Energy-efficient spatial-domain-based hybrid multidimensional coded-modulations enabling multi-Tb/s optical transport

In this keynote invited paper, to address key challenges for beyond 5G wireless technologies in a simultaneous manner, we propose an orbital angular momentum (OAM) based, energy-efficient multidimensional coded modulation. The key idea is to employ all available degrees of freedom (DOFs) to convey the information over the wireless links including amplitude, phase, polarization state, and spatial-domain DOFs. In particular, the OAM is associated with azimuthal phase dependence of the wavefront, and represents an underutilized DOF. Given that OAM eigenstates are orthogonal, an arbitrary number of bits per symbol can be transmitted. To implement OAM multiplexer and demultiplexer in the RF domain, we propose to use a properly designed antenna arrays. Monte Carlo simulations demonstrate high tolerance to fading effects of LDPC-coded multidimensional signaling schemes compared to conventional LDPC-coded QAM.

Future Internet technologies will be affected not only by limited bandwidth of information infrastructure, but also by its energy consumption. In order to solve both problems simultaneously, in this invited paper, we describe several energy-efficient (EE) hybrid coded-modulation (CM) schemes enabling Terabit optical Ethernet: EE 4D CM, EE generalized- OFDM, and EE spatial-domain-based CM. A common property of these is employment of EE modulations, various degrees of freedom and rate-adaptive coding. These EE schemes are called hybrid as all available degrees of freedom are used for transmission over optical fibers including amplitude, phase, polarization and OAM. Since the channel capacity is a linear function in number of dimensions, by increasing the number of basis functions, we can dramatically improve overall capacity. The EE problem is solved by properly designing multi-dimensional signal constellations such that transinformation is maximized, while taking the energy constraint into account.

We examine a novel method to control the orbital angular momentum (OAM) of lightwaves using forked polarization gratings (FPGs). We significantly improve the fabrication of FPGs and achieve substantially higher quality and efficiency than prior work. This is obtained by recording the hologram of two orthogonally polarized beams with phase singularities introduced by <i>q</i>-plates. As a single compact thin-film optical element, an FPG can control the OAM state of light with higher efficiency and better flexibility than current methods, which usually involve many bulky optical elements and are limited to lower OAM states. Our simulations confirm that FPGs work as polarization-controlled OAM state ladder operators that raise or lower the OAM states (charge l) of incident lightwaves, by the topological charge (<i>l_g</i>) on the FPGs, to new OAM states (charge <i>l ± l_g</i>). This feature allows us to generate, detect, and modify the OAM state with an arbitrary and controllable charge. An important application of FPGs are the essential state controlling elements in quantum systems based on OAM eigenstates, which may enable extreme high capacity quantum computation and communication.

Vector vortex beams are conventionally created as the superposition of orbital angular momentum (OAM) modes with orthogonal polarizations, limiting the available degrees of freedom (DoFs) to 2, while their creation by complex optical devices such as metasurfaces, liquid crystals, and interferometers has hindered their versatility. Here we demonstrate a new class of vector vortex beam constructed from four DoFs as multiple ray-like trajectories with wave-like properties, which we create by operating a simple anisotropic microchip laser in a frequency-degenerate state. Our new structure is obtained by the superposition of two stable periodic ray trajectories, simultaneously fulfilling a completed oscillation in the cavity. By a simple external modulation, we can transform our ray trajectories into vortex beams with large OAM, multiple singularities, as well as exotic helical star-shaped patterns. Our experimental results are complemented by a complete theoretical framework for this new class of beam, revealing parallels to hybrid SU(2) coherent states. Our approach offers in principle unlimited DoFs for vectorial structured light with concomitant applications, for example, in engineering classically entangled light and in vectorial optical trapping and tweezing.

In sensor networks, both energy and bandwidth are scarce resources. In the past, the energy efficient routing problem has been vastly studied in order to maximize network lifetime, but link bandwidth has been optimistically assumed to be abundant. As energy constraint affects how data should be routed, link bandwidth also affects not just the routing topology but also the allowed data rate on each link, which in turn affects lifetime. Previous works that focus on energy efficient operations in sensor networks with the sole objective of maximizing network lifetime only consider the energy constraint and ignore the bandwidth constraint. This article shows how infeasible these solutions could be if bandwidth does become a constraint, then provides a new mathematical model to tackle both energy and bandwidth constraints. Two efficient heuristics are proposed based on this model; Simulation results show these heuristics provide more feasible routing solutions than previous works, and provide significant improvement on throughput.

