Photonics

15 pages, 6478 KiB

Open AccessEditor’s ChoiceArticle

Nonlinear Optical Response of Dispersed Medium Based on Conjugates Single-Walled Carbon Nanotubes with Phthalocyanines

by Pavel N. Vasilevsky, Mikhail S. Savelyev, Alexander Yu. Tolbin, Artem V. Kuksin, Yulia O. Vasilevskaya, Andrey P. Orlov, Yury P. Shaman, Alexander A. Dudin, Alexander A. Pavlov and Alexander Yu. Gerasimenko

Photonics 2023, 10(5), 537; https://doi.org/10.3390/photonics10050537 - 6 May 2023

Cited by 6 | Viewed by 2183

Abstract

Nanosecond lasers have recently been widely involved in human activity. However, high-intensity laser radiation can cause severe damage to organs of vision and expensive photonic devices. Radiation in the near UV range is especially dangerous for human eyes, since it is strongly absorbed [...] Read more.

Nanosecond lasers have recently been widely involved in human activity. However, high-intensity laser radiation can cause severe damage to organs of vision and expensive photonic devices. Radiation in the near UV range is especially dangerous for human eyes, since it is strongly absorbed by biological media and is also invisible, i.e., the reaction time of the eye to such radiation is much lower than that of visible light. Passive limiters have high transmission (>70%) at a low light intensity and begin to “darken” only when the threshold value of the laser radiation intensity is reached. In this work, we studied liquid nanodispersed nonlinear optical limiters based on hybrids of single-walled carbon nanotubes (SWCNTs) with metal-free tetra(hydroxy)phthalocyanine (OH)₄PcHH). The value of the hydrodynamic radius of separate particles after (OH)₄PcHH binding increased from 288 ± 55 nm to 350 ± 60 nm, which confirms the attachment of phthalocyanine complexes to nanotubes. The third harmonic of a Nd:YAG nanosecond laser (355 nm, 20 ns) was used to study the nonlinear optical response. Based on a Z-scan with open-aperture and input-output dependence curves, third-order nonlinear optical absorption coefficients of 149, 236, and 229 cm/GW were obtained for dispersions of composites of SWCNTs and (OH)₄PcHH in water, dimethylformamide (DMF), and dimethylsulfoxide (DMSO), respectively. Threshold values did not exceed 100 mJ/cm². The Z-scan showed a gradual decrease in the duration of the laser pulse by 53%; however, near the focus, there was a sharp increase in the duration of the transmitted pulse, reaching a value of 29 ns in z = 0. This phenomenon confirms the occurrence of reverse saturable absorption in the investigated media and can be used in photonic devices to control the temporal characteristics of the signal. Thus, the possibility of protection of sensitive photonic devices and human eyes from nanosecond laser pulses in the near UV range by nanodispersed liquid media based on composites of SWCNTs with (OH)₄PcHH has been discussed in this paper. Full article

(This article belongs to the Special Issue Nonlinear and Ultrafast Optics: Fundamentals and Applications)

► Show Figures

Graphical abstract

11 pages, 2874 KiB

Open AccessEditor’s ChoiceCommunication

Valley-Selective Polarization in Twisted Bilayer Graphene Controlled by a Counter-Rotating Bicircular Laser Field

by Jiayin Chen, Candong Liu and Ruxin Li

Photonics 2023, 10(5), 516; https://doi.org/10.3390/photonics10050516 - 1 May 2023

Cited by 6 | Viewed by 2564

Abstract

The electron valley pseudospin in two-dimensional hexagonal materials is a crucial degree of freedom for achieving their potential application in valleytronic devices. Here, bringing valleytronics to layered van der Waals materials, we theoretically investigate lightwave-controlled valley-selective excitation in twisted bilayer graphene (tBLG) with [...] Read more.

The electron valley pseudospin in two-dimensional hexagonal materials is a crucial degree of freedom for achieving their potential application in valleytronic devices. Here, bringing valleytronics to layered van der Waals materials, we theoretically investigate lightwave-controlled valley-selective excitation in twisted bilayer graphene (tBLG) with a large twist angle. It is demonstrated that the counter-rotating bicircular light field, consisting of a fundamental circularly-polarized pulse and its counter-rotating second harmonic, can manipulate the sub-cycle valley transport dynamics by controlling the relative phase between two colors. In comparison with monolayer graphene, the unique interlayer coupling of tBLG renders its valley selectivity highly sensitive to duration, leading to a noticeable valley asymmetry that is excited by single-cycle pulses. We also describe the distinct signatures of the valley pseudospin change in terms of observing the valley-selective circularly-polarized high-harmonic generation. The results show that the valley pseudospin dynamics can still leave visible fingerprints in the modulation of harmonic signals with a two-color relative phase. This work could assist experimental researchers in selecting the appropriate protocols and parameters to obtain ideal control and characterization of valley polarization in tBLG. Full article

(This article belongs to the Special Issue Light Control and Particle Manipulation)

► Show Figures

Figure 1

12 pages, 1968 KiB

Open AccessEditor’s ChoiceReview

Toward Practical Optical Phased Arrays through Grating Antenna Engineering

by Youqiang Shuai, Zhiping Zhou and Hui Su

Photonics 2023, 10(5), 520; https://doi.org/10.3390/photonics10050520 - 1 May 2023

Cited by 8 | Viewed by 4208

Abstract

In recent years, using silicon-based waveguide grating antennas in optical phased array has become a research focus. To date, this technique has not been widely implemented in practical applications. In this paper, the basic principle of a waveguide grating antenna is described, and [...] Read more.

In recent years, using silicon-based waveguide grating antennas in optical phased array has become a research focus. To date, this technique has not been widely implemented in practical applications. In this paper, the basic principle of a waveguide grating antenna is described, and the researches on effective length, uniform emission and the directionality of diffraction are summarized. Through analysis, it is found that there is a trend to prepare grating antennas by using a SiN/Si hybrid integrated platform. A novel design of grating antenna using the hybrid integration technique is proposed. It is convenient to match with the antenna front-end components on the structural level and is more practical. Full article

(This article belongs to the Special Issue Advances in Optical 3D Integration)

► Show Figures

Figure 1

10 pages, 1031 KiB

Open AccessEditor’s ChoiceCommunication

Single Photon Approach for Chirality Sensing

by Fabrizio Sgobba, Arianna Elefante, Stefano Dello Russo, Mario Siciliani de Cumis and Luigi Santamaria Amato

Photonics 2023, 10(5), 512; https://doi.org/10.3390/photonics10050512 - 28 Apr 2023

Viewed by 2512

Abstract

We developed a high sensitivity optical sensor for circular birefringence using a heralded photon source. The sensor can be employed for chirality measurements and, being based on single photons, can be exploited for fragile biological sample or in metrological applications where the light [...] Read more.

We developed a high sensitivity optical sensor for circular birefringence using a heralded photon source. The sensor can be employed for chirality measurements and, being based on single photons, can be exploited for fragile biological sample or in metrological applications where the light intensity must be kept as low as possible. We found the best operational condition; then, we calibrated the sensor and tested its performance up to a very long acquisition time, obtaining excellent stability and a sub-ppm birefringence detection limit (for a 100 μm sample), thus paving the way for fundamental physics test as well. Full article

(This article belongs to the Topic Advances in Optical Sensors)

► Show Figures

Figure 1

12 pages, 5623 KiB

Open AccessEditor’s ChoiceCommunication

Surface-Emitting Lasers with Surface Metastructures

by Anjin Liu, Jing Zhang, Chenxi Hao, Minglu Wang and Wanhua Zheng

Photonics 2023, 10(5), 509; https://doi.org/10.3390/photonics10050509 - 27 Apr 2023

Cited by 5 | Viewed by 2403

Abstract

Vertical-cavity surface-emitting lasers (VCSELs) have been widely used in consumer electronics, light detection and ranging, optical interconnects, atomic sensors, and so on. In this paper, a VCSEL with the surface metastructure like one-dimensional high-contrast grating (HCG), based on the HCG-DBR vertical cavity, was [...] Read more.

Vertical-cavity surface-emitting lasers (VCSELs) have been widely used in consumer electronics, light detection and ranging, optical interconnects, atomic sensors, and so on. In this paper, a VCSEL with the surface metastructure like one-dimensional high-contrast grating (HCG), based on the HCG-DBR vertical cavity, was first designed and fabricated. The polarization characteristic of the HCG-VCSEL were experimentally studied. The p-doped top 4-pair DBR for the current spreading and the direction shift between the HCG and the elliptical oxide aperture may result in a low orthogonal polarization suppression ratio in the HCG-VCSEL. Then, the Bloch surface wave surface-emitting laser (BSW-SEL), based on the HCG-DBR metastructure, is proposed for single-mode, high-efficiency, and high-power output with a low divergence angle. The mode field and the far field profile of the BSW-SEL are calculated for verification. The surface-emitting lasers with surface metastructures are useful for the sensing applications and optical interconnects. Full article

(This article belongs to the Special Issue Nanophotonics Pioneer: Prof. Dr. Dieter Bimberg ‘Green Photonics Networks: From VCSELs to Nanophotonics’)

► Show Figures

Figure 1

12 pages, 3731 KiB

Open AccessEditor’s ChoiceArticle

High-Efficiency and Compact Polarization-Insensitive Multi-Segment Linear Silicon Nitride Edge Coupler

by Yuhao Zhang, Yi Liu, Xuhua Cao, Li Wang, Chunyuan Mu, Ming Li, Ninghua Zhu and Wei Chen

Photonics 2023, 10(5), 510; https://doi.org/10.3390/photonics10050510 - 27 Apr 2023

Cited by 7 | Viewed by 3519

Abstract

Edge couplers are widely utilized in photonic integrated circuits and are vital for ensuring efficient chip-to-fiber coupling. In this paper, we present a high-efficiency and compact polarization-insensitive multi-segment linear silicon nitride edge coupler for coupling to high numerical aperture fibers. By optimizing the [...] Read more.

Edge couplers are widely utilized in photonic integrated circuits and are vital for ensuring efficient chip-to-fiber coupling. In this paper, we present a high-efficiency and compact polarization-insensitive multi-segment linear silicon nitride edge coupler for coupling to high numerical aperture fibers. By optimizing the thickness of the up cladding and introducing air slots in the transverse direction, we have further modified the limiting effect of the mode field. This innovative edge coupler scheme boasts a compact structure and is compatible with existing mature standard processes, with a total length of only 38 μm. We numerically demonstrate that the proposed edge coupler exhibits a low coupling loss of 0.22 dB/0.31 dB for TE/TM modes at λ = 1550 nm. Furthermore, the proposed coupler displays high wavelength insensitivity within the range of 1400–1850 nm and maintains a coupling loss of less than 0.2 dB with a manufacturing deviation of ±20 nm. Full article

(This article belongs to the Topic Optical and Optoelectronic Materials and Applications)

► Show Figures

Figure 1

10 pages, 3695 KiB

Open AccessEditor’s ChoiceCommunication

Theoretical Study on Dual-Function Optical Phased Array of LiDAR and Optical Wireless Communication Based on Optically Injection-Locked Semiconductor Lasers

by Anh-Hang Nguyen, Hyo-Sang Jeong, Hyungsik Shin and Hyuk-Kee Sung

Photonics 2023, 10(5), 498; https://doi.org/10.3390/photonics10050498 - 26 Apr 2023

Viewed by 1950

Abstract

Light detection and ranging (LiDAR) and optical wireless communication (OWC) are in high demand and rapidly developing owing to the explosive growth of smart systems that require automotive and mobile devices. Optical phased arrays (OPA) have become a key technology in LiDAR and [...] Read more.

