Photonics

17 pages, 8874 KiB

Open AccessArticle

Adaptive DBP System with Long-Term Memory for Low-Complexity and High-Robustness Fiber Nonlinearity Mitigation

by Mingqing Zuo, Huitong Yang, Yi Liu, Zhengyang Xie, Dong Wang, Shan Cao, Zheng Zheng and Han Li

Photonics 2025, 12(7), 704; https://doi.org/10.3390/photonics12070704 - 11 Jul 2025

Abstract

Adaptive digital back-propagation (A-DBP) is a promising candidate for mitigating Kerr nonlinearity due to its ability to estimate the optimal nonlinear scaling factor adaptively. However, the adaptive process relying on the gradient-dependent algorithm is prone to fluctuation, leading to extra iterations or even [...] Read more.

Adaptive digital back-propagation (A-DBP) is a promising candidate for mitigating Kerr nonlinearity due to its ability to estimate the optimal nonlinear scaling factor adaptively. However, the adaptive process relying on the gradient-dependent algorithm is prone to fluctuation, leading to extra iterations or even divergence and resulting in huge computational efforts in A-DBP. In this paper, an improved A-DBP algorithm with long-term memory (LTM) is proposed, employing root mean square propagation (RMSProp) to achieve low-complexity and high-robustness compensation performances. The A-DBP-LTM algorithm based on RMSProp was numerically validated through the simulated transmission of 69 Gbaud DP-16QAM over 2000 km and further verified through an experiment involving 26-λ 63 Gbaud DP-16QAM transmission over 1200 km. Compared with conventional digital back-propagation and A-DBP based on a gradient-descent algorithm, our proposed method allows substantial complexity reductions of 31.35% and 58.47%, respectively. Furthermore, high robustness in only a few iterations and a 0.33 dB improvement in the optical signal–noise ratio penalty were also experimentally demonstrated. Full article

(This article belongs to the Special Issue Next-Generation Optical Networks Communication)

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14 pages, 3007 KiB

Open AccessArticle

Characterization of Chirp Properties of an 850 nm Single-Mode Multi-Aperture Vertical-Cavity Surface-Emitting Laser and Analysis of Transmission Performance over Multimode and Single-Mode Fibers

by Xin Chen, Nikolay Ledentsov, Jr., Abdullah S. Karar, Jason E. Hurley, Oleg Yu. Makarov, Hao Dong, Ahmad Atieh, Ming-Jun Li and Nikolay Ledentsov

Photonics 2025, 12(7), 703; https://doi.org/10.3390/photonics12070703 - 11 Jul 2025

Abstract

By measuring the transfer function of the single-mode multi-aperture vertical-cavity surface-emitting laser (SM MA VCSEL) transmitting over a long single-mode fiber at 850 nm, we confirm that the chirp of the SM MA VCSEL under study is dominated by transient chirp with an [...] Read more.

By measuring the transfer function of the single-mode multi-aperture vertical-cavity surface-emitting laser (SM MA VCSEL) transmitting over a long single-mode fiber at 850 nm, we confirm that the chirp of the SM MA VCSEL under study is dominated by transient chirp with an alpha value of −3.81 enabling a 19 GHz bandwidth over 10 km of single-mode fiber. The detailed measurement of the VCSEL with different bias currents also allows us to recover other key characteristics of the VCSEL, thereby enabling us to practically construct the optical eye diagrams that closely match the experimentally measured ones. The link-level transfer function can be obtained using an analytical equation including effects of modal dispersion and laser chirp–chromatic dispersion (CD) interaction for an MMF of a given length and bandwidth grade. The narrow linewidth and chirp characteristics of the SM MA VCSEL enable transmission performance that surpasses that of conventional MM VCSELs, achieving comparable transmission distances at moderate modal bandwidths for OM3 and OM4 fibers and significantly longer reaches when the modal bandwidth is higher. The transmission performance was also confirmed with the modeled eye diagrams using extracted VCSEL parameters. The chirp properties also provide sufficient bandwidth for SM MA VCSEL transmission over kilometer-scale lengths of single-mode fibers at a high data rate of 100G or above with sufficient optical power coupled into the fibers. Advanced transmission distances are possible over multimode and single-mode fibers versus chirp-free devices. Full article

