Free-Space Optical Communications and Networks)
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9 pages, 2694 KiB  
Article
Ultrasound-Guided Photoacoustic Imaging of Salivary Gland Hemodynamics in Rabbits
by Eftekhar Rajab Bolookat, Vui King Vincent-Chong, Laurie J. Rich, Anurag K. Singh and Mukund Seshadri
Photonics 2024, 11(3), 273; https://doi.org/10.3390/photonics11030273 - 20 Mar 2024
Viewed by 633
Abstract
Xerostomia (severe dry mouth) is a debilitating and often permanent side effect experienced by head and neck cancer patients due to radiation injury to salivary glands. In this study, we evaluated the potential of ultrasound (US)-guided photoacoustic imaging (PAI) to non-invasively assess early [...] Read more.
Xerostomia (severe dry mouth) is a debilitating and often permanent side effect experienced by head and neck cancer patients due to radiation injury to salivary glands. In this study, we evaluated the potential of ultrasound (US)-guided photoacoustic imaging (PAI) to non-invasively assess early changes in salivary gland hemodynamics following radiation therapy (RT). US-guided PAI was performed in New Zealand white rabbits to visualize and quantify the hemoglobin concentration (HbT) and oxygen saturation (%sO2) of parotid glands before and after RT. The imaging findings were validated with histology and sialometry. An early increase in parotid gland HbT and %sO2 was seen following RT. Consistent with the PAI observations, histology of salivary glands revealed dilated blood vessels, along with hemorrhaging and fibrosis. Sialometric analysis confirmed a significant reduction in stimulated saliva secretion in irradiated rabbits compared to controls. Collectively, our findings demonstrate the translational utility of US-guided PAI as a valuable tool for label-free functional imaging of salivary gland hemodynamics in vivo. Full article
(This article belongs to the Special Issue New Trends in Photoacoustic Imaging)
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9 pages, 3226 KiB  
Communication
Thermally Controlled Broadband Ge2Sb2Te5-Based Metamaterial Absorber for Imaging Applications
by Zifeng Qiu, Gui Jin and Bin Tang
Photonics 2024, 11(3), 272; https://doi.org/10.3390/photonics11030272 - 19 Mar 2024
Viewed by 611
Abstract
In this paper, we theoretically and numerically demonstrate a thermally controlled broadband absorber based on the phase change material Ge2Sb2Te5 (GST). When GST operates in the amorphous state, the proposed metamaterial acts as a broadband nearly perfect absorber. [...] Read more.
In this paper, we theoretically and numerically demonstrate a thermally controlled broadband absorber based on the phase change material Ge2Sb2Te5 (GST). When GST operates in the amorphous state, the proposed metamaterial acts as a broadband nearly perfect absorber. The absorption can reach more than 90% in the wavelength range from 0.9 to 1.41 μm. As an application of the GST-based metamaterial absorber, the near-field imaging effect is achieved by using the intensity difference of optical absorption. Moreover, the thermally controlled switchable imaging can be performed by changing the phase transition characteristics of GST, and the imaging quality and contrast can be adjusted by changing the geometrical parameters. This designed metamaterial may have potential applications in near-infrared temperature control imaging, optical encryption, and information hiding. Full article
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10 pages, 3166 KiB  
Article
All-Fiber Low-Modal-Crosstalk Demultiplexers for DSP-Free IM/DD LP-Mode MDM Transmission
by Yuyang Gao, Jian Cui, Xian Zhou, Zhangyuan Chen and Juhao Li
Photonics 2024, 11(3), 271; https://doi.org/10.3390/photonics11030271 - 19 Mar 2024
Viewed by 558
Abstract
Weakly coupled mode-division multiplexing (MDM) techniques supporting intensity modulation and direct detection (IM/DD) transmission are promising methods of enhancing the capacity of short-reach scenarios in which low-modal-crosstalk-mode demultiplexers for degenerate linear polarized (LP) modes are highly desired. In this paper, we review two [...] Read more.
