Photonics

18 pages, 592 KiB

Open AccessArticle

Enhancing the Cooling of a Rotating Mirror in a Laguerre–Gaussian Cavity Optorotational System via Nonlinear Cross-Kerr Interaction

by Xinyue Cao, Sumei Huang, Li Deng and Aixi Chen

Photonics 2024, 11(10), 960; https://doi.org/10.3390/photonics11100960 (registering DOI) - 13 Oct 2024

Abstract

The cooling of a macroscopic mechanical oscillator to its quantum ground state is an important step for achieving coherent control over mechanical quantum states. Here, we theoretically study the cooling of a rotating mirror in a Laguerre–Gaussian (L-G) cavity optorotational system with a [...] Read more.

The cooling of a macroscopic mechanical oscillator to its quantum ground state is an important step for achieving coherent control over mechanical quantum states. Here, we theoretically study the cooling of a rotating mirror in a Laguerre–Gaussian (L-G) cavity optorotational system with a nonlinear cross-Kerr (CK) interaction. We discuss the effects of the nonlinear CK coupling strength, the cavity detuning, the power of the input Gaussian beam, the topological charge (TC) of the L-G cavity mode, the mass of the rotating mirror, and the cavity length on the cooling of the rotating mirror. We find that it is only possible to realize the improvement in the cooling of the rotating mirror by the nonlinear CK interaction when the cavity detuning is less than the mechanical frequency. Compared to the case without the nonlinear CK interaction, we find that the cooling of the rotating mirror can be improved by the nonlinear CK interaction at lower laser powers, smaller TCs of the L-G cavity mode, larger masses of a rotating mirror, and longer optorotational cavities. We show that the cooling of the rotating mirror can be enhanced by the nonlinear CK interaction by a factor of about 23.3 compared to that without the nonlinear CK interaction. Full article

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

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11 pages, 3916 KiB

Open AccessArticle

Nonlinear Optical Microscopic Imaging for Real-Time Gaseous Chemical Sensing

by Gombojav O. Ariunbold, Bryan Semon, Logan Carlson and Thejesh N. Bandi

Photonics 2024, 11(10), 959; https://doi.org/10.3390/photonics11100959 (registering DOI) - 13 Oct 2024

Abstract

Nonlinear optical microscopic imaging techniques have advanced for chemically sensitive imaging of solid and liquid samples but lack advancements for gaseous samples. In this work, wide-field three-color ultrafast coherent anti-Stokes Raman scattering microscopy is implemented for selectively imaging the ambient nitrogen gas. Our [...] Read more.

Nonlinear optical microscopic imaging techniques have advanced for chemically sensitive imaging of solid and liquid samples but lack advancements for gaseous samples. In this work, wide-field three-color ultrafast coherent anti-Stokes Raman scattering microscopy is implemented for selectively imaging the ambient nitrogen gas. Our technique operates by capturing a series of spectrally selected images with a rate of 5–10 frames per second. The recorded data are analyzed both qualitatively and quantitatively. This technique has been demonstrated to be sensitive to a variation of approximately 10¹¹ nitrogen molecules in ambient air confined within a microscopic volume of 10 μm by 50 μm by 50 μm. We believe that our approach can potentially be extended toward real-time, in situ chemical imaging of the microscopic dynamics of gases, for example, in ammonia for nitrogen cycle, greenhouse gases for environmental pollution, plant fertilization regulation for precision agriculture, or byproducts produced from lower-temperature plasmas. Full article

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

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17 pages, 1254 KiB

Open AccessReview

Astronomical Intensity Interferometry

by Shufei Yi, Qichang An, Wenyi Zhang, Jincai Hu and Liang Wang

Photonics 2024, 11(10), 958; https://doi.org/10.3390/photonics11100958 (registering DOI) - 12 Oct 2024

Abstract

The development of astronomy relies heavily on advances in high-resolution imaging techniques. With the growing demand for high-resolution astronomical observations, conventional optical interferometry has gradually revealed various limitations, especially in coping with atmospheric phase fluctuations and long baseline observations. However, intensity interferometry is [...] Read more.

