Photonics

10 pages, 3577 KiB

Open AccessArticle

Material Structure Design of High-Gain and Low-Noise Multi-Gain-Stage Avalanche Photodiode

by Lihong Han, Meiqin Du, Xiaoning Guan, Tong Sun, Gang Liu and Pengfei Lu

Photonics 2024, 11(12), 1202; https://doi.org/10.3390/photonics11121202 (registering DOI) - 21 Dec 2024

In this work, the InGaAs/InAlAs multi-gain-stage APD model is established. The gain and the noise performance of multi-gain-stage APDs are analyzed based on DSMT. By studying the influence of different doping concentrations of the dropping layer and the charge layer on the gain [...] Read more.

In this work, the InGaAs/InAlAs multi-gain-stage APD model is established. The gain and the noise performance of multi-gain-stage APDs are analyzed based on DSMT. By studying the influence of different doping concentrations of the dropping layer and the charge layer on the gain and noise characteristics of the device, the photocurrent, dark current, noise, and gain characteristics of the device are analyzed, and the device structure is optimized. The results show that the maximum gain of the three-gain-stage APD is 416, and the noise factor is 3.5 when the gain is 100. The five-gain-stage APD has a maximum gain of 450 and a noise factor of 4.5 when the gain is 100. The maximum gain of the 10-gain-stage APD can reach more than 850, and the noise factor reaches 6.5 when the gain is 100. Full article

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37 pages, 1300 KiB

Open AccessReview

Photon-Based Innovations in Oncology: Precise Diagnostic Techniques and Advanced Therapies

by Emilia Kamizela, Jakub Oberda, Albert Chomątowski, Angelika Masiarz, Kacper Ponikowski, Monika Lejman and Joanna Zawitkowska

Photonics 2024, 11(12), 1201; https://doi.org/10.3390/photonics11121201 (registering DOI) - 20 Dec 2024

Abstract

In diagnostics, photons are used in basic methods such as computed tomography (CT) and positron emission tomography (PET), which are pivotal tools for high-resolution, non-invasive tumor detection, offering insights into tumor staging and progression. Mentioned techniques facilitate early diagnosis and the planning of [...] Read more.

In diagnostics, photons are used in basic methods such as computed tomography (CT) and positron emission tomography (PET), which are pivotal tools for high-resolution, non-invasive tumor detection, offering insights into tumor staging and progression. Mentioned techniques facilitate early diagnosis and the planning of therapeutic strategies. However, new methods are emerging, enhancing the precision and detail of diagnostics, such as ultra-weak photon emission (UPE) imagining, two-photon fluorescence imaging, photo acoustic imaging, and others. Therapeutically, external beam radiation therapy (EBRT) uses photons to target cancer cells while minimizing harm to healthy tissue. Photodynamic therapy (PDT), which uses light-sensitive compounds activated by specific wavelengths, represents a photon-based treatment applicable to certain malignancies. Other treatments include photo thermal therapy (PTT), radio dynamic therapy (RDT), intensity-modulated radiation therapy (IMRT), volumetric modulated arc therapy (VMAT), and more. These constantly evolving photon-driven technologies can be used to treat a broad spectrum of cancers, such as pancreatic, prostate, breast, and skin cancers. This review article discusses the latest photon-based methods in oncology, focusing on new possibilities, solutions, perspectives, and the potential disadvantages of these approaches. Full article

13 pages, 3995 KiB

Open AccessArticle

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

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

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

Abstract

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

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

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

17 pages, 1919 KiB

Open AccessArticle

A New Method for Indoor Visible Light Imaging and Positioning Based on Single Light Source

by Xinxin Cheng, Xizheng Ke and Huanhuan Qin

Photonics 2024, 11(12), 1199; https://doi.org/10.3390/photonics11121199 (registering DOI) - 20 Dec 2024

Abstract

Visible light positioning (VLP) can provide indoor positioning functions under LED lighting, and it is becoming a cost-effective indoor positioning solution. However, the actual application of VLP is limited by the fact that most positioning requires at least two or more LEDs. Therefore, [...] Read more.

