Modulation For Optical Communications Research Articles

In-phase and quadrature frequency-shift keying for low-power optical wireless communications

This article proposes using in-phase and quadrature frequency-shift keying (IQFSK) modulation for low-power optical wireless communications (OWC). IQFSK independently leverages both cosine and sine basis functions to enhance the system’s spectral efficiency (SE). It uses only the odd harmonic frequencies for these basis functions, allowing the clipping of negative amplitude excursions without losing information, making the waveform compatible with OWC The work presents optimal maximum likelihood and low-complexity sub-optimal detection mechanisms for IQFSK. The proposed scheme is analyzed analytically and with numerical simulations. The simulation and analytical results indicate that the proposed scheme is more energy-efficient, can attain a better energy and SE trade-off by exploiting the frame structure of the waveform, and has a lower minimum squared Euclidean distance relative to other state-of-the-art FSK-based counterparts, thus establishing it as one of the most efficient FSK approaches for low-power OWCs.

Generalized modulation for distance-aware optical camera communication beyond oversampled and undersampled schemes

Oversampled modulation (OM) and undersampled modulation (UM) are two commonly used optical camera communication (OCC) modulation schemes for high-speed communication in short-range and reliable communication at long distances, respectively. However, the relationship between these two schemes and the tradeoff in performance for arbitrary communication distances have not been thoroughly investigated. In this study, we analyze the impact of distance and modulation parameters on pixel efficiency and packet delivery rate performance, demonstrating the underlying unity of traditional OM and UM schemes. Furthermore, we propose a generalized modulation scheme that allows for achieving predefined link performance at a given distance by adjusting the modulation parameters, such as packet length and repetition counts. Simulation and experimental results show that the proposed generalized modulation scheme provides OCC with a unique distance-aware capability other than the traditional OM and UM schemes, which are two special cases focusing on effectiveness and reliability, respectively. This research enhances our understanding of OCC data modulation and establishes a theoretical foundation for achieving efficient and reliable OCC transmission in complex environments.

AC-FPSK modulation for low-energy and low data rate optical wireless communications

This article proposes an amalgam of phase-shift keying and asymmetrically clipped frequency-shift keying (i.e., AC-FPSK) for low-energy/low-data rate optical wireless communications. We design a near-optimal, low-complexity harmonic receiver by analyzing the AC-FPSK time-domain (TD) and frequency-domain (FD) waveforms. The Euclidean distance analysis between AC-FPSK waveform pairs yields lower and upper bounds for theoretical bit error probability. Using numerical simulations, we present an exhaustive evaluation of the optimal cardinality of PSK, which illustrates the superiority of AC-FPSK. We show that AC-FPSK provides better energy efficiency versus spectral efficiency trade-off than state-of-the-art AC-FSK, direct current-FPSK, pulse amplitude modulation, and asymmetrically clipped optical orthogonal frequency division multiplexing. We also demonstrate that by making a suitable choice for the cardinality of PSK, the proposed 2-tap harmonic receiver for AC-FPSK can reach a similar complexity as the AC-FSK harmonic receiver.

Free Space Optical Communication Networking Technology Based on a Laser Relay Station

Optical communication modulation technology and networking technology are two important technologies for constructing free-space optical (FSO) communication. In this paper, pulse width modulation (PWM) is used to realize free-space optical communication. The process of signal modulation and demodulation is implemented by means of a field programmable gate array (FPGA). An optical communication relay system is constructed to realize communication networking. The binary data bits in the communication process are converted into pulse signals of different widths, the data demodulation process is realized by sampling with a high-speed analog-to-digital converter (ADC), the data level is determined by counting the proportion of high and low voltages sampled in a pulse period. The relay system analyzes the routing target after receiving the pulse signal from the transmitter, and then sends the data to the target receiver. The experimental results show that the constructed system can achieve point-to-multipoint free-space optical communication. Additionally, using ADC to demodulate the received signal increases the stability of the free-space optical communication system. This system provides the design prototype system of FSO communication networking technology.