The development of astronomy relies heavily on advances in high-resolution imaging techniques. With the growing demand for high-resolution astronomical observations, conventional optical interferometry has gradually revealed various limitations, especially in coping with atmospheric phase fluctuations and long baseline observations. However, intensity interferometry is becoming an important method to overcome these challenges due to its high robustness to atmospheric phase fluctuations and its excellent performance in long-baseline observations. In this paper, the basic principles and key technologies of intensity interferometry are systematically described, and the remarkable potential of this technique for improving angular resolution and detection sensitivity is comprehensively discussed in light of the recent advances in modern photon detector and signal processing techniques. The results show that the intensity interferometry technique is capable of realizing high-precision observation of long-range and low-brightness targets, especially in the field of exoplanet detection, which shows a wide range of application prospects. In the future, with the continuous development of telescope arrays and adaptive optics, the intensity interferometry technique is expected to further promote the precision and breadth of astronomical observations, and provide new opportunities for revealing the mysteries of the universe. Full article

(This article belongs to the Special Issue Emerging Trends in Spectral Analysis with Optical Sensors: Modern Approaches and Applications)

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14 pages, 7519 KiB

Open AccessArticle

Research on Tracking Control Technology Based on Fuzzy PID in Underwater Optical Communication

by Dongliang Guan, Yang Liu, Jingyi Fu, Yunjie Teng, Yang Qian, Gongtan Wang, Sen Gu, Tongyu Liu and Wang Xi

Photonics 2024, 11(10), 957; https://doi.org/10.3390/photonics11100957 (registering DOI) - 12 Oct 2024

Abstract

In order to realize effective laser communication on underwater mobile platforms, the active tracking and alignment technology of underwater laser communication (UWLC) is studied. Firstly, the servo control principle of the UWLC system is analyzed. Secondly, aiming at underwater disturbance, an adaptive fuzzy [...] Read more.

In order to realize effective laser communication on underwater mobile platforms, the active tracking and alignment technology of underwater laser communication (UWLC) is studied. Firstly, the servo control principle of the UWLC system is analyzed. Secondly, aiming at underwater disturbance, an adaptive fuzzy PID controller is designed to realize parameter self-tuning, thereby improving the system’s anti-disturbance ability. The designed algorithm was simulated, and the simulation results show that the adaptive fuzzy PID algorithm has better anti-disturbance ability and tracking performance than the traditional PID. Finally, an experimental platform was built for dynamic tracking experiments, and the results show that the dynamic tracking accuracy of the designed control algorithm was improved by 30.29% compared with the traditional control algorithm, which provides a certain reference for the development of laser communication on underwater moving platforms. Full article

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

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13 pages, 7674 KiB

Open AccessArticle

Multilayer Metamaterials with Vertical Cavities for High-Efficiency Transmittance with Metallic Components in the Visible Spectrum

by Huiyu Li, Lin Zhao, Guangwei Chen, Guoqing Hu and Zhehai Zhou

Photonics 2024, 11(10), 956; https://doi.org/10.3390/photonics11100956 (registering DOI) - 11 Oct 2024

Abstract

Metasurfaces are opening promising flexibilities to reshape the wavefront of electromagnetic waves. Notable optical phenomena are observed with the tailored surface plasmon, which is excited by metallic components in the visible spectrum. However, metamaterial or metasurface devices utilizing metallic materials encounter the challenge [...] Read more.

Metasurfaces are opening promising flexibilities to reshape the wavefront of electromagnetic waves. Notable optical phenomena are observed with the tailored surface plasmon, which is excited by metallic components in the visible spectrum. However, metamaterial or metasurface devices utilizing metallic materials encounter the challenge of low transmission efficiency, particularly within the visible spectrum. This study proposes a multilayer design strategy to enhance their transmission efficiency. By incorporating additional metal layers for improvements in the transmission efficiency and dielectric layers as spacers, cavities are formed along the propagation direction, enabling the modulation of transmittance and reflection through a process mimicking destructive interference. An analytical model simplified with the assumption of deep-subwavelength-thick metal layers is proposed to predict the structural parameters with optimized transmittance. Numerical studies employing the rigorous coupled wave analysis method confirmed that the additional metal layers significantly improve the transmittance. The introduction of the extra metal and dielectric layers enhances the transmission efficiency in specific spectral regions, maintaining a controllable passband and transmittance. The results indicate that the precise control over the layers’ thicknesses facilitates the modulation of peak-to-valley ratios and the creation of comb-like filters, which can be further refined through controlled random variation in the thickness. Furthermore, when the thickness of the silver layer followed an arithmetic sequence, a multilayer structure with a transmittance of approximately 80% covering the entire visible spectrum could be achieved. Significantly, the polarization extinction ratio and the phase delay of the incident beams could still be modulated by adjusting the geometrical structure and parameters of the multilayer metamaterial for diversified functionalities. Full article

