Linear Polarization Modes Research Articles

Optical Performance Monitoring for Intra-LP-Mode Dispersion in Weakly-Coupled Mode-Division Multiplexed Systems

Weakly-coupled mode-division multiplexing (MDM) technique based on linearly-polarized (LP) modes in ultralow-modal-crosstalk few-mode fiber (FMF) has been considered as a promising candidate for capacity enhancement of next-generation optical transmission systems. Similar to polarization mode dispersion in single-mode fibers (SMFs), signals in weakly-coupled FMFs suffer from the impacts of intra-LP-mode dispersion (ILMD) for both degenerate and non-degenerate LP modes.In this paper, we propose an effective method for ILMD monitoring in weakly-coupled MDM systems. The ILMD evaluation method for degenerate LP modes is designed by the analysis of channel transfer matrix in a digital coherent receiver. Then the ILMD monitoring scheme is experimentally validated for all the 6 LP mods in a weakly-coupled FMF. The influences of chromatic dispersion, optical signal to noise ratio and inter-LP-mode crosstalk to the method are also investigated.

Single-Fed Triple-Mode Wideband Circularly Polarized Microstrip Antennas Using Characteristic Mode Analysis

A method of triple-mode operation by capacitive slot loading is proposed for bandwidth enhancement of single-fed circularly polarized (CP) patch antennas. Instead of using even-numbered linearly polarized (LP) modes with quadrature phase, three orthogonal LP modes are used for CP bandwidth enhancement, where the middle mode is shared by two cross-polarized modes with the same polarization. The advantages include reduced constraints, lower complexity, and a higher degree of freedom for antenna design. Guided by the method, a U-slot antenna and an E-shaped antenna are proposed and designed with characteristic mode analysis (CMA). Both antennas work with a TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub> -like mode and a TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> -like mode. Differently, the U-slot antenna works with an additional slot mode and the E-shaped antenna works with an additional TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> -like mode. The operating modes are manipulated by the slot loadings for creating phase difference. As a result, wideband CP radiation is achieved with a single feeding. CMA-based empirical formulas are derived for fast design. The proposed method and antennas are experimentally validated. Both antennas measure a bandwidth exceeding 21% for 10 dB return loss and 3 dB axial-ratio (AR), a significant improvement compared with conventional corner-truncated U-slot patch antennas of similar thickness or volume.

A mode‐combination‐based polarization‐reconfigurable antenna

In this article, a novel polarization-reconfigurable antenna based on the theory of linearly polarized (LP) modes combination is proposed. The antenna can electronically alter its polarization states between left-hand circular polarization, right-hand circular polarization and continuous linear polarization. A corner-truncated square patch with two isolated aperture-coupling feeds is used as the main radiator. By independently exciting the two feeding ports, the proposed antenna can work in two isolated orthogonal LP modes at 2.45 GHz, respectively. By simultaneously exciting the two feeding ports with different combination of amplitudes and phases, two circularly polarized (CP) modes and LP modes polarized at arbitrary azimuth can be obtained. The simulated and measured gains for all the polarization states are 7.3 dB at 2.5 GHz. The –10 dB impedance bandwidth ranging from 2.35 to 2.58 GHz is fully overlapped for all the polarization states.

A Liquid-Metal-Based Crossed-Slot Antenna With Polarization and Continuous-Frequency Reconfiguration

Aiming to the limitations of traditional reconfigurable antennas, this paper proposes a novel reconfigurable antenna that can achieve four polarizations and continuously-tunable frequency. The proposed antenna is based on liquid metal, i.e. eutectic gallium and indium (EGaIn). The pattern of the slots can be controlled by changing the amount of the liquid metal inserted in the 3-D printed channels. The antenna can work under four polarization modes: left-hand circular polarization (LHCP), right-hand circular polarization (RHCP), and two orthogonal linear polarization (LP) modes. Under the LP modes, the working frequency of the antenna can be tuned continuously from 1.82 GHz to 2.0 GHz. The measured results agree with the simulated ones, which validate the performance of proposed antenna.

