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Photonics, Volume 8, Issue 5 (May 2021) – 37 articles

Cover Story (view full-size image): The new principle of Master–Slave OCT is tested in an interferometer driven by an electrically tuned swept source, whose emitted spectrum presents non-linearities owing to its tuning principle, which leads to some of the optical frequencies being generated more than once in a sweep. We demonstrate that the Master–Slave OCT technique can utilize the spectrum as it is, with no correction needed. This simplifies the overall procedure, bypassing any spectrum cleaning step otherwise needed before applying Fourier Transforms to the photo-detected signal to deliver A-scans. Further on, by using the down-conversion variant of the Master–Slave procedure, decoding of the channeled spectrum modulation at optical path differences exceeding 10 cm can be performed using a modest sampling rate digitizer. View this paper
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Communication
Investigation on Speckle-Free Imaging at the Output of a Multimode Fiber under Various Mode Excitation Conditions
Photonics 2021, 8(5), 171; https://0-doi-org.brum.beds.ac.uk/10.3390/photonics8050171 - 20 May 2021
Viewed by 307
Abstract
Speckle-free imaging using a multimode fiber has been widely used for imaging systems. Generally, previous work has assumed that all the propagating modes of the fiber are uniformly excited, but the modal power distribution is actually affected by excitation conditions. Here, we propose [...] Read more.
Speckle-free imaging using a multimode fiber has been widely used for imaging systems. Generally, previous work has assumed that all the propagating modes of the fiber are uniformly excited, but the modal power distribution is actually affected by excitation conditions. Here, we propose the utilization of a modal analysis method to study the dependence of the speckle contrast on the modal power distribution by changing the tilt angle of the Gaussian beam and on the group delay time difference caused by different fiber lengths. The results of numerical simulations and experiments show that, with an increase in the tilt angle of the Gaussian beam, the modal power is transferred to higher-order modes and the maximum delay difference between excitation modes becomes larger. Therefore, the inter-mode interference effect is effectively weakened, and the speckle contrast is significantly reduced. The increase in fiber length will also make the delay difference between excitation modes larger and thus the speckle contrast is decreased. For the larger tilt angle of the Gaussian beam, only a shorter optical fiber is required to reduce the speckle contrast significantly. Our work further promotes the use of a multimode fiber to produce speckle-free patterns in laser imaging systems. Full article
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Article
Thickness Measurement for Glass Slides Based on Chromatic Confocal Microscopy with Inclined Illumination
Photonics 2021, 8(5), 170; https://0-doi-org.brum.beds.ac.uk/10.3390/photonics8050170 - 20 May 2021
Viewed by 485
Abstract
Chromatic confocal microscopy is a widely used method to measure the thickness of transparent specimens. In conventional configurations, both the illumination and imaging axes are perpendicular to the test specimen. The reflection will be very weak when measuring high-transparency specimens. In order to [...] Read more.
Chromatic confocal microscopy is a widely used method to measure the thickness of transparent specimens. In conventional configurations, both the illumination and imaging axes are perpendicular to the test specimen. The reflection will be very weak when measuring high-transparency specimens. In order to overcome this limitation, a special chromatic confocal measuring system was developed based on inclined illumination. This design was able to significantly improve the signal-to-noise ratio. Compared with conventional designs, the proposed system was also featured by its biaxial optical scheme, instead of a coaxial one. This biaxial design improved the flexibility of the system and also increased the energy efficiency by avoiding light beam splitting. Based on this design, a prototype was built by the authors’ team. In this paper, the theoretical model of this specially designed chromatic confocal system is analyzed, and the calculating formula for the thickness of transparent specimen is provided accordingly. In order to verify its measurement performance, two experimental methodology and results are presented. The experimental results show that the repeatability is better than 0.54 μm, and the axial measurement accuracy of the system could reach the micron level. Full article
(This article belongs to the Special Issue Glass Optics)
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Article
Optimized Omnidirectional High-Reflectance Using Octonacci Photonic Crystal for Thermographic Sensing Applications
Photonics 2021, 8(5), 169; https://0-doi-org.brum.beds.ac.uk/10.3390/photonics8050169 - 19 May 2021
Viewed by 433
Abstract
The transmittance of waves through one-dimensional periodic and Octonacci photonic structures was studied using the theoretical transfer matrix method for both wave-polarization-modes. The first structures were made up of the SiO2 and TiO2 materials. The objective here was to obtain a [...] Read more.
The transmittance of waves through one-dimensional periodic and Octonacci photonic structures was studied using the theoretical transfer matrix method for both wave-polarization-modes. The first structures were made up of the SiO2 and TiO2 materials. The objective here was to obtain a broad omnidirectional high reflector covering the infrared spectrum of a thermographic camera [1–14 µm] and, especially, to prevent the transmission of emitted human body peak radiation λmax = 9.341 µm. By comparing the periodic and Octonacci structures, we found that the last structure presented a main and wide photonic band gap near this human radiation. For that, we kept only the Octonacci structure for the rest of the study. The first structure did not give the aspired objective; thus, we replaced the TiO2 layers with yttrium barium copper oxide material, and a significant enhancement of the omnidirectional photonic band gap was found for both TE and TM polarization modes. It was shown that the width of this band was sensitive to the Octonacci iteration number and the optical thickness (by changing the reference wavelength), but it was not affected by the ambient temperature. The number of layers and the thickness of the structure was optimized while improving the omnidirectional high reflector properties. Full article
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Article
Deep Learning-Assisted Index Estimator for Generalized LED Index Modulation OFDM in Visible Light Communication
Photonics 2021, 8(5), 168; https://0-doi-org.brum.beds.ac.uk/10.3390/photonics8050168 - 19 May 2021
Cited by 2 | Viewed by 471
Abstract
In this letter, we present the first attempt of active light-emitting diode (LED) indexes estimating for the generalized LED index modulation optical orthogonal frequency-division multiplexing (GLIM-OFDM) in visible light communication (VLC) system by using deep learning (DL). Instead of directly estimating the transmitted [...] Read more.
