Refractive Index Difference Research Articles

Compact silica dual-band wavelength demultiplexer based on asymmetric-defined multimode interference coupler

Limited refractive index difference of silica waveguide brings great size challenges in wavelength demultiplexing. A silica dual-band wavelength demultiplexer (DBWD) based on an asymmetric multimode interference coupler (MMI) is demonstrated. The selective separation and output of target dual bands can be implemented using the proposed design method based on asymmetric-defined MMI couplers. The beam propagation method is adopted to verify the proposed design principle. Standard CMOS fabrication is used in demultiplexer preparation. At λ=1350 nm, insertion loss (IL) and crosstalk (CT) at port Output1 are 1.94 dB and −24.92 dB, respectively. At λ=1550 nm, IL and CT at port Output2 are 2.44 dB and −29.00 dB, respectively. The 3-dB bandwidth (BW3 dB) for Output1 and Output2 are 82 nm (1297-1379 nm) and 87 nm (1492-1579 nm), respectively. The corresponding CT for Output1 and Output2 are < −7.16 dB and < −12.9 dB, respectively. Due to the introduction of asymmetric MMI coupler, the footprint is reduced to 0.1 mm2 (25 µm × 4000 µm). Because of the periodic characteristic, the wavelength demultiplexing can be extended from O/C bands to E/L bands by a reshaped asymmetric-MMI coupler. Even by combining more asymmetric-MMI couplers with different etched sections, all 18 channels of coarse wavelength division multiplexing (CWDM) transmission (1270-1610 nm) can be covered. The favorable manufacturing tolerance facilitates large-scale integration and mass production.

Design of polarization-maintaining fiber with uniform doping concentration supporting 14 weakly coupled modes based on discrete point configuration optimization

In this work, we propose a polarization-maintaining, weakly coupled few-mode fiber with a uniform doping concentration, designed via a particle swarm optimization algorithm and a discrete point configuration method. The design features a circular central air hole and an irregular doped boundary. Across the C+L band, all 14 modes exhibit an effective refractive index difference exceeding 1.43×10−4. Compared to conventional polarization-maintaining few-mode fibers, the proposed design offers enhanced flexibility, ensuring mode isolation with a very non-complicated process. This approach can be readily extended to other inhomogeneous optical waveguides, offering a valuable framework for intelligent design in non-uniform waveguide systems.

Stretching the Limits of Refractometric Sensing in Water Using Whispering-Gallery-Mode Resonators

A novel application of microresonators for refractometric sensing in aqueous media is presented. To carry out this approach, microspheres of different materials and sizes were fabricated and doped with Nd3+ ions. Under 532 nm excitation, the microspheres presented typical NIR Nd3+ emission bands with superimposed sharp peaks, related to the Whispering Gallery Modes (WGMs), due to the geometry of the microspheres. When the microspheres were submerged in water with increasing concentrations of glycerol, spectral shifts for the WGMs were observed as a function of the glycerol concentration. These spectral shifts were studied and calibrated for three different microspheres and validated with the theoretical shifts, obtained by solving the Helmholtz equations for the electromagnetic field, considering the geometry of the system, and also by calculating the extinction cross-section. WGM shifts strongly depend on the diameter of the microspheres and their refractive index (RI) difference compared with the external medium, and are greater for decreasing values of the diameter and lower values of RI difference. Experimental sensitivities ranging from 2.18 to 113.36 nm/RIU (refractive index unit) were obtained for different microspheres. Furthermore, reproducibility measurements were carried out, leading to a repeatability of 2.3 pm and a limit of detection of 5 × 10−4 RIU. The proposed sensors, taking advantage of confocal microscopy for excitation and detection, offer a robust, reliable, and contactless alternative for environmental water analysis.

SnHPO4: A Layered Tin(II) Phosphate with Enhanced Birefringence.

