Photonics, Volume 10, Issue 5 (May 2023) – 122 articles

We show that the focusing of a random electromagnetic beam by a lens gives rise to a scintillation index at the geometrical focus that generally differs from that of the incident beam. In the examples we present, focusing produces a significant increase of the index. This observation is of particular relevance for optical communication systems in which scintillation is a major cause of signal degradation. Full article

A method to create compositionally disordered compounds with a high number of cations in the matrices, that utilize the cubic spatial symmetry of the garnet-type crystalline systems is demonstrated. Mixtures of the garnet-type powdered materials solely doped with Ce were used to create atomic compositions of high complexity. Several mixed systems, namely Gd₃Al₂Ga₃O₁₂/(Gd,Y)₃Al₂Ga₃O₁₂, Y₃Al₅O₁₂/Gd₃Al₂Ga₃O₁₂, and Y₃Al₅O₁₂/Y₃Al₂Ga₃O₁₂ were annealed, compacted and sintered in air. The materials were evaluated for structural, luminescence, and scintillation properties. It was demonstrated that the properties of the resulting ceramics are a little dependent on the granularity of powders when the median particle size is below ~5 μm. Full article

Bismuth shows outstanding optical properties, including a metal-like response in the ultraviolet-visible range and a dielectric character with a giant refractive index in the infrared range. In recent years, such unique properties have been employed to construct bismuth-based metamaterials with remarkable optical responses in these spectral regions, especially with cost-effective lithography-free methods. Such responses can be manipulated, both in an astatic way by suitable metamaterial design and in a dynamic way by harnessing the solid–liquid transition of bismuth. In this paper, we review the advances in this field and highlight the applications of such metamaterials to information technology production, energy harvesting and sensing. Full article

We discuss the properties of signal strength and integrated intensity in two-photon excitation confocal microscopy and image scanning microscopy. The resolution, optical sectioning and background rejection are all improved over nonconfocal two-photon microscopy. Replacing the pinhole of confocal two-photon microscopy with a detector array increases the peak intensity of the point spread function. The outer pixels of a detector array give signals from defocused regions, and thus the processing of these, such as through subtraction, can further improve optical sectioning and background rejection. Full article

In the study of the evolution of Gaussian beam in saturated nonlinear media, it is found that the probability of optical rogue waves changes with the change of nonlinearity. The light intensity distribution on the exit surface of nonlinear medium can be characterized by scintillation index, and the change of rogue wave corresponds to the evolution of scintillation index. The rogue wave probability shows a complex trend with the evolution of nonlinearity. The Lyapunov exponent and power spectrum method are used to determine that the probability of rogue wave is chaotic with nonlinear evolution. Full article

An optical fiber ring laser (FRL) cavity-based sensitive temperature and salinity sensor is proposed and experimentally demonstrated. The sensor consists of a Sagnac loop with a waist of 15 µm and a total length of 30 cm made of tapered polarization-maintaining fiber (PMF). Sagnac loop dual parameter sensing was theoretically modeled and presented. The salinity sensitivity of 0.173 nm/‰ was made possible by the efficient interaction between the tapered PMF cladding mode and the external refractive index. In addition, temperature sensitivity of 0.306 nm/°C was achieved through ultrahigh birefringence of PMF. Apart from that, the previous sensing system used a broadband light source (BBS) as the input light, resulting in a wide bandwidth and a poor signal-to-noise ratio (SNR). The Sagnac loop integrated into the FRL system can achieve a high SNR of approximately 50 dB and a narrow bandwidth of 0.15 nm while serving as the filter and sensor head. Additionally, the developed sensor has the advantages of simple design, low cost, and easy fabrication. It can also extend sensing distance indefinitely within a given range, which is anticipated to have positive effects on the testing of marine environments in laboratories. Full article

