Orbital Angular Momentum Modes Research Articles

Experimental demonstration of complex four-wave mixing processes with pump of orbital-angular-momentum superposition mode

Orbital angular momentum (OAM), characterized by a topological charge ℓ (ℓ integer), serves as a promising vehicle for carrying quantum information. Generating a nonclassical field involving two or more OAM modes can largely enhance the data-carrying capacity of quantum information processing. In this Letter, we present the implementation of a four-wave mixing (FWM) process, featuring a pump beam of a coherent OAM superposition mode and a probe beam of a single OAM mode. We generate twin beams encompassing multiple OAM modes and illustrate their quantum correlation when the pump beam assumes different coherent OAM superposition modes. Furthermore, we analyze the OAM mode components in each output field to deepen our understanding of the nonlinear interaction among different OAM modes. The obtained results show the rich structure of the nonlinear interaction of OAM modes in the FWM process and provide a perspective to study quantum information protocols.

Learning-enabled data transmission with up to 32 multiplexed orbital angular momentum channels through a commercial multi-mode fiber.

Multiplexing orbital angular momentum (OAM) modes enable high-capacity optical communication. However, the highly similar speckle patterns of adjacent OAM modes produced by strong mode coupling in common fibers prevent the utility of OAM channel demultiplexing. In this paper, we propose a machine learning-supported fractional OAM-multiplexed data transmission system to sort highly scattered data from up to 32 multiplexed OAM channels propagating through a commercial multi-mode fiber parallelly with an accuracy of >99.92%, which is the largest bit number of OAM superstates reported to date (to the best of our knowledge). Here, by learning limited samples, unseen OAM superstates during the training process can be predicted precisely, which reduces the explosive quantity of the dataset. To verify its application, both gray and colored images, encoded by the given system, have been successfully transmitted with error rates of <0.26%. Our work might provide a promising avenue for high-capacity OAM optical communication in scattering environments.

Simultaneous generation of first- to fourth-order OAM modes based on a cascaded preset-twist long-period fiber grating

Abstract We propose and demonstrate the simulation and fabrication of an all-fiber orbital angular momentum (OAM) mode converter capable of generating first- to fourth-order modes simultaneously, which is realized by inscribing a cascaded preset-twist long-period fiber grating (CPT-LPFG) in a six-mode fiber utilizing a CO2 laser. A new segmented Runge–Kutta method is proposed to simulate the preset-twist long-period fiber gratings. By calculating the twist angle and relative coupling coefficient for each pitch and then solving the coupled mode equations utilizing the Runge–Kutta algorithm. The simulation illustrates that the preset-twist method significantly improves the coupling coefficient of higher-order modes, thereby reducing coupling difficulty. In the experiment, by twisting the fiber at an angle of 1080° and fabricating cascaded gratings with periods of 745 μm, 310 μm, 204 μm, and 146 μm, it is feasible to generate first- to fourth-order OAM modes simultaneously, at wavelengths of 1635 nm, 1548 nm, 1460 nm, and 1334 nm, respectively. The insertion loss is less than 1 dB, and the mode purity is over 90 %. To the best of our knowledge, this is the first time that first- to fourth-order OAM modes are simultaneously generated utilizing a single long-period fiber grating.

High-Efficiency 4 × 4 × 10 Gbps Orbital Angular Momentum Modes Incorporated into Satellite–Ground–Underwater Optical Wireless System under Diverse Turbulences

With their unique capability to deal with a considerable geographic area, satellite–ground–underwater optical wireless communication (OWC) systems are an appealing alternative to meet the ever-increasing demand for end-to-end broadband services. Using four different Laguerre–Gaussian (LG) modes, an orbital angular momentum (OAM) multiplexing method was developed to enhance the spectral efficiency and system capacity of the satellite–ground–underwater OWC system. At an aggregate throughput of 160 Gbps, LG[0,0], LG[0,2], LG[0,4], and LG[0,8] were realized. Various atmospheric conditions, water types, and scintillation effects were used to evaluate the performance of two separate OWC links for satellite-to-ground and ground-to-underwater communication. A maximum OWC range of 21,500–30,000 km has been obtained under weak-to-strong turbulence for satellite-to-ground scenarios, and a range of 12–27 m underwater for ground-to-underwater scenarios under various scintillation effects. At LG[0,0], in pure sea, the maximum gain is −75.02 dB, the noise figure is 75.02 dB, the output signal is −78.32 dBm, and the signal-to-noise ratio is 21.67 dB. In comparison with other works in the literature, this system shows a superior performance.

