Orbital Angular Momentum Mode Transmission Research Articles

Improving orbital angular momentum mode transmission with imperfect ring–core fiber

Orbital angular momentum (OAM), inherent to light beams characterized by helical phase fronts, represents an alternative spatial degree of freedom for photons. In recent years, OAM-based communication has gained significant attention as a potential solution to increase the capacity and spectral efficiency of optical fiber communication systems. In optical fibers, OAM modes can be generated and manipulated by utilizing specialized optical components and techniques. One approach involves using specially designed optical fibers, such as ring–core fibers, which have the capability to support multiple OAM modes. In the manufacturing process, the ring fiber is hardly made perfectly, especially the doped-cores. The introduced defect breaks the symmetry of the ring–core, which has a serious impact on OAM mode transmission. Using a material with a slightly higher refractive index than the cladding to wrap the ring–core is effective in reducing impairment of defects. After numerical simulation, the proposed method improves the walk-off length by a factor of 2–40.

Anti-resonance fiber propagating 86 orbital angular momentum modes with high purity for a long distance

Future advances in information transmission technology require better capacities for fiber optics communication. As a new degree of freedom, the orbital angular momentum (OAM) has attracted extensive attention in the field of mode division multiplexing. Herein, an anti-resonance fiber (ARF) is designed with three different materials, SF11, LaSF09, and SiO2, for the negative curvature tube. Systematic analysis and optimization are carried out to determine the number of layers of the negative curvature tubes in the cladding, the number of second-layer cladding tubes, and thickness of the annular area. The finite element analysis software COMSOL is utilized to calculate the effective refractive index difference, dispersion, limiting loss, and other characteristics. The ARF can stably transmit 86 OAM modes in the wavelength range between 1.2 and 2.0[Formula: see text][Formula: see text]m, and the minimum refractive index difference is greater than [Formula: see text], which effectively reduces the mode abridgment. The nonlinear coefficients of the HE[Formula: see text] mode and the OAM purity of HE[Formula: see text] mode are 0.34[Formula: see text]km[Formula: see text][Formula: see text]⋅[Formula: see text]W[Formula: see text] and 99.43%, respectively, thus facilitating stable transmission of OAM modes for a long transmission distance. The anti-resonance fiber has significant application potential in long-distance network information transmission, relay communication, and other communication areas.

MIMO-free OAM-MDM transmission with a ring-core fiber recirculation loop

We experimentally demonstrate the record long-haul orbital angular momentum (OAM) mode-division multiplexed (MDM) transmission over 300-km assisted by a ring-core fiber (RCF) recirculating loop. The recirculation loop consisting of 50-km RCF is controlled mainly by two acousto-optic switches with opposite states, and the signals are cyclically transmitted in the single-span 50-km RCF to achieve long-haul transmission. The RCF features largely separated OAM mode groups, where the effective refractive index difference between adjacent high-order OAM mode groups is larger than 1.0 × 10-3. Due to the low-crosstalk feature of RCF incorporated into fiber optic links, multiple-input multiple-output (MIMO) equalization technology is not used in the OAM MDM system. We transmit 20-Gbit/s quadrature phase-shift keying signals over two high-order OAM modes, and the transmission performance is characterized. The measured average optical signal-to-noise ratio penalties at a bit-error rate of 1.5 × 10-2 are about 2 dB between single OAM mode transmission and OAM modes multiplexing transmission. The obtained experimental results indicate favorable OAM-based MDM transmission performance in a 300-km recirculating loop link.

Negative curvature fiber for suppressing high-order radial OAM modes transmission

The combination of terahertz (THz) wave and the orbital angular momentum (OAM) multiplexing technology can further improve the communication capacity. Compared to other outer cladding structures, the negative curvature structure has been proven to provide stronger confinement effect on electromagnetic waves. Here, we propose a novel polymer (COC TOPAS) fiber which consists a central hollow-core, annular region and single layer negative curvature circle tubes as outer cladding for terahertz OAM modes transmission. The mode map is established by mathematical analysis of cut-off conditions for the vector modes, and the excitation of high-order radial modes in the fiber is successfully suppressed. In addition, the effective refractive index, confinement loss, effective mode area, mode purity and dispersion characteristics of the fiber are investigated under the condition of ‘single mode’ operation in the frequency range of 2.0–3.0 THz.

