Orbital Angular Momentum Fiber Research Articles

Propagating data without MIMO over a weakly-coupled OAM fiber by multiplexing OAM mode group

Urged by the continually increasing requirement of channel capacity, orbital angular momentum (OAM) has become a promising candidate for extending the multiplexed physical domain, which is compatible with the traditional physical space. However, due to the serious crosstalk among OAM modes and propagation loss, the OAM-based optical fiber communication (OFC) is limited within a shorter scope. A common multiple-input-multiple-out (MIMO) algorithm are usually employed to equalize the multiplexed channels for reducing the crosstalk at the cost of inevitably increasing the complexity of the system. For overcoming the aforementioned issues, a weakly-coupled OAM fiber (WCOF) is designed and manufactured to propagate OAM beams without MIMO, where three OAM mode groups (OMGs) are employed for multiplexing. An experimental setup is also designed and established for validating the feasibility of the proposed WCOF. The captured intensity profiles demonstrate the transmitted OAM beams can be successfully received, and the measured bit-error-rate (BER) distributions show the simulated user data carried by each of three OMGs can be correctly received with/without MDM after the propagation of more than 100 km WCOF when the received power (RP) is set to be greater than −28.33/-27.5 dBm, respectively. Moreover, the measured BERs of OMG1/2/3 are all below the Hard FEC threshold at the C + L waveband, which demonstrates that the proposed scheme can be utilized in the whole C + L waveband. In addition, the influences of data velocity and WCOF length on the BER are also exploded, which shows that the detected BER performance declines mildly as the data velocity increases from 10 to 40 Gbit/s, and the measured BER performance also slightly degrades as the WCOF length increases from 100 to 140 km. Therefore, the proposed scheme can be potentially utilized without MIMO for long-haul OFC with higher channel capacity.

Ultrahigh-channel-count OAM mode conversion utilizing a hybrid few-mode fiber configuration.

We propose and demonstrate a hybrid few-mode fiber configuration (HFMFC) that enables ultrahigh-channel-count orbital angular momentum (OAM) mode conversion. The HFMFC consists of periodically twisted graded-index few-mode fiber segments and a step-index few-mode fiber segment. Our proposed HFMFC-based multichannel OAM mode converter (OAM-MC) offers an exceptionally high channel count in a wide bandwidth, with customizable channel spacing down to 50 GHz (∼0.4 nm), achieved through optimization of the structural parameters of the HFMFC. By employing this methodology, we have successfully demonstrated 10, 32, 117, and 233 channel OAM mode conversions covering the entire C + L band, representing the highest performance among all reported fiber-based multichannel OAM-MCs to date, for the first time to the best of our knowledge. The suggested ultrahigh-channel-count OAM-MC exhibits promising potential for applications in various fields such as OAM fiber communication, OAM holography, OAM information processing, and OAM metrology.

Designing an OAM fiber with a two-layer seven-core structure to support 322 OAM-mode transmissions

In this paper, a two-layer seven-core structural fiber (TLSCSF) is proposed to support and improve the propagation of more orbital angular momentum (OAM) mode beams, thus increasing the transmission capacity and spectrum efficiency (SE) of an optical communication system. The TLSCSF is composed of seven sub-cores, each containing two inner and outer layers of core rings and a central air hole. The two core rings are prepared using two materials with different doping concentrations. The supported OAM modes can be propagated in the fiber core ring. A finite element method (FEM) is used to analyze the designed fiber, and the calculated results show that the TLSCSF can stably transmit 322 OAM modes without higher order radial modes in the wavelength range of 1.5∼1.6 um. The corresponding effective refractive index difference between two adjacent vector modes (HE/EH) is more than 10−4. The measured effective mode areas (Aeff) and the confinement losses (CL) of HE or EH modes are larger than 54 um2 and smaller than 10−10 dB m−1, respectively. Moreover, the calculated dispersion variations and the mode qualities are lower than 230 ps nm−1 km−1 and more than 90%, respectively. Finally, the 10ps walk-off lengths of all vector modes supported by TLSCSF at a wavelength of 1.55 um are also evaluated, where the measured results show that except for HE2,1 and EH1,1, all other modes achieve a 10ps walk-off length of the order of 103 to 105. Consequently, the TLSCSF can contributes to improving the channel capacity and SE by supporting 322 fundamental radial OAM modes with robust performances in an optical communication system.