Visible light positioning (VLP) can provide indoor positioning functions under LED lighting, and it is becoming a cost-effective indoor positioning solution. However, the actual application of VLP is limited by the fact that most positioning requires at least two or more LEDs. Therefore, this paper introduces a positioning system based on a single LED lamp, using an image sensor as the receiver. Additionally, due to the high computational cost of image processing affecting system real-time performance, this paper proposes a virtual grid segmentation scheme combined with the Sobel operator to quickly search for the region of interest (ROI) in a lightweight image processing method. The LED position in the image is quickly determined. Finally, the position is achieved by utilizing the geometric features of the LED image. An experimental setup was established in a space of 80 cm × 80 cm × 180 cm to test the system performance and analyze the positioning accuracy of the receiver in horizontal and tilted conditions. The results show that the positioning accuracy of the method can reach the centimeter level. Furthermore, the proposed lightweight image processing algorithm reduces the average positioning time to 53.54 ms. Full article

16 pages, 1400 KiB

Open AccessArticle

On-Chip Sensor Utilizing Concatenated Micro-Ring with Enhanced Temperature Invariance Using Deep Learning

by Thomas Mikhail, Sarah Shafaay and Mohamed Swillam

Photonics 2024, 11(12), 1198; https://doi.org/10.3390/photonics11121198 (registering DOI) - 20 Dec 2024

Abstract

An approach to measuring chemical concentrations using a slotted micro-ring resonator (sMRR) is proposed which is robust to spectral shifts caused by temperature variations. Two 1-D Convolutional Neural Network architectures, ResNet34 and VGG20, were trained for regression, achieving mean squared errors (MSEs) of [...] Read more.

An approach to measuring chemical concentrations using a slotted micro-ring resonator (sMRR) is proposed which is robust to spectral shifts caused by temperature variations. Two 1-D Convolutional Neural Network architectures, ResNet34 and VGG20, were trained for regression, achieving mean squared errors (MSEs) of 1.1251

\times 10^{- 4}

and 1.2195

\times 10^{- 4}

, respectively. The models predict concentrations of water, ethanol, methanol, and propanol (0–100%) from the transmission spectra of a single-ring sMRR etched in heavily doped silicon, operating in the mid-infrared range (290–310 K). Transfer learning adapted the models for datasets with different temperature ranges, analytes (e.g., butanol), and sMRR designs, achieving comparable accuracy. Variations in accuracy across these datasets are also explored. Full article

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

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16 pages, 2122 KiB

Open AccessArticle

Frequency and Polarization Response of Multipolar Gold Nanoantennae

by Juan R. Moreno, Miguel G. Ramírez-Elías, Israel A. Rosales-Gallegos, Eliseo García-Ramírez, Fernando S. Chiwo, Martha Angélica Cano-Figueroa, Hugo Arcos-Gutiérrez, Isaías E. Garduño, Carlos A. Domínguez, Jesús A. Aboytes-González and Francisco J. González

Photonics 2024, 11(12), 1197; https://doi.org/10.3390/photonics11121197 - 20 Dec 2024

Abstract

Multipolarized nanoantennae are promising for terahertz applications due to their ability to support multiple resonance and polarization sensitivity. This study investigates the behavior of multipolarized gold nanoantennae placed on a Si-SiO₂ substrate and compares their performance with antennae in air. The results [...] Read more.

Multipolarized nanoantennae are promising for terahertz applications due to their ability to support multiple resonance and polarization sensitivity. This study investigates the behavior of multipolarized gold nanoantennae placed on a Si-SiO₂ substrate and compares their performance with antennae in air. The results show that the resonance frequency shifts from 27 THz in the air to 26 THz on the SiSiO₂ substrate. SEM and FTIR analysis confirmed the antenna dimensions and optical response, with experimental reflectance spectra revealing resonance at 21 THz and 26 THz. The multi-polarized antenna demonstrated stable performance across different polarization angles compared to traditional dipole antennae, making it suitable for various terahertz applications. These findings provide insights into optimizing nanoantenna design for advanced sensors and communication systems. Full article

11 pages, 6054 KiB

Open AccessArticle

A Novel Demodulation Algorithm for Micro-Displacement Measurement Based on FMCW Sinusoidal Modulation

by Zhen Xu, Yongjie Wang, Zhenqiang Li, Gaochao Li, Ke Li, Hongtao Zhang and Fang Li

Photonics 2024, 11(12), 1196; https://doi.org/10.3390/photonics11121196 - 20 Dec 2024

Abstract

Frequency-modulated continuous wave (FMCW) interferometry, an emerging laser interferometry technology, can be applied in the field of fibre-optic sensing to achieve high-precision micro-displacement measurements. To address nonlinearity issues in laser frequency modulation and localisation deviations of feature points in traditional algorithms, this paper [...] Read more.

