Long-distance Optical Communication Systems Research Articles

Semiconductor laser collimation and shaping design based on the combination of a rotationally symmetrical lens and a cylindrical lens

A dual-lens collimation and shaping system which is composed of a rotationally symmetric lens and an aspherical cylindrical lens is proposed in this paper. The system design method is discussed in detail, and the divergent elliptical laser beams are finally converted into collimated circular beams. The simulation results show that the maximum divergence angle of the beams can be collimated to 2.58 µrad, the standard deviation of the edge beam radius samples (SDRS) can be decreased from the original 4.227 to 0.135 mm, and the ratio of beam waist (RBW) drops from 1.554 to 1.0093 in the case of the output beam radius of 40 mm. The effects of transmission distance, astigmatism, wavelength deviation, light source offset, and lens offset on the collimation shaping effect are discussed. The collimation system can be widely used in long-distance optical communication systems and the design method in this paper can provide some new ideas for optical design researchers.

Improving fiber coupling efficiency by shaping the transmission trajectory of the vortex beam

Vortex beams with orbital angular momentum (OAM) have the potential to significantly increase the communication capacity in long-distance free-space optical communication systems. In order to obtain optimal coupling efficiency, few-mode fibers (FMFs) are employed to receive spatial light. This paper presents a computer model that describes the process of coupling vortex beams into FMFs after they have been transmitted via free space. We propose to improve the coupling efficiency by controlling the transmission trajectory of the vortex beam in free space. The coupling efficiency of a vortex beam into FMFs can be enhanced at a specific transmission distance by pre-designing the transmission trajectory of the optical field and tweaking the parameters of the coupling lens and fiber. This paper also examines the coupling correlation between higher-order OAM modes vortex beams and FMF modes. The numerical results of this study show that controlling the radial phase distribution to manage the transmission trajectory of the optical field is an effective and flexible method for improving the efficiency of coupling light between space and fiber. The results of this study can be applied in the domain of free-space optical communications.

Signal-to-noise ratio degradation analysis for optoelectronic feedback-based chaotic optical communication systems.

Chaotic optical communication encrypts transmitted signals through physical noise; this ensures high security while causing a certain decrease in the signal-to-noise ratio (SNR). Thus, it is necessary to analyze the SNR degradation of decrypted signals after chaotic encryption and the minimum requirements for the SNR of the fiber channel to meet the required bit error rate (BER) performance. Accordingly, an SNR model of decrypted signals for optoelectronic feedback-based chaotic optical communication systems is proposed. Under different channel SNRs, the SNR degradation of 40 Gbit/s phase chaos and intensity chaos models is investigated by simulation and experiment, respectively, with a 15 GHz wideband chaotic carrier. Comparing decrypted signals with original signals, the simulation results show that there is a 2.9 dB SNR degradation for both intensity chaos and phase chaos. Further, in experiments, SNR degradation from 4.5 dB to 5.6 dB, with various channel SNRs for intensity chaos, is analyzed, while there is an SNR degradation from 7.1 dB to 8.3 dB for phase chaos. The simulation and experimental results provide guidance for long-distance transmission chaotic optical communication systems.

Long-distance indoor optical camera communication using side-emitting fibers as distributed transmitters.

We present a design approach for a long-distance optical camera communication (OCC) system using side-emitting fibers as distributed transmitters. We demonstrate our approach feasibility by increasing the transmission distance by two orders up to 40 m compared to previous works. Furthermore, we explore the effect of the light-emitting diode (LED) modulation frequency and rolling shutter camera exposure time on inter-symbol interference and its effective mitigation. Our proposed OCC-fiber link meets the forward-error-correction (FEC) limit of 3.8 · 10-3 of bit error rate (BER) for up to 35 m (with BER= 3.35 · 10-3) and 40 m (with BER=1.13 · 10-3) using 2-mm and 3-mm diameter side-emitting fibers, respectively. Our results at on-off keying modulation frequencies of 3.54 kHz and 5.28 kHz pave the way to moderate-distance outdoor and long-distance indoor highly-reliable applications in the Internet of Things and OCC using side-emitting fiber-based distributed transmitters.

