Relative Refractive Index Difference Research Articles

Set of mathematical models for Bessel-Gauss beams coupling into the parabolic-index fiber under the influence of atmospheric turbulence and random jitter.

The expression of efficiency for Bessel-Gauss (BG) beams coupling into the parabolic fibers (PF) after passing through the Cassegrain antenna system is first derived. The effects of atmospheric turbulence and random jitter of the coupling lens on the efficiency are also taken into account to improve the practical applicability of our model. This article use a BG beam with a wavelength of 1550 mm and fiber with a core radius RF of 50 μm and a relative refractive index difference ζ of 0.01 for simulation testing. The optimal parameters of the antenna system are determined: the radius of the primary mirror and the secondary mirror is 8.33 cm and 1.25 cm, respectively. The coupling efficiency of BG beams of different orders reaches above 94% simultaneously when the lens's focal length is 7.8 cm. After taking into account the transmission efficiency of the antenna system, the system's total efficiency for BG beams of different orders averages 76.33%, when the transmission distance is 1 km. The results show that the same degree of turbulence and random jitter have different influences on the coupling efficiency of BG beams of different orders, and lower-order BG beams have better resistance to turbulence and jitter during propagation and coupling. Moreover, the effect of the guided mode field on the coupling efficiency and the resistance to turbulence varies for different values of mode radial index in the fiber p. The guided mode with p = 0 not only enables the BG beams of different orders to achieve the highest transmission efficiency in the coupling system almost simultaneously but also the random jitter and turbulence have less influence on the coupling efficiency of this mode. It means that the BG beams can have higher efficiency when coupled to the mode with p = 0 after long-distance transmission. This property of the fiber mode at p = 0 provides conditions for the simultaneous propagation of multiple BG beams in a parabolic fiber, which provides a theoretical basis for higher transmission capacity. This research work provides a theoretical model for the theoretical study of vortex beams and optical communication, which is beneficial for the design and application of vortex beams and has instructive meaning for practical engineering design.

Differential group delay behavior in homogeneous weakly coupled multicore fiber under bending and twisting condition

The impacts of core count/layout on the mode effective refractive index (\({n}_{\mathrm{eff}}\)), bend- and twist-induced differential group delay (DGD) of different cores, worst-case DGD, and intercore DGD (\({\mathrm{DGD}}_{\mathrm{IC}}\)), in weakly coupled homogeneous multicore fibers (WC-HMCFs), have been analyzed to select the proper core layout with low DGD value for a particular core count. The core counts such as 3-core, 4-core, 6-core, and 12-core and the core layouts namely linear, triangular, square, rectangular, circular one ring structure (ORS), circular dual ring structure (DRS), hexagonal ORS, and square lattice structure (SLS) have been considered in the investigation. Moreover, in order to calculate the \({\mathrm{DGD}}_{\mathrm{IC}}\) values, the generalized expressions of core pitch (Λ) for the different core layouts have been derived. Further, the impacts of core radius (\(a\)), relative refractive index difference (\(\Delta\)), and Λ on the \({n}_{\mathrm{eff}}\) and DGD levels for the 3-core WC-HMCFs, with linear and triangular core layouts, have been analyzed numerically by using the finite element method-based FemSIM simulation platform, and MATLAB. It is observed that for a certain core count value, the core layout and Λ have significant impacts on the DGD levels, while the variations in the values of \(a\) and \(\Delta\) have no significant impact on the DGD levels. Further, increase in core count normally increases the DGD levels in all core layouts. On the other hand, for the fixed values of core count, cladding diameter (D), and outer cladding thickness (OCT), the core layout which has the lowest core pitch leads to the lowest value of \({\mathrm{DGD}}_{\mathrm{IC}}\), as compared to the other core layouts. The investigations carried out in this work may provide the guidelines in different MCF applications, such as MCF-based multi-parameter sensing and signal processing in microwave photonics.