(This article belongs to the Special Issue Advances in Near-Field Optics: Fundamentals and Applications)

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12 pages, 2094 KiB

Open AccessArticle

Real-Time Reconfigurable Radio Frequency Arbitrary-Waveform Generation via Temporal Pulse Shaping with a DPMZM and Multi-Tone Inputs

by Yireng Chen, Chenxiao Lin, Shuna Yang and Bo Yang

Photonics 2024, 11(10), 955; https://doi.org/10.3390/photonics11100955 (registering DOI) - 11 Oct 2024

Abstract

Benefitting from a large bandwidth and compact configuration, a time-domain pulse-shaping (TPS) system provides possibilities for generating broadband radio frequency (RF) arbitrary waveforms based on the Fourier transform relationship between the input–output waveform pair. However, limited by the relatively low sampling rate and [...] Read more.

Benefitting from a large bandwidth and compact configuration, a time-domain pulse-shaping (TPS) system provides possibilities for generating broadband radio frequency (RF) arbitrary waveforms based on the Fourier transform relationship between the input–output waveform pair. However, limited by the relatively low sampling rate and bit resolution of an electronic arbitrary-waveform generator (EAWG), the diversity and fidelity of the output waveform as well as its reconfiguration rate are constrained. To remove the EAWG’s limitation and realize dynamic real-time reconfiguration of RF waveforms, we propose and demonstrate a novel approach of RF arbitrary-waveform generation based on an improved TPS system with an integrated dual parallel Mach–Zehnder modulator (DPMZM) and multi-tone inputs. By appropriately adjusting the DC bias voltages of DPMZM and the power values, as well as the center frequencies of the multi-tone inputs, any desired RF arbitrary waveform can be generated and reconfigured in real time. Proof-of-concept experiments on the generation of different user-defined waveforms with a sampling rate up to 27 GSa/s have been successfully carried out. Furthermore, the impact of modulation modes and higher-order dispersion on waveform fidelity is also discussed in detail. Full article

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

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10 pages, 1681 KiB

Open AccessArticle

Effects of Gain Saturation on Orbital Instability of Chaotic Laser Diode with External Pseudorandom Signal

by Satoshi Ebisawa

Photonics 2024, 11(10), 954; https://doi.org/10.3390/photonics11100954 - 11 Oct 2024

Abstract

In a laser diode (LD) system with optical injection, the effects of gain saturation of the LD on the orbital instability of the system are analyzed numerically. For the optical injection LD system without signal application, it is shown that the effect of [...] Read more.

In a laser diode (LD) system with optical injection, the effects of gain saturation of the LD on the orbital instability of the system are analyzed numerically. For the optical injection LD system without signal application, it is shown that the effect of optical injection is suppressed in the system with gain saturation and small optical injection, and that a higher amount of optical injection is necessary to obtain similar dynamics. Next, in the optical injection LD system with a pseudo-random signal applied to the LD drive current, it is confirmed that when the dynamics are a periodic window between chaotic and chaotic regions, chaotic dynamics are actualized as the standard deviation of the applied signal becomes larger. Furthermore, it is suggested that this phenomenon can be explained by linear stability analysis, and it is shown by introducing randomly varying tentative gain coefficients that gain fluctuations that lead to an expansion of the chaotic region. Hence, the results of this study provide research on the effects of gain saturation on chaotic oscillation in LDs with pseudo-random signals applied and contribute to the generation of more complex chaotic signals, chaotic secure communication, and random number generation. Full article

(This article belongs to the Special Issue Laser Technology and Applications)