Frequency-modulated continuous wave (FMCW) interferometry, an emerging laser interferometry technology, can be applied in the field of fibre-optic sensing to achieve high-precision micro-displacement measurements. To address nonlinearity issues in laser frequency modulation and localisation deviations of feature points in traditional algorithms, this paper proposes a demodulation algorithm suitable for sinusoidal frequency modulation schemes, incorporating the principle of orthogonal phase-locked amplification. The algorithm includes signal preprocessing, phase-locked amplification, error correction, and phase calculation. Experimental results show that the system achieves a measurement error standard deviation of 3.23 nanometres for static targets. The displacement measurement error at 100 μm is 0.057% F.S., and the linearity between the measured values and the actual displacement values is 0.99997. Compared with conventional methods, the approach introduced in this paper eliminates the need for separate nonlinear corrections of the current-to-optical frequency relationship and minimises the issue of feature point localization deviations, showing significant potential for practical applications. Full article

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

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17 pages, 4133 KiB

Open AccessArticle

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

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

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

Abstract

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

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

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

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

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

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

7.7 µ

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

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

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13 pages, 5338 KiB

Open AccessArticle

Curved Holographic Augmented Reality Near-Eye Display System Based on Freeform Holographic Optical Element with Extended Field of View

by Hong Xu, Yuan Xu, Changyu Wang and Juan Liu

Photonics 2024, 11(12), 1194; https://doi.org/10.3390/photonics11121194 - 19 Dec 2024

Abstract

At present, most near-eye display devices adopt flat substrates, which have problems such as limited field of view (FOV) and bulky shape, while the curved structure is expected to expand the FOV with appropriate volume. In this paper, we propose a curved holographic [...] Read more.

At present, most near-eye display devices adopt flat substrates, which have problems such as limited field of view (FOV) and bulky shape, while the curved structure is expected to expand the FOV with appropriate volume. In this paper, we propose a curved holographic augmented reality (AR) near-eye display system based on holographic optical element (HOE) with the ability to expand the FOV. The system includes a display source and a HOE with curved substrate. We analyze the system by exploiting the diffraction theory between plane and curved surface, and a layered and weighted FOV optimization method using particle swarm optimization algorithm is proposed to realize the optimization of the phase of freeform HOE. Numerical and experimental results show that the proposed curved holographic near-eye display system can realize cylindrical AR display and expand the FOV of the system. It is expected to be applied to the holographic AR near-eye display in the future. Full article

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

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19 pages, 5980 KiB

Open AccessArticle

Study on the Transient Extraction Transform Algorithm for Defect Detection in Welded Plates Based on Laser Vibrometer

by Yu Du, Xinke Xu, Longbiao Zhao, Dijian Yuan and Jinwen Wang

Photonics 2024, 11(12), 1193; https://doi.org/10.3390/photonics11121193 - 19 Dec 2024

Abstract

This paper addresses the issue of detecting welding defects in steel plates during the welding process by proposing a method that combines the laser vibrometer with transient feature extraction technology. The method employs a high-resolution laser vibrometer to collect vibration signals from excited [...] Read more.

This paper addresses the issue of detecting welding defects in steel plates during the welding process by proposing a method that combines the laser vibrometer with transient feature extraction technology. The method employs a high-resolution laser vibrometer to collect vibration signals from excited weld plates, followed by feature extraction and analysis for defect detection and identification. The focus of the research is on the optimization and application of the transient extraction transform algorithm, which plays a crucial role in signal feature extraction for defect recognition. By optimizing the short-time Fourier transform, we further propose the use of the transient extraction transform algorithm to effectively characterize and extract transient components from defect signals. To validate the proposed algorithm, we compare the defect recognition performance of several algorithms using quantitative metrics such as Rényi entropy and kurtosis. The results indicate that the proposed method yields a more centralized time–frequency representation and significantly increases the kurtosis of transient components, providing a new approach for detecting welding defects in steel plates. Full article

(This article belongs to the Special Issue Advances and Applications of Laser Measurements)

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12 pages, 5444 KiB

Open AccessArticle

Enhanced Chaos Generation in Mid-Infrared Interband Cascade Lasers Under Amplitude-Modulated Optical Injection

by Hong Han, Jiada Xu, Zhiwei Jia, Jianguo Zhang, Mingming Huo and K. Alan Shore

Photonics 2024, 11(12), 1192; https://doi.org/10.3390/photonics11121192 - 19 Dec 2024

Abstract

We numerically investigate the dynamics of an interband cascade laser (ICL) subjected to amplitude-modulated optical injection from a directly modulated master ICL. In comparison with steady-state optical injection, the proposed modulated optical injection significantly enlarges the chaos region. In excess of 10 GHz [...] Read more.

