High-capacity Optical Communication Research Articles

Theoretical investigation on communication bandwidth of an orthogonal symmetry-based photonic crystal waveguide for wavelength division multiplexing

To realize wavelength division multiplexing for high-capacity optical communications, we theoretically investigate the properties of a photonic crystal waveguide structure in which a row of air holes is displaced by a/2 (a is the lattice constant). We calculate transmission loss and coupling efficiency to circular defect cavities of this waveguide structure, which we call an orthogonal lattice waveguide (OLW). The communication bandwidth of the OLW is estimated to be about 20 nm, which is twice that of the conventional line-defect (W1) waveguide. This result indicates that improvements in characteristics of the wavelength division multiplexing are expected by using the OLW instead of W1.

Determining the topological charge of an obstructed vortex beam via reconstructed phase distribution

Determination of the topological charge (TC) of a vortex beam is of significant importance in various applications such as high-capacity optical communications and micromanipulations. Though many kinds of methods, until now, have been proposed to measure the TC, most of them will fail if the measured vortex beam is partially blocked by an opaque obstacle. In this Letter, we have introduced an efficient method to determine the TC of an incomplete vortex beam (partially blocked by an obstacle) using its reconstructed phase distribution under propagation. The numerical and experimental results showed that the sign and magnitude of the TC can be simultaneously determined by counting the number of phase singularities and the directions of the phase variations around the singularities occurring in the reconstructed phase pattern, respectively. Our method works even when half of the vortex beam is blocked.

On-Chip Orbital Angular Momentum Sorting With a Surface Plasmon Polariton Lens

Orbital angular momentum (OAM) of light provides a new dimension to increase the communication capacity. The compact and miniaturized devices are preferred because of the advantages in reliability and easy integration. In this work, a quarter-ring-shaped phase-gradient metasurface (QR-PGM) is proposed to sort various propagating waves into fully separated waveguide channels according to their OAM states. The principle is based on focal spot shifting arising from the constructive interference of surface plasmon polaritons with azimuthal phase modulated by the vortex beam. A thorough analysis of the SPP focusing properties was also carried out, which suggests the superiority of quarter-circular arc in OAM sorting (compared with half-circular arc). Focusing efficiency of the QR-PGM reaches to 25%, owing to dense sampling of incident light and unidirectional excitation of SPPs. By coupling spots to waveguides, the wide band on-chip sorting was demonstrated. In the broad wavelength band from 1.3 to 1.6 μm, the extinction ratio of each waveguide port keeps above 3 dB. The proposed structure is compact, efficient, wavelength-division-multiplexing-compatible, and easy to integrate with other on-chip devices. It will be a promising element for high capacity optical communication systems.

Wavelength-Switchable Multiple Type Bound Solitons in a Passively Mode-Locked Er-Doped Fiber Laser

We report on the experimental observation of wavelength-switchable bound solitons in a passively mode-locked erbium-doped fiber ring laser based on a tapered graded-index multimode fiber. Due to the multimode interference filtering effect, by adjusting the polarization controller while maintaining the pump power of 158 mW at 1480 nm, the center wavelength of the fiber laser output can be switched between 1565 nm and 1595 nm. At the same time, bound soliton pairs and bound soliton triplets can be both observed in these two wavelengths. Such a simple, compact all-fiber mode-locked laser with switchable wavelength has great potential for high capacity optical communication.

High-Accuracy and Fast Measurement of Optical Transfer Delay

Measurement of optical transfer delay (OTD) is crucial to applications such as fiber-distributed multiantenna systems, fiber-optic sensors, and high-capacity optical fiber communications. However, present OTD measurement techniques are inadequate for the demands of high accuracy, high speed, and large measurement range, simultaneously. Here, we propose a novel method based on nonlinear frequency sweeping and phase derived ranging to achieve all the above-mentioned performance. A continuous-wave light modulated by a microwave signal propagates in a device under test. Then, the OTD is mapped into the phase variation of the microwave signal by photodetection. A microwave phase discriminator is used to extract the phase variation from the microwave signal, while the nonlinear frequency sweeping and a novel phase unwrapping algorithm are proposed to resolve 2π phase ambiguity caused by phase detection. Frequencies of the microwave swept signals are set at four selected points in a range of 10 MHz, which ensures high speed and large measurement range. Our experiment results verify an accuracy of ±0.05 ps in measuring an ultrahigh-accuracy optical delay line. In addition, long fiber is also tested, which proves that a measurement range of at least 37 km (theoretically 100 km) can be achieved. Moreover, the measurement speed reaches milliseconds per measurement.