Design of photonic crystal fiber supporting 190 orbital angular momentum modes with low nonlinearity coefficient

A new photonic crystal fiber is proposed for the transmission of more orbital angular momentum modes with a rectangular air hole arrangement to increase the cladding air-filling rate. The results reveal that the fiber can support 190 orbital angular momentum modes in the C+L band, with the topological charge ordering up to 48, and the effective refractive index difference between adjacent eigenmodes is above 10−3. The effective mode area of all the eigenmodes is larger than 208.02 μm2, the nonlinearity coefficient is less than 0.43 W−1∙km−1, the confinement loss reaches up to 10−12-10−8 dB/m, the dispersion is flat and the minimum dispersion change is 3.15 ps∙nm−1∙km−1, with mode purity greater than 97.4%. Compared to existing typical orbital angular momentum transmission photonic crystal fibers, this fiber can support more orbital angular momentum modes for most purposes with lower nonlinearity coefficient, higher mode purity, better propagation characteristics but simpler structure. The highest nonlinearity coefficient is reduced by at least 0.3 W−1∙km−1 compared to existing typical orbital angular momentum transmission photonic crystal fibers. The effect of fabrication errors on the transmission quality of orbital angular momentum modes is also discussed, and the results reveal that the effect on mode transmission quality is small at 1–2% variations of air holes. Therefore, this photonic crystal fiber is promising in high-capacity communication systems.

Modulation effect of focusing mirror on beam propagation through anisotropic turbulence

Abstract In this study, we modulated the Bessel-Gaussian (BG) beam using a focusing mirror to enhance the performance of wireless optical communication (WOC) while considering the angles between turbulence cells and the transmitted direction when beams propagate through anisotropic atmospheric turbulence along a slant path. Numerical results reveal that when the BG beam is modulated by a focusing mirror with a focusing length of 1 km, the detection probability of orbital angular momentum (OAM) is increased by 9.06% compared with the unmodulated BG beam when the OAM quantum number is 1. Simultaneously, this modulation method effectively reduces the bit error rate and enhances the channel capacity in OAM-based WOC. Furthermore, we observed that the smaller the OAM number, the better the effect of the modulation. Through our analysis, we identified that the most significant distortions in OAM mode propagation occur at the exponential parameter approximately 3.1 in the modified von Karman spectrum. Moreover, we demonstrated that the effects of the anisotropic tilt angle can not be negligible for beams through a slant path. Therefore, the modulation of the focusing mirror, in combination with a well-designed propagation direction that takes appropriate angles into account, can significantly improve the overall quality and quality of optical communication systems.

The resilience of Hermite–Gaussian and orbital angular momentum modes for free space optical communication in atmospheric turbulence

The resilience of Hermite–Gaussian and orbital angular momentum modes for free space optical communication in atmospheric turbulence

The characteristics of vortex modes in twisted fiber with dielectrically chiral ring core

A novel twisted ring core fiber incorporating chiral material into the ring core is proposed, which supports orbital angular momentum (OAM) modes. Results show that between chirality and twist rate there exist a cooperative effect in birefringence and a competitive effect in mode index bifurcation between OAM modes with different helicity. The cooperative effect between chirality and twist rate also breaks the degeneracy within OAM modes of the same or opposite helicity while preserving the vectorial nature of the modes. The OAM modes generated by twisted dielectrically chiral fiber are less likely to have crosstalk. The dependences of OAM modes birefringence on chirality and twist rate are investigated. Additionally, the mode effective index bifurcation as a function of wavelength is also discussed under both kind of chirality. Owing to high birefringence, good refractive index bifurcation, we believe that this kind of fiber can find applications in new kind of OAM mode generators, OAM mode filters, optical communication, and sensing.

Mode reconfigurable OAM helical array antenna for higher mode generation

AbstractIn this paper, an orbital angular momentum (OAM) mode reconfigurable antenna based on a uniform circular array (UCA) is presented. The phase superposition method is presented to overcome the limitation of the number of array units on the mode order in the conventional four‐element UCA antenna, which is limited to generating first‐order OAM beams. The unit phase is introduced by a bifilar helical antenna in this method, and the unit phase is superimposed with the feed phase of the UCA to enable the generation of 3rd‐order OAM beams from the four‐element UCA. At the same time, the DC signal‐controlled feed network assigns three distinct feed phases to the UCA, which realizes the reconfigurability of the three OAM beams. To achieve mode reconfigurability, 20 p‐i‐n diodes were integrated as switches in the feed network for controlling the ON/OFF status of p‐i‐n diodes in the feed network using DC signals. Finally, simulated and measured results show that three OAM beams (lz = −1, −2, and −3) have been generated in the frequency band from 10.6 to 11.8 GHz.

