Long-distance Optical Transmission Research Articles

Diffraction-free distance enhancement of Bessel beams based on spatial domain phase modulation

Bessel beams have garnered significant interest due to their unique diffraction-free properties and extensive potential applications. In this work, we propose a spatial domain phase modulation theory to achieve diffraction-free distance enhancement of Bessel beams, overcoming the limitation of the traditional methods due to the inability to infinitely decrease the wave vector angle. The traditional formula for non-diffraction distance is also modified. Simulation results demonstrate that our proposed scheme can significantly increase the maximum diffraction-free distance of zero-order and higher-order Bessel beams by more than two times, while ensuring the self-healing property of Bessel beams. Furthermore, our proposed scheme is not restricted to specific systems or limited to the optical wavelength range. This implies that the results have great applicative potential in long-distance free-space optical communication and wireless energy transmission.

Compensation of emulated atmospheric disturbance and tracking of a moving object by a real-time digital phase conjugate mirror using a liquid crystal spatial light modulator

We developed a fast-response digital phase conjugate mirror using a 700 Hz high-speed liquid crystal spatial light modulator and a high-speed camera. The total delay from signal light acquisition to phase conjugate light generation was 9.7 ms at 1246 × 1024 and 5.9 ms at 640 × 512. The tracking experiment performed on a target moving at a constant distance perpendicular to the optical axis, produced an error of 2%. Furthermore, a heated soldering iron, used to compensate for artificially generated air disturbance, showed that beam wandering and intensity fluctuations were reduced by 86% and 55%, respectively, compared to a phase conjugate mirror with added delay. Phase conjugate light irradiation of a continuously moving target at a maximum speed of 0.9 mm s−1 was also performed. This study shows that real-time digital phase conjugate mirrors can correct wavefront distortion caused by air fluctuation, which is a major challenge in long-distance wireless optical transmission in the turbulent atmosphere, without complicated control, and prevent beam quality degradation in the presence of atmospheric disturbance.

Long-Distance High-Power Wireless Optical Energy Transmission Based on VECSELs

Wireless charging systems are critical for safely and efficiently recharging mobile electronic devices. Current wireless charging technologies involving inductive coupling, magnetic resonance coupling, and microwave transmission are bulky, require complicated systems, expose users to harmful radiation, and have very short energy transmission distances. Herein, we report on a long-distance optical power transmission system by optimizing the external cavity structure of semiconductor lasers for laser charging applications. An ultra-long stable oscillating laser cavity with a transmission distance of 10 m is designed. The optimal laser cavity design is determined by simulating the structural parameters for stable operation, and an improved laser cavity that produces an output of 2.589 W at a transmission distance of 150 cm is fabricated. The peak power attenuation when the transmission distance increases from 50 to 150 cm is only approximately 6.4%, which proves that this wireless power transfer scheme based on a vertical external cavity surface-emitting laser can be used to realize ultra-long-distance power transmission. The proposed wireless energy transmission scheme based on a VECSEL laser is the first of its kind to report a 1.5 m transmission distance output power that exceeds 2.5 W. Compared with other wireless energy transmission technologies, this simple, compact, and safe long-distance wireless laser energy transmission system is more suitable for indoor charging applications.

Incoherent Rayleigh Scattering Noise Depression for Single Laser Stable Radio Frequency Transmission

The stable radio frequency (RF) transmission scheme involving single wavelength has received increasing attention due to the feature of accurate phase information extraction. However in single laser system, the single side band (SSB) phase noise of output signal will be deteriorated by Rayleigh scattering during long-distance optical transmission. In this work, we propose an incoherent Rayleigh scattering noise depression method in the passive phase compensation stable RF transmission scheme with single laser. The carrier of scattering light is suppressed with narrow-band optical filter based on the Stimulated Brillouin scattering (SBS) effect, so that the noise generated by the beat of scattering itself will be significantly reduced. As a result, a 5-GHz RF signal stability transmission over 50-km single mode fiber is demonstrated experimentally. The SSB phase noise at 10 KHz offset is 10.9 dB lower than the scenarios without incoherent Rayleigh scattering noise depression. Meanwhile, the phase jitter mean square error is 1.12 ps during <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1\times 10^{4}$ </tex-math></inline-formula> seconds.