(This article belongs to the Special Issue Advances in Multimode Optical Fibers and Related Technologies)

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17 pages, 5754 KiB

Open AccessArticle

Simulation-Driven End-to-End Deep Learning Method for White-Light Interference Topography Reconstruction

by Xuan Qi, Yudong Lian, Yulei Wang and Zhiwei Lu

Photonics 2025, 12(7), 702; https://doi.org/10.3390/photonics12070702 - 11 Jul 2025

Abstract

White-light interferometry is essential for surface topography measurement in precision manufacturing, yet existing algorithms face challenges in accuracy, speed, and robustness. Motivated by the application of deep learning in optical metrology, this study presents a novel simulation-driven, end-to-end deep learning approach that significantly [...] Read more.

White-light interferometry is essential for surface topography measurement in precision manufacturing, yet existing algorithms face challenges in accuracy, speed, and robustness. Motivated by the application of deep learning in optical metrology, this study presents a novel simulation-driven, end-to-end deep learning approach that significantly advances white-light interference topography reconstruction. Validation with 200 simulated interferograms shows strong agreement with reference measurements. The neural network processes interferograms in <0.4 s with <0.3% calculation error, demonstrating real-time capability and noise robustness. Using simulated and experimental data from trapezoidal gratings, the method achieves a reconstruction error of 47.12 nm (<λ/8, λ ≈ 550 nm), outperforming traditional techniques by 9.0%. These results confirm the method’s superior accuracy, speed, and reliability for industrial metrology applications. Full article

(This article belongs to the Special Issue Advanced Fiber Laser Technology and Its Application)

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12 pages, 1896 KiB

Open AccessArticle

A 6 kW Level Linearly Polarized Near-Diffraction-Limited Monolithic Fiber Laser with a 0.43 nm Linewidth

by Zixiang Gao, Qiang Shu, Fang Li, Chun Zhang, Fengyun Li, Xingchen Jiang, Yu Wen, Cheng Chen, Li Li, Qiuhui Chu, Rumao Tao, Honghuan Lin, Zhitao Peng and Jianjun Wang

Photonics 2025, 12(7), 701; https://doi.org/10.3390/photonics12070701 - 11 Jul 2025

Abstract

A high-power, narrow-linewidth, all-fiber polarization-maintaining (PM) amplifier has been demonstrated. A lasing power of 5870 W has been delivered in master oscillator power amplifier architecture with cascaded white noise source (WNS) phase modulation and bidirectional pumping schemes. The maximal power was limited by [...] Read more.

A high-power, narrow-linewidth, all-fiber polarization-maintaining (PM) amplifier has been demonstrated. A lasing power of 5870 W has been delivered in master oscillator power amplifier architecture with cascaded white noise source (WNS) phase modulation and bidirectional pumping schemes. The maximal power was limited by the onset of stimulated Brillouin scattering. At the maximum power operation, the amplifier exhibited a 3 dB spectral linewidth of 0.43 nm with beam quality being M² < 1.33 and polarization extinction ratio (PER) being 16.3 dB. To the best of our knowledge, this represents the highest spectral brightness and PER achieved by PM fiber laser systems around 6 kW-level operation. Full article

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

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16 pages, 3084 KiB

Open AccessArticle

Generating Large Time–Bandwidth Product RF-Chirped Waveforms Using Vernier Dual-Optical Frequency Combs

by Mohammed S. Alshaykh

Photonics 2025, 12(7), 700; https://doi.org/10.3390/photonics12070700 - 11 Jul 2025

Abstract

Chirped radio-frequency signals are essential waveforms in radar systems. To enhance resolution and improve the signal-to-noise ratio through higher energy transmission, chirps with high time–bandwidth products are highly desirable. Photonic technologies, with their ability to handle broad electrical bandwidths, have been widely employed [...] Read more.