The influence of a metal rod inside a spherical Luneberg lens on its characteristics

The paper discusses the option of installing a load-bearing element inside a spherical Luneberg lens (LL). The presence of a metal element inside the LL will increase the structural rigidity of the spherical antenna and thereby expand the scope of use of the LL at various mobile communications and radar facilities with severe operating conditions. The influence of a metal rod inside the main spherical structure on the radiation pattern in two main planes in the linear polarization mode and on the diffraction pattern of the radiation field is estimated. The results of the analysis of the directional characteristics and the diffraction pattern of the LL radiation field were obtained using modeling in the Ansys Electronics Desktop (HFSS Design) working environment, as well as mathematical modeling using the Green tensor function method. At the same time, the obtained mathematical relations can be used in the future to solve the problem of finding the optimal size and position of a metal element in the sphere of LL.

Efficient silicon integrated four-mode edge coupler for few-mode fiber coupling

Here, we designed a broadband, low loss, compact, and fabrication-tolerant silicon-based four-mode edge coupler, composed of a 1×3 adiabatic mode-evolution counter-taper splitter and a triple-tip inverse taper. Based on mode conversion and power splitting, the proposed structure can simultaneously realize efficient mode coupling from TE0, TM0, TE1, and TM1 modes of multimode silicon waveguides to linearly polarized (LP), LP01,x , LP01,y , LP11a,x , and LP11a,y , modes in the few-mode fiber. To the best of our knowledge, we proposed the first scheme of four LP modes coupling, which is fully compatible with standard fabrication process. The 3D finite-difference time-domain simulation results show that the on-chip conversion losses of the four modes remain lower than 0.62 dB over the 200 nm wavelength range, and total coupling losses are 4.1 dB, 5.1 dB, 2.1 dB, and 2.9 dB for TE0-to-LP01,x , TM0-to-LP01,y , TE1-to-LP11a,x , and TM1-to-LP11a,y , respectively. Good fabrication tolerance and relaxed critical dimensions make the four-mode edge coupler compatible with standard fabrication process of commercial silicon photonic foundries.

Power efficient Space Division Multiplexing–Wavelength Division Multiplexing system using multimode EDFA with elevated refractive index profile

SummarySpace division multiplexing (SDM) is an endowment technology to increase the collective bandwidth of multimode transmission in wavelength division multiplexing (WDM) system. In this paper, four wavelength signals from 229.6 to 232.6 THz are transmitted. For each wavelength, three linearly polarized (LP) modes are multiplexed using SDM. These 12 LP modes are transmitted through four parallel Multimode Step Index (MSI) fibers and received by four optical network units (ONUs). A multimode erbium doped fiber amplifier (MMEDFA) with elevated refractive index profile (ERIP) is used as preamplifier to this SDM–WDM system. This ERIP is complying with ITU‐T G.651.1 standard fiber in dispersion performance. The pumping power of 10 mW is adequate for this MMEDFA which is very low and cost effective. LP modes are transmitted, and performance of proposed transmission system is evaluated in terms of spatial electric fields, pumping power requirement, output optical power, and bit error rate (BER).

Differential mode delay diagnostics for LP modes traversing a few-mode fiber.

We provide differential mode delay (DMD) diagnostics for linearly polarized (LP) modes traversing a few-mode fiber (FMF). We employ a modal interferometer method with a butt coupling mechanism to measure the DMD between any two modes guided in FMFs with step-index (SI) and depressed-cladding profiles. The measurement principle is based on investigating a transmitted spectrum through temporal decomposition by means of a Fourier transform. At least four different temporal waveforms are needed for an FMF supporting four LP modes to estimate the DMD between two different LP modes. A butt coupling mechanism with an air gap between an FMF and a dispersion-shifted fiber is employed for simultaneous DMD measurement. The present diagnostics requires some knowledge of the bending loss characteristics of the FMFs and the butt coupling properties of the circular symmetry modes or circular asymmetry modes in their electric fields. The DMD diagnostics are realized taking the bending loss and butt coupling characteristics into consideration. We present experimental results for the DMD and DMD slope as a function of the wavelength in a 1450-1625 nm telecommunication band.

Optimization of super-Gaussian thulium-doped fiber amplifier for S-band multimode division multiplexing

We have assigned the first four linear polarized (LP) modes of signals at the wavelength of 1470 nm as four channels of multimode division multiplexing (MDM) structure and for each channel we have used a few-mode thulium doped fiber amplifier (FM-TDFA) consisting a pump at the wavelength of 1050 nm and a few-mode thulium doped fiber (FM-TDF). The operation of each FM-TDFA has been investigated by changing the three parameters of super-Gaussian transverse distribution of FM-TDF in the presence of different kinds of LP modes of pumping with several values of input pump power. We have searched the whole space of variables through computation of the rate and power equations and calculated the overlap factors, gain and noise figure (NF) for each point of the space to find the global optimum condition. Also, we have found the best FM-TDFAs configurations by the average gain of 39.2 dB and average NF of 3.24 dB.

