Light Propagation Characteristics Research Articles

Depth-resolved imaging through scattering media using time-gated light field tomography.

We present a novel, to the best of our knowledge, approach to overcome the limitations imposed by scattering media using time-gated light field tomography. By integrating the time-gating technique with light field imaging, we demonstrate the ability to capture and reconstruct images with different depths through highly scattering environments. Our method exploits the temporal characteristics of light propagation to selectively isolate ballistic photons, enabling enhanced depth resolution and improved imaging quality. Through comprehensive experimental validation and analysis, we showcase the effectiveness of our technique in resolving depth information with high fidelity, even in the presence of significant scattering. The resultant system can simultaneously acquire multi-angled projections of the object without requiring prior knowledge of the media or the target. This advancement holds promise for a wide range of applications, including non-invasive medical imaging, environmental monitoring, and industrial inspection, where imaging through scattering media is critical for an accurate and reliable analysis.

Research on visible light communication channel model in underground mines

Abstract In order to effectively model the Visible Light Communication (VLC) channel in underground mines, this paper delves into the challenges of the underground environment and develops a mathematical model by analyzing the propagation characteristics of visible light. The objective is to deepen our understanding of light signal behavior in this unique setting. Most existing research focuses on visible light channel modeling in indoor environments, with limited studies on underground mines. Our model takes into account the irregular surfaces of mine walls, as well as the effects of factors such as high dust concentration, extreme temperatures and pressures, light absorption by mine walls, and obstruction caused by large machinery on the propagation of visible light signals. This paper introduces refined optical propagation models that incorporate both scattering and light absorption considerations, resulting in a maximum received power of -46.12 dBm, which is significantly lower than that in indoor environments. These models enhance our ability to analyze and improve channel performance, thereby optimizing communication efficiency.

Composite photonic lattice with a broad channel to sustain topological interface states.

In the field of topological photonics, one goal is to seek specialized structures with topological protection that can support the stable propagation of light. We have designed a topological configuration featuring a broad channel to sustain edge or interface states. The topological properties are elucidated by analyzing the energy spectrum, eigenstates, and winding numbers. Furthermore, the propagation characteristics of light within our structure are examined through the computation of intensities derived from the coupled mode equations. Our findings reveal that the structure is capable of confining light to the central region, facilitating stable and robust propagation for large-sized beams. Additionally, simulations conducted using commercial software have substantiated the theoretical analysis. Our finding may have significant implications for the modulation of structured light and the development of photonic devices with wide channel capabilities.

Modulation instability in nonidentical metamaterial waveguide arrays by graph Laplacian approach

We theoretically investigate modulation instability (MI) in a nonidentical waveguide array, which is made up of positive and negative index metamaterial waveguides. The unit cell of the optical waveguide array consists of three waveguides arranged in a triangular manner. Waveguides 1 and 3 are made up of positive index material (PIM) channels and waveguide 2 is by negative index material (NIM) channels, as a result, they show different light propagation characteristics. We model this array of waveguides using a generalized nonlinear Schro¨dinger equation, replacing the Laplacian operator with the graph Laplacian. Following linear stability analysis, we will discuss MI for different values of transverse wave number, as it determines the order of the Brillouin zone. We also discuss the effect of input power on periodic MI in normal and anomalous dispersion regimes. Thus we report a comprehensive study on the MI and hence the better ways to generate and manipulate the solitons or ultra-short pulses in NIM PIM waveguide arrays.

Estimation of Photon Path Length and Penetration Depth in Articular Cartilage Zonal Architecture Over the Therapeutic Window.

