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Editor’s Choice Articles

Editor’s Choice articles are based on recommendations by the scientific editors of MDPI journals from around the world. Editors select a small number of articles recently published in the journal that they believe will be particularly interesting to readers, or important in the respective research area. The aim is to provide a snapshot of some of the most exciting work published in the various research areas of the journal.

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10 pages, 2940 KiB  
Communication
Intrusion Monitoring Based on High Dimensional Random Matrix by Using Ultra-Weak Fiber Bragg Grating Array
by Hongcan Gu, Junbing Huang, Su Wu, Ciming Zhou, Zhiqiang Zhang, Cong Liu and Yandong Pang
Photonics 2023, 10(7), 733; https://doi.org/10.3390/photonics10070733 - 27 Jun 2023
Viewed by 786
Abstract
In order to ensure that a perimeter security system can work effectively, a convenient and effective event detection algorithm has an important engineering significance. Given the above background, in this paper, we propose a high reliability intrusion event recognition method and vibration sensing [...] Read more.
In order to ensure that a perimeter security system can work effectively, a convenient and effective event detection algorithm has an important engineering significance. Given the above background, in this paper, we propose a high reliability intrusion event recognition method and vibration sensing system, based on ultra-weak fiber Bragg grating array, by using high dimensional random matrix. We obtain a high sensitivity optical interference signal by constructing a patch-matched optical interference system, then compose the demodulated interference signal into a high-dimensional random matrix. The statistical characteristics of the matrix for the Marcenko-Pastur (M-P) law and ring law are used to confirm the presence of intrusion events efficiently, which can reflect the limit spectrum distribution of the high-dimensional random matrix; meanwhile, the abnormal state quantity and moment are obtained. Further, the average spectral radius value is used to judge the fault cause. Field experimental results show that the proposed method can effectively obtain the correct monitoring data for the sensor array. By comparing the monitoring results of normal operation and crusher operation, we can detect the intrusion event in 4.5 s, and the accuracy rate can reach more than 90%, which verifies that the proposed high-dimensional random matrix analysis method can work properly, proving a practical engineering application prospect. Full article
(This article belongs to the Special Issue Optically Active Nanomaterials for Sensing Applications)
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13 pages, 7244 KiB  
Article
First Experimental Demonstration of the Wide-Field Amplitude Surface Plasmon Resonance Microscopy in the Terahertz Range
by Vasiliy Valerievich Gerasimov, Oleg Eduardovich Kameshkov, Alexey Konstantinovich Nikitin, Ildus Shevketovich Khasanov, Alexey Georgievich Lemzyakov, Irina Veniaminovna Antonova, Artem Ilyich Ivanov, Nghiem Thi Ha Lien, Nguyen Trong Nghia, Le Tu Anh, Nguyen Quoc Hung and Ta Thu Trang
Photonics 2023, 10(7), 723; https://doi.org/10.3390/photonics10070723 - 24 Jun 2023
Viewed by 1168
Abstract
We have demonstrated the wide-field amplitude surface plasmon resonance (SPR) microscopy technique in the terahertz (THz) range. A Zeonex polymer prism was utilized to excite surface plasmon polaritons (SPPs) through attenuated total reflection (ATR) in an Otto configuration. Coherent quasimonochromatic radiation with a [...] Read more.
We have demonstrated the wide-field amplitude surface plasmon resonance (SPR) microscopy technique in the terahertz (THz) range. A Zeonex polymer prism was utilized to excite surface plasmon polaritons (SPPs) through attenuated total reflection (ATR) in an Otto configuration. Coherent quasimonochromatic radiation with a wavelength of approximately 197 μm, generated by the Novosibirsk free electron laser, was employed. Our results indicate that the SPR microscopy method is applicable for investigating the planar surfaces of semiconductors at THz frequencies, provided that the SPPs’ cutoff frequency is close to the probing radiation frequency. This condition ensures that the propagation length of the SPPs is comparable to the radiation wavelength. By varying the air gap between the prism and the surface under examination, we acquired images of a polypropylene coating 20 µm thick and a graphene coating 35 nm thick on a flat indium antimonide substrate. The boundary between the coated and uncoated regions can be precisely localized through determination of the kink in the reflection coefficient of the THz radiation beam that illuminates the boundary between the regions if the optimal conditions for the generation of the SPPs in the uncoated region are met. Full article
(This article belongs to the Special Issue Terahertz Spectroscopy and Imaging)
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16 pages, 4443 KiB  
Review
Sb-Based Low-Noise Avalanche Photodiodes
by Joe C. Campbell, John P. R. David and Seth R. Bank
Photonics 2023, 10(7), 715; https://doi.org/10.3390/photonics10070715 - 22 Jun 2023
Cited by 2 | Viewed by 1991
Abstract
Accurate detection of weak optical signals is a key function for a wide range of applications. A key performance parameter is the receiver signal-to-noise ratio, which depends on the noise of the photodetector and the following electrical circuitry. The circuit noise is typically [...] Read more.
Accurate detection of weak optical signals is a key function for a wide range of applications. A key performance parameter is the receiver signal-to-noise ratio, which depends on the noise of the photodetector and the following electrical circuitry. The circuit noise is typically larger than the noise of photodetectors that do not have internal gain. As a result, a detector that provides signal gain can achieve higher sensitivity. This is accomplished by increasing the photodetector gain until the noise associated with the gain mechanism is comparable to that of the output electrical circuit. For avalanche photodiodes (APDs), the noise that arises from the gain mechanism, impact ionization, increases with gain and depends on the material from which the APD is fabricated. Si APDs have established the state-of-the-art for low-noise gain for the past five decades. Recently, APDs fabricated from two Sb-based III-V compound quaternary materials, AlxIn1-xAsySb1-y and AlxGa1-xAsySb1-y, have achieved noise characteristics comparable to those of Si APDs with the added benefit that they can operate in the short-wave infrared (SWIR) and extended SWIR spectral regions. This paper describes the materials and device characteristics of these APDs and their performance in different spectral regions. Full article
(This article belongs to the Special Issue Advances in Avalanche Photodiodes)
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9 pages, 3397 KiB  
Communication
All-Polarization-Maintaining, Mode-Locking Fiber Front-End Laser Delivering Both the Picosecond Seed Laser and the Femtosecond Seed Laser
by Yinuo Zhang, Hao Zhang, Kong Gao, Wenchao Qiao, Tianli Feng, Xian Zhao and Yizhou Liu
Photonics 2023, 10(6), 665; https://doi.org/10.3390/photonics10060665 - 08 Jun 2023
Viewed by 1123
Abstract
An ytterbium-doped, mode-locking fiber front-end laser, delivering both a femtosecond seed laser and picosecond seed laser, was demonstrated. The fundamental repetition rate of the 1031 nm mode-locked laser was 32.77 MHz, realized with the all-polarization-maintaining (all-PM) nonlinear amplifying loop mirror (NALM). The femtosecond [...] Read more.
An ytterbium-doped, mode-locking fiber front-end laser, delivering both a femtosecond seed laser and picosecond seed laser, was demonstrated. The fundamental repetition rate of the 1031 nm mode-locked laser was 32.77 MHz, realized with the all-polarization-maintaining (all-PM) nonlinear amplifying loop mirror (NALM). The femtosecond seed laser and the picosecond seed laser were delivered after carefully optimizing the nonlinear amplification process. The corresponding pulse durations were 85 fs and 2.88 ps, with average power of 171 mW and 562.5 mW, respectively. Full article
(This article belongs to the Special Issue Ultrafast Laser Systems)
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21 pages, 9221 KiB  
Review
A Review of Advanced Transceiver Technologies in Visible Light Communications
by Cuiwei He and Chen Chen
Photonics 2023, 10(6), 648; https://doi.org/10.3390/photonics10060648 - 03 Jun 2023
Cited by 2 | Viewed by 1950
Abstract
Visible Light Communication (VLC) is an emerging technology that utilizes light-emitting diodes (LEDs) for both indoor illumination and wireless communications. It has the potential to enhance the existing WiFi network and connect a large number of high-speed internet users in future smart home [...] Read more.
Visible Light Communication (VLC) is an emerging technology that utilizes light-emitting diodes (LEDs) for both indoor illumination and wireless communications. It has the potential to enhance the existing WiFi network and connect a large number of high-speed internet users in future smart home environments. Over the past two decades, VLC techniques have made significant strides, resulting in transmission data rates increasing from just a few Mbps to several tens of Gbps. These achievements can be attributed to the development of various transceiver technologies. At the transmitter, LEDs should provide high-quality light for illumination and support wide modulation bandwidths. Meanwhile, at the receiver, optics systems should have functions such as optical filtering, light concentration, and, ideally, a wide field of view (FOV). The photodetector must efficiently convert the optical signal into an electrical signal. Different VLC systems typically consider various transceiver designs. In this paper, we provide a survey of some important emerging technologies used to create advanced optical transceivers in VLC. Full article
(This article belongs to the Special Issue Advances in Visible Light Communication)
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25 pages, 7076 KiB  
Review
A Review on Image Sensor Communication and Its Applications to Vehicles
by Ruiyi Huang and Takaya Yamazato
Photonics 2023, 10(6), 617; https://doi.org/10.3390/photonics10060617 - 26 May 2023
Cited by 3 | Viewed by 1548
Abstract
Image sensor communication (ISC), also known as optical camera communication, is a form of visible light communication that utilizes image sensors rather than a single photodiode, for data reception. ISC offers spatial separation properties and robustness to ambient noise, making it suitable for [...] Read more.
Image sensor communication (ISC), also known as optical camera communication, is a form of visible light communication that utilizes image sensors rather than a single photodiode, for data reception. ISC offers spatial separation properties and robustness to ambient noise, making it suitable for outdoor applications such as intelligent transportation systems (ITSs). This review analyzes the research trends in ISC, specifically concerning its application in ITSs. Our focus is on various ISC receivers, including rolling shutter cameras, global shutter high-speed cameras, optical communication image sensors, and event cameras. We analyze how each of these receivers is being utilized in ISC vehicular applications. In addition, we highlight the use of ISC in range estimation techniques and the ability to achieve simultaneous communication and range estimation. By examining these topics, we aim to provide a comprehensive overview of the role of ISC technology in ITSs and its potential for future development. Full article
(This article belongs to the Special Issue Advances in Visible Light Communication)
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10 pages, 1487 KiB  
Communication
Optical Chaos in Saturated Nonlinear Media
by Fuqiang Li, Ziyang Chen, Jie Song, Meng Li and Cibo Lou
Photonics 2023, 10(5), 600; https://doi.org/10.3390/photonics10050600 - 22 May 2023
Viewed by 921
Abstract
In the study of the evolution of Gaussian beam in saturated nonlinear media, it is found that the probability of optical rogue waves changes with the change of nonlinearity. The light intensity distribution on the exit surface of nonlinear medium can be characterized [...] Read more.
