Refractive Index Difference Research Articles

Highly Sensitive Dual-Core Microstructured Fiber Sensor Operating Near the Dispersion Turning Point

In this paper, a highly sensitive dual-core microstructured fiber (DC-MSF) sensor is proposed. Because of the adjustable dispersion characteristic of the DC-MSF, the group refractive index (RI) difference between the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</i> -polarization odd and even supermodes could achieve the zero value at the dispersion turning point, which makes the DC-MSF sensor have a high sensitivity. The simulation results show that the sensitivity can be up to 52875 nm/RIU around the RI of 1.4, the operating bandwidth of sensitivity over 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> nm/RIU is 625 nm, and the operating bandwidth of sensitivity over 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> nm/RIU is 100 nm. Moreover, the DC-MSF can also be used as a temperature sensor by coating the polydimethylsiloxane on the surface, and the sensitivity can be up to 23.12 nm/°C around 22 °C. The detectable RI and temperature ranges of the proposed DC-MSF sensor are 1.375-1.425 and -15-95 °C, respectively.

A Compact Sensor Integrated in a Single Hollow Core Bragg Fiber for Simultaneous Measurement of Temperature and Strain

A compact sensor integrated in a single hollow core Bragg fiber is proposed and demonstrated for simultaneous measurement of strain and temperature. The sensor is manufactured by splicing a 3 mm long homemade Hollow Core Bragg Fiber (HCBF) sandwiched between two segments of Single Mode Fibers (SMFs) with a lateral offset at one end. Experimental results demonstrate that both Mach-Zehnder Interferometer (MZI) and Bragg resonant mechanism are coexisting in the HCBF-based sensor. Two sensing mechanisms have different strain and temperature sensitivities with good linearity. The axial strain sensitivities of -0.4 pm/με and 0.33 pm/με and the temperature sensitivities of 40.8 pm/ °C and 26.5 pm /°C are obtained by monitoring the wavelength shifts with axial strain and temperature in temperature range of 60-80°C. Therefore, simultaneous measurement of temperature and strain can be achieved by using a characteristic matrix approach. Due to the large refractive index difference between the core and cladding of the Bragg fiber, the length of HCBF for forming mode interference in the sensor could be greatly reduced. This makes the sensor more compact than most previous reports for simultaneous measurement of temperature and strain.

Optimized segmented cladding fiber for extreme large mode area using latin hypercube sampling

In this paper a new design of octo-wing segmented cladding fiber (SCF) with a uniform core containing a ring and a non-uniform double cladding was proposed and optimized. The Latin hypercube sampling was applied to generate design cases as the input values for estimation of the objective functions as the output variables. Three physical parameters of the core assisted ring: thickness, radius, and refractive index difference were selected as the design variables. Twenty different design cases were numerically simulated by finite element method. To optimize the effective mode area (EMA) as the objective function, response surface methodology was applied. The fiber offers the single mode operation with a confinement loss ratio of over 100 between the fundamental mode (FM) and the higher order mode (HOM). Operating at the wavelength of 1.550μm, the EMA is significantly enlarged to about 884μm2. We have investigated the properties of the proposed optimized ring assisted SCF (RA-SCF) in terms of confinement losses of FM and first HOM considering a perfect matched layer. The proposed RA-SCF provides the potential applications in compact high power fiber lasers.

Design and manufacturing of aspherical GaAs-SILs for a versatile optical semiconductor failure analysis system

In this study, we have developed an aspherical solid immersion lens (SIL) made of gallium arsenide (GaAs) for semiconductor failure analysis. A SIL is an optical component to dramatically increase the numerical aperture and the spatial resolution of a microscopy system by attaching it to the backside of the device-under-test. To further enhance the spatial resolution of the optical microscopy system, a shorter wavelength light source has also been introduced. In the past, we have developed a spherical SIL made of silicon (Si), which is the same material as the device under test. However, Si is not transparent for wavelengths shorter than around 1100 nm. To enable the use of a SIL at shorter wavelengths, we chose GaAs as SIL material, which is transparent at shorter wavelengths than Si and has a refractive index very close to that of Si. However, a spherical GaAs SIL features a more pronounced spherical aberration due to the difference in refractive index between the device-under-test (Si) and that of the SIL (GaAs) deteriorating the optical resolution of the microscopy system. We have therefore designed an aspherical GaAs-SIL which makes it possible to correct spherical aberration. We applied diamond tooling as a technique for manufacturing the SIL shape with a required surface irregularity below 100 nm peak-to-valley, resulting in a diffraction limited performance of the optical system for semiconductor failure analysis with a performance at around 0.18 μm cutoff.