Light detection and ranging (LiDAR) and optical wireless communication (OWC) are in high demand and rapidly developing owing to the explosive growth of smart systems that require automotive and mobile devices. Optical phased arrays (OPA) have become a key technology in LiDAR and OWC owing to their nonmechanical beam steering capabilities. However, using separate LiDAR and OWC platforms in one system creates problems, such as spectrum congestion, resource consumption, and high complexity. We propose a dual-function OPA that enables LiDAR and OWC to function on a single platform based on the simultaneous amplitude and phase modulation of optically injection-locked semiconductor lasers. We numerically demonstrated that the primary LiDAR and secondary OWC function simultaneously by independent control of the main and side lobes in the OPA signal. The variation in side lobe levels is controlled at 20 or 25 dB to realize low- and high-level data for OWC function as well as maintaining the main beam LiDAR function. We successfully achieved wide-opening eye patterns of 10 Gbps data transmission of the OWC operation. Full article

(This article belongs to the Special Issue Challenges and Opportunities in Optical and Wireless Communication Networks)

► Show Figures

Figure 1

24 pages, 2183 KiB

Open AccessEditor’s ChoiceReview

The Optical Inverse Problem in Quantitative Photoacoustic Tomography: A Review

by Zeqi Wang, Wei Tao and Hui Zhao

Photonics 2023, 10(5), 487; https://doi.org/10.3390/photonics10050487 - 24 Apr 2023

Cited by 12 | Viewed by 2963

Abstract

Photoacoustic tomography is a fast-growing biomedical imaging modality that combines rich optical contrast with a high acoustic resolution, at depths in tissues. Building upon the foundation of this technique, novel quantitative photoacoustic tomography fully leverages its advantages while further delivering improved quantification capabilities [...] Read more.

Photoacoustic tomography is a fast-growing biomedical imaging modality that combines rich optical contrast with a high acoustic resolution, at depths in tissues. Building upon the foundation of this technique, novel quantitative photoacoustic tomography fully leverages its advantages while further delivering improved quantification capabilities to produce high-accuracy concentration estimates, which has attracted substantial research interest in recent years. The kernel challenge associated with quantitative photoacoustic tomography is an optical inverse problem aiming to recover the absorption coefficient distribution from the conventional photoacoustic image. Although the crucial importance of the optical inversion has been widely acknowledged, achieving it has remained a persistent challenge due to the inherent non-linearity and non-uniqueness. In the past decade, numerous methods were proposed and have made noticeable progress in addressing this concern. Nevertheless, a review has been conspicuously absent for a long time. Aiming to bridge this gap, the present study comprehensively investigates the recent research in this field, and methods identified with significant value are introduced in this paper. Moreover, all included methods are systematically classified based on their underlying principles. Finally, we summarize each category and highlight its remaining challenges and potential future research directions. Full article

(This article belongs to the Special Issue Photoacoustic Imaging: Applications, Approaches, and Systems)

► Show Figures

Figure 1

11 pages, 1987 KiB

Open AccessEditor’s ChoiceCommunication

All-Fiber Laser Feedback Interferometry for Sequential Sensing of In-Plane and Out-of-Plane Displacements

by Zhanwu Xie, Meng Zhang, Jie Li, Wei Xia and Dongmei Guo

Photonics 2023, 10(4), 480; https://doi.org/10.3390/photonics10040480 - 21 Apr 2023

Cited by 1 | Viewed by 1747

Abstract

In this paper, an all-fiber laser feedback interferometer (LFI) with a diffraction grating was developed for sequential measurement of in-plane and out-of-plane displacements without changing the optical arrangement. When the light emitted from an erbium-doped fiber ring laser is incident on a reflection [...] Read more.

In this paper, an all-fiber laser feedback interferometer (LFI) with a diffraction grating was developed for sequential measurement of in-plane and out-of-plane displacements without changing the optical arrangement. When the light emitted from an erbium-doped fiber ring laser is incident on a reflection grating at the Littrow angle, the diffracted light will return into the laser cavity along the original path, thus generating laser feedback interference. Experimental results reveal that the all-fiber system could achieve a precision of 40 nm in both in-plane and out-of-plane displacements sensing. Compared with the traditional all-fiber LFI, the proposed sensing system transfers the measuring scale from laser wavelength to grating period, and it has the advantages of good anti-interference performance and reliability. Full article

(This article belongs to the Special Issue Advanced Photonic Sensing and Measurement)

► Show Figures

Figure 1

15 pages, 5576 KiB

Open AccessEditor’s ChoiceReview

GaN-Based VCSELs with A Monolithic Curved Mirror: Challenges and Prospects

by Tatsushi Hamaguchi

Photonics 2023, 10(4), 470; https://doi.org/10.3390/photonics10040470 - 20 Apr 2023

Cited by 12 | Viewed by 4508

Abstract

In this paper, we introduce how gallium nitride-based (GaN-based) VCSELs with curved mirrors have evolved. The discussion starts with reviewing the fundamentals of VCSELs and GaN-based materials and then introducing the curved-mirror cavity’s principle and history and the latest research where the structure [...] Read more.

In this paper, we introduce how gallium nitride-based (GaN-based) VCSELs with curved mirrors have evolved. The discussion starts with reviewing the fundamentals of VCSELs and GaN-based materials and then introducing the curved-mirror cavity’s principle and history and the latest research where the structure is applied to GaN-based materials to form VCSELs. We prepared these parts so that readers understand how VCSELs with this cavity work and provide excellent characteristics such as efficiency, life, stabilized mode behavior, etc. Finally, we discussed the challenges and prospects of these devices by touching on their potential applications. Full article

(This article belongs to the Special Issue Nanophotonics Pioneer: Prof. Dr. Dieter Bimberg ‘Green Photonics Networks: From VCSELs to Nanophotonics’)

► Show Figures

Figure 1

17 pages, 5386 KiB

Open AccessEditor’s ChoiceReview

Progress in Short Wavelength Energy-Efficient High-Speed Vertical-Cavity Surface-Emitting Lasers for Data Communication

by Si-Cong Tian, Mansoor Ahamed and Dieter Bimberg

Photonics 2023, 10(4), 410; https://doi.org/10.3390/photonics10040410 - 6 Apr 2023

Cited by 12 | Viewed by 3307

Abstract

The current progress of energy-efficient high-speed VCSELs based on GaAs substrates is presented. Novel approaches for the design of VCSELs are presented, potentially leading to larger bandwidth bit rates and lower power consumption. The first approach is based on the optimization of the [...] Read more.

The current progress of energy-efficient high-speed VCSELs based on GaAs substrates is presented. Novel approaches for the design of VCSELs are presented, potentially leading to larger bandwidth bit rates and lower power consumption. The first approach is based on the optimization of the VCSEL photon lifetime. The second one introduces a novel design based on oxidizing the apertures from multiple etched holes of varying geometries. These designs are also essential for improving the energy efficiency of future modules by optimizing the match of the electronic driver and the photonic device. Full article

(This article belongs to the Special Issue Nanophotonics Pioneer: Prof. Dr. Dieter Bimberg ‘Green Photonics Networks: From VCSELs to Nanophotonics’)

► Show Figures

Figure 1

12 pages, 5621 KiB

Open AccessEditor’s ChoiceCommunication

Interpretation of Localized Surface Plasmonic Resonances of Gold Nanoparticles Covered by Polymeric Coatings

by Alkeos Stamatelatos, Maria Tsarmpopoulou, Dimitrios Geralis, Alexandros G. Chronis, Vagelis Karoutsos, Dimitrios Ntemogiannis, Dionysios M. Maratos, Spyridon Grammatikopoulos, Mihail Sigalas and Panagiotis Poulopoulos

Photonics 2023, 10(4), 408; https://doi.org/10.3390/photonics10040408 - 5 Apr 2023

Cited by 4 | Viewed by 6138

Abstract

Plasmonic materials currently have a plethora of applications. How would a dielectric matrix, such as diblock copolymers, tune plasmonic properties? In this work, self-assembled gold nanoparticles were fabricated in medium vacuum conditions on heated Corning glass substrates (kept at 440 ^°C) under [...] Read more.

Plasmonic materials currently have a plethora of applications. How would a dielectric matrix, such as diblock copolymers, tune plasmonic properties? In this work, self-assembled gold nanoparticles were fabricated in medium vacuum conditions on heated Corning glass substrates (kept at 440 ^°C) under the coexistence of argon and air by means of DC magnetron sputtering. These samples were compared structurally and optically to samples deposited at room temperature and post annealed. Subsequently, the better of the two preparations, those deposited on heated glass, were covered with three different polymers, namely: Polystyrene-block-polybutadiene-block-polystyrene (PS-b-PBD-b-PS); Polystyrene-co-methyl methacrylate (PS-co-PMMA); and Polystyrene-block-polyisoprene-block-polystyrene (PS-b-PI-b-PS), by means of spin coating. Localized surface plasmon resonances were recorded and analyzed, respectively, for polymer-covered gold nanoparticles, with the width, intensity, and position of the resonances changing according to multiple factors, such as the nanoparticles size and the refractive index of each polymer. Lastly, for purposes of justification and comparison with the experimental results, rigorous theoretical calculations have been carried out. Full article

(This article belongs to the Special Issue Integrated Plasmonic Devices)

► Show Figures

Figure 1

17 pages, 3410 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Requirements and Solutions for Robust Beam Alignment in Fiber-Coupled Free-Space Optical Systems

by Manuel M. Freitas, Marco A. Fernandes, Paulo P. Monteiro, Fernando P. Guiomar and Gil M. Fernandes

Photonics 2023, 10(4), 394; https://doi.org/10.3390/photonics10040394 - 2 Apr 2023

Cited by 8 | Viewed by 4601

Abstract

The continuous growth of Internet data traffic is pushing the current radio-frequency wireless technologies up to their physical limits. To overcome the upcoming bandwidth bottleneck, Free-Space Optics (FSO) is currently deemed as a key breakthrough toward next-generation ultra-high-capacity wireless links. Despite its numerous [...] Read more.

