Photonics

15 pages, 2612 KiB

Open AccessArticle

Modeling of Complex Integrated Photonic Resonators Using the Scattering Matrix Method

by Di Jin, Sian Ren, Junkai Hu, Duan Huang, David J. Moss and Jiayang Wu

Photonics 2024, 11(12), 1107; https://doi.org/10.3390/photonics11121107 (registering DOI) - 22 Nov 2024

We propose a universal approach for modeling complex integrated photonic resonators based on the scattering matrix method. By dividing devices into basic elements including directional couplers and connecting waveguides, our approach can be used to model integrated photonic resonators with both unidirectional and [...] Read more.

We propose a universal approach for modeling complex integrated photonic resonators based on the scattering matrix method. By dividing devices into basic elements including directional couplers and connecting waveguides, our approach can be used to model integrated photonic resonators with both unidirectional and bidirectional light propagation, with the simulated spectral response showing good agreement with experimental results. A simplified form of our approach, which divides devices into several independent submodules such as microring resonators and Sagnac interferometers, is also introduced to streamline the calculation of spectral transfer functions. Finally, we discuss the deviations introduced by approximations in our modeling, along with strategies for improving modeling accuracy. Our approach is universal across different integrated platforms, providing a useful tool for designing and optimizing integrated photonic devices with complex configurations. Full article

4 pages, 141 KiB

Open AccessEditorial

Advancing the Frontier of Photonics: Exploring Micro–Nano Optical Devices—An Overview of the Special Issue on Micro–Nano Optical Devices

by Yanfeng Zhang and Yan Shen

Photonics 2024, 11(12), 1106; https://doi.org/10.3390/photonics11121106 (registering DOI) - 22 Nov 2024

Abstract

The relentless pursuit of miniaturization in the field of photonics has led to the emergence of micro–nano optical devices as a cornerstone of modern technology [...] Full article

(This article belongs to the Special Issue Micro-Nano Optical Devices)

15 pages, 8416 KiB

Open AccessArticle

Interference Measurements Across Vacuum and Atmospheric Environments for Characterization of Space-Borne Telescope

by Yi-Kai Huang and Cheng-Huan Chen

Photonics 2024, 11(12), 1105; https://doi.org/10.3390/photonics11121105 - 22 Nov 2024

Abstract

A space-borne telescope is used for Earth observation at about 500 km above sea level in the thermosphere where the air density is very low and the temperature increases significantly during daytime. If the telescopes are aligned and characterized on the ground with [...] Read more.

A space-borne telescope is used for Earth observation at about 500 km above sea level in the thermosphere where the air density is very low and the temperature increases significantly during daytime. If the telescopes are aligned and characterized on the ground with standard temperature and pressure (STP) conditions, different from that of the thermosphere, their performance could drift during their mission. Therefore, they are usually placed in a thermal vacuum chamber during ground testing in order to verify the system can perform well and withstand the harsh environment such as a high vacuum level and large temperature variations before being launched. Nevertheless, it remains a challenge to build up an in situ optical measurement system for a large aperture telescope in a thermal vacuum chamber due to the finite internal space of the chamber, limited aperture size of the vacuum view port and thermal dissipation problem of the measuring instruments. In this paper, a novel architecture of an interferometer whose light path travels across a vacuum chamber and an atmospheric environment has been proposed to resolve all of these technical issues. The major feature of the architecture is the diverger lens being located within the vacuum chamber, leaving the rest of the interferometer outside. The variation of the interference fringe due to the relocation of the diverger lens has been investigated with optical simulations and the solutions for compensation have also been proposed. Together with a specific alignment procedure for the proposed architecture, the interferogram has been successfully acquired from a prototype testbed. Full article

(This article belongs to the Special Issue Optical Systems for Astronomy)

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11 pages, 3542 KiB

Open AccessArticle

Bend Distortion and Thermal Lensing Effect on Transverse Mode Instability

by Dan Cheng, Qing Zhong, Yujun Feng, Kun Zhang, Zhaochen Cheng, Dayong Zhang and Hong Zhao

Photonics 2024, 11(12), 1104; https://doi.org/10.3390/photonics11121104 - 22 Nov 2024

Abstract

In this work, we conducted a numerical analysis to investigate the combined effect of thermal lensing and bending-induced mode distortion on transverse mode instability in conventional large-mode-area (LMA) step-index fibers. Utilizing the finite element method, conformal mapping, and thermal conduction equations, we simulated [...] Read more.