In this letter, we present the first attempt of active light-emitting diode (LED) indexes estimating for the generalized LED index modulation optical orthogonal frequency-division multiplexing (GLIM-OFDM) in visible light communication (VLC) system by using deep learning (DL). Instead of directly estimating the transmitted binary bit sequence with DL, the active LEDs at the transmitter are estimated to maintain acceptable complexity and improve the performance gain compared with those of previously proposed receivers. Particularly, a novel DL-based estimator termed index estimator-based deep neural network (IE-DNN) is proposed, which can employ three different DNN structures with fully connected layers (FCL) or convolution layers (CL) to recover the indexes of active LEDs in a GLIM-OFDM system. By using the received signal dataset generated in simulations, the IE-DNN is first trained offline to minimize the index error rate (IER); subsequently, the trained model is deployed for the active LED index estimation and signal demodulation of the GLIM-OFDM system. The simulation results show that the IE-DNN significantly improves the IER and bit error rate (BER) compared with those of conventional detectors with acceptable run time. Full article
(This article belongs to the Special Issue Visible Light Communication (VLC))
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Article
Extended L-Band InAs/InP Quantum-Dash Laser in Millimeter-Wave Applications
Photonics 2021, 8(5), 167; https://0-doi-org.brum.beds.ac.uk/10.3390/photonics8050167 - 19 May 2021
Viewed by 436
Abstract
We report on the generation and transmission of a millimeter-wave (MMW) signal with a frequency of 28 GHz by employing an InAs/InP quantum-dash dual-wavelength laser diode (QD-DWL) emitting in the ~1610 nm extended L-band window. The self-injection locking (SIL) technique has been engaged [...] Read more.
We report on the generation and transmission of a millimeter-wave (MMW) signal with a frequency of 28 GHz by employing an InAs/InP quantum-dash dual-wavelength laser diode (QD-DWL) emitting in the ~1610 nm extended L-band window. The self-injection locking (SIL) technique has been engaged to improve the linewidth and reduce the noise of the optical tone. Besides, the transmission of a 2 Gbits/s quadrature phase-shift keying (QPSK)-modulated 28-GHz MMW beat tone over a hybrid 20-km radio-over-fiber combined with 5-m radio-over-free-space-optics and up to 6-m radio frequency wireless link has been demonstrated. Moreover, comparing the proposed QD-DWL with a commercial laser showcased similar performance characteristics, making the QD-DWL a candidate source for MMW applications. Full article
(This article belongs to the Special Issue Optical Network and Access Technologies)
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Article
Cobalt and Carbon Complex as Counter Electrodes in Dye-Sensitized Solar Cells
Photonics 2021, 8(5), 166; https://0-doi-org.brum.beds.ac.uk/10.3390/photonics8050166 - 19 May 2021
Viewed by 339
Abstract
We developed cobalt and carbon complex materials as counter electrodes (CEs) for dye-sensitized solar cells (DSSCs) to replace conventional platinum (Pt) CEs. Co12 and Co15, both of which are basic cobalt derivatives, showed good redox potential with a suitable open-circuit voltage (VOC [...] Read more.
We developed cobalt and carbon complex materials as counter electrodes (CEs) for dye-sensitized solar cells (DSSCs) to replace conventional platinum (Pt) CEs. Co12 and Co15, both of which are basic cobalt derivatives, showed good redox potential with a suitable open-circuit voltage (VOC); however, their poor electrical conductivity engendered a low short-circuit current (JSC) and fill factor (FF). Mixing them with carbon black (CB) improved the electrical conductivity of the CE; in particular, JSC and FF were considerably improved. Further improvement was achieved by combining cobalt derivatives and CB through thermal sintering to produce a novel CoCB material as a CE. CoCB had good electrical conductivity and electrocatalytic capability, and this further enhanced both JSC and VOC. The optimized device exhibited a power conversion efficiency (PCE) of 7.44%, which was higher than the value of 7.16% for a device with a conventional Pt CE. The conductivity of CoCB could be further increased by mixing it with PEDOT:PSS, a conducting polymer. The device’s JSC increased to 18.65 mA/cm2, which was considerably higher than the value of 14.24 mA/cm2 for the device with Pt CEs. The results demonstrate the potential of the cobalt and carbon complex as a CE for DSSCs. Full article
(This article belongs to the Special Issue Optoelectronic Materials and Their Applications)
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Article
Adaptive Gradient Estimation Stochastic Parallel Gradient Descent Algorithm for Laser Beam Cleanup
Photonics 2021, 8(5), 165; https://0-doi-org.brum.beds.ac.uk/10.3390/photonics8050165 - 19 May 2021
Viewed by 395
Abstract
For a high-power slab solid-state laser, obtaining high output power and high output beam quality are the most important indicators. Adaptive optics systems can significantly improve beam qualities by compensating for the phase distortions of the laser beams. In this paper, we developed [...] Read more.
For a high-power slab solid-state laser, obtaining high output power and high output beam quality are the most important indicators. Adaptive optics systems can significantly improve beam qualities by compensating for the phase distortions of the laser beams. In this paper, we developed an improved algorithm called Adaptive Gradient Estimation Stochastic Parallel Gradient Descent (AGESPGD) algorithm for beam cleanup of a solid-state laser. A second-order gradient of the search point was introduced to modify the gradient estimation, and it was introduced with the adaptive gain coefficient method into the classical Stochastic Parallel Gradient Descent (SPGD) algorithm. The improved algorithm accelerates the search for convergence and prevents it from falling into a local extremum. Simulation and experimental results show that this method reduces the number of iterations by 40%, and the algorithm stability is also improved compared with the original SPGD method. Full article
(This article belongs to the Special Issue Semiconductor Lasers)
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Communication
All-Metal Terahertz Metamaterial Absorber and Refractive Index Sensing Performance
Photonics 2021, 8(5), 164; https://0-doi-org.brum.beds.ac.uk/10.3390/photonics8050164 - 14 May 2021
Viewed by 449
Abstract
This paper presents a terahertz (THz) metamaterial absorber made of stainless steel. We found that the absorption rate of electromagnetic waves reached 99.95% at 1.563 THz. Later, we analyzed the effect of structural parameter changes on absorption. Finally, we explored the application of [...] Read more.
