Photonics

12 pages, 1285 KiB

Open AccessEditor’s ChoiceArticle

Performance Analysis of Space-to-Ground Downlink for Polarization Shift Keying Optical Communications with a Gaussian-Schell Model Beam

by Jiajie Wu, Yuwei Zhang, Qingyan Li, Siyuan Yu and Jianjie Yu

Photonics 2025, 12(7), 643; https://doi.org/10.3390/photonics12070643 - 24 Jun 2025

Viewed by 162

Abstract

Free-space optical communication has emerged as a pivotal technology for space-to-ground downlinks; however, signal degradation caused by atmospheric turbulence continues to pose a significant challenge. In this study, a model for the polarization transmission characteristics of a Gaussian-Schell model (GSM) beam in downlink [...] Read more.

Free-space optical communication has emerged as a pivotal technology for space-to-ground downlinks; however, signal degradation caused by atmospheric turbulence continues to pose a significant challenge. In this study, a model for the polarization transmission characteristics of a Gaussian-Schell model (GSM) beam in downlink was established, and conditions sufficient for maintaining the polarization transmission characteristics were derived. The impact of the source spatial coherence on the performance of optical communication systems using circular polarization shift keying (CPolSK) modulation was investigated. Additionally, models for the probability density distribution and scintillation index of the optical intensity under atmospheric turbulence were developed along with a bit error rate model for the optical communication system. The effects of the laser spatial coherence on these models were also analyzed. The results indicate that the optimal performance in the turbulent downlink is achieved with fully coherent light, where the GSM-beam-based CPolSK-modulated system demonstrates a reduction of 1.51 dB in the required power compared to that of an on–off keying system. The implications of this study suggest that optimizing spatial coherence could significantly enhance the reliability of space-to-ground communication systems under atmospheric disturbances. Full article

(This article belongs to the Special Issue Advanced Methods in Exploring Light–Matter Interactions and Nonlinear Effects Optics Applications)

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23 pages, 890 KiB

Open AccessEditor’s ChoiceReview

Space–Time Duality in Optics: Its Origin and Applications

by Govind P. Agrawal

Photonics 2025, 12(6), 611; https://doi.org/10.3390/photonics12060611 - 13 Jun 2025

Viewed by 256

Abstract

The concept of space–time duality in optics was originally based on the mathematical connection between the diffraction of beams in space and the dispersion of pulses in time. This concept has been extended in recent years from the temporal analog of reflection for [...] Read more.

The concept of space–time duality in optics was originally based on the mathematical connection between the diffraction of beams in space and the dispersion of pulses in time. This concept has been extended in recent years from the temporal analog of reflection for optical pulses to photonic time crystals in a medium where refractive index varies with time in a periodic fashion. In this review, I discuss how the concept of space–time duality and the use of nonlinear optics has led to many advances in recent years. Starting from the historical origin of space–time duality, time lenses and their applications are reviewed first. Later sections cover phenomena such as soliton-induced temporal reflection, time-domain waveguiding, and the formation of spatiotemporal Bragg gratings. Full article

(This article belongs to the Special Issue Recent Advances in Nonlinear Optics: From Fundamentals to Applications)

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13 pages, 2693 KiB

Open AccessEditor’s ChoiceCommunication

Prediction of Aluminum Alloy Surface Roughness Through Nanosecond Pulse Laser Assisted by Continuous Laser Paint Removal

by Jingyi Li, Rongfan Liang, Han Li, Junjie Liu and Jingdong Sun

Photonics 2025, 12(6), 575; https://doi.org/10.3390/photonics12060575 - 6 Jun 2025

Viewed by 304

Abstract

Reducing surface roughness can enhance the mechanical properties of processed materials. The variation law of the aluminum alloy surface roughness induced by continuous-nanosecond combined laser (CL) with different continuous laser power densities and laser delay is investigated experimentally. A back propagation neural network [...] Read more.

Reducing surface roughness can enhance the mechanical properties of processed materials. The variation law of the aluminum alloy surface roughness induced by continuous-nanosecond combined laser (CL) with different continuous laser power densities and laser delay is investigated experimentally. A back propagation neural network (BPNN) coupled with a sparrow search algorithm (SSA) is employed to predict surface roughness. The nanosecond laser energy density, continuous laser power density and laser delay are input parameters, while the surface roughness is output parameter. The lowest surface roughness is achieved with completely paint film removed by the CL while the nanosecond laser energy density is 1.99 J/cm², the continuous laser power density is 2118 W/cm² and the laser delay is 1 ms. Compared to the original target and the target irradiated by nanosecond pulse laser (ns laser), the reductions in the surface roughness are 20.62% and 12.00%, respectively. The SSA-BPNN model demonstrates high prediction accuracy, with a correlation coefficient (R²) of 0.98628, root mean square error (RMSE) of 0.024, mean absolute error (MAE) of 0.020 and mean absolute percentage error (MAPE) of 1.30% on the test set. These results indicate that the SSA-BPNN demonstrates higher-precision surface roughness prediction with limited experimental data than BPNN. Furthermore, the findings confirm that the CL can effectively reduce surface roughness. Full article

(This article belongs to the Special Issue Advanced Methods in Exploring Light–Matter Interactions and Nonlinear Effects Optics Applications)

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13 pages, 1630 KiB

Open AccessEditor’s ChoiceArticle

Raman Hyperspectroscopy and Chemometric Analysis of Blood Serum for Diagnosing Celiac Disease in Adults

by Entesar Al-Hetlani, Lamyaa M. Almehmadi and Igor K. Lednev

Photonics 2025, 12(6), 553; https://doi.org/10.3390/photonics12060553 - 30 May 2025

Viewed by 345

Abstract

Celiac disease (CD) is a chronic autoimmune disorder triggered by an abnormal immune response to gluten, a protein found in wheat, barley, and rye. Current diagnostic methods, including serological assessments and biopsies, can be challenging due to the disease’s heterogeneous nature, creating a [...] Read more.

Celiac disease (CD) is a chronic autoimmune disorder triggered by an abnormal immune response to gluten, a protein found in wheat, barley, and rye. Current diagnostic methods, including serological assessments and biopsies, can be challenging due to the disease’s heterogeneous nature, creating a need for a reliable, noninvasive diagnostic approach. Here, in this study, we aimed to extend the Raman peak area ratios approach to the adult population. However, our findings indicate no significant differences in Raman peak area ratios between healthy and diseased adults based on blood serum samples. Nevertheless, genetic algorithm combined with partial least squares discriminant analysis (GA-PLS-DA) allowed differentiation with 92% sensitivity and 96% specificity at the spectral level in external validation. Receiver operating characteristic (ROC) analysis showed 100% classification at the donor level in external validation. These results demonstrate further that Raman spectroscopy, combined with chemometrics, is a promising, noninvasive tool for CD diagnosis. Full article

(This article belongs to the Special Issue Biomedical Photonics)

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13 pages, 7359 KiB

Open AccessEditor’s ChoiceArticle

Tabletop 3D Display with Large Radial Viewing Angle Based on Panoramic Annular Lens Array

by Min-Yang He, Cheng-Bo Zhao, Xue-Rui Wen, Yi-Jian Liu, Qiong-Hua Wang and Yan Xing

Photonics 2025, 12(5), 515; https://doi.org/10.3390/photonics12050515 - 21 May 2025

Viewed by 295

Abstract

Tabletop 3D display is an emerging display form that enables multiple users to share viewing around a central tabletop, making it promising for the application of collaborative work. However, achieving an ideal ring-shaped viewing zone with a large radial viewing angle remains a [...] Read more.

Tabletop 3D display is an emerging display form that enables multiple users to share viewing around a central tabletop, making it promising for the application of collaborative work. However, achieving an ideal ring-shaped viewing zone with a large radial viewing angle remains a challenge for most current tabletop 3D displays. This paper presents a tabletop 3D display based on a panoramic annular lens array to realize a large radial viewing angle. Each panoramic annular lens in the array is designed with a block-structured panoramic front unit and a relay lens system, enabling the formation of a ring-shaped viewing zone and increasing the radial angle of the outgoing light. Additionally, the diffusion characteristics of the optical diffusing screen component are analyzed under large angles of incidence after light passes through the panoramic annular lens array. Then, a method for generating the corresponding elemental image array is presented. The results of the simulation experiments demonstrate that the viewing range is improved to −78.4–−42.2° and 42.6–78.9°, resulting in a total radial viewing angle of up to 72.5°, and the proposed 3D display can present a 360° viewable 3D image with correct perspective and parallax. Full article

(This article belongs to the Special Issue Research on Optical Materials and Components for 3D Displays)

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11 pages, 2561 KiB

Open AccessEditor’s ChoiceArticle

Generation of Longitudinal Bessel Beam Based on Complex Amplitude Metasurface

by Lei Zhang, Qiang Jiang, Xuedian Zhang and Songlin Zhuang

Photonics 2025, 12(5), 478; https://doi.org/10.3390/photonics12050478 - 13 May 2025

Viewed by 404

Abstract

Bessel beams occupy an important position in optical research due to their characteristics of long focal depth, self-healing ability, and diffraction-free propagation. Traditional methods for generating Bessel beams suffer from complexity, a large size, low uniformity, and limited NA. Metasurfaces are considered to [...] Read more.

Bessel beams occupy an important position in optical research due to their characteristics of long focal depth, self-healing ability, and diffraction-free propagation. Traditional methods for generating Bessel beams suffer from complexity, a large size, low uniformity, and limited NA. Metasurfaces are considered to be a new technology for the miniaturization of optical devices due to their ability to regulate optical fields at subwavelength scales flexibly. Here, we generated Bessel beams by a complex-amplitude (CA) metasurface. The polarization conversion efficiency was controlled by the geometric size, while the phase value from 0 to 2π was manipulated based on the Pancharatnam–Berry (PB) phase. This approach enabled precise control over the axial intensity distribution of the optical field, which facilitated the generation of sub-millimeter-scale Bessel beams. Axial light field control based on CA metasurfaces has great potential for applications in a variety of fields, such as particle manipulation, large-depth-of-field imaging, and laser processing. Full article

(This article belongs to the Special Issue Advanced Photonics Metamaterials and Metasurfaces: Science and Applications, 2nd Edition)

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21 pages, 6467 KiB

Open AccessEditor’s ChoiceArticle

Research on High-Precision Time–Frequency Phase-Synchronization Transmission Technology for Free-Space Optical Communication Systems on Mobile Platforms

by Fengrui Liu, Ning Sun, Jia Wei, Yingkai Zhao, Xingfa Wang, Weijie Zhang and Jianguo Liu

Photonics 2025, 12(5), 467; https://doi.org/10.3390/photonics12050467 - 10 May 2025

Viewed by 380

Abstract

This paper proposes a free-space time–frequency phase (TFP)-synchronization transmission architecture based on optoelectronic hybrid technology, addressing the high-precision TFP synchronization and high-speed communication requirements between mobile platforms in distributed collaborative positioning and other applications. The proposed scheme utilizes symmetric free-space optical (FSO) links [...] Read more.