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16 pages, 8258 KiB

Open AccessArticle

Multi-Source Fusion Deformation-Monitoring Accuracy Calibration Method Based on a Normal Distribution Transform–Convolutional Neural Network–Self Attention Network

by Xuezhu Lin, Bo Zhang, Lili Guo, Wentao Zhang, Jing Sun, Yue Liu and Shihan Chao

Photonics 2024, 11(10), 953; https://doi.org/10.3390/photonics11100953 - 10 Oct 2024

Abstract

In multi-source fusion deformation-monitoring methods that utilize fiber Bragg grating (FBG) data and other data types, the lack of FBG constraint points in edge regions often results in inaccuracies in fusion results, thereby impacting the overall deformation-monitoring accuracy. This study proposes a multi-source [...] Read more.

In multi-source fusion deformation-monitoring methods that utilize fiber Bragg grating (FBG) data and other data types, the lack of FBG constraint points in edge regions often results in inaccuracies in fusion results, thereby impacting the overall deformation-monitoring accuracy. This study proposes a multi-source fusion deformation-monitoring calibration method and develops a calibration model that integrates vision and FBG multi-source fusion data. The core of this model is a normal distribution transform (NDT)–convolutional neural network (CNN)–self-attention (SA) calibration network. This network enhances continuity between points in point clouds using the NDT module, thereby reducing outliers at the edges of the fusion results. Experimental validation shows that this method reduces the absolute error to below 0.2 mm between multi-source fusion calibration results and high-precision measured point clouds, with a confidence interval of 99%. The NDT-CNN-SA network offers significant advantages, with a performance improvement of 36.57% over the CNN network, 14.39% over the CNN–gated recurrent unit (GRU)–convolutional block attention module (CBAM) network, and 9.54% over the CNN–long short term memory (LSTM)–SA network, thereby demonstrating its superior generalization, accuracy, and robustness. This calibration method provides smoother and accurate structural deformation data, supports real-time deformation monitoring, and reduces the impact of assembly deviation on product quality and performance. Full article

(This article belongs to the Topic Advance and Applications of Fiber Optic Measurement: 2nd Edition)

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9 pages, 3380 KiB

Open AccessCommunication

Silicon Optical Phased Array Hybrid Integrated with III–V Laser for Grating Lobe-Free Beam Steering

by Jingye Chen, Shi Zhao, Wenlei Li, Xiaobin Wang, Xiang’e Han and Yaocheng Shi

Photonics 2024, 11(10), 952; https://doi.org/10.3390/photonics11100952 - 10 Oct 2024

Abstract

A silicon photonics-based optical phased array (OPA) is promising for realizing solid-state and miniature beam steering. In our work, a 1 × 16 silicon optical phased array (OPA) hybrid integrated with a III–V laser is proposed and demonstrated. The III–V laser chip is [...] Read more.

A silicon photonics-based optical phased array (OPA) is promising for realizing solid-state and miniature beam steering. In our work, a 1 × 16 silicon optical phased array (OPA) hybrid integrated with a III–V laser is proposed and demonstrated. The III–V laser chip is vertically coupled with a silicon OPA chip based on a chirped grating coupler with a large bandwidth. The coupling efficiency reaches up to 90% through utilizing the metal reflector underneath the silicon oxide layer. The one-dimensional antenna array comprising silicon waveguides with half-wavelength spacing enables beam steering with none high-order grating lobes in a 180° field of view. The measured beam steering angle of the hybrid integrated OPA chip is ±25°, without grating lobes, and the suppression ratio of the side-lobes is larger than 9.8 dB with phase calibration. Full article

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

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19 pages, 5647 KiB

Open AccessArticle

Experimental State Observer of the Population Inversion of a Multistable Erbium-Doped Fiber Laser

by Daniel Alejandro Magallón-García, Didier López-Mancilla, Rider Jaimes-Reátegui, Juan Hugo García-López, Guillermo Huerta Cuellar, Luis Javier Ontañon-García and Fabian Soto-Casillas

Photonics 2024, 11(10), 951; https://doi.org/10.3390/photonics11100951 - 10 Oct 2024

Abstract

In this work, numerical and experimental implementation of a state observer applied to an erbium-doped fiber laser (EDFL) has been developed. The state observer is designed through the mathematical model of the EDFL to estimate the non-measurable variable; however, in numerical estimation, the [...] Read more.