An arrayed optofluidic system for three-dimensional (3D) focal control via electrowetting.

A new lens capability for three-dimensional (3D) focal control is presented using an optofluidic system consisting of n × n arrayed liquid prisms. Each prism module contains two immiscible liquids in a rectangular cuvette. Using the electrowetting effect, the shape of the fluidic interface can be rapidly adjusted to create its straight profile with the prism's apex angle. Consequently, an incoming ray is steered at the tilted interface due to the refractive index difference between two liquids. To achieve 3D focal control, individual prisms in the arrayed system are simultaneously modulated, allowing incoming light rays to be spatially manipulated and converged on a focal point located at Pfocal (fx, fy, fz) in 3D space. Analytical studies were conducted to precisely predict the prism operation required for 3D focal control. Using three liquid prisms positioned on the x-, y-, and 45°-diagonal axes, we experimentally demonstrated 3D focal tunability of the arrayed optofluidic system, achieving focal tuning along lateral, longitudinal, and axial directions as wide as 0 ≤ fx ≤ 30 mm, 0 ≤ fy ≤ 30 mm, and 500 mm ≤ fz ≤ ∞. This focal tunability of the arrayed system allows for 3D control of the lens's focusing power, which could not be attained by solid-type optics without the use of bulky and complex mechanical moving components. This innovative lens capability for 3D focal control has potential applications in eye-movement tracking for smart displays, autofocusing of smartphone cameras, or solar tracking for smart photovoltaic systems.

Simultaneous multichannel RZ to NRZ format conversion for LP01 and LP11 using a few-mode fiber Bragg grating.

We propose a novel format conversion scheme, which can implement multichannel format conversion from return-to-zero (RZ) to non-return-to-zero (NRZ) for both LP01 and LP11 simultaneously by designing a few-mode fiber Bragg grating (FM-FBG) with comb spectra. To achieve filtering for all channels of the two modes, the FM-FBG response spectra of LP11 is designed to shift with that of LP01 by the WDM-MDM channel spacing. This approach is realized by carefully selecting the specifications of the few-mode fiber (FMF) to fulfill the requirements of the effective refractive index difference between LP01 and LP11. Each single-channel outline of the FM-FBG response spectra is designed according to the algebraic difference between the RZ and NRZ spectra. Numerical results show that both LP01 and LP11 channels with 300-GHz-spaced RZ signals at 40 Gbit/s can be converted into NRZ signals simultaneously, and the converted NRZ signals have high Q-factor and their eye diagrams are clean and open.

A Weakly-Coupled Double Bow-Tie Multi-Ring Elliptical Core Multi-Mode Fiber for Mode Division Multiplexing across C+L+U Band

We herein present a weakly-coupled double bow-tie multi-ring elliptical core multi-mode fiber (DBT-MREC-MMF) supporting 22 eigenmodes for mode division multiplexing across the C+L+U band. The proposed fiber introduces a multi-ring elliptical core, bow-tie air holes, and bow-tie stress-applying areas to effectively split adjacent eigenmodes. By utilizing the finite element method (FEM), we accordingly optimized the fiber to support the 22 modes under the weakly-coupled condition. We evaluated the impact of fiber parameters on the minimum effective refractive index difference (min Δneff) between adjacent eigenmodes, model birefringence (Bm), and bending loss at a wavelength of 1550 nm. Additionally, broadband performance metrics, such as effective modal index (neff), effective index difference (Δneff), effective mode area (Aeff), differential mode delay (DMD), and chromatic dispersion (D), were comprehensively studied over the entire C+L+U band, ranging from 1530 to 1675 nm. The proposed fiber is capable of supporting 22 completely separated eigenmodes with a min Δneff between adjacent eigenmodes larger than 3.089 × 10−4 over the entire C+L+U band. The proposed DBT-MREC-MMF holds great potential for use in short-haul communication systems that require MDM to improve transmission capacity and expand bandwidth.

Dual-parameter fiber sensor based on mode-division demultiplexing BOTDR system

We present and experimentally validate a neoteric and cost-effective dual-parameter distributed fiber sensor based on a Brillouin optical time-domain reflectometry in a general four-mode graded-index few-mode fiber. The spontaneous Brillouin scattering signals of different modes in the sensing fiber can be obtained simultaneously by a low-loss mode selective photonic lantern, on account of the difference of mode refractive indexes of different modes. By characterizing and evaluating the Brillouin scattering spectrum widths and the Brillouin frequency shift to temperature and strain sensitivities of different modes, the performance of the dual-parameter distributed sensor can be optimized. The proposed sensor achieves reliable differentiation of temperature and strain on a 1 km general four-mode graded-index few-mode fiber, with the temperature and strain accuracy of 1.6 °C and 39.2 με, respectively.