Learning-based digital back propagation to compensate for fiber nonlinearity considering self-phase and cross-phase modulation for wavelength-division multiplexed systems

We propose a learning-based digital back propagation (LDBP) technique that addresses self-phase modulation (SPM) and cross-phase modulation (XPM) of a wavelength-division multiplexed (WDM) optical signal. The LDBP has a structure defined for each of the individual channels of a WDM signal, and connections between the channels address the XPM to effectively compensate for nonlinear waveform distortion of the signal in long-distance optical transmission systems. We attempt to optimize the limited number of parameters used in the structure such as the dispersion, nonlinear coefficient, and walkoff parameter to compensate for the nonlinear phase shift induced by the XPM. We derive equations to update the parameters through an iterative process based on the stochastic gradient descent algorithm. We verify the effectiveness of the proposed LDBP technique through a transmission experiment that uses an 11-channel WDM, 32-Gbaud, dual-polarization 16 quadrature amplitude modulation (QAM), and probabilistically shaped (PS) 64QAM signals. With the focus on the LDBP with a 1-step/span configuration, we operate the learning process using the received 16QAM signal in the experiment. We confirmed the successful convergence of particular parameters of the model of the transmission line after the developed learning procedure. We apply the LDBP with fixed optimized parameters to the received waveforms of the PS-64QAM signals and compare the performance with some DBPs. We observed that the proposed LDBP technique that considers XPM with 1-step/span configuration exhibits the best performance in compensating for nonlinear waveform distortion. Additionally, the learning process is effective for the case considering both SPM and XPM compared with the case of SPM only. Finally, we investigate the computational complexity of the LDBP and reveal that the total calculation cost is of the same order as that of a conventional DBP considering only SPM with a 2-step/span configuration.

Implementation of Soft Decoding Mechanism for Addressing Nonlinearities in Long-Distance Optical Transmission System

A soft decoding technique is discussed in this paper to improve the performance of long-distance optical networks (LDOTNs). LDOTNs are affected by phase noise and nonlinearities generated inside the fiber. The investigations of the proposed LDOTN were carried out by dual-polarization 16-quadrature amplitude modulation (DP-16QAM), DP-64QAM over single-mode fiber (SMF) and digital signal processing (DSP) methodologies. The improved performance of the presented mechanism is discussed over SMF based on constellation shaping (CS). The CS of the presented LDOTN is then compared to the standard 16-QAM and 64-QAM using international telecommunication union-telecommunication (ITU-T) standard G-652.D and G-657.A1 SMF. The soft detecting procedure enables the LDOTNs to attain significant outcomes.

Research on Ultra-high-speed Mach-Zehnder Electro-optic Modulators for Long-distance Optical Transmission of 1 Terabit per Second

Research on Ultra-high-speed Mach-Zehnder Electro-optic Modulators for Long-distance Optical Transmission of 1 Terabit per Second

Robust Far-Field Optical Image Transmission with Structured Random Light Beams

The ability to overcome the adverse effect induced by the obstacles within the transmission link is a central challenge in long-distance optical image transmission and is significantly crucial in free-space optical communication. In this work, we introduce an efficient protocol to realize the robust far-field optical image-signal transmission by modulating the spatial-coherence structure of a partially coherent random light source. The image information encoded in the spatial-coherence structure can be stably transmitted to the far field and can resist the influence of obstructions within the communication link. This is due to the self-reconstruction property of the spatial-coherence structure embedded with the cross phase in the far field. We demonstrate experimentally that the image information can be recovered well by measuring the second-order spatial-coherence structure of the obstructed random light in the far field. Our findings open a door for robust optical signal transmission through the complex environment and may find application in optical communication through a turbulent atmosphere.

Photon-counting schemes for MIMO underwater wireless optical communication with arrayed PMTs.