Design of New Photonic Crystal Fiber for Orbital Angular Momentum Modes Transmission

In order to achieve the flexibility and miniaturization of orbital angular momentum (OAM) fiber coupler, it is necessary to continuously design and optimize the base fiber to cooperate with the efficient transmission of multiplexed beam. It is more advantageous to use the photonic crystal fiber (PCF) instead of ring fiber in the coupler design. We have proposed a new fiber based on circular PCF structure to support high performance OAM modes transmission. The optimal parameters are selected to realize the stable transmission of 4 OAM mode groups with high mode quality. In the wavelength range of 1.2 μm∼2.0 μm, the confinement loss of low order mode is less than 10−8 dB/m, and the mode purity is more than 81.7%. Because of its flexible structure and low loss characteristics, the proposed OAM fiber can be widely used in the design of optical fiber devices in modular division multiplexing (MDM) communication systems.

Dual-guided-mode-region circular photonic crystal fiber with low nonlinear coefficients and flat dispersion for transmission of orbital angular momentum modes

As a new degree of freedom in optical fiber communication, the transmission of orbital angular momentum (OAM) modes has attracted extensive attention in recent years. Herein, a photonic crystal fiber with a dual-guided mode field was designed for the transmission of OAM modes in the wavelength range between 1.50 and 1.60 μm. The effective index difference, dispersion, nonlinear coefficient, and isolation parameters were analyzed systematically by the finite element method. The results showed that the effective index difference of all OAM modes was greater than 1 × 10 − 4, the nonlinear coefficient was as low as 0.492 km − 1 · W − 1, and dispersion was very flat with a minimum variation of 4.033 ps / ( km · nm ) . The fiber with outstanding properties has great potential and provides insights into how to improve the efficiency of optical communication.

Effects of air holes in the cladding of photonic crystal fibers on dispersion and confinement loss of orbital angular momentum modes

In the development of orbital angular momentum (OAM) mode division multiplexing, the capacity of optical fiber communication must be improved. However, owing to dispersion and confinement loss, many OAM modes do not propagate stably over a long distance in optical fibers. In this work, the effects of the size, number, shape, number of layers, and layer spacing of air holes in the cladding of the fiber on the dispersion and confinement loss are analyzed based on a simple structure. The trends are studied and summarized to facilitate the design of optical fibers to achieve stable transmission of OAM modes over a long distance.

19-ring-air-core fiber supporting thousands of OAM modes for spatial division multiplexing.

We propose and design a 19-ring-air-core fiber that can support about 3000 orbital angular momentum (OAM) modes (156 modes in each ring) with <-80 dB inter-ring cross talk across the entire C and L bands after 100-km fiber propagation. Moreover, the eigenmodes are all separated from their adjacent modes by effective index differences >2.67 × 10-4 and mode groups by > 1.90 × 10-2, which can guarantee the stable transmission of OAM modes. This designed fiber is a potential candidate for applications in spatial division multiplexing (SDM) of optical channels to improve the capacity of next-generation high-speed optical communication systems, especially in short-distance applications. In this Letter, we also show the relationship between supported OAM mode numbers, total cross talk, and effective refractive index of intra-ring modes during the optimization of fiber through numerical simulations. It can provide a related reference for the future design of multi-ring-core fibers.

Design of pure silica-based photonic crystal fiber for supporting 114 OAM modes transmission

A novel photonic crystal fiber (PCF) based on pure silica is designed to transmit 114 orbital angular momentum (OAM) modes in the wavelength range of 1.2 μm–1.8 μm. The PCF composed of a central round air-hole, ring-shaped core, and air-hole array ensures effective refractive index differences of the constituent vector modes above 10−4 and stable transmission of the OAM modes. The finite element method is implemented to analyze the PCF. The confinement losses and nonlinear coefficients of all the eigenmodes at 1550 nm are less than 3.03 × 10−7 dB m−1 and 1.30 w−1 km−1, respectively. In addition, the PCF shows higher effective refractive index difference, lower confinement loss, and nonlinear coefficient thus holding large promise in high-performance fiber communication systems.