In this work, we conducted a numerical analysis to investigate the combined effect of thermal lensing and bending-induced mode distortion on transverse mode instability in conventional large-mode-area (LMA) step-index fibers. Utilizing the finite element method, conformal mapping, and thermal conduction equations, we simulated the mode profiles in LMA 20/400 and 25/400 fibers subjected to both bending and thermal lensing effects; the corresponding evolution of mode loss and effective area were explored as well. Additionally, by introducing the derived mode profiles to the TMI coefficient calculations, we analyzed the influence of bending and thermal lensing (TL) on TMI; the simulation results indicate that the mode distortion caused by bending and the TL effect, under the bending conditions commonly encountered in practice, do not have pronounced impacts on TMI coefficient and TMI threshold. Full article

(This article belongs to the Special Issue The Emerging Science and Applications of Fiber Laser Technology)

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16 pages, 1242 KiB

Open AccessArticle

Logic Gate Generation in a Monostable Optical System: Improving the Erbium-Doped Fiber Laser Reconfigurable Logic Operation

by Samuel Mardoqueo Afanador-Delgado, José Luis Echenausía-Monroy, Guillermo Huerta-Cuellar, Juan Hugo García-López, Erick Emiliano Lopez-Muñoz and Rider Jaimes-Reátegui

Photonics 2024, 11(12), 1103; https://doi.org/10.3390/photonics11121103 - 22 Nov 2024

Abstract

A logic gate is typically an electronic device with a Boolean or other type of function, e.g., adding or subtracting, including or excluding according to its logical properties. They can be used in electronic, electrical, mechanical, hydraulic, and pneumatic technology. This paper presents [...] Read more.

A logic gate is typically an electronic device with a Boolean or other type of function, e.g., adding or subtracting, including or excluding according to its logical properties. They can be used in electronic, electrical, mechanical, hydraulic, and pneumatic technology. This paper presents a new method for generating logic gates based on optical systems with an emission frequency equal to that used in current telecommunications systems. It uses an erbium-doped fiber laser in its monostable operating region, in contrast to most results published in the literature, where multistable behavior is required to induce dynamic changes, and where a DC voltage signal in the laser pump current provides the control between obtaining the different logic operations. The proposed methodology facilitates the generation of the gates, since it does not require taking the optical system to critical power levels that could damage the components. It is based on using the same elements that the EDFL requires to operate. The result is a system capable of generating up to five stable and robust logic gates to disturbances validated in numerical simulation and experimental setup. This eliminates the sensitivity to the initial conditions affecting the possible logic gates generated by the system and the need to add noise to the system (as is performed in works based on stochastic logic resonance). The experimental observations confirm the numerical results and open up new aspects of using chaotic systems to generate optical logic gates without bistable states. Full article

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12 pages, 4480 KiB

Open AccessArticle

He-Kr Gas-Discharge Laser Based on Hollow-Core Fiber

by Igor Bufetov, Dmitry Komissarov, Sergey Nefedov, Alexey Kosolapov, Vladimir Velmiskin, Alexander Mineev and Alexey Gladyshev

Photonics 2024, 11(12), 1102; https://doi.org/10.3390/photonics11121102 - 22 Nov 2024

Abstract

Recently, two completely different types of lasers—a fiber laser and a gas-discharge laser—were combined into a single device by demonstrating 2.03 µm laser generation in He-Xe plasma that was produced by a microwave discharge directly inside a hollow-core fiber. This new type of [...] Read more.