The dual-polarized staggered stacked patches antenna

A new dual-polarized staggered and stacked patches antenna with wide impedance bandwidth and high isolation is proposed. The antenna consists of two groups of radiation patches, in 7 layers, and uses the orthogonal adjacent coupling structure on staggered layer to excite a pair of linear polarization modes. Thanks to the staggered feeder mode, it has increased the isolation performance between ports and compressed the transverse size of the antenna. As a result of the combination of staggered stack-up between the patches and the stepped gradient shape of the main radiating patches, it has effectively expanded the impedance bandwidth of the antenna. The proposed antenna is simulated, fabricated and measured. The staggered feeding structure effectively reduces the cross-sectional area of the antenna, and greatly improves the isolation between feeding ports. The measurement results show that the impedance bandwidths for vertical and horizontal polarization modes are 40.2% (638–960 MHz) and 40.0% (645968 MHz) respectively when the return loss is lower than −10 dB, and the isolation between feeding ports is better than −30 dB. Meanwhile, the antenna has a stable and symmetrical radiation pattern across the working band, therefore making it suitable to be used as antenna and antenna array element of mobile wireless communication base stations.

Analysis and Measurement of Intra-LP-Mode Dispersion for Weakly-Coupled FMF

Weakly-coupled mode division multiplexing (MDM) technique based on linearly-polarized (LP) modes in ultralow-modal-crosstalk few-mode fibers (FMF) has been considered as a promising candidate for capacity enhancement of short-reach transmission applications. Similar to the polarization mode dispersion (PMD) in single-mode fibers (SMF), signals in weakly-coupled FMFs suffer from the influence of intra-LP-mode dispersion (ILMD). In this paper, the generation mechanism of ILMD is firstly analyzed and the mean ILMDs in a typical step-index circular-core FMF are calculated. Then numerical simulations are carried out to investigate its impact and the severe performance degradation indicates that it may be one of the major transmission impairments for intensity-modulation/direct-detection (IM/DD) MDM systems. Moreover, an Improved Fixed Analyzer Method (IFAM) is proposed for ILMD measurement, based on which a weakly-coupled FMF supporting 6 LP modes is measured. Experimental results show that the ILMD for each circularly-symmetric LP mode behaviors like PMD in a SMF, while it may be up to tens of times larger for the non-circularly-symmetric LP modes. This work evaluates for the first time the ILMD impact and is beneficial for the design of weakly-coupled FMFs and IM/DD MDM systems.

Performance optimization of multi-plane light conversion (MPLC) mode multiplexer by error tolerance analysis.

The linear polarized (LP) mode multiplexer based on the inverse designed multi-plane light conversion (MPLC) has the advantages of low insertion loss and low mode crosstalk. However, the multiplexer also requires the fabrication and alignment accuracy in experiments, which have not been systematically analyzed. Here, we perform the error tolerance analysis of the MPLC and summarize the design rules for the LP mode multiplexer/demultiplexer. The error tolerances in the fabrication process and experimental demonstration are greatly released with proper parameters of the input/output optical beam waist, the pitch of optical beam array, and the propagation distances between the phase plane. To proof this design rule, we experimentally demonstrate the LP mode multiplexer generating LP01, LP11a, LP11b, LP21 modes and coupling to the few mode fiber, with the insertion loss lower than -5 dB. The LP modes are demultiplexed by MPLC, with the crosstalk of different mode groups lower than -10 dB. LP modes carrying 10 Gbit/s on-off keying signals transmit in a 5 km few mode fiber. The measured bit error rates (BER) curves of the LP01, LP11a, LP21 modes have the power penalties lower than 12 dB.