Chirped radio-frequency signals are essential waveforms in radar systems. To enhance resolution and improve the signal-to-noise ratio through higher energy transmission, chirps with high time–bandwidth products are highly desirable. Photonic technologies, with their ability to handle broad electrical bandwidths, have been widely employed in the generation, filtering, processing, and detection of broadband electrical waveforms. In this work, we propose a photonics-based large-TBWP RF chirp generator utilizing dual optical frequency combs with a small difference in the repetition rate. By employing dispersion modules for frequency-to-time mapping, we convert the spectral interferometric patterns into a temporal RF sinusoidal carrier signal whose frequency is swept through the optical shot-to-shot delay. We derive analytical expressions to quantify the system’s performance under various design parameters, including the comb repetition rate and its offset, the second-order dispersion, the transform-limited optical pulse width, and the photodetector’s bandwidth limitations. We benchmark the expected system performance in terms of RF bandwidth, chirp duration, chirp rate, frequency step size, and TBWP. Using realistic dual-comb source parameters, we demonstrate the feasibility of generating RF chirps with a duration of 284.44

μ

s and a bandwidth of 234.05 GHz, corresponding to a TBWP of

3.3 \times 10^{7}

. Full article

(This article belongs to the Section New Applications Enabled by Photonics Technologies and Systems)

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16 pages, 5752 KiB

Open AccessArticle

Hybrid-Integrated Multi-Lines Optical-Phased-Array Chip

by Shengmin Zhou, Mingjin Wang, Jingxuan Chen, Zhaozheng Yi, Jiahao Si and Wanhua Zheng

Photonics 2025, 12(7), 699; https://doi.org/10.3390/photonics12070699 - 10 Jul 2025

Abstract

We propose a hybrid-integrated III–V-silicon optical-phased-array (OPA) based on passive alignment flip–chip bonding technology and provide new solutions for LiDAR. To achieve a large range of vertical beam steering in a hybrid-integrated OPA, a multi-lines OPA in a single chip is introduced. The [...] Read more.

We propose a hybrid-integrated III–V-silicon optical-phased-array (OPA) based on passive alignment flip–chip bonding technology and provide new solutions for LiDAR. To achieve a large range of vertical beam steering in a hybrid-integrated OPA, a multi-lines OPA in a single chip is introduced. The system allows parallel hybrid integration of multiple dies onto a single wafer, achieving a multi-fold improvement in tuning efficiency. In order to increase the range of horizontal beam steering, we propose a half-wavelength pitch waveguide emitter with non-uniform width to reduce the crosstalk, which can remove the higher-order grating lobes in free space. In this work, we simulate OPA individually for four-lines and eight-lines. As a result, we simultaneously achieved a beam steering with nearly ±90° (horizontal) × 17.2° (vertical, when four-line OPA) or 39.6° (vertical, when eight-line OPA) field of view (FOV) and a high tuning efficiency with 1.13°/nm (when eight-line OPA). Full article

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18 pages, 1900 KiB

Open AccessArticle

Recovery of Optical Transport Coefficients Using Diffusion Approximation in Bilayered Tissues: A Theoretical Analysis

by Suraj Rajasekhar and Karthik Vishwanath

Photonics 2025, 12(7), 698; https://doi.org/10.3390/photonics12070698 - 10 Jul 2025

Abstract

Time-domain (TD) diffuse reflectance can be modeled using diffusion theory (DT) to non-invasively estimate optical transport coefficients of biological media, which serve as markers of tissue physiology. We employ an optimized N-layer DT solver in cylindrical geometry to reconstruct optical coefficients of bilayered [...] Read more.

Time-domain (TD) diffuse reflectance can be modeled using diffusion theory (DT) to non-invasively estimate optical transport coefficients of biological media, which serve as markers of tissue physiology. We employ an optimized N-layer DT solver in cylindrical geometry to reconstruct optical coefficients of bilayered media from TD reflectance generated via Monte Carlo (MC) simulations. Optical properties for 384 bilayered tissue models representing human head or limb tissues were obtained from the literature at three near-infrared wavelengths. MC data were fit using the layered DT model to simultaneously recover transport coefficients in both layers. Bottom-layer absorption was recovered with errors under 0.02 cm⁻¹, and top-layer scattering was retrieved within 3 cm⁻¹ of input values. In contrast, recovered bottom-layer scattering had mean errors exceeding 50%. Total hemoglobin concentration and oxygen saturation were reconstructed for the bottom layer to within 10 μM and 5%, respectively. Extracted transport coefficients were significantly more accurate when obtained using layered DT compared to the conventional, semi-infinite DT model. Our results suggest using improved theoretical modeling to analyze TD reflectance analysis significantly improves recovery of deep-layer absorption. Full article

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

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23 pages, 3777 KiB

Open AccessArticle

Influence of Uncertainties in Optode Positions on Self-Calibrating or Dual-Slope Diffuse Optical Measurements

by Giles Blaney, Angelo Sassaroli, Tapan Das and Sergio Fantini

Photonics 2025, 12(7), 697; https://doi.org/10.3390/photonics12070697 - 10 Jul 2025

Abstract

Self-calibrating and dual-slope measurements have been used in the field of diffuse optics for robust assessment of absolute values or temporal changes in the optical properties of highly scattering media and biological tissue. These measurements employ optical probes with a minimum of two [...] Read more.

