Free-space Transmission Research Articles

Bandwidth enhancement with DAC-enabled pre-equalization and real-valued precoding for a FBMC-VLC

Modulation bandwidth limitation is one of the major drawbacks in light-emitting diode (LED)-based visible light communication (VLC) systems. Various advanced physical-layer modulation formats with digital/analog pre-equalization or adaptive modulation techniques are usually employed to improve the transmission rate of VLC systems. In this Letter, we use a radio frequency (RF) digital-to-analog converter (DAC) to directly realize signal pre-equalization without additional digital or analog signal processing. In addition, low-complexity real-valued precoding techniques are applied to reduce the peak-to-average power ratio (PAPR) and equalize the subcarrier signal-to-noise ratio (SNR). The proposed hybrid scheme is experimentally investigated in a spectral-efficient filter bank multi-carrier (FBMC)-based VLC transmission system. The results exhibit that the PAPR can be reduced by more than 3 dB at the complementary cumulative distribution function of 1 × 10-4 with the real-valued precoding techniques. By using the proposed hybrid scheme, the modulation bandwidth can be increased to 515 MHz for 16QAM-FBMC with a bit error rate below 3.8 × 10-3 after 2.3-m free-space transmission. The net bit rate is improved by more than 45% compared with the conventional FBMC-VLC.

150 Gbit/s 1 km high-sensitivity FSO communication outfield demonstration based on a soliton microcomb.

A high-sensitivity and large-capacity free space optical (FSO) communication scheme based on the soliton microcomb (SMC) is proposed. Using ultra-large bandwidth stabilized SMC with a frequency interval of 48.97 GHz as the laser source, 60 optical wavelengths modulated by 2.5 Gbit/s 16-Pulse position modulation (PPM) are transmitted in parallel. A corresponding outfield high-sensitivity 150 Gbit/s FSO communication experiment based on the SMC was carried out with 1 km space distance. Our experimental results show that the best sensitivity of the single comb wavelength which has higher OSNR can reach -52.62 dBm, and the difference is only 1.38 dB from the theoretical limit under the BER of 1 × 10-3 without forward error correction (FEC). In addition, at BER of 1 × 10-3, 16-PPM has a higher received sensitivity of 6.73dB and 3.72dB compared to on-off keying (OOK) and differential phase shift keying (DPSK) respectively. Meanwhile, taking the advantage of multi-channel SMC, 60 × 2.5 Gbit/s can achieve 150 Gbit/s large-capacity free-space transmission. For comparison, commercially available single-wavelength laser based FSO communication system have also been performed in the outfield. The outfield experimental results demonstrated the feasibility of high-sensitivity, large-capacity PPM FSO communication based on SMCs and provided a new perspective for the future development of large-capacity, long-haul FSO communication.

Quantum Shot Noise Limit in a Rydberg RF Receiver Compared to Thermal Noise Limit in a Conventional Receiver

Electromagnetically induced transparency in a vapor of Rydberg atoms can be used for radio frequency (RF) electric field sensing. Such sensors can be traceable, tunable, compact, and electrically passive. One application for these electric field sensors is as a receiver for RF communications, converting a 5G transmission in free space to a modulated optical signal in a fixed network. Different figures of merit are used for an electric-field sensor and a 5G receiver; so it is not straightforward to determine under which operating conditions a Rydberg RF receiver may outperform a conventional antenna and receiver. Here, we consider two fundamental limiting sources of noise, calculating the quantum shot noise limit in a Rydberg RF receiver and comparing this to the thermal noise limit in a conventional macroscopic receiver. For a typical experimental setup, we show that the quantum shot noise can only fall below the thermal noise floor when the symbol rate is below 1−2 MSymbol/s. Higher order signal modulation will be needed to further increase data rate whilst using this constrained symbol rate to limit the fundamental noise floor.

Long-range wireless optical power transfer system using an EDFA

A wireless optical power transfer (WOPT) system using an erbium-doped fiber amplifier as an optical power source is proposed to achieve long range, high power, and hazard-free power delivery in the air. The transmitter generates a wide band of amplified spontaneous emission around the central wavelength of 1550 nm. A wavelength division multiplexing (WDM) filter (λ=1552.25 nm) is deployed to obtain a safe narrowband beam illuminating the receiver units. A ball lens retroreflector reflects a small portion of the incident beam back to the transmitter, establishing a closed ring resonance loop. An improved safety mechanism is proposed to terminate the resonance when an obstacle blocks the transmitter-receiver line of sight. The measured incident power of 1 W decreases to 0.79 mW after the WDM filter is deployed which is well within defined maximum permissible exposure standards. For the demonstration of free-space transmission, transmitter-receiver separation is extended to 30 m. The experimental results show that a single-channel WOPT system provides an optical power of 400 mW with a channel linewidth of 1.027 nm over 30 m and an electrical power of 85 mW is acquired using a gallium antimonide photovoltaic.