The next generation of lidar systems needs to adapt to variable environments with broadened bandwidth for increased resolution. Due to their digital components, conventional lidar systems, especially imaging lidar systems, suffer from limited detector bandwidth and sampling frequency. However, photonics devices can provide a reliable technical solution with high precision and ultra-broad bandwidth. This paper presents a photonic signal processing structure for a phase-coded lidar system. Two acousto-optic modulators (AOMs) are adopted in the proposed architecture. One is used for phase-coded laser signal modulation, and the other is used for demodulation. The echo laser signal is directed to the AOM performing demodulation before the sampling of the detector, accomplishing the multiplication of the echo laser signal and the electric reference signal. The detector is controlled to accumulate the demodulated laser signal. The AOM and detector transfer the correlation calculation from electrical signals processing to photonic signals processing. This photonics-based structure greatly decreases the sampling frequency of the detector without extending the width of the laser pulses, which achieves high resolution with low sampling speed. Photonic signal processing has the promising potential of simultaneously processing signals of multiple pixels. It is going to be an effective solution for imaging lidar systems to increase resolution with available low-cost devices. Full article

Laser Direct Writing (LDW), also known as 3D multi-photon laser lithography of resins, is a promising technique for fabricating complex free-form elements, including micro-optical functional components. Regular organic or hybrid (organic–inorganic) resins are often used, with the latter exhibiting better optical characteristics, as well as having the option to be heat-treated into inorganic glass-like structures particularly useful for resilient micro-optics. This work is a continuation of our SZ2080™ calcination development of micro-optics, specifically studying the Laser-Induced Damage Threshold (LIDT). Such sol–gel-derived glass 3D micro-structures, particularly those that undergo heat treatment, have not been well-characterized in this respect. In this pilot study, we investigated the LIDT using the Series-on-One (S-on-1) protocol of functional micro-lenses produced via LDW and subsequently calcinated. Our results demonstrate that the LIDT can be significantly increased, even multiple times, by this approach, thus enhancing the resilience and usefulness of these free-form micro-optics. This work represents the first investigation in terms of LIDT into the impact of calcination on LDW-produced, sol–gel-derived glass micro-structures and provides important insights for the development of robust micro-optical devices. Full article

Underwater wireless optical communication (UWOC) is acknowledged as a useful way to transmit data in the ocean for short-distance applications. Carrying a UWOC device on mobile platforms is quite practical in ocean engineering, which is helpful to exploit its advantages. In application, such a platform needs a camera to observe the surroundings and guide its action. Since the majority of ocean is always dark, active illumination is necessary to imaging. When UWOC works in such an environment, its performance is affected by the illumination light noise. In this paper, we study the influence of underwater LED illumination on bidirectional UWOC with the Monte Carlo method. We simulate forward noise from LED illumination to the opposite receiver in the cooperative terminal, and the backscattering noise on the adjacent receiver in the same terminal. The results show that the forward noise is reduced with the increase of theabsorption coefficient, scattering coefficient, transmitting distance, and separated distance between receiver and the optical axis of LED. However, it becomes greater with the field of view (FOV) of the receiver. The backscattering noise is reduced with the increase of the absorption coefficient and separated distance between receiver and LED. However, it becomes greater with the FOV and scattering coefficient, while it has little relation with transmitting distance. In order to reduce these two kinds of noises, besides inserting an optical filter in the receivers and narrowing their FOV, the optical axis of LED light should keep away from the receivers. The results in this paper are helpful for UWOC application. Full article

Large survey telescopes are vital for mapping dark energy and dark matter in the deep universe. This study presents a fiber-linked internal motion metrology system that aligns the mirrors and large lenses in the telescopes to enhance alignment accuracy by improving the image quality at a lower weight, volume, power, and cost. The internal motion system comprises a photonic laser beam projector capable of projecting multiple Gaussian beams onto the detector of the telescope. The specific spatial frequency aberration component is determined by combining Gaussian beam location and the geometry model of the telescope. Furthermore, integrating the proposed system with the curvature-sensing wavefront system enables more precise alignment and camera sensing. In the experimental tests, the location precision was within 10 μm, and the rotation precision improved to 5 arcsecs, fulfilling the alignment and motion monitoring requirements of large survey telescopes. The results of this study can be used as a reference to improve the performance of closed-loop bandwidth systems and active camera optics. Full article