Self-calibrating and dual-slope measurements have been used in the field of diffuse optics for robust assessment of absolute values or temporal changes in the optical properties of highly scattering media and biological tissue. These measurements employ optical probes with a minimum of two source positions and a minimum of two detector positions. This work focuses on a quantitative analysis of the impact of errors in these source and detector positions on the assessment of optical properties. We considered linear, trapezoidal, and rectangular optode arrangements and theoretical computations based on diffusion theory for semi-infinite homogeneous media. We found that uncertainties in optodes’ positions may have a greater impact on measurements of absolute scattering versus absorption coefficients. For example, a 4.1% and 19% average error in absolute absorption and scattering, respectively, can be expected by displacing every optode in a linear arrangement by 1 in any direction. The impact of optode position errors is typically smaller for measurements of absorption changes. In each geometrical arrangement (linear, trapezoid, rectangular), we identify the direction of the position uncertainty for each optode that has minimal impact on the optical measurements. These results can guide the optimal design of optical probes for self-calibrating and dual-slope measurements. Full article

(This article belongs to the Special Issue Photonics: 10th Anniversary)

10 pages, 294 KiB

Open AccessArticle

Delayed Choice for Entangled Photons

by Rolando Velázquez, Linda López-Díaz, Leonardo López-Hernández, Eduardo Hernández, L. M. Arévalo-Aguilar and V. Velázquez

Photonics 2025, 12(7), 696; https://doi.org/10.3390/photonics12070696 - 10 Jul 2025

Abstract

The wave–particle duality is the quintessence of quantum mechanics. This duality gives rise to distinct behaviors depending on the experimental setup, with the system exhibiting either wave-like or particle-like properties, depending on whether the focus is on interference (wave) or trajectory (particle). In [...] Read more.

The wave–particle duality is the quintessence of quantum mechanics. This duality gives rise to distinct behaviors depending on the experimental setup, with the system exhibiting either wave-like or particle-like properties, depending on whether the focus is on interference (wave) or trajectory (particle). In the interaction with a beam splitter, photons with particle behavior can transform into a wave behavior and vice versa. In Wheeler’s delayed-choice gedanken experiment, this interaction is delayed so that the wave that initially travels through the interferometer can become a particle, avoiding the interaction. We show that this contradiction can be resolved using polarized entangled photon pairs. An analysis of Shannon’s entropy supports this proposal. Full article

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

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28 pages, 7517 KiB

Open AccessReview

A Review of the Research Progress on Optical Fiber Sensors Based on C-Type Structures

by Zhijun Gao, Zhenbo Li and Yu Ying

Photonics 2025, 12(7), 695; https://doi.org/10.3390/photonics12070695 - 10 Jul 2025

Abstract

With the continuous advancement of optical fiber micromachining technology, C-type optical fibers have demonstrated significant potential in the field of optical fiber sensing. By partially or completely removing specific regions of the cladding, a “leakage window” is created, enabling interaction between the optical [...] Read more.

With the continuous advancement of optical fiber micromachining technology, C-type optical fibers have demonstrated significant potential in the field of optical fiber sensing. By partially or completely removing specific regions of the cladding, a “leakage window” is created, enabling interaction between the optical field and external substances. This structure has facilitated the development of various sensors. This paper reviews recent progress in the research and applications of C-type optical fibers in optical sensing. Based on sensing principles and application scenarios, C-type optical fiber sensors can be categorized into two main types: interferometric and photonic crystal types. This article discusses the fundamental operating principles and structural characteristics of each type, and provides a detailed comparison of their respective advantages and disadvantages. Studies have shown that sensors based on C-type fiber structures offer notable benefits such as simple fabrication, excellent mechanical performance, strong anti-interference capability, and high sensitivity. Therefore, they hold great promise for applications in intelligent monitoring, environmental detection, and healthcare. Finally, this review outlines future research directions for C-type fiber sensors. As technology continues to evolve, future studies are expected to focus on improving sensor stability, expanding application scenarios, and addressing challenges in current fabrication techniques. Full article