In the study of the evolution of Gaussian beam in saturated nonlinear media, it is found that the probability of optical rogue waves changes with the change of nonlinearity. The light intensity distribution on the exit surface of nonlinear medium can be characterized by scintillation index, and the change of rogue wave corresponds to the evolution of scintillation index. The rogue wave probability shows a complex trend with the evolution of nonlinearity. The Lyapunov exponent and power spectrum method are used to determine that the probability of rogue wave is chaotic with nonlinear evolution. Full article
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11 pages, 16655 KiB  
Article
Study on the Influence of Underwater LED Illumination on Bidirectional Underwater Wireless Optical Communication
by Kelin Sun, Biao Han, Jingchuan Yang, Bo Li, Bin Zhang, Kaibin Liu and Chen Li
Photonics 2023, 10(5), 596; https://doi.org/10.3390/photonics10050596 - 21 May 2023
Cited by 4 | Viewed by 923
Abstract
Underwater wireless optical communication (UWOC) is acknowledged as a useful way to transmit data in the ocean for short-distance applications. Carrying a UWOC device on mobile platforms is quite practical in ocean engineering, which is helpful to exploit its advantages. In application, such [...] Read more.
Underwater wireless optical communication (UWOC) is acknowledged as a useful way to transmit data in the ocean for short-distance applications. Carrying a UWOC device on mobile platforms is quite practical in ocean engineering, which is helpful to exploit its advantages. In application, such a platform needs a camera to observe the surroundings and guide its action. Since the majority of ocean is always dark, active illumination is necessary to imaging. When UWOC works in such an environment, its performance is affected by the illumination light noise. In this paper, we study the influence of underwater LED illumination on bidirectional UWOC with the Monte Carlo method. We simulate forward noise from LED illumination to the opposite receiver in the cooperative terminal, and the backscattering noise on the adjacent receiver in the same terminal. The results show that the forward noise is reduced with the increase of theabsorption coefficient, scattering coefficient, transmitting distance, and separated distance between receiver and the optical axis of LED. However, it becomes greater with the field of view (FOV) of the receiver. The backscattering noise is reduced with the increase of the absorption coefficient and separated distance between receiver and LED. However, it becomes greater with the FOV and scattering coefficient, while it has little relation with transmitting distance. In order to reduce these two kinds of noises, besides inserting an optical filter in the receivers and narrowing their FOV, the optical axis of LED light should keep away from the receivers. The results in this paper are helpful for UWOC application. Full article
(This article belongs to the Special Issue Focus on Nonlinear Processing and Detection Technologies of Weak Optical Signal)
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9 pages, 1501 KiB  
Communication
Photoresponse of Graphene Channel in Graphene-Oxide–Silicon Photodetectors
by Kuo-Chih Lee, Yu-Hsien Chuang, Chen-Kai Huang, Hui Li, Guo-En Chang, Kuan-Ming Hung and Hung Hsiang Cheng
Photonics 2023, 10(5), 568; https://doi.org/10.3390/photonics10050568 - 12 May 2023
Viewed by 1310
Abstract
Graphene-on-silicon photodetectors exhibit broadband detection capabilities with high responsivities, surpassing those of their counterpart semiconductors fabricated purely using graphene or Si. In these studies, graphene channels were considered electrically neutral, and signal amplification was typically attributed to the photogating effect. By contrast, herein, [...] Read more.
Graphene-on-silicon photodetectors exhibit broadband detection capabilities with high responsivities, surpassing those of their counterpart semiconductors fabricated purely using graphene or Si. In these studies, graphene channels were considered electrically neutral, and signal amplification was typically attributed to the photogating effect. By contrast, herein, we show graphene channels to exhibit p-type characteristics using a structure wherein a thin oxide layer insulated the graphene from Si. The p-type carrier concentration is higher (six-times) than the photoaging-induced carrier concentration and dominates the photocurrent. Additionally, we demonstrate photocurrent tunability in the channel. By operating this device under a back-gated bias, photocurrent tuning is realized with not only amplification but also attenuation. Gate amplification produces a current equal to the photogating current at a low bias (0.2 V), and it is approximately two orders of magnitude larger at a bias of 2 V, indicating the operation effectiveness. Meanwhile, photocurrent attenuation enables adjustments in the detector output for compatibility with read-out circuits. A quantification model of gate-dependent currents is further established based on the simulation model used for metal–oxide–semiconductor devices. Thus, this study addresses fundamental issues concerning graphene channels and highlights the potential of such devices as gate-tunable photodetectors in high-performance optoelectronics. Full article
(This article belongs to the Special Issue Advanced Photonic Sensing and Measurement)
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15 pages, 3379 KiB  
Review
Review of 2 × 2 Silicon Photonic Switches
by Wencheng Yue, Yan Cai and Mingbin Yu
Photonics 2023, 10(5), 564; https://doi.org/10.3390/photonics10050564 - 11 May 2023
Cited by 4 | Viewed by 2747
Abstract
With the advent of 5G, artificial intelligence (AI), Internet of Things (IoT), cloud computing, Internet plus, and so on, data traffic is exploding and higher requirements are put forward for information transmission and switching. Traditional switching requires optical/electrical/optical conversions, which brings additional power [...] Read more.
With the advent of 5G, artificial intelligence (AI), Internet of Things (IoT), cloud computing, Internet plus, and so on, data traffic is exploding and higher requirements are put forward for information transmission and switching. Traditional switching requires optical/electrical/optical conversions, which brings additional power consumption and requires the deployment of large amounts of cooling equipment. This increases the cost and complexity of the system. Moreover, limited by the electronic bottleneck, electrical switching will suffer from many problems such as bandwidth, delay, crosstalk, and so on, with the continuous reduction in device footprint. Optical switching does not require optical/electrical/optical conversions and has lower power consumption, larger capacity, and lower cost. Silicon photonic switches received much attention because of their compatibility with the complementary metal-oxide-semiconductor (CMOS) process and are anticipated to be potential candidates to replace electrical switches in many applications such as data center and telecommunication networks. 2 × 2 silicon photonic switches are the basic components to build the large-scale optical switching matrices. Thus, this review article mainly focuses on the principle and state of the art of 2 × 2 silicon photonic switches, including electro-optic switches, thermo-optic switches, and nonvolatile silicon photonic switches assisted by phase-change materials. Full article
(This article belongs to the Section Optical Communication and Network)
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11 pages, 3700 KiB  
Article
Quantum Dot Lasers Directly Grown on 300 mm Si Wafers: Planar and In-Pocket
by Kaiyin Feng, Chen Shang, Eamonn Hughes, Andrew Clark, Rosalyn Koscica, Peter Ludewig, David Harame and John Bowers
Photonics 2023, 10(5), 534; https://doi.org/10.3390/photonics10050534 - 06 May 2023
Cited by 3 | Viewed by 3219
Abstract
We report for the first time the direct growth of quantum dot (QD) lasers with electrical pumping on 300 mm Si wafers on both a planar template and in-pocket template for in-plane photonic integration. O-band lasers with five QD layers were grown with [...] Read more.
We report for the first time the direct growth of quantum dot (QD) lasers with electrical pumping on 300 mm Si wafers on both a planar template and in-pocket template for in-plane photonic integration. O-band lasers with five QD layers were grown with molecular beam epitaxy (MBE) in a 300 mm reactor and then fabricated into standard Fabry–Perot ridge waveguide cavities. Edge-emitting lasers are demonstrated with high yield and reliable results ready for commercialization and scaled production, and efforts to make monolithically integrated lasing cavities grown on silicon-on-insulator (SOI) wafers vertically aligned and coupled to SiN waveguides on the same chip show the potential for 300 mm-scale Si photonic integration with in-pocket direct MBE growth. Full article
(This article belongs to the Special Issue Nanophotonics Pioneer: Prof. Dr. Dieter Bimberg ‘Green Photonics Networks: From VCSELs to Nanophotonics’)
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15 pages, 6478 KiB  
Article
Nonlinear Optical Response of Dispersed Medium Based on Conjugates Single-Walled Carbon Nanotubes with Phthalocyanines
by Pavel N. Vasilevsky, Mikhail S. Savelyev, Alexander Yu. Tolbin, Artem V. Kuksin, Yulia O. Vasilevskaya, Andrey P. Orlov, Yury P. Shaman, Alexander A. Dudin, Alexander A. Pavlov and Alexander Yu. Gerasimenko
Photonics 2023, 10(5), 537; https://doi.org/10.3390/photonics10050537 - 06 May 2023
Cited by 6 | Viewed by 1440
Abstract
Nanosecond lasers have recently been widely involved in human activity. However, high-intensity laser radiation can cause severe damage to organs of vision and expensive photonic devices. Radiation in the near UV range is especially dangerous for human eyes, since it is strongly absorbed [...] Read more.
Nanosecond lasers have recently been widely involved in human activity. However, high-intensity laser radiation can cause severe damage to organs of vision and expensive photonic devices. Radiation in the near UV range is especially dangerous for human eyes, since it is strongly absorbed by biological media and is also invisible, i.e., the reaction time of the eye to such radiation is much lower than that of visible light. Passive limiters have high transmission (>70%) at a low light intensity and begin to “darken” only when the threshold value of the laser radiation intensity is reached. In this work, we studied liquid nanodispersed nonlinear optical limiters based on hybrids of single-walled carbon nanotubes (SWCNTs) with metal-free tetra(hydroxy)phthalocyanine (OH)4PcHH). The value of the hydrodynamic radius of separate particles after (OH)4PcHH binding increased from 288 ± 55 nm to 350 ± 60 nm, which confirms the attachment of phthalocyanine complexes to nanotubes. The third harmonic of a Nd:YAG nanosecond laser (355 nm, 20 ns) was used to study the nonlinear optical response. Based on a Z-scan with open-aperture and input-output dependence curves, third-order nonlinear optical absorption coefficients of 149, 236, and 229 cm/GW were obtained for dispersions of composites of SWCNTs and (OH)4PcHH in water, dimethylformamide (DMF), and dimethylsulfoxide (DMSO), respectively. Threshold values did not exceed 100 mJ/cm2. The Z-scan showed a gradual decrease in the duration of the laser pulse by 53%; however, near the focus, there was a sharp increase in the duration of the transmitted pulse, reaching a value of 29 ns in z = 0. This phenomenon confirms the occurrence of reverse saturable absorption in the investigated media and can be used in photonic devices to control the temporal characteristics of the signal. Thus, the possibility of protection of sensitive photonic devices and human eyes from nanosecond laser pulses in the near UV range by nanodispersed liquid media based on composites of SWCNTs with (OH)4PcHH has been discussed in this paper. Full article
(This article belongs to the Special Issue Nonlinear and Ultrafast Optics: Fundamentals and Applications)
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12 pages, 1968 KiB  
Review
Toward Practical Optical Phased Arrays through Grating Antenna Engineering
by Youqiang Shuai, Zhiping Zhou and Hui Su
Photonics 2023, 10(5), 520; https://doi.org/10.3390/photonics10050520 - 01 May 2023
Cited by 2 | Viewed by 2055
Abstract
In recent years, using silicon-based waveguide grating antennas in optical phased array has become a research focus. To date, this technique has not been widely implemented in practical applications. In this paper, the basic principle of a waveguide grating antenna is described, and [...] Read more.