In this work, numerical and experimental implementation of a state observer applied to an erbium-doped fiber laser (EDFL) has been developed. The state observer is designed through the mathematical model of the EDFL to estimate the non-measurable variable; however, in numerical estimation, the state variables can be measurable given the mathematical model. Only the laser intensity variable was experimentally measured. The state observer estimated the population inversion through the obtained experimental laser intensity time series fitted with their numerical laser intensity using the mean square error (MSE) tool. A bifurcation diagram of the population inversion time series local maximum was built from the state observer. The state space of the experimental laser intensity versus observed population inversion was built. Full article

(This article belongs to the Special Issue Fiber Lasers: Recent Advances and Applications)

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9 pages, 3046 KiB

Open AccessArticle

Molybdenum Truncated Cone Arrays with Localized Surface Plasmon Resonance for Surface-Enhanced Raman Scattering Application

by Cheng Wang, Tao Cui, Zhe Liu, Yu Lin, Shuai Tang, Lei Shao, Huanjun Chen, Yan Shen and Shaozhi Deng

Photonics 2024, 11(10), 950; https://doi.org/10.3390/photonics11100950 - 10 Oct 2024

Abstract

Plasmonic materials have been extensively explored for surface-enhanced Raman scattering (SERS) due to their high tunability and excellent localized electric field enhancement. Most research for now has focused on noble metals, with limited investigation into corrosion-resistant materials for SERS effects. In this study, [...] Read more.

Plasmonic materials have been extensively explored for surface-enhanced Raman scattering (SERS) due to their high tunability and excellent localized electric field enhancement. Most research for now has focused on noble metals, with limited investigation into corrosion-resistant materials for SERS effects. In this study, a photolithography process is firstly used to create a patterned dot array on a silicon substrate. Next, magnetron sputtering is employed to deposit molybdenum films, finally resulting in the molybdenum truncated cone array substrates for SERS applications. The fabricated truncated cone array experimentally facilitates the coupling of localized surface plasmon polaritons, consistent with simulation results obtained via the finite-difference time-domain method. The formation of hot spots between the cone unit cell arrays leads to the improved Raman signals and can act as traps for target molecules. This study demonstrates that molybdenum-based micro-nano structures can serve as reliable SERS substrates for sensitive molecular sensing applications in highly corrosive environments. Full article

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

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18 pages, 3527 KiB

Open AccessArticle

ZEROES: Robust Derivative-Based Demodulation Method for Optical Camera Communication

by Maugan De Murcia, Hervé Boeglen and Anne Julien-Vergonjanne

Photonics 2024, 11(10), 949; https://doi.org/10.3390/photonics11100949 - 9 Oct 2024

Abstract

Most of Optical Camera Communication (OCC) systems benefit from the rolling shutter mechanism of Complementary Metal-Oxide Semiconductor (CMOS) cameras to record the brightness evolution of the Light-Emitting Diode (LED) through dark and bright strips within images. While this technique enhances the maximum achievable [...] Read more.

Most of Optical Camera Communication (OCC) systems benefit from the rolling shutter mechanism of Complementary Metal-Oxide Semiconductor (CMOS) cameras to record the brightness evolution of the Light-Emitting Diode (LED) through dark and bright strips within images. While this technique enhances the maximum achievable data rate, the main difficulty lies in the demodulation of the signal extracted from images, subject to blooming effect. Thus, two main approaches were proposed to deal with this issue, using adaptive thresholds whose value evolves according to amplitude changes or detecting signal variations with the first-order derivative. As the second method is more robust, a new demodulation method based on the detection of the zeros of the first-order derivative of the extracted signal was proposed in this paper. Obtained results clearly show an improvement in the extracted signal demodulation compared to other methods, achieving a raw Bit Error Rate (BER) of 10⁻³ around 50 cm in a Line-Of-Sight scenario, and increasing the maximum communication distance by 43.5%, reaching 330 cm in the case of a Non-Line-Of-Sight transmission. Full article

(This article belongs to the Special Issue Optical Wireless Communications (OWC) for Internet-of-Things (IoT))