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

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12 pages, 1199 KiB

Open AccessArticle

The Potential of Tunable Femtosecond Laser Light to Prevent Melanoma A375 Cell Growth: An In Vitro Investigation

by Safaa Taha, Khalid T. Nawaf, Hala M. Rifaat, Ahmed O. El-Gendy and Tarek Mohamed

Photonics 2025, 12(7), 694; https://doi.org/10.3390/photonics12070694 - 10 Jul 2025

Abstract

The incidence and mortality rates associated with melanoma are increasing. Due to their high proliferation rate, ability to self-renew, and resistance mechanisms, cancer cells often withstand conventional therapies such as radiation and chemotherapy. Therefore, further research is required to develop novel melanoma therapies [...] Read more.

The incidence and mortality rates associated with melanoma are increasing. Due to their high proliferation rate, ability to self-renew, and resistance mechanisms, cancer cells often withstand conventional therapies such as radiation and chemotherapy. Therefore, further research is required to develop novel melanoma therapies with fewer adverse effects, but effective therapeutic impacts. This study aims to investigate how femtosecond laser treatment affects melanoma cells using the A375 cell line as an in vitro model. A375 melanoma cells were plated at a concentration of 10⁴ cells per well in 96-well plates and incubated overnight; then, they were subjected to femtosecond laser irradiation for durations of 3, 5, or 10 min, maintaining a steady power of 100 mW. The laser operated across different wavelengths in the ultraviolet, visible, and infrared ranges. Cell viability was evaluated 24 h after irradiation using the MTT assay. The results showed the significant inhibition of melanoma cell growth with various femtosecond laser parameters, particularly at 380 and 400 nm. At 380 nm, the cell viability was reduced by approximately 90%, and at 400 nm by 73%, after 10 min of exposure. Additional reductions were observed at 420 nm (47%) and 440 nm (18%), while no significant effects were found at 700–780 nm. The most effective exposure time was 10 min. Femtosecond laser radiation exerts a noteworthy anticancer effect on A375 cells, particularly at specific wavelengths and exposure durations, underscoring the potential of femtosecond laser therapy for treating melanoma. Exploring the underlying mechanisms of these effects and evaluating the clinical potential of this treatment modality requires further research. Full article

(This article belongs to the Section Biophotonics and Biomedical Optics)

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27 pages, 1665 KiB

Open AccessArticle

A Heuristic Optical Flow Scheduling Algorithm for Low-Delay Vehicular Visible Light Communication

by Zhengying Cai, Shumeng Lei, Jingyi Li, Chen Yu, Junyu Liu and Guoqiang Gong

Photonics 2025, 12(7), 693; https://doi.org/10.3390/photonics12070693 - 9 Jul 2025

Abstract

Vehicular visible light communication (VVLC) with ultralow electromagnetic interference has great potential to propel the growth of the Internet of Vehicles (IoV). However, ensuring quick response times and minimal delays in VVLC is a significant challenge brought on by fast-moving vehicles. In response [...] Read more.

Vehicular visible light communication (VVLC) with ultralow electromagnetic interference has great potential to propel the growth of the Internet of Vehicles (IoV). However, ensuring quick response times and minimal delays in VVLC is a significant challenge brought on by fast-moving vehicles. In response to this problem, we propose a heuristic optical flow scheduling algorithm. First, the optical flow scheduling problem of VVLC is built as a multi-objective optimization model considering the makespan, delay, schedulable ratio, and bandwidth utilization with non-conflict constraints. Second, an improved artificial plant community (APC) algorithm with enhanced global and local search capabilities is proposed to achieve low-delay communication for time-sensitive optical flows. Finally, a series of benchmark experiments are conducted to show that the proposed algorithm can efficiently schedule optical flows with minimal delay. The cost of this algorithm is very low, and it is suitable for deployment on edge computing platforms such as vehicles. Full article

(This article belongs to the Special Issue New Advances in Optical Wireless Communication)