Monte Carlo and deterministic calculation of the mutual information of the fission source in k-eigenvalue calculations

Reactions for which the reactant (r)-to-product (p) mass ratio (mu(r)/mu(p)) is high, the well-known H + LH --> HH + L processes, convert most of available energy to product rotation, while that disposed as recoil is often regarded as negligible. In angular momentum (AM) terms, however, this recoil orbital AM (l(p)) is shown to be a critical component of the overall AM balance. For products of light mu(p), the maximum value of l(p) is energy limited and as a result the formation of products in low rotational (j(p)) states is severely restricted. Here energy constraints on recoil orbital AM and the consequent restrictions on j(p)-state populations are quantified using novel diagrammatic methods that illustrate how constraints on l(p) determine the j(p) states that are allowed or forbidden by the need to conserve energy and AM for each state-to-state transition. The method accurately predicts j(BaI)j (v=0,1,2) peaks from crossed-beam Ba + HI experiments, providing a quantitative and physically transparent rationale for the observed BaI rotational distributions. Extension to a wider range of reactions having mu(r)mu(p)>1 shows that at least some j(p) are formally forbidden for each given reactant relative velocity or, more accurately, l(r). The fraction of inaccessible product states for a given initial velocity rises rapidly with mu(r)/mu(p) (>96% in Ba + HI). The method is also used to demonstrate that recoil orbital AM will be strongly aligned parallel to product rotational AM for high mu(r)/mu(p), although this correlation is generally lost in the low j(p) region as the parallel vector requirement is relaxed.

Angular momentum of light can be divided into spin angular momentum and orbital angular momentum (OAM). Due to the theoretically unlimited orthogonal states, the physical dimension of OAM provides a potential solution to boost the information capacity. The OAM multiplexing and modulation techniques have been implemented to meet the continuous growth of bandwidth requirements, resulting in the concept of OAM optical communication. However, the performances of the traditional optical OAM detection techniques degrade seriously in the practical application of OAM optical communications. Thanks to the powerful data analysis advantages, the cutting-edge machine learning (ML) algorithms have been widely used in the field of image processing, laying the technical foundation for OAM recognition. This paper reviews the recent advances on OAM optical communications that are enhanced by ML methods. More than the traditional OAM detection methods, the OAM demodulation methods based on multiple network architectures, including the support vector machine, self-organizing map, feed-forward neural network, convolutional neural network, and diffractive deep optical neural network (D2NN), have been summarized. We also discuss the development of the spiking neural network and on-chip D2NN, opening a possible way to facilitate the future ultra-low power and ultra-fast OAM demodulation technology.

Traditional light sources cannot emit an electromagnetic (EM) field with an orbital angular momentum (OAM), limiting their applications in modern optics. The recent development of the OAM laser, mainly based on micro- and nanostructures, can satisfy the increasing requirements for on-chip photonics and information capacities. Nevertheless, the photonic structures have fixed parameters that prevent these OAM lasers from being dynamically tuned. Here, we propose tunable vortex lasing from a microring cavity integrated by a phase change material, Ge2Sb2Te5 (GST225). By modulating the complex refractive index to create an exceptional point (EP) to break the degeneracy of whispering gallery modes with opposite orientations, the microlaser working at the EP can impart an artificial angular momentum, thus emitting vortex beams with well-defined OAM. The grating scatter on the edge of the microring can provide efficient vertical radiation. The vortex laser wavelength from the GST225/InGaAsP dual-layered microring cavity can be dynamically tuned by switching the state of GST225 between amorphous and crystalline without changing the microring geometry. We construct an electric-thermal model to show the tuning range of operating wavelengths (EPs) from 1544.5 to 1565.9 nm in ~25 ns. Our study on high-speed tunable PT-symmetry vortex lasers facilitates the next generation of integrated optoelectronic devices for optical computing and communications in both classical and quantum regions.