As promising optoelectronic functional materials in the short-wavelength spectral region, such as ultraviolet (UV) and deep UV, phosphates have recently received increased attention. However, phosphate materials commonly suffer from limited birefringence owing to the highly symmetrical PO4 tetrahedra. We herein report a layered tin(II) phosphate with improved birefringence. By employing a polarizing microscope, the measured refractive index difference determined on a (010) wafer is 0.042@550 nm, which is very close to the calculated refractive index difference of 0.039@550 nm between nx and nz through the density functional theory (DFT) method. The largest birefringence appears on the (100) plane, which is theoretically determined to be 0.078@550 nm. Single crystals of SnHPO4 measuring 20 × 4 × 3 mm3 can be easily grown by a hydrothermal method. In addition, SnHPO4 is UV transparent with a short UV absorption cutoff edge of 242 nm and an optical band gap of 4.50 eV, implying that it could be a potential UV birefringent material.

Research on a Low-Loss and Low-Crosstalk Supermode Demultiplexer

ABSTRACT A novel demultiplexer with mode-degenerate output (DMDO) for the six-core supermode fiber (SSF) using finite element method (FEM) and beam propagation method (BPM) is proposed, which can achieve the multiple-input multiple-output free (MIMO-FREE) operations. The DMDO is composed of four directional mode-selective couplers, where the pure silica fiber cores in the couplers significantly minimize fiber attenuation and splicing loss. The main channel with the SSF core (SSFC) supports the supermodes LP01, LP11, LP21, and LP31, where large effective refractive index difference (ERID) between neighboring supermodes enables low crosstalk. The DMDO can achieve mode-degenerate outputs of four supermodes LP01, LP11, LP21, and LP31, which solves the mode rotation issue in the propagation system. In the range of 1530–1565 nm, extinction ratios (ERs) of supermodes LP01, LP11, LP21, and LP31are respectively maintained above 27.48, 29.07, 28.18, and 36.65 dB, while the ER of supermode LP31 is up to a maximum of 39.25 dB. The corresponding coupling efficiencies (CEs) can respectively reach −0.42, −0.22, −0.17, and −0.40 dB. According to principle of light wave reversibility, the DMDO can be used as a multiplexer.

Low-loss weakly coupled four-mode hollow-core antiresonant fiber based on centrosymmetric nested structure

Abstract This paper designs a novel centrosymmetric double-layer nested antiresonant fiber (CDNAF) with few-mode transmission characteristics. The cladding structure of the CDNAF incorporates glass plates and multilayer capillaries, effectively mitigating the coupling between various core modes and cladding modes. Simulation results show that the CDNAF can support four modes of low-loss transmission from LP01 to LP02 at 1550 nm. The confinement loss (CL) of the four modes is less than 0.008 dB/km, 0.07 dB/km, 0.42 dB/km, and 4.8 dB/km, respectively, and the CDNAF has a sizeable differential group delay (DGD) and weak bending sensitivity. The loss of high-order modes is much greater than that of the other four transmission modes, ensuring high-purity transmission of the four modes. In the wavelength range of 1200–1700 nm, the CDNAF ensures few-mode characteristics with at least two low-loss transmission modes. Furthermore, the effective refractive index difference (Δneff) between adjacent modes is much greater than 10^(-4), eliminating or significantly reducing the need for multiple-input multiple-output digital signal processing (MIMO-DSP) techniques in optical communication systems. Additionally, the results demonstrate that CDNAF can transmit three modes at an ultimate bending radius of 5 cm and can transmit four modes like a straight fiber at a bending radius of 18 cm, which demonstrates that CDNAF has excellent bending performance. These excellent characteristics give the CDNAF great potential for application in large-capacity optical communication systems.

Iridescent structural color by using ultra-low refractive index aerogel as optical cavity dielectric

Iridescent color-shift pigments have been used in some industrial applications, e.g., for cosmetics and packaging. To achieve environmental-friendly and lasting color, thin-film interference is used to generate structural color. By maximizing the refractive index (RI) difference between the thin films (i.e., using an ultralow RI film), super-iridescent structural color can be produced. While the lowest refractive index of a naturally occurring solid dielectric is close to 1.37 (i.e., MgF2), we synthesized highly porous dielectric SiO2 aerogel to achieve ultralow-RI (n ~ 1.06) and demonstrated a high-refractive index/low-refractive index/absorber (HLA) trilayer structural color. The achieved structural color is highly iridescent and capable of tracing a near-closed loop in CIE color space. By tuning the refractive index, thickness, and geometry of the aerogel layer, we control the reflection dip’s shape, therefore producing a wide range of vivid and iridescent colors.