Weakly coupled mode-division multiplexing (MDM) techniques supporting intensity modulation and direct detection (IM/DD) transmission are promising methods of enhancing the capacity of short-reach scenarios in which low-modal-crosstalk-mode demultiplexers for degenerate linear polarized (LP) modes are highly desired. In this paper, we review two degenerate-mode reception schemes. Firstly, a low-modal-crosstalk orthogonal combined reception method for degenerate modes is proposed based on all-fiber mode-selective couplers, in which signals in both degenerate modes are demultiplexed into the LP01 mode of single-mode fibers and then are multiplexed into the mutually orthogonal LP01 and LP11 modes of a two-mode fiber (TMF) for simultaneous detection. Secondly, a novel degenerate-mode-selective coupler consisting of an input few-mode fiber and an output TMF is proposed, which could demultiplex degenerate LP modes without any digital signal processing (DSP). Both demultiplexers are achieved based on the taper and polish process. The fabricated devices are characterized and compared. The results show that the proposed schemes can pave the way to the practical implementation of DSP-free IM/DD LP-mode MDM transmission systems. Full article
(This article belongs to the Special Issue Space Division Multiplexing Techniques)
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33 pages, 12311 KiB  
Review
Applications and Development of Multi-Core Optical Fibers
by Weiping Chen, Lei Yuan, Bo Zhang, Qianqin Yu, Zhenggang Lian, Yabin Pi, Chongxin Shan and Perry Ping Shum
Photonics 2024, 11(3), 270; https://doi.org/10.3390/photonics11030270 - 19 Mar 2024
Viewed by 787
Abstract
The rapid development of information and communication technology has driven the demand for higher data transmission rates. Multi-core optical fiber, with its ability to transmit multiple signals simultaneously, has emerged as a promising solution to meet this demand. Additionally, due to its characteristics [...] Read more.
The rapid development of information and communication technology has driven the demand for higher data transmission rates. Multi-core optical fiber, with its ability to transmit multiple signals simultaneously, has emerged as a promising solution to meet this demand. Additionally, due to its characteristics such as multi-channel transmission, high integration, spatial flexibility, and versatility, multi-core optical fibers hold vast potential in sensing applications. However, the manufacturing technology of multi-core fiber is still in its early stages, facing challenges such as the design and fabrication of high-quality cores, efficient coupling between cores, and the reduction of crosstalk. In this paper, an overview of the current status and future prospects of multi-core fiber manufacturing technology has been presented, and their limitations will be discussed. Some potential solutions to overcome these challenges will be proposed. Their potential applications in optical fiber sensing will also be summarized. Full article
(This article belongs to the Special Issue Specialty Optical Fibers: Advance and Sensing Application)
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10 pages, 10743 KiB  
Communication
Narrow Linewidth 510 nm Laser via Single-Pass Frequency-Tripling by Waveguide PPLNs
by Yanlin Chen, Jing Zhang, Xiaolang Qiu, Suo Wang, Chuanchuan Li, Haiyang Yu and Xin Wei
Photonics 2024, 11(3), 269; https://doi.org/10.3390/photonics11030269 - 18 Mar 2024
Viewed by 632
Abstract
A single-frequency narrow linewidth green laser at 510 nm is a vital component for the study of Cesium Rydberg atoms. Here, we demonstrate a 510 nm laser based on single-pass second-harmonic generation (SHG) and sum-frequency generation (SFG) via waveguide Periodically Poled Lithium Niobate [...] Read more.
A single-frequency narrow linewidth green laser at 510 nm is a vital component for the study of Cesium Rydberg atoms. Here, we demonstrate a 510 nm laser based on single-pass second-harmonic generation (SHG) and sum-frequency generation (SFG) via waveguide Periodically Poled Lithium Niobate (PPLN) seeded with a common C-band laser (1530 nm). The final linewidth measured using the delayed self-heterodyne method reaches a narrow linewidth of 4.8 kHz. And, the optical-to-optical conversion efficiency is up to 13.1% and reaches an output power up to 200 mW. Full article
(This article belongs to the Special Issue Narrow Linewidth Laser Sources and Their Applications)
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16 pages, 1234 KiB  
Article
A Best-Path Approach to the Design of a Hybrid Space–Ground Quantum Network with Dynamic Constraints
by David L. Bakker, Yannick Jong, Bob P. F. Dirks and Gustavo C. Amaral
Photonics 2024, 11(3), 268; https://doi.org/10.3390/photonics11030268 - 18 Mar 2024
Viewed by 734
Abstract
The design and operation of quantum networks are both decisive in the current push towards a global quantum internet. Although space-enabled quantum connectivity has already been identified as a beneficial candidate for long-range quantum channels for over two decades, the architecture of a [...] Read more.