This paper proposes a free-space time–frequency phase (TFP)-synchronization transmission architecture based on optoelectronic hybrid technology, addressing the high-precision TFP synchronization and high-speed communication requirements between mobile platforms in distributed collaborative positioning and other applications. The proposed scheme utilizes symmetric free-space optical (FSO) links to effectively suppress drift errors, integrating the high bandwidth of optical links and the high stability of microwave links, enabling one-to-many networking synchronization between mobile platforms. The system adopts optical wireless transmission technology based on pseudo-code regenerative ranging, integrating 1.5 Gbps high-speed data transmission with high-precision TFP-synchronization functionality. An experimental system consisting of a main station and two auxiliary stations was established in an outdoor mobile platform scenario. Experimental results show that while achieving high-speed communication, the frequency synchronization precision is 0.0131 ppb, frequency stability is in the order of 10⁻¹⁰@1 s, and phase synchronization precision is approximately 3.56°. The system achieves time synchronization precision at the picosecond level. The proposed technology is highly suitable for high-precision synchronization communication in scenarios lacking fiber-optic infrastructure, effectively fulfilling rigorous requirements in mobile platform applications such as distributed collaborative positioning. Full article

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36 pages, 5120 KiB

Open AccessEditor’s ChoiceReview

Enhancing Optoelectronic Performance Through Rare-Earth-Doped ZnO: Insights and Applications

by Shagun Sood, Pawan Kumar, Isha Raina, Mrinmoy Misra, Sandeep Kaushal, Jyoti Gaur, Sanjeev Kumar and Gurjinder Singh

Photonics 2025, 12(5), 454; https://doi.org/10.3390/photonics12050454 - 8 May 2025

Viewed by 1231

Abstract

Rare-earth (RE) doping has been found to be a potent method to improve the structural, optical, electronic, and magnetic properties of ZnO, positioning it as a versatile material for future optoelectronic devices. This review herein thoroughly discusses the latest developments in RE-doped ZnO [...] Read more.

Rare-earth (RE) doping has been found to be a potent method to improve the structural, optical, electronic, and magnetic properties of ZnO, positioning it as a versatile material for future optoelectronic devices. This review herein thoroughly discusses the latest developments in RE-doped ZnO based on the role of the dopant type, concentration, synthesis method, and consequences of property modifications. The 4f electronic states of rare-earth elements create strong visible emissions, control charge carriers, and design defects. These structural changes lead to tunable bandgap energies and increased light absorption. Also, RE doping considerably enhances ZnO’s performance in electronic devices, like UV photodetectors, LEDs, TCOs, and gas sensors. Though, challenges like solubility constraints and lattice distortions at higher doping concentrations are still key challenges. Co-doping methodologies and new synthesis techniques to further optimize the incorporation of RE into ZnO matrices are also reviewed in this article. By showing a systematic comparison of different RE-doped ZnO systems, this paper sheds light on their future optoelectronic applications. The results are useful for the design of advanced ZnO-based materials with customized functionalities, which will lead to enhanced device efficiency and new photonic applications. Full article

(This article belongs to the Special Issue Advances in Micro and Nano-Photonics: Emerging Materials and Applications)

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38 pages, 4091 KiB

Open AccessEditor’s ChoiceArticle

Mitigating the Impact of Satellite Vibrations on the Acquisition of Satellite Laser Links Through Optimized Scan Path and Parameters

by Muhammad Khalid, Wu Ji, Deng Li and Li Kun

Photonics 2025, 12(5), 444; https://doi.org/10.3390/photonics12050444 - 4 May 2025

Viewed by 565

Abstract

In the past two decades, there has been a tremendous increase in demand for services requiring a high bandwidth, a low latency, and high data rates, such as broadband internet services, video streaming, cloud computing, IoT devices, and mobile data services (5G and [...] Read more.

In the past two decades, there has been a tremendous increase in demand for services requiring a high bandwidth, a low latency, and high data rates, such as broadband internet services, video streaming, cloud computing, IoT devices, and mobile data services (5G and beyond). Optical wireless communication (OWC) technology, which is also envisioned for next-generation satellite networks using laser links, offers a promising solution to meet these demands. Establishing a line-of-sight (LOS) link and initiating communication in laser links is a challenging task. This process is managed by the acquisition, pointing, and tracking (APT) system, which must deal with the narrow beam divergence and the presence of satellite platform vibrations. These factors increase acquisition time and decrease acquisition probability. This study presents a framework for evaluating the acquisition time of four different scanning methods: spiral, raster, square spiral, and hexagonal, using a probabilistic approach. A satellite platform vibration model is used, and an algorithm for estimating its power spectral density is applied. Maximum likelihood estimation is employed to estimate key parameters from satellite vibrations to optimize scan parameters, such as the overlap factor and beam divergence. The simulation results show that selecting the scan path, overlap factor, and beam divergence based on an accurate estimation of satellite vibrations can prevent multiple scans of the uncertainty region, improve target satellite detection, and increase acquisition probability, given that the satellite vibration amplitudes are within the constraints imposed by the scan parameters. This study contributes to improving the acquisition process, which can, in turn, enhance the pointing and tracking phases of the APT system in laser links. Full article

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11 pages, 3727 KiB

Open AccessEditor’s ChoiceArticle

Dynamically Tunable Singular States Through Air-Slit Control in Asymmetric Resonant Metamaterials

by Yeong Hwan Ko and Robert Magnusson

Photonics 2025, 12(5), 403; https://doi.org/10.3390/photonics12050403 - 22 Apr 2025

Viewed by 282

Abstract

This study presents a novel method for dynamically tuning singular states in one-dimensional (1D) photonic lattices (PLs) using air-slit-based structural modifications. Singular states, arising from symmetry-breaking-induced resonance radiation, generate diverse spectral features through interactions between resonance modes and background radiation. By strategically incorporating [...] Read more.

This study presents a novel method for dynamically tuning singular states in one-dimensional (1D) photonic lattices (PLs) using air-slit-based structural modifications. Singular states, arising from symmetry-breaking-induced resonance radiation, generate diverse spectral features through interactions between resonance modes and background radiation. By strategically incorporating air slits to break symmetry in 1D PLs, we demonstrated effective control of resonance positions, enabling dual functionalities including narrowband band pass and notch filtering. These singular states originate from asymmetric guided-mode resonances (aGMRs), which can be interpreted by analytical modeling of the equivalent slab waveguide. Moreover, the introduction of multiple air slits significantly enhances spectral tunability by inducing multiple folding behaviors in the resonance bands. This approach allows for effective manipulation of optical properties through simple adjustments of air-slit displacements. This work provides great potential for designing multifunctional photonic devices with advanced metamaterial technologies. Full article

(This article belongs to the Special Issue Optical Metasurfaces: Applications and Trends)

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18 pages, 6501 KiB

Open AccessEditor’s ChoiceArticle

Airborne Constant Ground Resolution Imaging Optical System Design

by Zhiqiang Yang, Shizhen Gao, Qianxi Chen, Bohan Wu, Qiang Xu, Lei Gong and Lihong Yang

Photonics 2025, 12(4), 390; https://doi.org/10.3390/photonics12040390 - 16 Apr 2025

Viewed by 333

Abstract

When an unmanned aerial vehicle (UAV) tilts to capture an image of a ground target, variations in object distance may lead to uneven resolution distribution, with the focal length ranging from zero to the full field of view. The field-of-view focal length (FFL), [...] Read more.

When an unmanned aerial vehicle (UAV) tilts to capture an image of a ground target, variations in object distance may lead to uneven resolution distribution, with the focal length ranging from zero to the full field of view. The field-of-view focal length (FFL), which is a function of the field of view, characterizes the optical properties of the system for each viewing angle. The field-of-view focal length (FFL) quantifies the incremental change in image height resulting from marginal rays exiting the optical system, with infinitesimal angular variations at the field boundary. The optical aberration manifests as an effective focal length variation that exhibits field-dependent characteristics. Through systematic calculation and optimization of the field-of-view focal lengths (FFLs) for ground resolution (GR) control, a mid-wave infrared (MWIR) optical system has been successfully designed, featuring a 10° × 8° field of view (FOV) with an F-number of 3. The optical system implements field-adapted focal length adjustment across distinct viewing angles to ensure consistent ground resolution preservation throughout the full field of view. The designed optical system achieves near-diffraction-limited modulation transfer function (MTF) performance across the full field of view, with all dispersion spots consistently confined within the Airy disk at every viewing angle. The optical system demonstrates superior imaging performance with all dispersion spots confined within the Airy disk radius, fully complying with stringent image quality specifications. Featuring a compact structural configuration, the system exhibits optimal suitability for airborne ground-target reconnaissance applications. Full article

(This article belongs to the Special Issue Advances in Optical System Design)

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12 pages, 2383 KiB

Open AccessEditor’s ChoiceArticle

Novel Focusing Performances of High-Numerical-Aperture Micro-Fresnel Zone Plates with Selective Occlusion

by Qiang Liu, Yunpeng Wu, Yuanhao Deng, Junli Wang, Wenshuai Liu and Xiaomin Yao

Photonics 2025, 12(4), 372; https://doi.org/10.3390/photonics12040372 - 13 Apr 2025

Viewed by 398

Abstract

In this study, novel focusing performances of high-numerical-aperture (NA) micro-Fresnel zone plates (FZPs) with selective occlusion are identified and investigated through numerical calculations based on vectorial angular spectrum (VAS) theory, and further rigorously validated using the finite-difference time-domain (FDTD) method. The central occlusion [...] Read more.

In this study, novel focusing performances of high-numerical-aperture (NA) micro-Fresnel zone plates (FZPs) with selective occlusion are identified and investigated through numerical calculations based on vectorial angular spectrum (VAS) theory, and further rigorously validated using the finite-difference time-domain (FDTD) method. The central occlusion of a standard micro-FZP can significantly extend the depth of focus while keeping the lateral size of the focusing spot essentially unchanged. When a standard micro-FZP only retains two separated transparent rings and all other rings are obstructed, it will result in multi-focus phenomena; at the same time, the number of focal points is equal to the difference in number between the two separated transparent rings. Furthermore, a focusing light needle can be generated by combining the central occlusion and wavelength shift of a standard micro-FZP. This study not only provides new ideas for the design and optimization of micro-FZPs but also provides reference for the expansion of practical applications of FZPs. Full article

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8 pages, 1952 KiB

Open AccessEditor’s ChoiceCommunication

Structured Optical Toroidal Vortices with Rotational Symmetry

by Jinzhan Zhong and Qiwen Zhan

Photonics 2025, 12(3), 288; https://doi.org/10.3390/photonics12030288 - 20 Mar 2025

Viewed by 406

Abstract

Toroidal vortices, as intriguing topological structures, play a fundamental role across a wide range of physical fields. In this study, we theoretically propose a family of structured optical toroidal vortices as generalized forms of toroidal vortices in paraxial continuous wave beams. These structured [...] Read more.