Distorted Laser Spot Center Positioning Based on Captured Image Under Laser-Camera Misalignment in UOWC

The laser spot center positioning is an essential step for camera-based terminal positioning and alignment in a long-distance laser-based underwater optical wireless communication (UOWC) system. In this paper, the laser spot profile distorted by laser-camera misalignment is examined and a principal component analysis (PCA)-based spot center estimation algorithm is proposed. The centroid offset of the distorted laser spot under different deflection angles, transmitted optical powers and camera exposure values is experimentally demonstrated and analyzed. The estimation performance and the impact of the three parameters are also studied. Experimental results show that the proposed PCA-based algorithm significantly outperforms conventional gray centroid-based positioning method under different conditions.

Quantum-enhanced receiver for quadrature phase shift keying using conjugated homodyne detection

Coherent detection (homodyne detection/heterodyne detection) and single photon detection are commonly applied in classical communication and quantum communication/receiver respectively. In this paper, we firstly propose a quantum-enhanced receiver based on conjugated homodyne detection, which is consisted of two classical balanced homodyne detectors, for discrimination among quadrature-phase-modulated weak coherent states. Our detection part works as a photon counter in every single-shot measurement during the process of detection and feedback, which could help our quantum-enhanced receiver surpass the standard quantum limit when an optimized detection threshold τ from 0.05 to 0.3 is selected. Moreover, our proposed quantum-enhanced receiver with conjugated homodyne detectors can obtain the same low error probability as conventional quantum-enhanced receivers (with its superconducting nanowire single photon detectors’ detection efficiency 60% for pulse separation M=8 and 70% for M=12 under the same adaptive feedback countermeasures.) Meanwhile, our scheme of the quantum-enhanced receiver gains its incomparable advantages on room-temperature working condition and low cost due to the application of conjugated homodyne detectors. As far as we concerned, this is the first time to explore the performance of the quantum-enhanced receiver with commercial homodyne detectors. And the analysis in this paper may pave the way to reduce the cost of the quantum-enhanced receiver and make it more adaptable for long-distance optical communication systems.

Design and Theoretical Analysis of Highly Negative Dispersion-Compensating Photonic Crystal Fibers with Multiple Zero-Dispersion Wavelengths

This paper presents a highly negative dispersion-compensating photonic crystal fiber (DC-PCF) with multiple zero dispersion wavelengths (ZDWs) within the telecommunication bands. The multiple ZDWs of the PCF may lead to high spectral densities than those of other PCFs with few ZDWs. The full-vectorial finite element method with a perfectly matched layer (PML) is used to investigate the optical properties of the PCFs. The numerical analysis shows that the proposed PCF, i.e., PCF (b), exhibits multiple ZDWS and also achieves a high negative chromatic dispersion of −15089.0 ps/nm·km at 1.55 μ m wavelength, with the multiple ZDWs occurring within the range from 0.8 to 2.0 μ m range. Other optical properties such as the confinement loss of 0.059 dB/km, the birefringence of 4.11 × 10 − 1 , the nonlinearity of 18.92 W − 1 k m − 1 , and a normalized frequency of 2.633 was also achieved at 1.55 μ m wavelength. These characteristics make the PCF suitable for high-speed, long-distance optical communication systems, optical sensing, soliton pulse transmission, and polarization-maintaining applications.

Experimental demonstration of 201.6-Gbit/s coherent probabilistic shaping QAM transmission with quantum noise stream cipher over a 1200-km standard single mode fiber.

