Free-space Transmission Research Articles

Broadband Port-Selective Silicon Beam Scanning Device for Free-Space Optical Communication

Indoor free space optical (FSO) communication technology that provides high-speed connectivity to edge users is expected to be introduced in the near future mobile communication system, where the silicon photonics solid-state beam scanning device is a promising tool because of its low cost, long-term reliability, and other beneficial properties. However, the current two-dimensional beam scanning devices using grating coupler arrays have difficulty in increasing the transmission capacity because of bandwidth regulation. To solve the problem, we have introduced a broadband surface optical coupler, “elephant coupler,” which has great potential for combining wavelength and spatial division multiplexing technologies into the beam scanning device, as an alternative to grating couplers. The prototype port-selective silicon beam scanning device fabricated using a 300 mm CMOS pilot line achieved broadband optical beam emission with a 1 dB-loss bandwidth of 40 nm and demonstrated beam scanning using an imaging lens. The device has also exhibited free-space signal transmission of non-return-to-zero on-off-keying signals at 10 Gbps over a wide wavelength range of 60 nm. In this paper, we present an overview of the developed beam scanning device. Furthermore, the theoretical design guidelines for indoor mobile FSO communication are discussed.

High-speed spatial light modulation enabling 25-Gbit/s twisted light encoding/decoding and 260-m security free-space data transmission.

Spatial domain of light beam is an important degree of freedom to be extensively explored. As a set of spatial domains, twisted lights have some natural properties such as orthogonality and security, providing great potentials in optical communications especially for data encoding/decoding. However, the speed of traditional spatial light modulators has always been criticized. Here we present a hundred-meter security free-space data transmission based on high-speed spatial light modulation by exploiting temporal-to-spatial domain mapping. We demonstrate 25-Gbit/s twisted light encoding/decoding and 260-m security free-space data transmission in the experiment. The encoding/decoding link will lead to 3-dB improvement in bit error rate (BER) performance compared with a single channel in theory and ∼1-dB optical signal-to-noise ratio (OSNR) penalty at the forward error correction (FEC) threshold of 3.8e-3 in practice. The experiment results also show favorable security performance of the proposed encoding/decoding link system.

Atmospheric turbulence characterization with simultaneous measurement of phase, angle of arrival, and intensity in a retroreflected optical link.

Free-space optical transmission through the Earth's atmosphere is applicable to high-speed data transmission and optical clock comparisons, among other uses. Fluctuations in the refractive index of the atmosphere limit the performance of atmospheric optical transmission by inducing phase noise, angle-of-arrival variation, and scintillation. The statistics of these deleterious effects are predicted by models for the spatial spectrum of the atmospheric refractive index structure. We present measurements of phase fluctuations, angle-of-arrival variations, and scintillation, taken concurrently and compared with models for the atmospheric refractive index structure. The measurements are also cross-compared by deriving independent estimates of the turbulence structure constant $C_n^2$. We find agreement within an order of magnitude for derived $C_n^2$ values for all three metrics.

High Spectral Efficiency Long-Wave Infrared Free-Space Optical Transmission With Multilevel Signals

This study explores the potential of long-wave infrared free-space optical (FSO) transmission that leverages multilevel signals to attain high spectral efficiency. The FSO transmission system consists of a directly modulated-quantum cascade laser (DM-QCL) operating at 9.15 μm and a mercury cadmium telluride (MCT) detector. To fully understand the system, we conduct measurements on the DM-QCL chip and MCT detector and assess the overall amplitude response of the DM-QCL, MCT detector, and all electrical components. We apply various signals, including on-off keying (OOK), 4-level pulse amplitude modulation (PAM4), 6-level PAM (PAM6), and 8-level PAM (PAM8) to maximize the bit rate and spectral efficiency of the FSO transmission. Through a two-dimensional sweeping of the laser bias current and MCT detector photovoltage, we optimize the transmission performance. At the optimal operation point, the FSO system achieved impressive results which are up to 6 Gbaud OOK, 3.5 Gbaud PAM4, 3 Gbaud PAM6, and 2.7 Gbaud PAM8 signal transmissions, with a bit error rate performance below 6.25% overhead hard decision-forward error correction limit when the DM-QCL operates at 10°C. We also evaluate the eye diagrams and stability of the system to showcase its remarkable transmission performance. Our findings suggest that the DM-QCL and MCT detector-based FSO transceivers offer a highly competitive solution for the next generation of optical wireless communication systems.

Light-guiding-light-based temporal integration of broadband terahertz pulses in air

The next generation of all-optical computation platforms prefers the light-guiding-light (LGL) scheme inside a medium that envisions circuitry-free and rapidly reconfigurable systems powered by dynamic interactions between light beams. Currently, suitable LGL materials and corresponding mechanisms are in urgent need. In this work, we proposed ubiquitous air as a restorable LGL signal manipulation medium with transient air-plasma waveguide circuits. Briefly, by focusing femtosecond laser beams in free space, the created atmospheric plasma filament array via photoionization was able to guide terahertz (THz) pulses along its epsilon-near-zero zone with a 1/f-profile spectral response. Consequently, this achieved a time-domain integration of the THz pulse in broad bandwidth. When the pumping laser was sequentially turned off and on, this air-plasma multi-filament structure was erased and rebuilt within nano- and femto-seconds, respectively, allowing rapid and repeated rearrangements of the all-optical stage. Furthermore, this air-based LGL information processing approach is promising to pave the way toward all-optical calculations during free-space directional transmission of THz waves, in which way the delivered THz signal can be remotely controlled.