Dispersion compensation: impact of integration of soliton transmission and cascaded apodized FBGs

In this paper, we present a tanh apodized FBG as a dispersion compensator in optical fiber communication systems. This grating is integrated as a hybrid system of four cascaded apodized FBGs with soliton transmission technique. This enhances the linewidth of the light source to get the most out of transmission per-channel bit rate of 9.687 Tbps at a temperature of 20 °C, a relative refractive index difference of 0.002, a Germania doping ratio of 0.2, and a signal wavelength of 1.65 µm.

Factors Influencing Recognition Capability of Inverse Opal Structured Photonic Crystal Sensors

Nowadays, many kinds of colloidal photonic crystal (PC) sensors with inverse opal (IO) structures have been developed. However, there are few systematic studies on the factors influencing their recognition capability and responsiveness capability. In this paper, the relationships between recognition capability of IO structured PC sensors and all the parameters in Bragg–Snell’s law have been explored. In addition, research on the recognition ability of PC sensors typically focuses only on the refractive index difference between the identified substances. Herein, we define two concepts, namely the absolute refractive index difference and the relative refractive index difference, and prove that the recognition ability not only relies on the absolute refractive index between the identified substances, but also on the relative refractive index. Bragg–Snell’s law analysis confirms that the responsiveness capability is directly proportional to the void size of the IO structure, which is also confirmed by the finite difference time domain (FDTD) method. It is believed that these systematic studies have important guiding significance for creating advanced IO structured PC sensors.

Analysis of supermode properties in three-core homogeneous strongly coupled multicore fiber

We have investigated the supermode properties, namely, mode effective refractive index (RI) (neff), effective area (Aeff), and chromatic dispersion (CD), in three-core homogeneous strongly coupled multicore fibers (SC-MCFs) for different core RI profiles (step index and RI dip) and in different possible core arrangements (triangular and linear). Further, the impacts of fiber parameters, namely, core radius (a), relative RI difference between core and cladding (Δ), and core pitch (Λ), on the differential group delay (DGD) between different supermodes have also been analyzed for all the considered configurations of SC-MCFs. The analysis presented has been done numerically using the FemSIM simulation platform. It has been observed that core arrangement has significant impact on the levels of neff, Aeff, CD, and DGD, and it also affects the degeneracy of the supermodes in a three-core homogeneous SC-MCF. Further, there exist certain core pitches in which the Aeff values of fundamental supermodes in different core layouts are equal. Furthermore, there exists a certain relative RI difference between the core and cladding (Δ) values in which DGD values in all the considered SC-MCF configurations are equal. On the other hand, over a range of Δ values, DGD in a linear layout is flattened compared to a triangular layout. Incorporating the RI dip structure in the cores of SC-MCFs affects the CD levels significantly. Therefore, by careful control of the fiber parameters and core arrangement, large mode effective area and low and flattened dispersion SC-MCF can be designed that may be suitable in dense wavelength division multiplexing application.

Low-loss orbital angular momentum ring-core fiber: design, fabrication and characterization

We design, fabricate, and characterize two types of low-loss weakly guiding ring-core fibers (RCFs) supporting orbital angular momentum (OAM) modes. The ring core and cladding have a relative refractive index difference of 1%. The RCFs feature largely separated OAM mode groups (MGs), where the first MG only contains two fundamental modes and the other MGs each have four OAM modes. The effective refractive index difference (Δ neff ) between all MGs in the type-1 RCF is larger than 1.5 × 10−3. For the type-2 RCF, except the first two MGs, the Δ neff between the other MGs is also larger than 1.0 × 10−3. These enable relatively low-level inter-group crosstalk and facilitate multiple-input multiple-output (MIMO) simplified OAM multiplexing transmission systems, where small-scale MIMO assisted intra-group OAM multiplexing and MIMO-free inter-group OAM multiplexing can be considered to take full use of all OAM modes in RCFs. The propagation loss of all supported OAM modes in the fabricated RCFs is measured to be less than 0.25 dB/km. We study the modal effective area and anti-bending performance of the guided OAM modes in RCFs. The effective refractive index, group velocity dispersion, differential group delay, crosstalk, and stability of RCFs are all characterized showing favorable performance. Moreover, using the fabricated two types of low-loss RCFs and adopting 8-ary quadrature amplitude modulation (8-QAM) signals, we also demonstrate data-carrying two OAM mode group multiplexing transmission over 25.32 km type-1 RCF and 50 km type-2 RCF free of MIMO. The demonstrations show a great potential to use low-loss RCFs in long-distance fiber-optic OAM multiplexing communications.