Impact of Equalization-Enhanced Phase Noise on Digital Nonlinearity Compensation in High-Capacity Optical Communication Systems.

Equalization-enhanced phase noise (EEPN) can severely degrade the performance of long-haul optical fiber transmission systems. In this paper, the impact of EEPN in Nyquist-spaced dual-polarization quadrature phase shift keying (DP-QPSK), dual-polarization 16-ary quadrature amplitude modulation (DP-16QAM), and DP-64QAM optical transmission systems is investigated considering the use of electrical dispersion compensation (EDC) and multi-channel digital backpropagation (MC-DBP). Our results demonstrate that full-field DBP (FF-DBP) is more susceptible to EEPN compared to single-channel and partial-bandwidth DBP. EEPN-induced distortions become more significant with the increase of the local oscillator (LO) laser linewidth, and this results in degradations in bit-error-rates (BERs), achievable information rates (AIRs), and AIR-distance products in optical communication systems. Transmission systems using higher-order modulation formats can enhance information rates and spectral efficiencies, but will be more seriously degraded by EEPN. It is found that degradations on AIRs, for the investigated FF-DBP schemes, in the DP-QPSK, the DP-16QAM, and the DP-64QAM systems are 0.07 Tbit/s, 0.11 Tbit/s, and 0.57 Tbit/s, respectively, due to the EEPN with an LO laser linewidth of 1 MHz. It is also seen that the selection of a higher-quality LO laser can significantly reduce the bandwidth requirement and the computational complexity in the MC-DBP.

Dual-mode and two-signal-wavelength femtosecond optical parametric oscillator based on LiB3O5.

We demonstrate a tunable femtosecond dual-beam-mode (cylindrical vector beam [CVB] and Gaussian beam [GB]), dual-signal-wavelength optical parametric oscillator based on a temperature-tuned lithium triborate crystal, synchronously pumped by a frequency-doubled mode-locked Yb-doped fiber laser. When fixing the CVB wavelength at 780 nm, the central wavelength of the GB signal could be continuously tuned from 664 to 722 nm. The maximum total signal output power is 515 mW at a 4 W pump with dual-wavelength operation (664 and 780 nm). All the measured signal pulse durations are around 150 fs. Moreover, sum-frequency-generation with Gaussian mode tuning from 548 to 588 nm is obtained, with the maximum power of 52 mW at 548 nm. Thanks to the dual-channel configuration, the wavelengths of a CVB and GB can be tuned independently. Such a flexible and versatile configuration makes it a practical tool for many applications such as high-resolution microscopy and high-capacity optical communication.

Differential Gain Comparison of Optical Planar Amplifier on Silica Glasses Doped with Bi-Ge and Er, Yb Ions

The paper presents the measurement and calculation of the optical amplifier gain and the optimal length of the active optical amplifier waveguide doped with Bi-Ge radiation complexes compared to an optical amplifier with active doping with ions Er3+, Yb3+. At present are using optical amplifiers for the high-capacity optical communication systems in the narrow spectral region of 1530–1560 nm, determined by the gain bandwidth of erbium-doped fiber amplifiers (EDFA) or erbium-doped optical planar amplifiers (EDPA) realized as active planar waveguides in the optical integrated circuits technique. However, it is possible increase wavelength region up to 1610 nm, where optical losses of telecommunication fibers are less than 0.3 dB per km, if appropriate amplifiers are available. In this regard, the development of novel optical amplifiers operating in this spectral region have of great importance. The paper summarizes the results measurement of the attenuation and emission spectra for net gain spectra calculation of the new ion exchange Ag+ – Na+ optical Er3+ and Yb3+ doped active planar waveguides realized on silica glass substrates and parameters of novel optical amplifiers for extension of the bandwidth from 1530 to 1610 nm doped by bismuth – erbium ions.

Characterization of off-axis phase singular optical vortex and its nonlinear wave-mixing to generate control broad OAM spectra

We theoretically and numerically studied the off-axis Gaussian vortex beam (GVB) characteristics. The off-axis GVB scheme is utilized to generate tunable broad OAM modes using a single helical phase. The single helical phase can be easily created in the high peak power lasers using a spiral phase plate (SPP). The high peak power GVBs greatly enhances the efficiency of nonlinear wave-mixing to tune the wavelength as well as OAM modes of laser beams. Further, we propose type-II second harmonic generation (SHG) of off-axis GVB to tune the wavelength and to increase the number of OAM modes of vortex beam. Polarization-controlled broad OAM spectra is generated at SHG wavelength. This scheme is effective for control generation of a broad range of OAM spectra in the required wavelength for high-capacity and multi-channel optical communication.