A probabilistic shaping (PS) quadrature amplitude modulation (QAM) based on Y-00 quantum noise stream cipher (QNSC) has been proposed. We experimentally demonstrated this scheme with data rate of 201.6Gbit/s over a 1200-km standard single mode fiber (SSMF) under a 20% SD-FEC threshold. Accounting for the 20% FEC and 6.25% pilot overhead, the achieved net data rate is ∼160Gbit/s. In the proposed scheme, a mathematical cipher (Y-00 protocol) is utilized to convert the original low-order modulation PS-16 (22 × 22) QAM into ultra-dense high-order modulation PS-65536 (28 × 28) QAM. Then, the physical randomness of quantum (shot) noise at photodetection and amplified spontaneous emission (ASE) noise from optical amplifiers are employed to mask the encrypted ultra-dense high-order signal for further improving the security. We further analyze the security performance by two metrics known in the reported QNSC systems, namely the number of masked signals (NMS) of noise and the detection failure probability (DFP). Experimental results show it is difficult or even impossible to extract transmission signals from quantum or ASE noise for an eavesdropper (Eve). We believe that the proposed PS-QAM/QNSC secure transmission scheme has the potential to be compatible with existing high-speed long-distance optical fiber communication systems.

50 m/187.5 Mbit/s real-time underwater wireless optical communication based on optical superimposition

In this paper, an optical pulse amplitude modulation with 4 levels (PAM-4) using a fiber combiner is proposed to enhance the data rate of a field-programmable gate-array-based long-distance real-time underwater wireless optical communication system. Two on–off keying signals with different amplitudes are used to modulate two pigtailed laser diodes, respectively, and the generated optical signals are superimposed into optical PAM-4 signals by a fiber combiner. The optical PAM-4 scheme can effectively alleviate the nonlinearity, although it reduces the peak-to-peak value of the emitting optical power by 25%. A real-time data rate of 187.5 Mbit/s is achieved by using the optical PAM-4 with a transmission distance of 50 m. The data rate is increased by about 25% compared with the conventional electrical PAM-4 in the same condition.

Method Analysis of Compensating Polarization Mode Dispersion in Optical Fiber Communication System

At present, optical fiber communication systems are developing in the direction of large capacity, high speed, long distance, etc., making nonlinear effects and polarization mode dispersion (PMD) and other optical fiber performance defects that were negligible for low-rate systems become the main factors limiting system upgrades and transmission distances. Polarization mode dispersion is an important factor affecting the performance of optical fiber communication systems, and it has become one of the main obstacles to the development of high-speed and long-distance optical fiber communication systems. It can cause pulse widening in the digital transmission system, thereby limiting the transmission rate of the system. Therefore, the influence of polarization mode dispersion on the transmission system and the compensation method are analyzed in this paper.

Analytical soliton solutions of the perturbed fractional nonlinear Schrödinger equation with space–time beta derivative by some techniques

Nonlinear fractional-order evolution equations are fundamental strategies for simulating nonlinear phenomena on a large scale in technology, science, and engineering. This article considers the nonlinear space–time fractional perturbed nonlinear Schrödinger (NLS) equation defined in the sense of the beta-derivative, a widely used model to simulate nonlinear optics, ultra-short pulse lasers, optical communication systems, plasmas, and other domains. The model is converted into a nonlinear equation defined by fractional wave transformation. We construct diverse sorts of soliton solutions in the integration of rational, trigonometric, exponential, and hyperbolic functions with open parameters using the typical generalized exponential rational function (GERF) and the improved Bernoulli sub-equation function (IBSEF) schemes. Some standard shapes of waveforms, including parabolic soliton, kink, flat kink, bright-dark soliton, periodic soliton, breather, breathing periodic, singular periodic, breathing-like soliton, flat parabolic, and several other types of solitons, are determined. Periodic solitons under perturbations are required to understand ultra-short pulse lasers and long-distance optical communication systems. To validate the physical characteristics of the established solitons, we sketch 3D, 2D, and contour plots of the solutions using consistent values of the parameters. The techniques used in this study to extract inclusive and standard solutions are approachable, efficient, and speedier to compute the soliton solutions for nonlinear models in communication engineering and optics.

Hybrid OAM-Amplitude multiplexing and demultiplexing of incoherent optical states

We propose multiplexing and demultiplexing of Orbital Angular Momentum (OAM) states realized with incoherent beams to increase the effective transmission area in a free space optical communication link. The overlapped states of the beams have been effectively distinguished using a hybrid OAM-Amplitude scheme exploiting only a small portion of the entire wavefront at the receiver. The method has been theoretically formalized and the transmission experimentally tested up to 1 Mbit/s. The proposed method is suitable for high density transmission in long-distance free space optical communication systems.