Ultra-short laser pulses (1030 nm/230 fs) were used to laser ablate the surface of crystalline sapphire (Al₂O₃) at high intensity per pulse 20–200 TW/cm${}^2$/pulse. Laser-ablated patterns were annealed at a high temperature of 1500 ${}^{°}$C. Surface reconstruction took place, removing the ablation debris field at the edges of ablated pits in oxygen flow (O₂ flow). Partial reconstruction of ripples was also observed when multi-pulse ablated surfaces were annealed at high temperature in O₂ flow. Back-side ablation of a 0.5-mm-thick Al₂O₃ produced high surface roughness ∼$1\mathsf{\mu }$m which was reduced to ∼$0.2\mathsf{\mu }$m by high-temperature annealing at 1500 ${}^{°}$C for 2 h in O₂. Improvement of surface quality was due to restructuring of the crystalline surface and sublimation, while the defined 3D shape of a micro-lens was not altered after HTA (no thermal morphing). Full article

The integrated cavity absorption meter is designed to measure the seawater absorption coefficient spectra which are necessary for studying ocean productivity and heat balance. The performed numerical simulations of a light field structure made it possible to improve the measurement technique. Its results showed that the use of the Lambertian model allows to reduce the calculation time by two orders of magnitude with an acceptable loss of accuracy for these calculations. It is shown that in the case of an integrating sphere made of fluorilon, the use of different volume scattering functions does not affect the calculation result, which is not true in the case of using a sphere with a mirror coating. The effect of an air layer between quartz and fluorilon is considered, and the applicability of the diffusion approximation is verified. Examples of field measurements of the seawater absorption coefficient and its components performed in different water areas of the World Ocean in 2020–2022 are presented. Full article

Plasmonic metamaterials can exhibit a variety of physical optical properties that offer extraordinary nonlinear conversion efficiency for ultra-compact nanodevice applications. Furthermore, the optical-rectification effect from the plasmonic nonlinear metasurfaces (NLMSs) can be used as a compact source of deep-subwavelength thickness to radiate broadband terahertz (THz) signals. Meanwhile, a novel dual-mode metasurface consisting of a split-ring resonator (SRR) array and an epsilon-near-zero (ENZ) layer was presented to boost the THz conversion efficiency further. In this paper, to explore the mechanism of THz generation from plasmonic NLMSs, the Maxwell-hydrodynamic multiphysics model is adopted to investigate complex linear and intrinsic nonlinear dynamics in plasmonics. We solve the multiphysics model using the finite-difference time-domain (FDTD) method, and the numerical results demonstrate the physical mechanism of the THz generation processes which cannot be observed in our previous experiments directly. The proposed method reveals a new approach for developing new types of high-conversion-efficiency nonlinear nanodevices. Full article

A high-sensitivity axial force sensor with a large measurement range based on a dual-peak long-period fiber grating (LPFG) is proposed and experimentally demonstrated. Previously, the relationship between the grating period and the dual-peak wavelengths has been investigated based on the coupled-mode theory. In our experiment, the LPFG was fabricated in our laboratory by illuminating the fiber core with the aid of a 213 nm UV laser. The sensitivity of the proposed axial force sensor can reach −14.047 nm/N in the force range from 0.490 N to 4.508 N. Taking the advantages of a compact size, low cost, and large measurement range, our force sensor has more applicable abilities in harsh environments. Full article

In recent years, various solutions for augmented reality (AR) head-mounted displays have been proposed. In order to achieve the dual functions of reflective focusing on virtual images and transparency to the real world, the optical design of AR eyepieces is particularly critical. Designs based on traditional optics still face the problems of huge volume and a limited field of view. Due to their extraordinary phase control ability, portability, easy integration, and other advantages, metalenses have triggered extensive research and found many applications, including providing an innovative solution for AR eyepieces. In this work, we propose a single-layer trans-reflective RGB-achromatic metalens with a large field of view of 90°. The metalens reflects the oblique incident virtual image while maintaining balanced transparency of real-world light. Through simulation, the ability of the metalens to focus light at the wavelengths of 488 nm, 532 nm, and 633 nm with the same focal length and balanced efficiency is validated. Moreover, the metalens is polarization-insensitive to the incident light, thus allowing the elimination of the polarization modulation components, which greatly simplifies the optical structure. Our work demonstrates the great potential of metalenses for AR eyepiece applications. Full article

