Photonics

10 pages, 4214 KiB

Open AccessArticle

SBS Mitigation by Manipulating the Injecting Polarization Direction in a High-Power Monolithic PM Amplifier

by Yu Wen, Chun Zhang, Chenxu Liu, Qiuhui Chu, Lingli Huang, Yuan Zhu, Haoyu Zhang, Rumao Tao, Honghuan Lin and Jianjun Wang

Photonics 2024, 11(9), 890; https://doi.org/10.3390/photonics11090890 (registering DOI) - 21 Sep 2024

Abstract

The polarization direction-dependent SBS threshold was investigated, and the terminal polarization control technique was demonstrated to restore the linear polarization state. By increasing the relative angle of the injecting polarization direction from 0° to 90°, the measured SBS threshold increased until reaching a [...] Read more.

The polarization direction-dependent SBS threshold was investigated, and the terminal polarization control technique was demonstrated to restore the linear polarization state. By increasing the relative angle of the injecting polarization direction from 0° to 90°, the measured SBS threshold increased until reaching a maximum value, beyond which it decreased in a nearly symmetrical trend. The highest SBS threshold was achieved with the relative polarization angle being 45°, delivering a 67% threshold enhancement compared with that at 0°. A quarter-wave-plate was used to restore the polarization state of the output laser manually from an elliptic to a linearly polarized state, and temperature-dependent polarization fluctuation was observed, which intensified as the laser power was scaled. By reducing the cooling temperature, a 1 kW laser with a linearly polarized state was demonstrated using a 45° polarization direction-injected monolithic PM amplifier. Full article

(This article belongs to the Special Issue High-Power Fiber Lasers)

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13 pages, 2844 KiB

Open AccessArticle

Multi-Fresnel-Lens Pumping Approach for Simultaneous Emission of Seven TEM₀₀-Mode Beams with 3.73% Conversion Efficiency

by Hugo Costa, Dawei Liang, Ana Matos and Joana Almeida

Photonics 2024, 11(9), 889; https://doi.org/10.3390/photonics11090889 (registering DOI) - 20 Sep 2024

Abstract

TEM₀₀-mode operation is a requirement in many laser-based applications due to the small divergence and high-power density of the emitted laser beam. A solar laser scheme was designed and numerically studied with the goal of increasing the solar-to-laser power conversion efficiency [...] Read more.

TEM₀₀-mode operation is a requirement in many laser-based applications due to the small divergence and high-power density of the emitted laser beam. A solar laser scheme was designed and numerically studied with the goal of increasing the solar-to-laser power conversion efficiency in the TEM₀₀-mode operation. The collection and primary concentration of sunlight was performed via twelve sets of folding mirrors and Fresnel lenses, toward a laser head composed of a fused silica torus volume and seven Ce:Nd:YAG rods, in a side-pump configuration. With this scheme, TEM₀₀-mode laser power totaling 212.39 W could potentially be produced from seven beams, with six of them being 32.60 W each and with

M_{x}^{2}

= 1.00,

M_{y}^{2}

= 1.01 quality factors. Notably, 35.40 W/m² collection efficiency and, most importantly, 3.73% solar-to-laser power conversion efficiency were numerically achieved. The latter efficiency value represents a 1.81-time improvement over the experimental record, established with a prototype that had a single Ce:Nd:YAG rod in an end-side pump configuration. Full article

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

13 pages, 1582 KiB

Open AccessArticle

Iron-Modified Nano-TiO₂: Comprehensive Characterization for Enhanced Photocatalytic Properties

by Élida M. Margalho, Orlando Lima, Jr., Cátia Afonso, Iran Rocha Segundo, Salmon Landi, Jr., Elisabete Freitas, Manuel F. M. Costa and Joaquim Carneiro

Photonics 2024, 11(9), 888; https://doi.org/10.3390/photonics11090888 (registering DOI) - 20 Sep 2024

Abstract

This study investigates the effect of iron-modified nano-TiO₂, using the co-precipitation method with different concentrations of FeCl₃ (0.1, 1, and 10%), to improve its photocatalytic properties for outdoor applications. To this end, modified and unmodified nano-TiO₂ were characterized using [...] Read more.