Coherent chaotic optical communication of 30 Gb/s over 340-km fiber transmission via deep learning.

Chaotic optical communication has attracted much attention as a hardware encryption method in the physical layer. Limited by the requirements of chaotic hardware synchronization, fiber transmission impairments are restrictedly compensated in the optical domain. There has been little experimental demonstration of high-speed and long-distance chaotic optical communication systems. Here, we propose a method to overcome such limitations. Using a deep-learning model to realize chaotic synchronization in the digital domain, fiber transmission impairments can be compensated by digital-signal processing (DSP) algorithms with coherent detection. A successful transmission of 30 Gb/s quadrature phase-shift keying messages hidden in a 15 GHz wideband chaotic optical carrier was experimentally demonstrated over a 340-km fiber link. Meanwhile, the chaotic receiver can be significantly simplified without compromising security. The proposed method is a possible way to promote the practical application of chaotic optical communications.

Low-Complexity Chromatic Dispersion Equalization FIR Digital Filter for Coherent Receiver

This paper proposes a novel and efficient low-complexity chromatic dispersion equalizer (CDE) based on finite impulse response (FIR) filter architecture for polarization-multiplexed coherent optical communication systems. The FIR filter coefficients are optimized by weights to reduce the energy leakage caused by the truncation effect, and then quantization is used uniformly to reduce the number of real number additions and real number multiplications by utilizing the diversity of the quantized coefficients. Using Optisystem 15 to build a coherent optical communication system for simulation and experimental demonstration, the results show that after the filter coefficients are optimized by weights. Compared with the time-domain chromatic dispersion equalizer (TD-CDE), the proposed design has a lower bit error rate (BER) and better equalization effect. When the transmission distance is 4000 km and the system quantization stages M = 16, the multiplication operation and addition operations reduce computing resources by 99% and 43%, and the BER only increases by 5%. Compared with frequency-domain chromatic dispersion equalizer (FD-CDE), widely used in long-distance communication, the multiplication operation reduces computing resources by 30%. The proposed method provides a new idea for high-performance CDE in long-distance coherent optical communication systems.

Coupling efficiency of a plane wave to few-mode fiber in the presence of beam random angular jitter

Using few-mode fiber to receive spatial light is an important method to obtain high coupling efficiency in long-distance free space optical communication systems. Here, we deal with the influence of random angular jitter on few-mode fiber coupling. First, the expression of mean-coupling efficiency from a spatial plane wave to few-mode fiber in the presence of random angular jitter is given. We find that few-mode fiber can obtain higher mean-coupling efficiency than single-mode fiber. Under the same coupling efficiency requirement, the tolerance of random angular jitter standard deviation of four-mode fiber and seven-mode fiber is more than twice that of single-mode fiber. Furthermore, the effect of random angular jitter on the optimal f -number of an optical fiber coupling system is studied. The theoretical models and results reported in this paper will be helpful to the design and optimization of the few-mode fiber coupling system.

A design of novel photonic crystal fiber with low and flattened dispersion for supporting 84 orbital angular momentum modes

A dual-guided photonic crystal fiber (PCF) with low and flattened dispersion is designed, which can support a large number of orbital angular momentum (OAM) modes. The properties of the proposed PCF are systematically analyzed through the finite element method. The results show that the proposed PCF can support up to 84 OAM modes with 600 nm bandwidth ranging from 1000 to 1600 nm. All values of mode purity are above 91.7%, the isolation parameters are larger than 67 dB and the maximum value up to 145 dB, the lowest confinement loss is only 5 × 10−13 dB·m−1. More importantly, the values of dispersion for all modes are less than 40 ps·km−1·nm−1, and the lowest dispersion variation is only 0.7 ps·km−1·nm−1. These superior optical properties make the proposed PCF have great advantage in stable transmissions of data and long-distance optical fiber communication system with large capacity.