Toroidal vortices, as intriguing topological structures, play a fundamental role across a wide range of physical fields. In this study, we theoretically propose a family of structured optical toroidal vortices as generalized forms of toroidal vortices in paraxial continuous wave beams. These structured optical toroidal vortices exhibit unique rotational symmetry while preserving the topological properties of standard toroidal vortices. The three-dimensional topological structures demonstrate l-fold rotational symmetry, which is closely related to the topological charges. Structured toroidal vortices introduce additional topological invariants within the toroidal light field. These topological light fields hold significant potential applications in the synthesis of complex topological structure and optical information encoding. Full article

(This article belongs to the Special Issue Fundamentals and Applications of Vortex Beams)

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11 pages, 2121 KiB

Open AccessEditor’s ChoiceArticle

Miniaturized and Wide-Range Microwave-Permittivity Sensor Based on Electromagnetic-Induced Transparency

by Siyuan Liu and Feng Xue

Photonics 2025, 12(3), 283; https://doi.org/10.3390/photonics12030283 - 19 Mar 2025

Viewed by 261

Abstract

In this paper, we had designed a microwave band permittivity sensor based on analog electromagnetic-induced transparency (A-EIT). By comparing the S-parameter changes of the tested sample before and after measurement, we can calculate the permittivity of the tested sample then distinguish material types [...] Read more.

In this paper, we had designed a microwave band permittivity sensor based on analog electromagnetic-induced transparency (A-EIT). By comparing the S-parameter changes of the tested sample before and after measurement, we can calculate the permittivity of the tested sample then distinguish material types with similar appearances. The transmission line had used impedance transformation structure, and the open circuit branch is vertically connected to the transmission line. The open circuit branch will have a coupling effect with the spiral cross structure and can also simulate the A-EIT phenomenon. The above design has potential applications in the miniaturization of sensors. Full article

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12 pages, 2631 KiB

Open AccessEditor’s ChoiceArticle

Exciton-Resonance-Enhanced Two-Photon Absorption in Three-Dimensional Hybrid Organic–Inorganic Perovskites

by Xing Ran, Xin Xiang, Feng Zhou and Shunbin Lu

Photonics 2025, 12(3), 261; https://doi.org/10.3390/photonics12030261 - 13 Mar 2025

Viewed by 677

Abstract

Three-dimensional (3D) hybrid organic–inorganic perovskites (HOIPs) have attracted tremendous interest due to strong excitonic effects and large optical nonlinearities. Taking the advantages, 3D HOIPs show great potential for applications in excitonic and nonlinear devices. However, understanding the relevant mechanisms of exciton-associated nonlinear optical [...] Read more.

Three-dimensional (3D) hybrid organic–inorganic perovskites (HOIPs) have attracted tremendous interest due to strong excitonic effects and large optical nonlinearities. Taking the advantages, 3D HOIPs show great potential for applications in excitonic and nonlinear devices. However, understanding the relevant mechanisms of exciton-associated nonlinear optical phenomena in 3D perovskites is still challenging. Here, we apply the quantum perturbation theory to calculate the exciton-associated degenerate 2PA spectra of 3D HOIPs. The calculated 2PA spectra of twelve 3D HOIPs are predicted to exhibit resonance peaks at both the sub-band and band edges. The exciton-resonance-associated 2PA coefficients are at least one order of magnitude larger than those of band-to-band transitions and are comparable to those of low-dimensional perovskites. To validate our model, we carried out measurements of the static light-intensity-dependent transmission on MAPbBr₃ single crystals. Enhancements of 2PA coefficients are predicted theoretically and observed experimentally with a resonant peak at 1100 nm, indicating intrinsic two-photon transitions to excitonic states in MAPbBr₃ single crystals. Full article

(This article belongs to the Special Issue Novel Two-Dimensional Materials Based on Nonlinear Photonics)

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42 pages, 5831 KiB

Open AccessEditor’s ChoiceReview

Harnessing Ultra-Intense Long-Wave Infrared Lasers: New Frontiers in Fundamental and Applied Research

by Igor V. Pogorelsky and Mikhail N. Polyanskiy

Photonics 2025, 12(3), 221; https://doi.org/10.3390/photonics12030221 - 28 Feb 2025

Viewed by 874

Abstract

This review explores two main topics: the state-of-the-art and emerging capabilities of high-peak-power, ultrafast (picosecond and femtosecond) long-wave infrared (LWIR) laser technology based on CO₂ gas laser amplifiers, and the current and advanced scientific applications of this laser class. The discussion is [...] Read more.

This review explores two main topics: the state-of-the-art and emerging capabilities of high-peak-power, ultrafast (picosecond and femtosecond) long-wave infrared (LWIR) laser technology based on CO₂ gas laser amplifiers, and the current and advanced scientific applications of this laser class. The discussion is grounded in expertise gained at the Accelerator Test Facility (ATF) of Brookhaven National Laboratory (BNL), a leading center for ultrafast, high-power CO₂ laser development and a National User Facility with a strong track record in high-intensity physics experiments. We begin by reviewing the status of 9–10 μm CO₂ laser technology and its applications, before exploring potential breakthroughs, including the realization of 100 terawatt femtosecond pulses. These advancements will drive ongoing research in electron and ion acceleration in plasma, along with applications in secondary radiation sources and atmospheric energy transport. Throughout the review, we highlight how wavelength scaling of physical effects enhances the capabilities of ultra-intense lasers in the LWIR spectrum, expanding the frontiers of both fundamental and applied science. Full article

(This article belongs to the Special Issue High-Power Ultrafast Lasers: Development and Applications)

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15 pages, 4797 KiB

Open AccessEditor’s ChoiceArticle

Analytical Investigation of DNA Hybridization Sensing Using Integrated Photonic Micro-Ring Resonators

by Shalini Vardhan and Ritu Raj Singh

Photonics 2025, 12(3), 216; https://doi.org/10.3390/photonics12030216 - 28 Feb 2025

Cited by 1 | Viewed by 598

Abstract

The study of infected biological cells is crucial in modern biomedical research. This work presents a passive sensing approach using optical resonators, designed to detect malignant diseases within a refractive index (RI) range of 1 to 1.5. A comprehensive theoretical analysis is conducted, [...] Read more.

The study of infected biological cells is crucial in modern biomedical research. This work presents a passive sensing approach using optical resonators, designed to detect malignant diseases within a refractive index (RI) range of 1 to 1.5. A comprehensive theoretical analysis is conducted, yielding an expected limit of detection (LoD) ranging from 0.03 nm/RIU to 0.92 nm/RIU. Furthermore, an in-depth investigation of DNA hybridization is performed, incorporating a 1.8 nm linker layer at the analyte boundary. The refractive indices of single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA) are 1.456 and 1.529, respectively. The novelty of this work lies in the renaturation process of ssDNA to dsDNA, demonstrated through a labeled sensing modality with a measurable shift in the resonance wavelength spectrum. The proposed surface-functionalized resonators, designed using Silicon-on-Insulator (SOI) technology, include (a) a Rectangular Waveguide-based Ring Resonator (RWRiR), (b) a Rectangular Waveguide-based Racetrack Resonator (RWRaR), (c) a Slot Waveguide-based Ring Resonator (SWRiR), and (d) a Slot Waveguide-based Racetrack Resonator (SWRaR). Among these, the SWRiR exhibits the best performance for DNA sensing, achieving a quality factor (Q-factor) of 2216.714, a sensitivity (S) of 54.282 nm/RIU, and a normalized sensitivity (S’) of 0.0349. Full article

(This article belongs to the Special Issue Optical Fiber Sensors: Shedding More Light with Machine Learning)

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14 pages, 4295 KiB

Open AccessEditor’s ChoiceArticle

ZEMAX Simulations and Experimental Validation of Laser Interferometers

by Muddasir Naeem and Tayyab Imran

Photonics 2025, 12(3), 206; https://doi.org/10.3390/photonics12030206 - 27 Feb 2025

Cited by 1 | Viewed by 1423

Abstract

This study presents the design, simulation, and experimental validation of six fundamental laser interferometer types: Sagnac, Mach–Zehnder, Michelson, Twyman–Green, Fizeau, and Fabry–Pérot. Using ZEMAX OpticStudio in non-sequential mode with the physical optics propagation (POP) algorithm, the simulations provide detailed insights into the optical [...] Read more.

This study presents the design, simulation, and experimental validation of six fundamental laser interferometer types: Sagnac, Mach–Zehnder, Michelson, Twyman–Green, Fizeau, and Fabry–Pérot. Using ZEMAX OpticStudio in non-sequential mode with the physical optics propagation (POP) algorithm, the simulations provide detailed insights into the optical performance of these interferometers. A direct comparison is made between the simulated and experimental fringe patterns, coherent irradiance distributions, and phase plots, demonstrating strong agreement and validating the accuracy of computational modeling for interferometric analysis. The Mach–Zehnder and Michelson configurations exhibit high adaptability and measurement precision, while the Fabry–Pérot interferometer achieves superior spectral resolution. Twyman–Green interferometry proves particularly effective in mapping surface irregularities for optical testing. The results confirm the reliability of ZEMAX OpticStudio for high-precision optical system design and analysis. The novelty of this work lies in the comparative study between ZEMAX simulations and experimental interferometric results, particularly fringe patterns and phase distributions. This approach provides a clearer understanding of interferometer performance and enhances the accuracy of optical metrology, offering valuable insights for both theoretical modeling and practical applications. Full article

(This article belongs to the Special Issue Advances in Interferometric Optics and Applications)

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12 pages, 8798 KiB

Open AccessEditor’s ChoiceArticle

Influence of Thickness and Mass Ratio on Terahertz Spectra and Optical Parameters of Yttria-Stabilized Zirconia

by Miao Yu, Chenxi Liu, Yinxiao Miao, Lin Liu, Dawei Wei, Fangrong Hu, Haiyuan Yu, Hao Mei, Yong Shang, Yang Feng, Yanling Pei and Shengkai Gong

Photonics 2025, 12(3), 201; https://doi.org/10.3390/photonics12030201 - 26 Feb 2025

Viewed by 518

Abstract

Yttria-Stabilized Zirconia (YSZ) is an important material in thermal barrier coatings (TBCs), which are widely applied in aviation engines and ground gas turbines. Therefore, the quality inspection of the YSZ layer is of great significance for the safety of engines and gas turbines. [...] Read more.

