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Photonics, Volume 9, Issue 2 (February 2022) – 73 articles

Cover Story (view full-size image): Active optics and elasticity theory of thin plates have led us to the discovery of three geometrical configurations of variable curvature mirrors (VCMs) that are either cycloid-like or tulip-like thickness distributions. Starting from analytic theory, modeling with Nastran finite element analysis allows considering a realistic geometry by including, in a single piece, the plate with a thin outer collarette linked to a rigid ring. One shows that 3D optimizations provide a more accurate cycloid-like thickness distribution. For the ESO-VLTI array, such VCMs are inevitable components, which have since 1998 provided a 3″ co-phased field-of-view for high-angular resolution astronomy. From the ESO-AMU approved convention of making 10 VCM spares up to 2024, the present modeling should provide a diffraction-limited extended field-of-view for a zoom range from f/∞ to f/3.6. View this paper
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Article
Fast Estimation of the Spectral Optical Properties of Rabbit Pancreas and Pigment Content Analysis
Photonics 2022, 9(2), 122; https://doi.org/10.3390/photonics9020122 - 21 Feb 2022
Viewed by 336
Abstract
The pancreas is a highly important organ, since it produces insulin and prevents the occurrence of diabetes. Although rare, pancreatic cancer is highly lethal, with a small life expectancy after being diagnosed. The pancreas is one of the organs less studied in the [...] Read more.
The pancreas is a highly important organ, since it produces insulin and prevents the occurrence of diabetes. Although rare, pancreatic cancer is highly lethal, with a small life expectancy after being diagnosed. The pancreas is one of the organs less studied in the field of biophotonics. With the objective of acquiring information that can be used in the development of future applications to diagnose and treat pancreas diseases, the spectral optical properties of the rabbit pancreas were evaluated in a broad-spectral range, between 200 and 1000 nm. The method used to obtain such optical properties is simple, based almost on direct calculations from spectral measurements. The optical properties obtained show similar wavelength dependencies to the ones obtained for other tissues, but a further analysis on the spectral absorption coefficient showed that the pancreas tissues contain pigments, namely melanin, and lipofuscin. Using a simple calculation, it was possible to retrieve similar contents of these pigments from the absorption spectrum of the pancreas, which indicates that they accumulate in the same proportion as a result of the aging process. Such pigment accumulation was camouflaging the real contents of DNA, hemoglobin, and water, which were precisely evaluated after subtracting the pigment absorption. Full article
(This article belongs to the Special Issue Tissue Optics)
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Article
Tailoring of Inverse Energy Flow Profiles with Vector Lissajous Beams
Photonics 2022, 9(2), 121; https://doi.org/10.3390/photonics9020121 - 20 Feb 2022
Viewed by 495
Abstract
In recent years, structured laser beams for shaping inverse energy flow regions: regions with a direction of energy flow opposite to the propagation direction of a laser beam, have been actively studied. Unfortunately, many structured laser beams generate inverse energy flow regions with [...] Read more.
In recent years, structured laser beams for shaping inverse energy flow regions: regions with a direction of energy flow opposite to the propagation direction of a laser beam, have been actively studied. Unfortunately, many structured laser beams generate inverse energy flow regions with dimensions of the order of the wavelength. Moreover, there are significant limitations to the location of these regions. Here, we investigate the possibility of controlling inverse energy flow distributions by using the generalization of well-known cylindrical vector beams with special polarization symmetry—vector Lissajous beams (VLBs)—defined by two polarization orders (p, q). We derive the conditions for the indices (p, q) in order, not only to shape separate isolated regions with a reverse energy flow, but also regions that are infinitely extended along a certain direction in the focal plane. In addition, we show that the maximum intensity curves of the studied VLBs are useful for predicting the properties of focused beams. Full article
(This article belongs to the Special Issue Polarized Light and Optical Systems)
Communication
An All-MRR-Based Photonic Spiking Neural Network for Spike Sequence Learning
Photonics 2022, 9(2), 120; https://doi.org/10.3390/photonics9020120 - 20 Feb 2022
Viewed by 548
Abstract
Photonic spiking neural networks (SNN) have the advantages of high power efficiency, high bandwidth and low delay, but limitations are encountered in large-scale integration. The silicon photonics platform is a promising candidate for realizing large-scale photonic SNN because it is compatible with the [...] Read more.
Photonic spiking neural networks (SNN) have the advantages of high power efficiency, high bandwidth and low delay, but limitations are encountered in large-scale integration. The silicon photonics platform is a promising candidate for realizing large-scale photonic SNN because it is compatible with the current mature CMOS platforms. Here, we present an architecture of photonic SNN which consists of photonic neuron, photonic spike timing dependent plasticity (STDP) and weight configuration that are all based on silicon micro-ring resonators (MRRs), via taking advantage of the nonlinear effects in silicon. The photonic spiking neuron based on the add-drop MRR is proposed, and a system-level computational model of all-MRR-based photonic SNN is presented. The proposed architecture could exploit the properties of small area, high integration and flexible structure of MRR, but also faces challenges caused by the high sensitivity of MRR. The spike sequence learning problem is addressed based on the proposed all-MRR-based photonic SNN architecture via adopting supervised training algorithms. We show the importance of algorithms when hardware devices are limited. Full article
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Article
Trade-Off Asymmetric Profile for Extended-Depth-of-Focus Ocular Lens
Photonics 2022, 9(2), 119; https://doi.org/10.3390/photonics9020119 - 19 Feb 2022
Viewed by 412
Abstract
We explore the possibility of extending the depth of focus of an imaging lens with an asymmetric quartic phase-mask, while keeping the aberration within a relatively low level. This can be intended, for instance, for ophthalmic applications, where no further digital processing can [...] Read more.