End-to-end learning strategy based on a frequency domain feature decoupling network emulator with joint probabilistic shaping and equalization for a 300-Gbit/s OAM mode division multiplexing transmission

Mode coupling and device nonlinear impairment appear to be a long-standing challenge in the orbital angular momentum (OAM) mode division multiplexing (MDM) of intensity modulation direct detection (IM/DD) transmission systems. In this paper, we propose an end-to-end (E2E) learning strategy based on a frequency domain feature decoupling network (FDFDnet) emulator with joint probabilistic shaping (PS) and equalization for an OAM-MDM IM/DD transmission with three modes. Our FDFDnet emulator can accurately build a complex nonlinear model of an OAM-MDM system by separating the signal into features from different frequency domains. Furthermore, a FDFDnet-based E2E strategy for joint PS and equalization is presented with the aim of compensating the signal impairment for the OAM-MDM IM/DD system. An experiment is carried out on a 300 Gbit/s carrierless amplitude phase-32 (CAP-32) signal with three OAM modes over a 10 km ring-core fiber transmission, and the results show that the proposed FDFDnet emulator outperforms the traditional CGAN emulator, with improvements in the modelling accuracy of 30.8%, 26.3% and 31% for the three OAM modes. Moreover, the receiver sensitivity of the proposed E2E learning strategy is higher than for the CGAN emulator by 3, 2.5, 2.2 dBm and the real channel by 5.5, 5.1, and 5.3 dBm for the three OAM modes, respectively. Our experimental results demonstrate that the proposed FDFDnet emulator-based E2E learning strategy is a promising contender for achieving ultra-high-capacity interconnectivity between data centers.

ExtraTrees-hidden Markov model based equalizer for mode division multiplexing ring-core fiber communication with non-orthogonal MultiCAP modulation

This paper presents an orbital angular momentum mode division multiplexing (OAM-MDM) ring-core fiber transmission method based on non-orthogonal multiband carrierless amplitude and phase (NMCAP) modulation with an extremely randomized trees-hidden Markov model (ExtraTrees-HMM)-based equalizer. The ExtraTrees-HMM equalizer uses the statistical characteristics of the received distorted signals to model the nonlinear channel of the system to classify these distorted signals into corresponding constellation classes. Experiments were conducted using a 216 Gbit/s OAM-MDM NMCAP modulation optical fiber communication system with 2 km ring-core fiber transmission and the results show that compared with a conventional Volterra nonlinear equalizer (VNE), the proposed ExtraTrees-HMM equalizer could improve the receiver sensitivity by 1 dB for OAM mode l =  + 2, and 0.6 dB for OAM mode l =  + 3. In addition, the computational complexity of the proposed equalizer was reduced by 43.94% compared with the VNE. In brief, the ExtraTrees-HMM is a promising equalization candidate for ultra-high-capacity inter-data-center interconnections.

Vortex electromagnetic wave imaging with orbital angular momentum and waveform degrees of freedom

The vortex electromagnetic wave has shown great prospects of radar applications, due to the orbital angular momentum (OAM) degree of freedom. However, the radiation energy convergence of the OAM beam remains a hard problem to be solved for radar target imaging in realistic scenario. In this paper, an OAM beam generation method is developed exploiting the OAM and waveform degrees of freedom simultaneously, which can collimate the beams with different OAM modes. Furthermore, the echo demodulation and the imaging methods are proposed to reconstruct the target profiles in the range and azimuth domain. Simulation and experimental results both validate that the OAM-based radar imaging can achieve azimuthal super-resolution beyond the diffraction limit of the array aperture. This work can advance the system design of vortex electromagnetic wave radar and its real-world applications.

Broadband Mode Division Multiplexing of OAM‐Modes by a Micro Printed Waveguide Structure

AbstractA light beam carrying orbital angular momentum (OAM) is characterized by a helical phase‐front that winds around the center of the beam. These beams have unique properties that have found numerous applications. In the field of data transmission, they represent a degree of freedom that could potentially increase capacity. While an efficient method for (de)composing beams based on their OAM exists for free‐space optics, a device capable of performing this (de)composition in an integrated, compact fiber application without the use of external active optical elements and for multiple OAM modes and multiple wavelengths simultaneously has not been reported. In this study, two mechanisms are combined in a waveguide structure to demonstrate as a proof of principle that this can serve as a broadband fiber OAM (de)multiplexer. The structure design is based on the adiabatic principle used in photonic lanterns for highly efficient conversion of spatially separated single modes into eigenmodes of a few‐mode fiber. In addition, an artificial magnetic field is introduced by twisting the structure, which removes the degeneracy between modes having the same absolute OAM. This structure could simplify, stabilize, and miniaturize the creation or decomposition of OAM beams, making them useful for various applications.