Misalignment Analysis of a High-Speed Uplink OWC System Based on a 940-nm VCSEL

In this letter, we experimentally proposed a high-speed long-distance uplink optical wireless communication (OWC) system based on a commercial 940-nm VCSEL for the first time. To the best of our knowledge, the system can reach a record-high data rate of 4.81 Gbps over 12-m transmission with a BER lower than the FEC floor of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.8\times 10 ^{-3}$ </tex-math></inline-formula> among existing commercial VCSEL based long-distance OWC systems. Orthogonal frequency division multiplexing (OFDM) with adaptive bit-power-loading scheme is applied to improve the performance. We also built an mathematically equivalent model over 1.3 m to facilitate setting up optical link, reducing impact of link variation, and analyzing effect of link misalignment for a relatively long distance. Based on experimental and theoretical results, we presented a closed-form expression of the data rates versus the receiver offset distance by curve fitting. Newly calculated results differ from the actual measurements by no more than 4%, which demonstrates the validity of this empirical evaluation model of link misalignment.

Design of Solar Blind Photodetectors for Communication with Green Signal (λ= 0.532µm) in Space. Part II

The areas of Free Space Optical (FSO) communication and high-speed Visible Light Communication (VLC) offer potential for very high-speed data transmission. Favorable attributes including high frequency and wider bandwidths that can enable transmission speeds of the order 100Gb/s, associated with the visible region of the electromagnetic spectrum make it preferable for allowing communication among satellites and ground stations, under-water communication, etc. However, limitations associated with spectral absorption characteristics of the propagating media have stymied further development of such technologies.&#x0D; Aims: Commercial lasers operating in red, green and blue lights combined with three photodetectors, each sensitive to selected wavelengths (colors) present basics of long-distance optical communication system. The current study (Part II) depicts the design and operation of a solar-blind photodetector capable to work explicitly with green wavelength of 532nm.&#x0D; Study Design and Results: The structure of the solar-blind photodetector consists of two sections made of InxGa1-xN (Indium Gallium Nitride) heterostructure, (where x denotes the mole fraction)-a filter and a double barrier tunneling diode. The topmost approximately 1µm thick section acts as a filter and as a p-i-n solar cell providing the required voltage bias to the photodiode. The filter with Eg (Energy Band Gap)=2.33eV absorbs all photons having wavelengths shorter than 532nm. The double barrier tunneling photodiode which comprises the lower section operates with Eg=2.28eV. It consists of an n-type lightly doped quantum well having a width of 2.5nm housed between two lightly doped barriers of 10nm thickness. The 0.12µm topmost and bottom regions of the photodiode are doped with p and n (2×105cm-3) type impurities, respectively. The illuminated cross-section area of the device is finalized at 1mm2.

The application of OFDM in optical fiber communication systems

Orthogonal frequency division multiplexing (OFDM) is a special multi-carrier transmission technology. It utilizes digital signal processing (DSP) to perform inverse Fast Fourier transform (IFFT) and generates a set of orthogonal subcarriers for parallel transmission of low-rate digital signals, achieving the transmission of high-speed digital signals. It has the advantages of high spectral efficiency and anti-dispersion, and has broad applications in large-capacity and long-distance optical fiber communication systems and optical access networks. In this paper, the working principle, structure and application of OFDM in optical fiber communication systems are investigated. Then the problems which should be solved in the application of OFDM in optical communication systems are discussed. Finally, the development of OFDM system is prospected.

Phase shift, oscillation and collision of the anti-dark solitons for the (3+1)-dimensional coupled nonlinear Schrödinger equation in an optical fiber communication system

In the long-distance optical fiber communication system, we consider the (3+1)-dimensional coupled nonlinear Schrodinger equation with perturbation functions, which controls the transverse effects, because of the complexity of nonlinear phenomena. Anti-dark two-soliton solutions are derived by the Hirota method. The perturbation function representing the dissipation rate is discussed. Five different transmission structures of solitons are presented. Besides, their propagation and collision dynamics are analyzed. Control methods for phase shift and oscillation are also suggested. We hope results in this paper are of guiding significance to the research of phase shifters, optical logic devices and ultrashort pulse lasers.