We explore the possibility of extending the depth of focus of an imaging lens with an asymmetric quartic phase-mask, while keeping the aberration within a relatively low level. This can be intended, for instance, for ophthalmic applications, where no further digital processing can take place, relying instead on the patient’s neural adaptation to their own aberrations. We propose a computational optimization method to derive the design-strength factor of the asymmetric profile. The numerical and experimental results are shown. The optical experiment was conducted by means of a modulo-2π phase-only spatial light modulator. The proposed combination of the asymmetric mask and the lens can be implemented in a single refractive element. An exemplary case of an extended-depth-of focus intraocular lens based on the proposed element is described and demonstrated with a numerical experiment. Full article
(This article belongs to the Special Issue Ocular Imaging for Eye Care)
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Article
Supporting Quadric Method for Designing Freeform Mirrors That Generate Prescribed Near-Field Irradiance Distributions
Photonics 2022, 9(2), 118; https://doi.org/10.3390/photonics9020118 - 18 Feb 2022
Viewed by 443
Abstract
We consider a version of the supporting quadric method for designing freeform mirrors that generate prescribed irradiance distributions in the near field. The method is derived for a general case of an incident beam with an arbitrary wavefront. As an example, for a [...] Read more.
We consider a version of the supporting quadric method for designing freeform mirrors that generate prescribed irradiance distributions in the near field. The method is derived for a general case of an incident beam with an arbitrary wavefront. As an example, for a practically important special case of a plane incident wavefront, we design a freeform mirror that generates a complex-shaped uniform irradiance distribution in the form of the abbreviation “IPSI” on a zero background. The designed mirror is fabricated and qualitatively investigated in a proof-of-concept optical experiment. The experimental results confirm the correctness of the proposed approach and demonstrate the manufacturability of the mirrors designed using the considered method. Full article
(This article belongs to the Special Issue Nonimaging Optics)
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Communication
Optical Switch Based on Ge2Sb2Se4Te1-Assisted Racetrack Microring
Photonics 2022, 9(2), 117; https://doi.org/10.3390/photonics9020117 - 18 Feb 2022
Cited by 1 | Viewed by 358
Abstract
In this work, we have proposed and designed a 1 × 1 optical switch based on the optical phase-change material, Ge2Sb2Se4Te1 (GSST), for GSST-assisted silicon racetrack microring. Its optical power can periodically be exchanged between the [...] Read more.
In this work, we have proposed and designed a 1 × 1 optical switch based on the optical phase-change material, Ge2Sb2Se4Te1 (GSST), for GSST-assisted silicon racetrack microring. Its optical power can periodically be exchanged between the straight silicon waveguide and the GSST/Si hybrid racetrack waveguide due to the formed directional coupling structure. By changing GSST from the crystalline state to the amorphous state, the switch shifts from the ON state to the OFF state, and vice versa. With finite-difference time-domain method optimization, the proposed switch shows an extinction ratio of 18 dB at 1547.4 nm. The insert losses at the ON and OFF states are both less than 1 dB. The proposed switch unit has the potential to build an N × N switch matrix. Full article
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Communication
Quantitative Photoacoustic Reconstruction of the Optical Properties of Intervertebral Discs Using a Gradient Descent Scheme
Photonics 2022, 9(2), 116; https://doi.org/10.3390/photonics9020116 - 18 Feb 2022
Viewed by 303
Abstract
The intervertebral discs (IVD) are among the essential organs of the human body, ensuring the mobility of the spine. These organs possess a high proportion of water. However, as the discs age, this content decreases, which can potentially lead to various diseases called [...] Read more.
The intervertebral discs (IVD) are among the essential organs of the human body, ensuring the mobility of the spine. These organs possess a high proportion of water. However, as the discs age, this content decreases, which can potentially lead to various diseases called degenerative disc diseases. This water content is therefore an important indicator of the well-being of the disc. In this paper, we propose photoacoustic imaging as a means of probing a disc and quantitatively recovering its molecular composition, which should allow concluding on its state. An adjoint-assisted gradient descent scheme is implemented to recover the optical absorption coefficient in the disc, from which, if spectroscopic measurements are performed, the molecular composition can be deduced. The algorithm was tested on synthetic measurements. A realistic numerical phantom was built from magnetic resonance imaging of an actual IVD of a pig. A simplified experiment, with a single laser source, was performed. Results show the feasibility of using photoacoustics imaging to probe IVDs. The influences of exact and approximate formulations of the gradient are studied. The impact of noise on the reconstructions is also evaluated. Full article
(This article belongs to the Special Issue Tissue Optics)
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Article
Two Polarization Comb Dynamics in VCSELs Subject to Optical Injection
Photonics 2022, 9(2), 115; https://doi.org/10.3390/photonics9020115 - 18 Feb 2022
Viewed by 497
Abstract
Optical frequency comb technologies have received intense attention due to their numerous promising applications ranging from optical communications to optical comb spectroscopy. In this study, we experimentally demonstrate a new approach of broadband comb generation based on the polarization mode competition in single-mode [...] Read more.