Full C-band covered and DWDM channelized high channel-count all-fiber orbital-angular-momentum mode generator based on the fiber gratings.

To generate the orbital-angular-momentum (OAM) modes at multiple wavelengths, which exactly fit with the dense-wavelength-division-multiplex (DWDM) channel grids, is important to the DWDM-based OAM mode-division-multiplex (MDM) fiber communication system. In this study, a full C-band covered and DWDM channelized OAM mode generator is firstly proposed and experimentally demonstrated, which is realized especially by using a broadband helical long-period fiber grating (HLPG) combined with a phase-only sampled multichannel fiber Bragg grating (MFBG). As a proof-of-concept example, the DWDM channelized two complementary 51-channel OAM mode generators have been successfully demonstrated, each of which has a channel spacing of 100 GHz (∼0.8 nm), an effective bandwidth of ∼40 nm, a high azimuthal-mode conversion efficiency of 90%, and high uniformities in both inter- and intra-channel spectra as well. To the best of our knowledge, this is the first time for proposal and experimental demonstration of such a high channel-count and DWDM channelized first-order OAM mode (l = 1) generator, which can also be used for multichannel higher-order OAM mode generation as long as the utilized HLPG is capable of generating a broadband higher-order OAM mode. The proposed device has potential applications to DWDM-based OAM fiber communications, OAM comb lasers, OAM holography, and OAM sensors as well.

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.

Orbital Angular Momentum in Fibers

Orbital angular momentum (OAM) of light beams with helical phase front, as an alternative spatial dimension resource of photons, has attracted extensive attention over the past decades. Very recently, OAM in fibers has facilitated rapid development of various fiber-based applications, such as optical communications, optical manipulation, optical sensing, optical imaging, nonlinear optics, and quantum science. Comprehensive and in-depth understanding of OAM in fibers is of great interest to researchers in this area. Here, we give a tutorial overview of OAM in fibers. We first introduce the basic theories, including the eigenmode analysis method, mode coupling theory, and spin-orbit mapping of light in fibers. We then give a detailed classification of OAM fibers and discuss versatile spatial mode bases in fibers. After that, we review various OAM fiber designs, including conventional single-mode fibers, conventional multi-mode fibers, ring-core fibers, air-core fibers, photonic crystal fibers, negative-curvature hollow-core fibers, multi-core fibers, active fibers (e.g., erbium-doped fibers), etc. Besides, we describe the state-of-the-art commercial fiber manufacturing processes as well as testing and characterization methods for OAM fibers. We also review various active and passive OAM fiber devices. Moreover, we introduce several typical advanced applications based on OAM fibers, such as capacity scaling fiber-optic communications, remote vectorial Doppler velocimetry, super-resolution optical imaging, and high-dimensional quantum cryptography. Finally, we briefly discuss future challenges and prospects of OAM in fibers.

Mitigating Crosstalk of Vortex Beam Within an OAM-Mode Group Over an OAM Fiber by Reversing Transmission Matrix

For enhancing channel capacity and spectrum efficiency, vortex beam carrying orbital angular momentum (OAM) has been ardently investigated in optical communication. However, the crosstalk among OAM modes induced by mode coupling has seriously hindered the development of vortex beam communication (OBC). A novel concept of mitigating crosstalk within an OAM-mode group for a short-haul optical fiber communication (OFC) is proposed by reversing the transmission matrix. To support more OAM modes propagation, a step-index OAM fiber with ring-shaped profile is also designed and fabricated, which can support 6 vector/OAM modes propagation. A proof-of-concept experiment is performed for verifying the feasibility of the proposed concept, where a quadrature-phase-shift-keying (QPSK) signal is utilized as an excitation signal. Bit-error-ratio (BER) and constellation diagram (CD) are employed to evaluate the performance of the proposed system. The captured intensity profiles and interference diagrams demonstrate that the propagated four OAM beams within the 2rd OAM-mode group are successfully received. The measured BER and CD illustrate that the crosstalk within the mode group can be significantly mitigated. Further, although the data speed or fiber length is increased, the system performance using the proposed concept is still superior to the case without increasing fiber length or data speed when the proposed concept is missed. Therefore, the proposed concept is effective for mitigating the crosstalk and can be then used for a short-haul OAM-based OFC.