Analysis of Optical Single-Event Transients in Integrated Silicon Photonics Mach-Zehnder Modulators for Space-based Optical Communications

Analysis of Optical Single-Event Transients in Integrated Silicon Photonics Mach-Zehnder Modulators for Space-based Optical Communications

Efficient and Low-Complexity Rate and Dimming Control of VLC for Industrial IoT Applications

Visible light communication (VLC) has emerged as a promising complementary technology to millimeter-wave communications for short-range communications to meet the increasing industrial Internet of Things (IIoT) capacity demands. Efficient illumination and data transmission face specific challenges, such as controlling dimming without compromising transmission capacity. In this article, we revisit multipulse pulse-position modulation (MPPM) for joint rate–dimming control in VLC. MPPM is widely considered a highly efficient modulation for optical communications. However, the computational complexity of current MPPM coding methods grows exponentially as the code rate is increased. To overcome this limitation, we propose novel encoding and soft-decision decoding strategies that allow fine-tuning dimming control and increase the data rate. Performance results show higher data rates with lower energy demands as compared to current standards. Moreover, the proposed encoding and decoding methods exhibit linear time complexity, which makes them suitable for implementation in low-cost VLC devices for massive IIoT deployments.

Thermally stable gold nanorod dispersed silicone composite with plasmonic resonance in the optical communication window

Gold nanorods (AuNRs) possess a high optical nonlinear coefficient, ultrafast optical response speed and widely tunable localized surface plasmon resonance (LSPR) wavelength covering the visible and near infrared region. Therefore, they are extensively investigated for many optical applications. However, the poor thermal stability of the AuNRs seriously restricts their practical performance. In addition, for many applications, such as optical communication or laser modulation, AuNRs have to be combined with transparent solids, for example polymers, glass or crystals to make devices. Here, we report on the preparation of 0.23 mg AuNR dispersed methyl silicone resin (MSR) with longitudinal LSPR (L-LSPR) wavelength (1450 nm) in the optical communication window. We found that AuNR-silicone composites possess high thermal stability. After calcination in ambient environment at a temperature of 250 °C for 10 h, the L-LSPR peak of the sample can remain longer than 1380 nm, implying that the NR shape of the Au particles was well maintained. Using the open-aperture Z-scan technique, the nonlinear absorption coefficient of the composites was measured as −11.71 cm GW−1, higher than many nonlinear materials. Thus, the thermally stable AuNR@SiO2-MSR composite with high nonlinearity is promising for practical applications in the optical communication window.

LiNbO3 crystals: from bulk to film

Lithium niobate on insulator (LNOI) technology, for electro-optical modulation and acoustic wave filtering in next-generation optical and wireless communications, is driving optoelectronic device performance to new heights. Hybrid integration allows LNOI to be an enabling technology for optical computation, microwave photonics, and quantum information, with large-scale photonic integration, high optical reconfigurability, and strong nonlinear interaction at the single photon level. Key areas of future research identified: large-size low-defect thin film lithium niobate wafer and high-performance device fabrication.

Multifunctional Optical Vortex Beam Generator via Cross-Phase Based on Metasurface

We propose a multifunctional optical vortex beam (OVB) generator via cross-phase based on a metasurface. Accordingly, we separately investigate the two different propagation characteristics of OVB modulated by the low-order cross-phase (LOCP) and the high-order cross-phase (HOCP) in a self-selected area. When LOCP modulation is added to OVB, topological charges can be measured for any order of OVB. Moreover, we achieve the rotation tunable performance successfully by adding the rotation component. Then, we realize the function of polygonal beam generation and singularities regulation with the HOCP. The order of the HOCP is exactly equal to the number of a polygon OVB’s sides. The waist radius and usable width of the beam lengthens as the distance of the self-selected area increases. When the conversion rate is doubled, the distance between singularities widens by about 0.5 μm. The proposed OVB generator provides a simple strategy for detecting the value of topological charges and achieving OVB shaping and singularity manipulation simultaneously. We hope this can open new horizons for promoting the development of photon manipulation, optical communication, and vortex beam modulation.