The design and operation of quantum networks are both decisive in the current push towards a global quantum internet. Although space-enabled quantum connectivity has already been identified as a beneficial candidate for long-range quantum channels for over two decades, the architecture of a hybrid space–ground network is still a work in progress. Here, we propose an analysis of such a network based on a best-path approach, where either fiber- or satellite-based elementary links can be concatenated to form a repeater chain. The network consisting of quantum information processing nodes, equipped with both ground and space connections, is mapped into a graph structure, where edge weights represent the achievable secret key rates, chosen as the figure of merit for the network analysis. A weight minimization algorithm allows for identifying the best path dynamically, i.e., as the weather conditions, stray light radiance, and satellite orbital position change. From the results, we conclude that satellite links will play a significant role in the future large-scale quantum internet, in particular when node distances exceed 500 km, and both a constellation of satellites—spanning 20 or more satellites—and significant advances in filtering technology are required to achieve continuous coverage. Full article
(This article belongs to the Special Issue Optical Satellite Communications for Quantum Networking)
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11 pages, 5024 KiB  
Article
Optimization of Grating Coupler over Single-Mode Silicon-on-Insulator Waveguide to Reach < 1 dB Loss through Deep-Learning-Based Inverse Design
by Chung-Chih Lin, Audrey Na, Yi-Kuei Wu, Likarn Wang and Neil Na
Photonics 2024, 11(3), 267; https://doi.org/10.3390/photonics11030267 - 15 Mar 2024
Viewed by 932
Abstract
Grating couplers are essential components in silicon photonics that facilitate the coupling of light between waveguides and fibers. Optimization of the grating couplers to reach <1 dB loss when coupling to single-mode fibers (SMFs) has been reported in the literature, but this was [...] Read more.
Grating couplers are essential components in silicon photonics that facilitate the coupling of light between waveguides and fibers. Optimization of the grating couplers to reach <1 dB loss when coupling to single-mode fibers (SMFs) has been reported in the literature, but this was based on silicon-on-insulator (SOI) waveguides supporting multi-modes. In this paper, using a deep-learning model combined with an inverse-design process, we achieve <1 dB losses for grating couplers implemented over single-mode SOI waveguides, i.e., a maximum efficiency of 80.5% (−0.94 dB) for gratings constrained with e-beam (EB) lithography critical dimension (CD), and a maximum efficiency of 77.9% (−1.09 dB) for gratings constrained with deep ultraviolet (DUV) lithography CD. To verify these results, we apply covariance matrix adaptation evolution strategy (CMA-ES) and find that while CMA-ES yields slightly better results, i.e., 82.7% (−0.83 dB) and 78.9% (−1.03 dB) considering e-beam and DUV, respectively, the spatial structures generated by CMA-ES are nearly identical to the spatial structures generated by the deep-learning model combined with the inverse-design process. This suggests that our approach can achieve a representative low-loss structure, and may be used to improve the performance of other types of nanophotonic devices in the future. Full article
(This article belongs to the Section Optical Communication and Network)
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8 pages, 1902 KiB  
Article
Modal Phase Modulators Based on Liquid Crystals with 3D-Printed Polymer Microstructures: Increasing Size and Complexity
by Alec Xu, Camron Nourshargh, Patrick S. Salter, Steve J. Elston, Stephen M. Morris and Martin J. Booth
Photonics 2024, 11(3), 266; https://doi.org/10.3390/photonics11030266 - 15 Mar 2024
Viewed by 661
Abstract
We present extended capabilities in simple liquid crystal-based devices that are applicable to adaptive optics and other related fields requiring wavefront manipulation. The laser-written devices can provide complex phase profiles, but are extremely simple to operate, requiring only a single electrode pair tuned [...] Read more.