Long-range acoustic communication is crucial to underwater applications such as collection of scientific data from benthic stations, ocean geology, and remote control of off-shore industrial activities. However, the transmission rate of acoustic communication is always limited by the narrow-frequency bandwidth of the acoustic waves because of the large attenuation for high-frequency sound in water. Here, we demonstrate a high-throughput communication approach using the orbital angular momentum (OAM) of acoustic vortex beams with one order enhancement of the data transmission rate at a single frequency. The topological charges of OAM provide intrinsically orthogonal channels, offering a unique ability to multiplex data transmission within a single acoustic beam generated by a transducer array, drastically increasing the information channels and capacity of acoustic communication. A high spectral efficiency of 8.0 ± 0.4 (bit/s)/Hz in acoustic communication has been achieved using topological charges between -4 and +4 without applying other communication modulation techniques. Such OAM is a completely independent degree of freedom which can be readily integrated with other state-of-the-art communication modulation techniques like quadrature amplitude modulation (QAM) and phase-shift keying (PSK). Information multiplexing through OAM opens a dimension for acoustic communication, providing a data transmission rate that is critical for underwater applications.

Data center interconnect has stimulated the research on the short-reach communications with data rate beyond 100G per wavelength and transmission distance of hundreds of kilometers. Aiming at the high-speed short-reach communications, we recently proposed the Stokes vector direct detection (SV-DD) that realizes a linear complex optical channel similar to the coherent detection. In SV-DD, the transmitter places the signal and the carrier onto the orthogonal polarizations, while the receiver achieves the polarization insensitive 3-D detection in the Stokes space with the digital signal processing enabled polarization acquisition. SV-DD achieves 100% spectral efficiency with reference to the single-polarization coherent detection, and simultaneously attains the receiver phase diversity and the cancellation of photo-detection nonlinearity. We experimentally demonstrate the SV-DD signal transmission over 160-km standard single-mode fiber at data rates of both 80 and 160-Gb/s. SV-DD significantly decreases both the system hardware and DSP complexity compared with the polarization multiplexed coherent detection, while increases the system spectrum efficiency compared with the conventional intensity modulation direct detection. Therefore, SV-DD offers a cost-effective solution for the 100G per wavelength and beyond metropolitan area network (MAN). It also owns the potentials to be deployed in the future high-speed passive optical network (PON).

We corroborate that a synergy effect may emerge when the dually superposed orbital angular momentum (OAM) states with two adjacent OAM numbers pass through a weak fluctuation regime of the atmospheric turbulence. We show that this novel effect closely depends on the transition probabilities between this two OAM eigenstates for superposition. While such probabilities become larger, the synergy effect becomes more obvious. Our examples show that the synergy effect enables OAM states to better resist phase fluctuation caused by the atmospheric turbulence.

Orbital angular momentum (OAM) mode division multiplexing and encoding have been widely investigated for achieving higher information capacity and performance in both classical and quantum communication systems [1]. A critical component in such systems is an OAM mode sorter that can efficiently sort different OAM modes exp(ilθ) according to the mode index l. To this end, an elegant scheme is to transform the vortex phase exp(ilθ) of OAM modes to a linear phase exp(ilv / β) of tilted plane waves through geometrical transformations, so that they can be separated in the focal plane of a lens. A log-polar transformation was firstly employed [2] and recently a spiral transformation has been proposed to achieve high-resolution OAM mode sorting by the authors [3].

We consider universal methods for obtaining (uniform) continuity bounds for characteristics of multipartite quantum systems. We pay special attention to infinite-dimensional multipartite quantum systems under the energy constraints. By these methods, we obtain continuity bounds for several important characteristics of a multipartite quantum state: the quantum (conditional) mutual information, the squashed entanglement, the c-squashed entanglement, and the conditional entanglement of mutual information. The continuity bounds for the multipartite quantum mutual information are asymptotically tight for large dimension/energy. The obtained results are used to prove the asymptotic continuity of the n-partite squashed entanglement, the c-squashed entanglement, and the conditional entanglement of mutual information under the energy constraints.

Smith–Purcell radiation (SPR) is the electromagnetic wave generated by free electrons passing above a diffraction grating, and it has played an important role in free-electron light sources and particle accelerators. Orbital angular momentum (OAM) is a new degree of freedom that can significantly promote the capacity of information carried by an electro-magnetic beam. In this paper, we propose an integrable method for generating vortex Smith–Purcell radiation (VSPR), namely, SPR carrying OAM, by having free-electron bunches pass on planar holographic gratings. VSPRs generated by different electron energies, with different topological charges of the OAM, radiation angles, and frequencies are demonstrated numerically. It is also found that, for high-order radiation, the topological charge of the OAM wave will be multiplied by the radiation order. This work introduces a new way to generate SPR with OAM and provides a method to achieve an integratable and tunable free-electron OAM wave source at different frequency regions.