Recently, two completely different types of lasers—a fiber laser and a gas-discharge laser—were combined into a single device by demonstrating 2.03 µm laser generation in He-Xe plasma that was produced by a microwave discharge directly inside a hollow-core fiber. This new type of laser—a gas-discharge fiber laser—provides excellent opportunities to greatly enrich the wavelength range of the operation of fiber lasers. In this work, we investigate a He-Kr gas mixture as an active medium of this new type of laser. As a result, a He-Kr gas-discharge fiber laser is demonstrated for the first time. The laser was pumped by a microwave discharge in a He:Kr (40:1) mixture that was filled into a revolver fiber with the hollow-core diameter of 130 µm. The total gas pressure was about 100 torr. With broadband mirrors of the laser resonator, generation was observed simultaneously at wavelengths of 2190 and 2523 nm. The output power of the He-Kr gas-discharge fiber laser was about 1 mW. Full article

(This article belongs to the Special Issue Advanced Solid-State and Fiber Mid-IR Lasers: Novel Materils, Components, Systems and Applications)

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9 pages, 5150 KiB

Open AccessArticle

Reliability Study of Fiber Coupling Efficiency of 980 nm Semiconductor Laser

by Gang Liu, Shuhao Pang, Xin Zhang, Mingzhi Tang, Lei Liang, Rui Li and Rui Huang

Photonics 2024, 11(12), 1101; https://doi.org/10.3390/photonics11121101 - 21 Nov 2024

Abstract

In order to improve the stability of semiconductor laser fiber coupling efficiency, based on the coupling principle, the optimal parameters for semiconductor laser fiber coupling were simulated to be θ = 45°, r = 3.25 μm, and z = 5.65 μm. By optimizing [...] Read more.

In order to improve the stability of semiconductor laser fiber coupling efficiency, based on the coupling principle, the optimal parameters for semiconductor laser fiber coupling were simulated to be θ = 45°, r = 3.25 μm, and z = 5.65 μm. By optimizing the structure and position of the lens fiber, it has been experimentally proven that the maximum fiber coupling efficiency of the 980 nm semiconductor laser can reach 87.1%, and the average coupling efficiency can also reach 84%. After temperature cycling and aging experiments, the average coupling efficiency of the device was 81.7%, indicating a decrease in coupling efficiency. At the same time, the effect of fiber stress on the reliability of coupling efficiency was analyzed, and the stability and consistency of the device before and after temperature cycling were explored. In future work, it will be necessary to further optimize the thermal stress caused by UV glue curing and tail pipe soldering, find suitable process parameters, and obtain stable and reliable coupling modules. Full article

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

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11 pages, 1995 KiB

Open AccessArticle

Angle-Tunable Method for Optimizing Rear Reflectance in Fabry–Perot Interferometers and Its Application in Fiber-Optic Ultrasound Sensing

by Yufei Chu, Mohammed Alshammari, Xiaoli Wang and Ming Han

Photonics 2024, 11(12), 1100; https://doi.org/10.3390/photonics11121100 - 21 Nov 2024

Abstract

With the introduction of advanced Fiber Bragg Grating (FBG) technology, Fabry–Pérot (FP) interferometers have become widely used in fiber-optic ultrasound detection. In these applications, the slope of the reflectance is a critical factor influencing detection results. Due to the intensity limitations of the [...] Read more.

