High-capacity Optical Communication Research Articles

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.

Silicon-based polarization analyzer by polarization-frequency mapping

Measuring states of polarizations (SOPs) is a fundamental requirement in high capacity optical communications, optical imaging, and material characterization. However, most of the existing methods focused on the assembly of spatial optical elements, making the system bulky and complex. Alternatively, the integrated methods were mainly presented by plasmonic nanostructures or metasurfaces, difficult to integrate with commonly used silicon photonic devices. For large-scale inter-chip optical interconnections, the silicon-based polarization analyzers are in demand and in its infancy. Here, a silicon-based polarization analyzer by polarization-frequency mapping is put forward. The basis vectors of polarization are mapped to two frequencies by thermally tuned phase shifters. The SOPs are retrieved from the frequency domain. The proposed polarization analyzer is demonstrated experimentally and can measure SOPs in the entire C-band. The scheme is compatible with the CMOS fabrication process, making it possible to be integrated with other silicon-based devices monolithically.

High-Capacity Symmetric Dynamic Indoor Optical Wireless Communication Equipped With User Localization

We report on a novel all-optical system for ultra-high-capacity indoor wireless communication with centralized light sources. Using optical cross-connect and reflective modulator photonic chips, bidirectional dynamic indoor wireless networks equipped with localization and tracking functionalities are realized. This novel system allows us to harvest the ultimate bandwidth of optical communication in the wireless domain in a more cost-/energy-efficient manner. Bidirectional transmission capacities in excess of 40 Gb/s per user are demonstrated experimentally.

Theoretical investigation of optical modulators based on graphene-coated side- polished fiber.

The effective mode index (EMI) of a graphene-coated side-polished fiber (GSPF) is calculated numerically. Whereby, the influences of graphene atom layer number, residual radius of SPF, light frequency, scattering rate of graphene, and temperature on the EMI are investigated comprehensively. Two types of mechanisms for the electro-optical absorption modulation are found for such GSPF-based modulator. One mechanism is Pauli blocking effect (PBE) and the other is plasmonic attenuation effect (PAE). With the optimal design parameters, a PBE-based modulator is theoretically predicted to have a 0.0072 dB/μm modulation depth, 2.92 V driving voltage swing, 6.35 nJ/bit power consumption, and 56.2 THz optical modulation bandwidth. It is also predicted that a PAE-based modulator could have a 0.0056 dB/μm modulation depth, 0.6 V driving voltage swing, 0.27 nJ/bit power consumption, and 2.5 THz optical modulation bandwidth. By further optimization, the modulator performance such as the relatively high power consumption and the narrow operation bandwidth can be improved. Owing to their seamless connection to optical fiber networks, the GSPF-based modulators have great potential to be used in fast and high-capacity optical communication systems.

High-Capacity Optical Wireless Communication Using Two-Dimensional IR Beam Steering

The free-space narrow infrared beams can offer unprecedented data capacity to devices individually, as they can provide non-shared connections that have a large link power budget. By means of a fully passive module based on a high port count arrayed waveguide grating router (AWGR), many infrared beams can be 2D steered individually using wavelength tuning. By applying the defocusing techniques, a compact beam steering module has been realized. A simultaneous communication at up to 112 Gbit/s PAM-4 per beam has been shown with an 80-ports AWGR, thus offering a total wireless throughput beyond 8.9 Tbit/s. The wireless provisioning of multiple ultrahigh-definition video streams has been demonstrated in a proof-of-concept laboratory setup.

Analytical comparison of photonic crystal fibers for dispersion compensation with different structures using FDTD method

In new generation of optical communication networks, simultaneous transmission of several information channels has become possible over a single optical fiber. But dispersion and dispersion slope of optical fibers are important factors, which limit high capacity optical fiber communication systems to perform high speed transmission.

Bidirectional Mode Slicing and Re-Combining for Mode Conversion in Planar Waveguides

Photonics integrated circuits and planar lightwave circuits are crucial in future high capacity optical communication networks. Such circuits can perform complicated and sophisticated signal processing functions’ on-chip level. However, they should have high bandwidth in order to handle ultrafast data flow and thus achieve high-capacity transmission. One way to achieve such high bandwidth is using few-mode waveguides. Therefore, it becomes mandatory to find a way to convert from current conventional single-mode planar waveguides into few-mode waveguides, and moreover to selectively excite the desired modes within the converted multimode waveguides. In this paper, a novel silica-glass planar waveguide converter is numerically demonstrated for the first time. This device can convert in a bidirectional fashion between single-mode and two, three, or four modes planar waveguides, in addition, to selectively excite the desired mode. The device principle of operation mainly depends on spatial modes slicing and re-combining to achieve the desired higher or lower order modes. That could be accomplished by cascading stages of V-shape and M-shape graded-index planar waveguides. The graded-indices allow for flexible and simple geometrical designs that can split or combine fundamental as well as higher order modes. The finite-difference time-domain numerical method is utilized to verify the device operation and evaluate its performance for each excited mode in bidirectional directions. The device shows good performance over the entire C-band wavelength range with the worst-case magnitude of insertion-loss ≅ 3 dB, polarization-dependent loss ≅ 0.6 dB, and return-loss ≅ 21 dB. It also shows a worst-case cross-talk of ≅ −24.1 dB among the excited high-order modes in the forward direction, and a worst-case mode-rejection ratio of ≅ −23 dB for non-excited modes in the reverse direction. Moreover, the device shows reasonable performance tolerance to variations in V-shape and M-shape waveguide design parameters.