This paper presents a terahertz (THz) metamaterial absorber made of stainless steel. We found that the absorption rate of electromagnetic waves reached 99.95% at 1.563 THz. Later, we analyzed the effect of structural parameter changes on absorption. Finally, we explored the application of the absorber in refractive index sensing. We numerically demonstrated that when the refractive index (n) is changing from 1 to 1.05, our absorber can yield a sensitivity of 74.18 μm/refractive index unit (RIU), and the quality factor (Q-factor) of this sensor is 36.35. Compared with metal–dielectric–metal sandwiched structure, the absorber designed in this paper is made of stainless steel materials with no sandwiched structure, which greatly simplifies the manufacturing process and reduces costs. Full article
(This article belongs to the Special Issue Advanced Metamaterials and Metadevices)
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Article
Design of a Free Space Optical Communication System for an Unmanned Aerial Vehicle Command and Control Link
Photonics 2021, 8(5), 163; https://0-doi-org.brum.beds.ac.uk/10.3390/photonics8050163 - 14 May 2021
Viewed by 551
Abstract
An electromagnetic immune Free Space Optical Communication (FSOC) system for an Unmanned Aerial Vehicle (UAV) command and control link is introduced in this paper. The system uses the scheme of omnidirectional receiving and ground scanning transmitting. It has a strong anti-turbulence ability by [...] Read more.
An electromagnetic immune Free Space Optical Communication (FSOC) system for an Unmanned Aerial Vehicle (UAV) command and control link is introduced in this paper. The system uses the scheme of omnidirectional receiving and ground scanning transmitting. It has a strong anti-turbulence ability by using a large area detector and short-focus lens. The design of omnidirectional communication improves the ability of anti-vibration and link establishment. Pure static reception has no momentum effect on the platform. The receiver is miniaturized under no use of a gimbal mirror system, beacon camera system, Four-Quadrant Photodetector (QPD) and multi-level lens system. The system can realize omnidirectional reception and the communication probability in 1 s is greater than 99.99%. This design strengthens the ability of the FSOC system, so it can be applied in the UAV command and control, the satellite submarine communication and other occasions where the size of the platform is restricted. Full article
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Review
Spatial-Frequency Domain Imaging: An Emerging Depth-Varying and Wide-Field Technique for Optical Property Measurement of Biological Tissues
Photonics 2021, 8(5), 162; https://0-doi-org.brum.beds.ac.uk/10.3390/photonics8050162 - 12 May 2021
Viewed by 430
Abstract
Measurement of optical properties is critical for understanding light-tissue interaction, properly interpreting measurement data, and gaining better knowledge of tissue physicochemical properties. However, conventional optical measuring techniques are limited in point measurement, which partly hinders the applications on characterizing spatial distribution and inhomogeneity [...] Read more.
Measurement of optical properties is critical for understanding light-tissue interaction, properly interpreting measurement data, and gaining better knowledge of tissue physicochemical properties. However, conventional optical measuring techniques are limited in point measurement, which partly hinders the applications on characterizing spatial distribution and inhomogeneity of optical properties of biological tissues. Spatial-frequency domain imaging (SFDI), as an emerging non-contact, depth-varying and wide-field optical imaging technique, is capable of measuring the optical properties in a wide field-of-view on a pixel-by-pixel basis. This review first describes the typical SFDI system and the principle for estimating optical properties using the SFDI technique. Then, the applications of SFDI in the fields of biomedicine, as well as food and agriculture, are reviewed, including burn assessment, skin tissue evaluation, tumor tissue detection, brain tissue monitoring, and quality evaluation of agro-products. Finally, a discussion on the challenges and future perspectives of SFDI for optical property estimation is presented. Full article
(This article belongs to the Special Issue Tissue Optics)
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Communication
Dispersion Optimization of Silicon Nitride Waveguides for Efficient Four-Wave Mixing
Photonics 2021, 8(5), 161; https://0-doi-org.brum.beds.ac.uk/10.3390/photonics8050161 - 11 May 2021
Viewed by 489
Abstract
Silicon nitride waveguides have emerged as an excellent platform for photonic applications, including nonlinear optical signal processing, owing to their relatively high Kerr nonlinearity, negligible two photon absorption, and wide transparent bandwidth. In this paper, we propose an effective approach using 3D finite [...] Read more.
Silicon nitride waveguides have emerged as an excellent platform for photonic applications, including nonlinear optical signal processing, owing to their relatively high Kerr nonlinearity, negligible two photon absorption, and wide transparent bandwidth. In this paper, we propose an effective approach using 3D finite element method to optimize the dispersion characteristics of silicon nitride waveguides for four-wave mixing (FWM) applications. Numerical studies show that a flat and low dispersion profile can be achieved in a silicon nitride waveguide with the optimized dimensions. Near-zero dispersion of 1.16 ps/km/nm and 0.97 ps/km/nm at a wavelength of 1550 nm are obtained for plasma-enhanced chemical vapor deposition (PECVD) and low-pressure chemical vapor deposition (LPCVD) silicon nitride waveguides, respectively. The fabricated micro-ring resonator with the optimized dimensions exhibits near-zero dispersion of −0.04 to −0.1 ps/m/nm over a wavelength range of 130 nm which agrees with the numerical simulation results. FWM results show that near-zero phase mismatch and high conversion efficiencies larger than −12 dB using a low pump power of 0.5 W in a 13-cm long silicon nitride waveguide are achieved. Full article
(This article belongs to the Special Issue Photonics for Emerging Applications in Communication and Sensing)
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Review
Precise Optical Modulation and Its Application to Optoelectronic Device Measurement
Photonics 2021, 8(5), 160; https://0-doi-org.brum.beds.ac.uk/10.3390/photonics8050160 - 11 May 2021
Viewed by 478
Abstract
Optoelectronic devices which play important roles in high-speed optical fiber networks can offer effective measurement methods for optoelectronic devices including optical modulators and photodetectors. Precise optical signal modulation is required for measurement applications. This paper focuses on high-speed and precise optical modulation devices [...] Read more.
