Effective Refractive Index Difference Research Articles

A Reduction of the Half-Wave Voltage for Lithium Niobate Electro-Optic Mach-Zehnder Modulator

In this research, it proposes theoretically model to controlling on half-wave voltage of modulator, in which it uses studying the factor of effective of refractive index difference, this using analysis distribution of refractive index for LN material before and after voltage is applied (i.e., with and without applied voltage), and uses analysis a factor of propagation constant difference ??, before and after voltage is applied where it depends on the wavelength, these leads to it controls on the half-wave voltage V? using a reduction half-wave voltage up to 1V. Also, it achieves an excellent solution for solve the fundamental optical mode with effective factor is neff. Finally, the modulator with an effect effective refractive index difference have lower applied voltage up to 10V, and with an effective propagation constant difference the operating voltage is from value of 20V.

Titanium Nitride Modified Fiber Optic Interferometer for Refractive Index Sensitivity Enhancement.

As one of the most well-established biocompatible transition metal nitrides, titanium nitride (TiN) has been widely applied for fiber waveguide coupling device applications. This study proposes a TiN-modified fiber optic interferometer. Benefiting from the unique properties of TiN, including ultrathin nanolayer, high refractive index, and broad-spectrum optical absorption, the refractive index (RI) response of the interferometer is greatly enhanced, which is desired all the time in the field of biosensing. The experimental results show that the deposited TiN nanoparticles (NPs) can enhance the evanescent field excitation and modulate the effective RI difference of the interferometer, which eventually results in the RI response enhancement. Besides, after incorporating the TiN with different concentrations, the resonant wavelength and the RI responses of the interferometer are enhanced to varying degrees. Benefitting from this advantage, the sensing performances, including sensitivity and measurement range, can be flexibly adapted based on different detection requirements. Since RI response can effectively reflect the detection ability of biosensors, the proposed TiN-sensitized fiber optic interferometer can be potentially applied for high-sensitive biosensing applications.

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.

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.

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.

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.

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.

A novel few-mode multi-core fiber with large effective mode area and low inter-core crosstalk

This paper proposes a novel 7-core 5-mode fiber with large mode area and low inter-core crosstalk(XT). The proposed optical fiber, by adopting a combination of air-trench structure and high-index ring, realizes large mode area and low XT. The numerical analyses reveal that the proposed fiber can stably transmit 5 LP modes in the C+L band; the effective mode area of each of the 5 LP modes is larger than 110 µm2; the XT of in each of the 5 LP modes is smaller than −79 dB/km; each mode has an effective refractive index difference bigger than 1.1 × 10−3. Therefore, a low XT result can still be achieved when the mode area is large. The proposed optical fiber may be well utilized to construct a high-capacity fiber optic transmission system.

Strain and curvature optical fiber laser sensor based on an accurate RI-regulated FMF with high modal power balance

In this paper, a ring fiber laser sensor based on a three layered-core few modes fiber (TL-FMF) is proposed. The home-made TL-FMF is spliced with two pieces of single mode fiber (SMF) at both sides as the sensing and filtering component. In the TL-FMF, an intermediate with higher refractive index (RI) is sandwiched between a central dip and a sub-high RI layer. Due to the special RI, the TL-FMF has an equidistant modal effective RI difference and a balanced modal power of different modes, enabling a highly uniform interference spectrum. Then applying the TL-FMF in proposed ring fiber laser sensor, the sensing experimental results show that its sensitivities in strain and curvature are −2.78 pm/με and −6.17 nm/m−1, respectively, and shows an excellent characteristic of temperature insensitivity. This fiber laser sensor implements the dual-parameter measurement with anti-cross sensitivity, which offers a promising candidate in fiber fabrication monitoring, deformation measurement and bending detection.

Polymer negative curvature ring-core fiber for OAM modes guidance.

In this paper, a novel, to the best of our knowledge, polymer-based negative curvature ring-core fiber (NC-RCF) is proposed and investigated. The hollow-core NC-RCF is composed of TOPAS as background material. The inner and outer negative curvature structure layers are connected to the annular area, and the orbital angular momentum (OAM) modes can propagate in the annular core. In the frequency region of 1.0-1.5THz, the designed NC-RCF can stably transmit 82 OAM modes. Investigation results indicate that the effective refractive index differences between the corresponding HE and EH modes are above 10-4. The confinement losses of EH or HE modes are smaller than 10-8 d B/m, and the dispersion variations are lower than 0.31ps/THz/cm. Effective mode areas are larger than 5.14m m 2. Additionally, the highest mode purity of all vector modes is 99.78%. In addition, modal birefringence, also known as the walk-off length, has also been discussed. All these operation performances indicate that the designed NC-RCF make contributions to the optical communication systems.