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10 pages, 577 KiB

Open AccessArticle

Comparison of Dynamic Visual Acuity in Spectacles Prescribed to 0.05D Versus 0.25D Steps: A Self-Control, Randomized Study

by Zhixin Duan, Ningkai Tang and Yuexin Wang

Photonics 2025, 12(7), 692; https://doi.org/10.3390/photonics12070692 - 9 Jul 2025

Abstract

The research aims to compare the dynamic visual acuity (DVA) in myopic adults wearing spectacles prescribed to 0.05D and 0.25D steps. This double-blind, randomized, self-control study included 40 myopic participants aged 18–40. The participants were randomly assigned to receive spectacles with one 0.05D [...] Read more.

The research aims to compare the dynamic visual acuity (DVA) in myopic adults wearing spectacles prescribed to 0.05D and 0.25D steps. This double-blind, randomized, self-control study included 40 myopic participants aged 18–40. The participants were randomly assigned to receive spectacles with one 0.05D step lens and the contralateral lens of 0.25D step. The monocular horizontal and vertical motion DVA at 20 and 40 degrees per second (dps) was measured. The DVA was compared between eyes with 0.25D and 0.05D step lenses and further analyzed by eye dominance and test sequence. The result demonstrated no significant difference in DVA between two eyes with 0.25D or 0.05D step lenses at 20 and 40 dps horizontal and vertical motion test (p > 0.05, respectively). When the eye with a 0.25D step lens was the dominant eye (p = 0.004) or measured secondly (p = 0.002), it outperformed the contralateral eye with a 0.05D step lens in the 40 dps horizontal motion test. In conclusion, the horizontal and vertical motion DVA of the eye with 0.05D step lens spectacles was comparable to that of contralateral eyes corrected with 0.25D step lens. Full article

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

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20 pages, 5133 KiB

Open AccessReview

Photonics-Enabled High-Sensitivity and Wide-Bandwidth Microwave Phase Noise Analyzers

by Jingzhan Shi, Baojin Tu and Yiping Wang

Photonics 2025, 12(7), 691; https://doi.org/10.3390/photonics12070691 - 8 Jul 2025

Abstract

Phase noise constitutes a pivotal performance parameter in microwave systems, and the evolution of microwave signal sources presents new demands on phase noise analyzers (PNAs) regarding sensitivity and bandwidth. Traditional electronics-based PNAs encounter significant limitations in meeting these advanced requirements. This paper provides [...] Read more.

Phase noise constitutes a pivotal performance parameter in microwave systems, and the evolution of microwave signal sources presents new demands on phase noise analyzers (PNAs) regarding sensitivity and bandwidth. Traditional electronics-based PNAs encounter significant limitations in meeting these advanced requirements. This paper provides an overview of recent progress in photonics-based microwave PNA research. Microwave photonic (MWP) PNAs are categorized into two main types: phase-detection-based and frequency-discrimination-based architectures. MWP phase-detection-based PNAs utilize ultra-short-pulse lasers or optical–electrical oscillators as reference sources to achieve superior sensitivity. On the other hand, MWP frequency-discrimination-based PNAs are further subdivided into photonic-substitution-type PNA and MWP quadrature-frequency-discrimination-based PNA. These systems leverage innovative MWP technologies to enhance overall performance, offering broader bandwidth and higher sensitivity compared to conventional approaches. Finally, the paper addresses the current challenges faced in phase noise measurement technologies and suggests potential future research directions aimed at improving measurement capabilities. Full article

(This article belongs to the Special Issue Recent Advancement in Microwave Photonics)

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16 pages, 2510 KiB

Open AccessArticle

Aberration Theoretical Principle of Broadband Multilayer Refractive–Diffractive Optical Elements by Polychromatic Integral Aberration Method

by Ying Yang, Chongxing Liu, Wanting Yang, Yumin Wei, Mengyuan Liu, Bo Dong and Changxi Xue

Photonics 2025, 12(7), 690; https://doi.org/10.3390/photonics12070690 - 8 Jul 2025

Abstract

Multilayer diffractive optical elements (MLDOEs), which have broadband imaging performance, are widely used in lightweight and compact optical systems on the surface of the refractive lenses forming refractive–diffractive lenses. However, current research is generally limited to its broadband diffraction efficiency distribution and rarely [...] Read more.