Research on center-assisted ring-core few-mode fiber with an eccentric circle for mode degeneracy separation in space division multiplexing

An eccentric-circle-assisted ring-core fiber (ECRF) structure is proposed to effectively separate spatial-degenerated linear polarization (LP) modes while maintaining birefringence at the 10−6 level, which provides more degrees of freedom in the design of few-mode fiber employment in space division multiplexing (SDM) systems. Our simulation analysis demonstrates that the effective refractive index difference (Δneff) between the spatially degenerate modes in this fiber falls within the range of (2.44−6.42)×10−4 at 1530–1565 nm, given the refractive index difference level typical of conventional fiber core and cladding. In addition, the polarization separation level for each mode is on the order of 10−6 and below. The design requirements of significant separation of spatial degeneracy and basically no separation of polarization degeneracy are achieved. Based on the characteristics of spatially degenerate mode in this fiber, we extend the regulation law for spatially degenerate mode separation of the LP m n mode groups in center-assisted ring-core fibers. In comparison to the existing center-assisted ring-core fiber structure, the ECRF can further reduce the fabrication difficulty and increase the feasibility of preparation.

Low Loss Chip‐to‐Chip Couplers for High‐Density Co‐Packaged Optics

An experimentally demonstrated, vertical chip‐to‐chip evanescent coupler between silicon nitride (SiN) and silicon (Si) is presented with the coupler loss measured to be 0.39 1.06 dB at 1550 nm with a 1‐dB bandwidth of 160 nm extending across the C‐band, S‐band, and L‐band (1480–1640 nm). The average coupling loss is determined to be 0.73 dB for the 1480–1640 nm wavelength range with a 2σ tolerance of 0.92 dB. The 1‐dB lateral alignment tolerance is 1.56 0.14 μm at 1550 nm and the average tolerance is 1.38 0.24 μm across the 1480–1640 nm wavelength regime. In addition, the average coupling loss varies by less than 0.35 dB and the average 1‐dB alignment tolerance varies by less than 30 nm for temperatures varying from 23 to 60 °C. Finally, the average coupling loss range is less than 1.5 dB range across four sets of identically packaged die. This is the first experimental demonstration of an interchip, passively assembled evanescent coupler using standard complementary metal‐oxide‐semiconductor foundry processes for directly coupling between Si and SiN, overcoming a waveguide refractive index difference of = 1.32 without requiring taper tip widths of less than 100 nm.

Dispersion-engineered InGaAsP/InP waveguide for high efficiency all-optical on-chip wavelength conversion

InGaAsP/InP waveguide platform offers superiority for all-optical on-chip wavelength conversion due to strong optical nonlinearity. However, a small refractive index difference between the core and cladding makes it challenging to achieve sufficient structural dispersion for phase matching. In this work, the InGaAsP multilayer waveguide and the SiN-cladded InGaAsP waveguide are investigated for optimized wavelength conversion at telecom wavelengths. Structural dispersion is effectively engineered through the incorporation of additional slot layers and a heterogeneous passivation layer. Through comparative analysis, the wavelength conversion efficiency of -4.7 dB is achieved by the SiN-cladded waveguide, which shows an improvement of 22 dB compared to traditional nanowire waveguide.

Thermal Shielding Gd3TaO7-Based Thermal Barrier Ceramic with Ultralow NIR Transmittance.