Yttria-Stabilized Zirconia (YSZ) is an important material in thermal barrier coatings (TBCs), which are widely applied in aviation engines and ground gas turbines. Therefore, the quality inspection of the YSZ layer is of great significance for the safety of engines and gas turbines. In this work, the YSZ powder is mixed with Polytetrafluoroethylene (also known as teflon) in different mass ratios and pressed into tablets with different thicknesses. A terahertz time-domain spectroscopy system is used to obtain their time-domain spectra, and their frequency spectra are then obtained by fast Fourier transform. Based on theory formulas, we obtained the frequency-dependent curves of the absorption coefficient, refractive index, and absorbance of the YSZ tablets. The results show that the YSZ tablets have characteristic absorption peaks in the terahertz band, and these peaks are affected by the mass ratio of YSZ to teflon and the thickness of the tablets. Finally, we conducted a terahertz Raman spectroscopy test of the YSZ tablets for the first time. The results show that in the range from 0 to 1600 cm⁻¹, there are about ten strong Raman peaks. More importantly, these peaks are approximately independent of the mass ratio and the thickness of tablets. This study is of great significance for the nondestructive testing of TBC quality using terahertz spectroscopy technology. Full article

(This article belongs to the Special Issue Advancements in Terahertz Metamaterial Optics, Devices, and Applications)

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42 pages, 11528 KiB

Open AccessEditor’s ChoiceArticle

A Framework for Iterative Phase Retrieval Technique Integration into Atmospheric Adaptive Optics—Part II: High Resolution Wavefront Control in Strong Scintillations

by Mikhail A. Vorontsov and Ernst Polnau

Photonics 2025, 12(3), 185; https://doi.org/10.3390/photonics12030185 - 23 Feb 2025

Viewed by 630

Abstract

In this paper, we introduce atmospheric adaptive optics (AO) system architectures that utilize scintillation-resistant wavefront sensors based on iterative phase retrieval (IPR) techniques (described in detail in Part I) for closed-loop mitigation of atmospheric turbulence-induced wavefront aberrations in strong intensity scintillation conditions. The [...] Read more.

In this paper, we introduce atmospheric adaptive optics (AO) system architectures that utilize scintillation-resistant wavefront sensors based on iterative phase retrieval (IPR) techniques (described in detail in Part I) for closed-loop mitigation of atmospheric turbulence-induced wavefront aberrations in strong intensity scintillation conditions. The objective is to provide a framework (mathematical and numerical models, performance metrics, control algorithms, and wave-optics modeling and simulation results) for the potential integration of IPR-based wavefront sensing techniques into the following major atmospheric optics system types: directed energy laser beam projection, remote laser power delivery (remote power beaming), and free-space optical communications. Theoretical analysis and numerical simulation results demonstrate that the proposed closed-loop AO system architectures and control algorithms can be uniquely applicable for addressing one of the most challenging AO problems of turbulence effects mitigation in the presence of strong-intensity scintillations. Full article

(This article belongs to the Special Issue Challenges and Future Directions in Adaptive Optics Technology)

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10 pages, 5162 KiB

Open AccessEditor’s ChoiceArticle

Portable Filter-Free Lens-Free Incoherent Digital Holography System

by Tatsuki Tahara

Photonics 2025, 12(2), 167; https://doi.org/10.3390/photonics12020167 - 19 Feb 2025

Viewed by 622

Abstract

A portable incoherent digital holography system without a polarization filter or a refractive lens was developed. Phase-shifted self-interference incoherent holograms of light diffracted from an object were generated without attenuation due to a polarization filter using two polarization-sensitive phase-only spatial light modulators (TPP-SLMs). [...] Read more.

A portable incoherent digital holography system without a polarization filter or a refractive lens was developed. Phase-shifted self-interference incoherent holograms of light diffracted from an object were generated without attenuation due to a polarization filter using two polarization-sensitive phase-only spatial light modulators (TPP-SLMs). The number of optical elements in filter-free lens-free incoherent digital holography was reduced to make the system compact and portable. Experiments were conducted using the developed digital holography system set on a tripod stand and objects illuminated by a light-emitting diode. Full article

(This article belongs to the Special Issue Optical Imaging Innovations and Applications)

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14 pages, 1621 KiB

Open AccessEditor’s ChoiceReview

Interband Cascade Lasers from a Historic Perspective to a Future Outlook

by Rui Q. Yang and Michael B. Santos

Photonics 2025, 12(2), 155; https://doi.org/10.3390/photonics12020155 - 13 Feb 2025

Cited by 1 | Viewed by 1843

Abstract

Efficient, reliable, and low-cost mid-infrared interband cascade lasers (ICLs) are needed to meet the growing demands of many useful applications such as chemical sensing, environmental and greenhouse gas monitoring, detection of pipe leaks and explosives, food safety, medical diagnostics, and industrial process control. [...] Read more.

Efficient, reliable, and low-cost mid-infrared interband cascade lasers (ICLs) are needed to meet the growing demands of many useful applications such as chemical sensing, environmental and greenhouse gas monitoring, detection of pipe leaks and explosives, food safety, medical diagnostics, and industrial process control. We review the developments and status of ICLs from a historic perspective, discuss the lessons learnt from experience, and suggest considerations for future research and development. This review endeavors to include the most representative aspects and activities of ICLs, but cannot possibly describe every contribution in the 30 years since the initiation of ICLs. We present an overall picture of the ICL architecture and connect the fundamental principle and underlying physics to future activities. Full article

(This article belongs to the Special Issue The Three-Decade Journey of Quantum Cascade Lasers)

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15 pages, 4966 KiB

Open AccessEditor’s ChoiceArticle

Wavelength-Dependent Calcium Signaling Response to Photobiomodulation in Pancreatic Cells

by Kelli Fowlds, Anne M. Alsup, Abhidha Kunwar, Carly M. Darden, Jacob M. Luber, Michael C. Lawrence and Michael Cho

Photonics 2025, 12(2), 99; https://doi.org/10.3390/photonics12020099 - 23 Jan 2025

Viewed by 1120

Abstract

Diabetes mellitus is a metabolic disorder that is rapidly growing across the world. Our laboratory has recently demonstrated that photobiomodulation (PBM) can couple to its metabolic pathways by modulating calcium dynamics in islet cells, including α- and β-cells. Using computer vision algorithms, changes [...] Read more.

Diabetes mellitus is a metabolic disorder that is rapidly growing across the world. Our laboratory has recently demonstrated that photobiomodulation (PBM) can couple to its metabolic pathways by modulating calcium dynamics in islet cells, including α- and β-cells. Using computer vision algorithms, changes in PBM-induced calcium dynamics can be verified, and, more importantly, this led us to propose hypotheses that will likely advance our understanding of photostimulatory effects in islet cells. In our previous paper, we determined changes in calcium spiking in response to PBM at 810 nm by manually segmenting the cells and the calcium spiking patterns. We have since developed a computer vison pipeline to automate cell segmentation and subsequent image analyses. By using automated methods for segmentation, registration, tracking, and statistical analysis, we were able to improve the accuracy of previously observed changes in calcium spiking in response to PBM in both cell types. Moreover, this pipeline was applied to elucidate the wavelength-dependent modulation of calcium dynamics at 1064 nm. The extent of increase in calcium spiking appears to have been overestimated by manual analysis, and the machine learning pipeline was able to capture and segment nearly 3-fold more cells, suggesting improved accuracy in the analysis of calcium spiking in islet cells. Detailed calcium analysis also indicates a biphasic dose response among α- and β-cells in response to PBM therapy at different wavelengths. The current findings offer a novel hypothesis and may facilitate the use of translational PBM as a potential therapy for diabetes mellitus. Full article

(This article belongs to the Special Issue Phototheranostics: Science and Applications)

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20 pages, 8796 KiB

Open AccessEditor’s ChoiceArticle

Scattering Improves Temporal Resolution of Vision: A Pilot Study on Brain Activity

by Francisco J. Ávila

Photonics 2025, 12(1), 23; https://doi.org/10.3390/photonics12010023 - 30 Dec 2024

Viewed by 1117

Abstract

Temporal vision is a vital aspect of human perception, encompassing the ability to detect changes in light and motion over time. Optical scattering, or straylight, influences temporal visual acuity and the critical flicker fusion (CFF) threshold, with potential implications for cognitive visual processing. [...] Read more.

Temporal vision is a vital aspect of human perception, encompassing the ability to detect changes in light and motion over time. Optical scattering, or straylight, influences temporal visual acuity and the critical flicker fusion (CFF) threshold, with potential implications for cognitive visual processing. This study investigates how scattering affects CFF using an Arduino-based psychophysical device and electroencephalogram (EEG) recordings to analyze brain activity during CFF tasks under scattering-induced effects. A cohort of 30 participants was tested under conditions of induced scattering to determine its effect on temporal vision. Findings indicate a significant enhancement in temporal resolution under scattering conditions, suggesting that scattering may modulate the temporal aspects of visual perception, potentially by altering neural activity at the temporal and frontal brain lobes. A compensation mechanism is proposed to explain neural adaptations to scattering based on reduced electrical activity in the visual cortex and increased wave oscillations in the temporal lobe. Finally, the combination of the Arduino-based flicker visual stimulator and EEG revealed the excitatory/inhibitory stimulation capabilities of the high-frequency beta oscillation based on the alternation of an achromatic and a chromatic stimulus displayed in the CFF. Full article

(This article belongs to the Special Issue New Technologies for Human Visual Function Assessment)

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14 pages, 4205 KiB

Open AccessEditor’s ChoiceArticle

Ocular Aberrations and Retinal Thickness Variations After Moderate-Term Reading on Electronic Devices by Age

by María Arcas-Carbonell, Elvira Orduna-Hospital, María Mechó-García, María Munarriz-Escribano and Ana Sanchez-Cano

Photonics 2025, 12(1), 16; https://doi.org/10.3390/photonics12010016 - 27 Dec 2024

Viewed by 787

Abstract

Background: This study aims to evaluate subjective visual fatigue and objective optical and morphological changes in ocular structures after intermediate-duration reading on an iPad and an Ebook across different age groups. Methods: The sample included 108 right eyes from healthy subjects aged 18 [...] Read more.