Optoelectronic devices which play important roles in high-speed optical fiber networks can offer effective measurement methods for optoelectronic devices including optical modulators and photodetectors. Precise optical signal modulation is required for measurement applications. This paper focuses on high-speed and precise optical modulation devices and their application to device measurement. Optical modulators using electro-optic effect offers precise control of lightwaves for wideband signals. As examples, this paper describes frequency response measurement of photodetectors using high-precision amplitude modulation and wavelength domain measurement of optical filters using fast optical frequency sweep. Precise and high-speed modulation can be achieved by active trimming which compensates device structure imbalance due to fabrication error, where preciseness can be described by on-off extinction ratio. A Mach-Zehnder modulator with sub Mach-Zehnder interferometors can offer high extinction-ratio optical intensity modulation, which can be used for precise optoelectronic frequency response measurement. Precise modulation would be also useful for multi-level modulation schemes. To investigate impact of finite extinction ratio on optical modulation, duobinary modulation with small signal operation was demonstrated. For optical frequency domain analysis, single sideband modulation, which shifts optical frequency, can be used for generation of stimulus signals. Rapid measurement of optical filters was performed by using an optical sweeper consisting of an integrated Mach-Zehnder modulator for optical frequency control and an arbitrary waveform generator for generation of a source frequency chirp signal. Full article
(This article belongs to the Special Issue Advanced Ultra High Speed Optoelectronic Devices)
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Article
A QoS-Aware Dynamic Bandwidth Allocation Algorithm for Passive Optical Networks with Non-Zero Laser Tuning Time
Photonics 2021, 8(5), 159; https://0-doi-org.brum.beds.ac.uk/10.3390/photonics8050159 - 10 May 2021
Viewed by 419
Abstract
The deployment of new 5G services and future demands for 6G make it necessary to increase the performance of access networks. This challenge has prompted the development of new standardization proposals for Passive Optical access Networks (PONs) that offer greater bandwidth, greater reach [...] Read more.
The deployment of new 5G services and future demands for 6G make it necessary to increase the performance of access networks. This challenge has prompted the development of new standardization proposals for Passive Optical access Networks (PONs) that offer greater bandwidth, greater reach and a higher rate of aggregation of users per fiber, being Time- and Wavelength-Division Multiplexing (TWDM) a promising technological solution for increasing the capacity by up to 40 Gbps by using several wavelengths. This solution introduces tunable transceivers into the Optical Network Units (ONUs) for switching from one wavelength to the other, thus addressing the ever-increasing bandwidth demands in residential broadband and mobile fronthaul networks based on Fiber to the Home (FTTH) technology. This adds complexity and sources of inefficiency, such as the laser tuning time (LTT) delay, which is often ignored when evaluating the performance of Dynamic Bandwidth Allocation (DBA) mechanisms. We present a novel DBA algorithm that dynamically handles the allocation of bandwidth and switches the ONUs’ lasers from one wavelength to the other while taking LTT into consideration. To optimize the packet delay, we introduce a scheduling mechanism that follows the Longest Processing Time first (LPT) scheduling discipline, which is implemented over the Interleaved Polling with Adaptive Cycle Time (IPACT) DBA. We also provide quality of service (QoS) differentiation by introducing the Max-Min Weighted Fair Share Queuing principle (WFQ) into the algorithm. The performance of our algorithm is evaluated through simulations against the original IPACT algorithm, which we have extended to support multi-wavelengths. With the introduction of LPT, we obtain an improved performance of up to 73% reduction in queue delay over IPACT while achieving QoS differentiation with WFQ. Full article
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Article
Characteristics of Nonstatic Quantum Light Waves: The Principle for Wave Expansion and Collapse
Photonics 2021, 8(5), 158; https://0-doi-org.brum.beds.ac.uk/10.3390/photonics8050158 - 10 May 2021
Viewed by 476
Abstract
Nonstatic quantum light waves arise in time-varying media in general. However, from a recent report, it turned out that nonstatic waves can also appear in a static environment where the electromagnetic parameters of the medium do not vary in time. Such waves in [...] Read more.
Nonstatic quantum light waves arise in time-varying media in general. However, from a recent report, it turned out that nonstatic waves can also appear in a static environment where the electromagnetic parameters of the medium do not vary in time. Such waves in Fock states exhibit a belly and a node in turn periodically in the graphic of their evolution. This is due to the wave expansion and collapse in quadrature space, which manifest a unique nonstaticity of the wave. The principle for wave expansion and collapse is elucidated from rigorous analyses for the basic nonstatic waves which are dissipative and amplifying ones. The outcome of wave nonstaticity can be interpreted in terms of the coefficient of the quadratic exponent in the exponential function appearing in the wave eigenfunction; if the imaginary part of the coefficient is positive, the wave expands, whereas the wave collapses when it is negative. Using this principle, we further analyze novel nonstatic properties of light waves which exhibit complicated time behaviors, i.e., for the case that the waves not only undergo the periodical change of nodes and bellies but their envelopes exhibit gradual dissipation/expansion as well. Full article
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Article
Efficient Carrier Recombination in InGaN Pyramidal µ-LEDs Obtained through Selective Area Growth
Photonics 2021, 8(5), 157; https://0-doi-org.brum.beds.ac.uk/10.3390/photonics8050157 - 10 May 2021
Viewed by 609
Abstract
Pyramid-shaped InGaN/GaN micro-light-emitting diodes (μ-LEDs) were grown on a sapphire substrate using the selective area growth technique. A stable emission wavelength of a single μ-LED pyramid at 412 nm was observed under an injection current from 0.05 to 20 mA, [...] Read more.