Method for 10 Gbps near-ground quasi-static free-space laser transmission by nutation mutual coupling

This study proposes a method to reduce link loss in near-ground space laser communication with fiber-nutation-based mutual coupling. We designed a transmit-receive coaxial laser terminal with a 50 mm aperture and implemented beam acquisition tracking and nutation coupling using a single-detector and single-actuator. Following an indoor experiment and a 1 km field test, the single-ended nutation coupling and the two-way nutation mutual coupling theories were compared and analyzed. In conclusion, the proposed method could significantly increase the reception efficiency by ∼8 dB. The bit error rate of 10 Gbps pseudo-random code transmission was 2.478E-9. We also demonstrated video transmission.

3.76-Gbps yellow-light visible light communication system over 1.2 m free space transmission utilizing a Si-substrate LED and a cascaded pre-equalizer network.

Recently, visible light communication (VLC) has emerged as a promising communication method in 6G. To achieve 6G high-speed transmission, wavelength division multiplexing (WDM) based VLC systems are a highly promising candidate. However, the "yellow and green gap" greatly limits the yellow light efficiency of InGaN-based LEDs and also restricts the transmission rate of yellow LEDs. In addition, pre-equalization and post-equalization also have an important impact on high-speed communication. In this paper, we propose to employ a vertical InGaN-based Si-substrate yellow LED with bit-power loading discrete multitone (DMT) modulation and a novel cascaded pre-equalizer network to achieve a high-speed yellow-light VLC system. The proposed cascaded pre-equalizer network is based on a digital Zobel network and a partial nonlinear pre-equalizer (DZNPN). The microscopic time-domain transient response of the high-speed and large-amplitude signal is also investigated to show a severe impairment. Utilizing the DZNPN cascaded pre-equalizer network based on the third-order Volterra series, a record-breaking data rate of 3.764Gbps over 1.2 m free space and 3.808Gbps over 0.7 m are experimentally demonstrated under the hard decision-forward error correction (HD-FEC) threshold of 3.8 × 10-3. The rate can be improved from 2.818Gbps to 3.764Gbps with 650Mbaud compared to the un-preprocessed signal. This is the highest data rate ever reported for yellow-light VLC systems based on a single LED to the best of our knowledge.

Spherical lens aberration minimization method for short-range free-space light transmission

Spherical aberrations of lenses lead to increased spot radii on the focal plane. Several methods, such as optimizing thickness and radii or building lens groups, are known to minimize these spherical aberrations. Here, an innovative method for spherical aberration minimization is introduced; it can be used for short-range free-space optical communication systems, such as unit power afocal relay trains, lens waveguides, and periodic lens systems. This spherical aberration compensation principle is based on the combination of two identical spherical convex lenses at an optimal distance. Due to the higher refractive power for rays with an larger axis distance, rays from the outer area of the first lens intersect the inner area of the second lens, and vice versa. With this setup, the radial refractive power deviation of the two lenses compensate each other. Analytic calculations and numerical simulations are done to confirm this behavior, and measurements using two symmetric spherical lenses with polymer optical fibers as light feed confirm the calculations. Simulations and measurements show a very good matching behavior except for an unknown systematic error. At a lens distance of 300 mm, the optical attenuation decreased by ∼2 dB compared with a very small lens distance. This leads to the conclusion that this proposed spherical aberration minimization method works as theoretically predicted.

Light fidelity performance via hybrid free space optic/fiber optic communication under atmospheric disturbance

Light f idelity (Li-Fi) considers technology in optical wireless communication (OWC) that utilize visible light for transmission in free space, visible light communication (VLC) technology was chosen to overcome the interference issues of radio frequency (RF) communication and it also has higher bandwidth and is more secure. This paper aims to analyse the influence of atmospheric attenuation on the end-users of VLC system that consisted of five light-emitting diodes (LEDs) distributed in a room, which in t urn linked with the hybrid optical system includes a multi-channel of free space optic (FSO) connected with a fiber-optic channel (FOC). The hybrid system to the end-user was estimated in terms of bit error rate, quality factor, and signal power under dust-rain weather conditions. In the dust scenario, the suggested system achieved acceptable bit-error-rate (BER) to a distance of 1, 100 m. In rainy conditions, the performance accomplished acceptable BER to a distance of 3000 m. The VLC system's end-user also was evaluated under the effectiveness of the variation of irradiance and incident angles. Despite the harsh environmental conditions imposed on the communication network, VLC has proven its ability to maintain acceptable performance in end-user.