We numerically investigate the dynamics of an interband cascade laser (ICL) subjected to amplitude-modulated optical injection from a directly modulated master ICL. In comparison with steady-state optical injection, the proposed modulated optical injection significantly enlarges the chaos region. In excess of 10 GHz broadband mid-infrared chaos is obtained at large bias currents and with a high gain stage number for the appropriate choice of master-slave detuning, modulation frequency, and modulation depth. Full article

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

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13 pages, 2529 KiB

Open AccessArticle

A Filter-Free, Image-Reject, Sub-Harmonic Downconverted RoF Link Without Fiber-Dispersion-Induced Power Fading

by Yuanyuan Li, Qiong Zhao and Wu Zhang

Photonics 2024, 11(12), 1191; https://doi.org/10.3390/photonics11121191 - 19 Dec 2024

Abstract

A filter-free, image-reject, sub-harmonic downconverted RoF link is proposed based on a dual-polarization quadrature phase-shift keying (DP–QPSK) modulator. At the remote antenna unit, the receiving radio frequency signal is applied to the upper QPSK modulator to achieve carrier-suppressed single-sideband (CS–SSB) modulation. The local [...] Read more.

A filter-free, image-reject, sub-harmonic downconverted RoF link is proposed based on a dual-polarization quadrature phase-shift keying (DP–QPSK) modulator. At the remote antenna unit, the receiving radio frequency signal is applied to the upper QPSK modulator to achieve carrier-suppressed single-sideband (CS–SSB) modulation. The local oscillator (LO) is applied to the lower QPSK modulator, achieving sub-harmonic single-sideband (SH–SSB) modulation. The I/Q mixing is realized by exploiting a two-channel photonic microwave phase shifter, which mainly consists of a modulator, two polarization controllers, and two polarizers. The image interference signal can be rejected when combing the I and Q IF signals through a 90° electrical hybrid. Because the scheme is simple and filter-free, it has a good image-reject capability over a large frequency tunable range. Moreover, due to the special SH-SSB modulation, the modulated signals are immune to the chromatic dispersion-introduced power fading effect. Last, the sub-harmonic downconverter can decrease the frequency requirement of the LO signal. Experimental results show that an image rejection ratio (IRR) greater than 50 dB can be achieved when transmitted through a 25 km single-mode fiber (SMF). Simultaneously, under different RF signals and IF signals, the IRR has no periodic power fading, only small fluctuations. Image rejection capability of the scheme for the 50-MBaud 16-QAM wideband vector signal is also verified and the demodulation of the desired IF signal with a good EVM of less than 5% is realized. Full article

(This article belongs to the Special Issue New Perspectives in Microwave Photonics)

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8 pages, 1607 KiB

Open AccessArticle

CdSiP₂ Optical Parametric Oscillator Pumped by a Nanosecond Ho:LLF Laser at 2.06 µm with Non-Collinear Phase-Matching

by Marcin Piotrowski, Gerhard Spindler, Achille Bogas-Droy and Anne Hildenbrand-Dhollande

Photonics 2024, 11(12), 1190; https://doi.org/10.3390/photonics11121190 - 19 Dec 2024

Abstract

We report on a nanosecond-pulsed CdSiP₂ (CSP) optical parametric oscillator (OPO) pumped by a Q-switched 2.06 µm Ho:LiLuF₄ (Ho:LLF) laser in a compact linear cavity with a 10 mm long crystal. Utilizing a non-collinear type-I phase-matching configuration, the OPO achieved an [...] Read more.