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8 pages, 2637 KiB

Open AccessArticle

Wide-Spectrum Tuning and Narrowing of 780 nm Broad-Area Diode Laser with Littrow-Type Transmission Gratings

by Huizi Zhao, Longfei Jiang, Liang Li, Meng Li, Rui Wang, Zining Yang, Hongyan Wang, Weiqiang Yang, Chaofan Zhang, Weihong Hua and Xiaojun Xu

Photonics 2024, 11(10), 948; https://doi.org/10.3390/photonics11100948 - 9 Oct 2024

Abstract

Spectrum-narrowed and -locked broad-area diode lasers operating at 780 nm are essential for rubidium laser development. With the help of Littrow-type transmission gratings, we demonstrated a simple scheme with a narrow linewidth and the diode laser’s center wavelength locked without thermal drift, in [...] Read more.

Spectrum-narrowed and -locked broad-area diode lasers operating at 780 nm are essential for rubidium laser development. With the help of Littrow-type transmission gratings, we demonstrated a simple scheme with a narrow linewidth and the diode laser’s center wavelength locked without thermal drift, in contrast to volume Bragg gratings. By carefully collimating the diode laser beam, we realized a linewidth narrower than 0.17 nm and a side-mode suppression ratio over 20 dB. Furthermore, broad-spectrum tuning at 9 nm was demonstrated by grating angle tuning. This method could easily be adapted to other wavelength diode lasers. Full article

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

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16 pages, 7297 KiB

Open AccessArticle

Effects of Black Silicon Surface Morphology Induced by a Femtosecond Laser on Absorptance and Photoelectric Response Efficiency

by Xiaomo Zhang, Weinan Li, Chuan Jin, Yi Cao, Feng Liu, Na Wei, Bo Wang, Rundong Zhou, Xiangping Zhu and Wei Zhao

Photonics 2024, 11(10), 947; https://doi.org/10.3390/photonics11100947 - 9 Oct 2024

Abstract

In this study, the effects of variations in the height (

h

) and bottom radius (

r

) of black silicon microstructures on their absorptance and photoelectric response efficiency were analyzed. By using the relation

\cot \frac{θ}{2} = \frac{h}{r}

[...] Read more.

In this study, the effects of variations in the height (

h

) and bottom radius (

r

) of black silicon microstructures on their absorptance and photoelectric response efficiency were analyzed. By using the relation

\cot \frac{θ}{2} = \frac{h}{r}

to combine the parameters, it was found that changes in morphology affected the absorptance of black silicon microstructures, with

h

being directly proportional to the absorptance, while

r

was inversely proportional. A positive correlation was observed between

\cot \frac{θ}{2}

and absorptance. However, the correlation between

\cot \frac{θ}{2}

and photoelectric response efficiency was not significant. Through Raman spectroscopy analysis of the samples, it was concluded that as the laser ablation energy density increased, more lattice defects were introduced, weakening the charge carrier transport efficiency. This study further elucidated the mechanism by which microstructural changes impacted the absorptance and energy density of black silicon, providing valuable insights for optimizing its energy density. Full article

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

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16 pages, 29393 KiB

Open AccessArticle

Switchable Dual-Wavelength Fiber Laser with Narrow-Linewidth Output Based on Parity-Time Symmetry System and the Cascaded FBG

by Kaiwen Wang, Bin Yin, Chao Lv, Yanzhi Lv, Yiming Wang, Hao Liang, Qun Wang, Shiyang Wang, Fengjie Yu, Zhong Zhang, Ziwang Li and Songhua Wu

Photonics 2024, 11(10), 946; https://doi.org/10.3390/photonics11100946 - 8 Oct 2024

Abstract

In this paper, a dual-wavelength narrow-linewidth fiber laser based on parity-time (PT) symmetry theory is proposed and experimentally demonstrated. The PT-symmetric filter system consists of two optical couplers (OCs), four polarization controllers (PCs), a polarization beam splitter (PBS), and cascaded fiber Bragg gratings [...] Read more.