Structure and property evolution of Ag-doped GexAsySe1-x-y and its potential in infrared optics

Gradient refractive index (GRIN) lenses are an excellent solution for multispectral imaging. Here, we present a method to establish a series of gradient glasses with large refractive index differences (Δn) as well as similar thermal properties (glass transition temperature, Tg). A series of selenium-based gradient glasses were developed by tailoring the Ge-As-Se composition and external doping with different amounts of silver. These sulfur-free and tellurium-free selenium-based glasses cover an ultra-wide high transmission spectrum from 0.8 to 15 μm and possess a large Δn of 0.23 and a small Tg difference of 9 °C. These gradient glasses also exhibit excellent refractive index linearity. The evolution pattern of the glass network structure and its corresponding properties are analyzed in detail with the variation of the composition.

Phase wavefront perturbation calculation model for spectroscopic refractive index matching of hybrid materials.

A low-cost flexible spectroscopic refractive index matching (SRIM) material with bandpass filtering properties without incidence angle and polarization dependence by randomly dispersing inorganic C a F 2 particles in organic polydimethylsiloxane (PDMS) materials was proposed in our previous study. Since the micron size of the dispersed particles is much larger than the visible wavelength, the calculation based on the commonly used finite-difference time-domain (FDTD) method to simulate light propagation through the SRIM material is too bulky; however, on the other hand, the light tracing method based on Monte Carlo theory in our previous study cannot adequately explain the process. Therefore, a novel approximate calculation model, to the best of our knowledge, based on phase wavefront perturbation is proposed that can well explain the propagation of light through this SRIM sample material and can also be used to approximate the soft scattering of light through composite materials with small refractive index differences, such as translucent ceramics. The model simplifies the complex superposition of wavefront phase disturbances and the calculation of scattered light propagation in space. The scattered and nonscattered light ratios; the light intensity distribution after transmission through the spectroscopic material; and the influence of absorption attenuation of the PDMS organic material on the spectroscopic performance are also considered. The simulation results based on the model are in great agreement with the experimental results. This work is important to further improve the performance of SRIM materials.

Design and analysis of a weakly coupled few-mode hollow-core U-shaped tube nested antiresonant fiber

In this work, we proposed a weakly coupled few-mode hollow-core U-shaped tube nested antiresonant fiber (FM-UARF) for the potential large-capacity mode-division multiplexing transmission without multiple-in-multiple-out (MIMO) digital signal processing. Through theoretical analysis and numerical simulation, the six-tube FM-UARF can be a good candidate for two-mode and three-mode transmission with better performance. After parameter optimization, the weakly coupled condition of effective refractive index difference (Δneff) larger than 5×10−4 can be achieved. Under the two-mode case, the confinement loss (CL) of the LP01 and LP11 modes is less than 0.005 dB/km and 0.1 dB/km in the 700 nm bandwidth range (0.9−1.6µm), respectively. In addition, LP01 achieves the lowest CL of 0.00038 dB/km at a wavelength of 1.06 µm, which gives it potential applications in high-power laser transmission. Under the three-mode case, the CL of the LP01, LP11, and LP21 modes are less than 0.006 dB/km, 0.1 dB/km, and 10 dB/km, respectively, in the wavelength range of 0.95 to 1.65 µm. Both the CL ratios are larger than 150 (23 dB), which ensures the high purity of the supported fiber modes. The results show that the proposed FM-UARF with optimized parameters has the potential in the MIMO-less large-capacity data transmission and so on.

Enhanced Schlieren System for In Situ Observation of Dynamic Light–Resin Interactions in Projection-Based Stereolithography Process

Abstract Digital maskless lithography is gaining popularity due to its unique ability to quickly fabricate high-resolution parts without the use of physical masks. By implementing controlled grayscaling and exposure control, it has the potential to replace conventional lithography altogether. However, despite the existence of a theoretical foundation for photopolymerization, observing the voxel growth process in situ is a significant challenge. This difficulty can be attributed to several factors, including the microscopic size of the parts, the low refractive index difference between cured and uncured resin, and the rapid rate of photopolymerization once it crosses a certain threshold. As such, there is a pressing need for a system that can address these issues. To tackle these challenges, the paper proposes a modified Schlieren-based observation system that utilizes confocal magnifying optics to create a virtual screen at the camera's focal plane. This system allows for the visualization of the minute changes in refractive indices made visible by the use of Schlieren optics, specifically the deflection of light by the changing density-induced refractive index gradient. The use of focusing optics provides the system with the flexibility needed to position the virtual screen and implement optical magnification. The researchers employed single-shot binary images with different pixel numbers to fabricate voxels and examine the various factors affecting voxel shape, including chemical composition and energy input. The observed results were then compared against simulations based on Beer–Lambert's law, photopolymerization curve, and Gaussian beam propagation theory. The physical experimental results validated the effectiveness of the proposed observation system. The paper also briefly discusses the application of this system in fabricating microlenses and its advantages over theoretical model-based profile predictions.