Pyramid-shaped InGaN/GaN micro-light-emitting diodes (μ-LEDs) were grown on a sapphire substrate using the selective area growth technique. A stable emission wavelength of a single μ-LED pyramid at 412 nm was observed under an injection current from 0.05 to 20 mA, despite the non-uniformity of the thickness and composition of the multiple quantum wells (MQWs) on the sidewall. An efficient carrier confinement and, thus, a high luminescence intensity were demonstrated in the middle of the sidewall through spatial-resolved cathodoluminescence (CL) characterization and were predicted by theoretical simulations. An ultra-high output power density of 1.37 kW/cm2 was obtained from the single μ-LED pyramid, illustrating its great potential for application in high-brightness micro-displays and in virtual reality and augmented reality (VR and AR) applications. Full article
(This article belongs to the Special Issue Wide Bandgap Semiconductor Photonic Devices)
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Article
Generalized Resonance Sensor Based on Fiber Bragg Grating
Photonics 2021, 8(5), 156; https://0-doi-org.brum.beds.ac.uk/10.3390/photonics8050156 - 06 May 2021
Cited by 1 | Viewed by 548
Abstract
In response to the difficulty of weak detection of early bearing damage, resonance demodulation technology and the principle of fiber Bragg grating sensing strain were combined to design a fiber Bragg grating generalized resonance sensor, which can extract the weak pulse signal of [...] Read more.
In response to the difficulty of weak detection of early bearing damage, resonance demodulation technology and the principle of fiber Bragg grating sensing strain were combined to design a fiber Bragg grating generalized resonance sensor, which can extract the weak pulse signal of weak detection of early bearing’s early damage from rolling bearing. First, a principle of resonance dynamics of second-order mechanical systems based on fiber Bragg grating and generalized resonance principles is proposed. Second, the basic structure of the sensor is designed. Then, ANSYS finite element simulation is used to analyze the natural frequency of the sensor. Finally, the natural frequency value of the sensor was obtained through experiments. The experimental results of proof-of-principle show that the experimental results are consistent with the theoretical predictions. The theoretical model is accurate, which verifies the feasibility of the sensor. Full article
(This article belongs to the Special Issue Optical Sensing)
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Article
Graphene-Coated Highly Sensitive Photonic Crystal Fiber Surface Plasmon Resonance Sensor for Aqueous Solution: Design and Numerical Analysis
Photonics 2021, 8(5), 155; https://0-doi-org.brum.beds.ac.uk/10.3390/photonics8050155 - 06 May 2021
Viewed by 638
Abstract
This paper presents the design and analysis of a surface plasmon resonance (SPR) sensor in a photonic crystal fiber (PCF) platform, where graphene is used externally to attain improved sensing performance for an aqueous solution. The performance of the proposed sensor was analyzed [...] Read more.
This paper presents the design and analysis of a surface plasmon resonance (SPR) sensor in a photonic crystal fiber (PCF) platform, where graphene is used externally to attain improved sensing performance for an aqueous solution. The performance of the proposed sensor was analyzed using the finite element method-based simulation tool COMSOL Multiphysics. According to the simulation results, the proposed sensor exhibits identical linear characteristics as well as a very high figure of merit (FOM) of 2310.11 RIU−1 in the very low detection limit of 10−3. The analysis also reveals the maximum amplitude sensitivity of 14,847.03 RIU−1 and 7351.82 RIU−1 for the x and y polarized modes, respectively, which are high compared to several previously reported configurations. In addition, the average wavelength sensitivity is 2000 nm/RIU which is comparatively high for the analyte refractive index (RI) ranging from 1.331 to 1.339. Hence, it is highly expected that the proposed PCF-based SPR sensor can be a suitable candidate in different sensing applications, especially for aqueous solutions. Full article
(This article belongs to the Section Lasers, Light Sources and Sensors)
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Article
Electron Spill-Out Effect in Singular Metasurfaces
Photonics 2021, 8(5), 154; https://0-doi-org.brum.beds.ac.uk/10.3390/photonics8050154 - 05 May 2021
Viewed by 444
Abstract
The electron spill-out effect is considered in a singular metasurface. Using the hydrodynamic model, we found that electron spill-out effectively smears the sharp singularity. The introduction of the electron spill-out effect also significantly changes the reflection spectrum, charge distribution, field profile for a [...] Read more.
The electron spill-out effect is considered in a singular metasurface. Using the hydrodynamic model, we found that electron spill-out effectively smears the sharp singularity. The introduction of the electron spill-out effect also significantly changes the reflection spectrum, charge distribution, field profile for a singular metasurface. Therefore, this spill-out contribution is crucial and cannot be ignored for a realistic description of optical response in a singular system. Full article
(This article belongs to the Special Issue Plasmonic Metasurfaces)
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Article
Modulation Index and Phase Imbalance of Dual-Sideband Optical Carrier Suppression (DSB-OCS) in Optical Millimeter-Wave System
Photonics 2021, 8(5), 153; https://0-doi-org.brum.beds.ac.uk/10.3390/photonics8050153 - 04 May 2021
Viewed by 476
Abstract
This paper presents a Dual-sideband Optical Carrier Suppression (DSB-OCS) technique which is used to generate an optical millimeter-wave (mm-wave) signal in radio over fiber (RoF) systems. The proposed system employs a Dual-Electrode Mach-Zehnder Modulator (DE-MZM) and a carrier of 40 GHz mm-wave for [...] Read more.