Low crosstalk and large effective mode field area heterogeneous 13-core 6-mode fiber with double high refractive index rings

We propose a heterogeneous thirteen-core few-mode fiber, which has the excellent characteristics of low crosstalk (XT), large effective mode field area (Aeff) and low dispersion slope (Dslope). The double high refractive index ring can precisely control the effective refractive index difference between the LP mode in the core and reduce intra-core XT. The relative spatial efficiency (RSE) of the 13-core 6-mode fiber reaches 33.84, and the number of spatial channels is as high as 130. The inter-core crosstalk is less than −60 dB/km at 1550 nm. The results show that the proposed fiber with low crosstalk and large Aeff is sufficient to satisfy the requirements of long-distance transmission in the future.

A novel mode-division (de)multiplexer with degenerate modes output for MIMO-FREE applications

In this study, we propose a novel three-dimensional architecture mode (de)multiplexer with degenerate modes output using a pure silica FMF ring core transmission channel, which solves the problem caused by random mode rotation and can be used in multiple-input multiple-output free (MIMO-FREE) applications such as data center application in the future. By using the pure silica FMF ring core transmission channel and larger effective refractive index difference, the performance with low loss, high extinction ratio (ER) and low crosstalk is achieved. The main channel with a few-mode fiber (FMF) ring-core structure supports the modes LP01, LP11, and LP21, and the large effective refractive index difference between each mode in the core ensures low crosstalk characteristics between the modes. Using the pure silica core channel can effectively reduce propagation attenuation and fusion loss. Our proposed MUX/DEMUX with degenerate modes output is achieved when the degenerate modes LP11a/LP11b and LP21a/LP21b are transmitted as two independent mode signals, which can be used in MIMO-FREE applications. The extinction ratios (ERs) of the degenerate modes LP11 and LP21 are kept above 31.66 dB and 24.43 dB, respectively, and the ER of mode LP01 is kept above 38.72 dB in the C band. The coupling efficiency of mode LP01 is approximately 0 dB, which is almost unchanged with the increase of the wavelength. The coupling efficiency of LP11 is higher than −3.49 dB and that of LP21 is higher than −7.24 dB in the whole C-band. At 1550 nm, the coupling efficiencies of modes LP01, LP11, and LP21 are −0.002 dB, −0.052 dB, and −0.178 dB, respectively. The coupling efficiency and ER of LP01 mode are the best, and those of the degenerate mode LP11 are always better than those of mode LP21. Our proposed MUX/DEMUX achieves low crosstalk and high ER performance and solves the problem caused by the degenerate modes rotations during transmission.

Optimization of the nanostructured weakly coupled few-mode fiber for mode-division-multiplexed systems.

The objective of the study is to optimize the optical fiber structure for mode-division multiplexing systems using nanostructurization. The nanostructuring technique allows to fabricate fibers with arbitrarily designed (free-form) refractive index distribution based on two glasses. Three optimization schemes have been proposed. The nanostructuring method allows for designing fibers with optical properties similar and even better parameters impossible to produce by other methods. In this proposal, we examined four linearly polarized (LP) few-mode fibers. We report a high effective refractive index difference between modes while maintaining other important parameters for the weakly coupled approach.

Design and Characteristics of Diamond-Assisted Ring-Core Fiber for Space Division Multiplexing

In this paper, a novel diamond-assisted ring-core fiber (DRF) is proposed. With the introduction of a low-refractive-index diamond-shaped region located in the center of the core, the proposed fiber effectively eliminates spatial degeneracy of the LPmn mode groups and maintains a low level of birefringence. Under the fiber structure parameters proposed in this paper, the effective refractive index difference (Δneff) between the spatial modes supported by the fiber in the entire C-band is greater than 2.25 × 10−4, and the Δneff between adjacent modes falls within the scope of (2.11~9.41) × 10−4. The degree of degenerate separation between the two polarization modes of all modes is very low, which is 2~3 orders of magnitude lower than that of the spatial mode. By discussing the mode characteristics of DRF and several other center-assisted ring-core fibers, the method that can be used to manipulate the spatial mode degenerate separation with structural symmetry is obtained, which can be applied to provide guidance for similar fiber designs. The proposed fiber structure is a promising candidate in space division multiplexing systems.