Background: This study aims to evaluate subjective visual fatigue and objective optical and morphological changes in ocular structures after intermediate-duration reading on an iPad and an Ebook across different age groups. Methods: The sample included 108 right eyes from healthy subjects aged 18 to 66 years. The participants read for 20 min on an Ebook and another 20 min on an iPad under controlled illumination conditions. Aberrometry and retinal optical coherence tomography (OCT) measurements were taken before and after each reading session. Parameters such as total aberration, high-order aberration (HOA), low-order aberration (LOA), and retinal thickness in the nine Early Treatment Diabetic Retinopathy Study (ETDRS) areas were measured. The sample was analyzed as a whole and divided into five age groups by decade. Results: This study included 66 women (61.11%) and 42 men (38.89%), with an average age of 36.58 years (±14.83). The aberrometry results revealed significant differences in the total root mean square (RMS_TOTAL) after reading on both devices (p = 0.001). Low-order aberrations (RMS_LOA) also changed significantly (p = 0.001 for Ebook, p = 0.002 for the iPad), but high-order aberrations (RMS_HOA) did not. Central retinal thickness increased significantly after reading on the Ebook (p < 0.001) but not on the iPad. The peripheral retinal thickness did not change significantly. Conclusion: Moderate-duration reading increases LOA and central retinal thickness, with variations by age group and more pronounced effects from the Ebook, whereas HOA remains unaffected. Full article

(This article belongs to the Special Issue Advances in Optometric and Ophthalmic Technologies: Innovations and Applications)

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13 pages, 6904 KiB

Open AccessEditor’s ChoiceArticle

Considering Grayscale Process and Material Properties for Robust Multilevel Diffractive Flat Optics

by Diogo E. Aguiam, Ana Dias, Manuel J. L. F. Rodrigues, Aamod Shanker, Filipe Camarneiro, Joana D. Santos, Pablo Valentim, Joao Cunha and Patrícia C. Sousa

Photonics 2024, 11(12), 1200; https://doi.org/10.3390/photonics11121200 - 20 Dec 2024

Viewed by 852

Abstract

Arbitrarily designed flat optics directly manipulate the light wavefront to reproduce complex optical functions, enabling more compact optical assemblies and microsystem integration. Phase-shifting micro-optical devices rely on locally tailoring the optical path length of the wavefront through binary or multilevel surface relief micro- [...] Read more.

Arbitrarily designed flat optics directly manipulate the light wavefront to reproduce complex optical functions, enabling more compact optical assemblies and microsystem integration. Phase-shifting micro-optical devices rely on locally tailoring the optical path length of the wavefront through binary or multilevel surface relief micro- and nanostructures. Considering the resolution and tolerances of the production processes and the optical properties of the substrate and coating materials is crucial for designing robust multilevel diffractive flat optics. In this work, we evaluate the impact of the grayscale laser lithography resolution and geometry constraints on the efficiency of surface-relief diffractive lenses, and we analyze the process parameter space that limit lens performance. We introduce a spectral bandwidth metric to help evaluate the broad-spectrum performance of different materials. We simulate and experimentally observe that the diffractive focusing is dominated by the periodic wavelength-dependent phase discontinuities arising in the profile transitions of multilevel diffractive lenses. Full article

(This article belongs to the Special Issue Recent Advances in Diffractive Optics)

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17 pages, 4133 KiB

Open AccessEditor’s ChoiceArticle

MOCVD Grown InGaAs/InAlAs Quantum Cascade Lasers Emitting at 7.7 μm

by Maciej Bugajski, Andrzej Kolek, Grzegorz Hałdaś, Włodzimierz Strupiński, Iwona Pasternak, Walery Kołkowski and Kamil Pierściński

Photonics 2024, 11(12), 1195; https://doi.org/10.3390/photonics11121195 - 20 Dec 2024

Cited by 3 | Viewed by 1210

Abstract

In this paper, we report the growth of high-quality

{In}_{0.59} {Ga}_{0.41} As / {In}_{0.37} {Al}_{0.63} As

strain-balanced quantum cascade lasers (QCLs) in the low-pressure MOCVD production type multi-wafer planetary reactor addressing, in particular, quality and scaled manufacturing issues. Special [...] Read more.

In this paper, we report the growth of high-quality

{In}_{0.59} {Ga}_{0.41} As / {In}_{0.37} {Al}_{0.63} As

strain-balanced quantum cascade lasers (QCLs) in the low-pressure MOCVD production type multi-wafer planetary reactor addressing, in particular, quality and scaled manufacturing issues. Special attention was given to achieving the sharp interfaces (IFs), by optimizing the growth interruptions time and time of exposure of InAlAs layer to oxygen contamination in the reactor, which all result in extremely narrow IFs width, below 0.5 nm. The lasers were designed for emission at

7.7 µ

m. The active region was based on diagonal two-phonon resonance design with 40 cascade stages. For epitaxial process control, the High Resolution X-Ray Diffraction (HR XRD) and Transmission Electron Microscopy (TEM) were used to characterize the structural quality of the QCL samples. The grown structures were processed into mesa Fabry-Perot lasers using dry etching RIE ICP processing technology. The basic electro-optical characterization of the lasers is provided. We also present results of Green’s function modeling of QCLs and demonstrate the capability of non-equilibrium Green’s function (NEGF) approach for sophisticated, but still computationally effective simulation of laser’s characteristics. The sharpness of the grown IFs was confirmed by direct measurements of their chemical profiles and as well as the agreement between experimental and calculated wavelength obtained for the bandstructure with ideally abrupt (non-graded) IFs. Full article

(This article belongs to the Special Issue The Three-Decade Journey of Quantum Cascade Lasers)

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20 pages, 1038 KiB

Open AccessEditor’s ChoiceReview

Temporal Reflection from Ultrashort Solitons in Nonlinear Dispersive Medium: Impact of Raman Scattering

by Govind P. Agrawal

Photonics 2024, 11(12), 1189; https://doi.org/10.3390/photonics11121189 - 19 Dec 2024

Cited by 1 | Viewed by 914

Abstract

This review focuses on phenomena such as temporal reflection, total internal reflection, and waveguiding from ultrashort solitons forming inside a nonlinear dispersive medium such as an optical fiber. The case of wider solitons, moving at a constant speed inside the fiber, is discussed [...] Read more.

This review focuses on phenomena such as temporal reflection, total internal reflection, and waveguiding from ultrashort solitons forming inside a nonlinear dispersive medium such as an optical fiber. The case of wider solitons, moving at a constant speed inside the fiber, is discussed first to introduce the basic concepts. In the case of short solitons, the phenomenon of intrapulse Raman scattering shifts their spectrum toward longer wavelengths and decelerates them as they propagate through an optical fiber. These features lead to several novel effects such as temporal focusing and waveguiding by a single variable-speed Raman soliton. Recent experimental results are also discussed in this context. Full article

(This article belongs to the Special Issue Advances in Nonlinear Optics: From Fundamentals to Applications)

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14 pages, 7309 KiB

Open AccessEditor’s ChoiceArticle

Design Considerations for 1.6 Tbit/s Data Center Interconnects: Evaluating IM/DD and Coherent Transmission over O-Band Transmission Window

by Adrian A. Juarez, Yanjun Zhu, Xin Chen and Ming-Jun Li

Photonics 2024, 11(12), 1179; https://doi.org/10.3390/photonics11121179 - 14 Dec 2024

Cited by 1 | Viewed by 1496

Abstract

As data center interconnects surge towards a 1.6 Tbit/s data rate, achieving cost-effective and technically viable solutions present challenges. Intensity-modulation and direct-detection (IM/DD) transmission over O-Band using standard single-mode fiber has emerged as a promising low-cost option. However, understanding the limitations imposed by [...] Read more.

As data center interconnects surge towards a 1.6 Tbit/s data rate, achieving cost-effective and technically viable solutions present challenges. Intensity-modulation and direct-detection (IM/DD) transmission over O-Band using standard single-mode fiber has emerged as a promising low-cost option. However, understanding the limitations imposed by factors like chromatic dispersion (CD) and fiber non-linearity (FWM) is crucial, particularly in different scenarios, such as operating at 8 × 100 GBaud PAM4 in an LWDM-8 configuration. In this paper, we adopt a statistical approach to assess outage probability and consider practical fluctuations in link parameters. Numerical modeling suggests IM/DD can span distances up to 5 km with transmission power under 0 dBm using this architecture. In addition, we evaluate recently proposed architecture to achieve 800 Gbit/s and 1.6 Tbit/s using an LWDM4 configuration and assess the impact of FWM to understand the role of zero-dispersion wavelength (ZDW) of the fiber. Coherent transmission leverages more powerful signal processing capabilities which extends the transmission range. Yet, reducing coherent transmission complexity is desirable for cost-effective and power-efficient data center applications. By exploring dual wavelength transmission and DP-16 QAM transceivers, akin to IM/DD counterparts, the feasibility of streamlining this architecture is also studied. The analysis indicates that the complexity of the coherent approach can be reduced without significant penalties for distances up to 10 km. Full article

(This article belongs to the Special Issue Next-Generation High-Speed Direct Detection Optical Communication Systems)

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12 pages, 16440 KiB

Open AccessEditor’s ChoiceCommunication

Resolution Enhancement of Geometric Phase Self-Interference Incoherent Digital Holography Using Synthetic Aperture

by Youngrok Kim, KiHong Choi, Keehoon Hong and Sung-Wook Min

Photonics 2024, 11(12), 1170; https://doi.org/10.3390/photonics11121170 - 12 Dec 2024

Viewed by 1006

Abstract

Incoherent digital holography is a promising solution for acquiring three-dimensional information in the form of interference without a laser. Self-interference is the principal phenomenon of incoherent holography which splits the incident light wave into two waves and interferes with each one. The characteristics [...] Read more.

Incoherent digital holography is a promising solution for acquiring three-dimensional information in the form of interference without a laser. Self-interference is the principal phenomenon of incoherent holography which splits the incident light wave into two waves and interferes with each one. The characteristics of incoherent holograms are defined by the optical configuration of wavefront dividing units. In this paper, we propose the synthetic aperture method to achieve super-resolution of geometric phase lens-based self-interference incoherent digital holography. We analyze the resolution of incoherent holograms acquired from geometric phase lenses and experimentally demonstrate the resolution enhancement via the synthetic-aperture method. Moreover, the sparse synthetic-aperture method is proposed to ensure effective data capturing and the numerical optimization method is also proposed to fill the gap between the empty space of sparsely acquired holograms. Full article

(This article belongs to the Special Issue Optical Imaging Innovations and Applications)

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11 pages, 4846 KiB

Open AccessEditor’s ChoiceArticle

Vanadium Dioxide-Based Terahertz Metamaterials for Non-Contact Temperature Sensor

by Jin Leng, Yong Gong, Li Luo and Qiwu Shi

Photonics 2024, 11(12), 1148; https://doi.org/10.3390/photonics11121148 - 6 Dec 2024

Viewed by 1071

Abstract

Temperature sensors play important roles in wide-spreading human activities. The non-contact method of using temperature sensors offers significant advantages but faces challenges in detection precision. In this work, a double-layer asymmetric terahertz (THz) metamaterial combined with phase transition oxide was proposed to realize [...] Read more.