Most thermal barrier coating materials exhibit transparent/semi-transparent properties at higher temperatures, causing the surface heat flow to directly heat the substrate with infrared radiation, which significantly reduces the thermal barrier effectiveness. Herein, composite ceramic materials composed of GdFeO3 diffusely dispersed within the Gd3TaO7 are produced. Specifically, the 0.9Gd3TaO7/0.1GdFeO3 composition demonstrates an ultra-low near-infrared (NIR) transmittance of less than 0.1% across the 400-2500nm range. The introduction of variable-valence behavior and oxygen vacancies contribute to the narrow bandgap of GdFeO3. The incorporated GdFeO3 produces extra multimode vibrations, resulting in the strong infrared emissivity of Gd3TaO7/GdFeO3 composite ceramics. Besides, the refractive index difference between Gd3TaO7 and GdFeO3 can contribute to the potential photons scattering within the composites, severely impeding the infrared radiation penetration. The upward curvature of the thermal conductivity curve of 0.9Gd3TaO7/0.1GdFeO3 at high temperature is significantly suppressed, confirming its excellent IR radiation shielding properties. Moreover, the Gd3TaO7/ GdFeO3 composite ceramic exhibits thermal suitability, with a thermal expansion coefficient (9.47-9.67 × 10-6 K-1 at 1400°C) comparable to YSZ. These combined benefits position the Gd3TaO7/ GdFeO3 composite ceramic system as a promising candidate for the development of infrared radiation shielding and high-emissivity thermal radiation materials.

Beyond two-octave coherent OAM supercontinuum generation in air-core Ge-doped ring fiber

Orbital angular momentum (OAM) has emerged as a revolutionary technology for communication networks due to its ability to significantly increase the channel capacity. However, traditional optical fibers present significant hurdles to harnessing OAM’s full potential, including dispersion and limited bandwidth, which facilitates investigations on supercontinuum (SC) generation for OAM beams. In this paper, an air-core Ge-doped ring fiber is proposed and designed to support high-order OAM mode up to |l| = 24. To achieve this, the fiber has a high refractive index difference between the ring core and the cladding, enabling stable transmission of high-order OAM modes. A key feature of this design is the OAM24,1 mode, which exhibits near-zero and flat dispersion. This characteristic translates into high coherence and a remarkably broad SC generation. The generated SC spans over two octaves (2336 nm) within the infrared wavelength range (764 nm to 3100 nm) at a power level of −40 dB. Furthermore, by optimizing the structural parameters, we ensure near-zero and flat dispersion characteristics for the other OAM modes (|l| < 24), along with broad SC generation exceeding two octaves. This fiber design holds significant promise for future advancements in OAM beam transmission within the infrared spectrum.

Spatial and reconfigurable control of photoluminescence from single-layer MoS2 using a strained VO2-based Fabry–Pérot cavity

We investigated the photoluminescence (PL) from single-layer MoS2 on VO2 platelets grown on SiO2, where the insulating and metallic phases can coexist above a bulk transition temperature of 340 K, due to the inhomogeneous strain. We found that the intensity of PL from MoS2 on metallic VO2 is higher than that on the insulating counterpart, resulting in spatially varying PL even at the sub-micrometer scale. In contrast to the intensity, the PL peak energies were observed to be nearly identical on insulating and metallic VO2, indicating that the influences of charge transfer, strain, and dielectric screening on MoS2 are comparable, regardless of the phase state. Thus, the observed difference in PL intensity is due to the difference in refractive indices of insulating and metallic VO2, leading to the phase-dependent Fabry–Pérot interference effect. We performed numerical simulations for the emission from MoS2 supported on the VO2-based Fabry–Pérot interferometer. The calculated emission intensity ratio on insulating and metallic VO2 well reproduces the experimental observations. These results suggest a strategy for controlling PL from two-dimensional semiconductors in a spatial and reconfigurable manner.

High capacity performance signature of micro optical mechanical system switches based fiber Bragg grating scheme implementation in optical WDM data routed center networks

Abstract This paper demonstrated the high capacity performance signature of micro optical mechanical system switches based fiber Bragg grating scheme in optical wavelength division multiplexing data routed center networks. The FBG reflectivity is simulated versus near infrared operating wavelength region with relative refractive index difference of Δn = 0.1 %. Required optical pump power, temperature increment and FBG power dissipation or consumption are demonstrated versus FBG wavelength spacing at pumping wavelength of 1,480 nm for doped/undoped fiber. Required FBG mechanical switching voltage, and required FBG mechanical/thermal switching time are clarified at pumping wavelength of 1,480 nm for doped/undoped fiber. Grating period and Bragg wavelength is studied against effective refractive silica/polymer FBG index.