DETECTING MOISTURE IN BAUXITE USING MICROWAVES

AbstractMathematical modelling of microwaves travelling through bauxite ore provides a way to compute moisture content in the free space transmission method given data on signal attenuation, phase shift and variable bauxite depth. We extend a recently developed four-layer model that uses coupled ordinary differential wave equations for the electric field together with continuity boundary conditions at interfaces between ore, air and antenna to find a solution that incorporates multiple internal reflections in ore and air. The model provides good fits to data, depending on ore permittivity and conductivity.Our extensions are to use effective medium models to obtain electromagnetic properties of the ore mixture from moisture content and to incorporate the damping effects of scattering from the ore surface. Our model leads to a formula for the received signal showing how signal strengths SS and phase shifts depend on the moisture content of the bauxite ore, through the effects of moisture on permittivity and conductivity. We show that SS may be noninvertible, indicating that attenuation data alone cannot be used to infer moisture content. Combining with phase data typically corrects the noninvertibility. Reducing the operating frequency dramatically improves the usefulness of signal strength data for inferring moisture content.

Car Bumper Effects in ADAS Sensors at Automotive Radar Frequencies.

Radars in the W-band are being integrated into car bumpers for functionalities such as adaptive cruise control, collision avoidance, or lane-keeping. These Advanced Driving Assistance Systems (ADAS) enhance traffic security in coordination with Intelligent Transport Systems (ITS). This paper analyzes the attenuation effect that car bumpers cause on the signals passing through them. Using the free-space transmission technique inside an anechoic chamber, we measured the attenuation caused by car bumper samples with different material compositions. The results show level drops lower than 1.25 dB in all the samples analyzed. The signal attenuation triggered by the bumpers decreases with the frequency, with differences ranging from 0.55 dB to 0.86 dB when comparing the end frequencies within the radar band. Among the analyzed bumper samples, those with a thicker varnish layer or with talc in the composition seem to attenuate more. We also provide an estimation of the measurement uncertainty for the validation of the obtained results. Uncertainty analysis yields values below 0.21 dB with a 95% coverage interval in the measured frequency band. When comparing the measured value with its uncertainty, i.e., the relative uncertainty, the lower the frequency in the measured band, the more accurate the measurements seem to be.

Secure FSO communication based on optical frequency-hopping technology using delay interferometers

—A novel optical frequency-hopping (OFH) scheme using optical delay interferometers (DI) is proposed and demonstrated for secure transmission in free space. By performing carrier suppression modulation on the light wave emitted by the laser and connecting the phase modulator (PM) and DI in series, the conversion of the light wave modulated by the Mach-Zehnder modulator (MZM) from phase modulation to intensity modulation can be realized, and finally output the desired optical frequency-shift-keying (OFSK) carrier signal. Meanwhile, by controlling the positions of the frequencies of the positive and negative first-order sideband light waves on the DI frequency response curve, the OFSK signals output by the two ports of the DI can be complemented in the time domain. For the proposed OFH scheme, we carried out simulation experiments of 5 km free-space link transmission and back-to-back transmission with a communication rate of 10 Gbps, and the simulation results proved the feasibility of the scheme. Additionally, we also analyze the security performance of the proposed scheme and give the security space based on the eavesdropping probability.

Extinction of high-power laser radiation under adverse weather conditions.

The propagation of laser radiation over long distances can be significantly affected by atmospheric extinction due to precipitation as well as aerosol particles and molecules. The knowledge of the contribution of precipitation is critical to the operation of a variety of laser-based systems. The study of high-power laser transmission around 1µm is of particular interest because several atmospheric transmission windows are located in this region. To investigate the effect of adverse weather conditions on laser transmission, free-space laser transmission experiments are conducted on the DLR test range in Lampoldshausen, Germany. A high-power laser with a wavelength of 1.03µm is used for the transmission measurements in combination with calibrated power monitors. Local weather conditions are continuously monitored by meteorological instruments during the experiments. Extinction coefficients are derived from transmission measurements showing that the extinction for snow is 7 times higher than for rain, and the extinction for drizzle/rain is 4 times higher than for rain at a given precipitation rate of 1mm/h. For a mixture of rain and snow, the extinction is comparable to that of rain, indicating that the water content strongly influences the optical properties and thus the extinction of laser radiation in mixed precipitation. A good relationship is found between the measured extinction coefficient and visibility for drizzle and rain and a slightly larger scatter of the data for snow. Furthermore, measured extinction coefficients are compared to the extinction coefficients based on the experimental size distributions of precipitation particles and geometric optics. A reasonable agreement is obtained for rain, with no improvement taking the forward-scattering into the detector aperture into account, and a much better agreement is obtained for snow when the forward-scattering contribution is included.