Optical frequency comb technologies have received intense attention due to their numerous promising applications ranging from optical communications to optical comb spectroscopy. In this study, we experimentally demonstrate a new approach of broadband comb generation based on the polarization mode competition in single-mode VCSELs. More specifically, we analyze nonlinear dynamics and polarization properties in VCSELs when subject of optical injection from a frequency comb. When varying injection parameters (injection strength and detuning frequency) and comb properties (comb spacing), we unveil several bifurcation sequences enabling the excitation of free-running depressed polarization mode. Interestingly, for some injection parameters, the polarization mode competition induces a single or a two polarization comb with controllable properties (repetition rate and power per line). We also show that the performance of the two polarization combs depends crucially on the injection current and on the injected comb spacing. We explain our experimental findings by utilizing the spin-flip VCSEL model (SFM) supplemented with terms for parallel optical injection of frequency comb. We provide a comparison between parallel and orthogonal optical injection in the VCSEL when varying injection parameters and SFM parameters. We show that orthogonal comb dynamics can be observed in a wide range of parameters, as for example dichroism linear dichroism (γa=0.1 ns1 to γa=0.8 ns1), injection current (μ=2.29 to μ=5.29) and spin-flip relaxation rate (γs=50 ns1 to γs=2300 ns1). Full article
(This article belongs to the Special Issue Nonlinear Dynamics of Semiconductor Lasers and Their Applications)
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Article
Effect of Transmission-Line Contact Length on the 50-Gbit/s Data Encoding Performance of a Multimode VCSEL
Photonics 2022, 9(2), 114; https://doi.org/10.3390/photonics9020114 - 17 Feb 2022
Viewed by 473
Abstract
Directly modulated 850-nm multimode vertical-cavity surface-emitting lasers (MM-VCSELs) with different oxide apertures and transmission microstrip lengths are compared on the transmission performance of the non-return-to-zero on-off keying (NRZ-OOK) and four-level pulse amplitude modulation (PAM-4) data formats. In this work, intrinsic and extrinsic responses [...] Read more.
Directly modulated 850-nm multimode vertical-cavity surface-emitting lasers (MM-VCSELs) with different oxide apertures and transmission microstrip lengths are compared on the transmission performance of the non-return-to-zero on-off keying (NRZ-OOK) and four-level pulse amplitude modulation (PAM-4) data formats. In this work, intrinsic and extrinsic responses of the MM-VCSEL are also discussed concurrently. By tuning the length of the transmission microstrip in VCSEL, the low reflection coefficient and the enhanced 3-dB modulation bandwidth are achieved. The inductance of the transmission microstrip in the series connection with the capacitance in the active region is optimized to reduce the power loss induced by imaginary impedance. The different oxide aperture sizes for MM-VCSEL are also studied to control the capacitance and photon density. More importantly, the 3-dB modulation bandwidth, impedance matching, slope efficiency, relative intensity noise (RIN), and mode partition noise (MPN) for the MM-VCSEL with various designs are discussed to determine the best device with the high-speed transmission capability. The optimal MM-VCSEL with a diameter of 7 µm oxide aperture and a length of 25 µm transmission microstrip successfully demonstrates 50-Gbit/s OOK and 84-Gbit/s PAM4 after using the pre-emphasis technique for future data-center applications. Full article
(This article belongs to the Special Issue Vertical-Cavity Surface-Emitting Lasers (VCSELs))
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Review
Review of Virus Inactivation by Visible Light
Photonics 2022, 9(2), 113; https://doi.org/10.3390/photonics9020113 - 17 Feb 2022
Cited by 1 | Viewed by 615
Abstract
The COVID-19 pandemic is driving the search for new antiviral techniques. Bacteria and fungi are known to be inactivated not only by ultraviolet radiation but also by visible light. Several studies have recently appeared on this subject, in which viruses were mainly irradiated [...] Read more.
The COVID-19 pandemic is driving the search for new antiviral techniques. Bacteria and fungi are known to be inactivated not only by ultraviolet radiation but also by visible light. Several studies have recently appeared on this subject, in which viruses were mainly irradiated in media. However, it is an open question to what extent the applied media, and especially their riboflavin concentration, can influence the results. A literature search identified appropriate virus photoinactivation publications and, where possible, viral light susceptibility was quantitatively determined in terms of average log-reduction doses. Sensitivities of enveloped viruses were plotted against assumed riboflavin concentrations. Viruses appear to be sensitive to visible (violet/blue) light. The median log-reduction doses of all virus experiments performed in liquids is 58 J/cm2. For the non-enveloped, enveloped and coronaviruses only, they were 222, 29 and 19 J/cm2, respectively. Data are scarce, but it appears that (among other things) the riboflavin concentration in the medium has an influence on the log-reduction doses. Experiments with DMEM, with its 0.4 mg/L riboflavin, have so far produced results with the greatest viral susceptibilities. It should be critically evaluated whether the currently published virus sensitivities are really only intrinsic properties of the virus, or whether the medium played a significant role. In future experiments, irradiation should be carried out in solutions with the lowest possible riboflavin concentration. Full article
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Editorial
Latest Advances in Nanoplasmonics and Use of New Tools for Plasmonic Characterization
Photonics 2022, 9(2), 112; https://doi.org/10.3390/photonics9020112 - 17 Feb 2022
Viewed by 381
Abstract
Nanoplasmonics is a research topic that takes advantage of the light coupling to electrons in metals, and can break the diffraction limit for light confinement into subwavelength zones allowing strong field enhancements [...] Full article
Article
Piston Error Extraction from Dual-Wavelength Interference Patterns Using Phase Retrieval Technique
Photonics 2022, 9(2), 111; https://doi.org/10.3390/photonics9020111 - 16 Feb 2022
Viewed by 553
Abstract
As next-generation large-aperture telescopes, synthetic aperture is a promising method for realizing high resolution observations. Co-phasing the misaligned segmented aperture is an important procedure for high-resolution observations with segmented telescopes. In this paper, a piston error detection method is proposed based on two [...] Read more.