The continuous growth of Internet data traffic is pushing the current radio-frequency wireless technologies up to their physical limits. To overcome the upcoming bandwidth bottleneck, Free-Space Optics (FSO) is currently deemed as a key breakthrough toward next-generation ultra-high-capacity wireless links. Despite its numerous advantages, FSO also entails several particular challenges regarding the mitigation of the stochastic impairments induced by turbulence and the strict alignment requirements. One of the main issues of FSO communication systems is the mitigation of pointing errors and angle-of-arrival (AoA) fluctuations, which arise from misalignments induced by atmospheric turbulence and vibrations at the transmitting and receiving stations. A common approach to mitigate the impact of pointing errors is the use of an acquisition, tracking and pointing (ATP) system on one or both ends of the FSO link. In this paper, we present a characterization of the pointing errors and the AoA impact on the power budget of the FSO link to quantify the misalignment impairments. Afterwards, we experimentally demonstrate an FSO link with an ATP mechanism at both ends, managed by a control plane that enables the continuous and accurate alignment of the FSO link. To increase the misalignment tolerance, the ATP mechanism comprises two stages: the first one is based on a spatial diversity method provided by a quadrant detector, while the second stage maximizes the optical received power. Lastly, the impact of the beam misalignment on the achievable information rate of a coherent optical wireless system is theoretically addressed and characterized. Full article

(This article belongs to the Special Issue Free Space Optics-Based 6G Non-terrestrial Networks)

► Show Figures

Figure 1

20 pages, 16085 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Comparative Performance Analysis of Femtosecond-Laser-Written Diode-Pumped Pr:LiLuF₄ Visible Waveguide Lasers

by Davide Baiocco, Ignacio Lopez-Quintas, Javier R. Vázquez de Aldana, Mauro Tonelli and Alessandro Tredicucci

Photonics 2023, 10(4), 377; https://doi.org/10.3390/photonics10040377 - 29 Mar 2023

Cited by 8 | Viewed by 2370

Abstract

In this work, we present the operation of a femtosecond-laser-written diode-pumped visible waveguide laser based on praseodymium-doped lithium lutetium fluoride. The refractive index modification induced by the femtosecond laser in the crystal exhibits an anisotropic behavior, thus enabling the fabrication of different types [...] Read more.

In this work, we present the operation of a femtosecond-laser-written diode-pumped visible waveguide laser based on praseodymium-doped lithium lutetium fluoride. The refractive index modification induced by the femtosecond laser in the crystal exhibits an anisotropic behavior, thus enabling the fabrication of different types of waveguides from single-track structures to stress-induced waveguides and depressed cladding structures. All the waveguides were characterized by realizing transmission measurements and the waveguide design was optimized to obtain extremely low propagation losses, equal to 0.12

d

B

/

c

m

. Lasing has been achieved at 604

n

m

and 721

n

m

from different waveguides. In addition, stable continuous-wave lasing at 698

n

m

has been obtained in a depressed cladding waveguide. This wavelength corresponds to the one needed for the transition of the atomic clock based on the neutral strontium atom. In the end, we report the observation of laser emission at 645

n

m

from a depressed cladding waveguide. Full article

(This article belongs to the Special Issue Novel Applications of Solid-State Laser and Future Prospects)

► Show Figures

Figure 1

11 pages, 10853 KiB

Open AccessEditor’s ChoiceCommunication

Fiber-Fed 3D Printing of Germanate Glass Optics

by Zhihan Hong, Tao Luo, Shibin Jiang and Rongguang Liang

Photonics 2023, 10(4), 378; https://doi.org/10.3390/photonics10040378 - 29 Mar 2023

Cited by 8 | Viewed by 3004

Abstract

In recent years, 3D printing glass optics has gained massive attention in industry and academia since glass could be an ideal material to make optical elements, including the lens. However, the limitation of materials and printing methods has prevented 3D printing glass optics [...] Read more.

In recent years, 3D printing glass optics has gained massive attention in industry and academia since glass could be an ideal material to make optical elements, including the lens. However, the limitation of materials and printing methods has prevented 3D printing glass optics progress. Therefore, we have developed a novel printing strategy for germanate glass printing instead of pure silica. Moreover, compared with traditional multi-component quartz glass, germanate glass has unmatched advantages for its mid-infrared (MIR) transparency and outstanding visible light imaging performance. Furthermore, compared with non-oxide glass (fluoride glass and chalcogenide glass), germanate glass has much better mechanical, physical, and chemical properties and a high refractive index. Germanate glass has been widely applied in remote sensing, ranging, environmental detection, and biomedical detection. However, it is difficult to shape, cast, polish, and grind for optical and photonics applications such as imaging optics and laser-collimation optics. These drawbacks have made germanate glass inaccessible to complex optical elements and greatly increased their cost. In this report, we use germanate glass fibers with a diameter of 125 µm based on fiber-fed laser heating technology to fabricate an mm-size optical application. In this paper, we combine the fiber-fed laser heating technology with an optimized temperature control process to manufacture high-precision optical elements. Germanate glass optics can be printed with excellent visible light and IR transparency and a smooth surface with roughness under 4 nm. By optimizing the layer-by-layer 3D printing process and the thermal feedback in the printing process, we avoid cracks and minimize surface deformation. This work shows the possibility of the mm-size glass optical elements 3D printing and widens its application for IR optics. Full article

(This article belongs to the Special Issue Direct Laser Writing for Photonic Applications)

► Show Figures

Figure 1

11 pages, 3190 KiB

Open AccessEditor’s ChoiceArticle

Bandwidth Expansion of Zero-Power-Consumption Visible Light Communication System

by Yiwu Xu, Xiongbin Chen and Yufeng Wang

Photonics 2023, 10(4), 376; https://doi.org/10.3390/photonics10040376 - 28 Mar 2023

Cited by 2 | Viewed by 2187

Abstract

The data rate of the zero-power-consumption visible light communication system is limited due to the solar cell bandwidth limit. A new method has been proposed to improve the data rate of communication systems. The predistortion circuit based on the nonlinear compensation principle of [...] Read more.

The data rate of the zero-power-consumption visible light communication system is limited due to the solar cell bandwidth limit. A new method has been proposed to improve the data rate of communication systems. The predistortion circuit based on the nonlinear compensation principle of the communication system is added between the transmitter and the receiver to equalize the amplitude–frequency response of the solar cell. Based on the measurement of the original amplitude–frequency response of the system, the zero-pole of the predistortion circuit is calculated, and an accurate equalization circuit is designed by using separate components to expand the 3 dB bandwidth of the system. The test results show that the 3 dB bandwidth of the visible light self-powered communication system is increased from 85 kHz to 750 kHz, and the system bandwidth is expanded by nearly 10 times. Full article

(This article belongs to the Special Issue Visible Light Communications)

► Show Figures

Figure 1

15 pages, 4633 KiB

Open AccessEditor’s ChoiceArticle

Analysis of the Emission Features in CdSe/ZnS Quantum Dot-Doped Polymer Fibers

by Xuefeng Peng, Zhijian Wu, Chen Ye, Yang Ding and Wei Liu

Photonics 2023, 10(3), 327; https://doi.org/10.3390/photonics10030327 - 18 Mar 2023

Cited by 4 | Viewed by 2634

Abstract

The emission features of Cdse/ZnS quantum dots doped step-index polymer optical fibers are computationally analyzed in this paper. Spontaneous emission and amplified spontaneous emission were calculated by a theoretical model based on the rate equations in terms of time, fiber length, and wavelength. [...] Read more.

The emission features of Cdse/ZnS quantum dots doped step-index polymer optical fibers are computationally analyzed in this paper. Spontaneous emission and amplified spontaneous emission were calculated by a theoretical model based on the rate equations in terms of time, fiber length, and wavelength. All the calculated parameters are derived from experiments. Through the comparative analysis of the calculated and experimental results of spontaneous emission, we found that the pump power and overlap between the emission and absorption cross-sections may be the two main reasons for the red shift of the output spectra. When the pump power exceeds the threshold of amplified spontaneous emission, the width of the output spectra will rapidly decrease, the output wavelength will fall back toward the maximum emission cross-section, and the final output wavelength will still be affected by the doping concentration and pump power, while amplified spontaneous emission will not be generated when the total number of doped quantum dots is less than 1.27 × 10¹². Full article

(This article belongs to the Special Issue Advances in Photonic Materials and Technologies)

► Show Figures

Figure 1

14 pages, 4464 KiB

Open AccessEditor’s ChoiceArticle

Adaptive Weighted K-Nearest Neighbor Trilateration Algorithm for Visible Light Positioning

by Kaiyao Wang, Yi He, Xinpeng Huang and Zhiyong Hong

Photonics 2023, 10(3), 319; https://doi.org/10.3390/photonics10030319 - 16 Mar 2023

Cited by 4 | Viewed by 2790

Abstract

An adaptive weighted K-nearest neighbor (AWKNN) trilateration positioning algorithm fused with the channel state information (CSI) is proposed to optimize the accuracy of the visible light positioning. The core concept behind this algorithm is to combine the WKNN algorithm with ranging based on [...] Read more.

An adaptive weighted K-nearest neighbor (AWKNN) trilateration positioning algorithm fused with the channel state information (CSI) is proposed to optimize the accuracy of the visible light positioning. The core concept behind this algorithm is to combine the WKNN algorithm with ranging based on the CSI. The direct path distance estimated by the CSI is utilized to construct a position set consisting of multiple positions and a corresponding distance database containing multiple distance vectors. The error parameters of the weighted combinations of different distance vectors are calculated iteratively to evaluate the impact of different K-values and weights on the positioning accuracy. The proposed algorithm can achieve high-precision trilateration positioning by adaptively selecting the K-value and weight. A typical 4 m × 4 m × 3 m indoor multipath scene with four LEDs is established to simulate the positioning performance. The simulation results reveal that the mean error of the CSI-based AWKNN algorithm achieves 1.84 cm, with a root mean square error (RMSE) of 2.13 cm. Compared with the CSI-based least squares (LS) method, the CSI-based nonlinear LS method, and the CSI-based WKNN method, the average error of this method is decreased by 29%, 16%, and 17%, whereas the RMSE is reduced by 35%, 14%, and 19%. Full article

(This article belongs to the Special Issue Optical Wireless Communications and Applications)

► Show Figures

Figure 1

6 pages, 5566 KiB

Open AccessEditor’s ChoiceCommunication

Ultrabroadband OPA in YCOB with a sub-ps Pump Source

by Hugo Pires, Joana Alves, Victor Hariton, Mario Galletti, Celso João and Gonçalo Figueira

Photonics 2023, 10(3), 253; https://doi.org/10.3390/photonics10030253 - 28 Feb 2023

Cited by 2 | Viewed by 3579

Abstract

We demonstrate the broadband optical parametric amplification of near-infrared laser pulses using a single yttrium calcium oxyborate (YCOB) crystal pumped in a noncollinear geometry by a sub-picosecond, milijoule-level source. The crystal uses an optimized orientation for phase matching outside of the principal planes, [...] Read more.