Beam steering using delays generated from an optical OAM mode shifting recirculating loop

This paper investigates the performance of generated delays from an orbital angular momentum mode shifting recirculating delay loop (OAM-SL) in simulation using Kirchhoff-Fresnel diffraction (KFD) and the delays applicability for RF signal beam steering applications. OAM-SL is a novel structure that has been recently reported to produce multiple delayed signal replicas on independent OAM carriers using a single compact system. The objective of this research is to explore the impact of several limiting factors on the quality of OAM-SL delays for beam steering applications, such as where there is (i) displacement of the OAM mode shifter, (ii) displacement of the center of the OAM mode demultiplexer at the loop output, and (iii) the limited resolution. The methodology includes: (i) simulating the beam propagation using KFD, (ii) producing the output as a coherent summation of the recirculating beams, and (iii) characterizing the SCR by dividing the power of the recirculating beam (ρ) on its OAM mode by the power accumulated from the other recirculating beams. Our findings show that delays generated from a system with lin=−30, lshift=+1, and a resolution of 1024 pixels can steer a 100 MHz signal at a steering angle of 11.5° in a four antenna system with 0.1° error if loop alignment is perfect. Also, for steering the 100 MHz signal in an eight antenna system, we analyze the system assuming the system loss can be compensated, and a similar result of 0.1° error is obtained. Additionally, we report the beam steering performance at 23.6° and 36.8°.

Realization of 4 × 200 Gbps 4-QAM OFDM-OWC System Using Higher Order OAM Modes for HAP-to-Satellites Scenario

Recently, there has been an increase in interest in using optical wireless communication (OWC) links on high-altitude platforms (HAPs) for satellite applications. We implement an orbital angular momentum (OAM) multiplexed orthogonal frequency division multiplexing (OFDM) system using an OWC link. A space-to-air scenario is considered in which transmission pointing errors, geometric loss, turbulence, and additional link losses are taken into account to extend the transmission range, system capacity, and throughput. At 200 Gbps per channel data rate, four different OAM modes are implemented with higher order Laguerre–Gaussian (LG) modes of [0,0], [0,13], [0,40], and [0,80]. An aggregate 800 Gbps data rate can deliver a maximum OWC range of 3300–5000 km for all channels. The maximum received power over the 1000 km range is −19.34 to −32.59 dBm with an additional gain of 0–2.5 dB. It is also possible to obtain a better performance over large distances of 500–3500 km with an error vector magnitude of 2.98–17.5%. Furthermore, a high gain of −40.80 dB, a signal-to-noise ratio (SNR) of 55.21 dB, and an optical SNR of 67.25 dB can be achieved for varied transmitter pointing errors of 0.1 rad. As compared to other literature, this system exhibits a superior performance.

Longitudinal encoding of focused vector vortex beams empowered by silicon metasurfaces

Vector vortex beams (VVBs) have attracted considerable applications due to their unique properties, such as vortex filtering, particle trapping, and quantum information. Recently, information coding methods based on metasurface strategies show excellent performance in remote sensing and communication, but the implementation of the longitudinal dimension of the optical field in information coding remains to be developed. Here, we provide a strategy for constructing all-dielectric terahertz (THz) metasurface designs for longitudinal codecs, with the individual focal planes carrying specific encrypted information. Benefiting from the joint modulation of the geometrical and propagation phases, the physical mechanism of spin-polarization multiplexing realizes the coherent superposition of multidimensional focused orbital angular momentum (OAM) modes. As a bridge between Poincaré sphere (PS) and the higher-order Poincaré sphere (HOPS), the proposed design produces a longitudinally controllable variation of OAM mode superposition states. A series of metasurface examples for generating specific coding sequences are characterized by manipulating topological charges and phase factors embedded within orthogonal circularly polarized channels. The longitudinal dimension, as a new coding degree of freedom, can be further expanded for meta-optical applications with the assistance of the VVB pattern.