With the introduction of advanced Fiber Bragg Grating (FBG) technology, Fabry–Pérot (FP) interferometers have become widely used in fiber-optic ultrasound detection. In these applications, the slope of the reflectance is a critical factor influencing detection results. Due to the intensity limitations of the laser source in fiber-optic ultrasound detection, the reflectance of the FBG is generally increased to enhance the signal-to-noise ratio (SNR). However, increasing reflectance can cause the reflectance curve to deviate from a sinusoidal shape, which in turn affects the slope of the reflectance and introduces greater errors. This paper first investigates the relationship between the transmission curve of the FP interferometer and reflectance, with a focus on the errors introduced by simplified assumptions. Further research shows that in sensors with asymmetric reflectance slopes, their transmittance curves deviate significantly from sinusoidal signals. This discrepancy highlights the importance of achieving symmetrical slopes to ensure consistent and accurate detection. To address this issue, this paper proposes an innovative method to adjust the rear-end reflectance of the FP interferometer by combining stress modulation, UV adhesive, and a high-reflectivity metal disk. Additionally, by adjusting the rear-end reflectance to ensure that the transmittance curve approximates a sinusoidal signal, the symmetry of the slope is maintained. Finally, through practical ultrasound testing, by adjusting the incident wavelength to the positions of slope extrema (or zero) at equal intervals, the expected ultrasound signals at extrema (or zero) can be detected. This method converts the problem of approximating a sinusoidal signal into a problem of the slope adjustment of the transmittance curve, making it easier and more direct to determine its impact on detection results. The proposed method not only improves the performance of fiber-optic ultrasound sensors but also reduces costs, paving the way for broader applications in medical diagnostics and structural health monitoring. Full article

(This article belongs to the Special Issue Optical Sensing Technologies, Devices and Their Data Applications)

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18 pages, 12480 KiB

Open AccessArticle

Bionic Compass Method Based on Atmospheric Polarization Optimization in Non-Ideal Clear Condition

by Yuyang Li, Xia Wang, Min Zhang, Ruiqiang Li and Qiyang Sun

Photonics 2024, 11(12), 1099; https://doi.org/10.3390/photonics11121099 - 21 Nov 2024

Abstract

The bionic polarization compass is a fascinating subject in the navigation domain. Existing polarization navigation models are primarily based on Rayleigh scattering theory, which is applicable to high-altitude, dry, and clear weather conditions. In most scenarios, it is difficult to meet such ideal [...] Read more.

The bionic polarization compass is a fascinating subject in the navigation domain. Existing polarization navigation models are primarily based on Rayleigh scattering theory, which is applicable to high-altitude, dry, and clear weather conditions. In most scenarios, it is difficult to meet such ideal clear conditions. This paper proposes a bionic navigation method based on atmospheric polarization optimization to improve heading accuracy under non-ideal clear conditions. A signal model under non-ideal clear conditions was firstly established to introduce disturbances of aerosols and other particles into the raw signal function acquired by a camera. Then, an energy functional optimization model was constructed to eliminate the disturbances caused by large particle scattering and restore the original sky polarization pattern. Subsequently, the heading angle was calculated using astronomical data, enhancing accuracy under non-ideal conditions. Finally, we constructed a polarization compass system and conducted field experiments. The results demonstrate that the proposed algorithm effectively mitigates the impact of scattering from aerosols and other particles, reducing the heading angle error to within 2° under sunny, cloudy, overcast and sandy conditions. Full article

(This article belongs to the Special Issue Polarization Optics)

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10 pages, 2370 KiB

Open AccessCommunication

Extra-Cavity Modulating a Soliton Molecule with Chirped Gaussian Pulse Shape

by Daqian Tang, Junxiao Zhan, Dayu Wang, Haoming Wang, Yangyang Peng, Zian Cheak Tiu and Yan Zhou

Photonics 2024, 11(12), 1098; https://doi.org/10.3390/photonics11121098 - 21 Nov 2024

Abstract

In this work, we theoretically simulate the modulation of a soliton molecule that has an initial chirped Gaussian pulse shape in a 1 μm extra-cavity optical fiber modulation system. Different soliton parameters in orthogonal polarizations are applied to achieve controllable optical solitons’ output [...] Read more.