Unidirectional electromagnetic modes have significant potential for routing electromagnetic radiation and are highly desirable for various applications, such as isolators, splitters, and switches. In this study, we theoretically investigate surface magnetoplasmons (SMPs) in a four-layer structure consisting of a perfect magnetic conductor (PMC)–semiconductor–dielectric–metal, which exhibits complete unidirectional propagation. We extend this structure to a 3D model by decreasing the width of the PMC-semiconductor part to an appropriate value and demonstrate that the SMPs in the proposed 3D waveguide retain complete unidirectional propagation. Our findings indicate that the unidirectional SMPs are robust to backscattering caused by surface roughness and defects. Moreover, the proposed 3D waveguide can be efficiently coupled to conventional microstrip line waveguides. Our results (based on the numerical method) demonstrate that SMPs based on semiconductors offer a promising approach to creating devices with new functionalities in the terahertz regime below the diffraction limit. Full article

In this paper, infrared polarization detection information acquisition technology is proposed, and the polarization characteristics of oil spills are modeled and studied. A set of long-wave infrared polarization detection equipment for oil spills is designed and built, and modeling research on oil spill polarization characteristics is carried out to accurately detect and identify oil spill types and for the faster processing of oil spill events. Oil spill accuracy is increased by defining the polarization maintenance method of the polarization optical system and reducing the polarization measurement error brought on by the imaging system. As a result, a higher than 3% contrast exists between the polarization degree image and the corrected infrared intensity image. Outdoor tests using oil, palm oil, crude oil, gasoline, and diesel oil spill types are carried out in a controlled environment to collect data on the polarization of various oil species. According to the findings, each oil species’ infrared polarization contrast with seawater is typically greater than its infrared intensity contrast. However, the polarization data of saltwater, diesel, and palm oil, which are difficult to identify in intensity data, show a noticeable difference, further proving the viability of utilizing polarization to discern oil spills. Full article

In this paper, a polarization selective broad/triple-band metamaterial absorber based on SiO₂ all-dielectric is designed and studied. The absorber works in a long infrared band (8–14 μm). It is composed of cuboid and trapezoidal silica structures in the upper layer and metal plates in the lower layer. We calculate the absorption results of the metamaterial absorber at different polarization angles as the polarization angle of incident light increases from 0° to 90°; that is, the light changes from Ex polarization to Ey polarization. The results show that the absorption rate of the structure is more than 90% in the range of 8.16 to 9.61 μm when the polarization angle is 0°. When the polarization angle of the incident light is less than 45°, the absorption result of the absorber does not change significantly. When the polarization angle of the incident light is greater than 45°, three absorption peaks appear in the long infrared band, realizing the selectivity of the polarization of the incident light. When the polarization angle increases to 90°, the absorptivity of the two absorption peaks at λ = 9.7 μm and 12.3 μm reaches more than 85%. In addition, the sensitivity analysis of the length, width, and thickness of the all-dielectric metamaterial absorber and the calculation of the electric field of this structure are also carried out. The designed all-dielectric metamaterial absorber has polarization selection and perfect absorption characteristics and has a broad application prospect. Full article

An erbium-doped Lu₃Ga₅O₁₂(LuGG) single crystal was grown by the Czochralski method. The crystalline phase in the grown crystal was analyzed by powder X-ray diffraction. The erbium-ion emission spectra of the crystal were acquired. The erbium-ion emission cross-section (ECS) spectrum was computed from the acquired emission spectrum. The erbium-ion absorption cross-section (ACS) spectrum was computed using the McCumber relationship. The results are discussed in contrast to those computed from the acquired absorption spectrum, and the comparison shows that both methods give consistent results. The temporal characteristics of the emissions were also studied based on 0.98 μm pulse pumping. The study shows that the infrared emissions at 1.0, 1.5, and 2.8 μm show mono-exponentially temporal behavior. Instead, the decays of two visible emissions at 0.56 and 0.67 μm show considerable non-exponential features; each trace can be fitted double-exponentially. The non-exponential behavior is associated with those erbium ions that are present in the form of clusters, which enables non-radiative upconversion depopulation and hence additional contribution to the decay through cross relaxation between the erbium ions in clusters. The study also shows that about half of the erbium ions are present in the cluster state in the studied crystal. Full article