The key characteristics of light propagation are the average penetration depth, average maximum penetration depth, average maximum lateral spread, and average path length of photons. These parameters depend on tissue optical properties and, thus, on the pathological state of the tissue. Hence, they could provide diagnostic information on tissue integrity. This study investigates these parameters for articular cartilage which has a complex structure. We utilize Monte Carlo simulation to simulate photon trajectories in articular cartilage and estimate the average values of the light propagation parameters (penetration depth, maximum penetration depth, maximum lateral spread, and path length) in the spectral band of 400-1400 nm based on the optical properties of articular cartilage zonal layers and bulk tissue. Our findings suggest that photons in the visible band probe a localized small volume of articular cartilage superficial and middle zones, while those in the NIR band penetrate deeper into the tissue and have larger lateral spread. In addition, we demonstrate that a simple model of articular cartilage tissue, based on the optical properties of the bulk tissue, is capable to provide an accurate description of the light-tissue interaction in articular cartilage. The results indicate that as the photons in the spectral band of 400-1400 nm can reach the full depth of articular cartilage matrix, they can provide viable information on its pathological state. Therefore, diffuse optical spectroscopy holds significant importance for objectively assessing articular cartilage health. In this study, for the first time, we estimate the light propagation parameters in articular cartilage.

Vehicle light communication design using an efficient learning model

Vehicle communication is preferable in vehicular visible light communication (VVLC) owing to its low complexity, secure, interference-free characteristics, line of sight exploitation, vehicle light emitting diodes, and vehicle light propagation characteristics. The channels of VVLC are stochastic and give less accuracy during path loss prediction without considering external environmental factors. There is no wireless channel adopted for channel-based frequency response and node multi-access. This work concentrates on modeling a machine learning-based model for VVLC channel modeling and enhancing the accuracy by specifically concentrating on frequency response via multiple variables like environmental factors and vehicle mobility. The proposed model considers variables with the adoption of an Extreme learning model (ELM) with feed-forward neural networks (FFNN). This model performs information processing over the VLC channel to enhance prediction. The accuracy of the proposed model relies on channel modeling and multi-access relays among nodes. The proposed model performance is evaluated with prevailing approaches to produce distance prediction and response among the relay nodes. Some metrics like BER, SNR, MSE, received power, impulse, and frequency response, and received optical power are evaluated and compared with other approaches.

Photonics of plant chloroplasts

It is shown that the features of light propagation in plant leaves depend on the long-range ordering in chloroplasts and spectral characteristics of pigments. It has been established that, with regard of the dispersion of the chlorophyll absorption spectrum, the photonic density of states increases and the spectral peak shifts to the effective photosynthesis wavelength range, which enhances the probability of the photosynthesis processes.

DFFA-Net: A Differential Convolutional Neural Network for Underwater Optical Image Dehazing

This paper proposes DFFA-Net, a novel differential convolutional neural network designed for underwater optical image dehazing. DFFA-Net is obtained by deeply analyzing the factors that affect the quality of underwater images and combining the underwater light propagation characteristics. DFFA-Net introduces a channel differential module that captures the mutual information between the green and blue channels with respect to the red channel. Additionally, a loss function sensitive to RGB color channels is introduced. Experimental results demonstrate that DFFA-Net achieves state-of-the-art performance in terms of quantitative metrics for single-image dehazing within convolutional neural network-based dehazing models. On the widely-used underwater Underwater Image Enhancement Benchmark (UIEB) image dehazing dataset, DFFA-Net achieves a peak signal-to-noise ratio (PSNR) of 24.2631 and a structural similarity index (SSIM) score of 0.9153. Further, we have deployed DFFA-Net on a self-developed Remotely Operated Vehicle (ROV). In a swimming pool environment, DFFA-Net can process hazy images in real time, providing better visual feedback to the operator. The source code has been open sourced.