Design of High Speed Optical Fiber Cables and Transmission Techniques in Advanced Optical Networks

In the present paper, design of high velocity fiber-optic communication networks with ultra wide-dense wavelength division multiplexing (UW-DWDM), space division multiplexing (SDM) and optical transmission techniques is presented. All connects in the (SDM) are split into subgroups of various chemical and geometrical characteristics .The parameters have been designed at: i) Δn, the relative refractive index difference for both the core and also the clad, ii) x%, the percentage of doping in silicon material by Germanium material. The Δn and x% parameters are also preserved during their technological boundaries of attention. UW-DWDM has 1200 channels that transferred at the fiber-optic wavelengths from 1450 nm up to 1650 nm within 140 connects from UW-SDM wherever every connect is prepared to own the same compressed chromatic dispersion. From this technique, it is noticed that the connect design parameters suffer more nonlinearity with the number of connects. So, two different propagation techniques have been used to investigate the transmitted bit-rate as a criterion to enhance the system performance. The first technique is Soliton propagation, where the control parameters lead to equilibrium between the pulse spreading due to dispersion and the pulse shrinking because of nonlinearity. The second technique is the maximum time division multiplexing (MTDM) technique where the parameters are adjusted to lead to minimum dispersion. Two cases are investigated; no dispersion cancellation and dispersion cancellation. The investigations are conducted over an enormous range of the set of controlling parameters. Thermal effects are considered through three basic quantities, namely: the transmission bit rate, the dispersion characteristics and the spectral losses.

OAM mode multiplexing in weakly guiding ring-core fiber with simplified MIMO-DSP.

We present a low-loss weakly guiding ring-core fiber for orbital angular momentum (OAM) mode group multiplexing (MGM) transmission. This special fiber design supports 50 radially fundamental modes divided into 13 mode groups with only 0.7% relative refractive index difference between the fiber ring core and cladding. Except the first two groups with 10-5 mode spacing, the other mode groups are separated with each other with effective refractive index difference (Δneff) larger than 10-4, indicating relatively low-level inter-group crosstalk. One can directly use different OAM mode groups for MGM communications without multiple-input multiple-output digital signal processing (MIMO-DSP) technique. Besides, one can employ different OAM modes among the same mode group to carry different data information assisted by small-scale MIMO technique. The target fiber exhibits small and flat dispersion within (14.3, 39.7) ps/nm/km which is comparable to that in the standard single-mode fiber (SMF), and extremely large mode area within (787.9, 841.2) µm2 over the whole C + L band. MIMO equalization complexities for modified small-scale MIMO-DSP assisted intra-group modes multiplexing combined with MIMO-free inter-group modes multiplexing method in both time and frequency domain are much simpler compared to traditional 50×50 MIMO equalization.

Analytical evaluation of coupling coefficients of a nine-core homogeneous optical fiber

Coupling coefficients due to inter-core coupling in a 9-core homogeneous multi-core optical fiber (MCF) are evaluated analytically. Variation of coupling coefficients with respect to the fiber parameters, like, core-to-core separation (pitch), core radius and relative refractive index difference are investigated. Results show that the coupling coefficients are inversely related to the core pitch and index contrast, but it increases with the increase of core radius. It is also observed that while keeping the pitch and index contrast unchanged, the core radius plays a significant role to control the coupling among the cores. Variation of coupling coefficients with relative index difference are compared with similar results reported earlier.