This study investigates the effect of iron-modified nano-TiO₂, using the co-precipitation method with different concentrations of FeCl₃ (0.1, 1, and 10%), to improve its photocatalytic properties for outdoor applications. To this end, modified and unmodified nano-TiO₂ were characterized using different techniques. The optical properties were characterized by diffuse reflectance spectroscopy (DRS) followed by band gap calculation. X-ray diffraction (XRD) was used to analyze the crystalline structure. Chemical and morphological characterization were carried out using energy-dispersive X-ray spectroscopy (EDS) and scanning electron microscopy (SEM). The photocatalytic activity was investigated by decolorizing Rhodamine B aqueous solutions under similar sunlight irradiation. The results indicate that the modification improved light absorption in the UV range for all iron concentrations; however, only the concentration of TiO₂: FeCl₃ (10%) shifted the absorption to the visible region. Also, including Fe³⁺ in TiO₂ decreased the band gap energy from 3.14 to up to 2.80 eV. There were variations in crystallite size from 21.13 to up to 40.07 nm. The nano-TiO₂ morphology analysis showed that it did not change after iron modification. EDS showed an FeCl₃ peak only at higher concentrations (10%). In addition, the 0.1% Fe-modified TiO₂ exhibited the highest activity in the photocatalytic process, with an efficiency of 95.23% after 3 h of irradiation. Full article

(This article belongs to the Special Issue Current Advances on Applications of Optics and Photonics – Selected Papers from the 6th International Conference on Applications of Optics and Photonics, AOP2024)

12 pages, 3246 KiB

Open AccessArticle

Long-Range Imaging through Scattering Media Using Deep Learning

by Ying Jin, Cangtao Zhou and Wanjun Dai

Photonics 2024, 11(9), 887; https://doi.org/10.3390/photonics11090887 (registering DOI) - 20 Sep 2024

Abstract

Imaging through scattering media is an important and challenging problem, and the technology has been used in many fields, such as autonomous driving, industrial inspections, remote sensing imaging, and biomedical imaging. However, most of the previous experiments used numbers or letters for close-range [...] Read more.

Imaging through scattering media is an important and challenging problem, and the technology has been used in many fields, such as autonomous driving, industrial inspections, remote sensing imaging, and biomedical imaging. However, most of the previous experiments used numbers or letters for close-range imaging, while objects in life are colorful. In this study, a new deep learning network, DesUNet, was constructed to image realistic objects at medium and long distances under sunlight through scattering media, and to realize object recognition. In addition, this study also compares the imaging results of different neural networks, and the results show that the DesUNet network improves the feature information storage ability and enhances the image reconstruction. It not only clearly restores the original appearance of the object, but also extracts the physical information about the object. In order to further verify the power of the DesUNet network, this study also conducted indoor near distance and outdoor medium distance imaging experiments. For indoor reconstructed objects, the appearance of the objects could be clearly identified. For outdoor reconstructed objects, the confidence level could reach above 0.9 through YOLO. The experiments show that the DesUNet network has good robustness and generalization. Full article

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11 pages, 3871 KiB

Open AccessArticle

A Bidirectional Mode-Locked Fiber Laser for Asynchronous Noise-like Pulse Generation

by Yongguo Zheng, Anshuang Wang, Yue Cai, Fan Zhang and Xinhai Zhang

Photonics 2024, 11(9), 886; https://doi.org/10.3390/photonics11090886 (registering DOI) - 20 Sep 2024

Abstract

We demonstrate a bidirectional fiber laser with an artificial saturable absorber of a step-index multimode fiber sandwiched by single-mode fibers. Two asynchronous noise-like pulse sequences with a repetition frequency difference of 3.16 kHz are obtained. The average power of the bidirectional asynchronous noise-like [...] Read more.