In recent years, using silicon-based waveguide grating antennas in optical phased array has become a research focus. To date, this technique has not been widely implemented in practical applications. In this paper, the basic principle of a waveguide grating antenna is described, and the researches on effective length, uniform emission and the directionality of diffraction are summarized. Through analysis, it is found that there is a trend to prepare grating antennas by using a SiN/Si hybrid integrated platform. A novel design of grating antenna using the hybrid integration technique is proposed. It is convenient to match with the antenna front-end components on the structural level and is more practical. Full article
(This article belongs to the Special Issue Advances in Optical 3D Integration)
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11 pages, 2874 KiB  
Communication
Valley-Selective Polarization in Twisted Bilayer Graphene Controlled by a Counter-Rotating Bicircular Laser Field
by Jiayin Chen, Candong Liu and Ruxin Li
Photonics 2023, 10(5), 516; https://doi.org/10.3390/photonics10050516 - 01 May 2023
Cited by 5 | Viewed by 1519
Abstract
The electron valley pseudospin in two-dimensional hexagonal materials is a crucial degree of freedom for achieving their potential application in valleytronic devices. Here, bringing valleytronics to layered van der Waals materials, we theoretically investigate lightwave-controlled valley-selective excitation in twisted bilayer graphene (tBLG) with [...] Read more.
The electron valley pseudospin in two-dimensional hexagonal materials is a crucial degree of freedom for achieving their potential application in valleytronic devices. Here, bringing valleytronics to layered van der Waals materials, we theoretically investigate lightwave-controlled valley-selective excitation in twisted bilayer graphene (tBLG) with a large twist angle. It is demonstrated that the counter-rotating bicircular light field, consisting of a fundamental circularly-polarized pulse and its counter-rotating second harmonic, can manipulate the sub-cycle valley transport dynamics by controlling the relative phase between two colors. In comparison with monolayer graphene, the unique interlayer coupling of tBLG renders its valley selectivity highly sensitive to duration, leading to a noticeable valley asymmetry that is excited by single-cycle pulses. We also describe the distinct signatures of the valley pseudospin change in terms of observing the valley-selective circularly-polarized high-harmonic generation. The results show that the valley pseudospin dynamics can still leave visible fingerprints in the modulation of harmonic signals with a two-color relative phase. This work could assist experimental researchers in selecting the appropriate protocols and parameters to obtain ideal control and characterization of valley polarization in tBLG. Full article
(This article belongs to the Special Issue Light Control and Particle Manipulation)
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10 pages, 1031 KiB  
Communication
Single Photon Approach for Chirality Sensing
by Fabrizio Sgobba, Arianna Elefante, Stefano Dello Russo, Mario Siciliani de Cumis and Luigi Santamaria Amato
Photonics 2023, 10(5), 512; https://doi.org/10.3390/photonics10050512 - 28 Apr 2023
Viewed by 1119
Abstract
We developed a high sensitivity optical sensor for circular birefringence using a heralded photon source. The sensor can be employed for chirality measurements and, being based on single photons, can be exploited for fragile biological sample or in metrological applications where the light [...] Read more.
We developed a high sensitivity optical sensor for circular birefringence using a heralded photon source. The sensor can be employed for chirality measurements and, being based on single photons, can be exploited for fragile biological sample or in metrological applications where the light intensity must be kept as low as possible. We found the best operational condition; then, we calibrated the sensor and tested its performance up to a very long acquisition time, obtaining excellent stability and a sub-ppm birefringence detection limit (for a 100 μm sample), thus paving the way for fundamental physics test as well. Full article
(This article belongs to the Topic Advances in Optical Sensors)
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12 pages, 5623 KiB  
Communication
Surface-Emitting Lasers with Surface Metastructures
by Anjin Liu, Jing Zhang, Chenxi Hao, Minglu Wang and Wanhua Zheng
Photonics 2023, 10(5), 509; https://doi.org/10.3390/photonics10050509 - 27 Apr 2023
Cited by 2 | Viewed by 1269
Abstract
Vertical-cavity surface-emitting lasers (VCSELs) have been widely used in consumer electronics, light detection and ranging, optical interconnects, atomic sensors, and so on. In this paper, a VCSEL with the surface metastructure like one-dimensional high-contrast grating (HCG), based on the HCG-DBR vertical cavity, was [...] Read more.
Vertical-cavity surface-emitting lasers (VCSELs) have been widely used in consumer electronics, light detection and ranging, optical interconnects, atomic sensors, and so on. In this paper, a VCSEL with the surface metastructure like one-dimensional high-contrast grating (HCG), based on the HCG-DBR vertical cavity, was first designed and fabricated. The polarization characteristic of the HCG-VCSEL were experimentally studied. The p-doped top 4-pair DBR for the current spreading and the direction shift between the HCG and the elliptical oxide aperture may result in a low orthogonal polarization suppression ratio in the HCG-VCSEL. Then, the Bloch surface wave surface-emitting laser (BSW-SEL), based on the HCG-DBR metastructure, is proposed for single-mode, high-efficiency, and high-power output with a low divergence angle. The mode field and the far field profile of the BSW-SEL are calculated for verification. The surface-emitting lasers with surface metastructures are useful for the sensing applications and optical interconnects. Full article
(This article belongs to the Special Issue Nanophotonics Pioneer: Prof. Dr. Dieter Bimberg ‘Green Photonics Networks: From VCSELs to Nanophotonics’)
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12 pages, 3731 KiB  
Article
High-Efficiency and Compact Polarization-Insensitive Multi-Segment Linear Silicon Nitride Edge Coupler
by Yuhao Zhang, Yi Liu, Xuhua Cao, Li Wang, Chunyuan Mu, Ming Li, Ninghua Zhu and Wei Chen
Photonics 2023, 10(5), 510; https://doi.org/10.3390/photonics10050510 - 27 Apr 2023
Cited by 1 | Viewed by 1601
Abstract
Edge couplers are widely utilized in photonic integrated circuits and are vital for ensuring efficient chip-to-fiber coupling. In this paper, we present a high-efficiency and compact polarization-insensitive multi-segment linear silicon nitride edge coupler for coupling to high numerical aperture fibers. By optimizing the [...] Read more.
Edge couplers are widely utilized in photonic integrated circuits and are vital for ensuring efficient chip-to-fiber coupling. In this paper, we present a high-efficiency and compact polarization-insensitive multi-segment linear silicon nitride edge coupler for coupling to high numerical aperture fibers. By optimizing the thickness of the up cladding and introducing air slots in the transverse direction, we have further modified the limiting effect of the mode field. This innovative edge coupler scheme boasts a compact structure and is compatible with existing mature standard processes, with a total length of only 38 μm. We numerically demonstrate that the proposed edge coupler exhibits a low coupling loss of 0.22 dB/0.31 dB for TE/TM modes at λ = 1550 nm. Furthermore, the proposed coupler displays high wavelength insensitivity within the range of 1400–1850 nm and maintains a coupling loss of less than 0.2 dB with a manufacturing deviation of ±20 nm. Full article
(This article belongs to the Topic Optical and Optoelectronic Materials and Applications)
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10 pages, 3695 KiB  
Communication
Theoretical Study on Dual-Function Optical Phased Array of LiDAR and Optical Wireless Communication Based on Optically Injection-Locked Semiconductor Lasers
by Anh-Hang Nguyen, Hyo-Sang Jeong, Hyungsik Shin and Hyuk-Kee Sung
Photonics 2023, 10(5), 498; https://doi.org/10.3390/photonics10050498 - 26 Apr 2023
Viewed by 1125
Abstract
Light detection and ranging (LiDAR) and optical wireless communication (OWC) are in high demand and rapidly developing owing to the explosive growth of smart systems that require automotive and mobile devices. Optical phased arrays (OPA) have become a key technology in LiDAR and [...] Read more.
Light detection and ranging (LiDAR) and optical wireless communication (OWC) are in high demand and rapidly developing owing to the explosive growth of smart systems that require automotive and mobile devices. Optical phased arrays (OPA) have become a key technology in LiDAR and OWC owing to their nonmechanical beam steering capabilities. However, using separate LiDAR and OWC platforms in one system creates problems, such as spectrum congestion, resource consumption, and high complexity. We propose a dual-function OPA that enables LiDAR and OWC to function on a single platform based on the simultaneous amplitude and phase modulation of optically injection-locked semiconductor lasers. We numerically demonstrated that the primary LiDAR and secondary OWC function simultaneously by independent control of the main and side lobes in the OPA signal. The variation in side lobe levels is controlled at 20 or 25 dB to realize low- and high-level data for OWC function as well as maintaining the main beam LiDAR function. We successfully achieved wide-opening eye patterns of 10 Gbps data transmission of the OWC operation. Full article
(This article belongs to the Special Issue Challenges and Opportunities in Optical and Wireless Communication Networks)
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24 pages, 2183 KiB  
Review
The Optical Inverse Problem in Quantitative Photoacoustic Tomography: A Review
by Zeqi Wang, Wei Tao and Hui Zhao
Photonics 2023, 10(5), 487; https://doi.org/10.3390/photonics10050487 - 24 Apr 2023
Cited by 7 | Viewed by 1658
Abstract
Photoacoustic tomography is a fast-growing biomedical imaging modality that combines rich optical contrast with a high acoustic resolution, at depths in tissues. Building upon the foundation of this technique, novel quantitative photoacoustic tomography fully leverages its advantages while further delivering improved quantification capabilities [...] Read more.