Long Period Grating Imprinted on a Flat-Shaped Plastic Optical Fiber for Refractive Index Sensing

A corrugated surface long period grating (LPG) was fabricated on a flat-shaped plastic optical fiber (POF) as a refractive index (RI) sensor by a simple pressing with the heat pressure and mechanical die press print method. The light propagation characteristics of an LPG imprinted on a multi-mode POF were analyzed by the method of geometrical optics. Theoretical and experimental results showed that the structural parameters of the sensor affected the RI sensing performance, and the sensor with a thinner flat thickness, a deeper groove depth of the corrugated surface LPG, and a longer LPG exhibited better RI sensing performance. When the POF with a diameter of 1 mm was pressed with the heat pressure to a flat shape with a thickness of 600 µm, an LPG with a period of 300 µm, a groove depth of 200 µm, and a length of 6 cm was fabricated on it, and the RI sensitivity of 1447%/RIU was obtained with a resolution of 5.494×10−6 RIU. In addition, the influences of the POF cladding, tilting of LPG, and bending of the sensing structure were investigated. The results demonstrated that after removing the cladding and tilting or bending the LPG, the RI sensing performance was improved. When the LPG imprinted on the flat-shaped POF was bent with a curvature radius of 6/π cm, the highest sensitivity of 6 563%/RIU was achieved with a resolution of 2.487×10−9 RIU in the RI range of 1.3330–1.4230. The proposed sensor is a low-cost solution for RI measurement with the features of easy fabrication, high sensitivity, and intensity modulation at the visible wavelengths.

Three-dimensional modelling of blur property for conventional optical microscopes

Intensity diffusion caused by optical diffraction limits the imaging resolution of conventional optical microscopes, therefore modelling and measuring the intensity transmission and distribution property of the light sources is a significant research topic in system development and pattern recognition. However, the complicated wave propagation process in optical imaging makes it difficult to provide a direct, analytical and simple mathematical model to measure the relationship between the blur degree and various camera parameters. In this study, an improved intensity transmission and distribution calculation method for conventional optical microscopes was proposed; furthermore, a simple mathematical relation between the blur degree and camera parameters was achieved based on the proposed method. First, the light intensity distribution and propagation characteristics of a conventional optical microscope were modeled based on the property of the Fresnel circular hole diffraction combined with the practical optical parameters. Second, by analyzing the property of intensity distribution and blurring imaging, a quantitative simplified mathematical relationship between the blur degree and camera parameters in optical microscope imaging was obtained, and the three-dimensional (3D) blur property in the optical imaging process was analyzed under different conditions. Third, the connection between diffractive optics and geometric optics was obtained by summarizing and generalizing the 3D blur property curve of each monochromatic light source. Finally, the proposed method was verified through a series of simulations and experiments.

The dependence of light extraction improvement on optimized surface microstructure for AlGaN-based UVC-LEDs considering TM-polarized emission

In order to improve the light extraction of AlGaN-based short wavelength ultraviolet light emitting diodes (DUC-LEDs), a type of microstructure with high aspect ratio is introduced and optimized on the AlN substrate surface. And, particle swarm optimization (PSO) algorithm is used to inverse design of the surface microstructure to maximize the light extraction efficiency (LEE). Considering that the propagation characteristics of TM-polarized light are different from that of TE-polarized light, the optical field distribution and LEE is analyzed for the UVC-LEDs with different TE-polarized component when the optimized surface microstructure is applied. Furthermore, the preparation process tolerance of the high aspect ratio structure is discussed by calculating the LED's LEE when the structural deviation occurs or morphology changes. Simulation results show that, by using the optimized surface microstructure based on parabola cone array, the LEDs' LEE is increased from 4.4% to 8.7% and from 0.4% to 3.7% for TE-polarized and TM-polarized emission, respectively. In addition, it is demonstrated that the light extraction improvement by the surface microstructure has a good tolerance to the structural deviation and morphology. The results are significant for improving light extraction and realizing high efficient short wavelength AlGaN-based UVC-LEDs by designing surface microstructures.

Theoretical and computational study of the effects of the complex chirality parameter in chiral-coated planar waveguides

A theoretical and computational study of the effects of the real and imaginary parts of the complex chirality parameter on the characteristics of light propagation in planar waveguides is presented. Symmetric and asymmetric structures using complex chiral media in the cladding region are analyzed. The guided modes are hybrid modes due to coupling between electric and magnetic fields in the chiral medium. Changes in the chirality parameter in magnitude and handedness produce changes in the dispersion curves and electromagnetic fields of the guided modes. The characteristics of the guided modes are evidenced in simulations of the transmission and absorption spectra of the structures. The results obtained indicate that the light propagation characteristics of these structures could be used to design and fabricate novel photonic devices.