Temperature sensors play important roles in wide-spreading human activities. The non-contact method of using temperature sensors offers significant advantages but faces challenges in detection precision. In this work, a double-layer asymmetric terahertz (THz) metamaterial combined with phase transition oxide was proposed to realize non-contact temperature sensor with high sensitivity. The metamaterial exhibited band-stop filtering effects in the simulated transmission spectra. Temperature changes induced a reversible phase transition in VO₂, resulting in altered conductivity. The numerical results indicated that the S21 parameter increases from −44.33 dB to −4.78 dB at a frequency of 1.22 THz as the conductivity of the VO₂ film increases from 10 to 5000 S/m, achieving a modulation depth of 89%. In addition, the 86 nm thick VO₂ film underwent a phase transition in the temperature range of 54.93 °C to 66.93 °C, achieving a sensitivity of 1.82 dB/°C for temperature sensing. This work provided great insights into the development of metamaterials based on high-precision temperature measurement. Full article

(This article belongs to the Special Issue Advanced Photonics Metamaterials and Metasurfaces: Science and Applications, 2nd Edition)

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18 pages, 1606 KiB

Open AccessEditor’s ChoiceReview

Recent Development of Fourier Domain Mode-Locked Laser

by Lu Chen, Hongcui Zhang, Song Yu, Bin Luo and Tianwei Jiang

Photonics 2024, 11(12), 1131; https://doi.org/10.3390/photonics11121131 - 29 Nov 2024

Cited by 1 | Viewed by 2555

Abstract

Since the advent of Fourier Domain Mode-Locked (FDML) lasers, they have demonstrated outstanding performance in several fields. They achieve high-speed, narrow-linewidth laser output with the new mode-locking mechanism, which has been intensively researched in the past decades. Compared with conventional wavelength-scanning light sources, [...] Read more.

Since the advent of Fourier Domain Mode-Locked (FDML) lasers, they have demonstrated outstanding performance in several fields. They achieve high-speed, narrow-linewidth laser output with the new mode-locking mechanism, which has been intensively researched in the past decades. Compared with conventional wavelength-scanning light sources, FDML lasers have successfully increased the scanning rate of frequency-sweeping lasers from kHz to MHz. They are widely used in optical coherence tomography, spectral analysis, microscopy, and microwave photonics. With the deepening research on FDML lasers, several performance metrics have been optimized and improved, offering superior performance for FDML laser-based applications. This paper reviews the principles and key performance indicators of FDML lasers, as well as the recent progress made in some important applications, and highlights further research directions for FDML lasers in the future. Full article

(This article belongs to the Special Issue Cutting-Edge Developments in Fiber Laser)

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13 pages, 13317 KiB

Open AccessEditor’s ChoiceArticle

Toward Single-Shot Real-Time Spectroscopy of Dynamic Materials via White-Light and Supercontinuum Light Sources

by Zhanibek Bolatbek and Imad Agha

Photonics 2024, 11(12), 1119; https://doi.org/10.3390/photonics11121119 - 26 Nov 2024

Viewed by 865

Abstract

In conventional camera or monochromator-based spectroscopy, different wavelengths, spanning from short ultraviolet region to long infrared region, are calculated under steady-state conditions due to the temporally multiplexed nature of conventional CMOS/CCD-equipped spectrometers, which limit the refresh rates to the order of milliseconds for [...] Read more.

In conventional camera or monochromator-based spectroscopy, different wavelengths, spanning from short ultraviolet region to long infrared region, are calculated under steady-state conditions due to the temporally multiplexed nature of conventional CMOS/CCD-equipped spectrometers, which limit the refresh rates to the order of milliseconds for most tools available on the market. These refresh rates might not be suitable for most temporally dynamic effects that govern the behavior of disparate effects, such as phase transition in phase-change materials, conformal changes in molecules, and microbial community evolution, among others. Pump-probe methods are often presented as a solution to the capture speed limitation, but in themselves are not applicable universally and are not truly “real-time”. In this work, we present an evolution to the conventional spectrometers, increasing its speed by over 4 orders of magnitude while maintaining reasonable spectral resolution. We additionally present a path that combines our technique with supercontinuum light sources for even more ambitious future applications. Full article

(This article belongs to the Special Issue Optical Technologies for Measurement and Metrology)

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8 pages, 4421 KiB

Open AccessEditor’s ChoiceArticle

Chromatic Aberration in Wavefront Coding Imaging with Trefoil Phase Mask

by Miguel Olvera-Angeles, Justo Arines and Eva Acosta

Photonics 2024, 11(12), 1117; https://doi.org/10.3390/photonics11121117 - 26 Nov 2024

Viewed by 1026

Abstract

The refractive index of the lenses used in optical designs varies with wavelength, causing light rays to fail when focusing on a single plane. This phenomenon is known as chromatic aberration (CA), chromatic distortion, or color fringing, among other terms. Images affected by [...] Read more.

The refractive index of the lenses used in optical designs varies with wavelength, causing light rays to fail when focusing on a single plane. This phenomenon is known as chromatic aberration (CA), chromatic distortion, or color fringing, among other terms. Images affected by CA display colored halos and experience a loss of resolution. Fully achromatic systems can be achieved through complex and costly lens designs and/or computationally when digital sensors capture the image. In this work, we propose using the wavefront coding (WFC) technique with a trefoil-shaped phase modulation plate in the optical system to effectively increase the resolution of images affected by longitudinal chromatic aberration (LCA), significantly simplifying the optical design and reducing costs. Experimental results with three LEDs simulating RGB images verify that WFC with trefoil phase plates effectively corrects longitudinal chromatic aberration. Transverse chromatic aberration (TCA) is corrected computationally. Furthermore, we demonstrate that the optical system maintains depth of focus (DoF) for color images. Full article

(This article belongs to the Special Issue Adaptive Optics Imaging: Science and Applications)

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15 pages, 15053 KiB

Open AccessEditor’s ChoiceArticle

Simple Direct Measurement of the Orbital Stokes Parameters in Structured Vortex Beams

by Alexander Volyar, Mikhail Bretsko, Server Khalilov and Yana Akimova

Photonics 2024, 11(11), 1095; https://doi.org/10.3390/photonics11111095 - 20 Nov 2024

Cited by 1 | Viewed by 882

Abstract

An analogy with the polarization Stokes parameters and symplectic methods of the second-order intensity moment matrix allowed us to develop a simple technique for measuring the orbital Stokes parameters followed by mapping the structured beam states onto the orbital Poincaré sphere. The measurement [...] Read more.

An analogy with the polarization Stokes parameters and symplectic methods of the second-order intensity moment matrix allowed us to develop a simple technique for measuring the orbital Stokes parameters followed by mapping the structured beam states onto the orbital Poincaré sphere. The measurement process involves only two shots of the beam intensity patterns in front of a cylindrical lens and in its double focus. Such a simple measurement approach is based on the reciprocity effect between the experimentally measured cross-intensity element

W_{x y}

and the orbital angular momentum of the intensity moment matrix. For experiments, we chose two types of two-parameter structured beams, namely, structured Laguerre–Gaussian beams and binomial beams. We obtained a good agreement between our theoretical background and the experiments, as well as the results of other authors. Full article

(This article belongs to the Special Issue Recent Advances in Diffractive Optics)

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13 pages, 4582 KiB

Open AccessEditor’s ChoiceArticle

Dual-Wavelength Confocal Laser Speckle Contrast Imaging Using a Deep Learning Approach

by E Du, Haohan Zheng, Honghui He, Shiguo Li, Cong Qiu, Weifeng Zhang, Guoqing Wang, Xingquan Li, Lan Ma, Shuhao Shen and Yuan Zhou

Photonics 2024, 11(11), 1085; https://doi.org/10.3390/photonics11111085 - 18 Nov 2024

Cited by 4 | Viewed by 1694

Abstract

This study developed a novel dual-wavelength confocal laser speckle imaging platform. The system includes both visible and near-infrared lasers and two imaging modes: confocal and wide-field laser speckle contrast imaging. The experimental results confirm that the proposed system can be used to measure [...] Read more.

This study developed a novel dual-wavelength confocal laser speckle imaging platform. The system includes both visible and near-infrared lasers and two imaging modes: confocal and wide-field laser speckle contrast imaging. The experimental results confirm that the proposed system can be used to measure not only blood flow but also blood oxygen saturation. Additionally, we proposed a blood flow perfusion imaging method called BlingNet (a blood flow imaging CNN) based on the laser speckle contrast imaging technique and deep learning approach. Compared to the traditional nonlinear fitting method, this method has superior accuracy and robustness with higher imaging speed, making real-time blood flow imaging possible. Full article

(This article belongs to the Special Issue New Perspectives in Biomedical Optics and Optical Imaging)

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11 pages, 2926 KiB

Open AccessEditor’s ChoiceArticle

Remote Detection and Visualization of Surface Traces of Nitro-Group-Containing Explosives

by Sergei Bobrovnikov, Evgeny Gorlov and Viktor Zharkov

Photonics 2024, 11(11), 1065; https://doi.org/10.3390/photonics11111065 - 14 Nov 2024

Viewed by 917

Abstract

This paper presents the results of an experimental study of the possibility of remote visualization of traces of some nitro-group-containing explosives (TNT, RDX, HMX, Composition-B, and Tetryl) on the surface of aluminum foil using the laser fragmentation/laser-induced fluorescence (LF/LIF) method. A tunable excimer [...] Read more.

This paper presents the results of an experimental study of the possibility of remote visualization of traces of some nitro-group-containing explosives (TNT, RDX, HMX, Composition-B, and Tetryl) on the surface of aluminum foil using the laser fragmentation/laser-induced fluorescence (LF/LIF) method. A tunable excimer KrF laser with a narrow generation line was used to fragment explosives and excite fluorescence of their NO fragments (nitric oxide molecules) from the second vibrationally excited state (v″ = 2). When recording optical responses, spectral selection of the γ(0, 0) fluorescence band of NO was carried out. The LF/LIF method is shown to be promising for creating scanning detectors that will allow remote detection of trace amounts of explosives with a concentration of up to 1 μg/cm² on the surfaces of objects at a distance of several meters and simultaneously determine their location. The sensitivity of the one-color LF/LIF detection method can be increased by increasing the energy density of the probing radiation and/or by optimizing the LF/LIF excitation process. Full article

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15 pages, 4852 KiB

Open AccessEditor’s ChoiceArticle

Characterization of Holmium-Doped Fiber Using AOM and Considering Pair-Induced Quenching and Fiber Length

by Yuri Barmenkov, Pablo Muniz-Cánovas, José-Luis Cruz and Miguel V. Andrés

Photonics 2024, 11(11), 1043; https://doi.org/10.3390/photonics11111043 - 7 Nov 2024

Viewed by 1206

Abstract

In this paper, we present the results of an experimental study on the characterization of holmium-doped silica fiber. A standard acousto-optic modulator controls the output power of the ytterbium-doped fiber laser operating at 1134.5 nm and serving as a pump source of the [...] Read more.