Fiber Amplifiers and Fiber Lasers Based on Stimulated Raman Scattering: A Review.

Nowadays, in fiber optic communications the growing demand in terms of transmission capacity has been fulfilling the entire spectral band of the erbium-doped fiber amplifiers (EDFAs). This dramatic increase in bandwidth rules out the use of EDFAs, leaving fiber Raman amplifiers (FRAs) as the key devices for future amplification requirements. On the other hand, in the field of high-power fiber lasers, a very attractive option is provided by fiber Raman lasers (FRLs), due to their high output power, high efficiency and broad gain bandwidth, covering almost the entire near-infrared region. This paper reviews the challenges, achievements and perspectives of both fiber Raman amplifier and fiber Raman laser. They are enabling technologies for implementation of high-capacity optical communication systems and for the realization of high power fiber lasers, respectively.

Fully Passive User Localization for Beam-Steered High-Capacity Optical Wireless Communication System

By means of narrow infrared beams very high capacity wireless links can be set up to users individually, while also offering high privacy and high energy efficiency. Accurate device localization for steering the beams is required. This has been achieved within the 10 cm pico-cell resolution of a 2D beam-steered optical wireless communication system by applying a matrix of miniature corner-cube retro-reflectors. The localization process is self-calibrating, and does not need power-consuming elements at the receiver. In an indoor laboratory system setup, real-time transfer of multiple 10GbE high-definition video streams has been demonstrated.

Capacity of turbulent ocean links with carrier Bessel-Gaussian localized vortex waves.

Based on the power spectrum of the index fluctuation with the outer scale of seawater turbulence, we develop the channel capacity of oceanic turbulence links with carrier Bessel-Gaussian vortex localized waves. By this capacity model, we investigate the influences of seawater turbulence and carrier parameters on the channel capacity. The results show that a higher rate of dissipation of kinetic energy per unit mass of fluid, larger inner scale, or lower dissipation rate of the mean-squared temperature causes the higher channel capacity; the Bessel-Gaussian localized vortex wave with a larger source transverse size, smaller Bessel cone angle, lower orbital angular momentum quantum number, or broader initial half-pulse width has stronger resistance to oceanic turbulent perturbation. This work provides a theoretical basis for realizing high-capacity oceanic optical communication with carrier Bessel-Gaussian vortex localized waves.

Special Issue on Advanced DSP Techniques for High-Capacity and Energy-Efficient Optical Fiber Communications

The rapid proliferation of the Internet has been driving communication networks closer and closer to their limits, while available bandwidth is disappearing due to ever-increasing network loads [...]

Performance enhancement of high-capacity coherent DWDM free-space optical communication link using digital signal processing

In this paper, 1.28 Tbps (32 × 40 Gbps) high-capacity DWDM-FSO link has been investigated for performance enhancement using coherent detection and digital signal processing (DSP). The DP-16QAM-modulated proposed DWDM-FSO link has been analyzed for both adverse weather and turbulent atmospheric conditions. It is observed that when link is subjected to strong turbulence along with adverse weather conditions, the DSP-aided coherent DWDM-FSO receiver achieves target bit error rate (BER) of 10−4 at signal-to-noise ratio (SNR) of 36.4 dB, while for similar conditions, the SNR requirements for IM/DD-based DWDM-FSO link shoots by 12.8 dB to 49.2 dB. Also, in terms of operational link range, the proposed link even under strong turbulent conditions serves 1.88 km, whereas IM/DD link was restricted to mere 1.12 km for target of BER of 10−4, thus producing a decent range increment of 760 m. The proposed link has been designed and investigated using OptiSystem™ 14.2.

All-dielectric metasurfaces for generation and detection of multi-channel vortex beams

We theoretically investigate the generation and detection of multi-channel vortex beams (VBs) carrying multiple orbital angular momenta by a monolayer all-dielectric metasurface. The metasurface can realize simultaneous manipulation of a two-level amplitude and continuous phase by an artificial design. The multi-channel VBs’ complex electric field is acquired by holography and the corresponding phase and amplitude are, respectively, encoded to the spatial orientation and geometry of nanofins. The maximum polarization conversion efficiency exceeds 98% and the uniformity is 0.0714. The ultra-compact, highly efficient and versatile metasurface is anticipated to promote development for high-capacity optical communication and integrated chip–level optical systems involving VBs.