Dynamic Network Formation for FSO Satellite Communication

Satellite network optimization is essential, particularly since the cost of manufacturing, launching and maintaining each satellite is significant. Moreover, classical communication optimization methods, such as Minimal Spanning Tree, cannot be applied directly in dynamic scenarios where the satellite constellation is constantly changing. Motivated by the rapid growth of the Star-Link constellation that, as of Q4 2021, consists of over 1600 operational LEO satellites with thousands more expected in the coming years, this paper focuses on the problem of constructing an optimal inter-satellite (laser) communication network. More formally, given a large set of LEO satellites, each equipped with a fixed number of laser links, we direct each laser module on each satellite such that the underlying laser network will be optimal with respect to a given objective function and communication demand. In this work, we present a novel heuristic to create an optimal dynamic optical network communication using an Ant Colony algorithm. This method takes into account both the time it takes to establish an optical link (acquisition time) and the bounded number of communication links, as each satellite has a fixed amount of optical communication modules installed. Based on a large number of simulations, we conclude that, although the underlying problem of bounded-degree-spanning-tree is NP-hard (even for static cases), the suggested ant-colony heuristic is able to compute cost-efficient solutions in semi-real-time.

Freeform wavelength division multiplexing metagrating based on topology optimization

Metasurfaces consist of arrays of artificial atoms arranged on a subwavelength scale, and have significant advantages in modulating the phase, amplitude, and polarization of optical field. Limited by the discrete sampling principle and the assumption of periodicity, the conventional forward design method suffers unavoidable design errors, which easily leads the device performance to degrade. In this paper, a freeform wavelength division multiplexing (WDM) metagrating with a large deflection angle and polarization-insensitive characteristics is inversely designed by using an adjoint multi-objective topology optimization method. The simulation results show that the topology-optimized WDM metagrating has superior polarization in sensitivity compared with the discrete regular structure, with a deflection angle of 70.8° at 510 nm, an absolute deflection efficiency of 48%, and a transmission efficiency of 98% for 852 nm incident light. On this basis, the absolute deflection efficiency can be optimized to more than 70% by using a random initial structure. The freeform WDM metagrating designed in this paper has the advantages of large deflection angle, high efficiency, and low spatial crosstalk, and has potential applications in optical communication, micro and nano-optical field modulation, and Rydberg atom-based microwave measurements.

Design of an On-Chip Plasmonic Modulator Based on Hybrid Orthogonal Junctions Using Vanadium Dioxide.

We present the design of a plasmonic modulator based on hybrid orthogonal silver junctions using vanadium dioxide as the modulating material on a silicon-on-insulator. The modulator has an ultra-compact footprint of 1.8 μm × 1 μm with a 100 nm × 100 nm modulating section based on the hybrid orthogonal geometry. The modulator takes advantage of the large change in the refractive index of vanadium dioxide during its phase transition to achieve a high modulation depth of 46.89 dB/μm. The simulated device has potential applications in the development of next generation high frequency photonic modulators for optical communications which require nanometer scale footprints, large modulation depth and small insertion losses.

PAM4 rolling-shutter demodulation using a pixel-per-symbol labeling neural network for optical camera communications.

The typical optical camera communication (OCC) modulation scheme is based on binary intensity modulation. To increase the transmission data rate, multi-level modulation format is highly desirable. In this work, we bring forward and demonstrate a rolling shutter 4-level pulse amplitude modulation (PAM4) demodulation scheme for OCC systems using pixel-per-symbol labeling neural network (PPSL-NN) for the first time up to the authors' knowledge. A bit-rate distance product of 28.8 kbit/s • m per color is achieved. The proposed scheme is to calculate and re-sample the pixel-per-symbol (PPS) to make sure the same number of pixels in each PAM4 symbol is corresponding to a label for the neural network. Experiment results reveal that the proposed scheme can efficiently demodulate high speed PAM4 signal in the rolling shutter OCC pattern.