We present extended capabilities in simple liquid crystal-based devices that are applicable to adaptive optics and other related fields requiring wavefront manipulation. The laser-written devices can provide complex phase profiles, but are extremely simple to operate, requiring only a single electrode pair tuned between 0 and 10 V RMS. Furthermore, the devices operate in the transmissive mode for easy integration into the optical path. We present here as examples three such devices: the first device reproduces the defocus Zernike polynomial; the second device reproduces a seventh-order Zernike polynomial, tertiary coma; and the last example is of a primary spherical aberration. All devices offer wavelength-scale wavefront manipulation up to more than 2π radians peak-to-peak phase at a wavelength of 660 nm. The coma correction device is significantly more complex, reproducing a mode two orders higher than previous demonstrations, while the spherical device is nearly a full order of magnitude larger, measuring 2 mm in diameter. Full article
(This article belongs to the Special Issue Liquid Crystals in Photonics)
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13 pages, 6350 KiB  
Article
Efficient Structure Transformation Based on Sensitivity-Oriented Structure Adjustment for Inverse-Designed Devices
by Yuchen Chen, Jifang Qiu, Zhenli Dong, Lihang Wang, Lan Wu, Suping Jiao, Hongxiang Guo and Jian Wu
Photonics 2024, 11(3), 265; https://doi.org/10.3390/photonics11030265 - 14 Mar 2024
Viewed by 633
Abstract
Inverse-designed devices with thousands of degrees of freedom could achieve high performance in compact footprints, but typically have complex structure topologies that contain many irregular and tiny features and sharp corners, which tend to lead to a poor robustness to fabrication errors. In [...] Read more.
Inverse-designed devices with thousands of degrees of freedom could achieve high performance in compact footprints, but typically have complex structure topologies that contain many irregular and tiny features and sharp corners, which tend to lead to a poor robustness to fabrication errors. In order to effectively transform the structure of inverse-designed nanophotonic devices into simple structure topologies that have high robustness to fabrication errors without sacrificing device performance, in this paper, we propose a structure adjustment method that innovatively adjusts the structures of inverse-designed devices by introducing their structural sensitivity to the optical performance, extracting the device substructures with high sensitivity and eliminating those with low sensitivity, and, finally, transforming the device structures into simple structure topologies with high robustness and better performance. Two devices (90° crossing and T-junction) were designed and fabrication tolerance simulation was conducted to verify the method. The results show that the devices designed using the proposed method achieved better performance and were more robust to under/over-etched errors. Full article
(This article belongs to the Special Issue Design and Applications of Novel Nanophotonics Devices)
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25 pages, 19395 KiB  
Review
Femtosecond Laser Microfabrication of Artificial Compound Eyes
by Fan Zhang, Huacheng Xu, Qing Yang, Yu Lu, Guangqing Du and Feng Chen
Photonics 2024, 11(3), 264; https://doi.org/10.3390/photonics11030264 - 14 Mar 2024
Viewed by 840
Abstract
Over millions of years of evolution, arthropods have intricately developed and fine-tuned their highly sophisticated compound eye visual systems, serving as a valuable source of inspiration for human emulation and tracking. Femtosecond laser processing technology has attracted attention for its excellent precision, programmable [...] Read more.
Over millions of years of evolution, arthropods have intricately developed and fine-tuned their highly sophisticated compound eye visual systems, serving as a valuable source of inspiration for human emulation and tracking. Femtosecond laser processing technology has attracted attention for its excellent precision, programmable design capabilities, and advanced three-dimensional processing characteristics, especially in the production of artificial bionic compound eye structures, showing unparalleled advantages. This comprehensive review initiates with a succinct introduction to the operational principles of biological compound eyes, providing essential context for the design of biomimetic counterparts. It subsequently offers a concise overview of crucial manufacturing methods for biomimetic compound eye structures. In addition, the application of femtosecond laser technology in the production of biomimetic compound eyes is also briefly introduced. The review concludes by highlighting the current challenges and presenting a forward-looking perspective on the future of this evolving field. Full article
(This article belongs to the Special Issue New Advances in Ultrashort Pulse Fiber Lasers and Their Applications)
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19 pages, 12490 KiB  
Article
Determining Topological Charge of Bessel-Gaussian Beams Using Modified Mach-Zehnder Interferometer
by Mansi Baliyan and Naveen K. Nishchal
Photonics 2024, 11(3), 263; https://doi.org/10.3390/photonics11030263 - 14 Mar 2024
Viewed by 647
Abstract
The orbital angular momentum (OAM) associated with structured singular beams carries vital information crucial for studying various properties and applications of light. Determining OAM through the interference of light is an efficient method. The interferogram serves as a valuable tool for analyzing the [...] Read more.