We demonstrate a bidirectional fiber laser with an artificial saturable absorber of a step-index multimode fiber sandwiched by single-mode fibers. Two asynchronous noise-like pulse sequences with a repetition frequency difference of 3.16 kHz are obtained. The average power of the bidirectional asynchronous noise-like pulses is greater than 36 mW, and the pulse energy is greater than 3 nJ. The laser we demonstrate has potential applications in micromachining, supercontinuum spectrum generation, and signal processing. Full article

(This article belongs to the Special Issue Nonlinear Optics and Hyperspectral Polarization Imaging)

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14 pages, 3284 KiB

Open AccessArticle

Low Complexity Parallel Carrier Frequency Offset Estimation Based on Time-Tagged QPSK Partitioning for Coherent Free-Space Optical Communication

by Siqi Zhang, Liqian Wang, Kunfeng Liu and Shuang Ding

Photonics 2024, 11(9), 885; https://doi.org/10.3390/photonics11090885 (registering DOI) - 20 Sep 2024

Abstract

To effectively mitigate the effects of atmospheric turbulence in free space optical (FSO) communication, we propose a parallel carrier frequency offset estimation (FOE) scheme based on time-tagged QPSK partitioning (TTQP). This scheme can be applied to spatial diversity polarization multiplexing (PM) coherent FSO [...] Read more.

To effectively mitigate the effects of atmospheric turbulence in free space optical (FSO) communication, we propose a parallel carrier frequency offset estimation (FOE) scheme based on time-tagged QPSK partitioning (TTQP). This scheme can be applied to spatial diversity polarization multiplexing (PM) coherent FSO communication systems. Specifically, the TTQP scheme performs QPSK partitioning by time-tagging signal points, accurately recording the time intervals between signals, and significantly reducing implementation complexity through a modified Mth power algorithm. The simulation results for the PM 16-quadrature amplitude modulation (QAM) validate the effectiveness of the proposed scheme. Compared to traditional QPSK partitioning algorithms, the TTQP algorithm achieves high accuracy, low complexity, and multi-format versatility in high-speed coherent FSO communication. Full article

(This article belongs to the Special Issue Challenges and Opportunities in Free Space Optical Communication)

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9 pages, 2402 KiB

Open AccessCommunication

Study on the Influence of Atmospheric Light Intensity Scintillation Effect on Optical Fiber Coupling Efficiency

by Xiaoying Ding and Xin Zhao

Photonics 2024, 11(9), 884; https://doi.org/10.3390/photonics11090884 (registering DOI) - 20 Sep 2024

Abstract

Light intensity flicker is the most basic and important effect of turbulence. Fiber coupling efficiency is the primary parameter that ensures the system’s communication quality. The light intensity flicker effect caused by atmospheric turbulence significantly affects the coupling efficiency of space light in [...] Read more.

Light intensity flicker is the most basic and important effect of turbulence. Fiber coupling efficiency is the primary parameter that ensures the system’s communication quality. The light intensity flicker effect caused by atmospheric turbulence significantly affects the coupling efficiency of space light in a single-mode optical fiber. Based on the principle of fiber coupling efficiency, this paper first establishes the relationship between light intensity flicker and spatial coherence radius, then analyzes the influence of light intensity flicker on fiber coupling efficiency through the spatial coherence radius. A laser communication system was built, and real-time measurement experiments on atmospheric light intensity flicker and optical fiber coupling efficiency at different transmission distances and altitudes above the ground were completed. The experimental results show that the constructed experimental system can simultaneously measure the scintillation index and fiber coupling efficiency. When the communication distance is 12,000 m and the height is 600 m above the ground, the scintillation index is measured to be 0.63, and the coupling efficiency is 0.05. The results of the study provide an experimental and theoretical basis and data support to promote the development of atmospheric laser communications. Full article

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

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14 pages, 841 KiB

Open AccessArticle

A Closed-Form Analytical Conversion between Zernike and Gatinel–Malet Basis Polynomials to Present Relevant Aberrations in Ophthalmology and Refractive Surgery

by Masoud Mehrjoo, Damien Gatinel, Jacques Malet and Samuel Arba Mosquera

Photonics 2024, 11(9), 883; https://doi.org/10.3390/photonics11090883 (registering DOI) - 20 Sep 2024

Abstract

The Zernike representation of wavefronts interlinks low- and high-order aberrations, which may result in imprecise clinical estimates. Recently, the Gatinel–Malet wavefront representation has been introduced to resolve this problem by deriving a new, unlinked basis originating from Zernike polynomials. This new basis preserves [...] Read more.