Comparison Study of Multi-Slot Designs in Epsilon-Near-Zero Waveguide-Based Electro-Optical Modulators

We present a systematical comparative research on the modulation performance and the optimization of the multi-slot waveguide modulator at the telecom wavelength of 1.55 μm. It is found that the existence of epsilon-near-zero indium tin oxide slots in modulators can enhance the optical confinement, thus yielding a high extinction ratio (ER) and low insertion loss. Among the designed four types of slot modulators, the single-slot modulator exhibits the widest modulation bandwidth of 78.75 GHz and lowest energy consumption of 1.15 pJ/bit, while the dual-slot one shows the moderate performance. For the more-slot designs, the tri-slot has the advantages of a maximum figure of merit of ~106, and the quadri-slot one has the highest ER of 1.38 dB/μm. By integrating the multi-slot modulator on the silicon waveguide, the quadri-slot modulator waveguide exhibits broad optical bandwidth of 83 nm from 1479 to 1560 nm and large modulation depth (~18.3 dB). The performance of four types of slot modulators can be further improved from different aspects by the optimization of their geometric parameters. The results of this work could be useful in the design and selection of high performance on-chip modulators for optical communications and ultrafast data processing.

Generalized Time Slot Index Modulation for Optical Wireless Communications

A novel modulation scheme for an indoor optical wireless communication (OWC) system referred to as generalized time slot index modulation (GTIM) is proposed for single-carrier frequency domain equalization (SC-FDE) systems. This scheme is further referred to as SC-GTIM. In SC-GTIM, information bits are flexibly encoded onto both the amplitudes and positions of pulses. A set partitioning algorithm is proposed to construct a codebook for the SC-GTIM system. In this paper, SC-GTIM will be compared with the conventional pulse amplitude modulation-based SC-FDE (SC-PAM) and time slot index modulation-based SC-FDE (SC-TIM). By considering a multipath optical wireless channel and the limited dynamic range of a light emitting diode (LED), a considerable bit error ratio (BER) gain can be achieved by SC-GTIM. It is concluded that SC-GTIM is a promising technique for low to medium spectral efficiency transmission and uplink schemes.

Research on Key Technologies of Mode Multiplexing / Demultiplexing

The rapid growth of Internet capacity has driven the continuous progress of optical fiber communication technology. The transmission capacity of the current optical fiber communication system is close to the limit of single-mode optical fiber transmission. The mode-division multiplexing technology based on few-mode optical fiber can increase the communication capacity exponentially, which is the current research hotspot. Mode multiplexing and demultiplexing are one of the key technologies to realize modular division multiplexing. This paper expounds the existing mode multiplexing / demultiplexing schemes and their corresponding research progress, and discusses the current problems and future development application prospects of optical fiber communication module division multiplexing technologies.

Robust Multidimensional Optical Modulation Based on Hybrid Subcarrier/Amplitude/Phase/Dual Polarization for Wavelength-Division Multiplexing Systems

Here, we propose a novel scheme based on advanced techniques of digital modulation in optical communications to achieve a single-channel transmission rate above 100 Gb/s. We utilize a hybrid scheme amplitude/phase/frequency/dual polarization, combined with multidimensional dual lattice and a low-density parity-check-coded modulation. The Stokes parameters are applied to the proposed scheme to map the four-dimensional classical polarization I X , Q X , I Y , Q Y in a three-dimensional space. In addition, in the proposed system, the packing theory is applied to the bit interleaver process. Three wavelengths are packaged before being transmitted over a wavelength-division multiplexing optical channel. This modulation process is carried out using symmetrical geometric shapes, such as a hypercube or a polyhedron, based on the molecular links theory using a grouping of 12 and 13/15 bits for the cubic and spherical lattices, respectively. The proposed technique is evaluated in the context of long-distance communications over distances up to 100km. The bit error rate (BER) results showed that the optical signal-to-noise ratio was approximately 4dB over a distance of 50km. In addition, the power spectral efficiency was found to be 3 lambdas, which is considered good performance considering the effects of distance and the non-linear effects influencing the number of lambdas. Also, we use an optical time-division multiplexing scheme (OTDM) in order to achieve a transmission rate beyond 1Tbit/s, where the speed effect is evaluated, taking into consideration that the power spectral efficiency is degraded.