In this paper, a dual-wavelength narrow-linewidth fiber laser based on parity-time (PT) symmetry theory is proposed and experimentally demonstrated. The PT-symmetric filter system consists of two optical couplers (OCs), four polarization controllers (PCs), a polarization beam splitter (PBS), and cascaded fiber Bragg gratings (FBGs), enabling stable switchable dual-wavelength output and single longitudinal-mode (SLM) operation. The realization of single-frequency oscillation requires precise tuning of the PCs to match gain, loss, and coupling coefficients to ensure that the PT-broken phase occurs. During single-wavelength operation at 1548.71 nm (λ₁) over a 60-min period, power and wavelength fluctuations were observed to be 0.94 dB and 0.01 nm, respectively, while for the other wavelength at 1550.91 nm (λ₂), fluctuations were measured at 0.76 dB and 0.01 nm. The linewidths of each wavelength were 1.01 kHz and 0.89 kHz, with a relative intensity noise (RIN) lower than −117 dB/Hz. Under dual-wavelength operation, the maximum wavelength fluctuations for λ₁ and λ₂ were 0.03 nm and 0.01 nm, respectively, with maximum power fluctuations of 3.23 dB and 2.38 dB. The SLM laser source is suitable for applications in long-distance fiber-optic sensing and coherent LiDAR detection. Full article

(This article belongs to the Special Issue Single Frequency Fiber Lasers and Their Applications)

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16 pages, 3604 KiB

Open AccessArticle

High-Strength Welding of Silica Glass Using Double-Pulse Femtosecond Laser under Non-Optical Contact Conditions

by Zheng Gao, Jiahua He, Xianshi Jia, Zhaoxi Yi, Cheng Li, Shifu Zhang, Cong Wang and Ji’an Duan

Photonics 2024, 11(10), 945; https://doi.org/10.3390/photonics11100945 - 8 Oct 2024

Abstract

Ultrafast laser welding technology for transparent materials has developed rapidly in recent years; however, high-strength non-optical contact transparent material welding has been a challenge. This work presents a welding method for silica glass using a double-pulse femtosecond (fs) laser and optimizes the laser [...] Read more.

Ultrafast laser welding technology for transparent materials has developed rapidly in recent years; however, high-strength non-optical contact transparent material welding has been a challenge. This work presents a welding method for silica glass using a double-pulse femtosecond (fs) laser and optimizes the laser processing parameters to enhance the welding performance. The welding characteristics of silica glass are analyzed under different time delays by controlling the pulse delay of double pulses. In addition to comprehensively study the influence of various experimental conditions on double-pulse fs laser welding, multi-level tests are designed for five factors, including average laser power, pulse delay, scanning interval, scanning speed, and repetition rate. Finally, by optimizing the parameters, a welding strength of 57.15 MPa is achieved at an average power of 3500 mW, repetition rate of 615 kHz, pulse delay of 66.7 ps, scanning interval of 10 µm, and scanning speed of 1000 µm/s. This work introduces a new approach to glass welding and presents optimal parameters for achieving higher welding strength, which can be widely used in aerospace, microelectronic packaging, microfluidics, and other fields. Full article

(This article belongs to the Special Issue Combined Pulse Laser: A Reliable Tool for High-Quality, High-Efficiency Material Processing)

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16 pages, 2036 KiB

Open AccessArticle

NOMA Visible Light Communications with Distinct Optical Beam Configurations

by Jupeng Ding, Chih-Lin I, Jintao Wang and Hui Yang

Photonics 2024, 11(10), 944; https://doi.org/10.3390/photonics11100944 - 7 Oct 2024

Abstract

Visible light communication (VLC) has been viewed as one promising candidate to mitigate the challenging spectrum crisis and radio frequency interference in future 6G mobile communications and networking. Due to the relatively limited baseband modulation bandwidth of VLC light sources—typically, light-emitting diodes—non-orthogonal multiple [...] Read more.