The orbital angular momentum (OAM) associated with structured singular beams carries vital information crucial for studying various properties and applications of light. Determining OAM through the interference of light is an efficient method. The interferogram serves as a valuable tool for analyzing the wavefront of structured beams, especially identifying the order of singularity. In this study, we propose a modified Mach–Zehnder interferometer architecture to effectively determine the topological charge of Bessel–Gaussian (BG) beams. Several numerically generated self-referenced interferograms have been used for analysis. Moreover, this study examines the propagation property and phase distribution within BG beams after they are obstructed by an aperture in the interferometer setup. Full article
(This article belongs to the Special Issue Structured Light Beams: Science and Applications)
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13 pages, 4144 KiB  
Article
Experimental Demonstration to Enhance the Curvature Sensitivity of a Fiber Mach–Zehnder Interferometer Based on a Waist-Enlarged Technique Using Polymer
by Laura G. Martinez-Ramirez, Iván Hernández-Romano, Cipriano Guzmán-Cano, Sigifredo Marrujo-García, Arturo A. Fernandez-Jaramillo, Julian M. Estudillo-Ayala, Roberto Rojas-Laguna and Juan M. Sierra-Hernandez
Photonics 2024, 11(3), 262; https://doi.org/10.3390/photonics11030262 - 14 Mar 2024
Viewed by 660
Abstract
A fiber curvature sensor based on a Mach–Zehnder Interferometer (MZI) constructed using the waist-enlarged technique to splice a segment of non-zero dispersion-shifted fiber (NZ-DSF) between two segments of single mode fiber (SMF) is proposed and experimentally demonstrated. All fabricated sensors presented an improvement [...] Read more.
A fiber curvature sensor based on a Mach–Zehnder Interferometer (MZI) constructed using the waist-enlarged technique to splice a segment of non-zero dispersion-shifted fiber (NZ-DSF) between two segments of single mode fiber (SMF) is proposed and experimentally demonstrated. All fabricated sensors presented an improvement in their curvature sensitivity when they were coated with polydimethylsiloxane (PDMS) polymer. The sensor that exhibited the best performance was 6.5 cm long, with a curvature sensitivity of 8.27 nm/m−1 in a range of 0.69 m−1 (from 1.08 to 1.77 m−1). This sensitivity is 3.22 times higher than that of the sensor without polymer. Additionally, the sensor coated with polymer exhibited cross-sensitivity that is 2.23 times smaller than the sensor without polymer. The easy fabrication and notable performance of this device makes it alluring for structural health monitoring. Full article
(This article belongs to the Section Lasers, Light Sources and Sensors)
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9 pages, 3798 KiB  
Communication
Saturated Gain-Induced Non-Reciprocal Transmission and Broadband On-Chip Optical Isolator
by Mingyuan Xue, Haojiang Tong, Hao Dong and Meijia Wang
Photonics 2024, 11(3), 261; https://doi.org/10.3390/photonics11030261 - 14 Mar 2024
Viewed by 669
Abstract
To overcome the limitation of dynamic reciprocity, a new method for designing broadband on-chip optical isolators is proposed and demonstrated based on saturated gain, which is able to support simplex and duplex operation modes. By connecting a saturated gain waveguide to an appropriate [...] Read more.
To overcome the limitation of dynamic reciprocity, a new method for designing broadband on-chip optical isolators is proposed and demonstrated based on saturated gain, which is able to support simplex and duplex operation modes. By connecting a saturated gain waveguide to an appropriate linear loss waveguide, broadband isolation is predicted and proved theoretically through saturated gain-induced non-reciprocal transmission. The proposed isolator is numerically demonstrated with an operating band of 59 nm and an isolation ratio of −20 dB at the central wavelength of 1550 nm. It is noteworthy that when the current pump changes, the isolator still works well and keeps the high isolation ratio at a different input power. The footprint of the whole device is 465 μm × 0.35 μm which satisfies the requirement of photonic integrated circuits. The proposed isolator, with the combined advantages of compact footprint, broadband, duplex operation and high isolation, can enable on-chip unidirectional transmission and complex topological routing designation. Full article
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19 pages, 1045 KiB  
Review
Transcranial Photobiomodulation and Chronic Traumatic Brain Injury
by Naomi L. Gaggi, Nathaniel Lewis Roy, Xiaotong Song, Anna Leigh Peterson, Dan V. Iosifescu, Ramon Diaz-Arrastia, Paolo Cassano and Junghoon J. Kim
Photonics 2024, 11(3), 260; https://doi.org/10.3390/photonics11030260 - 13 Mar 2024
Viewed by 1110
Abstract
Traumatic brain injury (TBI) is a common cause of neurologic morbidity for which few effective therapies exist, especially during the chronic stage. A potential therapy for chronic TBI is transcranial photobiomodulation (tPBM). tPBM is a noninvasive neuromodulation technique that uses light to stimulate [...] Read more.