Underwater wireless optical communication (UWOC) is a promising means of realizing large capacity and high rate in aquatic media. In this paper, a photomultiplier tube (PMT)-based multiple-input multiple-output (MIMO) UWOC system is investigated. Photon counting is an effective technique used to detect very low-level light. A PMT with an excellent photon-counting mode is adopted, and the performance in terms of the bit error rate is discussed. The received optical power can be predicted based on the detected photocount in each symbol period, and the received photocount distribution may be simulated through MATLAB. Furthermore, the optical link model and energy per bit with on-off keying are evaluated for different water types at a 10 m optical link distance. This MIMO-UWOC system combines the advantages of PMTs and the MIMO scheme and has the potential to realize long-distance optical link transmission.

Guided acoustic Brillouin scattering measurements in optical communication fibers.

Guided acoustic Brillouin (GAWBS) noise is measured using a novel, homodyne measurement technique for four commonly used fibers in long-distance optical transmission systems. The measurements are made with single spans and then shown to be consistent with separate multi-span long-distance measurements. The inverse dependence of the GAWBS noise on the fiber effective area is confirmed by comparing different fibers with the effective area varying between 80 µm2 and 150 µm2. The line broadening effect of the coating is observed, and the correlation between the width of the GAWBS peaks to the acoustic mode profile is confirmed. An extensive model of the GAWBS noise in long-distance fibers is presented, including corrections to some commonly repeated mistakes in previous reports. It is established through the model and verified with the measurements that the depolarized scattering caused by TR2m modes contributes twice as much to the optical noise in the orthogonal polarization to the original source, as it does to the noise in parallel polarization. Using this relationship, the polarized and depolarized contributions to the measured GAWBS noise is separated for the first time. As a result, a direct comparison between the theory and the measured GAWBS noise spectrum is shown for the first time with excellent agreement. It is confirmed that the total GAWBS noise can be calculated from fiber parameters under certain assumptions. It is predicted that the level of depolarized GAWBS noise created by the fiber may depend on the polarization diffusion length, and consequently, possible ways to reduce GAWBS noise are proposed. Using the developed theory, dependence of GAWBS noise on the location of the core is calculated to show that multi-core fibers would have a similar level of GAWBS noise no matter where their cores are positioned.

Performance Analysis of DPSK Technique for Optical Transmission Systems

The long-distance high data rate transmission is accompanied by many transmission defects, which reduce the performance of optical communication systems. Therefore, due to the need to increase data capacity for the end user, signal regeneration techniques were used to mitigate these non-linear defects in the electric or optical field. Several techniques used to modify the optical signal, which is the possible solution to reduce the limit of non-linear transmission defects, especially before converting the signal from the optical field to the electric field. Since most of the techniques used to reduce the attenuation in the optical signal do not have the ability to reduce the error rate to reach a bit error rate (BER) less than 10-10 with a transmission capacity less than -10dBm. In this paper, the performance evaluation of DPSK has been validated for its adequate use in transmission systems. Nevertheless, the performance of the DPSK optical system was assessed to rely on a different coding format such as: Return to Zero (RZ) &amp; Non-Return to Zero (NRZ-DPSK). The system is designed to withstand high data rate on long-distance optical transmission systems. The system investigated was simulated with OptiSystemTM 13.0 commercial software. The investigated system demonstrated the ability to mitigate nonlinear impairments from the noisy optical DPSK system with high bit-error rate (BER) improvement at low power transmission. Optical DPSK demonstrates the ability to send high-rate data transmission systems for use in the next generation of optical networks.

Neural Network Nonlinear Equalizer in Long-Distance Coherent Optical Transmission Systems

Recently, neural network (NN) nonlinear equalizers which are expected to reduce computational complexity have attracted attention as fiber nonlinear compensation methods for coherent optical transmission systems. However, these fiber nonlinear compensation methods have problems that training of NN is not easy for long-distance transmission systems because of accumulated phase noise. In this letter, we propose a training method of NN nonlinear equalizer using the target outputs including phase noise which is produced by received signals. The phase noise is estimated by the inverse modulation of received signals and filtering. The estimated phase noise is added to transmitter signals, and the target outputs are obtained. The target outputs allow training of NN in long-distance coherent optical transmission systems. Since the NN trained by this proposed method compensates only for fiber nonlinearities, the phase locked loop (PLL) is placed after the NN to compensate for phase noise. The performances are evaluated by the simulation of 32 Gbaud 16QAM 4000 km long-distance coherent optical transmission. These results indicate that the proposed training method is effective in training NN nonlinear equalizer in long-distance coherent optical transmission in the presence of phase noise.