Visible light communication (VLC) has been viewed as one promising candidate to mitigate the challenging spectrum crisis and radio frequency interference in future 6G mobile communications and networking. Due to the relatively limited baseband modulation bandwidth of VLC light sources—typically, light-emitting diodes—non-orthogonal multiple access (NOMA) techniques have been proposed and explored to enhance the spectral efficiency (SE) of VLC systems. However, almost all reported NOMA VLC schemes focus on well-discussed applications employing a Lambertian light beam configuration and ignore the potential applications with distinct non-Lambertian optical beam configurations. To address this issue, in this work, the performance of non-Lambertian optical beam configuration-based NOMA VLC is comparatively investigated for future 6G mobile networks. The numerical results demonstrate that, for a fundamental two-user application scenario with the far user located at the corner position, a maximum sum rate gain of about 15.6 Mbps could be provided by the investigated distinct non-Lambertian beam-based NOMA VLC, compared with the maximum sum rate of about 93.3 Mbps for the conventional Lambertian configuration with the same power splitting factor. Full article

(This article belongs to the Special Issue Advancements in Wireless Optical Communication: Integrating Visible Light and Beyond)

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12 pages, 1919 KiB

Open AccessArticle

Learning Gradient-Based Feed-Forward Equalizer for VCSELs

by Muralikrishnan Srinivasan, Alireza Pourafzal, Stavros Giannakopoulos, Peter Andrekson, Christian Häger and Henk Wymeersch

Photonics 2024, 11(10), 943; https://doi.org/10.3390/photonics11100943 - 7 Oct 2024

Abstract

Vertical cavity surface-emitting laser (VCSEL)-based optical interconnects (OI) are crucial for high-speed data transmission in data centers, supercomputers, and vehicles, yet their performance is challenged by harsh and fluctuating thermal conditions. This paper addresses these challenges by integrating an ordinary differential equation (ODE) [...] Read more.

Vertical cavity surface-emitting laser (VCSEL)-based optical interconnects (OI) are crucial for high-speed data transmission in data centers, supercomputers, and vehicles, yet their performance is challenged by harsh and fluctuating thermal conditions. This paper addresses these challenges by integrating an ordinary differential equation (ODE) solver within the VCSEL communication chain, leveraging the adjoint method to enable effective gradient-based optimization of pre-equalizer weights. We propose a machine learning (ML) approach to optimize feed-forward equalizer (FFE) weights for VCSEL transceivers, which significantly enhances signal integrity by managing inter-symbol interference (ISI) and reducing the symbol error rate (SER). Full article

(This article belongs to the Special Issue Machine Learning Applied to Optical Communication Systems)

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25 pages, 12866 KiB

Open AccessReview

Advances in and Future Perspectives on High-Power Ceramic Lasers

by Vinay Rastogi and Shivanand Chaurasia

Photonics 2024, 11(10), 942; https://doi.org/10.3390/photonics11100942 - 7 Oct 2024

Abstract

Advancements in laser glass compositions and manufacturing techniques has allowed the development of a new category of high-energy and high-power laser systems which are being used in various applications, such as for fundamental research, material processing and inertial confinement fusion (ICF) technologies research. [...] Read more.

Advancements in laser glass compositions and manufacturing techniques has allowed the development of a new category of high-energy and high-power laser systems which are being used in various applications, such as for fundamental research, material processing and inertial confinement fusion (ICF) technologies research. A ceramic laser is a remarkable revolution in solid state lasers. It exhibits crystalline properties, high yields, better thermal conductivity, a uniformly broadened emission cross-section, and a higher mechanical constant. Polycrystalline ceramic lasers combine the properties of glasses and crystals, which offer the unique advantages of high thermal stability, excellent optical transparency, and the ability to incorporate active laser ions homogeneously. They are less expensive and have a similar fabrication process to glass lasers. Recent developments in these classes of lasers have led to improvements in their efficiency, beam quality, and wavelength versatility, making them suitable for a broad range of applications, such as scientific research requiring ultra-fast laser pulses, medical procedures like laser surgery and high-precision cutting and welding in industrial manufacturing. The future of ceramic lasers looks promising, with ongoing research focused on enhancing their performance, developing new doping materials and expanding their functional wavelengths. The ongoing progress in high-power ceramic lasers is continuously expanding the limits of laser technology, therefore allowing the development of more powerful and efficient systems for a wide range of advanced and complex applications. In this paper, we review the advances, limitations and future perspectives of ceramic lasers. Full article

(This article belongs to the Special Issue Recent Advances and Future Perspectives in Solid-State Lasers)

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