Photoacoustic tomography is a fast-growing biomedical imaging modality that combines rich optical contrast with a high acoustic resolution, at depths in tissues. Building upon the foundation of this technique, novel quantitative photoacoustic tomography fully leverages its advantages while further delivering improved quantification capabilities to produce high-accuracy concentration estimates, which has attracted substantial research interest in recent years. The kernel challenge associated with quantitative photoacoustic tomography is an optical inverse problem aiming to recover the absorption coefficient distribution from the conventional photoacoustic image. Although the crucial importance of the optical inversion has been widely acknowledged, achieving it has remained a persistent challenge due to the inherent non-linearity and non-uniqueness. In the past decade, numerous methods were proposed and have made noticeable progress in addressing this concern. Nevertheless, a review has been conspicuously absent for a long time. Aiming to bridge this gap, the present study comprehensively investigates the recent research in this field, and methods identified with significant value are introduced in this paper. Moreover, all included methods are systematically classified based on their underlying principles. Finally, we summarize each category and highlight its remaining challenges and potential future research directions. Full article
(This article belongs to the Special Issue Photoacoustic Imaging: Applications, Approaches, and Systems)
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11 pages, 1987 KiB  
Communication
All-Fiber Laser Feedback Interferometry for Sequential Sensing of In-Plane and Out-of-Plane Displacements
by Zhanwu Xie, Meng Zhang, Jie Li, Wei Xia and Dongmei Guo
Photonics 2023, 10(4), 480; https://doi.org/10.3390/photonics10040480 - 21 Apr 2023
Viewed by 1141
Abstract
In this paper, an all-fiber laser feedback interferometer (LFI) with a diffraction grating was developed for sequential measurement of in-plane and out-of-plane displacements without changing the optical arrangement. When the light emitted from an erbium-doped fiber ring laser is incident on a reflection [...] Read more.
In this paper, an all-fiber laser feedback interferometer (LFI) with a diffraction grating was developed for sequential measurement of in-plane and out-of-plane displacements without changing the optical arrangement. When the light emitted from an erbium-doped fiber ring laser is incident on a reflection grating at the Littrow angle, the diffracted light will return into the laser cavity along the original path, thus generating laser feedback interference. Experimental results reveal that the all-fiber system could achieve a precision of 40 nm in both in-plane and out-of-plane displacements sensing. Compared with the traditional all-fiber LFI, the proposed sensing system transfers the measuring scale from laser wavelength to grating period, and it has the advantages of good anti-interference performance and reliability. Full article
(This article belongs to the Special Issue Advanced Photonic Sensing and Measurement)
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15 pages, 5576 KiB  
Review
GaN-Based VCSELs with A Monolithic Curved Mirror: Challenges and Prospects
by Tatsushi Hamaguchi
Photonics 2023, 10(4), 470; https://doi.org/10.3390/photonics10040470 - 20 Apr 2023
Cited by 5 | Viewed by 2528
Abstract
In this paper, we introduce how gallium nitride-based (GaN-based) VCSELs with curved mirrors have evolved. The discussion starts with reviewing the fundamentals of VCSELs and GaN-based materials and then introducing the curved-mirror cavity’s principle and history and the latest research where the structure [...] Read more.
In this paper, we introduce how gallium nitride-based (GaN-based) VCSELs with curved mirrors have evolved. The discussion starts with reviewing the fundamentals of VCSELs and GaN-based materials and then introducing the curved-mirror cavity’s principle and history and the latest research where the structure is applied to GaN-based materials to form VCSELs. We prepared these parts so that readers understand how VCSELs with this cavity work and provide excellent characteristics such as efficiency, life, stabilized mode behavior, etc. Finally, we discussed the challenges and prospects of these devices by touching on their potential applications. Full article
(This article belongs to the Special Issue Nanophotonics Pioneer: Prof. Dr. Dieter Bimberg ‘Green Photonics Networks: From VCSELs to Nanophotonics’)
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17 pages, 5386 KiB  
Review
Progress in Short Wavelength Energy-Efficient High-Speed Vertical-Cavity Surface-Emitting Lasers for Data Communication
by Si-Cong Tian, Mansoor Ahamed and Dieter Bimberg
Photonics 2023, 10(4), 410; https://doi.org/10.3390/photonics10040410 - 06 Apr 2023
Cited by 5 | Viewed by 1937
Abstract
The current progress of energy-efficient high-speed VCSELs based on GaAs substrates is presented. Novel approaches for the design of VCSELs are presented, potentially leading to larger bandwidth bit rates and lower power consumption. The first approach is based on the optimization of the [...] Read more.
The current progress of energy-efficient high-speed VCSELs based on GaAs substrates is presented. Novel approaches for the design of VCSELs are presented, potentially leading to larger bandwidth bit rates and lower power consumption. The first approach is based on the optimization of the VCSEL photon lifetime. The second one introduces a novel design based on oxidizing the apertures from multiple etched holes of varying geometries. These designs are also essential for improving the energy efficiency of future modules by optimizing the match of the electronic driver and the photonic device. Full article
(This article belongs to the Special Issue Nanophotonics Pioneer: Prof. Dr. Dieter Bimberg ‘Green Photonics Networks: From VCSELs to Nanophotonics’)
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12 pages, 5621 KiB  
Communication
Interpretation of Localized Surface Plasmonic Resonances of Gold Nanoparticles Covered by Polymeric Coatings
by Alkeos Stamatelatos, Maria Tsarmpopoulou, Dimitrios Geralis, Alexandros G. Chronis, Vagelis Karoutsos, Dimitrios Ntemogiannis, Dionysios M. Maratos, Spyridon Grammatikopoulos, Mihail Sigalas and Panagiotis Poulopoulos
Photonics 2023, 10(4), 408; https://doi.org/10.3390/photonics10040408 - 05 Apr 2023
Cited by 2 | Viewed by 3479
Abstract
Plasmonic materials currently have a plethora of applications. How would a dielectric matrix, such as diblock copolymers, tune plasmonic properties? In this work, self-assembled gold nanoparticles were fabricated in medium vacuum conditions on heated Corning glass substrates (kept at 440 °C) under [...] Read more.
Plasmonic materials currently have a plethora of applications. How would a dielectric matrix, such as diblock copolymers, tune plasmonic properties? In this work, self-assembled gold nanoparticles were fabricated in medium vacuum conditions on heated Corning glass substrates (kept at 440 °C) under the coexistence of argon and air by means of DC magnetron sputtering. These samples were compared structurally and optically to samples deposited at room temperature and post annealed. Subsequently, the better of the two preparations, those deposited on heated glass, were covered with three different polymers, namely: Polystyrene-block-polybutadiene-block-polystyrene (PS-b-PBD-b-PS); Polystyrene-co-methyl methacrylate (PS-co-PMMA); and Polystyrene-block-polyisoprene-block-polystyrene (PS-b-PI-b-PS), by means of spin coating. Localized surface plasmon resonances were recorded and analyzed, respectively, for polymer-covered gold nanoparticles, with the width, intensity, and position of the resonances changing according to multiple factors, such as the nanoparticles size and the refractive index of each polymer. Lastly, for purposes of justification and comparison with the experimental results, rigorous theoretical calculations have been carried out. Full article
(This article belongs to the Special Issue Integrated Plasmonic Devices)
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17 pages, 3410 KiB  
Article
Requirements and Solutions for Robust Beam Alignment in Fiber-Coupled Free-Space Optical Systems
by Manuel M. Freitas, Marco A. Fernandes, Paulo P. Monteiro, Fernando P. Guiomar and Gil M. Fernandes
Photonics 2023, 10(4), 394; https://doi.org/10.3390/photonics10040394 - 02 Apr 2023
Cited by 4 | Viewed by 2538
Abstract
The continuous growth of Internet data traffic is pushing the current radio-frequency wireless technologies up to their physical limits. To overcome the upcoming bandwidth bottleneck, Free-Space Optics (FSO) is currently deemed as a key breakthrough toward next-generation ultra-high-capacity wireless links. Despite its numerous [...] Read more.
The continuous growth of Internet data traffic is pushing the current radio-frequency wireless technologies up to their physical limits. To overcome the upcoming bandwidth bottleneck, Free-Space Optics (FSO) is currently deemed as a key breakthrough toward next-generation ultra-high-capacity wireless links. Despite its numerous advantages, FSO also entails several particular challenges regarding the mitigation of the stochastic impairments induced by turbulence and the strict alignment requirements. One of the main issues of FSO communication systems is the mitigation of pointing errors and angle-of-arrival (AoA) fluctuations, which arise from misalignments induced by atmospheric turbulence and vibrations at the transmitting and receiving stations. A common approach to mitigate the impact of pointing errors is the use of an acquisition, tracking and pointing (ATP) system on one or both ends of the FSO link. In this paper, we present a characterization of the pointing errors and the AoA impact on the power budget of the FSO link to quantify the misalignment impairments. Afterwards, we experimentally demonstrate an FSO link with an ATP mechanism at both ends, managed by a control plane that enables the continuous and accurate alignment of the FSO link. To increase the misalignment tolerance, the ATP mechanism comprises two stages: the first one is based on a spatial diversity method provided by a quadrant detector, while the second stage maximizes the optical received power. Lastly, the impact of the beam misalignment on the achievable information rate of a coherent optical wireless system is theoretically addressed and characterized. Full article
(This article belongs to the Special Issue Free Space Optics-Based 6G Non-terrestrial Networks)
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11 pages, 10853 KiB  
Communication
Fiber-Fed 3D Printing of Germanate Glass Optics
by Zhihan Hong, Tao Luo, Shibin Jiang and Rongguang Liang
Photonics 2023, 10(4), 378; https://doi.org/10.3390/photonics10040378 - 29 Mar 2023
Cited by 2 | Viewed by 1696
Abstract
In recent years, 3D printing glass optics has gained massive attention in industry and academia since glass could be an ideal material to make optical elements, including the lens. However, the limitation of materials and printing methods has prevented 3D printing glass optics [...] Read more.