We demonstrate the broadband optical parametric amplification of near-infrared laser pulses using a single yttrium calcium oxyborate (YCOB) crystal pumped in a noncollinear geometry by a sub-picosecond, milijoule-level source. The crystal uses an optimized orientation for phase matching outside of the principal planes, enabling ultrabroadband amplification (gain of ∼800) in the range of 750–950 nm and supporting down to 7 fs pulses. Full article

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

► Show Figures

Figure 1

16 pages, 1726 KiB

Open AccessEditor’s ChoiceArticle

Towards Microbial Food Safety of Sprouts: Photodynamic Decontamination of Seeds

by Andreas Fellner, Christoph Hamminger, Michael Fefer, Jun Liu and Kristjan Plaetzer

Photonics 2023, 10(3), 239; https://doi.org/10.3390/photonics10030239 - 22 Feb 2023

Cited by 1 | Viewed by 3043

Abstract

The climate crisis is one of the biggest challenges for humanity in the 21st century. Production and consumption of meat contributes to global warming by causing emissions of climate-relevant gases. Freshly grown sprouts are part of an alternative, as they are less polluting [...] Read more.

The climate crisis is one of the biggest challenges for humanity in the 21st century. Production and consumption of meat contributes to global warming by causing emissions of climate-relevant gases. Freshly grown sprouts are part of an alternative, as they are less polluting but still a nutritious food. However, warm humid sprouting conditions may cause pathogenic microorganisms to thrive. Decontamination methods for raw sprouts are therefore relevant. Photodynamic Inactivation (PDI) is a novel approach that uses photoactivatable molecules (photosensitisers, PS) and visible or near-infrared light to produce reactive oxygen species (ROS). These ROS kill microorganisms by oxidative processes. Here, we test the application of PDI based on sodium-magnesium-chlorophyllin (Chl, approved as food additive E140) for photo-decontamination of mung bean, radish, and buckwheat seeds. Seeds were contaminated with Listeria innocua, serving as a model system for Listeria monocytogenes, subjected to PDI using an LED array with 395 nm and tested for remaining bacterial contamination by CFU counting. PDI based on 100 µM Chl reduces the bacterial load of mung bean and radish seeds by 99.9% (radiant exposure 56.4 J/cm² and 28.2 J/cm², respectively), and of buckwheat seeds by <90% reduction after illumination with 28.2 J/cm². Neither weight nor the germination rates of seeds are affected by PDI. Interestingly, the effect of PDI on seeds is partially maintained on stored sprouts after germination: The bacterial load on mung bean sprouts is reduced by more than 99.9% after phototreatment of seeds with 100 µM Chl and illumination at 56.4 J/cm². In conclusion, we suggest PDI based on Chl as an effective and biocompatible method for the decontamination of seeds and sprouts for human consumption from Listeria. Full article

(This article belongs to the Special Issue Light-Based Technologies and Spectroscopic Techniques for Photo-Sensing and Photoinactivation of Microorganisms, Virus, and Cancer Cells)

► Show Figures

Graphical abstract

19 pages, 1619 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Revisiting Experimental Signatures of the Ponderomotive Force

by Bjorn Manuel Hegelich, Lance Labun and Ou Z. Labun

Photonics 2023, 10(2), 226; https://doi.org/10.3390/photonics10020226 - 20 Feb 2023

Cited by 5 | Viewed by 4216

Abstract

The classical theory of single-electron dynamics in focused laser pulses is the foundation of both the relativistic ponderomotive force (RPF), which underlies models of laser-collective-plasma dynamics, and the discovery of novel strong-field radiation dynamics. Despite this bedrock importance, consensus eludes the community as [...] Read more.

The classical theory of single-electron dynamics in focused laser pulses is the foundation of both the relativistic ponderomotive force (RPF), which underlies models of laser-collective-plasma dynamics, and the discovery of novel strong-field radiation dynamics. Despite this bedrock importance, consensus eludes the community as to whether acceleration of single electrons in vacuum has been observed in experimental conditions. We analyze an early experiment on the RPF with respect to several features that were neglected in modeling and that can restore consistency between theory predictions and experimental data. The right or wrong pulse profile function, laser parameters, or initial electron distribution can each make or break the agreement between predictions and data. The laser phase at which the electron’s interaction with the pulse begins has a large effect, explaining why much larger energies are achieved by electrons liberated in the focal region by photoionization from high-Z atoms and by electrons ejected from a plasma mirror. Finally, we compute the difference in a typical electron spectrum arising from fluctuating focal spot size in state-of-the-art ultra-relativistic laser facilities. Our results emphasize the importance of thoroughly characterizing laser parameters in order to achieve quantitatively accurate predictions and the precision required for discovery science. Full article

(This article belongs to the Special Issue Progress in Laser Accelerator and Future Prospects)

► Show Figures

Figure 1

16 pages, 2857 KiB

Open AccessEditor’s ChoiceReview

Development and Application of THz Gyrotrons for Advanced Spectroscopic Methods

by Svilen Sabchevski and Mikhail Glyavin

Photonics 2023, 10(2), 189; https://doi.org/10.3390/photonics10020189 - 10 Feb 2023

Cited by 14 | Viewed by 3103

Abstract

Nowadays, gyrotrons are used in numerous and diverse fields of research and technology. Their most prominent application is to electron cyclotron resonance plasma heating and current drive-in reactors for controlled thermonuclear fusion. Another matured field is the thermal microwave treatment of materials in [...] Read more.

Nowadays, gyrotrons are used in numerous and diverse fields of research and technology. Their most prominent application is to electron cyclotron resonance plasma heating and current drive-in reactors for controlled thermonuclear fusion. Another matured field is the thermal microwave treatment of materials in industrial-grade gyrotron-based technological systems. The unique spectral properties of gyrotron radiation, frequency tunability, and the possibility for precise control and modulation of both the output power and frequency have made the gyrotrons attractive and appropriate radiation sources for various novel advanced spectroscopic techniques. Among them are ESR (electron spin resonance), NMR-DNP (nuclear magnetic resonance with a signal enhancement through dynamic nuclear polarization), XDMR (X-ray detected magnetic resonance), acoustic molecular spectroscopy, as well as high-precision spectroscopy for measuring the SFS (super-fine splitting of the energy levels of positronium). In this review paper, we present both the current status and the most remarkable recent achievements of these methods implemented in gyrotron-based spectroscopy systems and discuss the main trends in the development of their dedicated radiation sources operating in the THz frequency range. Full article

(This article belongs to the Special Issue Terahertz Spectroscopy and Imaging)

► Show Figures

Figure 1

19 pages, 13958 KiB

Open AccessEditor’s ChoiceArticle

Observation of Giant Angular Goos-Hanchen Shifts Enhanced by Surface Plasmon Resonance in Subwavelength Grating

by Nikolai I. Petrov, Yuri M. Sokolov, Vladimir V. Stoiakin, Viktor A. Danilov, Vladimir V. Popov and Boris A. Usievich

Photonics 2023, 10(2), 180; https://doi.org/10.3390/photonics10020180 - 8 Feb 2023

Cited by 8 | Viewed by 2349

Abstract

The effect of the Goos-Hanchen (GH) angular shift for a visible light beam under the excitation of surface plasmon resonance (SPR) in a metal subwavelength grating has been investigated. Extremely large angular GH shifts have been demonstrated for a subwavelength grating with an [...] Read more.

The effect of the Goos-Hanchen (GH) angular shift for a visible light beam under the excitation of surface plasmon resonance (SPR) in a metal subwavelength grating has been investigated. Extremely large angular GH shifts have been demonstrated for a subwavelength grating with an optimal depth. The high sensitivity of the beam shape transformation and the GH shift to a change in the angle of incidence near the SPR has been shown by rigorous electromagnetic simulation and demonstrated experimentally. The focusing of the reflected beam near the subwavelength grating surface has been demonstrated. Full article

(This article belongs to the Special Issue Integrated Plasmonic Devices)

► Show Figures

Figure 1

14 pages, 6531 KiB

Open AccessEditor’s ChoiceArticle

Distortion Detection of Lithographic Projection Lenses Based on Wavefront Measurement

by Tian Li, Jian Wang, Shaolin Zhou, Haiyang Quan, Lei Chen, Junbo Liu, Jing Du, Xianchang Zhu and Song Hu

Photonics 2023, 10(2), 168; https://doi.org/10.3390/photonics10020168 - 4 Feb 2023

Viewed by 4755

Abstract

As with the decreasing feature size prompted by Moore’s law and the continuous technological advancements in the semiconductor industry, the distortion of the projection lens is an important factor that affects the overlay. In this paper, we propose a wavefront-measurement-based method to detect [...] Read more.

As with the decreasing feature size prompted by Moore’s law and the continuous technological advancements in the semiconductor industry, the distortion of the projection lens is an important factor that affects the overlay. In this paper, we propose a wavefront-measurement-based method to detect the projection lens distortion in the lithographic system. By normalizing the coordinates of the Shack–Hartmann system with the image displacements represented in the Z₂ and Z₃ terms of Zernike coefficients, the offsets between the actual image points and the ideal image points can be determined. By offset collection at an array of 7 × 7 field points to establish an overdetermined system of equations, the proposed method can simultaneously detect the distortions of translation, magnification, rotation, decentering distortion, thin prism distortion, and third-order radial distortion. This distortion measurement method is highly flexible for distortion measurement with portable and compactly integrated sensors, enabling the real-time and cost-efficient measurement of wave aberration and distortion. For proof-of-concept experiments, a projection lens with a numerical aperture (NA) of 0.58 for i-line (365 nm) is used for experimental testing. The results reveal that the repeatability accuracy of distortion detection is 51 nm and the 72 h long-term reproducibility is 143 nm. Full article

(This article belongs to the Special Issue Optical Measurement Systems)

► Show Figures

Figure 1

17 pages, 4797 KiB

Open AccessEditor’s ChoiceReview

The Progress and Trend of Heterogeneous Integration Silicon/III-V Semiconductor Optical Amplifiers

by Wenqi Shi, Canwen Zou, Yulian Cao and Jianguo Liu

Photonics 2023, 10(2), 161; https://doi.org/10.3390/photonics10020161 - 3 Feb 2023

Cited by 13 | Viewed by 8209

Abstract

Silicon photonics is a revolutionary technology in the integrated photonics field which has experienced rapid development over the past several decades. High-quality III-V semiconductor components on Si platforms have shown their great potential to realize on-chip light-emitting sources for Si photonics with low-cost [...] Read more.

Silicon photonics is a revolutionary technology in the integrated photonics field which has experienced rapid development over the past several decades. High-quality III-V semiconductor components on Si platforms have shown their great potential to realize on-chip light-emitting sources for Si photonics with low-cost and high-density integration. In this review, we will focus on semiconductor optical amplifiers (SOAs), which have received considerable interest in diverse photonic applications. SOAs have demonstrated high performance in various on-chip optical applications through different integration technologies on Si substrates. Moreover, SOAs are also considered as promising candidates for future light sources in the wavelength tunable laser, which is one of the key suitable components in coherent optical devices. Understanding the development and trends of heterogeneous integration Silicon/III-V SOA will help researchers to come up with effective strategies to combat the emerging challenges in this family of devices, progressing towards next-generation applications. Full article

(This article belongs to the Special Issue Integrated Microwave Photonics)

► Show Figures

Figure 1

12 pages, 2010 KiB

Open AccessEditor’s ChoiceArticle

Real-Time Chlorophyll-a Pigment Monitoring of Chlamydomonas reinhardtii in a Controlled Environment Using Pulsed LED Fluorescence LiDAR System

by Jumar G. Cadondon, Edgar A. Vallar, Tatsuo Shiina and Maria Cecilia D. Galvez

Photonics 2023, 10(2), 144; https://doi.org/10.3390/photonics10020144 - 30 Jan 2023

Cited by 5 | Viewed by 2143

Abstract

The real-time chlorophyll-a pigment monitoring of C. reinhardtii is studied using our developed LED fluorescence light detection and ranging (LiDAR) system. It features a portable set-up that uses a pulsed LED module with an excitation wavelength of 385 nm. We were able to [...] Read more.