Crosstalk Limitations due to Intercore Coupling on the BER Performance of an Optical Communication System with Homogeneous Multi-core Fiber

Abstract The effect of crosstalk due to inter-core coupling on the bit error rate (BER) performance of a fiber optic communication link with a homogeneous multi-core optical fiber (MCF) and a direct detection receiver with optical preamplifier is analytically evaluated and presented in this paper. Coupling coefficients due to inter-core coupling in a 7-core MCF is evaluated numerically with as a function of core-to-core distance (pitch) and relative refractive index contrast. Coupling length is determined against various index contrasts and at different core pitches. Crosstalk power, Signal to Crosstalk plus Noise Ratio (SCNR), BER and power penalty at any output core with light launched into other core of a 7-core MCF are determined analytically. The results show that there is mentionable deterioration in BER performance due to coupling induced crosstalk, and system suffers distinct power penalty at a given BER. For example, power penalty at BER of 10e-9 is found to be 4.0 dB and 4.3 dB for a MCF link of 10 km corresponding to relative refractive index difference of 0.0036 and 0.0030, respectively, for a pitch value of 30 µm and core radius of 4.5 µm.

Analysis of power confinement in Trench-Assisted Fiber for Sensing Applications

In this paper, we have the analysis on the effect of power confinement in trench assisted fiber. In single mode trench-assisted optical fibers at a certain value of trench width, all power propagates through the cladding. Single mode operation in the fiber is realized by the small diameter of the core and low mode field diameter gives tight confinement. The optimized design parameters are obtained by introducing trench between inner cladding and the outer cladding layer. The analysis shows that the relative refractive index difference in trench affects the properties of optical fiber for sensing applications.

A GaN‐Based VCSEL with a Convex Structure for Optical Guiding

In this paper, room‐temperature continuous‐wave operation of a GaN‐based vertical‐cavity surface‐emitting laser (VCSEL) with a convex structure for optical guiding is demonstrated. The GaN‐based VCSEL is designed and fabricated with the convex structure containing a 15‐nm step between the center area and the peripheral area of the VCSEL with a Nb2O5 spacer layer. In theory the convex structure with a 1.5% relative refractive index difference between the areas could provide more than 10 high‐order modes. Experimental emission spectra and near field pattern images clearly suggested a multi‐mode operation of the VCSEL with the convex structure. In addition, the VCSEL shows a lower threshold current, 2 mA, and a higher maximum light output power, 0.88 mW, which are superior to those of our standard VCSEL fabricated simultaneously without the convex structure. These results indicate that the optical guiding with the convex structure plays an important role in developments of GaN‐based VCSELs.

Designing suitable dispersion decreasing active fibers to generate parabolic pulses in presence of macrobending

The present work deals with the performances of straight and bent active normal dispersion decreasing fibers (NDDFs) to create parabolic pulses. Since these optical fibers are generally kept in the form of fiber rolls, they may suffer from unavoidable bend losses. For the NDDFs, the core radius and hence transverse field distribution inevitably change along the fiber length; thereby bend loss is no longer an invariable quantity throughout the NDDF length. The spatial variation of bend loss for a parabolic index NDDF is incorporated for the first time to investigate the effect of bend loss analytically and numerically on parabolic pulse generation. It is illustrated that suitable choice of relative refractive index difference and bend radius of curvature of the fibers would lead to parabolic pulse formation by a bent NDDF. The comparative study on the straight and bent NDDFs will be helpful for the fiber optic manufacturers to design and fabricate NDDFs with proper choice of optimized parameters and index differences so that parabolic pulses can be generated even under the influence of practically unavoidable bending.

Few-mode and large-mode-area fiber with circularly distributed cores

In this paper, a novel few-mode large-mode-area fiber is proposed. This type of fiber consists of 11 conventional cores and 8 air-hole cores circularly arranged around the center core. Few-mode condition equal to strict dual-mode here is available by appropriate adjusting on corepitch, relative refractive index difference and core radius. Large effective area of fundamental mode around 1403.561μm2 could be achieved by optimization of structural parameters. Bending loss less than 10−3dB/m is realized when effective area is over 1400μm2.