Optimization on the design of nano-patterned ZnS:Cu LED surface using FDTD simulation

Communication technology is one of the most important parts of human history and is fast developing. Visible Light Communication (VLC) can be categorized as a Light Fidelity (Li-Fi) communication that uses visible light through a Light Emitting Diode (LED) to transfer data and information. Even though LEDs are considered as very good light sources, they also have problems with effectiveness, which is influenced by their surface structures. The LEDs must also be able to focus the transmitted data and greatly reduce the signal-to-noise ratio. One of the problems encountered with LEDs is the presence of Total Internal Reflection (TIR) due to the large difference in refractive index between the LED and air, which causes photons to be trapped in the LED. Some trapped photons' energy may change into heat, thus reducing the LED's Light Extraction Efficiency (LEE). Using nanopatterns on the surface of LEDs is one way to reduce TIR on LEDs and enhance photon extraction from LEDs. Many parameters can influence the performance of nanopatterns on LEDs, so modelling efforts are needed before fabrication to save time and cost. Ansys Lumerical FDTD can be used to simulate and model the effects of nanopatterns on LED surfaces on LEE and beam focusing effect. In this study, simulations were carried out using Ansys Lumerical FDTD with variations in nanopattern shape parameters in the form of grating, blaze grating, triangular grating, and hemisphere. Apart from that, variations were also made to the height and width parameters of the grating to see the effect of these parameters on the efficiency of the LED. The FDTD simulation codes were validated using the Ansys database. The optimization results show that the most optimum shape is the grating nanopattern with a width of 216 nm and a height of 300 nm, which produces a peak wavelength of 480 nm in the far field pattern and has the highest increase in the LEE of 300 times in ±15° and 5500 times in ±5°. The huge spike in LEE enhancement at ±5° indicates that the nanopattern caused a focusing effect. The simulation results were compared using previous experimental data of Fabricated ZnS:Cu LED. It is shown that the simulation results are in line with the experimental data. The result shows that the simulation is very useful in designing the nano-patterned ZnS:Cu LED surface to achieve the best performance in the LEE and LED focusing effect for a specific application such as the VLC.

INNV-05. DEVELOPMENT OF A LABEL-FREE PLASMONIC OPTICAL BIOSENSOR FOR INTRAOPERATIVE DETECTION OF TUMOR TISSUE IN GLIOBLASTOMA

Abstract Safe maximal resection of patients with glioblastoma remains a surgical challenge due to the invasive nature of the tumor and the difficulty in distinguishing tumor margins during resection. Several tools are currently used for this purpose, such as, confocal laser endomicroscopy and fluorescence-guided surgery. In this context, biosensors based on plasmonic technology using the extraordinary optical transmission (EOT) phenomenon can help to distinguish between tumor and surrounding tissue based on their optical properties. We developed a biosensor utilizing EOT through a nanostructured gold matrix with 220 nm diameter holes and conducted a prospective study in 35 GBM patients. Paired samples of tumor and peritumoral tissue were collected during surgery. Each sample’s optical imprint was analyzed, revealing significant differences in refractive indices (RI). Tumor tissue showed higher RI values (1.350, IQR 1.344–1.363) compared to peritumoral tissue (1.341, IQR 1.339–1.349) with a p-value of 0.0047. The receiver operating characteristic (ROC) curve indicated the biosensor’s ability to differentiate the tissues (AUC = 0.8779, p &amp;lt; 0.0001). An optimal RI cut-off point of 0.003 was determined, with the biosensor achieving 81% sensitivity and 80% specificity. Our results indicate that this plasmonic technology-based biosensor could have a significant impact on the surgical treatment of glioblastoma by providing a label-free tool for tumor margin discrimination and improving the accuracy of glioblastoma surgery. Future research will focus on optimizing the nanoarchitecture of the biosensor to further improve its sensitivity and specificity, as well as exploring its applicability to other cancer types by analyzing the biomarkers responsible for the different RIs of the footprints.

Ultra-broadband multimode fiber-to-chip edge coupler based on periodically segmented waveguides.