WDM-VLLC and White-Lighting Ring Networks With Optical Add-Drop Multiplexing Scheme

A wavelength-division-multiplexing (WDM)-visible laser light communication (VLLC) and white-lighting ring network with a 150-Gbit/s accumulative transmission rate at central station (CS), a 50-Gbit/s transmission rate at optical node, and a 604-lux white-lighting at CS is successfully demonstrated. Red, green, and blue laser diodes employing two-stage injection locking and optoelectronic feedback techniques are utilized to afford the dual functions of high-speed VLLC links and white-light illumination at reading/writing-level. By deploying optical add-drop multiplexing scheme, this newly-built WDM-VLLC and white-lighting ring network is promising since it allows a point-to-multipoint link and a two-way operation in free-space transmission. Impressively low bit error rate of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−9</sup> , clear PAM4 eye diagrams, and qualified white-lighting for reading and writing are achieved through 7.5 m/9 m/10.5 m/12 m VLLC link with 5 m plastic optical fiber transmission. Such constructed WDM-VLLC and white-lighting ring network shows a potential one for the evolution of high-speed VLLC with white-lighting for reading/writing.

The Improved Precoding Method in the VLC-Based Intelligent Transportation System

Visible light communication (VLC) transmission system, combined with lighting and communication, has a great application value in the field of intelligent transportation. To efficiently improve the performance of VLC-based intelligent transportation system, an improved precoding scheme is proposed and experimentally verified. Partial data-carrying subcarrier (PDS)-precoding scheme can balance the signal-to-noise ratio (SNR) and combat frequency selective fading (FSF) in the VLC-based intelligent transportation system. For ∼3.52 Gb/s DMT-VLC system with 1.9-m free space transmission, the OCT-based PDS-precoding signal shows almost the same BER performance as full data-carrying subcarriers precoding ones. Compared with the conventional scheme, the BER is reduced from 2e-3 to 8.14e-5 by using the proposed PDS-precoding. PDS-precoding is a nice choice to degrade the unbalanced damage for data-carrying subcarriers (SCs) in the bandwidth-limited VLC-based intelligent transportation system.

Computational Soliton Modulated Nonlinear Microring Add-drop Multiplexer and Potential Applications

This paper proposed the design of the multiplexer node for the free-space transmission link. Various soliton modulated signal categories are numerically studied. The proposed system is a modified micro-optical add-drop multiplexer. The cross phase modulated signals induced into the main ring by the two side rings can give more signal complexity, which can lead to having the securely transmitted signal requirement. The high optical power obtained from the soliton aspect can give long-distance communication, where the required frequency band can be improved and suitable for the required applications. For an instant, the use for the side road connection of the vehicle-to-everything (V2X) can be implemented in either cable or wireless(free-space) connection. The results obtained have shown promising applications such as free-space transmission link in both above and underground nodes, quantum cryptography, and communication security based on nonlinear aspects. Moreover, the electro-optic conversion can also be used by the stacked layers of silicon-graphene-gold, in which the signal conversion can be applied suitably.

High bandwidth series-biased green micro-LED array toward 6 Gbps visible light communication.

In this Letter, a record modulation bandwidth of 1.31 GHz was achieved by a 10 µm c-plane green micro light emitting diode (micro-LED) at a current density of 41.4 kA/cm2. Furthermore, by designing a series-biased 20 µm micro-LED with higher light output power, combined with an orthogonal frequency division multiplexing modulation scheme, a maximum data rate of 5.789 Gbps was achieved at a free-space transmission distance of 0.5 m. This work demonstrates the prospect of c-plane polar green micro-LED in ultrahigh-speed visible light communication, which is expected to realize a high-performance wireless system in the future.