Reduction in Bend Losses of a Buried Waveguide on a Silicon Substrate by Adjusting the Core Location

A new technique for reducing pure bend and transition losses in a buried dielectric waveguide is discussed, using the imaginary-distance beam-propagation method. It is numerically shown that only the adjustment of a core location enables us to reduce the bend losses. The reason why a proper location of the core offers minimum bend losses regardless of a bending radius is explained in terms of the generation and suppression of a leaky wave. Significant loss reduction is achieved over a wide spectral range from 1.3 to 1.65 $\mu {\rm m}$ . It is also revealed that a minimum pure bend loss is obtained regardless of the relative refractive index difference, when the air-cladding interface is placed at the position, where the field amplitude decays to approximately 12% of its peak.

Polarization Beam Splitter With Different Core Widths and Its Application to Dual-Polarization Optical Hybrid

We investigated the core-width dependence of the birefringence of silica-based waveguides with different relative refractive index differences (Δ), namely a 0.75%-Δ GeO2-doped waveguide and a 5%-Δ Ta2O5-doped waveguide. We revealed that the birefringence of the 0.75%-Δ GeO2-doped waveguide is almost the same as the stress-induced birefringence and that the 5%-Δ Ta2O5-doped waveguide has a large geometrical birefringence. By utilizing the core-width dependence of the birefringence, we have successfully demonstrated a 0.75%-Δ polarization beam splitter (PBS) with an operating wavelength range of more than 150 nm and a dual-polarization optical hybrid including a compact 5%-Δ PBS with a polarization extinction ratio of more than 20 dB over the C-band.

New geometry of photonic crystal fiber for improved dispersion compensation

A new design is proposed for a dispersion compensation photonic crystal fiber (DCPCF). Large negative dispersion has been achieved by using a combination of square and circular ‘holes’ arranged in a hexagonal lattice, with background index equal to 1.45 (silica). Square-shaped holes have been utilized for the second ring only, whereas the remaining structure has circular holes. High index difference and geometrical parameters, such as radius, square width, pitch of PCF, and air hole configuration, allow a PCF to act as an efficient dispersion-compensating device. Large negative dispersion up to –1000 ps/(nm km) has been achieved without any doping. Moreover, the relative refractive index difference (%) obtained in this work is much higher than conventional methods at an optical frequency of 193 THz.

Recent Developments and Signal Processing of Low Driving Voltage and High Modulation Efficiency Electro absorption Modulators (EAMs)

Electro-absorption (EA) modulators are very attractive devices for optical fiber communications because of their very low driving voltage, very high modulation efficiency and integratibility with lasers. However, conventional EA modulators are lumped electrode devices, whose speeds are limited by the total parasitics of the devices, which restricts the devices to very short length for high speed operation. This paper has presented the important transmission characteristics of EA modulators such as transmission performance efficiency, modulation photocurrent, insertion loss, extinction ratio, relative refractive index difference, and signal transmission quality, over wide range of the affecting parameters for different selected electro-absorption materials to be the major of interest.

Influence of Fiber Parameters on Stress-Induced Group Delay Variation in Hole-Assisted Fibers

We have studied a specific type of hole-assisted fibers (HAFs) for parallel transmission systems. A fiber ribbon using HAFs will be suitable for parallel transmission systems when used in a small space. Additional skew occurs when a fiber ribbon is used under nonuniform stress environment. We focus on stress-induced group delay variations. Influence of fiber parameters, such as a core diameter 2a and a relative refractive index difference Δ, on stress-induced group delay variations is investigated using a finite-element method. We also study the influence of hole-structure variations.

Study on Coupling Coefficient and Fabrication of a Kind of Symmetric Twin-Core Silica Fiber

The coupling coefficient of twin-core fiber is discussed with coupled-mode theory in this paper. It shows that the coupling coefficient is related with the pump wavelength, the core diameter, the distance between the centers of the two cores and the relative refractive index difference. And the parameters of the twin-core fibers can be optimized to satisfy different requirements. Using the theory, the twin-core fibers are fabricated by an improved method. The measurement results have an obvious progress than previous work.