A scanning phased array based on a polarization reconfigurable antenna with compact aperture size

A compact antenna with tri-polarization switching is presented. The antenna is mainly composed of a monopole slot and a shorted loop, and the two radiating elements are connected in series and placed in a metallic box filled with dielectrics. To realize polarization switching, four PIN diodes are introduced into the antenna structure, and they cannot only suppress the excitation of the loop antenna but also vary the direction of the current flowing on the loop. Both simulated and measured results indicate that the proposed antenna can be operated in one linear polarization and dual circular polarization modes within an impedance bandwidth of around 15%; moreover, the aperture size of the antenna is about 0.09 λ02. Based on the polarization reconfigurable antenna, a 1 × 8 phased array with an aperture size of 0.3 λ0 × 2.8 λ0 is then developed. Simulated results show that the phased array operating in each polarization mode can give a gain fluctuation of smaller than 1.5 dB and a side lobe level of less than −10 dB when the beam is scanned from −60° to 60°. Experiments are also carried out, and their results have good agreements with the simulated data.

A low crosstalk multi-core few-mode fiber with composite refractive index profile and air-hole embedded trench assistance

In order to reduce the fiber crosstalk for the space division multiplexing (SDM), this paper proposes a novel multi-core few-mode fiber of composite refractive index profile and dual assistance structure. On the one hand, by adopt dual assistance of air-hole embedment and trench wrapping around all the seven cores, inter-core crosstalk is greatly reduced. On the other hand, a non-circularly symmetrical refractive index (RI) profile is innovatively proposed for the six outer-ring cores, which not only weakens the inter-mode coupling but also has a suppression effect on inter-core crosstalk. The finite element method (FEM) is used to analyze the fiber structure. In the case of transmitting six spatial modes, the minimum inter-mode effective RI difference (Δneff) between the linear polarization (LP) modes of the center core and between the spatial modes of the outer-ring core are greater than 0.5 × 10−3, the inter-core crosstalk of the sixth mode LP02 at 1550 nm is less than -66dB/100km, and the relative core multiplexing factor reaches 29.24. The characteristics of this structure are significantly better than that of current commercial optical fibers, promising broad application prospects in large-capacity SDM optical communication system.

Enhancement of OAM and LP modes based on double guided ring fiber for high capacity optical communication

This article introduced a double guided ring-based spiral photonic crystal fiber (S-PCF) supporting both the orbital angular momentum (OAM) and linear polarization (LP) mode simultaneously with the efficient transmission. The outcomes exhibition that the designed S-PCF have good optical features such as less confinement loss (4.05342 × 10−12 dB/m for the EH9,1 mode), better mode quality and ISO (up to 98.87% and 126 dB, respectively), supported a decent amount of modes which is numerically 64 OAM and 6 LP modes, maintaining the large refractive index differences (>10-4) of all the eigenmodes, and a relatively flat dispersion variation (0.0743 ps/km-nm for the EH1,1 mode). Also, we obtained the crosstalk less than −12 dB and the power fraction is approximately 93%-99% of the S-PCF. All the optimizing properties are obtained using the full-vector finite element method (FEM) and also the perfectly matched layer (PML) as a boundary condition. Thus, the above mentioned optimizing propagation characteristic based S-PCF has effective applications like reliable long-distance communication into the space division multiplexing (SDM) process of the large-capacity fiber communication systems.

A Weakly-Coupled Few-Mode Optical Fiber With a Graded Concave High-Index-Ring

This paper proposes a novel refractive index profile design based on few-mode fibers (FMFs) which can support 4 linear polarization (LP) modes. We first present a FMF whose core is dually assisted by a nano-hole (NH) and a high-index-ring (HIR), and then substitute the dual assistance by an innovative graded concave HIR (GC-HIR) assisted structure. Using the finite element method (FEM), the parameters of the NH-HIR and GC-HIR optical fiber are adjusted to investigate their respective minimum effective refractive index difference (minΔn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> ), which is 2.012 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> for the former, and 2.532 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> for the latter. Both optical fibers have a significant improvement on the crosstalk suppression effect, and the GC-HIR optical fiber is even better. The proposed GC-HIR FMF with special refractive index profile has potential application prospects in mode division multiplexing (MDM) optical fiber communication system, and can provide theoretical foundation for the design and analysis of subsequent optical fibers and key components.