Broadband dispersion compensating ring-core fiber for orbital angular momentum modes.

A well designed ring-core fiber can theoretically support numerous orbital angular momentum (OAM) modes with low crosstalk for space-division-multiplexing (SDM) data transmission, which is considered as a promising solution for overcoming the capacity crunch in optical communication network. However, the accumulated chromatic dispersion in OAM-fiber could limit the data speed and transmission distance of communication systems. A potential solution is to insert a dispersion compensation ring-core fiber with opposite-sign of dispersion in the transmission fiber along the fiber link. In this work, we propose a triple ring-core fiber with broadband negative dispersion. A highest negative dispersion of -24.47 ps/(nm·km) at 1550 nm and an average dispersion slope in the C band from -0.182 ps/(nm2·km) to 0.065 ps/(nm2·km) can be achieved to compensate multi-order dispersion. The effects of Ge-doping concentration fluctuation in the high-index ring core and fabrication errors on fiber geometric structures are also investigated. Furthermore, the effective mode area decreases as the widths of high-index rings increase due to the enhanced confinement ability. The designed triple ring-core fiber could offer potential for compensating OAM fiber links with positive dispersions.

Design for Terahertz Circular-Core Photonic Crystal Fiber Supporting Orbital Angular Momentum Modes

We propose a new terahertz fiber based on a circular-core photonic crystal fiber (PCF) structure to support high-performance orbital angular momentum (OAM) modes transmission. The modal characteristics of the proposed terahertz fiber were thoroughly analyzed to vary the parameters of air holes radius and the annular thickness by the full-vector finite element method (FEM). The optimal parameters are selected to realize the stable transmission of five-order OAM mode with high mode quality, low confinement loss and wide bandwidth. The mode purity is in excess of 91%, and the confinement loss is less than 10−7 dB/m over the 0.2 THz to 0.55 THz band. Furthermore, the design of this PCF is relatively simple and flexible, since it consists only of circular air holes. Due to its excellent transmission characteristics, the proposed OAM fiber has a potential application in terahertz mode division multiplexing (MDM) communication system.

OAM Beams Generation Technology in Optical Fiber: A Review

In optical fiber, the Orbital Angular Momentum (OAM) mode is a special mode with spatially infinite orthogonality. It provides a new multiplexing method to the communication capacity shortage problem and solves the complexity and high energy loss of OAM beams in space. In the past few years, lots of researchers studied OAM generation technology, including phase element coupling method, fiber grating method, optical coupling conversion method, and photonic crystal fiber method. This article provides a comprehensive review of the basic principles of OAM fiber design, the generation technology of OAM beams in optical fibers, and finally discusses the challenges and application prospects. The purpose of this paper is to provide reference and guidance for related experiments of OAM beams in the future.

Spin-orbit coupling suppressed high-capacity dual-step-index ring-core OAM fiber.

We design and fabricate a dual-step-index ring-core fiber (RCF) for orbital angular momentum (OAM) mode transmission. It has been proven that the proposed novel, to the best of our knowledge, dual-step-index ring-core structure can suppress the spin-orbit coupling and cut off the radial higher-order modes when the mode number becomes very large. In experiments, we demonstrate that the fabricated fiber can support OAM8,1 with the interferometric method, where four higher-order mode groups are weakly-coupled. We also measure the loss of each mode according to the cut-back method and the loss can achieve <0.3 dB/km for the OAM modes with an order from |l| = 1 to |l| = 5. The exploration of this novel optical fiber structure may provide ideas and knowledge for the improvement of the optical fiber communication capacity.