As next-generation large-aperture telescopes, synthetic aperture is a promising method for realizing high resolution observations. Co-phasing the misaligned segmented aperture is an important procedure for high-resolution observations with segmented telescopes. In this paper, a piston error detection method is proposed based on two interference patterns. Two interference patterns are generated by using a lens placed across two adjacent pupils in the exit pupil plane at two wavelengths and a method based on phase retrieval technique is proposed to extract the piston error from the two interference patterns. The introduction of dual-wavelength in the scheme overcomes the 2π ambiguities problem and expands the piston error detection range. Meanwhile, the proposed piston error extraction method based on phase retrieval technique allows high precision measurement of the piston error and is robust to offset lens. Various simulations are demonstrated and the feasibility of the proposed piston error detection method is validated. Full article
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Article
Deep Neural Network Based Reconciliation for CV-QKD
Photonics 2022, 9(2), 110; https://doi.org/10.3390/photonics9020110 - 15 Feb 2022
Viewed by 699
Abstract
High-speed reconciliation is indispensable for supporting the continuous-variable quantum key distribution (CV-QKD) system to generate the secure key in real-time. However, the error correction process’s high complexity and low processing speed limit the reconciliation speed. Therefore, reconciliation has also become the bottleneck of [...] Read more.
High-speed reconciliation is indispensable for supporting the continuous-variable quantum key distribution (CV-QKD) system to generate the secure key in real-time. However, the error correction process’s high complexity and low processing speed limit the reconciliation speed. Therefore, reconciliation has also become the bottleneck of system performance. In this paper, we proposed a high-speed reconciliation scheme that uses the deep neural network to optimize the decoding process of the low-density parity-check (LDPC) code. We first introduced a network structure of decoding implementation based on the deep neural network, which can be applied to decoding algorithms of parallel strategy and significantly reduce the decoding complexity. Subsequently, we proposed two improved decoding algorithms based on this structure, named linear fitting algorithm and deep neural network-assisted decoding algorithm. Finally, we introduced a high-speed reconciliation scheme based on the CPU-GPU hybrid platform. Simulation results show that the proposed reconciliation scheme reduces the complexity and enables us to realize the high-speed CV-QKD system. Furthermore, the improved decoding algorithm can also reduce the FER, thereby increasing the secret key rate. Full article
(This article belongs to the Topic Fiber Optic Communication)
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Concept Paper
Simultaneous Up-Conversion Based on a Co- & Counter-Directions SOA-MZI Sampling Mixer with Standard & Differential Modulation Modes
Photonics 2022, 9(2), 109; https://doi.org/10.3390/photonics9020109 - 14 Feb 2022
Viewed by 552
Abstract
Simulation and experimental performance analyses of simultaneous up-converted signals, for the first time, were investigated utilizing a semiconductor optical amplifier Mach–Zehnder interferometer (SOA-MZI) sampling mixer in co- and counter-directions for standard and differential modulation modes. An optical pulse source at a sampling frequency [...] Read more.
Simulation and experimental performance analyses of simultaneous up-converted signals, for the first time, were investigated utilizing a semiconductor optical amplifier Mach–Zehnder interferometer (SOA-MZI) sampling mixer in co- and counter-directions for standard and differential modulation modes. An optical pulse source at a sampling frequency of fs = 15.6 GHz was used as a sampling signal. The IF signal channels carrying quadrature phase shift keying (QPSK) data at frequencies fm were up-converted at different mixing frequencies up to 195.5 GHz. Using the Virtual Photonics Inc. (VPI) simulator, we realized mixed QPSK signals and studied their characteristics through a conversion gain and an error vector magnitude (EVM). Simulations of up mixing operated in a frequency range up to 158 GHz. For the standard modulation in the co-direction, the conversion gain decreased from 43.3 dB at the mixing frequency of 16.6 GHz to 21.8 dB at 157 GHz for the first channel and from 43 dB at 17.6 GHz to 21 dB at 158 GHz for the second channel. The use of the differential modulation principle improved the conversion gain by about 10 dB at 195.5 GHz compared to standard modulations in co- and counter-directions. The EVM reached, respectively, 15.5 and 17.5% for the differential modulation in both configurations, at the bit rate of 100 Gbit/s at 195.5 GHz. The benefit provided by the differential modulation was that EVM values were shifted by 20% for all channels in both configurations at 100 Gbit/s. In the real measurement, we confirmed that co-directional conversion exhibited a better performance than the counter-directional operation. In addition, the real mixed signal exhibited lower efficiency and quality in comparison with simulated signals due to the sensitivity of the receiver. Full article
(This article belongs to the Topic Optical and Optoelectronic Materials and Applications)
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Article
Reduction of Compton Background Noise for X-ray Fluorescence Computed Tomography with Deep Learning
Photonics 2022, 9(2), 108; https://doi.org/10.3390/photonics9020108 - 14 Feb 2022
Viewed by 582
Abstract
For bench-top X-ray fluorescence computed tomography (XFCT), the X-ray tube source will bring extreme Compton background noise, resulting in a low signal-to-noise ratio and low contrast detection limit. In this paper, a noise2noise denoising algorithm based on the UNet deep learning network is [...] Read more.