Low cross talk homogeneous few-mode multi-core fiber with a suspended air-trench and V-type index core

We propose a homogeneous 13-core 4 linear-polarized modes fiber based on a suspended air-trench V-type index core (SA-VC) structure that can suppress inter-core cross talk (ICXT) and inter-mode cross talk. The SA structure is used to control ICXT, and the VC structure is used to control inter-mode cross talk. Cross talk and other properties of the fiber are calculated by coupled power theory and the finite element method. The numerical results demonstrate that the SA-VC structure has stronger cross talk suppression ability compared with traditional trench structure, the ICXT meets the requirement of lower than − 30 d B / 100 k m , the effective refractive index difference ( Δ n e f f ) between modes is greater than 1.52 × 10 − 3 , the maximum intrinsic loss is 0.189 dB/km, and the relative core multiplicity factor reaches 49.09. Based on the current optical fiber fabrication technology, a manufacturing scheme for this fiber preform is proposed. This design structure aims to be applied to communication systems to increase the transmission capacity of optical fibers.

Panda type separated-circles-formed elliptical ring core few-mode fiber

To support large transmission and reduce inter-mode crosstalk, we designed a polarization-maintaining few-mode fiber (PM-FMF) with a separated-circles-formed elliptical ring core and two stress rods. This fiber can support ten different polarization modes with tunable chromatic dispersion. With the stress-rods-induced birefringence and the high refractive index difference of 0.03 between the core and the cladding, the fiber can separate adjacent guided modes from each other. The effective refractive index differences between these modes all remain above 1.2 × 10−4 over the entire C + L band. In addition, the special separated-circles-formed elliptical ring structure of this few-mode fiber can provide a new degree of freedom for the regulation of chromatic dispersion. The chromatic dispersion can be simply adjusted by changing the radius of the small circles.

Numerical analysis of a photonic crystal fiber with elliptical air hole for supporting 80 orbital angular momentum modes

• We design photonic crystal fiber with elliptical air holes, which has good orbital angular momentum transmission performance. • The influence of elliptic air hole eccentricity on photonic crystal fiber transmission performance is analyzed. • This paper provides some ideas for new photonic crystal fiber design and air hole shape error analysis. In this paper, a new photonic crystal fiber with elliptical air hole (EAH-PCF) is designed, which can support stable transmission of 80 orbital angular momentum (OAM) modes at 1550 nm. The effective refractive index differences between vector modes are all above 0.002. Most of the modes have a confinement loss between 10 −10 dB/m and 10 −8 dB/m. The effective mode area of the vector modes are all greater than 70 μm 2 and the nonlinear coefficients are all less than 1.8 W −1 /km. These properties guarantee high quality OAM. Finally, we discuss the effect of EAH’s eccentricity on the PCF. Simulations show that increasing the EAH’s eccentricity will increase the degree of separation between the vector modes, but causes a slight decrease in the quality of OAM.

Design and characteristics of ellipse-assisted ring-core fiber for space division multiplexing

• Elliptical-assisted ring-core fiber with central low-refractive-index elliptical region. • 9 effectively separated spatial modes have uniform Δn eff with values greater than 3.14 × 10 −4 at 1550 nm. • The level of polarization separation is lower than 3.33 × 10 −6 . • Good scalability and fabrication error tolerance. • This fiber design could be promising candidates toward high-capacity SDM systems. We propose a novel ellipse-assisted ring-core fiber characterized by an elliptical region of low refractive index in the core. The proposed fiber proves effective in eliminating the spatial degeneracy of the LP mn mode groups and keeping the birefringence properties at a low level. This fiber, with optimized structural parameters, can guide 9 modes over the entire C-band. The simulation results show that the effective refractive index difference (Δn eff ) between adjacent modes at 1530–1565 nm falls within the scope of (3.1 ∼ 6.64) × 10 −4 . At the same time, the polarization separation level of each mode is lower than 3.23 × 10 −6 , which is 2 orders of magnitude lower than the degree of spatial mode degeneracy separation. This fiber design has good application prospects in space division multiplexing systems.

A cascaded triple waist-enlarged taper few-mode fiber temperature sensor with beaded structure

A cascaded triple waist-enlarged taper few-mode fiber (FMF) temperature sensor with beaded structure is proposed and fabricated. The main structure of the sensor is a waist-enlarged taper cascaded by multi-segment FMF. The diameter of the waist-enlarged taper structure affects the effective refractive index difference and therefore affects the sensitivity of the sensor. The effective refractive index difference of the sensor is 0.00199841. The diameters of the three waist-enlarged taper structures used in the experiment are 206.56 µm, 208.58 µm, and 208.58 µm. The function of the waist-enlarged taper cascaded structure is to couple the light transmission mode. Therefore, interference from other modes is reduced and the sensitivity of the sensor is improved. The experimental results show that in the range of 30–90 °C, the spectrum appears a red-shift phenomenon with the increase of temperature. The temperature sensitivity can reach 89.41 pm/°C, and there is a good linear relationship between wavelength shift and temperature change. The sensor has the advantages of simple structure, high sensitivity and good stability, it can be used for temperature detection in the fields of fire alarm, biomedicine and chemical sensing.