We report on a nanosecond-pulsed CdSiP₂ (CSP) optical parametric oscillator (OPO) pumped by a Q-switched 2.06 µm Ho:LiLuF₄ (Ho:LLF) laser in a compact linear cavity with a 10 mm long crystal. Utilizing a non-collinear type-I phase-matching configuration, the OPO achieved an average output power exceeding 3 W in the mid-IR region with good beam quality. The idler and signal waves can be tuned in a wide range of 3.5–4.7 µm by pump tilt and crystal rotation. Full article

(This article belongs to the Special Issue Nonlinear Optics and Optical Parametric Oscillators: From Fundamentals to Cutting-Edge Research)

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20 pages, 1038 KiB

Open AccessReview

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

by Govind P. Agrawal

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

Abstract

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

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

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

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9 pages, 2760 KiB

Open AccessArticle

Bandwidth-Tunable Optical Amplifier with Narrowband Filtering Function Enabled by Parity-Time Symmetry at Exceptional Points

by Kunpeng Zhu, Xiaoyan Zhou, Yinxin Zhang, Zhanhua Huang and Lin Zhang

Photonics 2024, 11(12), 1188; https://doi.org/10.3390/photonics11121188 - 19 Dec 2024

Abstract

Integrated optical amplifiers are the building blocks of on-chip photonic systems, and they are often accompanied by a narrowband filter to limit noise. In this sense, a bandwidth-tunable optical amplifier with narrowband filtering function is crucial for on-chip optical circuits and radio frequency [...] Read more.

Integrated optical amplifiers are the building blocks of on-chip photonic systems, and they are often accompanied by a narrowband filter to limit noise. In this sense, a bandwidth-tunable optical amplifier with narrowband filtering function is crucial for on-chip optical circuits and radio frequency systems. The intrinsic loss and coupling coefficients between resonator and waveguide inherently limit the bandwidth. The parity-time symmetric coupled microresonators operating at exceptional points enable near zero bandwidth. In this study, we propose a parity-time symmetric coupled microresonators system operating near EPs to achieve a bandwidth of 46.4 MHz, significantly narrower than bandwidth of 600.0 MHz and 743.2 MHz achieved by two all-pass resonators with identical gain/loss coefficients. This system also functions as an optical bandwidth-tunable filter. The bandwidth tuning ranges from 175.7 MHz to 7.8 MHz as gain coefficient adjusts from 0.2 dB/cm to 0.4 dB/cm. Our scheme presents a unique method to obtain narrow bandwidth from two broadband resonators and serves as an optical bandwidth-tunable filter, thereby paving a new avenue for exploring non-Hermitian light manipulation in all-optical integrated devices. Full article

(This article belongs to the Special Issue Group IV Photonics: Advances and Applications)

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12 pages, 2407 KiB

Open AccessCommunication

A Motion Compensation Method for Terahertz SAR Imaging with a Large Squint

by Yuanfeng Li, Qi Yang, Xiaoqiang Hua and Hongqiang Wang

Photonics 2024, 11(12), 1187; https://doi.org/10.3390/photonics11121187 - 18 Dec 2024

Abstract

Terahertz-band squint synthetic aperture radars (SARs) can obtain high-resolution images and have application potential in airborne radar systems. However, airborne radars usually have a large squint, which has led to traditional SAR algorithms no longer being applicable to airborne SARs. Additionally, terahertz radar [...] Read more.

Terahertz-band squint synthetic aperture radars (SARs) can obtain high-resolution images and have application potential in airborne radar systems. However, airborne radars usually have a large squint, which has led to traditional SAR algorithms no longer being applicable to airborne SARs. Additionally, terahertz radar imaging systems are more susceptible to the error induced by the platform’s motion. This paper proposes a motion compensation method for terahertz SAR imaging with a large squint angle. First, the signal model of motion compensation is derived, and the processing flow of imaging and motion compensation is detailed. Second, some simulations and experiments are conducted, and the results are reported. The results indicate that the proposed method can effectively correct the motion errors, and the signal model and processing flow are verified. Full article

(This article belongs to the Special Issue Terahertz Advancements in Fibers, Waveguides and Devices)

7 pages, 1979 KiB

Open AccessCommunication

Excess Intensity Noise in a Nonlinear Amplifying Loop-Mirror-Based Mode-Locked Laser from a Non-Reciprocal Phase Bias

by Dohyeon Kwon

Photonics 2024, 11(12), 1186; https://doi.org/10.3390/photonics11121186 - 18 Dec 2024

Abstract

We demonstrate a low-intensity-noise, nonlinear amplifying loop-mirror-based mode-locked fiber laser by optimizing the polarization of the non-reciprocal phase bias and the pump current. If the angle of the waveplate in the non-reciprocal phase bias to the polarization axis of a polarization-maintaining fiber is [...] Read more.