For bench-top X-ray fluorescence computed tomography (XFCT), the X-ray tube source will bring extreme Compton background noise, resulting in a low signal-to-noise ratio and low contrast detection limit. In this paper, a noise2noise denoising algorithm based on the UNet deep learning network is proposed. The network can use noise image learning to convert the noise image into a clean image. Two sets of phantoms (high concentration Gd phantom and low concentration Bi phantom) are used for scanning to simulate the imaging process under different noise levels and generate the required data set. Additionally, the data set is generated by Geant4 simulation. In the training process, the L1 loss function is used for its good convergence. The image quality is evaluated according to CNR and pixel profile, which shows that our algorithm is better than BM3D, both visually and quantitatively. Full article
(This article belongs to the Special Issue X-ray Luminescence and Fluorescence)
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Article
Recent Advances in 850 nm VCSELs for High-Speed Interconnects
Photonics 2022, 9(2), 107; https://doi.org/10.3390/photonics9020107 - 13 Feb 2022
Cited by 2 | Viewed by 936
Abstract
Vertical-cavity surface-emitting lasers (VCSELs) have made remarkable progress, are being used across a wide range of consumer electronic applications, and have particularly received much attention from the telecom and datacom industries. However, several constraints are thus currently being tackled to improve the device [...] Read more.
Vertical-cavity surface-emitting lasers (VCSELs) have made remarkable progress, are being used across a wide range of consumer electronic applications, and have particularly received much attention from the telecom and datacom industries. However, several constraints are thus currently being tackled to improve the device characteristics and modulation formats to meet the various demanding requirements of the future 800 GbE and 1.6 TbE Ethernet standards. This manuscript discusses the device characteristics and the key considerations in the device designs and optimizations. Finally, we elucidate the latest developments and vital features of modern 850 nm VCSELs for high-speed interconnects. Full article
(This article belongs to the Special Issue Advanced Ultra High Speed Optoelectronic Devices)
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Article
Variable Repetition Rate Picosecond Master Oscillator for Photoelectron Gun
Photonics 2022, 9(2), 106; https://doi.org/10.3390/photonics9020106 - 13 Feb 2022
Viewed by 455
Abstract
In this work, aiming at the master oscillator of the photoelectron gun with a variable repetition rate of electron bunches, a picosecond solid-state laser subject to delayed optoelectronic feedback and RF loss modulation is studied. Loss modulation is performed using an electro-optical modulator [...] Read more.
In this work, aiming at the master oscillator of the photoelectron gun with a variable repetition rate of electron bunches, a picosecond solid-state laser subject to delayed optoelectronic feedback and RF loss modulation is studied. Loss modulation is performed using an electro-optical modulator with zero bias at the second accelerator frequency subharmonic. Optoelectronic negative feedback uses an intracavity electro-optical modulator and a fast high-voltage photodiode mounted as close as possible to the modulator crystal. An analytical formula is obtained for the pulse duration, and estimates are given for Nd and Yb based media and L, S, C and X-band used in modern linear accelerators. Numerical simulation proves that the control is suitable for pulse-repetitive operation. The proposed approach solves the problem of laser pulse shortening and locking the master oscillator, and therefore, electron bunches in photoelectron guns, to the high-stable RF generator controlling accelerator functioning. Full article
(This article belongs to the Special Issue Optoelectronics)
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Communication
High Power and Efficient 4.43 μm BaGa4Se7 Optical Parametric Oscillator Pumped at 1064 nm
Photonics 2022, 9(2), 105; https://doi.org/10.3390/photonics9020105 - 11 Feb 2022
Viewed by 625
Abstract
A high power and efficiency mid-infrared optical parametric oscillator based on a BaGa4Se7 crystal is demonstrated in this paper. It was pumped by a 500 Hz Q-switched Nd:YAG laser at room temperature. Without cooling, up to 0.76 W output power [...] Read more.
A high power and efficiency mid-infrared optical parametric oscillator based on a BaGa4Se7 crystal is demonstrated in this paper. It was pumped by a 500 Hz Q-switched Nd:YAG laser at room temperature. Without cooling, up to 0.76 W output power at 4.43 μm was generated with respect to the incident pump power (1064 nm) of 5.52 W, corresponding to an optical-to-optical conversion efficiency of 13.7%. The corresponding slope efficiency was as high as 18.7%. The pulse width of the signal wave was 5.2 ns at the pump pulse of 13.7 ns. To the best of our knowledge, this is to date the highest output power achieved at 4–5 μm from a 1064 nm pumped BGSe OPO laser. Considering that no additional cooling system was applied, this work provides a good solution for a high-efficient, compact or even portable mid-infrared solid-state laser device. Full article
(This article belongs to the Special Issue Recent Advances in Nonlinear Optics and Nonlinear Optical Materials)
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Article
Performance of Surface Plasmon Resonance Sensors Using Copper/Copper Oxide Films: Influence of Thicknesses and Optical Properties
Photonics 2022, 9(2), 104; https://doi.org/10.3390/photonics9020104 - 11 Feb 2022
Cited by 1 | Viewed by 671
Abstract
Surface plasmon resonance sensors (SPR) using copper for sensitive parts are a competitive alternative to gold and silver. Copper oxide is a semiconductor and has a non-toxic nature. The unavoidable presence of copper oxide may be of interest as it is non-toxic, but [...] Read more.