This paper presents a Dual-sideband Optical Carrier Suppression (DSB-OCS) technique which is used to generate an optical millimeter-wave (mm-wave) signal in radio over fiber (RoF) systems. The proposed system employs a Dual-Electrode Mach-Zehnder Modulator (DE-MZM) and a carrier of 40 GHz mm-wave for data transmission through the RoF systems. Characteristics determining the performance of the system, among which are the modulation index, phase imbalance and dispersion parameters, are included. The performance evaluations of the system show that the mm-wave signal output power follows MZM’s transfer function when the modulation index is raised. Moreover, the generated optical mm-wave signal power is affected by phase imbalance and optical splitting ratio. It is observed that the optical fiber dispersion influences the DSB-OCS system by decreasing the amplitude of the mm-wave and the signal-to-noise ratio (SNR). Full article
(This article belongs to the Special Issue Radio over Fiber)
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Communication
Direct Writing of Silicon Oxide Nanopatterns Using Photonic Nanojets
Photonics 2021, 8(5), 152; https://0-doi-org.brum.beds.ac.uk/10.3390/photonics8050152 - 03 May 2021
Viewed by 611
Abstract
The ability to create controllable patterns of micro- and nanostructures on the surface of bulk silicon has widespread application potential. In particular, the direct writing of silicon oxide patterns on silicon via femtosecond laser-induced silicon amorphization has attracted considerable attention owing to its [...] Read more.
The ability to create controllable patterns of micro- and nanostructures on the surface of bulk silicon has widespread application potential. In particular, the direct writing of silicon oxide patterns on silicon via femtosecond laser-induced silicon amorphization has attracted considerable attention owing to its simplicity and high efficiency. However, the direct writing of nanoscale resolution is challenging due to the optical diffraction effect. In this study, we propose a highly efficient, one-step method for preparing silicon oxide nanopatterns on silicon. The proposed method combines femtosecond laser-induced silicon amorphization with a subwavelength-scale beam waist of photonic nanojets. We demonstrate the direct writing of arbitrary nanopatterns via contactless scanning, achieving patterns with a minimum feature size of 310 nm and a height of 120 nm. The proposed method shows potential for the fabrication of multifunctional surfaces, silicon-based chips, and silicon photonics. Full article
(This article belongs to the Special Issue Photonic Jet: Science and Application)
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Communication
Label-Free Observation of Micrometric Inhomogeneity of Human Breast Cancer Cell Density Using Terahertz Near-Field Microscopy
Photonics 2021, 8(5), 151; https://0-doi-org.brum.beds.ac.uk/10.3390/photonics8050151 - 01 May 2021
Viewed by 666
Abstract
Terahertz-light imaging is attracting great attention as a new approach in non-invasive/non-staining biopsy of cancerous tissues. Positively, terahertz light has been shown to be sensitive to the cell density, the hydration content, and the chemical composition of biological samples. However, the spatial resolution [...] Read more.
Terahertz-light imaging is attracting great attention as a new approach in non-invasive/non-staining biopsy of cancerous tissues. Positively, terahertz light has been shown to be sensitive to the cell density, the hydration content, and the chemical composition of biological samples. However, the spatial resolution of terahertz imaging is typically limited to several millimeters, making it difficult to apply the technology to image biological tissues which have sub-terahertz-wavelength-scale inhomogeneity. For overcoming the resolution, we have recently developed a terahertz near-field microscope with a spatial resolution of 10 µm, named scanning point terahertz source (SPoTS) microscope. In contrast to conventional far-field terahertz techniques, this microscope features the near-field interactions between samples and point terahertz sources on a sub-terahertz-wavelength scale. Herein, to evaluate the usefulness of terahertz imaging in cancer tissue biopsy in greater detail, we performed terahertz near-field imaging of a paraffin-embedded human-breast-cancer section having sub-terahertz-wavelength-scale inhomogeneity of the cancer cell density using the SPoTS microscope. The observed terahertz images successfully visualized local (~250 µm) inhomogeneities of the cell density in breast invasive ductal carcinoma. These results may bypass the terahertz limitation in terms of spatial resolution and may further motivate the application of terahertz light to cancer tissue biopsy. Full article
(This article belongs to the Section Biophotonics and Biomedical Optics)
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Article
Theoretical Study of Tunable Optical Resonators in Periodic and Quasiperiodic One-Dimensional Photonic Structures Incorporating a Nematic Liquid Crystal
Photonics 2021, 8(5), 150; https://0-doi-org.brum.beds.ac.uk/10.3390/photonics8050150 - 01 May 2021
Viewed by 498
Abstract
In this work, the transfer matrix method (TMM) is employed to investigate the optical properties of one-dimensional periodic and quasiperiodic photonic crystals containing nematic liquid crystal (NLC) layers. This structure is expressed as (ABC)J(CBA)J and made of alternated layers of isotropic dielectrics SiO [...] Read more.