The possibility to control both spectral and polarization properties of seed THz pulses in strongly nonequilibrium elongated magnetized plasma channels formed via intense UV femtosecond laser pulses in nitrogen (air) is analyzed. The physical mechanism of THz pulse control is based on cyclotron resonance, which can strongly reconstruct electrodynamical plasma features and, in particular, its ability to amplify the radiation of different spectral bands and polarization states. In particular, the formation of quasiunipolar pulses with a non-zero electric area and a specific polarization state is discussed. This study is based on the self-consistent solution of the kinetic Boltzmann equation for the electron velocity distribution function (EVDF) in the plasma channel and the second-order wave equation for THz pulse propagation. Full article

The quantum Cramer–Rao bound (QCRB) provides an ultimate precision limit in parameter estimation. The sensitivity of spatial measurements can be improved by using the higher-order Hermite–Gaussian mode. However, to date, the QCRB-saturating tilt measurement has not been realized. Here, we experimentally demonstrate tilt measurements using a higher-order $H{G}_{40}$ mode as the probe beam. Using the balanced homodyne detection with an optimal local beam, which involves the superposition of high-order $H{G}_{30}$ and $H{G}_{50}$ modes, we demonstrate the precision of the tilt measurement approaching the QCRB. The signal-to-noise ratio of the tilt measurement is enhanced by 9.2 dB compared with the traditional method using $H{G}_{00}$ as the probe beam. This scheme is more practical and robust to losses, which has potential applications in areas such as laser interferometer gravitational-wave observatories and high-sensitivity atomic force microscopes. Full article

Photons Counted Integral Imaging (PCII) reconstructs 3D scenes with both focused and off-focused voxels. The off-focused portions do not contain or convey any visually valuable information and are therefore redundant. In this work, for the first time, we developed a six-ensembled Deep Neural Network (DNN) to identify and remove the off-focused voxels from both the conventional computational integral imaging and PCII techniques. As a preprocessing step, we used the standard Otsu thresholding technique to remove the obvious and unwanted background. We then used the preprocessed data to train the proposed six ensembled DNNs. The results demonstrate that the proposed methodology can efficiently discard the off-focused points and reconstruct a focused-only 3D scene with an accuracy of 98.57%. Full article

Double-track microstructures were induced in the bulk of a z-cut lithium niobate crystal by 1030 nm 240 fs ultrashort laser pulses with a repetition rate of 100 kHz at variable pulse energies exceeding the critical Kerr self-focusing power. The microstructure topography was characterized by atomic force microscopy in piezoelectric response mode. The spatial positions of laser-induced modification regions inside lithium niobate in the case of laser beam propagation along the crystal optical axis can be directly predicted by simple analytical expressions under the paraxial approximation. A dimensional analysis of the morphology of the double-track structures revealed that both their length and width exhibit a monotonous increase with the pulse energy. The presented results have important implications for direct laser writing technology in crystalline dielectric birefringent materials, paving the way to control the high spatial resolution by means of effective energy deposition in modified regions. Full article

In the field of compressed imaging, many attempts have been made to use the high-resolution digital micromirror array (DMD) in combination with low-resolution detectors to construct imaging systems by collecting low-resolution compressed data to reconstruct high-resolution images. However, the difficulty of achieving micrometer-level alignment between DMD devices and detectors has resulted in significant reconstruction errors. To address this issue, we proposed a joint input generative adversarial network with an error correction function that simulates the degradation of image quality due to alignment errors, designed an optical imaging system, and incorporated prior imaging system knowledge in the data generation process to improve the training efficiency and reconstruction performance. Our network achieved the ability to reconstruct 4× high-resolution images with different alignment errors and performed outstanding reconstruction in real-world scenes. Compared to existing algorithms, our method had a higher peak signal-to-noise ratio (PSNR) and better visualization results, which demonstrates the feasibility of our approach. Full article

Optical fiber sensors that have a compact size and the capability for multi-parameter sensing are desired in various applications. This article reports a miniaturized optical fiber Fabry-Perot interferometric sensor with a length of hundreds of µm that is able to simultaneously measure variations of curvature, temperature, and strain. The sensor is easy to fabricate, requiring only the fusion splicing of a short section of the silica capillary tube between two single-mode fibers (SMFs). The combined mechanism of the Fabry-Perot interference occurred in the two interfaces between the capillary and the SMFs, and the antiresonant guidance induced by the capillary tube makes the device capable of realizing multi-parameter sensing. A simplified coefficient matrix approach is developed to decouple the contributions from different parameters. In addition, the capability of the device for multiplexing is investigated, where four such prototypes with different air cavity lengths are multiplexed in a system in parallel. The spectral behavior of an individual device for measuring curvature and strain is reconstructed and investigated, showing reliable responses and little crosstalk between different devices. The proposed device is easy to fabricate, cost-effective, robust, and could find potential applications in the field of structural health monitoring and medical and human–machine interactive sensing. Full article