Multilayer diffractive optical elements (MLDOEs), which have broadband imaging performance, are widely used in lightweight and compact optical systems on the surface of the refractive lenses forming refractive–diffractive lenses. However, current research is generally limited to its broadband diffraction efficiency distribution and rarely involves the study of the imaging quality of multilayer refractive–diffractive optical elements in the broadband. The lack of research on its aberration principle and the absence of methods on how to achieve average aberration control in the broadband have led to a decline in imaging quality when it is applied to the optical system. Therefore, in this paper, we have derived in detail the aberration theory of multilayer refractive–diffractive optical elements and proposed the polychromatic integral aberration (PIA) method to evaluate the aberration characteristics of multilayer diffraction optical elements in the whole broadband. First, we start with the aberrations of diffractive optical elements in the air, and then derive the overall aberrations applied to multilayer refractive–diffractive optical elements. Then, based on the performance of the average aberrations throughout the entire broadband, a broadband aberration evaluation method named PIA is proposed. Finally, the design of traditional multilayer diffraction optical elements, the design of refractive–diffractive multilayer optical elements based on the derivation, and the design of multilayer refraction diffraction optical elements under the PIA method are compared. The results show that the multilayer refractive–diffractive optical element designed by PIA can effectively achieve aberration control and balanced aberration performance in the whole broadband. This research provides a practical and feasible path for exploring the imaging quality of broadband multilayer refractive–diffractive optical elements. Full article

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10 pages, 4530 KiB

Open AccessArticle

A Switchable-Mode Full-Color Imaging System with Wide Field of View for All Time Periods

by Shubin Liu, Linwei Guo, Kai Hu and Chunbo Zou

Photonics 2025, 12(7), 689; https://doi.org/10.3390/photonics12070689 - 8 Jul 2025

Abstract

Continuous, single-mode imaging systems fail to deliver true-color high-resolution imagery around the clock under extreme lighting. High-fidelity color and signal-to-noise ratio imaging across the full day–night cycle remains a critical challenge for surveillance, navigation, and environmental monitoring. We present a competitive dual-mode imaging [...] Read more.

Continuous, single-mode imaging systems fail to deliver true-color high-resolution imagery around the clock under extreme lighting. High-fidelity color and signal-to-noise ratio imaging across the full day–night cycle remains a critical challenge for surveillance, navigation, and environmental monitoring. We present a competitive dual-mode imaging platform that integrates a 155 mm f/6 telephoto daytime camera with a 52 mm f/1.5 large-aperture low-light full-color night-vision camera into a single, co-registered 26 cm housing. By employing a sixth-order aspheric surface to reduce the element count and weight, our system achieves near-diffraction-limited MTF (>0.5 at 90.9 lp/mm) in daylight and sub-pixel RMS blur < 7 μm at 38.5 lp/mm under low-light conditions. Field validation at 0.0009 lux confirms high-SNR, full-color capture from bright noon to the darkest nights, enabling seamless switching between long-range, high-resolution surveillance and sensitive, low-light color imaging. This compact, robust design promises to elevate applications in security monitoring, autonomous navigation, wildlife observation, and disaster response by providing uninterrupted, color-faithful vision in all lighting regimes. Full article

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

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11 pages, 2180 KiB

Open AccessArticle

Impact of Mild Acid and Alkali Treatments on Cotton Fibers with Nonlinear Optical Imaging and SEM Analysis

by Huipeng Gao, Xiaoxiao Li, Rui Li, Chao Wang, Hsiang-Chen Chui and Quan Zhang

Photonics 2025, 12(7), 688; https://doi.org/10.3390/photonics12070688 - 8 Jul 2025

Abstract

This study investigates the structural effects of dilute acid and alkali treatments on cotton fibers, aiming to understand the influence of chemical pretreatment on cellulose morphology. Cotton samples were exposed to 1% sulfuric acid and 1% sodium hydroxide at 90 °C, and the [...] Read more.