The real-time chlorophyll-a pigment monitoring of C. reinhardtii is studied using our developed LED fluorescence light detection and ranging (LiDAR) system. It features a portable set-up that uses a pulsed LED module with an excitation wavelength of 385 nm. We were able to monitor the different growth phases of C. reinhardtii with specific cultivation parameters. The developed fluorescence LiDAR system showed the linear correlation of its chlorophyll-a signal with the optical density and EEM fluorescence measurements at 680 nm emission wavelength. Water quality and weather parameters were also measured, which explains the variation in the growth dynamics of C. reinhardtii during the sampling period. The results from the monitoring demonstrated a different technique that can be used in estimating algal biomass in the environment. Full article

(This article belongs to the Special Issue Micro Fluorescence Detectors/Sensors and Their Applications)

► Show Figures

Figure 1

8 pages, 12673 KiB

Open AccessEditor’s ChoiceCommunication

Experimental Demonstration of Attoseconds-at-Harmonics at the SASE3 Undulator of the European XFEL

by Andrei Trebushinin, Gianluca Geloni, Svitozar Serkez, Giuseppe Mercurio, Natalia Gerasimova, Theophilos Maltezopoulos, Marc Guetg and Evgeny Schneidmiller

Photonics 2023, 10(2), 131; https://doi.org/10.3390/photonics10020131 - 27 Jan 2023

Cited by 6 | Viewed by 3311

Abstract

We report on observations of single spike spectra (3–13% of events) upon employing a previously proposed method for single spike generation via harmonic conversion. The method was tested at the soft X-ray SASE3 undulator of the European XFEL. The first part of the [...] Read more.

We report on observations of single spike spectra (3–13% of events) upon employing a previously proposed method for single spike generation via harmonic conversion. The method was tested at the soft X-ray SASE3 undulator of the European XFEL. The first part of the undulator allows one to amplify bunching at the fundamental as well as the higher harmonics. The downstream undulator is tuned to a harmonic, the fourth in our case, to amplify pulses with a shorter duration. We estimate the generated pulse duration within such a subset of short pulses at a level of 650 as. Considering the demonstrated probability of single spike events, this method is attractive for high repetition-rate free electron lasers. Full article

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

► Show Figures

Figure 1

8 pages, 1230 KiB

Open AccessEditor’s ChoiceCommunication

Fiber Optic All-Polarization Weak Magnetic Field Sensor Based on Sagnac Interferometer

by Cui Liang, Zhihang Zhang, Dengwei Zhang, Tengchao Huang and Shuangliang Che

Photonics 2023, 10(2), 101; https://doi.org/10.3390/photonics10020101 - 17 Jan 2023

Cited by 4 | Viewed by 2581

Abstract

A novel fiber-optic magnetic field sensor, based on a Sagnac structure, is proposed with the approach of polarization interference detection. The sensor takes advantage of common path interference, combining with a high magnetic field sensitivity sensing unit, composed of magneto-optical crystal, and magnetic [...] Read more.

A novel fiber-optic magnetic field sensor, based on a Sagnac structure, is proposed with the approach of polarization interference detection. The sensor takes advantage of common path interference, combining with a high magnetic field sensitivity sensing unit, composed of magneto-optical crystal, and magnetic field concentrators, to achieve high resolution, high stability, and large dynamic measurement of DC magnetic field signals. In this paper, the theoretical model is established and the related theory is derived in detail. The key technologies in the system are thoroughly investigated and verified. Experimental research on the proposed system is demonstrated and the results show that a DC magnetic field resolution of 5.6 nT and a dynamic range of larger than 70 dB is achieved. Furthermore, the linearity of the system is greater than 99.8% and the instability is less than 0.5%. Full article

(This article belongs to the Special Issue Optical Measurement Systems and Instruments)

► Show Figures

Figure 1

17 pages, 6093 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Enhanced Performance of a Cascaded Receiver Consisting of a DNN-Based Waveform-to-Symbol Converter and Modified NN-Based DD-LMS in CAP Underwater VLC System

by Xianhao Lin, Fangchen Hu and Nan Chi

Photonics 2023, 10(1), 79; https://doi.org/10.3390/photonics10010079 - 10 Jan 2023

Cited by 7 | Viewed by 4227

Abstract

Underwater visible light communication (UVLC) based on LEDs has become a competitive candidate, which is able to provide high data rates, low latency and low cost for next-generation wireless communication technologies. However, it is still challenging to achieve high-speed communication because of bottleneck [...] Read more.

Underwater visible light communication (UVLC) based on LEDs has become a competitive candidate, which is able to provide high data rates, low latency and low cost for next-generation wireless communication technologies. However, it is still challenging to achieve high-speed communication because of bottleneck problems such as bandwidth limitation and linear and nonlinear distortions. Traditional Deep-learning Neural Network (DNN)-based waveform-to-symbol converter is verified to be an effective method to alleviate them, but impractical due to high complexity. To achieve a better tradeoff between communication performance and computation complexity, a cascaded receiver consisting of a DNN-based waveform-to-symbol converter and modified Neural Network (NN)-based decision-directed least mean square (DD-LMS) is then innovatively proposed. With fewer taps and nodes than the traditional converter, the front-stage converter could mitigate the majority of Inter-Symbol Interference (ISI) and signal nonlinear distortions. Then modified NN-based DD-LMS is cascaded to improve communication performance by reducing phase offset, making received constellation points more concentrated and closer to standard constellation points. Compared with the traditional converter, the cascaded receiver could achieve 89.6% of signal Vpp dynamic range with 12.4% of complexity in the 64APSK UVLC system. Moreover, the ratio of signal Vpp dynamic range and total trainable parameters is 1.24 × 10⁻¹ mV, while that of the traditional converter is 1.95 × 10⁻² mV. The cascaded receiver used in 64APSK UVLC systems is experimentally verified to achieve enhanced performance, thus as a promising scheme for future high-speed underwater VLC. Full article

(This article belongs to the Special Issue Visible Light Communications)

► Show Figures

Figure 1

7 pages, 1924 KiB

Open AccessEditor’s ChoiceCommunication

Intra-Cavity Raman Laser Operating at 1193 nm Based on Graded-Index Fiber

by Chunhua Hu and Ping Sun

Photonics 2023, 10(1), 33; https://doi.org/10.3390/photonics10010033 - 28 Dec 2022

Viewed by 1983

Abstract

Nonlinear Raman frequency conversion is an important technical scheme to obtain special optical band lasers based on conventional ion-doped lasers. In our work, we designed an intra-cavity Raman fiber laser based on graded index fiber (GRIF) as the Raman gain medium. Based on [...] Read more.

Nonlinear Raman frequency conversion is an important technical scheme to obtain special optical band lasers based on conventional ion-doped lasers. In our work, we designed an intra-cavity Raman fiber laser based on graded index fiber (GRIF) as the Raman gain medium. Based on the fundamental-frequency 1080-nanometer laser, efficient first-order and second-order Stokes Raman lasers were obtained, respectively. When the power of the fundamental-frequency 1080-nanometer laser was 33.4 W, the output power of the second-order 1193-nanometer laser was 11.39 W. The corresponding conversion efficiency was 34.1%. To our knowledge, this is the first report of a second-order Raman output based on a GRIF and intra-cavity structure. In the experiment, the spectrum-purification process with the increase in power was also observed. Our experimental results prove that the intracavity Raman-laser system based on graded index fiber with a high optical conversion efficiency has important application potential for obtaining new special-application bands. Full article

(This article belongs to the Special Issue High Power Fiber Laser and Amplifiers)

► Show Figures

Figure 1

18 pages, 4341 KiB

Open AccessEditor’s ChoiceArticle

Identification of Browning in Human Adipocytes by Partial Least Squares Regression (PLSR), Infrared Spectral Biomarkers, and Partial Least Squares Discriminant Analysis (PLS-DA) Using FTIR Spectroscopy

by Dong-Hyun Shon, Se-Jun Park, Suk-Jun Yoon, Yang-Hwan Ryu and Yong Ko

Photonics 2023, 10(1), 2; https://doi.org/10.3390/photonics10010002 - 21 Dec 2022

Cited by 3 | Viewed by 2459

Abstract

We aimed to identify the browning of white adipocytes using partial least squares regression (PLSR), infrared spectral biomarkers, and partial least squares discriminant analysis (PLS-DA) with FTIR spectroscopy instead of molecular biology. PLSR helps distinguish human beige adipocytes treated with norepinephrine and rosiglitazone. [...] Read more.

We aimed to identify the browning of white adipocytes using partial least squares regression (PLSR), infrared spectral biomarkers, and partial least squares discriminant analysis (PLS-DA) with FTIR spectroscopy instead of molecular biology. PLSR helps distinguish human beige adipocytes treated with norepinephrine and rosiglitazone. When PLSR was based on the selected regions of 3997–3656 and 1618–938 cm⁻¹, PLSR achieved an R² of cross-validation of 88.95, a root mean square error of cross validation (RMSECV) of 2.13, and a ratio performance deviation (RPD) of 3.01. Infrared spectral biomarkers [1635 cm⁻¹ (β-sheet amide I), 879–882, 860–3 cm⁻¹ (A-form helix), and 629–38 cm⁻¹ (OH out-of-plane bending)] were identified in human beige adipocytes based on spectral differences between human beige adipocytes and human white adipocytes, principal component analysis-linear discriminant analysis (PCA-LDA) cluster vector, U-test, and Fisher’s score per wavenumber. PLS-DA yielded a useful classification of adipocytes and expression distribution of adipogenesis genes in adipocytes. PLSR, infrared spectral biomarkers, and PLS-DA using FTIR spectroscopy are proposed as effective tools for identifying specific biological activities in a limited environment through features that do not require labeling and are relatively inexpensive in terms of time and labor. Full article

(This article belongs to the Special Issue Biomedical Spectroscopy: Techniques and Applications)

► Show Figures

Figure 1

26 pages, 4599 KiB

Open AccessEditor’s ChoiceReview

Line Field Optical Coherence Tomography

by Samuel Lawman, Zijian Zhang, Yao-Chun Shen and Yalin Zheng

Photonics 2022, 9(12), 946; https://doi.org/10.3390/photonics9120946 - 7 Dec 2022

Cited by 7 | Viewed by 4192

Abstract

The line field (LF) design choice for the lateral image formation mechanism (lateral format) has historically been a fraction of the whole optical coherence tomography (OCT) field. However, as the OCT technology develops, the parallelised acquisition of LF-OCT formats (LF-time domain (TD)-OCT, LF-spectral [...] Read more.