A physical model of an external-cavity tunable laser (ECTL) utilizing the vernier effect of a dual Fabry–Perot (FP) etalon is presented and simulated using the finite-difference traveling wave (FDTW) method. In this paper, we provide a detailed explanation of the physical principle and construction process of the model, as well as the simulation results for the laser. The model is precisely established by studying the time-dependent changes in the carrier concentration and optical field of different wavelengths inside the laser before reaching a steady state. By determining multiple parameters in the tuning region and gain region, the proposed model can calculate and predict various laser parameters, such as output power and side-mode suppression ratio (SMSR). Moreover, the FDTW method displays the change process of various parameters, such as carrier concentration and spectrum, in the convergence of various positions in the laser with femtosecond time resolution. This capability is promising for in-depth research on the inner mechanism of lasers. Full article

An ultra-broadband, compact and CMOS-compatible arbitrary ratio power splitter that is based on a directional coupler is proposed on the silicon-on-insulator (SOI) platform. The proposed device consists of an adiabatic sub-wavelength grating (ASWG) and a conventional directional coupler. The wavelength dependence is greatly reduced by introducing an ASWG in the coupling region of the directional coupler. Simulation results show that our proposed device has an operating bandwidth of 250 nm for arbitrary power splitting ratios, with a transmission power variation of less than 8.5%, covering the wavelength range from 1400 nm to 1650 nm. Meanwhile, the device footprint has been narrowed to less than 46 μm. In addition, the power splitters also exhibit a low excess loss of below 0.24 dB. Our proposed ASWG-assisted power splitters show excellent potential for application in large-scale photonic integrated circuits. Full article

A controllable helico-conical beam is proposed in this paper. The intensity patterns and the local spatial frequency of the controllable helico-conical beams in the focal region are analyzed in detail. The results show that the length of the helico-conical beams can be customized by the variable parameter k, and the angular dimension of the bored spiral trajectory is dependent on the proportion k/l. Moreover, the focal-field energy flow density and orbital angular momentum distributions of the controllable helico-conical beams are also analyzed. The proposed helico-conical beams with controllable lengths can be potentially applied in the field of optical guiding. Full article

Terahertz (THz) technology has great potential for applications in various fields, such as security imaging detection, optical communication, environmental quality monitoring, and life sciences. Most of these applications require THz detectors with high sensitivity, fast response, and a miniaturized size that can operate at room temperature. In this study, we present a graphene THz detector integrated with an asymmetric bowtie antenna. The asymmetric antenna confines the incident THz waves into the graphene active layer, leading to photocurrent generation and its directional flow. The maximum responsivity of this device can reach 19.6 V/W at 2.52 THz, with a noise–equivalent power (NEP) of 0.59 nW /Hz^0.5. Additionally, the response time is less than 21 μs, with an active area of less than 1500 μm². Such a small device enables THz imaging with a spatial resolution as small as 200 μm. These results provide a feasible way to design miniaturized and integrable two–dimensional material–based THz detectors. Full article

A new signal processing method named orthogonal signal phase multiplication (OSPM) is proposed, which is used to improve the precision of vibration measurement in a phase-modulating self-mixing interferometer (SMI). The modulated signal is acquired by an electro-optic modulator, which is placed in the external cavity. Higher measurement precision is realized by performing the phase multiplication algorithm on the orthogonal signals extracted from the harmonic components of the signal spectrum. Theoretically, the displacement reconstruction precision of OSPM is higher than that of conventional modulation methods, and it can be continuously improved by increasing the multiplication times. The feasibility and performance of the proposed method are verified by simulated signals and confirmed by experiments; the absolute error is less than 11 nm, and relative error is less than 0.75%, within the amplitude range from 661 nm to 2013 nm. This method does not involve additional optical elements, and its effectiveness meet the requirements for real-time high-precision measurements. Full article