In this work, we theoretically simulate the modulation of a soliton molecule that has an initial chirped Gaussian pulse shape in a 1 μm extra-cavity optical fiber modulation system. Different soliton parameters in orthogonal polarizations are applied to achieve controllable optical solitons’ output with specific properties in the time/frequency domain. For instance, when the phase difference is changed, both pulse shapes’ and corresponding optical spectra’s peak intensities will have a sudden change when the orthogonal phase difference is π/2. These simulation results provide a beneficial reference value for extra-cavity shaping of different solitons that come from nonlinear optical systems. Optimally, the reported results could pave the groundwork for industrial growth in ultrafast laser design. Full article

(This article belongs to the Special Issue Editorial Board Members' Collection Series: Nonlinear Photonics)

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12 pages, 1342 KiB

Open AccessArticle

Diffraction Losses in a Stack of Diamond X-Ray Lenses

by Nataliya Klimova and Anatoly Snigirev

Photonics 2024, 11(12), 1097; https://doi.org/10.3390/photonics11121097 - 21 Nov 2024

Abstract

Compound refractive lenses, crafted from single-crystal materials like diamond and silicon, are increasingly favored, particularly in cutting-edge facilities, such as free electron lasers and fourth-generation synchrotrons. These lenses are prized for their low parasitic scattering and resistance to significant radiation doses over extended [...] Read more.

Compound refractive lenses, crafted from single-crystal materials like diamond and silicon, are increasingly favored, particularly in cutting-edge facilities, such as free electron lasers and fourth-generation synchrotrons. These lenses are prized for their low parasitic scattering and resistance to significant radiation doses over extended periods. However, they do encounter a notable drawback known as the “glitch effect”, wherein undesired diffraction can occur across various X-ray energies. This phenomenon leads to a decrease in transmitted intensity, impacting experiments, particularly in spectroscopy. Typically, a series of lenses is employed to achieve optimal beam parameters, and each lens has its own spectrum of glitches. This paper presents experimentally measured glitches in stacks of 1, 4, 8, and 16 diamond compound refractive lenses, elucidating the theory behind glitch formation and offering strategies to predict and mitigate glitches in diverse focusing systems employing lenses made from single-crystal materials. Full article

(This article belongs to the Special Issue Advances in X-ray Optics for High-Resolution Imaging)

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12 pages, 2377 KiB

Open AccessArticle

Low-Cost Optical Filters Based on SiO_xC_y:H and Ag Thin Films Fabricated by Plasma Enhanced Chemical Vapor Deposition and Sputtering

by Ahmed Kotbi, Michael Lejeune, Pierre Barroy, Ilham Hamdi Alaoui, Wiaam El Hakim, Frederic Lamarque and Andreas Zeinert

Photonics 2024, 11(11), 1096; https://doi.org/10.3390/photonics11111096 - 20 Nov 2024

Abstract

Hexamethyldisiloxane (HMDSO) is an organosilicon compound with a modifiable bandgap, depending on the deposition conditions. This material has many unique properties due to its stability, low toxicity, and strong adhesion, making it useful as a protective barrier against corrosion, moisture, and oxidation. In [...] Read more.

Hexamethyldisiloxane (HMDSO) is an organosilicon compound with a modifiable bandgap, depending on the deposition conditions. This material has many unique properties due to its stability, low toxicity, and strong adhesion, making it useful as a protective barrier against corrosion, moisture, and oxidation. In this work, HMDSO films were deposited on glass substrates by the Plasma Enhanced Chemical Vapor Deposition (PECVD) technique at different deposition times. The optical properties of HMDSO films, such as dielectric permittivity, refractive index, extinction and absorption coefficients, and band gap energy, are inferred from transmission and reflection spectra. As the deposition time increased, the real part of the dielectric constant, the refractive index, and the bandgap energy showed a decrease, dropping from 4.24 to 3.40, from 2.06 to 1.84, and from 2.85 eV to 2.03 eV, respectively. The latter result is determined using classical models such as the O’Leary-Johnson-Lim (‘OJL’) interband transition and the harmonic oscillator model. HMDSO and Silver are used in this study for the fabrication of optical filters using two types of structures, a multiple cavity metal–dielectric (MCMD) and the Fabry–Perot structure. The silver layers are deposited by a sputtering process. The MCMD optical filter shows a higher transmittance of about 30%, but a wide range of wavelengths is transmitted. In contrast, the Fabry–Perot filter showed high contrast but a lower transmittance of about 20%. Full article