In this paper, we present the results of an experimental study on the characterization of holmium-doped silica fiber. A standard acousto-optic modulator controls the output power of the ytterbium-doped fiber laser operating at 1134.5 nm and serving as a pump source of the holmium-doped fiber under test. This technique allows us to measure the lifetimes of ⁵I₇ and ⁵I₈ energy levels of Ho³⁺ ions. The effects of the fiber length and the concentration-dependent pair-induced quenching on the accuracy of the fluorescence lifetime measurement are considered. The results of this study are compared with those obtained using the exponential and Förster decay functions used for such types of measurements. It is demonstrated that the knowledge of two fiber parameters, the pump saturation power and the fluorescence saturation power, together with the fiber absorption spectrum, permits one to obtain the absorption cross-sections at the pump and other key wavelengths, the effective concentration of the active ions, and the quantum efficiency of the fluorescence from the laser level. The results of this study are applicable to the reliable characterization of any type of heavily doped gain fibers and to the further numerical modeling and optimization of fiber lasers. Full article

(This article belongs to the Special Issue Fiber Lasers: Recent Advances and Applications)

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17 pages, 7091 KiB

Open AccessEditor’s ChoiceArticle

High-Efficiency and High-Monochromaticity Semitransparent Organic Solar Cells Based on Optical Tamm States

by Junwei Zhao, Senxuan Lin, Jinxin Zhou, Fuhao Gao, Jingfeng Liu, Yongbing Long and Haitao Xu

Photonics 2024, 11(11), 1030; https://doi.org/10.3390/photonics11111030 - 1 Nov 2024

Cited by 1 | Viewed by 1309

Abstract

Semitransparent organic solar cells (ST-OSCs) have garnered more interest and stand out as promising candidates for next-generation solar energy harvesters with their unique advantages. However, challenges remain for the advancement of colorful ST-OSCs, such as enhancing the light absorption and transmittance without considerable [...] Read more.

Semitransparent organic solar cells (ST-OSCs) have garnered more interest and stand out as promising candidates for next-generation solar energy harvesters with their unique advantages. However, challenges remain for the advancement of colorful ST-OSCs, such as enhancing the light absorption and transmittance without considerable power conversion efficiency (PCE) losses. Herein, an optical analysis of silver (Ag) electrodes and one-dimensional photonic crystals (1DPCs) was conducted by simulations, revealing the presence of optical Tamm states (OTSs) at the interface of Ag/1DPCs. Furthermore, the spectral and electrical properties were fine-tuned by modulating the OTSs through theoretical simulations, utilizing PM6:Y6 as the active layer. The structural parameters of the ST-OSCs were optimized, including the Ag layer thickness, the central wavelength of 1DPCs, the first WO₃ layer thickness, and the pair number of WO₃/LiF. The optimization resulted in the successful development of blue, violet-blue, and red ST-OSC devices, which exhibited transmittance peak intensities ranging from 31.5% to 37.9% and PCE losses between 1.5% and 5.2%. Notably, the blue device exhibited a peak intensity of 37.0% and a PCE of 15.24%, with only a 1.5% loss in efficiency. This research presents an innovative approach to enhancing the performance of ST-OSCs, achieving a balance between high transparency and high efficiency. Full article

(This article belongs to the Special Issue Recent Advances in Micro/Nano-Optics and Photonics)

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13 pages, 2748 KiB

Open AccessEditor’s ChoiceArticle

Photonic-Metamaterial-Based, Near-Field-Enhanced Biosensing Approach for Early Detection of Lung and Ovarian Cancer

by Shuo Geng, Xuguang Zhang, Haiyan Liang and Yi Zheng

Photonics 2024, 11(11), 1020; https://doi.org/10.3390/photonics11111020 - 30 Oct 2024

Cited by 2 | Viewed by 1339

Abstract

Early detection of lung and ovarian cancers relies heavily on identifying tumor biomarkers, but current methods require large blood samples and complex genetic testing. This study presents a novel photonic-metamaterial-based biosensing approach that leverages near-field radiative enhancement to detect cancer biomarkers (CA 125, [...] Read more.

Early detection of lung and ovarian cancers relies heavily on identifying tumor biomarkers, but current methods require large blood samples and complex genetic testing. This study presents a novel photonic-metamaterial-based biosensing approach that leverages near-field radiative enhancement to detect cancer biomarkers (CA 125, CEA, and CYFRA 21-1) with high sensitivity. By utilizing structured photonic metamaterials, we optimize specific wavelengths to identify these biomarkers in interstitial fluid, which can be easily collected via minimally invasive microneedle arrays. Integrating near-field interactions with wavelength-selective metamaterials amplifies the thermal response at the nanoscale, allowing for the detection of deficient concentrations of biomarkers. This photonic metamaterial technique provides a faster, more accessible, and affordable alternative to conventional blood-based methods, significantly improving early detection and monitoring of cancer. Ultimately, this approach offers a transformative tool for clinical and research applications in cancer diagnostics. Full article

(This article belongs to the Special Issue Optical Metasurfaces: Applications and Trends)

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11 pages, 5796 KiB

Open AccessEditor’s ChoiceArticle

Real-Time Resolution Enhancement of Confocal Laser Scanning Microscopy via Deep Learning

by Zhiying Cui, Yi Xing, Yunbo Chen, Xiu Zheng, Wenjie Liu, Cuifang Kuang and Youhua Chen

Photonics 2024, 11(10), 983; https://doi.org/10.3390/photonics11100983 - 19 Oct 2024

Cited by 1 | Viewed by 2377

Abstract

Confocal laser scanning microscopy is one of the most widely used tools for high-resolution imaging of biological cells. However, the imaging resolution of conventional confocal technology is limited by diffraction, and more complex optical principles and expensive optical-mechanical structures are usually required to [...] Read more.

Confocal laser scanning microscopy is one of the most widely used tools for high-resolution imaging of biological cells. However, the imaging resolution of conventional confocal technology is limited by diffraction, and more complex optical principles and expensive optical-mechanical structures are usually required to improve the resolution. This study proposed a deep residual neural network algorithm that can effectively improve the imaging resolution of the confocal microscopy in real time. The reliability and real-time performance of the algorithm were verified through imaging experiments on different biological structures, and an imaging resolution of less than 120 nm was achieved in a more cost-effective manner. This study contributes to the real-time improvement of the imaging resolution of confocal microscopy and expands the application scenarios of confocal microscopy in biological imaging. Full article

(This article belongs to the Special Issue Advanced Optical Microscopy and Imaging Technology)

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18 pages, 2319 KiB

Open AccessEditor’s ChoiceArticle

Propagation of a Partially Coherent Bessel–Gaussian Beam in a Uniform Medium and Turbulent Atmosphere

by Igor Lukin and Vladimir Lukin

Photonics 2024, 11(6), 562; https://doi.org/10.3390/photonics11060562 - 14 Jun 2024

Cited by 1 | Viewed by 2262

Abstract

In this paper, the coherent properties of partially coherent Bessel–Gaussian optical beams propagating through a uniform medium (free space) or a turbulent atmosphere are examined theoretically. The consideration is based on the analytical solution of the equation for the transverse second-order mutual coherence [...] Read more.

In this paper, the coherent properties of partially coherent Bessel–Gaussian optical beams propagating through a uniform medium (free space) or a turbulent atmosphere are examined theoretically. The consideration is based on the analytical solution of the equation for the transverse second-order mutual coherence function of the field of partially coherent optical radiation in a turbulent atmosphere. For the partially coherent Bessel–Gaussian beam, the second-order mutual coherence function of the source field is taken as a Gaussian–Schell model. In this approximation, we analyze the behavior of the coherence degree and the integral coherence scale of these beams as a function of the propagation pathlength, propagation conditions, and beam parameters, such as the radius of the Gauss factor of the beam, parameter of the Bessel factor of the beam, topological charge, and correlation width of the source field of partially coherent radiation. It was found that, as a partially coherent vortex Bessel–Gaussian beam propagates through a turbulent atmosphere, there appear not two (as might be expected: one due to atmospheric turbulence and another due to the partial coherence of the source field), but only one ring dislocation of the coherence degree (due to the simultaneous effect of both these factors on the optical radiation). In addition, it is shown that the dislocation of the coherence degree that significantly affects the beam coherence level is formed only for beams, for which the coherence width of the source field is larger than the diameter of the first Fresnel zone. Full article

(This article belongs to the Special Issue Recent Advances in Diffractive Optics)

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16 pages, 6620 KiB

Open AccessEditor’s ChoiceArticle

Long-Term Stability Test for Femtosecond Laser-Irradiated SnO₂-Nanowire Gas Sensor for C₇H₈ Gas Sensing

by Sanghoon Ahn, Kang Woo Chun and Changkyoo Park

Photonics 2024, 11(6), 550; https://doi.org/10.3390/photonics11060550 - 11 Jun 2024

Cited by 5 | Viewed by 1696

Abstract

In this study, femtosecond (FS) laser irradiation with different laser energy densities of 138, 276, and 414 mJ/cm² is applied to SnO₂-nanowire (NW) gas sensors, and the effect of the FS laser irradiation on the gas sensor response toward toluene [...] Read more.

In this study, femtosecond (FS) laser irradiation with different laser energy densities of 138, 276, and 414 mJ/cm² is applied to SnO₂-nanowire (NW) gas sensors, and the effect of the FS laser irradiation on the gas sensor response toward toluene (C₇H₈) gas is investigated. The FS laser irradiation causes oxygen deficiency in the SnO₂ NWs and forms SnO and SnO_x. Moreover, an embossing surface with multiple nano-sized bumps is created on the SnO₂ NW surface because of the FS laser irradiation. The FS laser-irradiated SnO₂-NW gas sensor exhibits superior sensing performance compared with the pristine SnO₂-NW gas sensor. Moreover, the FS laser energy density significantly affects gas-sensing performance, and the highest sensor response is achieved by the gas sensor irradiated at 138 mJ/cm². The long-term stability test of the laser-irradiated SnO₂-NW gas sensor is performed by comparing fresh and 6-month-old gas sensors in different gas concentrations and relative humidity levels. Comparable gas-sensing behaviors are examined between the fresh and 6-month-old gas sensor, and this verifies the robustness of the laser-irradiated SnO₂-NW gas sensor. Full article

(This article belongs to the Special Issue Advances in Ultrafast Laser Science and Applications)

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18 pages, 24888 KiB

Open AccessEditor’s ChoiceArticle

Miniaturized Multi-Platform Free-Space Laser-Communication Terminals for Beyond-5G Networks and Space Applications

by Alberto Carrasco-Casado, Koichi Shiratama, Dimitar Kolev, Fumie Ono, Hiroyuki Tsuji and Morio Toyoshima

Photonics 2024, 11(6), 545; https://doi.org/10.3390/photonics11060545 - 7 Jun 2024

Cited by 6 | Viewed by 4474

Abstract

Beyond-5G (B5G) technology plays a pivotal role in the next generation of communication infrastructure to support the future Society 5.0, a concept introduced in the 5th Basic Plan for Science and Technology by the Japanese Cabinet to define the long-term growth strategy for [...] Read more.