Numerical study of microstructured fiber with concentric dual rings for supporting orbital angular momentum mode propagation

A microstructured fiber with two concentric high-refractive-index rings is proposed to support more orbital angular momentum (OAM) beam propagations and OAM-based mode-division multiplexing (MDM) for improving the channel capacity and spectral efficiency. The two ring cores can be considered as two independent OAM channels without interference for enhancing more OAM channels. A finite element method (FEM) is employed to numerically study the properties of the designed fiber. A large effective refractive index difference between the adjacent OAM modes is achieved, which indicates that the propagated OAM modes can be well separated without multiple input, multiple output in each of the two rings. The calculated results demonstrate that a total of 80 OAM modes with a mode quality above 70% is achieved without any high-order radial modes from 1.5 to 1.6 μm, which contains 18 inner-ring OAM modes and 62 outer-ring OAM modes. Moreover, the designed fiber possesses low confinement loss (<2.2 × 10−7 dB m−1) and can be potentially utilized for high-capacity communications by OAM-based MDM.

Characterization of the photoelastic dispersion coefficient using polarized digital holography.

The photoelastic dispersion coefficient represents the relationship between stress and the differences in refractive indices in a birefringent material. However, determining the coefficient using photoelasticity is challenging, as it is difficult to determine the refractive indices within photoelastic samples that are under tension. Here we present, for the first time, to our knowledge, the use of polarized digital holography to investigate the wavelength dependence of the dispersion coefficient in a photoelastic material. A digital method is proposed to analyze and correlate the differences in mean external stress with differences in mean phase. The results confirm the wavelength dependence of the dispersion coefficient, with an accuracy improvement of 25% compared to other photoelasticity methods.

Proposal and numerical verification of an ultrasmall terahertz source using integrated photonic crystal waveguides for highly efficient differential frequency generation

We propose and numerically investigate integrated photonic crystal waveguides (PhC-WGs) formed in a semiconductor slab to realize an ultrasmall and highly efficient terahertz (THz) wave source. The structure consists of a straight PhC-WG with low-group-velocity and low-dispersion (LVLD) for efficient difference frequency generation (DFG) connected to two PhC-WGs to introduce two fundamental lights into the LVLD PhC-WG. The fundamental light propagating through each PhC-WG designed to enhance their electric fields by the slow-light effect is efficiently coupled to the LVLD PhC-WG owing to the reduced refractive index differences at the boundaries of the heterostructures. The DFG from the two fundamental lights was numerically simulated, and a temporal intensity oscillation corresponding to the difference in frequency was clearly observed. By comparing the DFG intensities of the integrated structures with an LVLD PhC-WG and a strip WG, the estimated DFG intensity from the LVLD PhC-WG was more than 100 times higher than that from the strip WG. These results indicate the effectiveness of the proposed heterostructure in the application of a highly efficient THz source with an ultrasmall footprint compared with conventional materials.

Multi-Parametric Birefringence Control in Ultrashort-Pulse Laser-Inscribed Nanolattices in Fluorite

An ultrashort-pulse laser inscription of embedded birefringent microelements was performed inside bulk fluorite in pre-filamentation (geometrical focusing) and filamentation regimes as a function of laser wavelength, pulsewidth and energy. The resulting elements composed of anisotropic nanolattices were characterized by retardance (Ret) and thickness (T) quantities, using polarimetric and 3D-scanning confocal photoluminescence microscopy, respectively. Both parameters exhibit a monotonous increase versus pulse energy, going over a maximum at 1-ps pulsewidth at 515 nm, but decrease versus laser pulsewidth at 1030 nm. The resulting refractive-index difference (RID) Δn = Ret/T ~ 1 × 10-3 remains almost constant versus pulse energy and slightly decreases at a higher pulsewidth, generally being higher at 515 nm. The birefringent microelements were visualized using scanning electron microscopy and chemically characterized using energy-dispersion X-ray spectroscopy, indicating the increase of calcium and the contrary decrease of fluorine inside them due to the non-ablative inscription character. Dynamic far-field optical diffraction of the inscribing ultrashort laser pulses also demonstrated the accumulative inscription character, depending on the pulse energy and the laser exposure. Our findings revealed the underlying optical and material inscription processes and demonstrated the robust longitudinal homogeneity of the inscribed birefringent microstructures and the facile scalability of their thickness-dependent retardance.