The Zernike representation of wavefronts interlinks low- and high-order aberrations, which may result in imprecise clinical estimates. Recently, the Gatinel–Malet wavefront representation has been introduced to resolve this problem by deriving a new, unlinked basis originating from Zernike polynomials. This new basis preserves the classical low and high aberration subgroups’ structure, as well as the orthogonality within each subgroup, but not the orthogonality between low and high aberrations. This feature has led to conversions relying on separate wavefront reconstructions for each subgroup, which may increase the associated numerical errors. This study proposes a robust, minimised-error (lossless) analytical approach for conversion between the Zernike and Gatinel–Malet spaces. This method analytically reformulates the conversion as a nonhomogeneous system of linear equations and computationally solves it using matrix factorisation and decomposition techniques with high-level accuracy. This work fundamentally demonstrates the lossless expression of complex wavefronts in a format that is more clinically interpretable, with potential applications in various areas of ophthalmology, such as refractive surgery. Full article

(This article belongs to the Special Issue Visual Optics)

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10 pages, 6069 KiB

Open AccessArticle

Robust Sensing Based on Exceptional Points in Detuned Non-Hermitian Scattering System

by Jipeng Xu, Yuanhao Mao, Ken Liu and Zhihong Zhu

Photonics 2024, 11(9), 882; https://doi.org/10.3390/photonics11090882 - 20 Sep 2024

Abstract

Non-Hermitian optics has revealed a series of counterintuitive phenomena with profound implications for sensing, lasing, and light manipulation. While the non-Hermiticity of Hamiltonians has been intensively investigated, recent advancements in the non-Hermitian scattering matrix have given birth to a lot of unique phenomena, [...] Read more.

Non-Hermitian optics has revealed a series of counterintuitive phenomena with profound implications for sensing, lasing, and light manipulation. While the non-Hermiticity of Hamiltonians has been intensively investigated, recent advancements in the non-Hermitian scattering matrix have given birth to a lot of unique phenomena, such as simultaneous lasing and anti-lasing, reflectionless scattering modes (RSMs), and coherent chaos control. Despite these developments, the investigation has predominantly focused on static and symmetric configurations, leaving the dynamic properties of non-Hermitian scattering in detuned systems, which is essential for applications in sensing and beyond, largely unexplored. Here, we extend the stationary behaviors associated with the RSMs to resonant detuned systems. Contrary to the common belief of exceptional point (EP) sensors as being susceptible to parametric disturbances, we induce an RSM EP in a one-dimensional optical cavity and demonstrate its robustness in displacement sensing against laser frequency drifts up to 10 MHz. Our findings not only contribute to the broader understanding of non-Hermitian scattering phenomena but also pave the way for the next generation of non-Hermitian sensors. Full article

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

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13 pages, 3642 KiB

Open AccessArticle

A Fast Solution of the Dynamic Rate Equation for a High-Power Fiber Laser

by Lei Duan, Runqin Xu, Shudan Tan, Xiongxin Tang and Fanjiang Xu

Photonics 2024, 11(9), 881; https://doi.org/10.3390/photonics11090881 - 19 Sep 2024

Abstract

In the study of dynamic behaviors, such as nonlinear effects, power evolution, and pulse evolution of light in fiber gain media, solving dynamic rate equations in fiber laser systems is involved, which is computationally intensive and directly affects overall computational efficiency. A modified [...] Read more.