Beyond-5G (B5G) technology plays a pivotal role in the next generation of communication infrastructure to support the future Society 5.0, a concept introduced in the 5th Basic Plan for Science and Technology by the Japanese Cabinet to define the long-term growth strategy for reconciling economic development with the resolution of social issues through the promotion of science and technologies. Free-space laser communication is a key element in boosting the data transmission capabilities required for B5G applications. The NICT will complete in 2024 the first fully functional prototypes of a series of miniaturized laser-communication terminals for multiple platforms. These terminals are designed to adapt to a wide range of requirements to address scenarios where laser communications can offer a competitive, enhanced solution compared to existing technologies. This paper provides an overview of these terminals’ capabilities and the plans for their functional validation, as well as preliminary data from the first full-system tests. A number of innovations integrated into the terminals are introduced, such as the manufacture of the smallest miniaturized EDFA with integrated HPA and LNA and full space qualification to date, the first-ever integration of a beam-divergence control system in a practical communication terminal, the development of the most compact Tbit/s-class modem prototype documented in the literature, and the smallest gimbal design integrated in a lasercom terminal. Furthermore, this paper outlines the mid-term plans for demonstration in the most significant realistic scenarios, emphasizing the use of High-Altitude Platform Stations (HAPSs) and ultra-small satellites. Full article

(This article belongs to the Special Issue Recent Advances in Optical Wireless Communication Systems and Networks)

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22 pages, 1619 KiB

Open AccessEditor’s ChoiceArticle

Optimisation of the Transmitter Layout in a VLP System Using an Aperture-Based Receiver

by José Miguel Menéndez and Heidi Steendam

Photonics 2024, 11(6), 517; https://doi.org/10.3390/photonics11060517 - 28 May 2024

Cited by 1 | Viewed by 1221

Abstract

In this paper, we consider a visible light positioning (VLP) system, where an array of photo diodes combined with apertures is used as a directional receiver and a set of inexpensive and energy-efficient light-emitting diodes (LEDs) is used as transmitters. The paper focuses [...] Read more.

In this paper, we consider a visible light positioning (VLP) system, where an array of photo diodes combined with apertures is used as a directional receiver and a set of inexpensive and energy-efficient light-emitting diodes (LEDs) is used as transmitters. The paper focuses on the optimisation of the layout of the transmitter, i.e., the number and placement of the LEDs, to meet the wanted position estimation accuracy levels. To this end, we evaluate the Cramer–Rao bound (CRB), which is a lower bound on the mean-squared error (MSE) of the position estimate, to analyse the influence of the LEDs’ placement. In contrast to other works, where only the location of the LEDs was considered and/or the optimisation was carried out through simulations, in this work, the optimisation is carried out analytically and considers all the parameters involved in the VLP system as well as the illumination. Based on our results, we formulate simple rules of thumb with which we can determine the spacing between LEDs and the minimum number of LEDs, as well as their position on the ceiling, while also taking into account the requirements for the illumination. Full article

(This article belongs to the Special Issue Advanced Technologies in Optical Wireless Communications)

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15 pages, 5146 KiB

Open AccessEditor’s ChoiceArticle

A Fast Time Synchronization Method for Large Scale LEO Satellite Networks Based on A Bionic Algorithm

by Yue Xu, Tao Dong, Jie Yin, Ziyong Zhang, Zhihui Liu, Hao Jiang and Jing Wu

Photonics 2024, 11(5), 475; https://doi.org/10.3390/photonics11050475 - 19 May 2024

Cited by 1 | Viewed by 2138

Abstract

A fast time synchronization method for large-scale LEO satellite networks based on a bionic algorithm is proposed. Because the inter-satellite links are continuously established and interrupted due to the relative motion of the satellites, the topology of the LEO satellite networks is time [...] Read more.

A fast time synchronization method for large-scale LEO satellite networks based on a bionic algorithm is proposed. Because the inter-satellite links are continuously established and interrupted due to the relative motion of the satellites, the topology of the LEO satellite networks is time varying. Firstly, according to the ephemeris information in navigation messages, a connection table which records the connections between satellites is generated. Then, based on the connection table, the current satellite network topology is calculated and generated. Furthermore, a bionic algorithm is used to select some satellites as time source nodes and calculate the hierarchy of the clock transmission tree. By taking the minimum level of the time transmission tree as the optimization objective, the time source nodes and the clock stratums of the whole satellite networks are obtained. Finally, the onboard computational center broadcasts the time layer table to all the satellites in the LEO satellite networks and the time synchronization links can be established or recovered fast. Full article

(This article belongs to the Special Issue Novel Advances in Optical Communications)

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23 pages, 19086 KiB

Open AccessEditor’s ChoiceArticle

Analysis of the Polarization Distribution and Spin Angular Momentum of the Interference Field Obtained by Co-Planar Beams with Linear and Circular Polarization

by Svetlana N. Khonina, Andrey V. Ustinov, Alexey P. Porfirev and Sergey V. Karpeev

Photonics 2024, 11(5), 478; https://doi.org/10.3390/photonics11050478 - 19 May 2024

Cited by 3 | Viewed by 1263

Abstract

Interference of two and four light beams with linear or circular polarization is studied analytically and numerically based on the Richards–Wolf formalism. We consider such characteristics of the interference fields as the distribution of intensity, polarization, and spin angular momentum density. The generation [...] Read more.

Interference of two and four light beams with linear or circular polarization is studied analytically and numerically based on the Richards–Wolf formalism. We consider such characteristics of the interference fields as the distribution of intensity, polarization, and spin angular momentum density. The generation of light fields with 1D and 2D periodic structure of both intensity and polarization is demonstrated. We can control the periodic structure both by changing the polarization state of the interfering beams and by changing the numerical aperture of focusing. We consider examples with a basic configuration, as well as those with a certain symmetry in the polarization state of the interfering beams. In some cases, increasing the numerical aperture of the focusing system significantly affects the generated distributions of both intensity and polarization. Experimental results, obtained using a polarization video camera, are in good agreement with the simulation results. The considered light fields can be used in laser processing of thin films of photosensitive (as well as polarization-sensitive) materials in order to create arrays of various ordered nano- and microstructures. Full article

(This article belongs to the Special Issue Structured Light Beams: Science and Applications)

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12 pages, 15402 KiB

Open AccessEditor’s ChoiceArticle

Compact Low Loss Ribbed Asymmetric Multimode Interference Power Splitter

by Yanfeng Liang, Huanlin Lv, Baichao Liu, Haoyu Wang, Fangxu Liu, Shuo Liu, Yang Cong, Xuanchen Li and Qingxiao Guo

Photonics 2024, 11(5), 472; https://doi.org/10.3390/photonics11050472 - 17 May 2024

Cited by 2 | Viewed by 2019

Abstract

Optical power splitters (OPSs) are utilized extensively in integrated photonic circuits, drawing significant interest in research on power splitters with adjustable splitting ratios. This paper introduces a compact, low-loss 1 × 2 asymmetric multimode interferometric (MMI) optical power splitter on a silicon-on-insulator (SOI) [...] Read more.

Optical power splitters (OPSs) are utilized extensively in integrated photonic circuits, drawing significant interest in research on power splitters with adjustable splitting ratios. This paper introduces a compact, low-loss 1 × 2 asymmetric multimode interferometric (MMI) optical power splitter on a silicon-on-insulator (SOI) platform. The device is simulated using the finite difference method (FDM) and eigenmode expansion solver (EME). It is possible to attain various output power splitting ratios by making the geometry of the MMI central section asymmetric relative to the propagation axis. Six distinct optical power splitters are designed with unconventional splitting ratios in this paper, which substantiates that the device can achieve any power splitter ratios (PSRs) in the range of 95:5 to 50:50. The dimensions of the multimode section were established at 2.9 × (9.5–10.9) μm. Simulation results show a range of unique advantages of the device, including a low extra loss of less than 0.4 dB, good fabrication tolerance, and power splitting ratio fluctuation below 3% across the 1500 nm to 1600 nm wavelength span. Full article

(This article belongs to the Special Issue Optical Fiber Communication Systems and Networks)

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11 pages, 4949 KiB

Open AccessEditor’s ChoiceArticle

The Generation of Circularly Polarized Isolated Attosecond Pulses with Tunable Helicity from CO Molecules in Polarization Gating Laser Fields

by Shiju Chen, Hua Yuan, Feng Wang, Jiahang Song, Yue Zhao, Chunhui Yang, Tianxin Ou, Ru Zhang, Qiang Chang and Yuping Sun

Photonics 2024, 11(5), 464; https://doi.org/10.3390/photonics11050464 - 15 May 2024

Viewed by 1683

Abstract

We theoretically demonstrate a scheme to generate circularly polarized (CP) isolated attosecond pulses (IAPs) with tunable helicity using a polarization gating laser field interacting with the CO molecule. The results show that a broadband CP supercontinuum is produced from the oriented CO molecule, [...] Read more.

We theoretically demonstrate a scheme to generate circularly polarized (CP) isolated attosecond pulses (IAPs) with tunable helicity using a polarization gating laser field interacting with the CO molecule. The results show that a broadband CP supercontinuum is produced from the oriented CO molecule, which supports the generation of an IAP with an ellipticity of 0.98 and a duration of 90 as. Furthermore, the helicity of the generated harmonics and IAP can be effectively controlled by modulating the laser field and the orientation angle of the CO molecule. Our method will advance research on chiral-specific dynamics and magnetic circular dichroism on the attosecond timescale. Full article

(This article belongs to the Topic Recent Advances in Nonlinear Optics and Nonlinear Optical Materials)

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16 pages, 3641 KiB

Open AccessEditor’s ChoiceReview

Features of Adaptive Phase Correction of Optical Wave Distortions under Conditions of Intensity Fluctuations

by Vladimir Lukin

Photonics 2024, 11(5), 460; https://doi.org/10.3390/photonics11050460 - 14 May 2024

Cited by 2 | Viewed by 1757

Abstract

An analysis of the features of measurements and correction of phase distortions in optical waves propagating in the atmosphere at various levels of turbulence was performed. It is shown that with increasing intensity fluctuations, the limiting capabilities of phase correction decrease, and the [...] Read more.

An analysis of the features of measurements and correction of phase distortions in optical waves propagating in the atmosphere at various levels of turbulence was performed. It is shown that with increasing intensity fluctuations, the limiting capabilities of phase correction decrease, and the phase of an optical wave that has passed through a turbulence layer consists of two components: potential and vortex. It was found that even in the region of weak fluctuations there is an overlap of spectral filtering functions for intensity and phase fluctuations. Areas of turbulence inhomogeneities have been identified that will have mutual influence and negatively affect the operation of the phase meter. It is noted that correlation functions, both phase and intensity, are less susceptible to this compared to structural functions. The results of experimental studies on the reconstruction of the wavefront of laser radiation distorted by atmospheric turbulence using a Shack–Hartmann wavefront sensor during vignetting and central screening of the entrance pupil in the optical system are presented. Studies have been carried out on the propagation of laser radiation along a horizontal atmospheric path for various levels of turbulence. The results are analyzed in terms of Zernike polynomials. Full article

(This article belongs to the Special Issue Advances in Structured Light Generation and Manipulation)

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