(This article belongs to the Section Optoelectronics and Optical Materials)

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15 pages, 15053 KiB

Open AccessArticle

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

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

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

Abstract

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

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

W_{x y}

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

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

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19 pages, 11818 KiB

Open AccessArticle

Enhancing Alignment Accuracy in Laser Wireless Power Transmission Systems Using Integrated Target Detection and Perturbation-Observation Method

by Jinsong Kang, Liangrong Sun, Yanping Zhou and Yunfeng Bai

Photonics 2024, 11(11), 1094; https://doi.org/10.3390/photonics11111094 - 20 Nov 2024

Abstract

This paper introduces a high-precision alignment method for laser wireless power transmission (LWPT) systems, integrating neural network-based target detection with a perturbation-observation technique. The objective is to enhance the alignment accuracy between the laser spot and the photovoltaic array, thereby improving energy transfer [...] Read more.

This paper introduces a high-precision alignment method for laser wireless power transmission (LWPT) systems, integrating neural network-based target detection with a perturbation-observation technique. The objective is to enhance the alignment accuracy between the laser spot and the photovoltaic array, thereby improving energy transfer efficiency. The method’s key feature is its ability to achieve these results without requiring additional optical components, simplifying system design. Continual assessment and adjustment based on real-time output power data ensure optimal alignment, maximizing the photovoltaic array power output. Experimental results demonstrated that the proposed method achieved an initial alignment precision with pixel errors below 3%, translating to a physical error of approximately 7 mm. Fine-tuning through the perturbation-observation method further optimized the alignment, resulting in a photovoltaic array power output of 98.70% of its maximum potential. This hybrid approach provides a reliable solution for boosting the performance of LWPT systems, offering significant potential for practical applications. Full article

(This article belongs to the Special Issue Latest Papers Related to OWPT 2024 on the Topics of Devices, Components and Systems)

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11 pages, 2679 KiB

Open AccessArticle

Analysis of Randomization Capacity in Quantum Noise Randomized Cipher System

by Mingrui Zhang, Shuang Wei, Yuang Li, Yajie Li, Yongli Zhao and Jie Zhang

Photonics 2024, 11(11), 1093; https://doi.org/10.3390/photonics11111093 - 20 Nov 2024

Abstract

We propose and verify a method for analyzing the randomization capacity in a 160 km quantum noise randomized cipher system with different data modulation formats. The randomization capacity is defined as the difference in mutual information between Alice and Bob while the randomization [...] Read more.

We propose and verify a method for analyzing the randomization capacity in a 160 km quantum noise randomized cipher system with different data modulation formats. The randomization capacity is defined as the difference in mutual information between Alice and Bob while the randomization level is at 0 and at its maximum, under the condition of error-free transmission. Our experimental analysis examines the capacity of quantum noise randomized cipher systems under different optical signal-to-noise ratios for each modulation format. Additionally, we analyze the noise masking values while the randomization reaches its capacity. The experimental results indicate that the binary phase shift-keying-based quantum noise randomized cipher system achieves the highest randomization capacity and highest noise masking value. Full article

(This article belongs to the Special Issue Photonics for Emerging Applications in Communication and Sensing II)