Efficient Recognition of the Propagated Orbital Angular Momentum Modes in Turbulences With the Convolutional Neural Network

The vortex beam carrying orbital angular momentum (OAM) has attracted great attentions in optical communication field, which can extend the channel capacity of communication system due to the orthogonality between different OAM modes. Generally, atmospheric turbulence can distort the helical phase fronts of OAM beams, which presents a critical challenge to the effective recognition of OAM modes. Recently, convolutional neural network (CNN), as a model of deep learning, has been widely applied to machine vision. In this paper, based on the CNN theory, we make a tradeoff between the computational complexity of the system and the efficiency of recognition by establishing a specially designed six-layer CNN structure in CPU station to efficiently achieve the recognition of OAM mode in turbulent environment through the feature extraction of the received Laguerre-Gaussian beam's intensity distributions. Furthermore, we examine the performances of our designed CNN with respect to various turbulence levels, transmission distances, mode spacings, and we have also compared the performances of recognizing single OAM mode with multiplexed OAM modes. The numerical simulation shows that basing on CNN method, the coaxial multiplexed OAM modes can obtain higher recognizing accuracy about 96.25% even under long transmission distance with strong turbulence. It is anticipated that the results might be helpful for future implementation of high-capacity OAM-based optical communication technology.

Design of low-loss and near-zero ultraflattened dispersion PCF for broadband optical communication

A 2D finite difference time domain method is used to design a single-mode photonic crystal fiber (PCF) with engineered parameters that is studied with the aim of simultaneous achievement of low loss (below 10−14dB/km) and low chromatic dispersion over a broad band for future optical communication. The superiority of the proposed structure over recently reported structures is shown by the near-zero ultraflattened chromatic dispersion of 1.69±0.08 ps/(km nm) obtained over a broad wavelength range from 1 to 2 μm, covering the entire optical communication bands (O, E, S, C, L, and U). Also, the large effective area of more than 130 μm2 and as a result the low nonlinearity in addition to the near-zero ultraflattened chromatic dispersion and ultralow loss achieved for the proposed PCF make the structure a good candidate for high-capacity optical communication with a high bitrate over long distances for future ultradense wavelength-division multiplexing.

Optical orbital-angular-momentum-multiplexed data transmission under high scattering

Multiplexing multiple orbital angular momentum (OAM) channels enables high-capacity optical communication. However, optical scattering from ambient microparticles in the atmosphere or mode coupling in optical fibers significantly decreases the orthogonality between OAM channels for demultiplexing and eventually increases crosstalk in communication. Here, we propose a novel scattering-matrix-assisted retrieval technique (SMART) to demultiplex OAM channels from highly scattered optical fields and achieve an experimental crosstalk of –13.8 dB in the parallel sorting of 24 OAM channels after passing through a scattering medium. The SMART is implemented in a self-built data transmission system that employs a digital micromirror device to encode OAM channels and realize reference-free calibration simultaneously, thereby enabling a high tolerance to misalignment. We successfully demonstrate high-fidelity transmission of both gray and color images under scattering conditions at an error rate of <0.08%. This technique might open the door to high-performance optical communication in turbulent environments.

Elliptical hollow-core optical fibers for polarization-maintaining few-mode guidance

Abstract We propose polarization-maintaining few-mode elliptical hollow-core optical fibers with simple triple-layer structure toward space-division-multiplexing based high-capacity optical communication systems. The proposed fibers include a central elliptical air hole, a concentric elliptical ring core and a circular cladding. The structure-dependent polarization-maintaining performance is comprehensively investigated via numerical simulations. With appropriate structural parameters, 10 distinct polarization-maintaining modes can be guided in this fiber. The effective refractive index differences (Δneff) between all adjacent modes are beyond 1.93 × 10−4 at 1550 nm, and the values are higher than 1.8 × 10−4 over 1510–1630 nm. The modal chromatic dispersions are analyzed subsequently. Furthermore, the number of guided polarization-maintaining modes can be expanded to 14 with modified structural parameters meanwhile keeping all Δneff values no less than 1.33 × 10−4 in range of 1510–1630 nm.

Perfect optical vortex array for optical communication based on orbital angular momentum shift keying

We proposed a simple and efficient method to create a 2D perfect optical vortex (POV) array based on the Fourier shift theorem and a 2D POV array hexadecimal encoding/decoding method to realize high capacity data transmission. The encoding data sequence contains the position and topological charge information of each POV, which can be modulated by adding a phase shift factor to a specially designed hybrid phase-only grating. Based on the position and topological charge information, the 2D POV array can realize spatial division multiplexing and orbital angular momentum shift keying. The hexadecimal encoding/decoding method was investigated both theoretically and experimentally. The experimental results show that this method has potential applications in high capacity optical communications.