Effects of the Intramode-Group Coupling Induced by the Elliptical Deformation of Orbital Angular Momentum Fibers

Based on the coupling mode theory and numerical simulation, intramode-group coupling in orbital angular momentum (OAM) fibers with elliptical deformation is analyzed in depth. The elliptical deformation not only introduces a time delay difference between even and odd modes of the same high-order vector mode but also destroys the structure of the high-order vector modes; the structural damage depends on the strength of the elliptical deformation. When the elliptical deformation is relatively small, the structural damage of high-order vector modes can be ignored and coupling can be thought as occurring only between two OAM modes composed of the same high-order vector mode. While the elliptical deformation becomes larger, the structural damage cannot be ignored, coupling appears among four OAM modes in the same OAM mode group, and the coupling between two OAM modes composed of the same high-order vector mode is more serious. The results will pave the way for the establishment of accurate propagation models and the optimization of the OAM fiber design.

Distributed multi-parameter sensing based on the Brillouin scattering effect in orbital angular momentum guiding fiber

The orbital angular momentum (OAM) guiding fiber is used as a sensing element to measure strain and ambient temperature, sensing information simultaneously in a classical BOTDR configuration, due to its higher-order acoustic modes and high stimulated Brillouin threshold. The Brillouin threshold, the Brillouin gain coefficient and the Brillouin gain spectrum (BGS) of OAM fiber at 1.5 µm are characterized and demonstrated theoretically and experimentally. Taking advantage of the special acoustic properties of the peaks caused by the hard cladding-core interface in the Brillouin scattering process, the distributed multi-parameter sensing (e.g., strain and/or ambient temperature) is verified over a 1-km OAM guiding fiber, with the respective errors of strain and temperature of 18.2 µɛ and 0.93 °C, respectively.

Amplification of 14 orbital angular momentum modes in ring-core erbium-doped fiber with high modal gain.

We propose and demonstrate an orbital angular momentum (OAM) fiber amplifier supporting 14 OAM modes based on a fabricated ring-core erbium-doped fiber (RC-EDF) with a tailored erbium-doped profile. Theoretical analyses and numerical simulations suggest that the proposed RC-EDF provides relatively uniform gain larger than 20 dB for all 14 modes. With a core pump configuration, we experimentally characterize the performance of the RC-EDF-assisted OAM fiber amplifier, which can acquire a high modal gain up to 33.4 dB and low differential modal gain less than 1.8 dB at the wavelength of 1550 nm. The obtained results indicate successful implementation of the RC-EDF-assisted OAM fiber amplifier for 14 OAM modes with favorable performance.

Design of negative curvature fiber carrying multiorbital angular momentum modes for terahertz wave transmission

We design a novel negative curvature fiber (NCF) to support terahertz orbital angular momentum (OAM) modes. Numerical models are constructed to evaluate the transmission performances of vector modes in the frequency range of 0.40–0.80 THz. The designed fiber made from only one kind of heat-resistance resin (HTL) material can support 50–52 OAM modes with large effective index differences above 10−4 and low confinement loss (in the level of 10−15 dB/cm). Some other significant characteristics of the designed NCF including waveguide dispersion, OAM purity, and effective mode field area (Aeff), are also calculated and analyzed with their effects and applications on terahertz OAM guidance. The optimized absolute value of dispersion is between 0.33 and 0.8 ps/(THz × cm) in the range of 0.45–0.75 THz. The high mode purity (85%−95%) in the range of 0.60–0.80 THz further promotes the mode division multiplexing in OAM fiber. The proposed NCF can offer great potential in high-capacity terahertz communication based on OAM multiplexing.

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.