Surface plasmon resonance sensors (SPR) using copper for sensitive parts are a competitive alternative to gold and silver. Copper oxide is a semiconductor and has a non-toxic nature. The unavoidable presence of copper oxide may be of interest as it is non-toxic, but it modifies the condition of resonance and the performance of the sensor. Therefore, the characterization of the optical properties of copper and copper oxide thin films is of interest. We propose a method to recover both the thicknesses and optical properties of copper and copper oxide from absorbance curves over the (0.9;3.5) eV range, and we use these results to numerically investigate the surface plasmon resonance of copper/copper oxide thin films. Samples of initial copper thicknesses 10, 30 and 50 nm, after nine successive oxidations, are systematically studied to simulate the signal of a Surface Plasmon Resonance setup. The results obtained from the resolution of the inverse problem of absorbance are used to discuss the performance of a copper-oxide sensor and, therefore, to evaluate the optimal thicknesses. Full article
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Communication
Experimental and Numerical Study of Locking of Low-Frequency Fluctuations of a Semiconductor Laser with Optical Feedback
Photonics 2022, 9(2), 103; https://doi.org/10.3390/photonics9020103 - 11 Feb 2022
Cited by 1 | Viewed by 563
Abstract
We study the output of a semiconductor laser with optical feedback operated in the low-frequency fluctuations (LFFs) regime and subject to weak sinusoidal current modulation. In the LFF regime, the laser intensity exhibits abrupt drops, after which it recovers gradually. Without modulation, the [...] Read more.
We study the output of a semiconductor laser with optical feedback operated in the low-frequency fluctuations (LFFs) regime and subject to weak sinusoidal current modulation. In the LFF regime, the laser intensity exhibits abrupt drops, after which it recovers gradually. Without modulation, the drops occur at irregular times, while, with weak modulation, they can lock to the external modulation and they can occur, depending on the parameters, every two or every three modulation cycles. Here, we characterize experimentally the locking regions and use the well-known Lang–Kobayashi model to simulate the intensity dynamics. We analyze the effects of several parameters and find that the simulations are in good qualitative agreement with the experimental observations. Full article
(This article belongs to the Special Issue Nonlinear Dynamics of Semiconductor Lasers and Their Applications)
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Communication
A Low-Frequency Magnetic Field Sensor Based on Fiber Bragg Gratings
Photonics 2022, 9(2), 102; https://doi.org/10.3390/photonics9020102 - 11 Feb 2022
Viewed by 548
Abstract
A new type of low-frequency magnetic-field sensor based on fiber Bragg gratings (FBGs) was experimentally demonstrated for measuring the DC to low-frequency magnetic field. The operating mechanism of this AC magnetic sensor is based on the strain exerted by a loaded magnet on [...] Read more.
A new type of low-frequency magnetic-field sensor based on fiber Bragg gratings (FBGs) was experimentally demonstrated for measuring the DC to low-frequency magnetic field. The operating mechanism of this AC magnetic sensor is based on the strain exerted by a loaded magnet on the sensing structure, which causes center-wavelength shifts of FBG. The achieved sensitivity was 8.16 pm/G with a resolution of 3 Gauss at ambient conditions. The proposed FBG magnetic sensor is easy to use, compact, and suitable for DC to low-frequency magnetic sensing applications. Full article
(This article belongs to the Special Issue Advancements in Fiber Bragg Grating Research II)
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Article
Mapping the Stability and Dynamics of Optically Injected Dual State Quantum Dot Lasers
Photonics 2022, 9(2), 101; https://doi.org/10.3390/photonics9020101 - 10 Feb 2022
Viewed by 516
Abstract
Optical injection is a key nonlinear laser configuration both for applications and fundamental studies. An important figure for understanding the optically injected laser system is the two parameter stability mapping of the dynamics found by examining the output of the injected laser under [...] Read more.
Optical injection is a key nonlinear laser configuration both for applications and fundamental studies. An important figure for understanding the optically injected laser system is the two parameter stability mapping of the dynamics found by examining the output of the injected laser under different combinations of the injection strength and detuning. We experimentally and theoretically generate this map for an optically injected quantum dot laser, biased to emit from the first excited state and optically injected near the ground state. Regions of different dynamical behaviours including phase-locking, excitability, and bursting regimes are identified. At the negatively detuned locking boundary, ground state dropouts and excited state pulses are observed near a hysteresis cycle for low injection strengths. Higher injection strengths reveal μs duration square wave trains where the intensities of the ground state and excited state operate in antiphase. A narrow region of extremely slow oscillations with periods of several tens of milliseconds is observed at the positively detuned boundary. Two competing optothermal couplings are introduced and are shown to reproduce the experimental results extremely well. In fact, the dynamics of the system are dominated by these optothermal effects and their interplay is central to reproducing detailed features of the stability map. Full article
(This article belongs to the Special Issue Nonlinear Dynamics of Semiconductor Lasers and Their Applications)
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Communication
Tunable Triple Plasmonically Induced Transparency in Triangular Cavities Coupled with an MDM Waveguide
Photonics 2022, 9(2), 100; https://doi.org/10.3390/photonics9020100 - 09 Feb 2022
Viewed by 497
Abstract
In this paper, a side-coupled triangle cavity in a plasmonic waveguide structure is proposed and numerically analyzed by the finite-difference time-domain (FDTD) method and coupled mode theory (CMT). Triple plasmonically induced transparency (PIT) was achieved when an extra triangle was added into the [...] Read more.