In this work, the transfer matrix method (TMM) is employed to investigate the optical properties of one-dimensional periodic and quasiperiodic photonic crystals containing nematic liquid crystal (NLC) layers. This structure is expressed as (ABC)J(CBA)J and made of alternated layers of isotropic dielectrics SiO2 (A), BGO (B) and nematic liquid crystal (C). The simulation study shows that the proposed ternary configuration exhibits tunable defect mode within the photonic band gap (PBG) that can be manipulated by adjusting the thicknesses of NLC layers in order of the periodic lattice. In addition, the optimized structure permits for strong confinement light giving rise to an optical microcavity. The application of an applied voltage into NLC layers enables improving the sensitivity by guiding the local defect mode. It has been also shown that by applying quasiperiodic inflation according to Rudin Shapiro Sequence (RSS) scheme to main periodic structure, several tunable resonant modes appear within the PBG. The presence of such sharp resonant peaks reflects that the quasiperiodic NLC-based structure behaves like multiple microcavites with strong light-matter coupling. Full article
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Article
A Straightness Error Compensation System for Topography Measurement Based on Thin Film Interferometry
Photonics 2021, 8(5), 149; https://0-doi-org.brum.beds.ac.uk/10.3390/photonics8050149 - 30 Apr 2021
Viewed by 459
Abstract
Straightness error compensation is a critical process for high-accuracy topography measurement. In this paper, a straightness measurement system was presented based on the principle of fringe interferometry. This system consisted of a moving optical flat and a stationary prism placed close to each [...] Read more.
Straightness error compensation is a critical process for high-accuracy topography measurement. In this paper, a straightness measurement system was presented based on the principle of fringe interferometry. This system consisted of a moving optical flat and a stationary prism placed close to each other. With a properly aligned incident light beam, the air wedge between the optical flat and the prism would generate the interferogram, which was captured by a digital camera. When the optical flat was moving with the motion stage, the variation in air wedge thickness due to the imperfect straightness of the guideway would lead to a phase shift of the interferogram. The phase shift could be calculated, and the air wedge thickness could be measured accordingly using the image processing algorithm developed in-house. This air wedge thickness was directly correlated with the straightness of the motion stage. A commercial confocal sensor was employed as the reference system. Experimental results showed that the repeatability of the proposed film interferometer represented by σ was within 25 nm. The measurement deviation between the film interferometer and the reference confocal sensor was within ±0.1 µm. Compared with other interferometric straightness measurement technologies, the presented methodology was featured by a simplified design and good environment robustness. The presented system could potentially be able to measure straightness in both linear and angular values, and the main focus was to analyze its linear value measurement capability. Full article
(This article belongs to the Special Issue Glass Optics)
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Article
Low Noise Short Wavelength Infrared Avalanche Photodetector Using SB-Based Strained Layer Superlattice
Photonics 2021, 8(5), 148; https://0-doi-org.brum.beds.ac.uk/10.3390/photonics8050148 - 30 Apr 2021
Viewed by 618
Abstract
We demonstrate low noise short wavelength infrared (SWIR) Sb-based type II superlattice (T2SL) avalanche photodiodes (APDs). The SWIR GaSb/(AlAsSb/GaSb) APD structure was designed based on impact ionization engineering and grown by molecular beam epitaxy on a GaSb substrate. At room temperature, the device [...] Read more.
We demonstrate low noise short wavelength infrared (SWIR) Sb-based type II superlattice (T2SL) avalanche photodiodes (APDs). The SWIR GaSb/(AlAsSb/GaSb) APD structure was designed based on impact ionization engineering and grown by molecular beam epitaxy on a GaSb substrate. At room temperature, the device exhibits a 50% cut-off wavelength of 1.74 µm. The device was revealed to have an electron-dominated avalanching mechanism with a gain value of 48 at room temperature. The electron and hole impact ionization coefficients were calculated and compared to give a better prospect of the performance of the device. Low excess noise, as characterized by a carrier ionization ratio of ~0.07, has been achieved. Full article
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Article
New Methods to Seismic Monitoring: Laboratory Comparative Study of Michelson Fiber-Optic Interferometer and Pneumatic Measurement Systems
Photonics 2021, 8(5), 147; https://0-doi-org.brum.beds.ac.uk/10.3390/photonics8050147 - 28 Apr 2021
Cited by 1 | Viewed by 455
Abstract
New possibilities of vibration monitoring can be found in completely different physical approaches, where all measuring technology is currently based on sensors in the electrical domain. This paper presents two different promising alternative approaches to vibration measurement, specifically in the field of fiber-optics [...] Read more.
New possibilities of vibration monitoring can be found in completely different physical approaches, where all measuring technology is currently based on sensors in the electrical domain. This paper presents two different promising alternative approaches to vibration measurement, specifically in the field of fiber-optics and pneumatic sensors. The proposed solution uses a Michelson fiber-optic interferometer designed without polarization fading and with operationally passive demodulation technique using three mutually phase-shifted optical outputs. Experimentally developed sensor systems for the registration of anthropogenic seismic phenomena were complemented by standard instrumentation for measuring seismicity used as a standard. The measurement was performed under simplified conditions using a calibrated stroke as a source of dynamic loading. In addition to alternative systems, the paper also presents the results of recalculation of the measured values in a time domain and basic relationships for the conversion to basic units derived from the SI (International System of Units) system and used internationally in the field of seismic engineering. The results presented demonstrate that even systems operating on a different physical principle have great potential to replace the existing seismic devices. The correlation coefficients for both sensory devices were high (above 0.9) and the average deviations from the measured values of the amplitude of the oscillation velocity did not exceed the value of 0.02, neither with the fiber-optic or pneumatic sensor. Full article
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Article
Linear Combinations of the Complex Degrees of Coherence
Photonics 2021, 8(5), 146; https://0-doi-org.brum.beds.ac.uk/10.3390/photonics8050146 - 28 Apr 2021
Viewed by 388
Abstract
We propose a method for structuring the spatial coherence state of light via mixed linear combinations of N complex degrees of coherence (CDC) and specify the conditions under which such combinations represent a valid CDC. Several examples demonstrate that this method opens previously [...] Read more.