In recent years, 3D printing glass optics has gained massive attention in industry and academia since glass could be an ideal material to make optical elements, including the lens. However, the limitation of materials and printing methods has prevented 3D printing glass optics progress. Therefore, we have developed a novel printing strategy for germanate glass printing instead of pure silica. Moreover, compared with traditional multi-component quartz glass, germanate glass has unmatched advantages for its mid-infrared (MIR) transparency and outstanding visible light imaging performance. Furthermore, compared with non-oxide glass (fluoride glass and chalcogenide glass), germanate glass has much better mechanical, physical, and chemical properties and a high refractive index. Germanate glass has been widely applied in remote sensing, ranging, environmental detection, and biomedical detection. However, it is difficult to shape, cast, polish, and grind for optical and photonics applications such as imaging optics and laser-collimation optics. These drawbacks have made germanate glass inaccessible to complex optical elements and greatly increased their cost. In this report, we use germanate glass fibers with a diameter of 125 µm based on fiber-fed laser heating technology to fabricate an mm-size optical application. In this paper, we combine the fiber-fed laser heating technology with an optimized temperature control process to manufacture high-precision optical elements. Germanate glass optics can be printed with excellent visible light and IR transparency and a smooth surface with roughness under 4 nm. By optimizing the layer-by-layer 3D printing process and the thermal feedback in the printing process, we avoid cracks and minimize surface deformation. This work shows the possibility of the mm-size glass optical elements 3D printing and widens its application for IR optics. Full article
(This article belongs to the Special Issue Direct Laser Writing for Photonic Applications)
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20 pages, 16085 KiB  
Article
Comparative Performance Analysis of Femtosecond-Laser-Written Diode-Pumped Pr:LiLuF4 Visible Waveguide Lasers
by Davide Baiocco, Ignacio Lopez-Quintas, Javier R. Vázquez de Aldana, Mauro Tonelli and Alessandro Tredicucci
Photonics 2023, 10(4), 377; https://doi.org/10.3390/photonics10040377 - 29 Mar 2023
Cited by 1 | Viewed by 1424
Abstract
In this work, we present the operation of a femtosecond-laser-written diode-pumped visible waveguide laser based on praseodymium-doped lithium lutetium fluoride. The refractive index modification induced by the femtosecond laser in the crystal exhibits an anisotropic behavior, thus enabling the fabrication of different types [...] Read more.
In this work, we present the operation of a femtosecond-laser-written diode-pumped visible waveguide laser based on praseodymium-doped lithium lutetium fluoride. The refractive index modification induced by the femtosecond laser in the crystal exhibits an anisotropic behavior, thus enabling the fabrication of different types of waveguides from single-track structures to stress-induced waveguides and depressed cladding structures. All the waveguides were characterized by realizing transmission measurements and the waveguide design was optimized to obtain extremely low propagation losses, equal to 0.12 dB/cm. Lasing has been achieved at 604 nm and 721 nm from different waveguides. In addition, stable continuous-wave lasing at 698 nm has been obtained in a depressed cladding waveguide. This wavelength corresponds to the one needed for the transition of the atomic clock based on the neutral strontium atom. In the end, we report the observation of laser emission at 645 nm from a depressed cladding waveguide. Full article
(This article belongs to the Special Issue Novel Applications of Solid-State Laser and Future Prospects)
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11 pages, 3190 KiB  
Article
Bandwidth Expansion of Zero-Power-Consumption Visible Light Communication System
by Yiwu Xu, Xiongbin Chen and Yufeng Wang
Photonics 2023, 10(4), 376; https://doi.org/10.3390/photonics10040376 - 28 Mar 2023
Viewed by 1458
Abstract
The data rate of the zero-power-consumption visible light communication system is limited due to the solar cell bandwidth limit. A new method has been proposed to improve the data rate of communication systems. The predistortion circuit based on the nonlinear compensation principle of [...] Read more.
The data rate of the zero-power-consumption visible light communication system is limited due to the solar cell bandwidth limit. A new method has been proposed to improve the data rate of communication systems. The predistortion circuit based on the nonlinear compensation principle of the communication system is added between the transmitter and the receiver to equalize the amplitude–frequency response of the solar cell. Based on the measurement of the original amplitude–frequency response of the system, the zero-pole of the predistortion circuit is calculated, and an accurate equalization circuit is designed by using separate components to expand the 3 dB bandwidth of the system. The test results show that the 3 dB bandwidth of the visible light self-powered communication system is increased from 85 kHz to 750 kHz, and the system bandwidth is expanded by nearly 10 times. Full article
(This article belongs to the Special Issue Visible Light Communications)
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15 pages, 4633 KiB  
Article
Analysis of the Emission Features in CdSe/ZnS Quantum Dot-Doped Polymer Fibers
by Xuefeng Peng, Zhijian Wu, Chen Ye, Yang Ding and Wei Liu
Photonics 2023, 10(3), 327; https://doi.org/10.3390/photonics10030327 - 18 Mar 2023
Viewed by 1544
Abstract
The emission features of Cdse/ZnS quantum dots doped step-index polymer optical fibers are computationally analyzed in this paper. Spontaneous emission and amplified spontaneous emission were calculated by a theoretical model based on the rate equations in terms of time, fiber length, and wavelength. [...] Read more.
The emission features of Cdse/ZnS quantum dots doped step-index polymer optical fibers are computationally analyzed in this paper. Spontaneous emission and amplified spontaneous emission were calculated by a theoretical model based on the rate equations in terms of time, fiber length, and wavelength. All the calculated parameters are derived from experiments. Through the comparative analysis of the calculated and experimental results of spontaneous emission, we found that the pump power and overlap between the emission and absorption cross-sections may be the two main reasons for the red shift of the output spectra. When the pump power exceeds the threshold of amplified spontaneous emission, the width of the output spectra will rapidly decrease, the output wavelength will fall back toward the maximum emission cross-section, and the final output wavelength will still be affected by the doping concentration and pump power, while amplified spontaneous emission will not be generated when the total number of doped quantum dots is less than 1.27 × 1012. Full article
(This article belongs to the Special Issue Advances in Photonic Materials and Technologies)
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14 pages, 4464 KiB  
Article
Adaptive Weighted K-Nearest Neighbor Trilateration Algorithm for Visible Light Positioning
by Kaiyao Wang, Yi He, Xinpeng Huang and Zhiyong Hong
Photonics 2023, 10(3), 319; https://doi.org/10.3390/photonics10030319 - 16 Mar 2023
Viewed by 1520
Abstract
An adaptive weighted K-nearest neighbor (AWKNN) trilateration positioning algorithm fused with the channel state information (CSI) is proposed to optimize the accuracy of the visible light positioning. The core concept behind this algorithm is to combine the WKNN algorithm with ranging based on [...] Read more.
An adaptive weighted K-nearest neighbor (AWKNN) trilateration positioning algorithm fused with the channel state information (CSI) is proposed to optimize the accuracy of the visible light positioning. The core concept behind this algorithm is to combine the WKNN algorithm with ranging based on the CSI. The direct path distance estimated by the CSI is utilized to construct a position set consisting of multiple positions and a corresponding distance database containing multiple distance vectors. The error parameters of the weighted combinations of different distance vectors are calculated iteratively to evaluate the impact of different K-values and weights on the positioning accuracy. The proposed algorithm can achieve high-precision trilateration positioning by adaptively selecting the K-value and weight. A typical 4 m × 4 m × 3 m indoor multipath scene with four LEDs is established to simulate the positioning performance. The simulation results reveal that the mean error of the CSI-based AWKNN algorithm achieves 1.84 cm, with a root mean square error (RMSE) of 2.13 cm. Compared with the CSI-based least squares (LS) method, the CSI-based nonlinear LS method, and the CSI-based WKNN method, the average error of this method is decreased by 29%, 16%, and 17%, whereas the RMSE is reduced by 35%, 14%, and 19%. Full article
(This article belongs to the Special Issue Optical Wireless Communications and Applications)
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6 pages, 5566 KiB  
Communication
Ultrabroadband OPA in YCOB with a sub-ps Pump Source
by Hugo Pires, Joana Alves, Victor Hariton, Mario Galletti, Celso João and Gonçalo Figueira
Photonics 2023, 10(3), 253; https://doi.org/10.3390/photonics10030253 - 28 Feb 2023
Viewed by 1792
Abstract
We demonstrate the broadband optical parametric amplification of near-infrared laser pulses using a single yttrium calcium oxyborate (YCOB) crystal pumped in a noncollinear geometry by a sub-picosecond, milijoule-level source. The crystal uses an optimized orientation for phase matching outside of the principal planes, [...] Read more.
We demonstrate the broadband optical parametric amplification of near-infrared laser pulses using a single yttrium calcium oxyborate (YCOB) crystal pumped in a noncollinear geometry by a sub-picosecond, milijoule-level source. The crystal uses an optimized orientation for phase matching outside of the principal planes, enabling ultrabroadband amplification (gain of ∼800) in the range of 750–950 nm and supporting down to 7 fs pulses. Full article
(This article belongs to the Special Issue Ultrafast Lasers: Science and Applications)
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16 pages, 1726 KiB  
Article
Towards Microbial Food Safety of Sprouts: Photodynamic Decontamination of Seeds
by Andreas Fellner, Christoph Hamminger, Michael Fefer, Jun Liu and Kristjan Plaetzer
Photonics 2023, 10(3), 239; https://doi.org/10.3390/photonics10030239 - 22 Feb 2023
Viewed by 1898
Abstract
The climate crisis is one of the biggest challenges for humanity in the 21st century. Production and consumption of meat contributes to global warming by causing emissions of climate-relevant gases. Freshly grown sprouts are part of an alternative, as they are less polluting [...] Read more.
The climate crisis is one of the biggest challenges for humanity in the 21st century. Production and consumption of meat contributes to global warming by causing emissions of climate-relevant gases. Freshly grown sprouts are part of an alternative, as they are less polluting but still a nutritious food. However, warm humid sprouting conditions may cause pathogenic microorganisms to thrive. Decontamination methods for raw sprouts are therefore relevant. Photodynamic Inactivation (PDI) is a novel approach that uses photoactivatable molecules (photosensitisers, PS) and visible or near-infrared light to produce reactive oxygen species (ROS). These ROS kill microorganisms by oxidative processes. Here, we test the application of PDI based on sodium-magnesium-chlorophyllin (Chl, approved as food additive E140) for photo-decontamination of mung bean, radish, and buckwheat seeds. Seeds were contaminated with Listeria innocua, serving as a model system for Listeria monocytogenes, subjected to PDI using an LED array with 395 nm and tested for remaining bacterial contamination by CFU counting. PDI based on 100 µM Chl reduces the bacterial load of mung bean and radish seeds by 99.9% (radiant exposure 56.4 J/cm2 and 28.2 J/cm2, respectively), and of buckwheat seeds by <90% reduction after illumination with 28.2 J/cm2. Neither weight nor the germination rates of seeds are affected by PDI. Interestingly, the effect of PDI on seeds is partially maintained on stored sprouts after germination: The bacterial load on mung bean sprouts is reduced by more than 99.9% after phototreatment of seeds with 100 µM Chl and illumination at 56.4 J/cm2. In conclusion, we suggest PDI based on Chl as an effective and biocompatible method for the decontamination of seeds and sprouts for human consumption from Listeria. Full article
(This article belongs to the Special Issue Light-Based Technologies and Spectroscopic Techniques for Photo-Sensing and Photoinactivation of Microorganisms, Virus, and Cancer Cells)
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19 pages, 1619 KiB  
Article
Revisiting Experimental Signatures of the Ponderomotive Force
by Bjorn Manuel Hegelich, Lance Labun and Ou Z. Labun
Photonics 2023, 10(2), 226; https://doi.org/10.3390/photonics10020226 - 20 Feb 2023
Cited by 3 | Viewed by 2010
Abstract
The classical theory of single-electron dynamics in focused laser pulses is the foundation of both the relativistic ponderomotive force (RPF), which underlies models of laser-collective-plasma dynamics, and the discovery of novel strong-field radiation dynamics. Despite this bedrock importance, consensus eludes the community as [...] Read more.