We demonstrate a low-intensity-noise, nonlinear amplifying loop-mirror-based mode-locked fiber laser by optimizing the polarization of the non-reciprocal phase bias and the pump current. If the angle of the waveplate in the non-reciprocal phase bias to the polarization axis of a polarization-maintaining fiber is not carefully aligned, parasitic polarization is induced. The parasitic polarization affects the minimum pump power and dynamic range of pump power for mode-locking, the intensity noise, and the comb power. To reduce intensity noise, the angle of the waveplate for the non-reciprocal phase bias is adjusted, and then the pump power is adjusted. The waveplate angle minimizing the intensity noise maximizes the dynamic range of the pump power for mode-locking and output power. As a result, the relative intensity noise has been suppressed by more than 32 dB at 15 kHz Fourier frequency. The polarization extinction ratio at the non-reciprocal phase bias is critical since it can determine a cavity loss and quality factor of a laser oscillator. Therefore, the additional polarizers cannot improve the intensity noise once the angle is mismatched and the polarization extinction ratio is degraded. Full article

(This article belongs to the Special Issue Advancements in Fiber Lasers and Their Applications)

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11 pages, 381 KiB

Open AccessCommunication

Broadening the Bandwidth of Epsilon-Near-Zero Metamaterials with Embedded Square Frames

by Anatoliy V. Goncharenko and Vyacheslav M. Silkin

Photonics 2024, 11(12), 1185; https://doi.org/10.3390/photonics11121185 - 18 Dec 2024

Abstract

A narrow frequency bandwidth of epsilon-near-zero metamaterials limits the use of many optical, microwave, and electronic devices. In this paper, we propose a recipe to broaden the operational bandwidth by employing a structure of properly tailored square frames nested within each other. To [...] Read more.

A narrow frequency bandwidth of epsilon-near-zero metamaterials limits the use of many optical, microwave, and electronic devices. In this paper, we propose a recipe to broaden the operational bandwidth by employing a structure of properly tailored square frames nested within each other. To illustrate this effect, we derive the effective permittivity for the considered frame geometry. Then, we show that combining constituent materials with loss and materials with gain enables us to achieve the effective permittivity over a frequency band as small as desired. This technique may prove valuable for various applications including invisibility cloaks, camouflage, shielding, and sensorics. Full article

(This article belongs to the Special Issue Advances in Epsilon-Near-Zero Photonics)

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14 pages, 4047 KiB

Open AccessArticle

Measurement of Atmospheric Coherence Length from a Shack–Hartmann Wavefront Sensor with Extended Sources

by Junrui Zhang, Yuling Zhao, Leqiang Yang, Jie Liu, Wenyu Wang, Zhengwei Li, Jianli Wang and Tao Chen

Photonics 2024, 11(12), 1184; https://doi.org/10.3390/photonics11121184 - 18 Dec 2024

Abstract

Free Space Optical Communication (FSOC) is a wireless communication method that utilizes laser beams for high speed and secure data transmission. Its performance is affected by various factors, among which atmospheric turbulence causes random fluctuations in the atmospheric refractive index, significantly impacting the [...] Read more.

Free Space Optical Communication (FSOC) is a wireless communication method that utilizes laser beams for high speed and secure data transmission. Its performance is affected by various factors, among which atmospheric turbulence causes random fluctuations in the atmospheric refractive index, significantly impacting the reliability of communication links. The atmospheric coherence length is a key parameter describing the coherence properties of a laser signal as it propagates through the atmosphere, and accurately measuring it is crucial for assessing the quality of FSOC links. This paper proposes a novel strategy that utilizes extended sources directly as the information sources, combining the wavefront phase variance method with the extended source offset algorithm based on Shack–Hartmann wavefront sensors to directly measure atmospheric coherence length. Existing methods in extended scenarios typically rely on deploying laser beacons to aid in the calibration of atmospheric coherence length but setting up suitable beacons on horizontal communication links is challenging. Additionally, these approaches can be costly in terms of equipment and measurement expenses. Compared to traditional measurement methods, the algorithm proposed in this paper can measure directly based on extended scenarios in horizontal links, thereby effectively reducing system complexity and equipment costs. To verify the feasibility and effectiveness of this method, targeted simulations and experiments were conducted, and the results show that the coherence length measured by the algorithm is highly consistent with that measured by the Differential Image Motion Monitor (DIMM), with a deviation of less than 2% from actual values, effectively demonstrating the algorithm’s feasibility in coherence length assessment. Full article

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

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