Statistically-anisotropic tensor bispectrum from inflation

We develop a possibility of generating tensor non-Gaussianity in a kind of anisotropic inflation, where a (1) gauge field is kinetically coupled to a spectator scalar field.Owing to this coupling, the coherent mode of the electric field appears and softly breaks the isotropy of the Universe.We compute the bispectrum of linearly-polarized tensor perturbations sourced by the gauge field and find that it is strongly red-tiltedand has distinctive statistical anisotropies including higher-order multipole moments.Interestingly, the tensor bispectra with the specific combinations of linear polarization modes are dominant, and their amplitudes depend on the different sets of multipole moments.This new type of statistically-anisotropic tensor non-Gaussianity can be potentially testable with the upcoming cosmic microwave background B-mode polarization experiments.

Fluidic Stub-Loaded Patch Antenna for Frequency-Tunable Polarization Reconfiguration

This paper presents a fluidically loaded microstrip patch antenna on a multi-layered substrate with frequency and polarization reconfigurability in the 2400 MHz band. A square patch loaded at all the corners with short circuited λg/4 long stubs is utilized. An arrangement of fluidic channels fabricated by moulding Polydimethylsiloxane (PDMS) with 3D printed mould is incorporated beneath each stub. By injecting or withdrawing distilled water into the channels, variable loads are realized. It exhibits frequency tunable linearly polarized (LP) mode with symmetric loading or circularly polarized (CP) mode with asymmetric loading. The sense of CP can be switched by swapping the loading configuration along the diagonals of the patch. The equivalent circuit for the antenna is presented. A prototype is fabricated and measured. In LP mode, frequency tuning of 7.85% is achieved. The minimum realized gain is 5.67 dBi with cross-polarization (X-pol) isolation 20 dB. In the CP modes, the overlapping bandwidth for |S11| -10 dB and axial ratio 3 dB (ARBW) is 16 MHz for both LHCP/RHCP modes. Frequency tuning of 141 MHz (5.76%) is measured in the RHCP mode. The minimum realized gain is 4.86 dBic and total efficiency is > 60%.

Wideband Printed Half Bow-Tie Antenna Array Based on a Quad-Mode Reconfigurable Feeding Network for UAV Communications

In this article, a wideband printed half bow-tie antenna array based on a quad-mode reconfigurable feeding network for unmanned aerial vehicle (UAV) communications is presented. The proposed array consists of four printed half bow-tie antennas in the top substrate, two supports with a tapered balun, and a reflector with the reconfigurable feeding network in the bottom substrate. For UAV communications caused by wide beam coverage, the proposed reconfigurable feeding network using one single-pole-four-throw (SP4T) and two single-pole-three-throw (SP3T) switches can adjust the phase and amplitude to the antenna element. By controlling the switches, the proposed array can be operated on four linear polarization modes. The simulated and measured results prove the proposed array reconfigurability of the main beam in four directions with 45° steps in the azimuth plane. In addition, the proposed array achieves the wide 10-dB impedance bandwidth of approximately 29.6% (1507 MHz) from 4.1 to 5.6 GHz. The peak gain and half-power beamwidths (HPBWs) on azimuth and elevation planes of the proposed array are approximately 5.9 dBi, 240°, and 99°, respectively. The overall volume size of the proposed array is $60\times 60\times18.3$ mm3 ( $\sim \lambda _{0}\times \lambda _{0}\times 0.3\lambda _{0}$ at 5.1 GHz).

Numerical investigation of dual guided elliptical ring core few-mode fiber for space division multiplexing applications

In this article, dual elliptical ring core few-mode fiber (DE-RCF-FMF) profile with assisting dual cladding is investigated. By this approach, it is possible to combine the performance behavior two separate ring core fiber (RCF). Of course, elliptically core region is constructed for polarization maintaining properties. In this way, the maximum number of linearly polarized (LP) modes up to 20 can be generated. As dual channel separation, inner elliptical core region generates maximum of 8 L P modes and identified as LP01, LP11a, LP11b, LP21 for both X - and Y- polarizations. Similarly, outer elliptical core is supposed to generate maximum of 12 L P modes and identified as LP01, LP11, LP11, LP21, LP31, LP41, and LP51 for both x- and y- polarizations. Using finite element method (FEM), each mode characteristics is numerically simulated and the performance studies are compared with other LP modes. It is obviously well known as mode converter property and the conversion is only possible due to the mutual energy sharing between the neighborhood modes. In this design, LP01 mode is converted to LP11 and LP11 converted as LP21. The conversion region may happen for either ellipticity or wavelength variation. Anyhow, the property of mode conversion in the transverse direction is also considered as essential property of Space division multiplexing techniques.