Research on Key Technologies of large capacity and long distance optical transmission systems

In recent years, with the rapid development of high-bandwidth applications and the rapid growth of Internet data services, the capacity of communication networks is growing rapidly. Optical fiber has become an ideal transmission medium for communication networks due to its advantages such as large bandwidth and low loss. Large-capacity and long-distance optical fiber communication networks have become the key research directions of optical communication systems. In this paper, the main technical means to achieve large-capacity and long-distance optical transmission are investigated. First, the application of orthogonal frequency division multiplexing (OFDM) technology in large-capacity transmission is briefly introduced, including its principle, main structure, advantages and disadvantages. Subsequently, wavelength division multiplexing (WDM) technology based on optical frequency comb is introduced. Finally, the methods to realize long-distance transmission are discussed, including Digital Backward Propagation (DBP), Phase-Conjugate Twin Wave (PCTW) and advanced modulation format.

Kilometers Long Graphene-Coated Optical Fibers for Fast Thermal Sensing.

The combination of optical fiber with graphene has greatly expanded the application regimes of fiber optics, from dynamic optical control and ultrafast pulse generation to high precision sensing. However, limited by fabrication, previous graphene-fiber samples are typically limited in the micrometer to centimeter scale, which cannot take the inherent advantage of optical fibers—long-distance optical transmission. Here, we demonstrate kilometers long graphene-coated optical fiber (GCF) based on industrial graphene nanosheets and coating technique. The GCF shows unusually high thermal diffusivity of 24.99 mm2 s−1 in the axial direction, measured by a thermal imager directly. This enables rapid thermooptical response both in optical fiber Bragg grating sensors at one point (18-fold faster than conventional fiber) and in long-distance distributed fiber sensing systems based on backward Rayleigh scattering in optical fiber (15-fold faster than conventional fiber). This work realizes the industrial-level graphene-fiber production and provides a novel platform for two-dimensional material-based optical fiber sensing applications.

A Kind of New Designing Method About High Speed Optcial Network Auto Protect Switch System

In this paper, we research and develop a new type of optical fiber based on high-speed switching FPGA routing automatic protection switching device, its innovative USES high-speed electronic SWITCH high-speed exchange of FPGA design, will be 0 to 10 g digital optical signal photoelectric conversion amplification and rearrangement of the equipment selected by the high speed switching FPGA SWITCH protection, can for any important 0 to 10 g optical signal circuit after transformation (1 + 1) send after protect rearrangement of amplification, adopt concurrent optimization of transmission mode, automatic selection of high quality circuit signal in the microsecond time complete electrical SWITCH (far superior to the traditional common optical SWITCH SWITCH time), based on the development of commercial products for the national grid ehv transmission network users with high reliability, high performance of 0 to 10 g optical signal transmission channel protection. If there is no insertion loss, does not affect the original light path, also used in long-distance optical network transmission protection system in the west and subvert the traditional light path switching apparatus for at least 2 ~ 5 DBM large insertion loss. It overcomes the disadvantage of the traditional optical switch protection device.