Ghost Fiber Optic 3D Endoscopy

A new type of ghost fiber optic endoscopy has been proposed to obtain ghost images of three-dimensional optically transparent objects. The method is based on spatial and temporal correlation of light beams formed in a bundle of single-mode fibers exposed in the transverse direction to femtosecond laser pulses. Resolution in the depth of an object is ensured by an original algorithm to reconstruct images, which involves both the properties of femtosecond radiation and the features of light propagation in an inhomogeneous scattering medium. The effectiveness of the proposed method has been confirmed by a numerical simulation by an example of an octahedron with a layered structure.

Optical Current and Magnetic Field Sensor Using Multimodal Interference in Fiber Optics With Carbon Steel

This work proposes an experimental configuration for a magnetic field and electric current sensor using an optical fiber in a single-mode-multimode (SM-MM) structure. The method applied is based on multimodal interference effects, where the MM fiber was coated with carbon steel. A continuous-wave laser with a 1550 nm wavelength was used to feed the sensor. The signal at the MM fiber output is captured by a photodiode in order to measure the signal in terms of voltage using a digital multimeter. The alternating magnetic field causes the carbon steel coated optical fiber to oscillate, changing its light propagation characteristics and therefore the output of the transmitted light. An increase in the signal was observed at the sensor output with increases in the magnetic field. The sensitivity of the sensor was tested by placing the coil at 2 cm and 4 cm from the fiber, reaching values of 78.13 mV/mT and 79.80 mV/mT, respectively. The relationship between the optical transmission variation and the magnetic field intensity reached a linearity of 98.297%. Notably, the method proposed in this experiment is less complex than most state-of-the-art methods. Furthermore, it presents excellent characteristics of cost, compactness and structure, with potential applications for the measurement of alternating magnetic fields and electric currents.

Novel Measurement-Based Efficient Computational Approach to Modeling Optical Power Transmission in Step-Index Polymer Optical Fiber

Polymer optical fibers (POFs) are playing an important role in industrial applications nowadays due to their ease of handling and resilience to bending and environmental effects. A POF can tolerate a bending radius of less than 20 mm, it can work in environments with temperatures ranging from −55 °C to +105 °C, and its lifetime is around 20 years. In this paper, we propose a novel, rigorous, and efficient computational model to estimate the most important parameters that determine the characteristics of light propagation through a step-index polymer optical fiber (SI-POF). The model uses attenuation, diffusion, and mode group delay as functions of the propagation angle to characterize the optical power transmission in the SI-POF. Taking into consideration the mode group delay allows us to generalize the computational model to be applicable to POFs with different index profiles. In particular, we use experimental measurements of spatial distributions and frequency responses to derive accurate parameters for our SI-POF simulation model. The experimental data were measured at different fiber lengths according to the cut-back method. This method consists of taking several measurements such as frequency responses, angular intensity distributions, and optical power measurements over a long length of fiber (>100 m), then cutting back the fiber while maintaining the same launching conditions and repeating the measurements on the shorter lengths of fiber. The model derivation uses an objective function to minimize the differences between the experimental measurements and the simulated results. The use of the matrix exponential method (MEM) to implement the SI-POF model results in a computationally efficient model that is suitable for POF-based system-level studies. The efficiency gain is due to the independence of the calculation time with respect to the fiber length, in contrast to the classic analytical solutions of the time-dependent power flow equation. The robustness of the proposed model is validated by calculating the goodness-of-fit of the model predictions relative to experimental data.