The classical theory of single-electron dynamics in focused laser pulses is the foundation of both the relativistic ponderomotive force (RPF), which underlies models of laser-collective-plasma dynamics, and the discovery of novel strong-field radiation dynamics. Despite this bedrock importance, consensus eludes the community as to whether acceleration of single electrons in vacuum has been observed in experimental conditions. We analyze an early experiment on the RPF with respect to several features that were neglected in modeling and that can restore consistency between theory predictions and experimental data. The right or wrong pulse profile function, laser parameters, or initial electron distribution can each make or break the agreement between predictions and data. The laser phase at which the electron’s interaction with the pulse begins has a large effect, explaining why much larger energies are achieved by electrons liberated in the focal region by photoionization from high-Z atoms and by electrons ejected from a plasma mirror. Finally, we compute the difference in a typical electron spectrum arising from fluctuating focal spot size in state-of-the-art ultra-relativistic laser facilities. Our results emphasize the importance of thoroughly characterizing laser parameters in order to achieve quantitatively accurate predictions and the precision required for discovery science. Full article
(This article belongs to the Special Issue Progress in Laser Accelerator and Future Prospects)
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16 pages, 2857 KiB  
Review
Development and Application of THz Gyrotrons for Advanced Spectroscopic Methods
by Svilen Sabchevski and Mikhail Glyavin
Photonics 2023, 10(2), 189; https://doi.org/10.3390/photonics10020189 - 10 Feb 2023
Cited by 6 | Viewed by 1852
Abstract
Nowadays, gyrotrons are used in numerous and diverse fields of research and technology. Their most prominent application is to electron cyclotron resonance plasma heating and current drive-in reactors for controlled thermonuclear fusion. Another matured field is the thermal microwave treatment of materials in [...] Read more.
Nowadays, gyrotrons are used in numerous and diverse fields of research and technology. Their most prominent application is to electron cyclotron resonance plasma heating and current drive-in reactors for controlled thermonuclear fusion. Another matured field is the thermal microwave treatment of materials in industrial-grade gyrotron-based technological systems. The unique spectral properties of gyrotron radiation, frequency tunability, and the possibility for precise control and modulation of both the output power and frequency have made the gyrotrons attractive and appropriate radiation sources for various novel advanced spectroscopic techniques. Among them are ESR (electron spin resonance), NMR-DNP (nuclear magnetic resonance with a signal enhancement through dynamic nuclear polarization), XDMR (X-ray detected magnetic resonance), acoustic molecular spectroscopy, as well as high-precision spectroscopy for measuring the SFS (super-fine splitting of the energy levels of positronium). In this review paper, we present both the current status and the most remarkable recent achievements of these methods implemented in gyrotron-based spectroscopy systems and discuss the main trends in the development of their dedicated radiation sources operating in the THz frequency range. Full article
(This article belongs to the Special Issue Terahertz Spectroscopy and Imaging)
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19 pages, 13958 KiB  
Article
Observation of Giant Angular Goos-Hanchen Shifts Enhanced by Surface Plasmon Resonance in Subwavelength Grating
by Nikolai I. Petrov, Yuri M. Sokolov, Vladimir V. Stoiakin, Viktor A. Danilov, Vladimir V. Popov and Boris A. Usievich
Photonics 2023, 10(2), 180; https://doi.org/10.3390/photonics10020180 - 08 Feb 2023
Cited by 5 | Viewed by 1523
Abstract
The effect of the Goos-Hanchen (GH) angular shift for a visible light beam under the excitation of surface plasmon resonance (SPR) in a metal subwavelength grating has been investigated. Extremely large angular GH shifts have been demonstrated for a subwavelength grating with an [...] Read more.
The effect of the Goos-Hanchen (GH) angular shift for a visible light beam under the excitation of surface plasmon resonance (SPR) in a metal subwavelength grating has been investigated. Extremely large angular GH shifts have been demonstrated for a subwavelength grating with an optimal depth. The high sensitivity of the beam shape transformation and the GH shift to a change in the angle of incidence near the SPR has been shown by rigorous electromagnetic simulation and demonstrated experimentally. The focusing of the reflected beam near the subwavelength grating surface has been demonstrated. Full article
(This article belongs to the Special Issue Integrated Plasmonic Devices)
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14 pages, 6531 KiB  
Article
Distortion Detection of Lithographic Projection Lenses Based on Wavefront Measurement
by Tian Li, Jian Wang, Shaolin Zhou, Haiyang Quan, Lei Chen, Junbo Liu, Jing Du, Xianchang Zhu and Song Hu
Photonics 2023, 10(2), 168; https://doi.org/10.3390/photonics10020168 - 04 Feb 2023
Viewed by 2893
Abstract
As with the decreasing feature size prompted by Moore’s law and the continuous technological advancements in the semiconductor industry, the distortion of the projection lens is an important factor that affects the overlay. In this paper, we propose a wavefront-measurement-based method to detect [...] Read more.
As with the decreasing feature size prompted by Moore’s law and the continuous technological advancements in the semiconductor industry, the distortion of the projection lens is an important factor that affects the overlay. In this paper, we propose a wavefront-measurement-based method to detect the projection lens distortion in the lithographic system. By normalizing the coordinates of the Shack–Hartmann system with the image displacements represented in the Z2 and Z3 terms of Zernike coefficients, the offsets between the actual image points and the ideal image points can be determined. By offset collection at an array of 7 × 7 field points to establish an overdetermined system of equations, the proposed method can simultaneously detect the distortions of translation, magnification, rotation, decentering distortion, thin prism distortion, and third-order radial distortion. This distortion measurement method is highly flexible for distortion measurement with portable and compactly integrated sensors, enabling the real-time and cost-efficient measurement of wave aberration and distortion. For proof-of-concept experiments, a projection lens with a numerical aperture (NA) of 0.58 for i-line (365 nm) is used for experimental testing. The results reveal that the repeatability accuracy of distortion detection is 51 nm and the 72 h long-term reproducibility is 143 nm. Full article
(This article belongs to the Special Issue Optical Measurement Systems)
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17 pages, 4797 KiB  
Review
The Progress and Trend of Heterogeneous Integration Silicon/III-V Semiconductor Optical Amplifiers
by Wenqi Shi, Canwen Zou, Yulian Cao and Jianguo Liu
Photonics 2023, 10(2), 161; https://doi.org/10.3390/photonics10020161 - 03 Feb 2023
Cited by 5 | Viewed by 3852
Abstract
Silicon photonics is a revolutionary technology in the integrated photonics field which has experienced rapid development over the past several decades. High-quality III-V semiconductor components on Si platforms have shown their great potential to realize on-chip light-emitting sources for Si photonics with low-cost [...] Read more.
Silicon photonics is a revolutionary technology in the integrated photonics field which has experienced rapid development over the past several decades. High-quality III-V semiconductor components on Si platforms have shown their great potential to realize on-chip light-emitting sources for Si photonics with low-cost and high-density integration. In this review, we will focus on semiconductor optical amplifiers (SOAs), which have received considerable interest in diverse photonic applications. SOAs have demonstrated high performance in various on-chip optical applications through different integration technologies on Si substrates. Moreover, SOAs are also considered as promising candidates for future light sources in the wavelength tunable laser, which is one of the key suitable components in coherent optical devices. Understanding the development and trends of heterogeneous integration Silicon/III-V SOA will help researchers to come up with effective strategies to combat the emerging challenges in this family of devices, progressing towards next-generation applications. Full article
(This article belongs to the Special Issue Integrated Microwave Photonics)
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12 pages, 2010 KiB  
Article
Real-Time Chlorophyll-a Pigment Monitoring of Chlamydomonas reinhardtii in a Controlled Environment Using Pulsed LED Fluorescence LiDAR System
by Jumar G. Cadondon, Edgar A. Vallar, Tatsuo Shiina and Maria Cecilia D. Galvez
Photonics 2023, 10(2), 144; https://doi.org/10.3390/photonics10020144 - 30 Jan 2023
Cited by 1 | Viewed by 1306
Abstract
The real-time chlorophyll-a pigment monitoring of C. reinhardtii is studied using our developed LED fluorescence light detection and ranging (LiDAR) system. It features a portable set-up that uses a pulsed LED module with an excitation wavelength of 385 nm. We were able to [...] Read more.
The real-time chlorophyll-a pigment monitoring of C. reinhardtii is studied using our developed LED fluorescence light detection and ranging (LiDAR) system. It features a portable set-up that uses a pulsed LED module with an excitation wavelength of 385 nm. We were able to monitor the different growth phases of C. reinhardtii with specific cultivation parameters. The developed fluorescence LiDAR system showed the linear correlation of its chlorophyll-a signal with the optical density and EEM fluorescence measurements at 680 nm emission wavelength. Water quality and weather parameters were also measured, which explains the variation in the growth dynamics of C. reinhardtii during the sampling period. The results from the monitoring demonstrated a different technique that can be used in estimating algal biomass in the environment. Full article
(This article belongs to the Special Issue Micro Fluorescence Detectors/Sensors and Their Applications)
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8 pages, 12673 KiB  
Communication
Experimental Demonstration of Attoseconds-at-Harmonics at the SASE3 Undulator of the European XFEL
by Andrei Trebushinin, Gianluca Geloni, Svitozar Serkez, Giuseppe Mercurio, Natalia Gerasimova, Theophilos Maltezopoulos, Marc Guetg and Evgeny Schneidmiller
Photonics 2023, 10(2), 131; https://doi.org/10.3390/photonics10020131 - 27 Jan 2023
Cited by 2 | Viewed by 1896
Abstract
We report on observations of single spike spectra (3–13% of events) upon employing a previously proposed method for single spike generation via harmonic conversion. The method was tested at the soft X-ray SASE3 undulator of the European XFEL. The first part of the [...] Read more.