Terahertz Hollow-Core Optical Fibers for Efficient Transmission of Orbital Angular Momentum Modes

Terahertz (THz) radiation has recently received a lot of interests in science and technology. The potential of orbital angular momentum (OAM) of light in the THz region is a novel idea for researchers with increasing applications in broadband communications. THz waveguides have rarely been reported to generate and guide OAM modes. In this paper, we propose for the first time hollow-core optical fibers consist of circular air-hole arrays for guiding THz OAM modes with the topological charge order up to l = 3. Using numerical simulations, two low-loss transmission windows over 0.47 - 0.65 THz (window1) and 0.95 - 1.20 THz (window2) are recognized to guide OAM modes. Results show that both windows of the THz fiber exhibit the flat near-zero dispersion as well as low confinement loss (~ 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> dB/cm) with simultaneously high purity of the OAM modes (higher than 90%). Finally, two designs of THz optical fibers consist of one and two rings of air-hole arrays in their cladding are examined and it is shown that difference of the confinement loss and the modal purity between two designs are negligible. This implies that hollow-core optical fibers consist of one ring of air-hole array in their cladding are very promising for efficient transmission of OAM modes in the THz region.

Transmission and Generation of Orbital ANGULAR Momentum Modes in Optical Fibers

The orbital angular momentum (OAM) of light provides a new degree of freedom for carrying information. The stable propagation and generation of OAM modes are necessary for the fields of OAM-based optical communications and microscopies. In this review, we focus on discussing the novel fibers that are suitable for stable OAM mode transmission and conversion. The fundamental theory of fiber modes is introduced first. Then, recent progress on a multitude of fiber designs that can stably guide or generate OAM modes is reviewed. Currently, the mode crosstalk is regarded as the main issue that damages OAM mode stability. Therefore, the coupled-mode theory and coupled-power power theory are introduced to analyze OAM modes crosstalk. Finally, the challenges and prospects of the applications of OAM fibers are discussed.

Experimental demonstration of OAM-based transmitter mode diversity data transmission under atmosphere turbulence.

Twisted light carrying orbital angular momentum (OAM), which features helical phase front, has shown its potential applications in diverse areas, especially in optical communications. For OAM-based free-space optical (FSO) links, a significant challenge is the power fading induced by atmospheric turbulence. In this paper, we experimentally demonstrate the mitigation of atmospheric turbulence effects with an OAM-based transmitter mode diversity scheme. By designing multi-OAM phase patterns, we successfully generate multiple OAM modes (OAM-1,0,1, OAM+2,+3,+4, OAM+5,+6,+7) carrying the same data stream for transmitter diversity without adding system complexity. An intensity-modulated direct-detection (IM-DD) system with 39.06 Gbit/s discrete multi-tone (DMT) signal is employed to confirm the feasibility of the OAM-based transmitter mode diversity scheme under atmosphere turbulence. The obtained experimental results show that the received power fluctuation and average bit-error rate (BER) are decreased under moderate to strong turbulence compared to the traditional single OAM mode transmission. In addition, the required transmitted power at 10% interruption probability is relaxed by nearly 2 dB under moderate to strong turbulence.

Numerical Study of Photonic Crystal Fiber Supporting 180 Orbital Angular Momentum Modes With High Mode Quality and Flat Dispersion

We propose a photonic crystal fiber with a high refractive index ring, which supports up to 180 orbital angular momentum (OAM) modes without any high order radial modes from 1.5 um to 1.7 um. The finite element method is employed to numerically study the properties of the designed fiber. Further research shows that the refractive index differences of all the eigenmodes are above 2 ×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> - 3</sup> , which indicates that the corresponding eigenmodes can be well-separated and ensures the stable transmission of OAM modes. In addition, it is quite interesting that the high order radial modes are significantly suppressed via increasing the radius of the cladding in the design, while most of the OAM modes are without phase distortion, which is beneficial for the (de)multiplexing of the OAM modes. Particularly, the mode quality of all the eigenmodes is higher than 94.9% and the chromatic dispersion is flat with minimum value of the dispersion variation is 0.36 ps/(km·nm). Moreover, the designed fiber possesses various advantages such as low nonlinear coefficient (<; 0.7 /W/km) and confinement loss (on the order between 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-12</sup> and 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> - 7</sup> dB/m). The proposed fiber has shown great potential for high capacity OAM mode division multiplexing communications.