Orbital angular momentum communications based on standard multi-mode fiber (invited paper)

Orbital angular momentum (OAM) modes, having unique properties of a helical phase structure and doughnut intensity profile, have been widely studied in fiber-optic communications, in terms of OAM modulation and OAM multiplexing. In general, different types of specialty fibers with a ring-shape structure are preferred for more stable OAM transmission, which, however, may face greater manufacturing challenge and larger fiber loss compared to standard multi-mode fibers (MMFs). Therefore, the widely deployed and commercially available standard MMFs that can support hundreds of OAM modes have recently attracted great attention. In this paper, we review recent research progress in OAM communications based on standard MMFs. First, the basic concept of OAM and different types of specially designed OAM fibers are briefly introduced. Then, the OAM mode properties in MMFs and recent works, including OAM mode modulation, multiple-input multiple-output (MIMO)-free OAM mode group multiplexing, small-scale partial MIMO assisted OAM mode multiplexing, and OAM-based heterogeneous fiber-optic networks, are presented. The OAM communications using other widely deployed standard single-mode fibers are also briefly introduced as supplementary. Finally, key challenges and perspectives of OAM communications based on standard MMF are discussed and summarized.

High resolution spectral metrology leveraging topologically enhanced optical activity in fibers

Optical rotation, a form of optical activity, is a phenomenon employed in various metrological applications and industries including chemical, food, and pharmaceutical. In naturally-occurring, as well as structured media, the integrated effect is, however, typically small. Here, we demonstrate that, by exploiting the inherent and stable spin-orbit interaction of orbital angular momentum fiber modes, giant, scalable optical activity can be obtained, and that we can use this effect to realize a new type of wavemeter by exploiting its optical rotary dispersion. The device we construct provides for an instantaneous wavelength-measurement technique with high resolving power R = 3.4 × 106 (i.e., resolution < 0.3 pm at 1-μm wavelengths) and can also detect spectral bandwidths of known lineshapes with high sensitivity.

Design optimization of orbital angular momentum fibers using the gray wolf optimizer.

Optical data communication based on the orbital angular momentum (OAM) of light is a recently proposed method to enhance the transmission capacity of optical fibers. This requires a new type of optical fiber, the main part of the optical communication system, to be designed. Typically, these fibers have a ring-shaped refractive index profile. We aim to find an optimized cross section refractive index profile for an OAM fiber in which the number of supported OAM modes (channels), mode purity, and the effective refractive index separation of OAM modes to other fibers modes are maximized. However, the complexity of the relationship between structural parameters and optical transmission properties of these fibers has resulted in the lack of a comprehensive analytical method to design them. In this paper, we investigate the process of designing OAM fibers and propose a framework to design such fibers by using artificial intelligence optimizers. It is worth mentioning here that this problem is intrinsically a multiobjective optimization problem, and the actual solution for such problems is not unique and leads to a set of optimum solutions. Therefore, at the end of the optimization process, a wide range of optimal designs will be obtained in which a trade-off is established in each of the solutions. We solve this problem with the multiobjective gray wolf optimizer (GWO) and compare the results with that of the single-objective GWO. The framework can easily find many optimal designs that support more than 20 OAM modes. The obtained results show that the proposed method is comprehensive and can optimize the structure of any OAM fibers. No human involvement, simplicity, and being straightforward are the main advantages of the proposed framework.

Mode-division multiplexed transmission of wavelength-division multiplexing signals over a 100-km single-span orbital angular momentum fiber

We experimentally demonstrate mode-division multiplexed (MDM) transmission using eight orbital angular momentum (OAM) modes over a single span of 100-km low-attenuation and low-crosstalk ring-core fiber (RCF). Each OAM mode channel carries 10 wavelength-division multiplexing (WDM) signal channels in the C band, with each WDM channel in turn transmitting 16-GBaud quadrature phase-shift keying signal. An aggregate capacity of 2.56 Tbit/s and an overall spectral efficiency of 10.24 bit/(s · Hz) are realized. The capacity-distance product of 256 (Tbit/s) · km is the largest reported so far for OAM fiber communications systems to the best of our knowledge. Exploiting the low crosstalk between the OAM mode groups in the RCF, the scheme only requires the use of modular 4 × 4 multiple-input multiple-output processing, and it can therefore be scaled up in the number of MDM channels without increasing the complexity of signal processing.