This study investigates the structural effects of dilute acid and alkali treatments on cotton fibers, aiming to understand the influence of chemical pretreatment on cellulose morphology. Cotton samples were exposed to 1% sulfuric acid and 1% sodium hydroxide at 90 °C, and the resulting changes were evaluated using scanning electron microscopy and nonlinear optical imaging techniques. The results indicate that sulfuric acid causes significant fiber degradation, leading to fragmentation and reduced fiber thickness. In contrast, sodium hydroxide treatment results in a roughened, flaky surface while preserving the overall structural integrity, with fibers appearing fluffier and more accessible to enzymatic processes. Untreated cotton fibers maintained a smooth and uniform surface, confirming the chemical specificity of the observed changes. These findings are crucial for optimizing biomass pretreatment methods, demonstrating that dilute chemical treatments primarily affect macrostructural features without significantly disrupting the cellulose microfibrils. The study provides valuable insights for the development of efficient biorefining processes and sustainable bio-based materials, highlighting the importance of selecting appropriate chemical conditions to enhance enzymatic hydrolysis and biomass conversion while maintaining the core structure of cellulose. This research contributes to advancing the understanding of cellulose’s structural resilience under mild chemical pretreatment conditions. Full article

(This article belongs to the Section Optical Interaction Science)

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15 pages, 6000 KiB

Open AccessArticle

The Algorithm for Recognizing Superposition of Wave Aberrations from Focal Pattern Based on Partial Sums

by Sergey G. Volotovsky, Pavel A. Khorin, Aleksey P. Dzyuba and Svetlana N. Khonina

Photonics 2025, 12(7), 687; https://doi.org/10.3390/photonics12070687 - 7 Jul 2025

Abstract

In this paper, we investigate the possibility of recognizing a superposition of wave aberrations from a focal pattern based on a matrix of partial sums. Due to the peculiarities of the focal pattern, some types of the considered superpositions are recognized ambiguously from [...] Read more.

In this paper, we investigate the possibility of recognizing a superposition of wave aberrations from a focal pattern based on a matrix of partial sums. Due to the peculiarities of the focal pattern, some types of the considered superpositions are recognized ambiguously from the intensity pattern in the focal plane by standard error-reduction algorithms. It is numerically shown that when recognizing superpositions of Zernike functions from the intensity pattern in the focal plane, the use of step-by-step optimization in combination with the Levenberg–Marquardt algorithm yields good results only with an initial approximation close to the solution. In some cases, the root mean square reaches 0.3, which is unacceptable for precise detection in optical systems that require prompt correction of aberrations in real time. Therefore, to overcome this drawback, an algorithm was developed that considers partial sums, which made it possible to increase the convergence range and achieve unambiguous recognition results for aberrations (root mean square does not exceed 10⁻⁸) described by superpositions of Zernike functions up to n = 5. Full article

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13 pages, 3086 KiB

Open AccessArticle

Single-Polarization Single-Mode Hollow-Core Anti-Resonant Fiber with Low Loss and Wide Bandwidth

by Yong You, Wei Liu, Shuo Zhang, Jianxiong Wu, Yuanjiang Li, Huimin Shi and Haokun Yang

Photonics 2025, 12(7), 686; https://doi.org/10.3390/photonics12070686 - 7 Jul 2025

Abstract

Stable generation and propagation of single-polarization single-mode (SPSM) beams in hollow-core fiber (HCF) has become an important research direction. However, their routine use is yet to become a reality, a major obstacle is to maintain the polarization state of light at a sufficiently [...] Read more.

Stable generation and propagation of single-polarization single-mode (SPSM) beams in hollow-core fiber (HCF) has become an important research direction. However, their routine use is yet to become a reality, a major obstacle is to maintain the polarization state of light at a sufficiently long transmission distance in a wide spectral range. In the paper, a hollow-core anti-resonant fiber (HC-ARF) that can support SPSM beam transmission with an average loss of 15 dB/km in wavelengths beyond 1000 nm is proposed. SPSM guidance is achieved by setting the cladding tubes in the orthogonal direction to have different structures and material properties. Different cladding tube structures break the degeneracy of polarization modes, and different cladding tube materials make the polarization modes experience enough loss difference. In the range of more than 600 nm, the y-polarization loss ≈ 9.3 dB/km, while the x-polarization is > 500 dB/km, and the birefringence is > 1.7 × 10⁻⁵. In addition, the SPSM optimization process and bending losses in different directions are also discussed in detail. Full article

(This article belongs to the Special Issue Applications and Development of Optical Fiber Sensors)

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