Hollow Ring-Core Photonic Crystal Fiber With &gt;500 OAM Modes Over 360-nm Communications Bandwidth

We propose and design a hollow As2S3 ring-core photonic crystal fiber (PCF) with 514 radially fundamental orbital angular momentum (OAM) modes over 360 nm communications bandwidth across all the O, E, S, C, and L bands. The designed PCF with 40 $\mu \text{m}$ -radius air core and 150 nm-width As2S3 ring can support eigenmodes up to HE130,1 and EH128,1. The numerical analysis shows that the designed ring PCF has large effective refractive index contrast, and can transmit up to 874 OAM modes near 1.55 $\mu \text{m}$ . Simulation results show that in the C and L bands, the PCF with a hollow-core radius of 40 $\mu \text{m}$ and a ring width of 0.15 $\mu \text{m}$ can retain an $2.5\times 10 ^{-3}$ effective refractive index difference between the two highest order OAM modes, which achieves effective mode separation, thereby achieving stable OAM mode transmission. The $n_{eff}$ difference between the even and odd fiber eigenmodes and the intra-mode walk-off are also carefully studied under different bending radii. The results show that higher-order OAM modes has better tolerance to the fiber bending, compared with the lower-order modes. The fiber has the potential to support ultra-high capacity OAM mode division multiplexing in the optical fiber communication systems.

Low dispersion and confinement loss photonic crystal fiber for orbital angular momentum mode transmission

In this paper, we design a novel type of photonic crystal fiber (PCF) with a kind of typical micro-structure in its innermost cladding area, which can totally transmit 26 orbital angular momentum modes (OAMs). Over a bandwidth of 300 nm from 1.5 to 1.8 μm, this new type PCF shows a great improvement in dispersion property when it is compared with the total internal reflection PCF for reference. Moreover, other mode properties are also calculated and listed in curves, including effective indices, confinement loss and nonlinear coefficient. And basing on the test results, we further ameliorate the micro-structure and cladding of previous PCF structure and propose two effective ways to reduce the dispersion and confinement loss respectively. One is to insert typical type of micro-structure; the other is to change the arrangement and size of air holes in the cladding area. We finally propose an optimized structure which can support 30 OAM modes with the application of the two ways. Results show the optimized structure performs very well and achieves great progress in terms of dispersion and confinement loss.

Epsilon-near-zero photonic crystal fibers for a large mode separation of orbital angular momentum modes

An orbital angular momentum (OAM) mode is a good candidate as an orthogonal basis mode set for mode division multiplexing (MDM) owing to its unique phase properties. Photonic crystal fibers support large numbers of OAM modes with notable performances. However, the effective index difference between high order OAM modes is not large enough. In this work, we propose two types of epsilon-near-zero (ENZ) materials-embedded circular photonic crystal fibers for high mode separation. Compared with previously developed photonic crystal fibers, the designed fibers exhibit a high refractive index contrast owing to the inserted ENZ materials. The designed fibers support up to 42 OAM modes. Most of these OAM modes provide a large effective index difference (Δneff > 10−3) while maintaining a radial sing mode condition over a wide wavelength range from 1.3 μm to 1.8 μm. The proposed fibers could potentially be exploited in MDM for stable transmission of OAM modes and other OAM-based applications.