Review of Microwave Photonics Technique to Generate the Microwave Signal by Using Photonics Technology

AbstractMicrowave photonics system provides high bandwidth capabilities of fiber optic systems and also contains the ability to provide interconnect transmission properties, which are virtually independent of length. The low-loss wide bandwidth capability of optoelectronic systems makes them attractive for the transmission and processing of microwave signals, while the development of high-capacity optical communication systems has required the use of microwave techniques in optical transmitters and receivers. These two strands have led to the development of the research area of microwave photonics. So, we can considered microwave photonics as the field that studies the interaction between microwave and optical waves for applications such as communications, radars, sensors and instrumentations. In this paper we have thoroughly reviewed the microwave generation techniques by using photonics technology.

Theoretical proposal of a low-loss wide-bandwidth silicon photonic crystal fiber for supporting 30 orbital angular momentum modes.

We propose a novel four-ring hollow-core silicon photonic crystal fiber (PCF), and we systematically and theoretically investigate the properties of their vector modes. Our PCF can stably support 30 OAM states from the wavelength of 1.5 μm to 2.4 μm, with a large effective refractive index separation of above 1×10−4. The confinement loss is less than 1×10−9 dB/m at the wavelength of 1.55 μm, and the average confinement loss is less than 1×10−8 dB/m from the wavelength of 1.2 μm to 2.4 μm. Moreover, the curve of the dispersion tends to flatten as the wavelength increases. In addition, we comparably investigate PCFs with different hole spacing. This kind of fiber structure will be a potential candidate for high-capacity optical fiber communications and OAM sensing applications using fibers.

High-Capacity Free-Space Optical Communications Between a Ground Transmitter and a Ground Receiver via a UAV Using Multiplexing of Multiple Orbital-Angular-Momentum Beams

We explore the use of orbital-angular-momentum (OAM)-multiplexing to increase the capacity of free-space data transmission to moving platforms, with an added potential benefit of decreasing the probability of data intercept. Specifically, we experimentally demonstrate and characterize the performance of an OAM-multiplexed, free-space optical (FSO) communications link between a ground transmitter and a ground receiver via a moving unmanned-aerial-vehicle (UAV). We achieve a total capacity of 80 Gbit/s up to 100-m-roundtrip link by multiplexing 2 OAM beams, each carrying a 40-Gbit/s quadrature-phase-shift-keying (QPSK) signal. Moreover, we investigate for static, hovering, and moving conditions the effects of channel impairments, including: misalignments, propeller-induced airflows, power loss, intermodal crosstalk, and system bit error rate (BER). We find the following: (a) when the UAV hovers in the air, the power on the desired mode fluctuates by 2.1 dB, while the crosstalk to the other mode is −19 dB below the power on the desired mode; and (b) when the UAV moves in the air, the power fluctuation on the desired mode increases to 4.3 dB and the crosstalk to the other mode increases to −10 dB. Furthermore, the channel crosstalk decreases with an increase in OAM mode spacing.

Digital nonlinearity compensation in high-capacity optical communication systems considering signal spectral broadening effect

Nyquist-spaced transmission and digital signal processing have proved effective in maximising the spectral efficiency and reach of optical communication systems. In these systems, Kerr nonlinearity determines the performance limits, and leads to spectral broadening of the signals propagating in the fibre. Although digital nonlinearity compensation was validated to be promising for mitigating Kerr nonlinearities, the impact of spectral broadening on nonlinearity compensation has never been quantified. In this paper, the performance of multi-channel digital back-propagation (MC-DBP) for compensating fibre nonlinearities in Nyquist-spaced optical communication systems is investigated, when the effect of signal spectral broadening is considered. It is found that accounting for the spectral broadening effect is crucial for achieving the best performance of DBP in both single-channel and multi-channel communication systems, independent of modulation formats used. For multi-channel systems, the degradation of DBP performance due to neglecting the spectral broadening effect in the compensation is more significant for outer channels. Our work also quantified the minimum bandwidths of optical receivers and signal processing devices to ensure the optimal compensation of deterministic nonlinear distortions.

Compact fabrication-tolerant subwavelength-grating-based two-mode division (de)multiplexer.

Regarding the importance of bandwidth and capacity expansion in communication systems, a novel mode division (de)multiplexer based on subwavelength grating (SWG) is proposed. SWG-based devices could have smaller sizes, be much more fabrication-tolerant, and have much wider bandwidths due to the reduced confinement of the field and dispersion. It is also feasible to reduce the loss of a SWG-based device by tuning the duty cycle of the grating. Owing to these properties of SWGs, we have designed a compact fabrication-tolerant two-mode division (de)multiplexer. A flat-top transmission of >0.89 (loss <0.5 dB) is obtained over 65 nm from 1500 to 1565 nm, completely covering the entire C-band used for dense wavelength division multiplexing. Moreover, the cross-talk is <-10 dB for a broad bandwidth of ∼120 nm over which the loss of the complete device including both multiplexer and demultiplexer is <1 dB. Therefore, the proposed device is promising for high-capacity optical communications in both conventional and entirely SWG-based silicon photonic circuits.

Underwater optical communications using orbital angular momentum-based spatial division multiplexing

In this paper, we review high-capacity underwater optical communications using orbital angular momentum (OAM)-based spatial division multiplexing. We discuss methods to generate and detect blue–green optical data-carrying OAM beams as well as various underwater effects, including attenuation, scattering, current, and thermal gradients on OAM beams. Attention is also given to the system performance of high-capacity underwater optical communication links using OAM-based space division multiplexing. The paper closes with a discussion of a digital signal processing (DSP) algorithm to mitigate the inter-mode crosstalk caused by thermal gradients.