Wavefront distortion correction of superposed optical vortices based on deep multi-branch compensation network

Optical vortices with orbital angular momentum play a crucial role in many fields such as target detection, quantum entanglement, and space optical communication. However, the wavefront distortion of optical vortices caused by atmospheric turbulence during free-space transmission is a major obstacle to practical applications. In this work, a deep-learning-based method is proposed for wavefront correction of distorted superposed optical vortices under various turbulence intensities. First, the Zernike polynomial expansion method is used to numerically simulate the atmospheric turbulence phase screen conforming to the Kolmogorov statistical law which assumes that turbulence is locally uniform and isotropic, in which the first 10 Zernike polynomials are adopted. Then, according to the distinct wavefront distortion features of different polynomials, a deep multi-branch compensation network (DMCN) is designed to adaptively and more accurately learn the mapping relationship between the intensity distribution profile of distorted optical vortices and the corresponding turbulent phase screen. The distorted superposed optical vortices dataset for network training is obtained by experimental measurement. Finally, the turbulence compensation screen predicted by the trained DMCN is verified experimentally and the results show that our wavefront distortion correction method based on DMCN can accurately predict the turbulent compensation screen and improve the mode purity of superposed optical vortices to more than 80% within 40 ms when using a workstation computer under various turbulence intensities, which has apparent advantages compared with traditional algorithms.

Direct generation of 2D arrays of random numbers for high-fidelity optical ghost diffraction and information transmission through scattering media

In this paper, we propose a new approach for high-fidelity optical ghost diffraction and information transmission through scattering media in free space using a series of 2D arrays of random numbers as information carriers. A series of 2D arrays of random numbers are directly generated to encode the ghosts, i.e., original signals or images. The generated 2D arrays of random numbers are sequentially embedded into amplitude-only spatial light modulator (SLM), and are illuminated to propagate through scattering media in free space. Experimental results show feasibility and effectiveness of the proposed method, and different types of ghosts (e.g., analog signals and grayscale images) can be encoded and retrieved with high fidelity. In addition, comparisons using different algorithm parameters and different experimental parameters (e.g., different diffraction distances and wavelengths) are conducted to analyze and illustrate performance of the proposed method. It is found that the proposed method possesses high robustness against scattering media, and can realize high-fidelity ghost diffraction and optical information transmission. The proposed high-fidelity optical ghost diffraction could open up an avenue for many applications, e.g., free-space optical transmission through scattering media. @ 2022.

Demonstration of 10-channel mode- and polarization-division multiplexed free-space optical transmission with successive interference cancellation DSP.

We experimentally demonstrate 10-channel mode-division multiplexed free-space optical transmission with five spatial modes, each carrying 19.6925-Gbaud dual-polarization quadrature phase shift keying signals. Strong inter-mode cross talk is observed in our commercially available photonic lantern based system when using a complete orthogonal mode set as independent channels. A successive interference cancellation based multiple-input multiple-output digital signal processing (DSP) algorithm is first applied to mitigate the inter-mode cross talk in mode-division multiplexed systems. The DSP also supports unequal transmit and receive channel numbers to further improve the cross talk resiliency. Compared to the conventional minimum mean square error DSP, the required optical signal-to-noise ratio of the successive interference cancellation DSP is decreased by approximately 5 dB at the hard-decision forward error correction limit. As a result, this system demonstrates a record-high independent channel number of 10 and spectral efficiency of 13.7 b/s/Hz in mode-division multiplexed free-space optical systems.

Bridging the Terahertz Gap: Photonics-Assisted Free-Space Communications From the Submillimeter-Wave to the Mid-Infrared

Since about one and half centuries ago, at the dawn of modern communications, the radio and the optics have been two separate electromagnetic spectrum regions to carry data. Differentiated by their generation&#x002F;detection methods and propagation properties, the two paths have evolved almost independently until today. The optical technologies dominate the long-distance and high-speed terrestrial wireline communications through fiber-optic telecom systems, whereas the radio technologies have mainly dominated the short- to medium-range wireless scenarios. Now, these two separate counterparts are both facing a sign of saturation in their respective roadmap horizons, particularly in the segment of free-space communications. The optical technologies are extending into the mid-wave and long-wave infrared (MWIR and LWIR) regimes to achieve better propagation performance through the dynamic atmospheric channels. Radio technologies strive for higher frequencies like the millimeter-wave (MMW) and sub-terahertz (sub-THz) to gain broader bandwidth. The boundary between the two is becoming blurred and intercrossed. During the past few years, we witnessed technological breakthroughs in free-space transmission supporting very high data rates, many achieved with the assistance of photonics. This paper focuses on such photonics-assisted free-space communication technologies in both the lower and upper sides of the THz gap and provides a detailed review of recent research and development activities on some of the key enabling technologies. Our recent experimental demonstrations of high-speed free-space transmissions in both frequency regions are also presented as examples to show the system requirements for device characteristics and digital signal processing (DSP) performance.

Colloidal Quantum Dot Light Emitting Diodes at Telecom Wavelength with 18% Quantum Efficiency and Over 1MHz Bandwidth.