In the study of dynamic behaviors, such as nonlinear effects, power evolution, and pulse evolution of light in fiber gain media, solving dynamic rate equations in fiber laser systems is involved, which is computationally intensive and directly affects overall computational efficiency. A modified difference scheme is proposed to solve fiber dynamic rate equations efficiently. The advantages of the improved scheme and its convergence rate are analyzed. By incorporating a correction coefficient into the finite difference, the approximations of spatial and temporal derivatives are improved, greatly enhancing the performance of the numerical method. The computational results of the proposed method are compared with those of the conventional upwind difference scheme, demonstrating that the improved method is more stable and requires fewer sampling points to maintain a certain level of precision, thereby saving significant computation time and computational resources. The power and spectral evolutions of the fiber laser oscillator under different pump conditions are simulated and compared with experimental data, validating the applicability and reliability of the rapid solving method. Full article

(This article belongs to the Special Issue High-Power Fiber Lasers)

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8 pages, 956 KiB

Open AccessCommunication

Quality Assessment of Perovskite Solar Cells: An Industrial Point of View

by Nicolò Lago, Francesco Moretti, Noah Tormena, Alessandro Caria, Matteo Buffolo, Carlo De Santi, Nicola Trivellin, Andrea Cester, Gaudenzio Meneghesso, Enrico Zanoni, Matteo Meneghini, Fabio Matteocci, Jessica Barichello, Luigi Vesce, Aldo Di Carlo and Federico Quartiani

Photonics 2024, 11(9), 880; https://doi.org/10.3390/photonics11090880 - 19 Sep 2024

Abstract

The mass production of photovoltaic (PV) devices requires fast and reliable characterization methods and equipment. PV manufacturers produce a complete module roughly every 20 s, and the electrical performance assessment is typically completed in less than 1 s. Times are even more stringent [...] Read more.

The mass production of photovoltaic (PV) devices requires fast and reliable characterization methods and equipment. PV manufacturers produce a complete module roughly every 20 s, and the electrical performance assessment is typically completed in less than 1 s. Times are even more stringent during cell manufacturing. To be competitive in the PV market, perovskite solar cells and modules aim to the same target, i.e., fast and reliable quality assessment. This communication report discusses the limit of characterizing the current perovskite technology. Standard current vs voltage measurements are compared to maximum power point tracking (MPPT), and a fast MPPT procedure is developed to meet the highly demand standard for quality control in the industry of PV production. Full article

(This article belongs to the Special Issue Advances in Perovskite Solar Cells)

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7 pages, 2081 KiB

Open AccessCommunication

Improving the Performance of Bidirectional Communication System Using Second-Order Raman Amplifiers

by Zhongshuai Feng, Peili He, Wei Li, Kaijing Hu, Fei Tong and Xingrui Su

Photonics 2024, 11(9), 879; https://doi.org/10.3390/photonics11090879 - 19 Sep 2024

Abstract

In order to achieve low-cost scalability, the same-wavelength bidirectional (SWB) fiber communication system is a better solution. We present a detailed investigation of the performance of the different orders Raman amplifiers in same-wavelength bidirectional fiber communication systems. We discuss how to suppress the [...] Read more.

In order to achieve low-cost scalability, the same-wavelength bidirectional (SWB) fiber communication system is a better solution. We present a detailed investigation of the performance of the different orders Raman amplifiers in same-wavelength bidirectional fiber communication systems. We discuss how to suppress the main factor affecting system performance which is Rayleigh scattering noise (RSN). By using different Raman amplifiers to construct different quasi-lossless transmission, the performance changes in the same-wavelength bidirectional fiber optic communication system were studied. On this basis, multi-channel and same-wavelength single fiber bidirectional system experiments were conducted to compare the performance of second-order Raman systems and first-order Raman systems. The results indicate that the Rayleigh scattering suppression effect of second-order Raman systems is better, and compared to first-order Raman systems, the average signal-to-noise ratio (SNR) can be increased by 2.88 dB. Full article

(This article belongs to the Special Issue Advancements in Optical Sensing and Communication Technologies)

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16 pages, 7263 KiB

Open AccessArticle

Asymmetric Optical Scanning Holography Encryption with Elgamal Algorithm

by Chunying Wu, Yinggang Ding, Aimin Yan, Ting-Chung Poon and Peter Wai Ming Tsang

Photonics 2024, 11(9), 878; https://doi.org/10.3390/photonics11090878 - 19 Sep 2024

Abstract

This paper proposes an asymmetric scanning holography cryptosystem based on the Elgamal algorithm. The method encodes images with sine and cosine holograms. Subsequently, each hologram is divided into a signed bit matrix and an unsigned hologram matrix, both encrypted using the sender’s private [...] Read more.