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16 pages, 7263 KiB

Open AccessArticle

Inscription and Thermal Stability of Fiber Bragg Gratings in Hydrogen-Loaded Optical Fibers Using a 266 nm Pulsed Laser

by Xiangxi Zhu, Zixuan Xin, Haoming Zhu, Hongye Wang, Xin Cheng, Hwa-Yaw Tam, Hang Qu and Xuehao Hu

Photonics 2024, 11(11), 1092; https://doi.org/10.3390/photonics11111092 - 20 Nov 2024

Abstract

Fiber Bragg gratings (FBGs) have gained substantial research interest due to their exceptional sensing capabilities. Traditionally, FBG fabrication has required the use of pre-hydrogenated fibers and high-cost laser systems such as excimer lasers at 193 nm or femtosecond lasers. In this study, we [...] Read more.

Fiber Bragg gratings (FBGs) have gained substantial research interest due to their exceptional sensing capabilities. Traditionally, FBG fabrication has required the use of pre-hydrogenated fibers and high-cost laser systems such as excimer lasers at 193 nm or femtosecond lasers. In this study, we present the first instance of FBG inscription in hydrogen-loaded, standard single-mode silica optical fibers using a more affordable 266 nm solid-state pulsed laser combined with a scanning phase mask lithography technique. We systematically explored the effects of pulse energy and scanning speed on the quality and spectral characteristics of the gratings, achieving reflectivities as high as 99.81%. Additionally, we tracked the spectral evolution during the FBG inscription process, demonstrating uniform growth of the core mode. We also investigated the stability of the core mode during a 24-h thermal annealing process up to 150 °C. The sensitivity was 10.7 pm/°C in the range of 0 to 130 °C. Furthermore, strain measurement was conducted based on the FBG annealed at 100 °C, showing a sensitivity of 0.943 pm/µε in the range of 0 to 1667 µε. Full article

(This article belongs to the Special Issue Advances in Polymer Optical Fiber Sensors: Materials, Designs and Applications)

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17 pages, 5969 KiB

Open AccessArticle

Rapid Autofocus Method Based on LED Oblique Illumination for Metaphase Chromosome Microscopy Imaging System

by Changliang Yu, Fangqiu Ding, Zhenyu Ma and Yuguo Tang

Photonics 2024, 11(11), 1091; https://doi.org/10.3390/photonics11111091 - 20 Nov 2024

Abstract

In clinical practice, microscopes are commonly used for imaging chromosomes to diagnose genetic diseases. Achieving precise and rapid autofocusing is a significant challenge in the advancement of high-throughput chromosome imaging systems. Here, we introduce a rapid autofocus method based on LED oblique illumination [...] Read more.

In clinical practice, microscopes are commonly used for imaging chromosomes to diagnose genetic diseases. Achieving precise and rapid autofocusing is a significant challenge in the advancement of high-throughput chromosome imaging systems. Here, we introduce a rapid autofocus method based on LED oblique illumination for dual-objective configuration in metaphase chromosome imaging system. Our method utilizes a programmable LED array for sample illumination, employing a sequential activation of two LEDs from opposing angles to create oblique illumination. The defocus distance is ascertained through image cross-correlation calculation. Illumination with multiple LEDs in the array is used to acquire bright-field images after completing the focusing. Our method can perform continuous autofocus under a 10× objective and a 100× oil immersion objective, with average focusing errors of 1.29 μm and 0.12 μm, respectively. The total imaging time for a single sample has been significantly reduced from approximately 10 min with conventional methods to just 2 min. This study provides preliminary evidence supporting the viability of developing a next-generation high-throughput chromosome scanner employing a LED array. Full article

(This article belongs to the Section Optical Interaction Science)

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15 pages, 3783 KiB

Open AccessArticle

Exploring Changes in Ocular Aberrations for Different Fixation and Accommodation Stimuli

by María Mechó-García, Rute J. Macedo-de-Araújo, Paulo Fernandes and José Manuel González-Méijome

Photonics 2024, 11(11), 1090; https://doi.org/10.3390/photonics11111090 - 20 Nov 2024

Abstract

Background: Given the lack of standardization in stimulus types for assessing accommodation, we aimed to evaluate accommodative response (AR) and Zernike coefficients using four different stimuli. Methods: Sixteen healthy subjects aged 22–32 years participated. Four black transilluminated stimuli (Snellen 6/12 “E”, 6/6 “e”, [...] Read more.