Mode-division multiplexing (MDM) technology demonstrates a bright outlook for enhancing the capacity of chip-scale or fiber-based optical communication. Nevertheless, the fiber-to-chip MDM optical interconnects are hindered by the considerable mode mismatch and inter-modal cross talk between the few-mode fiber (FMF) and on-chip few-mode waveguide (FMW). In this Letter, a new, to the best of our knowledge, multimode coupling solution based on periodically segmented waveguides for the MDM system is proposed, which achieves efficient conversion between LP01, LP11a, and LP11b modes in FMF and E11, E21, and E12 modes in FMW with low refractive index difference. The simulation results show that the coupling loss is less than 0.41, 0.27, and 0.90 dB for the three modes, over the wavelength range of 1100-1800 nm. The fabricated device based on a polymer platform shows low fiber-to-chip coupling losses of less than 1.8, 1.7, and 3.0 dB, respectively, over a 130 nm wavelength. The presented scheme provides a competitive solution for realizing the ultra-efficient integration of prospective fiber-chip optical interconnections and communications.

Gradient refractive index tellurite glass-ceramics for miniaturized and lightweight MWIR imaging

Tellurite glasses are highly anticipated for creating gradient refractive index (GRIN) lenses, which have the potential to be applied in the lightweight and miniaturization of optical imaging systems to improve their performance. However, it is still challenging to precipitate pure crystalline phases in tellurite glasses after heat treatment to control the refractive index via spatial variation of nanocrystal distribution. Here, we present a novel method for fabricating axial telluride GRIN glass via a gradient heat-treatment technique. Through the selection of glass composition and control of the heat treatment process, pure phase Bi4TeO8 nanocrystals with a high refractive index (n) can be precipitated in telluride glass with a lower refractive index. This allows the axial telluride GRIN structure of maximum refractive index difference Δn ∼ 0.1 to be obtained by modulating the spatial distribution and concentration of Bi4TeO8 nanocrystals. This work not only demonstrates the feasibility of producing GRIN lenses from tellurite glass but also lays the foundation for the future development of medium wave infrared (MWIR) imaging.

Numerical Analysis of Optical Fiber Refractive Index in the Miniaturized Graded‐Index Fiber Probe

ABSTRACTWe analyze the impact of the refractive index of optical fibers on the focusing properties of a miniaturized graded‐index (GRIN) fiber probe. The ABCD ray transfer matrix and characteristic parameters are employed to characterize the working distance, focusing spot size, and the depth of field effectively. The three‐dimensional (3‐D) function diagrams and two‐dimensional (2‐D) graphs are used to illustrate the impact of the no‐core fiber (NCF) refractive index and the center refractive index of GRIN fiber on the focusing properties. Numerical analysis results show that when the length of the NCF is 0.36 mm and the GRIN fiber is 0.1 mm, the variation of the refractive index of the NCF between 1.44 and 1.52 leads to the variation of the working distance in the range of 0.02 mm, while the focusing spot size varies in the range of 5 μm. A comparison of 12 probe samples reveals that a 0.02 difference in the center refractive index of GRIN fiber could result in a 0.16 mm variation in working distance and a variation in focusing spot size of over 3 μm. The experimental results indicate that alterations in the length of the fibers have a considerable effect on the focusing properties of the probe. In contrast, the range of variation in focusing properties with fiber refractive index is relatively limited.

Forward design method for the design of panda polarization-maintaining few-mode optical fiber based on artificial neural network.

Panda polarization-maintaining few-mode optical fiber (PPMFMOF) has important research significance in the short distance optical transmission field owing to its advantages of weak nonlinear effects, which is benefit to reduce the use of digital signal processing equipment. Designing a high-performance PPMFMOF quickly and efficiently is expected and yet challenging. In this article, we demonstrated a forward design method for the design of PPMFMOF based on artificial neural network (ANN) to solve the problems of inefficient and time-consuming PPMFMOF design in traditional design method. By studying the influence of different ANN models on the fiber performance, the approximate range of the optimal value was obtained in advance, then the minimum effective refractive index difference (Δneff,min) between adjacent LP modes was used as the optimization object, finally design of PPMFMOF supporting 10 LP modes in C + L band was successfully realized. This method provided low time-consuming, high-efficiency and high-accuracy for the fast design of PPMFMOF and the maximum mean absolute percentage error (MAPE) of the ANN model to predict the effective refractive index (neff) of 10 LP modes is only 3.2211 × 10-7. We believe that the proposed method could also be quickly and accurately applied to other functional optical fiber designs.