The line field (LF) design choice for the lateral image formation mechanism (lateral format) has historically been a fraction of the whole optical coherence tomography (OCT) field. However, as the OCT technology develops, the parallelised acquisition of LF-OCT formats (LF-time domain (TD)-OCT, LF-spectral domain (SD)-OCT, LF-swept source (SS)-OCT) offers benefits and capabilities, which may mean it is now becoming more mainstream. Prior reviews on OCT have focused on scanning point (SP) and, to a lesser extent, full field (FF), lateral formats, with, to our knowledge, no prior review specifically on the LF lateral format. Here, we address this gap in the literature by reviewing the history of each LF-OCT format, identifying the applications it has had and providing generic system design overviews. We then provide an analysis and discussion of the benefits and drawbacks of the format. Full article

(This article belongs to the Special Issue Recent Advances in Optical Coherence Tomography)

► Show Figures

Figure 1

16 pages, 2735 KiB

Open AccessEditor’s ChoiceArticle

DeepGOMIMO: Deep Learning-Aided Generalized Optical MIMO with CSI-Free Detection

by Xin Zhong, Chen Chen, Shu Fu, Zhihong Zeng and Min Liu

Photonics 2022, 9(12), 940; https://doi.org/10.3390/photonics9120940 - 5 Dec 2022

Cited by 2 | Viewed by 1967

Abstract

Generalized optical multiple-input multiple-output (GOMIMO) techniques have been recently shown to be promising for high-speed optical wireless communication (OWC) systems. In this paper, we propose a novel deep learning-aided GOMIMO (DeepGOMIMO) framework for GOMIMO systems, wherein channel state information (CSI)-free detection can be [...] Read more.

Generalized optical multiple-input multiple-output (GOMIMO) techniques have been recently shown to be promising for high-speed optical wireless communication (OWC) systems. In this paper, we propose a novel deep learning-aided GOMIMO (DeepGOMIMO) framework for GOMIMO systems, wherein channel state information (CSI)-free detection can be enabled by employing a specially designed deep neural network (DNN)-based MIMO detector. The CSI-free DNN detector mainly consists of two modules: one is the preprocessing module, which is designed to address both the path loss and channel crosstalk issues caused by MIMO transmission, and the other is the feedforward DNN module, which is used for joint detection of spatial and constellation information by learning the statistics of both the input signal and the additive noise. Our simulation results clearly verify that, in a typical indoor 4 × 4 MIMO-OWC system using both generalized optical spatial modulation (GOSM) and generalized optical spatial multiplexing (GOSMP) with unipolar nonzero 4-level pulse-amplitude modulation (4-PAM) modulation, the proposed CSI-free DNN detector achieves near the same bit error rate (BER) performance as the optimal joint maximum-likelihood (ML) detector, but with much-reduced computational complexity. Moreover, because the CSI-free DNN detector does not require instantaneous channel estimation to obtain accurate CSI, it enjoys the unique advantages of improved achievable data rate and reduced communication time delay in comparison to the CSI-based zero-forcing DNN (ZF-DNN) detector. Full article

(This article belongs to the Special Issue Advances in Visible Light Communication)

► Show Figures

Figure 1

18 pages, 4110 KiB

Open AccessEditor’s ChoiceArticle

Modelling and Design of Holographic Optical Elements for Beam-Coupling Applications for a Range of Incident Beam Angles

by Dipanjan Chakraborty, Rosen Georgiev, Sinead Aspell, Vincent Toal, Izabela Naydenova, Dervil Cody and Suzanne Martin

Photonics 2022, 9(12), 936; https://doi.org/10.3390/photonics9120936 - 3 Dec 2022

Cited by 9 | Viewed by 3473

Abstract

Theoretical modelling has been used to calculate the holographic recording beam angles required in air (at any recording wavelength) to produce a Volume Holographic Optical Element (VHOE) for any defined input and output beam angles. The approach is used to facilitate the design [...] Read more.

Theoretical modelling has been used to calculate the holographic recording beam angles required in air (at any recording wavelength) to produce a Volume Holographic Optical Element (VHOE) for any defined input and output beam angles. The approach is used to facilitate the design and fabrication of diffractive coupling elements through a holographic process that avoids the use of coupling prisms during recording and will help in the design of recording arrangements that better suit the mass production of low-cost elements, especially those designed for non-normal incidence. In this study, the recording angles needed for a range of recording wavelengths were explored for VHOE couplers designed for input angles (in air) ranging from 0° to −55°. Then, in order to validate the model, holographic recording in Bayfol HX 200 photopolymer at 532 nm was used to fabricate photopolymer VHOE couplers for 633 nm light (−45° input angle in air). Bragg curves obtained experimentally for different probe wavelengths (403 nm, 532 nm and 633 nm) confirm the recording of the desired grating structures to a precision of ±1°, and coupling is demonstrated at 633 nm with a diffraction efficiency of up to 72%. Furthermore, the model is used to identify the origins of some weaker spurious gratings observed alongside the expected ones. Full article

(This article belongs to the Special Issue Materials, Methods and Models for Holographic Optical Elements)

► Show Figures

Figure 1

7 pages, 9380 KiB

Open AccessEditor’s ChoiceCommunication

A Deep Reinforcement Learning Algorithm for Smart Control of Hysteresis Phenomena in a Mode-Locked Fiber Laser

by Alexey Kokhanovskiy, Alexey Shevelev, Kirill Serebrennikov, Evgeny Kuprikov and Sergey Turitsyn

Photonics 2022, 9(12), 921; https://doi.org/10.3390/photonics9120921 - 30 Nov 2022

Cited by 6 | Viewed by 2346

Abstract

We experimentally demonstrate the application of a double deep Q-learning network algorithm (DDQN) for design of a self-starting fiber mode-locked laser. In contrast to the static optimization of a system design, the DDQN reinforcement algorithm is capable of learning the strategy of dynamic [...] Read more.

We experimentally demonstrate the application of a double deep Q-learning network algorithm (DDQN) for design of a self-starting fiber mode-locked laser. In contrast to the static optimization of a system design, the DDQN reinforcement algorithm is capable of learning the strategy of dynamic adjustment of the cavity parameters. Here, we apply the DDQN algorithm for stable soliton generation in a fiber laser cavity exploiting a nonlinear polarization evolution mechanism. The algorithm learns the hysteresis phenomena that manifest themselves as different pumping-power thresholds for mode-locked regimes for diverse trajectories of adjusting optical pumping. Full article

(This article belongs to the Special Issue Optical Fiber Lasers and Amplifiers. Practical Applications of Fiber-Based Laser Sources)

► Show Figures

Figure 1

18 pages, 5011 KiB

Open AccessEditor’s ChoiceArticle

Nonlinear Impairment Compensation Using Transfer Learning-Assisted Convolutional Bidirectional Long Short-Term Memory Neural Network for Coherent Optical Communication Systems

by Xueyuan Luo, Chenglin Bai, Xinyu Chi, Hengying Xu, Yaxuan Fan, Lishan Yang, Peng Qin, Zhiguo Wang and Xiuhua Lv

Photonics 2022, 9(12), 919; https://doi.org/10.3390/photonics9120919 - 29 Nov 2022

Cited by 8 | Viewed by 2350

Abstract

By combining the nonlinear impairment features derived from the first-order perturbation theory, we propose a nonlinear impairment compensation (NLC) scheme based on the transfer learning-assisted convolutional bidirectional long short-term Memory (CNN-BiLSTM) neural network structure. When considering the correlation of [...] Read more.

By combining the nonlinear impairment features derived from the first-order perturbation theory, we propose a nonlinear impairment compensation (NLC) scheme based on the transfer learning-assisted convolutional bidirectional long short-term Memory (CNN-BiLSTM) neural network structure. When considering the correlation of nonlinear impairment between preceding and succeeding consecutive adjacent symbols on the current moment symbol and integrating the multidimensional feature extraction and time memory characteristics of CNN-BiLSTM, the nonlinear impairment information contained in the input feature can be fully utilized to accurately predict the nonlinear impairment showing significant compensation effect. Meanwhile, transfer learning (TL) is introduced to greatly reduce the complexity of the scheme on the basis of high compensation performance. To verify the effectiveness of the proposed scheme, we construct single-channel (SC) and 5-channel 28 GBaud polarization division multiplexing 16 quadrature amplitude modulation (PDM-16QAM)/85 GBaud PDM-64QAM simulation systems, and SC and 3-channel 28 GBaud PDM-16QAM experimental systems. The experimental results show that when compared with simple recurrent neural network (SRNN) NLC and DBP 20 steps per span (DBP20StPs), the Q-factor gain of our scheme is about 1 dB and 1.7 dB in the SC system, and about 1.1 dB and 1.5 dB in the 3-channel system at the optimal launch power, respectively. It is interesting to highlight that, by applying TL to the simulation and experimental systems, our scheme based on only 5% of the training samples can achieve compensation performance comparable to or higher quality than retraining at various launch powers. Full article

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

► Show Figures

Figure 1

17 pages, 5635 KiB

Open AccessEditor’s ChoiceArticle

An Experimental and Numerical Study of the Impact of Ambient Light of SiPMs in VLC Receivers

by William Matthews and Steve Collins

Photonics 2022, 9(12), 888; https://doi.org/10.3390/photonics9120888 - 22 Nov 2022

Cited by 3 | Viewed by 2423

Abstract

Silicon photomultiplier’s relatively large area and ability to detect single photons makes them attractive as receivers for visible light communications. However, their non-linear response has a negative impact on the receiver performance, including making them particularly sensitive to ambient light. Experiments and Monte [...] Read more.

Silicon photomultiplier’s relatively large area and ability to detect single photons makes them attractive as receivers for visible light communications. However, their non-linear response has a negative impact on the receiver performance, including making them particularly sensitive to ambient light. Experiments and Monte Carlo simulations have been used to study this non-linearity. The resulting detailed understanding of the origins of the non-linear response leads to concerns over the accuracy of some previous simulations of SiPMs. In addition, it leads to simple methods to determine the maximum rate at which an SiPM can count photons and of determining the impact of a SiPMs non-linearity on its performance of a receiver. Finally, a method of determining which filters should be used to protect an SiPM from ambient light is proposed. Full article

(This article belongs to the Special Issue Advances in Visible Light Communication)

► Show Figures

Figure 1

8 pages, 3788 KiB

Open AccessEditor’s ChoiceCommunication

Experimental Detection of Initial System–Environment Entanglement in Open Systems

by Gaoyan Zhu, Dengke Qu, Lei Xiao and Peng Xue

Photonics 2022, 9(11), 883; https://doi.org/10.3390/photonics9110883 - 21 Nov 2022

Cited by 1 | Viewed by 2302

Abstract

We experimentally investigate how initial entanglement between the system and environment can be detected in an open system by using some prior knowledge of the joint evolutions. The protocol we employed requires classical optimization on the results after performing measurements on the system [...] Read more.