Spider Silk as a Flexible Light Waveguide for Temperature Sensing

We propose an approach to utilize the natural spider dragline silk (SDS) as a flexible light waveguide integrated into an optic device. SDS is utilized as an air-clad biocompatible light waveguide in the fabricated multimode interferometer. Based on the evanescent field coupling theory, the light propagating in the microfiber can be coupled into SDS, and then coupled to another microfiber connected with a spectrometer. The difference in the effective refractive index of the modes propagating in the silk fiber changes in different external environments, thus the output multimode interference spectrum shifts with the change of environment. Accordingly, the multimode interferometer is encapsulated in polydimethylsiloxane (PDMS), a kind of flexible and temperature-sensitive material, which makes the interferometer have competitive temperature sensitivity. With the increase in temperature, the refractive index of PDMS decreases, and thus the output multimode interference spectrum blue shifts. The interferometer shows repeatability, reversibility and stability in temperature sensing, with a sensitivity of −1.15 nm/°C. The presented interferometer proves that the SDS is a natural independent light guide with good mechanical properties to be applied in the fabrication of integrated optic devices.

Weakly coupled graded index heterogeneous nineteen-core few-mode fiber.

We propose a novel heterogeneous nineteen-core four-mode fiber. The heterogeneous core arrangement and trench-assisted structure can significantly suppress inter-core crosstalk (XT). In order to control the number of modes in the core, a low refractive index area is introduced in the core. The number of LP modes and the effective refractive index difference (Δneff) of adjacent modes in the core are controlled by changing the refractive index distribution of the core and the parameters of the low refractive index area in the core. And the mode state of low intra-core crosstalk is successfully realized in the graded index core. After the optimization of fiber parameters, each core can stably transmit four LP modes under the optimal fiber parameters, and the inter-core crosstalk of LP02 mode is less than -60 dB/km. Finally, the effective mode area (Aeff) and dispersion (D) of nineteen-core four-mode fiber in C+L band are described. The results show that the nineteen-core four-mode fiber is suitable for terrestrial and undersea communication systems, data centers, optical sensors and other fields.

Light-Matter Interactions Revealing Load-Induced Phase Mobility in Elastomers.

Elastomers with segmental microstructure are a fascinating class of shock-tolerant and impact-resistant materials. However, their technological potential remains untapped due to a vague understanding of the molecular contributions to their superior mechanical behavior. Herein, in situ light-matter interactions, to reveal the extent of microstructural mobility by temporally exploiting molecular processes during creep response, are leveraged. The segmental microstructure comprises aromatic hard domains embedded within an aliphatic soft matrix. High-resolution digital image correlation reveals the development of strain striations, mild anisotropy, and the mechanisms responsible for domain mobility, where the rate of hard segment mobility is found to be 60% slower than that of the soft segment. Terahertz spectral analyses pinpoint the contributions of interchain hydrogen bonding of the hard segments and their significant conformational changes by observing spectral features at ≈1.2THz and ≈1.67THz. Moreover, the domain mobility is examined using experimental and computational light scattering approaches, uncovering dynamic scattering and elucidating the difference in the complex refractive index of the soft and hard segments. The study unlocks the pathway for quantitative measurements of elusive molecular mobility and conformational changes during mechanical loading and sheds light on the origin of the shock tolerance in some elastomeric polymers with segmental microstructure.

Trench-assisted 12-core 5-LP mode fiber with a low refractive index circle and a high refractive index ring.

We propose a homogeneous five-mode twelve-core fiber with a trench-assisted structure, combining a low refractive index circle and a high refractive index ring (LCHR). The 12-core fiber utilizes the triangular lattice arrangement. The properties of the proposed fiber are simulated by the finite element method. The numerical result shows that the worst inter-core crosstalk (ICXT) can achieve at -40.14 dB/100 km, which is lower than the target value (-30 dB/100 km). Since adding the LCHR structure, the effective refractive index difference between LP21 and LP02 mode is 2.8 × 10-3, which illustrates that the LP21 and LP02 modes can be separated. In contrast to without the LCHR, the dispersion of LP01 mode has an apparent dropping, which is 0.16 ps/(nm·km) at 1550 nm. Moreover, the relative core multiplicity factor can reach 62.17, which indicates a large core density. The proposed fiber can be applied to the space division multiplexing system to enhance the fiber transmission channels and capacity.