Study on the variation of Bragg propagation period in elliptic bar chain structure

In this paper, the propagation of light along a long chain of two dimensional elliptic bars with infinite period is analyzed. Due to the instability of the shape of the ellipse, the propagation characteristics of light will be changed by changing the ratio of the short half axis length to the long half axis length of the ellipse. The Bragg propagation period is observed by the change of phase constant and attenuation constant with the normalized frequency. The waveguide mode, cut-off mode and leakage mode were explained by analyzing the numerical value and field distribution of the cycle results of a single layer elliptic cylinder with different ratios. The results also indicated that, as the ratio increased, the propagation period of Bragg wave became shorter and special multimodal phenomena emerged. In addition, 20 dielectric bar chain structures are used to visually show the electric field propagation.

A novel underwater image restoration method based on decomposition network and physical imaging model

Underwater image restoration is one of the significant research in marine engineering and aquatic robotics. However, due to the propagation characteristics of light and the serious turbidity in underwater, the captured images often have chromatic aberration and scattering blur, which brings great challenges to the restoration of the raw image. In this paper, a revised underwater imaging model is proposed first, which reanalyzes the generation of background light from the atmosphere to the underwater and provides important support for underwater color correction. And then a network framework via the revised model is designed, which can decompose the captured image into different components corresponding to the revised model. The proposed network consists of a decomposition architecture with residual blocks that learns a complete separation of clear image and transmittance features. These two features are used along with the raw image to predict the background light. Finally, combining three constraints of the imaging model, the proposed framework can converge rapidly along the desired direction. By comparison with the performance of the state-of-the-art algorithms, the designed network shows excellent visibility and is capable of removing water on both synthetic and real-world images in different water types.

The Role of Periodic Structures in Light Harvesting.

The features of light propagation in plant leaves depend on the long-period ordering in chloroplasts and the spectral characteristics of pigments. This work demonstrates a method of determining the hidden ordered structure. Transmission spectra have been determined using transfer matrix method. A band gap was found in the visible spectral range. The effective refractive index and dispersion in the absorption spectrum area of chlorophyll were taken into account to show that the density of photon states increases, while the spectrum shifts towards the wavelength range of effective photosynthesis.

High sensitivity temperature sensor based on a helically twisted photonic crystal fiber

Helically twisted photonic crystal fibers (HT-PCFs) provide additional opportunities for controlling the light propagation characteristics and improving the sensing performances. In this paper, a toluene and gold wire-filled HT-PCF was proposed and designed for high sensitivity temperature sensing. The influences of the structure parameters on the confinement loss, sensitivity, and resolution of the proposed HT-PCF were investigated. For the optimized HT-PCF, the average sensitivity is as high as 14.35 and 17.29 nm/℃ in the temperature range of −20 to 20 ℃ and 20 to 70 ℃, respectively. Moreover, the proposed HT-PCF-based temperature sensor is insensitive to the hydrostatic pressure. Finally, the detailed fabrication process of the toluene and gold wire-filled HT-PCF temperature sensor is proposed. It is believed that the proposed HT-PCF temperature sensor has potential applications in the fields of the environmental monitoring, medical diagnostics, etc.

Light scattering and absorption by two-dimensional arrays of nano and micrometer monodisperse spherical silver particles

The problem of light scattering and absorption is considered for a free-standing two-dimensional array (monolayer) of nano- and micrometer spherical silver particles. To solve this problem for short- and imperfect long-range ordered arrays the statistical theory of multiple scattering of waves is used. The key points of the developed semi-analytical method are described. The method is based on the quasicrystalline approximation and the multipole expansion of electromagnetic fields and the dyadic Green's function in terms of vector spherical wave functions. Results for the angular distribution of scattered light, the coefficients of coherent transmission and reflection, incoherent scattering, and absorption of a monolayer are obtained for a spectral range corresponding to the nanoparticle plasmonic resonance. Some features of light propagation through two-dimensional arrays of particles with different spatial order are investigated and discussed. The results are in close agreement with the existing experimental data for a partially ordered array and the calculations for a regular array obtained under the extended low-energy electron diffraction theory.