This paper proposes an asymmetric scanning holography cryptosystem based on the Elgamal algorithm. The method encodes images with sine and cosine holograms. Subsequently, each hologram is divided into a signed bit matrix and an unsigned hologram matrix, both encrypted using the sender’s private key and the receiver’s public key. The resulting ciphertext matrices are then transmitted to the receiver. Upon receipt, the receiver decrypts the ciphertext matrices using their private key and the sender’s public key. We employ an asymmetric single-image encryption method for key management and dispatch for securing imaging and transmission. Furthermore, we conducted a sensitivity analysis of the encryption system. The image encryption metrics, including histograms of holograms, adjacent pixel correlation, image correlation, the peak signal-to-noise ratio, and the structural similarity index, were also examined. The results demonstrate the security and stability of the proposed method. Full article

(This article belongs to the Special Issue Holographic Information Processing)

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9 pages, 3631 KiB

Open AccessArticle

Kilowatt-Level High-Efficiency Narrow-Linewidth All-Fiber Tm³⁺-Doped Laser

by Hongyu Wang, Qilai Zhao, Hang Liu, Yuxin Sun, Jialong Li, Junjie Zheng, Ye Yuan, Qianwen Zhang, Changsheng Yang, Yujun Feng, Yinhong Sun, Zhongmin Yang and Shanhui Xu

Photonics 2024, 11(9), 877; https://doi.org/10.3390/photonics11090877 - 19 Sep 2024

Abstract

In this study, a kilowatt-level high-efficiency narrow-linewidth all-fiber Tm³⁺-doped continuous-wave laser operating at 1.95 μm is demonstrated. Benefitting from an advanced boost design of a two-stage main amplifier, it not only effectively manages heat dissipation resulting from the high pump-induced quantum [...] Read more.

In this study, a kilowatt-level high-efficiency narrow-linewidth all-fiber Tm³⁺-doped continuous-wave laser operating at 1.95 μm is demonstrated. Benefitting from an advanced boost design of a two-stage main amplifier, it not only effectively manages heat dissipation resulting from the high pump-induced quantum defect but also realizes the controlled extraction of optical gain and improves the optical conversion efficiency. Finally, this laser system has realized an output power of 1018 W, a linewidth of 3.8 GHz, and a slope efficiency of 60.0% simultaneously. Moreover, a high optical signal-to-noise ratio of over 45 dB and excellent beam quality of M² factors 1.19 are obtained. To the best of our knowledge, this represents the narrowest linewidth and highest slope efficiency achieved in a kilowatt-level Tm³⁺-doped fiber laser. Such a high-performance laser is ideally suited for mid-infrared generation and remote sensing applications. Full article

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

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11 pages, 4907 KiB

Open AccessArticle

The Correction Method for Wavefront Aberration Caused by Spectrum-Splitting Filters in Multi-Modal Optical Imaging System

by Xiaolin Liu, Ying Huang, Xu Yan, Li Wang, Qiang Li, Tingcheng Zhang, Bin Hu, Wenping Lei, Shengbo Mu and Xiaohong Zhang

Photonics 2024, 11(9), 876; https://doi.org/10.3390/photonics11090876 - 19 Sep 2024

Abstract

In current biomedical and environmental detection, multi-modal optical imaging technology is playing an increasingly important role. By utilizing information from dimensions such as spectra and polarization, it reflects the detailed characteristics and material properties of the targets. However, as detection system performance becomes [...] Read more.