Design of optical-acoustic hybrid underwater wireless sensor network

High-speed underwater wireless transmission technologies are urgently demanded to transmit three-dimensional high-resolution observation data timely in ocean exploration. Currently, underwater acoustic sensor networks have obtained high transmission distance, but with low date rate, high power consumption, high cost and damage to marine mammals. Meanwhile, underwater optical communications can achieve high date rate, but with difficult networking and short communication distance. This paper proposed a kind of novel Optical-Acoustic hybrid Underwater Wireless Sensor Network (OA-UWSN) which employed optical communication for high-speed transmission at close range and employed acoustic communication for transmitting control commands and node localization. OA-UWSN accomplished long-distance optical transmission through directional optical communication and multi-hop transmission mechanism. The problem of difficult networking in optical communication was solved by designing the whole space optical repeater. With the characteristics of directional optical transmission, the Space Division Multiple Access (SDMA) technology was employed in Data Link Layer (DLL) to improve the data transmission efficiency. A routing protocol based on reverse route search was designed to improve the network life cycle and adapt to network topology which changed frequently. The paper proposed a solution for wireless transmission of real-time video and images in marine exploration and provided new methods for high-speed transmission of marine information detection.

Monolithic Silicon Photonic Integrated Circuits for Compact 100&lt;formula formulatype="inline"&gt;&lt;tex Notation="TeX"&gt; $^{+}$&lt;/tex&gt;&lt;/formula&gt;Gb/s Coherent Optical Receivers and Transmitters

We present silicon photonic integrated circuits (PICs) based coherent optical transmitters and receivers for high-speed long-distance fiber optical transmission. High-degree photonic integration is achieved by monolithically integrating silicon electro-optic modulators, germanium photo detectors, silicon nitride-assisted on-chip polarization rotators, thermal phase shifters, and various passive silicon optical devices on a single wafer platform. We demonstrate the use of these PICs for modulating and detecting 112-Gb/s polarization-division-multiplexed quadrature phase-shift keying (PDM-QPSK) and 224-Gb/s PDM 16-ary quadrature amplitude modulation (PDM-16-QAM) signals. Transmission and coherent detection of a 112-Gb/s PDM-QPSK signal over 2560-km standard single-mode fiber is also demonstrated. The high-degree photonic integration for silicon PICs promises small-form-factor and low-power-consumption transceivers for future coherent systems that demand high cost efficiency and energy efficiency.

Supermodes for optical transmission

In this paper, the concept of supermode is introduced for long-distance optical transmission systems. The supermodes exploit coupling between the cores of a multi-core fiber, in which the core-to-core distance is much shorter than that in conventional multi-core fiber. The use of supermodes leads to a larger mode effective area and higher mode density than the conventional multi-core fiber. Through simulations, we show that the proposed coupled multi-core fiber allows lower modal dependent loss, mode coupling and differential modal group delay than few-mode fibers. These properties suggest that the coupled multi-core fiber could be a good candidate for both spatial division multiplexing and single-mode operation.

Theoretical and experimental investigation of compatible SSB modulation for single channel long-distance optical OFDM transmission

We present the first experimental demonstration of compatible single-sideband (compatible-SSB) modulated OFDM optical system at 11.1Gb/s data rate for long distance transmission over 675 km uncompensated standard single-mode fiber. Compatible-SSB modulation employing a simple dual-drive Mach-Zehnder modulator (MZM) in the transmitter and direct-detection at the receiver provides an OFDM system implementation with reduced complexity. It does not require a guard-band between the carrier and the OFDM sideband and so makes full use of available digital-to-analog converter (DAC) bandwidth. We demonstrate digital pre-compensation applied in the transmitter to correct MZM nonlinear transfer function and to improve the system transmission performance. We show that optimum modulation index value decreases with increasing transmission distance. We have also investigated the impact of self-phase modulation (SPM) on the system performance, and show that a compatible-SSB modulated system is less vulnerable to SPM in comparison to the conventional offset-SSB modulation at the same data rate.

Long-distance soliton transmission through ultralong fiber lasers

We present the first experimental demonstration (to our knowledge) of long-distance unperturbed fundamental optical soliton transmission in conventional single-mode optical fiber. The virtual transparency in the fiber required for soliton transmission, over 15 complete periods, was achieved by using an ultralong Raman fiber laser amplification scheme. Optical soliton pulse duration, pulse bandwidth, and peak intensity are shown to remain constant along the transmission length. Frequency-resolved optical gating spectrograms and numerical simulations confirm the observed optical soliton dynamics.