Chaotic Polarization-Assisted ${L}$ DPSK-MPPM Modulation for Free-Space Optical Communications

In this paper, we present a polarization-assisted $L$ -ary differential phase-shift keying multipulse pulse-position modulation (PA. $L$ DPSK-MPPM) technique that is secured in the physical layer by a discrete-chaos system. The all-optical PA. $L$ DPSK-MPPM scheme benefits from the polarization as an additional degree of freedom which greatly reduces the system complexity relative to prior implementations. The discrete-chaos scrambling is based on a message-seeded two-dimensional chaotic map tailored for independent perturbation of the occupied time-slot positions (MPPM information) and their relative phase shift ( $L$ DPSK information). Synchronized and non-synchronized implementations of the chaotic PA. $L$ DPSK-MPPM technique are proposed with expressions for the corresponding spectral efficiencies being determined and compared with prior $L$ DPSK-MPPM setups. The performance of PA. $L$ DPSK-MPPM under gamma-gamma (GG) free-space optical (FSO) fading channels is analytically verified to outperform the prior designs for different FSO channel states which is supplemented by Monte Carlo (MC) simulations. The system security is numerically examined against various types of attacks, including brute-force, differential, and statistical attacks.

Ordered Sequence Detection and Barrier Signal Design for Digital Pulse Interval Modulation in Optical Wireless Communications

This paper proposes an ordered sequence detection (OSD) for digital pulse interval modulation (DPIM) in optical wireless communications. Leveraging the sparsity of DPIM sequences, OSD shows a comparable performance to the optimal maximum likelihood sequence detection with much lower complexity. Compared with the widely adopted sample-by-sample optimal threshold detection (OTD), it considerably improves the bit-error-rate (BER) performance by mitigating error propagation. Moreover, this paper proposes a barrier signal-aided digital pulse interval modulation (BDPIM), where the last of every $K$ symbols is allocated with more power as an inserted barrier signal. BDPIM with OSD (BDPIM-OSD) can limit the error propagation between two adjacent barriers. To reduce the storing delay when using OSD to detect extremely large packets, we propose BDPIM with a combination of OTD and OSD (BDPIM-OTD-OSD), within which long sequences are cut into pieces and separately detected. Approximate upper bounds of the average BER performance of DPIM-OTD, DPIM-OSD, BDPIM-OSD, and BDPIM-OTD-OSD are analysed. Simulations are conducted to corroborate our analysis. Optimal parameter settings are also investigated in uncoded and coded systems by simulations. Simulation results show that the proposed OSD and BDPIM bring significant improvement in uncoded and coded systems over various channels.

Ultrahigh‐Q Photonic Nanocavity Devices on a Dual Thickness SOI Substrate Operating at Both 1.31‐ and 1.55‐µm Telecommunication Wavelength Bands

AbstractA feasible method for integrating several silicon (Si) photonic devices with operating wavelengths separated by several hundred nanometers on a single chip will greatly help increasing capacities of small optical communication modules. This work demonstrates the integration of two photonic crystal nanocavity devices that exhibit ultrahigh quality factors (Q) and operate at the 1.31‐ and 1.55‐µm bands. A dual thickness Si‐on‐insulator substrate forms the base of the device. The two nanocavity patterns are defined by electron beam lithography on the thick and thin substrate regions and are transferred to the top Si layer by performing plasma etching only once. All dimensions of the fabricated 1.31‐µm nanocavity are ≈15.5% smaller (1–1.31/1.55) than those of the 1.55‐µm nanocavity; that is, they can be treated with the same photonic band diagram. Both nanocavities exhibit an ultrahigh Q > 2.0 × 106 and enable fabrication of nanocavity‐based Raman lasers for the 1.31/1.55‐µm bands with sub‐microwatt threshold.