We experimentally investigate how initial entanglement between the system and environment can be detected in an open system by using some prior knowledge of the joint evolutions. The protocol we employed requires classical optimization on the results after performing measurements on the system state. Such an approach does not require a full 2-qubit QST, and works in scenarios where one has access to the system only. We demonstrate the protocol on both pure entangled states and mixed entangled states. The obtained results show the experimental accessibility and validity of the protocol. Compared with the previous methods, which also assume access only to the system, this protocol is less demanding in terms of measurement and state preparation. The experimental results also show that, using the knowledge of the interaction, we can fine-tune the protocol, thus showing the potential of the protocol for developing experimentally feasible and practical entanglement detection methods. Full article

(This article belongs to the Section Quantum Photonics and Technologies)

► Show Figures

Figure 1

13 pages, 3112 KiB

Open AccessEditor’s ChoiceArticle

First-Passage-Time Analysis of the Pulse-Timing Statistics in a Two-Section Semiconductor Laser under Excitable and Noisy Conditions

by Daan Lenstra, Lukas Puts and Weiming Yao

Photonics 2022, 9(11), 860; https://doi.org/10.3390/photonics9110860 - 14 Nov 2022

Cited by 2 | Viewed by 1884

Abstract

A two-section semiconductor laser can exhibit excitability for certain parameter settings. When used as a photonic spiking neuron, it is relevant to investigate its sensitivity to noise due to, e.g., spontaneous emission. Under excitable conditions, the system emits irregularly timed noise-triggered pulses. Their [...] Read more.

A two-section semiconductor laser can exhibit excitability for certain parameter settings. When used as a photonic spiking neuron, it is relevant to investigate its sensitivity to noise due to, e.g., spontaneous emission. Under excitable conditions, the system emits irregularly timed noise-triggered pulses. Their statistics is analyzed in terms of a first-passage time distribution for the fluctuating intensity to reach the threshold for excitable response. Two analytic approximations valid for short and long times, respectively, are derived which very well explain measured and simulated pulse-repetition time distributions. This provides physical insight into the noise-triggered spiking mechanism. Full article

(This article belongs to the Special Issue Advancements in Semiconductor Lasers)

► Show Figures

Figure 1

9 pages, 4011 KiB

Open AccessFeature PaperEditor’s ChoiceCommunication

Ultra-Wideband Polarization Insensitive Angle Filter Based on ENZ Characteristics and Dynamic Antireflection Structures

by Baofei Wan, Haining Ye and Haifeng Zhang

Photonics 2022, 9(11), 854; https://doi.org/10.3390/photonics9110854 - 12 Nov 2022

Cited by 4 | Viewed by 2254

Abstract

Bandwidth expansion has always been an important dimension in investigating angle filters (AFs) and is critical for optical communication and radar detection. In this paper, the AF with strong selectivity is realized by using the epsilon-near-zero (ENZ) jump characteristic of YaBa₂Cu [...] Read more.

Bandwidth expansion has always been an important dimension in investigating angle filters (AFs) and is critical for optical communication and radar detection. In this paper, the AF with strong selectivity is realized by using the epsilon-near-zero (ENZ) jump characteristic of YaBa₂Cu₃O₇ material. At the same time, for both the TE and the TM waves in the range of 237~1000 THz, the transmissivity of the AF is stronger than 0.9 by using dynamic antireflection structures (AFSs). The transfer matrix method is suitable for theoretical calculation, and the impedance matching theory is introduced to analyze the features of the AF. The increment of the thickness of superconductor material can effectively enhance the selectivity of the AF structure, and the consequence is the attenuation of transmission performances. If the temperature is covered from 0 K to 85 K, the filtering performance higher than 0.9 can still be maintained for two polarization waves. For these explicit performances, the proposed design may provide a new idea for widening the frequency bandwidth of the AF. Full article

(This article belongs to the Special Issue Photonic Crystals: Physics and Devices)

► Show Figures

Figure 1

6 pages, 1415 KiB

Open AccessEditor’s ChoiceOpinion

Advances in Photovoltaic Technologies from Atomic to Device Scale

by Christin David and Robert Hussein

Photonics 2022, 9(11), 837; https://doi.org/10.3390/photonics9110837 - 8 Nov 2022

Cited by 1 | Viewed by 2136

Abstract

The question of how energy resources can be efficiently used is likewise of fundamental and technological interest. In this opinion, we give a brief overview on developments of harvesting solar energy across different length scales and address some strategies to tackle economic and [...] Read more.

The question of how energy resources can be efficiently used is likewise of fundamental and technological interest. In this opinion, we give a brief overview on developments of harvesting solar energy across different length scales and address some strategies to tackle economic and ecological challenges, in particular with a view to sustainability and toward a circular economy. On the mesoscopic scale, the emergence of thermodynamic laws in open quantum systems is of central importance and how they can be employed for efficient quantum thermal machines and batteries. The broad tunability of band gaps in quantum dot systems makes them attractive for hybrid photovoltaic devices. Complementary, machine learning-aided band gap engineering and the high-throughput screening of novel materials assist with improving absorption characteristics. On the device scale, hybrid concepts of optical control via metasurfaces enable a multitude of functionalities such as a directed re-emission of embedded photoluminescent materials or field enhancement effects from nanostructures. Advanced techniques in computational nanophotonics concern a topology optimization of nanostructured layers together with multiobjective optimization toward specific light management tasks. On the industrial level, modern manufacturers explore 3D printing and flexible solar cell platforms obtained from roll-to-roll technologies. The remote control of solar parks through applications via the Internet of Things opens up new strategies to expand to difficult terrain where human interaction is only required to a limited extent. Full article

(This article belongs to the Special Issue Advances in Photovoltaic Technologies from Atomic to Device Scale)

► Show Figures

Figure 1

11 pages, 3857 KiB

Open AccessEditor’s ChoiceArticle

Design and Characteristics of Diamond-Assisted Ring-Core Fiber for Space Division Multiplexing

by Yujing Song, Jingjing Zheng, Li Pei, Jing Huang, Tigang Ning, Jing Li, Jianshuai Wang and Bing Bai

Photonics 2022, 9(10), 766; https://doi.org/10.3390/photonics9100766 - 13 Oct 2022

Cited by 2 | Viewed by 2178

Abstract

In this paper, a novel diamond-assisted ring-core fiber (DRF) is proposed. With the introduction of a low-refractive-index diamond-shaped region located in the center of the core, the proposed fiber effectively eliminates spatial degeneracy of the LP_mn mode groups and maintains a low [...] Read more.

In this paper, a novel diamond-assisted ring-core fiber (DRF) is proposed. With the introduction of a low-refractive-index diamond-shaped region located in the center of the core, the proposed fiber effectively eliminates spatial degeneracy of the LP_mn mode groups and maintains a low level of birefringence. Under the fiber structure parameters proposed in this paper, the effective refractive index difference (Δn_eff) between the spatial modes supported by the fiber in the entire C-band is greater than 2.25 × 10⁻⁴, and the Δn_eff between adjacent modes falls within the scope of (2.11~9.41) × 10⁻⁴. The degree of degenerate separation between the two polarization modes of all modes is very low, which is 2~3 orders of magnitude lower than that of the spatial mode. By discussing the mode characteristics of DRF and several other center-assisted ring-core fibers, the method that can be used to manipulate the spatial mode degenerate separation with structural symmetry is obtained, which can be applied to provide guidance for similar fiber designs. The proposed fiber structure is a promising candidate in space division multiplexing systems. Full article

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

► Show Figures

Figure 1

25 pages, 2087 KiB

Open AccessEditor’s ChoiceArticle

Nonlinear Spectrum Modulation in the Anomalous Dispersion Regime Using Second- and Third-Order Solitons

by Thiago D. S. De Menezes, Chaoran Tu, Valentin Besse, Maurice O’Sullivan, Vladimir S. Grigoryan, Curtis R. Menyuk and Ivan T. Lima Jr.

Photonics 2022, 9(10), 748; https://doi.org/10.3390/photonics9100748 - 10 Oct 2022

Cited by 2 | Viewed by 1704

Abstract

We study the robustness of a nonlinear frequency-division multiplexing (NFDM) system, based on the Zakharov-Shabat spectral problem (ZSSP), that is comprised of two independent quadrature phase-shift keyed (QPSK) channels modulated in the discrete spectrum associated with two distinct eigenvalues. Among the many fiber [...] Read more.

We study the robustness of a nonlinear frequency-division multiplexing (NFDM) system, based on the Zakharov-Shabat spectral problem (ZSSP), that is comprised of two independent quadrature phase-shift keyed (QPSK) channels modulated in the discrete spectrum associated with two distinct eigenvalues. Among the many fiber impairments that may limit this system, we focus on determining the limits due to third-order dispersion, the Raman effect, amplified spontaneous emission (ASE) noise from erbium-doped fiber amplifiers (EDFAs), and fiber losses with lumped gain from EDFAs. We examine the impact of these impairments on a 1600-km system by analyzing the Q-factor calculated from the error vector magnitude (EVM) of the received symbols. We found that the maximum launch power due to these impairments is: 13 dBm due to third-order dispersion, 11 dBm due to the Raman effect, 3 dBm due to fiber losses with lumped gain, and 2 dBm due to these three impairments combined with ASE noise. The maximum launch power due to all these impairments combined is comparable to that of a conventional wavelength-division multiplexing (WDM) system, even though WDM systems can operate over a much larger bandwidth and, consequently, have a much higher data throughput when compared with NFDM systems. We find that fiber losses in practical fiber transmission systems with lumped gain from EDFAs is the most stringent limiting factor in the performance of this NFDM system. Full article

(This article belongs to the Section Optical Communication and Network)

► Show Figures

Figure 1

17 pages, 1777 KiB

Open AccessEditor’s ChoiceArticle

On the Optimisation of Illumination LEDs for VLP Systems

by José Miguel Menéndez and Heidi Steendam

Photonics 2022, 9(10), 750; https://doi.org/10.3390/photonics9100750 - 10 Oct 2022

Cited by 3 | Viewed by 2115

Abstract

Recent studies have explored the synergy of illumination and positioning using indoor lighting infrastructure. While these studies mainly focused on the analysis of the performance of visible light positioning, these works did not consider the illumination aspects of such combined systems. In this [...] Read more.