In current biomedical and environmental detection, multi-modal optical imaging technology is playing an increasingly important role. By utilizing information from dimensions such as spectra and polarization, it reflects the detailed characteristics and material properties of the targets. However, as detection system performance becomes more complex, issues such as aberrations introduced by multilayered lenses, signal attenuation, decreased polarization sensitivity, and latency can no longer be ignored. These factors directly affect the assessment of image details, influencing subsequent analyses. In this paper, we propose a method for designing and optimizing spectrum-splitting filters that considers the wavefront aberration and transmittance of the multi-modal optical imaging system. The method of optimizing coating phases to minimize scalar phase aberrations while maximizing system transmission leads to substantially improved imaging performance. Simulation and experimental results demonstrate that the method can improve the imaging performance. The proposed approach has potential applications in fields such as biomedical field, multi-spectral, remote sensing and microscopy. Full article

(This article belongs to the Special Issue Optical Technologies for Biomedical Science)

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13 pages, 5982 KiB

Open AccessArticle

Comparative Study on the Interest in Non-Uniform Rational B-Splines Representation versus Polynomial Surface Description in a Freeform Three-Mirror Anastigmat

by Clément Freslier, Guillaume Druart, Alice Fontbonne, Thierry Lépine, Christophe Buisset, Tibor Agocs, Arnaud Heliere, Fanny Keller, Jean-Baptiste Volatier, Stéphane Beaussier and Paul Jougla

Photonics 2024, 11(9), 875; https://doi.org/10.3390/photonics11090875 - 18 Sep 2024

Abstract

Novel freeform optical design methods can be classified in two categories, depending on whether they focus on the generation of a starting point or the development of new optimization tools. In this paper, we design a freeform three-mirror anastigmat (TMA) and compare different [...] Read more.

Novel freeform optical design methods can be classified in two categories, depending on whether they focus on the generation of a starting point or the development of new optimization tools. In this paper, we design a freeform three-mirror anastigmat (TMA) and compare different surface representations using either a differential ray tracer as a new optimization tool or a commercial ray tracer (ANSYS-ZEMAX OpticStudio). For differential ray tracing, we used FORMIDABLE (Freeform Optics Raytracer with Manufacturable Imaging Design cApaBiLitiEs), an optical design library with differential ray tracing and Non-Uniform Rational B-Splines (NURBS) optimization capabilities, available under the European Software Community License (ESCL). NURBS allow a freeform surface to be represented without needing any prior knowledge of the surface, such as the polynomial degree in polynomial descriptions. OpticStudio and other commercial optical design software are designed to optimize polynomial surfaces but are not well-suited to optimize NURBS surfaces, requiring a custom optical design library. In order to demonstrate the interest in using NURBS representation, we designed and independently optimized two freeform telescopes over different iteration cycles; with NURBS using FORMIDABLE or with XY polynomials using OpticStudio. We then compared the resulting systems using their root mean square field maps to assess the optimization quality of each surface representation. We also provided a full-system comparison, including mirror freeform departures. This study shows that NURBS can be a relevant alternative to XY polynomials for the freeform optimization of reflective three-mirror telescopes as it achieves more a uniform imaging quality in the field of view. Full article

(This article belongs to the Special Issue New Advances in Freeform Optics Design)

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20 pages, 11342 KiB

Open AccessArticle

Accurate Inspection and Super-Resolution Reconstruction for Additive Manufactured Defects Based on Stokes Vector Method and Deep Learning

by Shangrongxi Sun, Xing Peng and Hongbing Cao

Photonics 2024, 11(9), 874; https://doi.org/10.3390/photonics11090874 - 18 Sep 2024

Abstract

Defects in additive manufacturing processes are closely related to the mechanical and physical properties of the components. However, the extreme conditions of high temperatures, intense light, and powder during the manufacturing process present significant challenges for defect detection. Additionally, the high reflectivity of [...] Read more.