Design of Photonic Crystal Fiber Capable of Carrying Multiple Orbital Angular Momentum Modes Transmission

For the traditional photonic crystal fibers with circular air holes, rectangular air holes are added to the fiber cladding. The periodic arrangement of the inner rectangular air holes allows the fiber structure to better match the annular mode field distribution of the vortex beam. The fiber structure was analyzed and calculated by COMSOL Multiphysics 5.4 finite element software, and the characteristics of fiber were analyzed, such as the dispersion, confinement loss, effective mode area and nonlinear coefficient. The results reveal that the photonic crystal fiber structure capable of carrying 50 orbital angular momentum (OAM) modes at the wavelength of 1.15 to 2.0 μm (850 nm). The effective refractive index difference Δneff between vector modes can reach 1 × 10-3, and larger difference can effectively separate the vector modes and improve the transmission performance of OAM modes. Moreover, the fiber has good performance, such as flat dispersion distribution of the low-order modes, low confinement loss below 10-9 dB·m-1, large effective mode field area and small nonlinear coefficient in the 850 nm wavelength range. Therefore, this fiber structure can be applied to the high-capacity communication system of fiber multiplexing OAM. In addition, the good characteristics of this fiber structure are of great significance for the transmission of vortex beam in fiber.

Air-Core Ring Fiber With &gt;1000 Radially Fundamental OAM Modes Across O, E, S, C, and L Bands

In this work, an air-core ring fiber is designed with a record high 1322 orbital angular momentum (OAM) modes at 1550 nm while maintaining radially single-mode condition. Moreover, it can support over 1004 OAM modes across all O, E, S, C, and L bands, exploiting to our knowledge the highest number of OAM modes ever supported in the optical fiber within a wide wavelength range. Simulations show that, across the C and L bands, the fiber with 55-μm air-core radius and 0.45-μm ring width can preserve 3.3 × 10 -3 effective refraction index difference between the two highest-order OAM modes HE 340,1 and EH 271,1 . This enables efficient mode separation, and thus achieving stable OAM modes transmission. The effective refractive index differences between the even and odd fiber eigenmodes are also analyzed in the elliptical and bent fibers. We note that higher-order OAM modes are more tolerant to the fiber ellipticity and bending. This ring fiber design has the potential to increase the spectral efficiency and the overall capacity in fiber-based communications system.

Amplifying Orbital Angular Momentum Modes in Ring-Core Erbium-Doped Fiber.

Lots of research efforts have been devoted to increase the transmission capacity in optical communications using orbital angular momentum (OAM) multiplexing. To enable long-haul OAM mode transmission, an in-line OAM fiber amplifier is desired. A ring-core fiber (RCF) is considered to be a preferable design for stable OAM mode propagation in the fiber. Here, we demonstrate an OAM fiber amplifier based on a fabricated ring-core erbium-doped fiber (RC-EDF). We characterize the performance of the RC-EDF-assisted OAM fiber amplifier and demonstrate its use in OAM multiplexing communications with OAM modes carrying quadrature phase-shift keying (QPSK) and quadrature amplitude modulation (QAM) signals. The amplification of two OAM modes over four wavelengths is demonstrated in a data-carrying OAM-division multiplexing and wavelength-division multiplexing system. The obtained results show favorable performance of the RC-EDF-assisted OAM fiber amplifier. These demonstrations may open up new perspectives for long-haul transmission in capacity scaling fiber-optic communications employing OAM modes.

A Unified Statistical Model for Atmospheric Turbulence-Induced Fading in Orbital Angular Momentum Multiplexed FSO Systems

This paper proposes a unified statistical channel model to characterize the atmospheric turbulence induced distortions faced by orbital angular momentum (OAM) in free space optical (FSO) communication systems. In this channel model, the self-channel irradiance of OAM modes as well as crosstalk irradiances between different OAM modes are characterized by a Generalized Gamma distribution (GGD). The latter distribution is shown to provide an excellent match with simulated data for all regimes of atmospheric turbulence. Therefore, it can be used to overcome the computationally complex numerical simulations to model the propagation of OAM modes through atmospheric turbulent FSO channels. The GGD allows obtaining very simple tractable closed-form expressions for a variety of performance metrics. Indeed, the average capacity, the bit-error rate, and the outage probability are derived for FSO systems using single OAM mode transmission with direct detection. Furthermore, we extend our study to FSO systems using OAM mode diversity to improve the performance. By using a maximum ratio combining (MRC) at the receiver, the GGD is also shown to fit the simulated combined received optical powers. Finally, space-time (ST) coding is proposed to provide diversity and multiplexing gains, and the error probability is theoretically derived under the newly proposed generic model.