We report on observations of single spike spectra (3–13% of events) upon employing a previously proposed method for single spike generation via harmonic conversion. The method was tested at the soft X-ray SASE3 undulator of the European XFEL. The first part of the undulator allows one to amplify bunching at the fundamental as well as the higher harmonics. The downstream undulator is tuned to a harmonic, the fourth in our case, to amplify pulses with a shorter duration. We estimate the generated pulse duration within such a subset of short pulses at a level of 650 as. Considering the demonstrated probability of single spike events, this method is attractive for high repetition-rate free electron lasers. Full article
(This article belongs to the Special Issue Ultrafast Lasers: Science and Applications)
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8 pages, 1230 KiB  
Communication
Fiber Optic All-Polarization Weak Magnetic Field Sensor Based on Sagnac Interferometer
by Cui Liang, Zhihang Zhang, Dengwei Zhang, Tengchao Huang and Shuangliang Che
Photonics 2023, 10(2), 101; https://doi.org/10.3390/photonics10020101 - 17 Jan 2023
Cited by 2 | Viewed by 1730
Abstract
A novel fiber-optic magnetic field sensor, based on a Sagnac structure, is proposed with the approach of polarization interference detection. The sensor takes advantage of common path interference, combining with a high magnetic field sensitivity sensing unit, composed of magneto-optical crystal, and magnetic [...] Read more.
A novel fiber-optic magnetic field sensor, based on a Sagnac structure, is proposed with the approach of polarization interference detection. The sensor takes advantage of common path interference, combining with a high magnetic field sensitivity sensing unit, composed of magneto-optical crystal, and magnetic field concentrators, to achieve high resolution, high stability, and large dynamic measurement of DC magnetic field signals. In this paper, the theoretical model is established and the related theory is derived in detail. The key technologies in the system are thoroughly investigated and verified. Experimental research on the proposed system is demonstrated and the results show that a DC magnetic field resolution of 5.6 nT and a dynamic range of larger than 70 dB is achieved. Furthermore, the linearity of the system is greater than 99.8% and the instability is less than 0.5%. Full article
(This article belongs to the Special Issue Optical Measurement Systems and Instruments)
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17 pages, 6093 KiB  
Article
Enhanced Performance of a Cascaded Receiver Consisting of a DNN-Based Waveform-to-Symbol Converter and Modified NN-Based DD-LMS in CAP Underwater VLC System
by Xianhao Lin, Fangchen Hu and Nan Chi
Photonics 2023, 10(1), 79; https://doi.org/10.3390/photonics10010079 - 10 Jan 2023
Cited by 4 | Viewed by 2626
Abstract
Underwater visible light communication (UVLC) based on LEDs has become a competitive candidate, which is able to provide high data rates, low latency and low cost for next-generation wireless communication technologies. However, it is still challenging to achieve high-speed communication because of bottleneck [...] Read more.
Underwater visible light communication (UVLC) based on LEDs has become a competitive candidate, which is able to provide high data rates, low latency and low cost for next-generation wireless communication technologies. However, it is still challenging to achieve high-speed communication because of bottleneck problems such as bandwidth limitation and linear and nonlinear distortions. Traditional Deep-learning Neural Network (DNN)-based waveform-to-symbol converter is verified to be an effective method to alleviate them, but impractical due to high complexity. To achieve a better tradeoff between communication performance and computation complexity, a cascaded receiver consisting of a DNN-based waveform-to-symbol converter and modified Neural Network (NN)-based decision-directed least mean square (DD-LMS) is then innovatively proposed. With fewer taps and nodes than the traditional converter, the front-stage converter could mitigate the majority of Inter-Symbol Interference (ISI) and signal nonlinear distortions. Then modified NN-based DD-LMS is cascaded to improve communication performance by reducing phase offset, making received constellation points more concentrated and closer to standard constellation points. Compared with the traditional converter, the cascaded receiver could achieve 89.6% of signal Vpp dynamic range with 12.4% of complexity in the 64APSK UVLC system. Moreover, the ratio of signal Vpp dynamic range and total trainable parameters is 1.24 × 10−1 mV, while that of the traditional converter is 1.95 × 10−2 mV. The cascaded receiver used in 64APSK UVLC systems is experimentally verified to achieve enhanced performance, thus as a promising scheme for future high-speed underwater VLC. Full article
(This article belongs to the Special Issue Visible Light Communications)
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7 pages, 1924 KiB  
Communication
Intra-Cavity Raman Laser Operating at 1193 nm Based on Graded-Index Fiber
by Chunhua Hu and Ping Sun
Photonics 2023, 10(1), 33; https://doi.org/10.3390/photonics10010033 - 28 Dec 2022
Viewed by 1198
Abstract
Nonlinear Raman frequency conversion is an important technical scheme to obtain special optical band lasers based on conventional ion-doped lasers. In our work, we designed an intra-cavity Raman fiber laser based on graded index fiber (GRIF) as the Raman gain medium. Based on [...] Read more.
Nonlinear Raman frequency conversion is an important technical scheme to obtain special optical band lasers based on conventional ion-doped lasers. In our work, we designed an intra-cavity Raman fiber laser based on graded index fiber (GRIF) as the Raman gain medium. Based on the fundamental-frequency 1080-nanometer laser, efficient first-order and second-order Stokes Raman lasers were obtained, respectively. When the power of the fundamental-frequency 1080-nanometer laser was 33.4 W, the output power of the second-order 1193-nanometer laser was 11.39 W. The corresponding conversion efficiency was 34.1%. To our knowledge, this is the first report of a second-order Raman output based on a GRIF and intra-cavity structure. In the experiment, the spectrum-purification process with the increase in power was also observed. Our experimental results prove that the intracavity Raman-laser system based on graded index fiber with a high optical conversion efficiency has important application potential for obtaining new special-application bands. Full article
(This article belongs to the Special Issue High Power Fiber Laser and Amplifiers)
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18 pages, 4341 KiB  
Article
Identification of Browning in Human Adipocytes by Partial Least Squares Regression (PLSR), Infrared Spectral Biomarkers, and Partial Least Squares Discriminant Analysis (PLS-DA) Using FTIR Spectroscopy
by Dong-Hyun Shon, Se-Jun Park, Suk-Jun Yoon, Yang-Hwan Ryu and Yong Ko
Photonics 2023, 10(1), 2; https://doi.org/10.3390/photonics10010002 - 21 Dec 2022
Cited by 2 | Viewed by 1704
Abstract
We aimed to identify the browning of white adipocytes using partial least squares regression (PLSR), infrared spectral biomarkers, and partial least squares discriminant analysis (PLS-DA) with FTIR spectroscopy instead of molecular biology. PLSR helps distinguish human beige adipocytes treated with norepinephrine and rosiglitazone. [...] Read more.
We aimed to identify the browning of white adipocytes using partial least squares regression (PLSR), infrared spectral biomarkers, and partial least squares discriminant analysis (PLS-DA) with FTIR spectroscopy instead of molecular biology. PLSR helps distinguish human beige adipocytes treated with norepinephrine and rosiglitazone. When PLSR was based on the selected regions of 3997–3656 and 1618–938 cm−1, PLSR achieved an R2 of cross-validation of 88.95, a root mean square error of cross validation (RMSECV) of 2.13, and a ratio performance deviation (RPD) of 3.01. Infrared spectral biomarkers [1635 cm−1 (β-sheet amide I), 879–882, 860–3 cm−1 (A-form helix), and 629–38 cm−1 (OH out-of-plane bending)] were identified in human beige adipocytes based on spectral differences between human beige adipocytes and human white adipocytes, principal component analysis-linear discriminant analysis (PCA-LDA) cluster vector, U-test, and Fisher’s score per wavenumber. PLS-DA yielded a useful classification of adipocytes and expression distribution of adipogenesis genes in adipocytes. PLSR, infrared spectral biomarkers, and PLS-DA using FTIR spectroscopy are proposed as effective tools for identifying specific biological activities in a limited environment through features that do not require labeling and are relatively inexpensive in terms of time and labor. Full article
(This article belongs to the Special Issue Biomedical Spectroscopy: Techniques and Applications)
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26 pages, 4599 KiB  
Review
Line Field Optical Coherence Tomography
by Samuel Lawman, Zijian Zhang, Yao-Chun Shen and Yalin Zheng
Photonics 2022, 9(12), 946; https://doi.org/10.3390/photonics9120946 - 07 Dec 2022
Cited by 2 | Viewed by 2353
Abstract
The line field (LF) design choice for the lateral image formation mechanism (lateral format) has historically been a fraction of the whole optical coherence tomography (OCT) field. However, as the OCT technology develops, the parallelised acquisition of LF-OCT formats (LF-time domain (TD)-OCT, LF-spectral [...] Read more.
The line field (LF) design choice for the lateral image formation mechanism (lateral format) has historically been a fraction of the whole optical coherence tomography (OCT) field. However, as the OCT technology develops, the parallelised acquisition of LF-OCT formats (LF-time domain (TD)-OCT, LF-spectral domain (SD)-OCT, LF-swept source (SS)-OCT) offers benefits and capabilities, which may mean it is now becoming more mainstream. Prior reviews on OCT have focused on scanning point (SP) and, to a lesser extent, full field (FF), lateral formats, with, to our knowledge, no prior review specifically on the LF lateral format. Here, we address this gap in the literature by reviewing the history of each LF-OCT format, identifying the applications it has had and providing generic system design overviews. We then provide an analysis and discussion of the benefits and drawbacks of the format. Full article
(This article belongs to the Special Issue Recent Advances in Optical Coherence Tomography)
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16 pages, 2735 KiB  
Article
DeepGOMIMO: Deep Learning-Aided Generalized Optical MIMO with CSI-Free Detection
by Xin Zhong, Chen Chen, Shu Fu, Zhihong Zeng and Min Liu
Photonics 2022, 9(12), 940; https://doi.org/10.3390/photonics9120940 - 05 Dec 2022
Cited by 2 | Viewed by 1313
Abstract
Generalized optical multiple-input multiple-output (GOMIMO) techniques have been recently shown to be promising for high-speed optical wireless communication (OWC) systems. In this paper, we propose a novel deep learning-aided GOMIMO (DeepGOMIMO) framework for GOMIMO systems, wherein channel state information (CSI)-free detection can be [...] Read more.