Developing high performance, low‐cost solid‐state light emitters in the telecom wavelength bandwidth is of paramount importance for infrared light‐based communications. Colloidal quantum dot (CQD) based light emitting diodes (LEDs) have shown tremendous advances in recent times through improvement in synthesis chemistry, surface property, and device structures. Despite the tremendous advancements of CQD based LEDs in the visible range with efficiency reaching theoretical limits, their short‐wave infrared (SWIR) counterparts mainly based on lead chalcogenide CQDs, have shown lower performance (≈8%). Here the authors report on highly efficient SWIR CQD LEDs with a recorded EQE of 11.8% enabled by the use of a binary CQD matrix comprising QD populations of different bandgaps at the emission wavelength of 1550 nm. By further optimizing the optical out‐coupling via the use of a hemispherical lens to reduce optical waveguide loss, the EQE of the LED increased to 18.6%. The CQD LED has an electrical bandwidth of 2 MHz, which motivated them to demonstrate its use in the first SWIR free‐space optical transmission link based entirely on CQD technology (photodetector and light emitter) opening a new window of applications for CQD optoelectronics.

An LMI-based discrete time nonlinear observer for Light-Emitting Diode optical communication

Light-Emitting Diode (LED) optical wireless communication is a potentially low-cost, sustainable approach for enabling high-speed free-space and underwater transmissions within a limited communication range. Establishing a tightly controlled line of sight (LOS) between transmitter and receiver is a significant challenge because the angle of the alignment is not directly measured and has to be estimated. To address this problem, we propose a novel switched-gain discrete-time nonlinear observer for an LED-based optical communication model in which the nonlinear output functions are composed of nonlinear vector functions of multi-scalar combinations of the states. Lyapunov function-based analysis that ensures global stability is used to design the proposed observer in each piecewise monotonic region of the LED output functions. Furthermore, we prove via a quadratic Lyapunov-based approach that a constant stabilizing observer gain design approach has no feasible solution when the entire LED optical communication range is considered. Therefore, a suitable switched-gain nonlinear observer is derived for non-monotonic output measurement equations. Simulation results are shown together with extensive comparisons with the Extended Kalman Filter (EKF) to illustrate the performance of the proposed switched-gain observer design.

Direct Modulation and Free-Space Transmissions of up to 6 Gbps Multilevel Signals With a 4.65-$\mu$m Quantum Cascade Laser at Room Temperature

A roadmap for future wireless communications is expected to exploit all transmission-suitable spectrum bands, from the microwave to the optical frequencies, to support orders of magnitude faster data transfer with much lower latency than the deployed solutions nowadays. The currently under-exploited mid-infrared (mid-IR) spectrum is an essential building block for such an envisioned all-spectra wireless communication paradigm. Free-space optical (FSO) communications in the mid-IR region have recently attracted great interest due to their intrinsic merits of low propagation loss and high tolerance of atmospheric perturbations. Future development of viable mid-IR FSO transceivers requires a semiconductor source to fulfill the high bandwidth, low energy consumption, and small footprint requirements. In this context, quantum cascade laser (QCL) appears as a promisingtechnological choice. In this work, we present an experimental demonstration of a mid-IR FSO link enabled by a 4.65-μm directly modulated (DM) QCL operating at room temperature. We achieve a transmission data rate of up to 6 Gbps over a 0.5-m link distance. This achievement is enabled by system-level characterization and optimization of transmitter and receiver power level and frequency response and assisted with advanced modulation and digital signal processing (DSP) techniques. This work pushes the QCL-based FSO technology one step closer to practical terrestrial applications, such as the fixed wireless access and the wireless mobile backhaul. Such a QCL-based solution offers a promising way towards the futuristic all-spectra wireless communication paradigm by potentially supporting the whole spectrum from the MIR to the terahertz (THz).

High-efficiency and high-fidelity optical signal transmission in free space through scattering media using 2D random amplitude-only patterns and look-up table

A new approach using 2D random amplitude-only patterns and look-up table (LUT) is proposed to realize high-efficiency and high-fidelity optical signal transmission in free space through scattering media. Any digital signal only containing 0 and 1 can be optically transmitted with high fidelity in free space through scattering media using only 4 pre-generated 2D random amplitude-only patterns stored in a database. Any grayscale image with values ranging from 0 to 255 can also be transmitted with high fidelity in a direct mode using only 512 pre-generated 2D random amplitude-only patterns stored in a database. For analog signals, a LUT can also be constructed to facilitate optical signal transmission in free space through scattering media. Optical experiments are carried out to verify feasibility and effectiveness of the proposed method. The proposed method can provide a novel insight for high-efficiency and high-fidelity optical signal transmission in free space through scattering media.