In this paper, a side-coupled triangle cavity in a plasmonic waveguide structure is proposed and numerically analyzed by the finite-difference time-domain (FDTD) method and coupled mode theory (CMT). Triple plasmonically induced transparency (PIT) was achieved when an extra triangle was added into the structure, and the transmission characteristics were investigated. This novel structure has a maximal sensitivity of 933 nm/RIU when used as a sensor and a contrast ratio of 4 dB. Moreover, the tunability of PIT can be realized by filling the nematic liquid crystal (NLC) E7 into the triangles. The refractive index of E7 changes with the applied electric field. Given that E7 is also sensitive to temperature, this structure can be used as a temperature sensor with a sensitivity of 0.29 nm/°C. It is believed that this tunable structure with PIT may have potential applications in highly integrated optical circuits. Full article
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Review
Two-Membrane Cavity Optomechanics: Linear and Non-Linear Dynamics
Photonics 2022, 9(2), 99; https://doi.org/10.3390/photonics9020099 - 08 Feb 2022
Viewed by 509
Abstract
In this paper, we review the linear and non-linear dynamics of an optomechanical system made of a two-membrane etalon in a high-finesse Fabry–Pérot cavity. This two-membrane setup has the capacity to modify on demand the single-photon optomechanical coupling, and in the linearized interaction [...] Read more.
In this paper, we review the linear and non-linear dynamics of an optomechanical system made of a two-membrane etalon in a high-finesse Fabry–Pérot cavity. This two-membrane setup has the capacity to modify on demand the single-photon optomechanical coupling, and in the linearized interaction regime to cool simultaneously two mechanical oscillators. It is a promising platform for realizing cavity optomechanics with multiple resonators. In the non-linear regime, an analytical approach based on slowly varying amplitude equations allows us to derive a consistent and full characterization of the non-linear displacement detection, enabling a truthful detection of membrane displacements much above the usual linear sensing limited by the cavity linewidth. Such a high quality system also shows a pre-synchronization regime. Full article
(This article belongs to the Special Issue Optomechanics: Science and Applications)
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Article
Flexible Plenoptic X-ray Microscopy
Photonics 2022, 9(2), 98; https://doi.org/10.3390/photonics9020098 - 08 Feb 2022
Viewed by 736
Abstract
X-ray computed tomography (CT) is an invaluable technique for generating three-dimensional (3D) images of inert or living specimens. X-ray CT is used in many scientific, industrial, and societal fields. Compared to conventional 2D X-ray imaging, CT requires longer acquisition times because up to [...] Read more.
X-ray computed tomography (CT) is an invaluable technique for generating three-dimensional (3D) images of inert or living specimens. X-ray CT is used in many scientific, industrial, and societal fields. Compared to conventional 2D X-ray imaging, CT requires longer acquisition times because up to several thousand projections are required for reconstructing a single high-resolution 3D volume. Plenoptic imaging—an emerging technology in visible light field photography—highlights the potential of capturing quasi-3D information with a single exposure. Here, we show the first demonstration of a flexible plenoptic microscope operating with hard X-rays; it is used to computationally reconstruct images at different depths along the optical axis. The experimental results are consistent with the expected axial refocusing, precision, and spatial resolution. Thus, this proof-of-concept experiment opens the horizons to quasi-3D X-ray imaging, without sample rotation, with spatial resolution of a few hundred nanometres. Full article
(This article belongs to the Special Issue Advances in X-ray Optics)
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Article
Broadband Near-Field Near-Infrared Spectroscopy and Imaging with a Laser-Driven Light Source
Photonics 2022, 9(2), 97; https://doi.org/10.3390/photonics9020097 - 08 Feb 2022
Viewed by 644
Abstract
The scattering-type scanning near-field optical microscope (s-SNOM) has become a powerful imaging and nano-spectroscopy tool, which is widely used in the characterization of electronic and photonic devices, two-dimensional materials and biomolecules. However, in the published literature, nano-spectroscopy is mainly employed in the mid-infrared [...] Read more.
The scattering-type scanning near-field optical microscope (s-SNOM) has become a powerful imaging and nano-spectroscopy tool, which is widely used in the characterization of electronic and photonic devices, two-dimensional materials and biomolecules. However, in the published literature, nano-spectroscopy is mainly employed in the mid-infrared band, and the near-infrared (NIR) nano-spectroscopy with broadband spectral range has not been well discussed. In the present paper, we introduce a home-built near-field NIR spectroscopy and imaging set-up that is based on a laser-driven light source (LDLS). By mapping the Ge-Au periodic grating sample and the photonic topology device, a ~30 nm spatial resolution and the excellent capability of characterizing complex samples are demonstrated. Spectra obtained by experiment reveal the optical band-gap of Ge with a spectral resolution of 25 cm−1, and a spectral range from 900 to 2000 nm. This technology is expected to provide a novel and unique approach for near-field NIR spectroscopy and imaging. Full article
(This article belongs to the Special Issue Optical Instrumentation)
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Communication
Micro-Tapered Fiber Few-Mode Interferometers Incorporated by Molecule Self-Assembly Fiber Grating for Temperature Sensing Applications
Photonics 2022, 9(2), 96; https://doi.org/10.3390/photonics9020096 - 07 Feb 2022
Viewed by 629
Abstract
We demonstrate fiber few-mode interferometers based on a self-assembly surface corrugated grating using charged nano-particles. Initially, an abrupt taper (AT) was first created using a micro flame. The AT was then further outwardly stretched to make an elongated uniformed taper until the tapered [...] Read more.