Generalized optical multiple-input multiple-output (GOMIMO) techniques have been recently shown to be promising for high-speed optical wireless communication (OWC) systems. In this paper, we propose a novel deep learning-aided GOMIMO (DeepGOMIMO) framework for GOMIMO systems, wherein channel state information (CSI)-free detection can be enabled by employing a specially designed deep neural network (DNN)-based MIMO detector. The CSI-free DNN detector mainly consists of two modules: one is the preprocessing module, which is designed to address both the path loss and channel crosstalk issues caused by MIMO transmission, and the other is the feedforward DNN module, which is used for joint detection of spatial and constellation information by learning the statistics of both the input signal and the additive noise. Our simulation results clearly verify that, in a typical indoor 4 × 4 MIMO-OWC system using both generalized optical spatial modulation (GOSM) and generalized optical spatial multiplexing (GOSMP) with unipolar nonzero 4-level pulse-amplitude modulation (4-PAM) modulation, the proposed CSI-free DNN detector achieves near the same bit error rate (BER) performance as the optimal joint maximum-likelihood (ML) detector, but with much-reduced computational complexity. Moreover, because the CSI-free DNN detector does not require instantaneous channel estimation to obtain accurate CSI, it enjoys the unique advantages of improved achievable data rate and reduced communication time delay in comparison to the CSI-based zero-forcing DNN (ZF-DNN) detector. Full article
(This article belongs to the Special Issue Advances in Visible Light Communication)
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18 pages, 4110 KiB  
Article
Modelling and Design of Holographic Optical Elements for Beam-Coupling Applications for a Range of Incident Beam Angles
by Dipanjan Chakraborty, Rosen Georgiev, Sinead Aspell, Vincent Toal, Izabela Naydenova, Dervil Cody and Suzanne Martin
Photonics 2022, 9(12), 936; https://doi.org/10.3390/photonics9120936 - 03 Dec 2022
Cited by 4 | Viewed by 2129
Abstract
Theoretical modelling has been used to calculate the holographic recording beam angles required in air (at any recording wavelength) to produce a Volume Holographic Optical Element (VHOE) for any defined input and output beam angles. The approach is used to facilitate the design [...] Read more.
Theoretical modelling has been used to calculate the holographic recording beam angles required in air (at any recording wavelength) to produce a Volume Holographic Optical Element (VHOE) for any defined input and output beam angles. The approach is used to facilitate the design and fabrication of diffractive coupling elements through a holographic process that avoids the use of coupling prisms during recording and will help in the design of recording arrangements that better suit the mass production of low-cost elements, especially those designed for non-normal incidence. In this study, the recording angles needed for a range of recording wavelengths were explored for VHOE couplers designed for input angles (in air) ranging from 0° to −55°. Then, in order to validate the model, holographic recording in Bayfol HX 200 photopolymer at 532 nm was used to fabricate photopolymer VHOE couplers for 633 nm light (−45° input angle in air). Bragg curves obtained experimentally for different probe wavelengths (403 nm, 532 nm and 633 nm) confirm the recording of the desired grating structures to a precision of ±1°, and coupling is demonstrated at 633 nm with a diffraction efficiency of up to 72%. Furthermore, the model is used to identify the origins of some weaker spurious gratings observed alongside the expected ones. Full article
(This article belongs to the Special Issue Materials, Methods and Models for Holographic Optical Elements)
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7 pages, 9380 KiB  
Communication
A Deep Reinforcement Learning Algorithm for Smart Control of Hysteresis Phenomena in a Mode-Locked Fiber Laser
by Alexey Kokhanovskiy, Alexey Shevelev, Kirill Serebrennikov, Evgeny Kuprikov and Sergey Turitsyn
Photonics 2022, 9(12), 921; https://doi.org/10.3390/photonics9120921 - 30 Nov 2022
Cited by 6 | Viewed by 1508
Abstract
We experimentally demonstrate the application of a double deep Q-learning network algorithm (DDQN) for design of a self-starting fiber mode-locked laser. In contrast to the static optimization of a system design, the DDQN reinforcement algorithm is capable of learning the strategy of dynamic [...] Read more.
We experimentally demonstrate the application of a double deep Q-learning network algorithm (DDQN) for design of a self-starting fiber mode-locked laser. In contrast to the static optimization of a system design, the DDQN reinforcement algorithm is capable of learning the strategy of dynamic adjustment of the cavity parameters. Here, we apply the DDQN algorithm for stable soliton generation in a fiber laser cavity exploiting a nonlinear polarization evolution mechanism. The algorithm learns the hysteresis phenomena that manifest themselves as different pumping-power thresholds for mode-locked regimes for diverse trajectories of adjusting optical pumping. Full article
(This article belongs to the Special Issue Optical Fiber Lasers and Amplifiers. Practical Applications of Fiber-Based Laser Sources)
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18 pages, 5011 KiB  
Article
Nonlinear Impairment Compensation Using Transfer Learning-Assisted Convolutional Bidirectional Long Short-Term Memory Neural Network for Coherent Optical Communication Systems
by Xueyuan Luo, Chenglin Bai, Xinyu Chi, Hengying Xu, Yaxuan Fan, Lishan Yang, Peng Qin, Zhiguo Wang and Xiuhua Lv
Photonics 2022, 9(12), 919; https://doi.org/10.3390/photonics9120919 - 29 Nov 2022
Cited by 2 | Viewed by 1438
Abstract
By combining the nonlinear impairment features derived from the first-order perturbation theory, we propose a nonlinear impairment compensation (NLC) scheme based on the transfer learning-assisted convolutional bidirectional long short-term Memory (CNN-BiLSTM) neural network structure. When considering the correlation of [...] Read more.
By combining the nonlinear impairment features derived from the first-order perturbation theory, we propose a nonlinear impairment compensation (NLC) scheme based on the transfer learning-assisted convolutional bidirectional long short-term Memory (CNN-BiLSTM) neural network structure. When considering the correlation of nonlinear impairment between preceding and succeeding consecutive adjacent symbols on the current moment symbol and integrating the multidimensional feature extraction and time memory characteristics of CNN-BiLSTM, the nonlinear impairment information contained in the input feature can be fully utilized to accurately predict the nonlinear impairment showing significant compensation effect. Meanwhile, transfer learning (TL) is introduced to greatly reduce the complexity of the scheme on the basis of high compensation performance. To verify the effectiveness of the proposed scheme, we construct single-channel (SC) and 5-channel 28 GBaud polarization division multiplexing 16 quadrature amplitude modulation (PDM-16QAM)/85 GBaud PDM-64QAM simulation systems, and SC and 3-channel 28 GBaud PDM-16QAM experimental systems. The experimental results show that when compared with simple recurrent neural network (SRNN) NLC and DBP 20 steps per span (DBP20StPs), the Q-factor gain of our scheme is about 1 dB and 1.7 dB in the SC system, and about 1.1 dB and 1.5 dB in the 3-channel system at the optimal launch power, respectively. It is interesting to highlight that, by applying TL to the simulation and experimental systems, our scheme based on only 5% of the training samples can achieve compensation performance comparable to or higher quality than retraining at various launch powers. Full article
(This article belongs to the Special Issue Optical Fiber Communication Systems)
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17 pages, 5635 KiB  
Article
An Experimental and Numerical Study of the Impact of Ambient Light of SiPMs in VLC Receivers
by William Matthews and Steve Collins
Photonics 2022, 9(12), 888; https://doi.org/10.3390/photonics9120888 - 22 Nov 2022
Cited by 2 | Viewed by 1569
Abstract
Silicon photomultiplier’s relatively large area and ability to detect single photons makes them attractive as receivers for visible light communications. However, their non-linear response has a negative impact on the receiver performance, including making them particularly sensitive to ambient light. Experiments and Monte [...] Read more.
Silicon photomultiplier’s relatively large area and ability to detect single photons makes them attractive as receivers for visible light communications. However, their non-linear response has a negative impact on the receiver performance, including making them particularly sensitive to ambient light. Experiments and Monte Carlo simulations have been used to study this non-linearity. The resulting detailed understanding of the origins of the non-linear response leads to concerns over the accuracy of some previous simulations of SiPMs. In addition, it leads to simple methods to determine the maximum rate at which an SiPM can count photons and of determining the impact of a SiPMs non-linearity on its performance of a receiver. Finally, a method of determining which filters should be used to protect an SiPM from ambient light is proposed. Full article
(This article belongs to the Special Issue Advances in Visible Light Communication)
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8 pages, 3788 KiB  
Communication
Experimental Detection of Initial System–Environment Entanglement in Open Systems
by Gaoyan Zhu, Dengke Qu, Lei Xiao and Peng Xue
Photonics 2022, 9(11), 883; https://doi.org/10.3390/photonics9110883 - 21 Nov 2022
Cited by 1 | Viewed by 1574
Abstract
We experimentally investigate how initial entanglement between the system and environment can be detected in an open system by using some prior knowledge of the joint evolutions. The protocol we employed requires classical optimization on the results after performing measurements on the system [...] Read more.
We experimentally investigate how initial entanglement between the system and environment can be detected in an open system by using some prior knowledge of the joint evolutions. The protocol we employed requires classical optimization on the results after performing measurements on the system state. Such an approach does not require a full 2-qubit QST, and works in scenarios where one has access to the system only. We demonstrate the protocol on both pure entangled states and mixed entangled states. The obtained results show the experimental accessibility and validity of the protocol. Compared with the previous methods, which also assume access only to the system, this protocol is less demanding in terms of measurement and state preparation. The experimental results also show that, using the knowledge of the interaction, we can fine-tune the protocol, thus showing the potential of the protocol for developing experimentally feasible and practical entanglement detection methods. Full article
(This article belongs to the Section Quantum Photonics and Technologies)
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13 pages, 3112 KiB  
Article
First-Passage-Time Analysis of the Pulse-Timing Statistics in a Two-Section Semiconductor Laser under Excitable and Noisy Conditions
by Daan Lenstra, Lukas Puts and Weiming Yao
Photonics 2022, 9(11), 860; https://doi.org/10.3390/photonics9110860 - 14 Nov 2022
Cited by 1 | Viewed by 1323
Abstract
A two-section semiconductor laser can exhibit excitability for certain parameter settings. When used as a photonic spiking neuron, it is relevant to investigate its sensitivity to noise due to, e.g., spontaneous emission. Under excitable conditions, the system emits irregularly timed noise-triggered pulses. Their [...] Read more.
A two-section semiconductor laser can exhibit excitability for certain parameter settings. When used as a photonic spiking neuron, it is relevant to investigate its sensitivity to noise due to, e.g., spontaneous emission. Under excitable conditions, the system emits irregularly timed noise-triggered pulses. Their statistics is analyzed in terms of a first-passage time distribution for the fluctuating intensity to reach the threshold for excitable response. Two analytic approximations valid for short and long times, respectively, are derived which very well explain measured and simulated pulse-repetition time distributions. This provides physical insight into the noise-triggered spiking mechanism. Full article
(This article belongs to the Special Issue Advancements in Semiconductor Lasers)
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