Background: Given the lack of standardization in stimulus types for assessing accommodation, we aimed to evaluate accommodative response (AR) and Zernike coefficients using four different stimuli. Methods: Sixteen healthy subjects aged 22–32 years participated. Four black transilluminated stimuli (Snellen 6/12 “E”, 6/6 “e”, Maltese Cross 6/12 “X”, 6/6 “x”) were used to stimulate accommodation from 0 D to 5 D, in 1 D increments, using the irx3 aberrometer. From the results, AR was calculated with Seidel defocus and the change in Zernike coefficient value between the non-accommodative state and the fully accommodative state (5 D) was determined. Results: Larger pupils were observed with stimulus “E” (p-value < 0.05). The mean AR at the maximum accommodative level (5 D) for the different stimuli was −1.88 ± 1.00 for “E”, −2.60 ± 1.44 for “X”, −2.00 ± 1.32 for “e”, and −2.40 ± 1.27 for “x”. No statistically significant differences were found between AR and Zernike coefficients with the four different accommodative stimuli (p-value > 0.05, one-way ANOVA). Conclusions: The study evaluated accommodative stimulus design and size on AR and Zernike coefficients and found no significant differences. However, stimuli with higher spatial frequencies (“e” and “E”) provided larger ARs compared to the other stimuli. Full article

(This article belongs to the Special Issue Novel Techniques and Applications of Ophthalmic Optics)

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24 pages, 7512 KiB

Open AccessArticle

Color Reproduction of Chinese Painting Under Multi-Angle Light Source Based on BRDF

by Xinting Li, Jie Feng and Jie Liu

Photonics 2024, 11(11), 1089; https://doi.org/10.3390/photonics11111089 - 20 Nov 2024

Abstract

It is difficult to achieve high-precision color reproduction using traditional color reproduction methods when the angle is changed, and, for large-sized artefacts, it is also significantly difficult to collect a large amount of data and reproduce the colors. In this paper, we use [...] Read more.

It is difficult to achieve high-precision color reproduction using traditional color reproduction methods when the angle is changed, and, for large-sized artefacts, it is also significantly difficult to collect a large amount of data and reproduce the colors. In this paper, we use three Bidirectional Reflectance Distribution Function (BRDF) modeling methods based on spectral imaging techniques, namely, the five-parameter model, the Cook–Torrance model and the segmented linear interpolation model. We investigated the color reproduction of color chips with matte surfaces and Chinese paintings with rough surfaces under unknown illumination angles. Experiments have shown that all three models can effectively perform image reconstruction under small illumination angle intervals. The segmented linear interpolation model exhibits a higher stability and accuracy in color reconstruction under small and large illumination angle intervals; it can not only reconstruct color chips and Chinese painting images under any illumination angle, but also achieve high-quality image color reconstruction standards in terms of objective data and intuitive perception. The best test model (segmented linear interpolation) performs well in reconstruction, reconstructing Chinese paintings at 65° and 125° with an illumination angle interval of 10°. The average RMSE of the selected reference color blocks is 0.0450 and 0.0589, the average CIEDE2000 color difference is 1.07 and 1.50, and the SSIM values are 0.9227 and 0.9736, respectively. This research can provide a theoretical basis and methodological support for accurate color reproduction as well as the large-sized scientific prediction of artifacts at any angle, and has potential applications in cultural relic protection, art reproduction, and other fields. Full article

(This article belongs to the Special Issue Optical Imaging and Measurements: 2nd Edition)

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