We propose a method for structuring the spatial coherence state of light via mixed linear combinations of N complex degrees of coherence (CDC) and specify the conditions under which such combinations represent a valid CDC. Several examples demonstrate that this method opens previously unknown avenues for modeling random sources, radiating to light fields with unique features. Full article
(This article belongs to the Special Issue Structured Light Coherence)
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Article
Three-Dimensional Mapping of Retrograde Multi-Labeled Motor Neuron Columns in the Spinal Cord
Photonics 2021, 8(5), 145; https://0-doi-org.brum.beds.ac.uk/10.3390/photonics8050145 - 28 Apr 2021
Viewed by 468
Abstract
The quantification and distribution characteristics of spinal motor neurons play important roles in the study of spinal cord and peripheral nerve injury and repair. In most research, the sole retrograde labeling of each nerve or muscle could not simultaneously obtain the distributions of [...] Read more.
The quantification and distribution characteristics of spinal motor neurons play important roles in the study of spinal cord and peripheral nerve injury and repair. In most research, the sole retrograde labeling of each nerve or muscle could not simultaneously obtain the distributions of different motor neuron subpopulations. Therefore, it did not allow mapping of spatial relationships of different motor neuron columns for disclosing the functional relationship of different nerve branches. Here, we combined the multiple retrograde labeling, optical clearing, and imaging for three-dimensional (3D) visualization of motor neurons of multiple brachial plexus branches. After screening fluorescent tracers by the labeling feasibility of motor neurons and fluorescence compatibility with optical clearing, we performed mapping and quantification of the motor neurons of ulnar, median, and radial nerves in the spinal cord, then disclosed the relative spatial distribution among different neuronal subpopulations. This work will provide valuable mapping data for the understanding of the functional relationships among brachial plexus branches, hopefully facilitating the study of regeneration of axons and remodeling of motor neurons in peripheral nerve repair. Full article
(This article belongs to the Special Issue Tissue Optics)
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Article
Tumor Phantom with Incorporated SERS Tags: Detectability in a Turbid Medium
Photonics 2021, 8(5), 144; https://0-doi-org.brum.beds.ac.uk/10.3390/photonics8050144 - 26 Apr 2021
Viewed by 444
Abstract
Surface-enhanced Raman scattering (SERS) tags have proven to be excellent labels for tissue bioimaging because of their low interference from biological matrices, high photostability, and possibility for using as theranostic agents. Although SERS tags are widely used for the imaging of tumors in [...] Read more.
Surface-enhanced Raman scattering (SERS) tags have proven to be excellent labels for tissue bioimaging because of their low interference from biological matrices, high photostability, and possibility for using as theranostic agents. Although SERS tags are widely used for the imaging of tumors in vivo, in practice, the low contrast of the tag accumulation in the tissue and strong light scattering can significantly affect their detectability. In this work, we studied these effects by using a phantom of tumor tissue with incorporated SERS tags. The phantom is a 2 mm sphere of calcium alginate with incorporated SERS tags at a concentration of 0.625 × 108–2 × 109 cm−3. To simulate the surrounding medium with differing turbidities, the phantom was placed in a 4 mm thick agarose gel containing intralipid at a concentration of 0–1%. SERS bioimaging was carried out using standard backscattering geometry with different light focusing conditions. We found that shielding the phantom with a turbid medium led not only to a decrease in detectability but also to a decrease in the apparent size of the imaging object. Our results can help develop more accurate algorithms for processing SERS data for bioimaging. Full article
(This article belongs to the Special Issue Topical Problems of Biophotonics)
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Communication
Phase-Controlled Planar Metalenses for High-Resolution Terahertz Focusing
Photonics 2021, 8(5), 143; https://0-doi-org.brum.beds.ac.uk/10.3390/photonics8050143 - 26 Apr 2021
Viewed by 536
Abstract
We experimentally demonstrate that high-resolution terahertz focusing can be realized in planar metalenses, which consist of arrays of different V-shaped antenna units on a silicon substrate. Numerical results show that a larger numerical aperture of metalenses can provide smaller full width at half [...] Read more.
We experimentally demonstrate that high-resolution terahertz focusing can be realized in planar metalenses, which consist of arrays of different V-shaped antenna units on a silicon substrate. Numerical results show that a larger numerical aperture of metalenses can provide smaller full width at half maximum of field distribution, leading to higher spatial resolution. The measurement of fabricated metalenses samples was performed by a terahertz near-field imaging system, and experimental results agree well with the numerical prediction. Especially for 1.1 THz incident light, when the numerical aperture increases from 0.79 to 0.95, the full width at half maximum correspondingly decreases from 343 μm to 206 μm, offering an improvement of spatial resolution. Full article
(This article belongs to the Special Issue Plasmonic Metasurfaces)
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Article
Comparison of Pulse Wave Signal Monitoring Techniques with Different Fiber-Optic Interferometric Sensing Elements
Photonics 2021, 8(5), 142; https://0-doi-org.brum.beds.ac.uk/10.3390/photonics8050142 - 25 Apr 2021
Cited by 1 | Viewed by 550
Abstract
Pulse wave (PW) measurement is a highly prominent technique, used in biomedical diagnostics. Development of novel PW sensors with increased accuracy and reduced susceptibility to motion artifacts will pave the way to more advanced healthcare technologies. This paper reports on a comparison of [...] Read more.
Pulse wave (PW) measurement is a highly prominent technique, used in biomedical diagnostics. Development of novel PW sensors with increased accuracy and reduced susceptibility to motion artifacts will pave the way to more advanced healthcare technologies. This paper reports on a comparison of performance of fiber optic pulse wave sensors, based on Fabry–Perot interferometer, fiber Bragg grating, optical coherence tomography (OCT) and singlemode-multimode-singlemode intermodal interferometer. Their performance was tested in terms of signal to noise ratio, repeatability of demodulated signals and suitability of demodulated signals for extraction of information about direct and reflected waves. It was revealed that the OCT approach of PW monitoring provided the best demodulated signal quality and was most robust against motion artifacts. Advantages and drawbacks of all compared PW measurement approaches in terms of practical questions, such as multiplexing capabilities and abilities to be interrogated by portable hardware are discussed. Full article
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