Defects in additive manufacturing processes are closely related to the mechanical and physical properties of the components. However, the extreme conditions of high temperatures, intense light, and powder during the manufacturing process present significant challenges for defect detection. Additionally, the high reflectivity of metallic components can cause pixels in image sensors to become overexposed, resulting in the loss of many defect signals. Thus, this paper mainly focuses on proposing an accurate inspection and super-resolution reconstruction method for additive manufactured defects based on Stokes vector and deep learning, where the Stokes vectors, polarization degree, and polarization angles of the inspected defects are effectively utilized to suppress the high reflectivity of metallic surfaces, enhance the contrast of defect regions, and highlight the boundaries of defects. Furthermore, a modified SRGAN model designated SRGAN-H is presented by employing an additional convolutional layer and activation functions, including Harswish and Tanh, to accelerate the convergence of the SRGAN-H network and improve the reconstruction of the additive manufactured defect region. The experiment results demonstrated that the SRGAN-H model outperformed SRGAN and traditional SR reconstruction algorithms in terms of the images of Stokes vectors, polarization degree, and polarization angles. For the scratch and hole test sets, the PSNR values were 33.405 and 31.159, respectively, and the SSIM values were 0.890 and 0.896, respectively. These results reflect the effectiveness of the SRGAN-H model in super-resolution reconstruction of scratch and hole images. For the scratch and hole images chosen in this study, the PSNR values of SRGAN-H for single image super-resolution reconstruction ranged from 31.86786 to 43.82374, higher than the results obtained by the pre-improvement SRGAN algorithm. Full article

(This article belongs to the Special Issue New Perspectives in Optical Design)

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32 pages, 6923 KiB

Open AccessReview

A Comprehensive Exploration of Contemporary Photonic Devices in Space Exploration: A Review

by Muhammad A. Butt

Photonics 2024, 11(9), 873; https://doi.org/10.3390/photonics11090873 - 18 Sep 2024

Abstract

Photonics plays a pivotal role in propelling space exploration forward, providing innovative solutions to address the challenges presented by the unforgiving and expansive realm of outer space. Photonic-based devices, encompassing technologies such as lasers, optical fibers, and photodetectors, are instrumental in various aspects [...] Read more.

Photonics plays a pivotal role in propelling space exploration forward, providing innovative solutions to address the challenges presented by the unforgiving and expansive realm of outer space. Photonic-based devices, encompassing technologies such as lasers, optical fibers, and photodetectors, are instrumental in various aspects of space missions. A notable application is in communication systems, where optical communication facilitates high-speed data transfer, ensuring efficient transmission of information across vast interplanetary distances. This comprehensive review unveils a selection of the most extensively employed photonic devices within the realm of space exploration. Full article

(This article belongs to the Special Issue Innovations in Optical Wireless Communications: Challenges and Opportunities)

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17 pages, 3138 KiB

Open AccessReview

Advances in Photoacoustic Endoscopic Imaging Technology for Prostate Cancer Detection

by Ningning Wei, Huiting Chen, Bin Li, Xiaojun Dong and Bo Wang

Photonics 2024, 11(9), 872; https://doi.org/10.3390/photonics11090872 - 18 Sep 2024

Abstract

The rapid progress in biomedical imaging technology has generated considerable interest in new non-invasive photoacoustic endoscopy imaging techniques. This emerging technology offers significant benefits, including high spectral specificity, strong tissue penetration, and real-time multidimensional high-resolution imaging capabilities, which enhance clinical diagnosis and treatment [...] Read more.

The rapid progress in biomedical imaging technology has generated considerable interest in new non-invasive photoacoustic endoscopy imaging techniques. This emerging technology offers significant benefits, including high spectral specificity, strong tissue penetration, and real-time multidimensional high-resolution imaging capabilities, which enhance clinical diagnosis and treatment of prostate cancer. This paper delivers a thorough review of current prostate cancer screening techniques, the core principles of photoacoustic endoscopy imaging, and the latest research on its use in detecting prostate cancer. Additionally, the limitations of this technology in prostate cancer detection are discussed, and future development trends are anticipated. Full article

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

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