Recent studies have explored the synergy of illumination and positioning using indoor lighting infrastructure. While these studies mainly focused on the analysis of the performance of visible light positioning, these works did not consider the illumination aspects of such combined systems. In this paper, we analyse the illumination aspects based on the main illumination characteristics defined in the European Standard EN 12464-1, i.e., the horizontal illuminance and the uniformity of illuminance. As in the standard, we distinguish between a task area, where visual activities are performed that demand higher illuminance and uniformity, and a surrounding area that borders the former. In our analysis, we derive simple rules of thumb to determine the number and placement of LEDs to satisfy the constraints on the horizontal illuminance and uniformity for a given area. Full article

(This article belongs to the Special Issue Advances in Visible Light Communication)

► Show Figures

Figure 1

16 pages, 4884 KiB

Open AccessEditor’s ChoiceArticle

A Fast Accurate Attention-Enhanced ResNet Model for Fiber-Optic Distributed Acoustic Sensor (DAS) Signal Recognition in Complicated Urban Environments

by Xinyu Liu, Huijuan Wu, Yufeng Wang, Yunlin Tu, Yuwen Sun, Liang Liu, Yuanfeng Song, Yu Wu and Guofeng Yan

Photonics 2022, 9(10), 677; https://doi.org/10.3390/photonics9100677 - 21 Sep 2022

Cited by 6 | Viewed by 2443

Abstract

The fiber-optic distributed acoustic sensor (DAS), which utilizes existing communication cables as its sensing media, plays an important role in urban infrastructure monitoring and natural disaster prediction. In the face of a wide, dynamic environment in urban areas, a fast, accurate DAS signal [...] Read more.

The fiber-optic distributed acoustic sensor (DAS), which utilizes existing communication cables as its sensing media, plays an important role in urban infrastructure monitoring and natural disaster prediction. In the face of a wide, dynamic environment in urban areas, a fast, accurate DAS signal recognition method is proposed with an end-to-end attention-enhanced ResNet model. In preprocessing, an objective evaluation method is used to compare the distinguishability of different input features with the Euclidean distance between the posterior probabilities classified correctly and incorrectly; then, an end-to-end ResNet is optimized with the chosen time-frequency feature as input, and a convolutional block attention module (CBAM) is added, which can quickly focus on key information from different channels and specific signal structures and improves the system recognition performance further. The results show that the proposed ResNet+CBAM model has the best performance in recognition accuracy, convergence rate, generalization capability, and computational efficiency compared with 1-D CNN, 2-D CNN, ResNet, and 2-D CNN+CBAM. An average accuracy of above 99.014% can be achieved in field testing; while dealing with multi-scenario scenes and inconsistent laying or burying environments, it can still be kept above 91.08%. The time cost is only 3.3 ms for each signal sample, which is quite applicable in online long-distance distributed monitoring applications. Full article

(This article belongs to the Special Issue Optical Fiber Physical and Mechanical Sensors)

► Show Figures

Figure 1

11 pages, 1956 KiB

Open AccessEditor’s ChoiceCommunication

An All-Dielectric Color Filter, with a Wider Color Gamut

by Lizhen Zeng, Yuting Yang and Gongli Xiao

Photonics 2022, 9(10), 680; https://doi.org/10.3390/photonics9100680 - 21 Sep 2022

Cited by 3 | Viewed by 2241

Abstract

Due to their extraordinary abilities to manipulate light propagation at the nanoscale, dielectric resonators that generate electric and magnetic Mie resonances for minimal optical loss have recently attracted great interest. Based on an all-dielectric metasurface, made of H-type silicon nanoarrays, this study proposed [...] Read more.

Due to their extraordinary abilities to manipulate light propagation at the nanoscale, dielectric resonators that generate electric and magnetic Mie resonances for minimal optical loss have recently attracted great interest. Based on an all-dielectric metasurface, made of H-type silicon nanoarrays, this study proposed and constructed a visible-wavelength-range color filter, with high-quality Mie resonance and the ability to synthesize new colors. Using the finite-difference time-domain (FDTD) approach, we can create a larger color gamut by modifying the H-type array’s structural properties. The all-dielectric color filter suggested has a high color saturation and narrow bandwidth. The Mie resonance can be adjusted by manipulating the structural characteristics. By translating the reflectance spectrum into color coordinates and using the CIE1931 chromaticity diagram, a wide range of colors can be generated. This color filter offers a larger color range and saturation than other color filters. We produced color passband filters that span the visible spectrum using Mie resonator arrays, based on an H-type nanoresonator. This technology could have many applications, including high-resolution color printing, color-tunable switches, and sensing systems. Full article

(This article belongs to the Topic Optical and Optoelectronic Materials and Applications)

► Show Figures

Figure 1

7 pages, 1607 KiB

Open AccessEditor’s ChoiceCommunication

High Power Mid-Infrared Quantum Cascade Lasers Grown on Si

by Steven Slivken, Nirajman Shrestha and Manijeh Razeghi

Photonics 2022, 9(9), 626; https://doi.org/10.3390/photonics9090626 - 1 Sep 2022

Cited by 6 | Viewed by 2207

Abstract

This article details the demonstration of a strain-balanced, InP-based mid-infrared quantum cascade laser structure that is grown directly on a Si substrate. This is facilitated by the creation of a metamorphic buffer layer that is used to convert from the lattice constant of [...] Read more.

This article details the demonstration of a strain-balanced, InP-based mid-infrared quantum cascade laser structure that is grown directly on a Si substrate. This is facilitated by the creation of a metamorphic buffer layer that is used to convert from the lattice constant of Si (0.543 nm) to that of InP (0.587 nm). The laser geometry utilizes two top contacts in order to be compatible with future large-scale integration. Unlike previous reports, this device is capable of room temperature operation with up to 1.6 W of peak power. The emission wavelength at 293 K is 4.82 μm, and the device operates in the fundamental transverse mode. Full article

(This article belongs to the Section Lasers, Light Sources and Sensors)

► Show Figures

Figure 1

14 pages, 3126 KiB

Open AccessEditor’s ChoiceArticle

Plasma Dynamics Characterization for Improvement of Resonantly Enhanced Harmonics Generation in Indium and Tin Laser-Produced Plasmas

by Vyacheslav V. Kim, Jelena Butikova, Jurgis Grube, Anatolijs Sarakovskis and Rashid A. Ganeev

Photonics 2022, 9(9), 600; https://doi.org/10.3390/photonics9090600 - 24 Aug 2022

Cited by 3 | Viewed by 1779

Abstract

In this study, we characterize the properties of indium and tin laser-induced plasmas responsible for efficient high-order harmonics generation of the ultrashort pulses propagating through these media. The optimally formed plasma was determined using the analysis of the time-resolved variations in the spectral [...] Read more.

In this study, we characterize the properties of indium and tin laser-induced plasmas responsible for efficient high-order harmonics generation of the ultrashort pulses propagating through these media. The optimally formed plasma was determined using the analysis of the time-resolved variations in the spectral and morphological features of spreading indium and tin plasma components under different regimes of laser ablation. We report the measurements of plasma velocities under different regimes of ablation and correlate them with the optimal delay between the heating and probe laser pulses for the generation of harmonics with the highest yield. Electron temperatures and densities are determined using the integrated and time-resolved spectral measurements of plasmas. The resonance-enhanced harmonics are compared with other harmonics from the point of view of the modulation of plasma characteristics. The harmonics of 800 and 1200–2200 nm lasers and their second-harmonic fields were analyzed at optimal conditions of Sn and In plasma formation. The novelty of this work is the implementation of the diagnostics of the dynamics of plasma characteristics for the determination of the optimal plasma formation for harmonics generation. Such an approach allows for the demonstration of the maximal harmonic yield from the studied plasma and the definition of the various resonance-induced harmonic generation conditions. Full article

► Show Figures

Figure 1

17 pages, 3213 KiB

Open AccessEditor’s ChoiceArticle

Resonant Coupling of Hermite-Gaussian Transverse Modes in the Triangular Cavity of a Cavity Ring-down Spectroscope

by Yijie Ren, Changxiang Yan, Xiamiao Zhang and Chunhui Hu

Photonics 2022, 9(9), 595; https://doi.org/10.3390/photonics9090595 - 23 Aug 2022

Cited by 1 | Viewed by 2031

Abstract

During resonance in resonant cavities, such as those used in laser or cavity ring-down spectroscopes (CRDS), resonant coupling between higher-order transverse modes and fundamental modes can seriously affect the quality of the beam and introduce measurement errors. Several coupling models, such as thermal [...] Read more.

During resonance in resonant cavities, such as those used in laser or cavity ring-down spectroscopes (CRDS), resonant coupling between higher-order transverse modes and fundamental modes can seriously affect the quality of the beam and introduce measurement errors. Several coupling models, such as thermal deformation coupling and scattering coupling, have been established according to existing coupling theory and specific application scenarios; however, these coupling models have not been attributed to a unified theory. In this paper, we reveal that the same resonant coupling excitation factors exist under different types of environmental perturbation. The conditions and range of resonant coupling in a CRDS ring-down cavity are systematically analyzed, and a preferential coupling model of the middle-order modes is proposed. The time-domain characteristics of the CRDS are used in experiments to analyze the resonant coupling between the modes in a weak energy system. The order and coupling range of the middle-order modes involved in resonant coupling are verified using the modal filtering characteristics of the triangular cavity; this paper presents a unified explanation for various types of resonant coupling and also provides a new approach to resonant coupling experiments performed in high-finesse resonant cavities. Full article

(This article belongs to the Section Lasers, Light Sources and Sensors)

► Show Figures

Figure 1

13 pages, 4012 KiB

Open AccessEditor’s ChoiceArticle

Demonstration of Yb-Doped Fiber Amplifier Operating near 980 nm with the Slope Efficiency Close to the Theoretical Limit

by Zhaode Li, Shangde Zhou, Aimin Liu, Jianqiu Cao, Zhihe Huang and Jinbao Chen

Photonics 2022, 9(8), 571; https://doi.org/10.3390/photonics9080571 - 12 Aug 2022

Cited by 2 | Viewed by 2467

Abstract

In this paper, the scalability of slope efficiency of a Yb-doped fiber amplifier operating near 980 nm is studied with the core-pumping scheme. By means of numerical prediction, it is found that the theoretical limit of slope efficiency should be about 92.2%. Then, [...] Read more.

In this paper, the scalability of slope efficiency of a Yb-doped fiber amplifier operating near 980 nm is studied with the core-pumping scheme. By means of numerical prediction, it is found that the theoretical limit of slope efficiency should be about 92.2%. Then, the experiment study is carried out. An 85.3% slope efficiency of emission around 980 nm is achieved with the seed light around 976.5 nm, and the strong in-band amplified spontaneous emission (ASE) is supposed to be a factor limiting the upscaling of slope efficiency. In order to suppress the in-band ASE, the double-wavelength fiber oscillator near 980 nm is fabricated and used as the seed source, with which the slope efficiency is elevated to 90.7%. Such slope efficiency is very close to the theoretical limit and sets a new record of slope efficiency for the Yb-doped fiber amplifier operating near 980 nm, to the best of our knowledge. It is also revealed that the suppression of in-band ASE should be of great importance to elevate the slope efficiency of a Yb-doped fiber amplifier operating near 980 nm. Full article

(This article belongs to the Special Issue Rare-Earth-Doped Fiber Lasers and Amplifiers)

► Show Figures

Figure 1

Journal Menu

Journal Browser

Editor’s Choice Articles

Further Information

Guidelines

MDPI Initiatives

Follow MDPI