Traumatic brain injury (TBI) is a common cause of neurologic morbidity for which few effective therapies exist, especially during the chronic stage. A potential therapy for chronic TBI is transcranial photobiomodulation (tPBM). tPBM is a noninvasive neuromodulation technique that uses light to stimulate the cortex and increase blood flow and metabolism while also enhancing cognition and improving affect. There has been much work focusing on the efficacy of tPBM in acute TBI in small animals, but much less work has focused on chronic TBI. Patients with chronic TBI manifest microvascular injury, which may serve as a modifiable treatment target for tPBM. There is a need to study and improve tPBM, as the currently implemented protocols targeting microvascular injury have been relatively unsuccessful. This review includes 16 studies, which concluded that after tPBM application, there were improvements in neuropsychological outcomes in addition to increases in cerebral blood flow. However, these conclusions are confounded by differing tPBM parameters, small sample sizes, and heterogenous TBI populations. While these results are encouraging, there is a need to further understand the therapeutic potential of tPBM in chronic TBI. Full article
(This article belongs to the Section Biophotonics and Biomedical Optics)
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1 pages, 128 KiB  
Correction
Correction: Wu et al. A 1083 nm Narrow-Linewidth DFB Semiconductor Laser for Quantum Magnetometry. Photonics 2023, 10, 934
by Mengying Wu, Haiyang Yu, Wenyu Wang, Shaojie Li, Yulian Cao and Jianguo Liu
Photonics 2024, 11(3), 259; https://doi.org/10.3390/photonics11030259 - 13 Mar 2024
Viewed by 507
Abstract
Additional Affiliations [...] Full article
7 pages, 2544 KiB  
Communication
Double-Junction Cascaded GaAs-Based Broad-Area Diode Lasers with 132W Continuous Wave Output Power
by Jun Wang, Shaoyang Tan, Ye Shao, Wuling Liu, Kun Tian, Yao Xiao, Zhicheng Zhang, Yudan Gou, Lihong Zhu, Bangguo Wang and Shouhuan Zhou
Photonics 2024, 11(3), 258; https://doi.org/10.3390/photonics11030258 - 13 Mar 2024
Viewed by 957
Abstract
Improving the output power and efficiency of broad-area diode lasers is a prerequisite for the further development of fiber lasers, solid-state laser industries, and direct semiconductor laser applications. At present, the large amount of Joule heat generated by large drive currents and limited [...] Read more.
Improving the output power and efficiency of broad-area diode lasers is a prerequisite for the further development of fiber lasers, solid-state laser industries, and direct semiconductor laser applications. At present, the large amount of Joule heat generated by large drive currents and limited wall-plug efficiency presents the largest challenge for improving these lasers. In this paper, a multi-junction cascade laser with low Joule heat generation is demonstrated, showing large power and conversion efficiency. We fabricated devices with different junction numbers and compared their output power. We present double-junction lasers emitting at ~915 nm with an emitter width of 500 μm, delivering 132.5 W continuous wave output power at 70 A, which is the highest power reported so far for any single-emitter laser. The power conversion efficiencies are 66.7% and 60%, at 100 W and 132 W, respectively. Full article
(This article belongs to the Section Lasers, Light Sources and Sensors)
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36 pages, 12180 KiB  
Article
Development of Cryogenic Systems for Astronomical Research
by Yuri Balega, Oleg Bolshakov, Aleksandr Chernikov, Aleksandra Gunbina, Valerian Edelman, Mariya Efimova, Aleksandr Eliseev, Artem Krasilnikov, Igor Lapkin, Ilya Lesnov, Mariya Mansfeld, Mariya Markina, Evgenii Pevzner, Sergey Shitov, Andrey Smirnov, Mickhail Tarasov, Nickolay Tyatushkin, Anton Vdovin and Vyacheslav Vdovin
Photonics 2024, 11(3), 257; https://doi.org/10.3390/photonics11030257 - 13 Mar 2024
Viewed by 777
Abstract
The article presents a brief review of cooling systems that ensure various temperature levels (from 0.1 K to 230 K) for radio astronomical receivers of photonic and electronic (or optical and radio) devices. The features of various cooling levels and the requirements for [...] Read more.