Fiber Design and Performance Analyses for Optical Multiplexing: Terahertz Optical Communications

Abstract Multiplexing is the process of combining multiple signals at a single channel. Orbital Angular Momentum (OAM) is one of the main multiplexing techniques for optical data transmissions. This paper examines and suggests a hollow core with four layers of semilunar air-hole-shaped circular photonic crystal fiber (PCF) capable of transmitting terahertz (THz) OAM information-carrying modes. By using the full vector finite element method (FEM), OAM multiplexing is analyzed for the proposed fiber. All the THz OAM-based factors are analyzed at a frequency band ranging from 400 GHz to 800 GHz. For the first time, some important PCF factors such as effective refractive index difference (ERID), dispersion profile (DP), OAM purity, confinement loss (CL), effective mode area (ERA), and numerical aperture (NA) are quantitatively discussed with applications. The proposed design supports 50 OAM modes with ERID up to 10-3. The PCF has a CL of approximately 10- 10 dB/cm and the lowest dispersion profile is 0.3581 ps/THz/cm. Furthermore, the OAM purity is around 97%. Nonetheless, the proposed design can be used in THz-OAM transmission and high optical fiber communications.

All‐Dielectric Terahertz Metasurfaces for Multi‐Dimensional Multiplexing and Demultiplexing

AbstractTerahertz (THz) communication is an up‐and‐coming technology for the sixth‐generation wireless network. The realization of ultra‐high‐speed THz communication requires the combination of multi‐dimensional multiplexing schemes, including polarization division multiplexing (PDM), mode division multiplexing (MDM), and wavelength division multiplexing, to increase channel capacity. However, most existing devices for MDM in the THz regime are single‐purpose and incapable of multi‐dimensional modulation. Here, all‐dielectric metasurfaces are designed for 2D multiplexing/demultiplexing, which takes the lead in combining orbital angular momentum (OAM) MDM and PDM in the THz regime. The multi‐functional wavefront phase modulations and interleaved meta‐atom arrangements are used to realize polarization‐selective multichannel OAM mode (de)multiplexing, in which the linear‐polarized 4‐channel and circular‐polarized 6‐channel demultiplexing are experimentally demonstrated. Between different linear‐polarized channels, the measured maximum crosstalk is −16.88 dB, and the isolation of each channel can be greater than 10 dB in a range wider than 0.1 THz. This study paves the way for multi‐dimensional multiplexing in the THz regime, which may benefit extremely high‐capacity and integrated THz communication systems. The proposed design strategy is readily applied to multi‐functional metasurfaces for microwaves and far infrared light, facilitating the development of multiplexing technology and OAM‐related applications.

Vibration-insensitive polarimetric fiber optic current sensor based on orbital angular momentum modes in an air-core optical fiber.

Current or magnetic field sensing is usually achieved by exploiting the Faraday effect of an optical material combined with an interferometric probe that provides the sensitivity. Being interferometric in nature, such sensors are typically sensitive to several other environmental parameters such as vibrations and mechanical disturbances, which, however, inevitably impose the inaccuracy and instability of the detection. Here we demonstrate a polarimetric fiber optic current sensor based on orbital angular momentum modes of an air-core optical fiber. In the fiber, spin-orbit interactions imply that the circular birefringence, which is sensitive to applied currents or resultant magnetic fields, is naturally resilient to mechanical vibrations. The sensor, which effectively measures polarization rotation at the output of a fiber in a magnetic field, exhibits high linearity in the measured signal versus the applied current that induces the magnetic field, with a sensitivity of 0.00128 rad/A and a noise limit of 1×10-5/H z. The measured polarization varies within only ±0.1% under mechanical vibrations with the frequency of up to 800 Hz, validating the robust environmental performance of the sensor.

Improving fiber coupling efficiency by shaping the transmission trajectory of the vortex beam

Vortex beams with orbital angular momentum (OAM) have the potential to significantly increase the communication capacity in long-distance free-space optical communication systems. In order to obtain optimal coupling efficiency, few-mode fibers (FMFs) are employed to receive spatial light. This paper presents a computer model that describes the process of coupling vortex beams into FMFs after they have been transmitted via free space. We propose to improve the coupling efficiency by controlling the transmission trajectory of the vortex beam in free space. The coupling efficiency of a vortex beam into FMFs can be enhanced at a specific transmission distance by pre-designing the transmission trajectory of the optical field and tweaking the parameters of the coupling lens and fiber. This paper also examines the coupling correlation between higher-order OAM modes vortex beams and FMF modes. The numerical results of this study show that controlling the radial phase distribution to manage the transmission trajectory of the optical field is an effective and flexible method for improving the efficiency of coupling light between space and fiber. The results of this study can be applied in the domain of free-space optical communications.