We demonstrate fiber few-mode interferometers based on a self-assembly surface corrugated grating using charged nano-particles. Initially, an abrupt taper (AT) was first created using a micro flame. The AT was then further outwardly stretched to make an elongated uniformed taper until the tapered diameter achieved a micron scale. The high order core modes (HOCMs) were excited at the AT and the optical path difference (OPD) among the modes was enlarged through the uniformed taper to achieve the few-mode interference effects seen. However, to significantly enhance the interference effects with higher extinction ratios (ER) over such a short length of interferometer, an external assisted grating was made using charged nanoparticles to form surface corrugated grating with a period, Λ, of approximately 14 μm. This intermediate period of the fiber grating was helpful in scattering and attenuating some unwanted high-order modes to change the optical characteristics of the few-mode interferometer (FMI). This FMI with a self-assembly fiber grating (SAFG) was further used to make fiber temperature sensors, with a maximum resonant wavelength shift of 4.6 nm, over a temperature range from 20–60 °C. The temperature sensitivity achieved was 112.6 pm/°C and the coefficient of determination, R2, was as high as 0.99, which revealed the high linearity of the results. Full article
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Article
Accelerated Generation of a Pinhole-Type Holographic Stereogram Based on Human Eye Characteristics in Near-Eye Displays
Photonics 2022, 9(2), 95; https://doi.org/10.3390/photonics9020095 - 07 Feb 2022
Viewed by 483
Abstract
In near-eye displays (NEDs), issues such as weight, heat, and power consumption mean that the rendering and computing power is usually insufficient. Due to this limitation, algorithms need to be further improved for the rapid generation of holograms. In this paper, we propose [...] Read more.
In near-eye displays (NEDs), issues such as weight, heat, and power consumption mean that the rendering and computing power is usually insufficient. Due to this limitation, algorithms need to be further improved for the rapid generation of holograms. In this paper, we propose two methods based on the characteristics of the human eye in NEDs to accelerate the generation of the pinhole-type holographic stereogram (HS). In the first method, we consider the relatively fixed position of the human eye in NEDs. The number of visible pixels from each elemental image is very small due to the limited pupil size of an observing eye, and the calculated amount can be dramatically reduced. In the second method, the foveated region rendering method is adopted to further enhance the calculation speed. When the two methods are adopted at the same time, the calculation speed can be increased dozens of times. Simulations demonstrate that the proposed method can obviously enhance the generation speed of a pinhole-type HS. Full article
(This article belongs to the Special Issue Computer Holography)
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Communication
Interlayer Slope Waveguide Coupler for Multilayer Chalcogenide Photonics
Photonics 2022, 9(2), 94; https://doi.org/10.3390/photonics9020094 - 07 Feb 2022
Viewed by 604
Abstract
The interlayer coupler is one of the critical building blocks for optical interconnect based on multilayer photonic integration to realize light coupling between stacked optical waveguides. However, commonly used coupling strategies, such as evanescent field coupling, usually require a close distance, which could [...] Read more.
The interlayer coupler is one of the critical building blocks for optical interconnect based on multilayer photonic integration to realize light coupling between stacked optical waveguides. However, commonly used coupling strategies, such as evanescent field coupling, usually require a close distance, which could cause undesired interlayer crosstalk. This work presents a novel interlayer slope waveguide coupler based on a multilayer chalcogenide glass photonic platform, enabling light to be directly guided from one layer to another with a large interlayer gap (1 µm), a small footprint (6 × 1 × 0.8 µm3), low propagation loss (0.2 dB at 1520 nm), low device processing temperature, and a high bandwidth, similar to that in a straight waveguide. The proposed interlayer slope waveguide coupler could further promote the development of advanced multilayer integration in 3D optical communications systems. Full article
(This article belongs to the Special Issue Glass Optics)
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Article
Indoor High-Precision 3D Positioning System Based on Visible-Light Communication Using Improved Whale Optimization Algorithm
Photonics 2022, 9(2), 93; https://doi.org/10.3390/photonics9020093 - 06 Feb 2022
Cited by 1 | Viewed by 654
Abstract
Visible-light communication (VLC) is a promising method for indoor positioning. The received signal strength algorithm is the most widely used localization algorithm in visible-light positioning (VLP) systems. However, in a VLP system, the photodiode (PD) will have a small rotation angle during movement, [...] Read more.
Visible-light communication (VLC) is a promising method for indoor positioning. The received signal strength algorithm is the most widely used localization algorithm in visible-light positioning (VLP) systems. However, in a VLP system, the photodiode (PD) will have a small rotation angle during movement, which will result in a massive positioning error ignoring the angle. In this study, a three-dimensional (3D) indoor VLP system using an improved whale optimization algorithm (IWOA) is proposed to reduce the error caused by the PD rotation. Firstly, the model of the VLC system with the PD rotation angles is introduced. Secondly, a novel IWOA with an elite opposition-based learning strategy and Lévy flight strategy is proposed. The convergence speed and accuracy of the WOA are improved. Lastly, the IWOA algorithm is efficiently utilized to address the problem with the PD rotation in the indoor VLP system. Simulation results show that the average error of 3D positioning is 2.14 cm with no PD rotation. When the PD has a rotation angle, the average positioning error estimated by ignoring the rotation angle is 27.14 cm, while that estimated by considering the rotation angle is 7.85 cm. In the VLP system, the positioning error with the PD rotation angle is effectively reduced by the proposed algorithm, which can be applied in a variety of indoor location scenes. Full article
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