The article presents a brief review of cooling systems that ensure various temperature levels (from 0.1 K to 230 K) for radio astronomical receivers of photonic and electronic (or optical and radio) devices. The features of various cooling levels and the requirements for the design of the cooling systems are considered in detail, as well as the approaches to designing interfaces for cooled receivers: vacuum, cryogenic, electrical, mechanical, optical, and other interfaces required for effective operation. The presented approaches to design are illustrated by a series of joint developments of the authors carried out over the past 45 years, including those produced over the past year. Full article
(This article belongs to the Special Issue Optical Systems for Astronomy)
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12 pages, 691 KiB  
Article
Second-Order Sidebands and Group Delays in Coupled Optomechanical Cavity System with a Cubic Nonlinear Harmonic Oscillator
by Qiwen Zhao, Ying He, Yanfang Yang, Huifang Zhang and Yi Xu
Photonics 2024, 11(3), 256; https://doi.org/10.3390/photonics11030256 - 12 Mar 2024
Viewed by 828
Abstract
The generation of second-order sidebands and its associated group delay is an important subject in optical storage and switch. In this work, the efficiency of second-order sideband generation in a coupled optomechanical cavity system with a cubic nonlinear harmonic oscillator is theoretically investigated. [...] Read more.
The generation of second-order sidebands and its associated group delay is an important subject in optical storage and switch. In this work, the efficiency of second-order sideband generation in a coupled optomechanical cavity system with a cubic nonlinear harmonic oscillator is theoretically investigated. It is found that the efficiency of second-order sideband generation can be effectively enhanced with the decrease in decay rate of optomechanical cavity, the increase in coupling strength between two cavities and the power of probe field. The slow light effect (i.e., positive group delay) is also observed in the proposed optomechanical cavity system, and can be controlled with the power of control field. Full article
(This article belongs to the Special Issue Levitated Optomechanics)
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8 pages, 3254 KiB  
Article
Fano Resonance Thermo-Optic Modulator Based on Double T-Bus Waveguides-Coupled Micro-Ring Resonator
by Hongpeng Li, Lidan Lu, Guang Chen, Shuai Wang, Jianzhen Ou and Lianqing Zhu
Photonics 2024, 11(3), 255; https://doi.org/10.3390/photonics11030255 - 12 Mar 2024
Viewed by 822
Abstract
For the silicon optical computing chip, the optical convolution unit based on the micro-ring modulator has been demonstrated to have high integration and large computing density. To further reduce power consumption, a novel, simple Fano resonant thermo-optic modulator is presented with numerical simulation [...] Read more.
For the silicon optical computing chip, the optical convolution unit based on the micro-ring modulator has been demonstrated to have high integration and large computing density. To further reduce power consumption, a novel, simple Fano resonant thermo-optic modulator is presented with numerical simulation and experimental demonstration. This designed Fano resonator comprises double T-shaped waveguides and a micro-ring with a radius of 10 μm. Compared with the free use of bus waveguides, our double T-shaped waveguides generate a phase shift, along with a Fano-like line shape. The experimental results show that the resonant wavelength shift of the designed modulator is 2.4 nm with a driven power of 20 mW. In addition, the maximum spectral resolution and the extinction ratio are 70.30 dB/nm and 12.69 dB, respectively. For our thermo-optic modulator, the optical intensity power consumption sensitivity of 7.60 dB/mW is three times as large as that of the micro-ring modulator. This work has broad potential to provide a low-power-consumption essential component for large-scale on-chip modulation for optical computing with compatible metal oxygen semiconductor processes. Full article
(This article belongs to the Special Issue Integrated Waveguide-Based Photonic Devices)
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