A 3.84-Gbit/s FBMC-NOMA-based VLC transmission system using DAC-based pre-equalization and DHT precoding technique

The transmission data rate of visible light communication (VLC) systems is mainly limited by the modulation bandwidth of light-emitting diodes (LEDs). Pre-equalization (PE) techniques in combination with advanced modulation formats have been employed to enhance the data rate of LED-based VLC systems. In this paper, we experimentally demonstrated a high spectral-efficiency filter bank multi-carrier (FBMC) and non-orthogonal multiple access (NOMA)-based VLC transmission system using digital-to-analog converter-based PE (DAC-PE) and discrete Hartley transform (DHT) precoding technique. Two PE techniques, i.e., DAC-PE and digital pre-equalization (DPE), are comparatively investigated in both orthogonal frequency-division multiplexing (OFDM)-NOMA-based VLC systems and FBMC-NOMA-based VLC systems. The results exhibit that, with the help of DHT precoding, the DAC-PE scheme outperforms the DPE scheme at the optimal power ratios (PRs). Besides, the FBMC-NOMA scheme can provide better bit error rate (BER) performance and higher data rate than OFDM-NOMA. At the optimal PR of 6 dB, the FBMC-NOMA signal with a record net data rate of 3.84 Gbit/s after 2.3-m free-space link transmission can be achieved with the BER below 3.8 × 10−3.

High-fidelity and high-robustness free-space ghost transmission in complex media with coherent light source using physics-driven untrained neural network.

It is well recognized that it is challenging to realize high-fidelity and high-robustness ghost transmission through complex media in free space using coherent light source. In this paper, we report a new method to realize high-fidelity and high-robustness ghost transmission through complex media by generating random amplitude-only patterns as 2D information carriers using physics-driven untrained neural network (UNN). The random patterns are generated to encode analog signals (i.e., ghost) without any training datasets and labeled data, and are used as information carriers in a free-space optical channel. Coherent light source modulated by the random patterns propagates through complex media, and a single-pixel detector is utilized to collect light intensities at the receiving end. A series of optical experiments have been conducted to verify the proposed approach. Experimental results demonstrate that the proposed method can realize high-fidelity and high-robustness analog-signal (ghost) transmission in complex environments, e.g., around a corner, or dynamic and turbid water. The proposed approach using the designed physics-driven UNN could open an avenue for high-fidelity free-space ghost transmission through complex media.

Free-space transmission of picosecond-level, high-speed optical pulse streams in the 3 µm band.

The utilization of mid-infrared (mid-IR) light spanning the 3-5 µm range presents notable merits over the 1.5 µm band when operating in adverse atmospheric conditions. Consequently, it emerges as a promising prospect for serving as optical carriers in free-space communication (FSO) through atmospheric channels. However, due to the insufficient performance level of devices in the mid-IR band, the capability of mid-IR communication is hindered in terms of transmission capacity and signal format. In this study, we conduct experimental investigations on the transmission of time-domain multiplexed ultra-short optical pulse streams, with a pulse width of 1.8 ps and a data rate of up to 40 Gbps at 3.6 µm, based on the difference frequency generation (DFG) effect. The mid-IR transmitter realizes an effective wavelength conversion of optical time division multiplexing (OTDM) signals from 1.5 µm to 3.6 µm, and the obtained power of the 40 Gbps mid-IR OTDM signal at the optimum temperature of 54.8 °C is 7.4 dBm. The mid-IR receiver successfully achieves the regeneration of the 40 Gbps 1.5 µm OTDM signal, and the corresponding regenerated power at the optimum temperature of 51.5 °C is -30.56 dBm. Detailed results pertaining to the demodulation of regeneration 1.5 µm OTDM signal have been acquired, encompassing parameters such as pulse waveform diagram, bit error rate (BER), and Q factor. The estimated power penalty of the 40 Gbps mid-IR OTDM transmission is 2.4 dB at a BER of 1E-6, compared with the back-to-back (BTB) transmission. Moreover, it is feasible by using chirped PPLN crystals with wider bandwidth to increase the data rate to the order of one hundred gigabits.

Two-Stage Link Loss Optimization of Divergent Gaussian Beams for Narrow Field-of-View Receivers in Line-of-Sight Indoor Downlink Optical Wireless Communication (Invited)

The predominant focus of research in high-speed optical wireless communication (OWC) lies in line-of-sight (LOS) links with narrow infrared beams. However, the implementation of precise tracking and steering necessitates delicate active devices, thereby presenting a formidable challenge in establishing a cost-effective wireless transmission. Other than using none-line-of-sight (NLOS) links with excessive link losses and multi-path distortions, the simplification of the tracking and steering process can be alternatively achieved through the utilization of divergent optical beams in LOS. This paper addresses the issue by relaxing the stringent link budget associated with divergent Gaussian-shaped optical beams and narrow field-of-view (FOV) receivers in LOS OWC through the independent optimization of geometrical path loss and fiber coupling loss. More importantly, the geometrical path loss is effectively mitigated by modifying the transverse intensity distribution of the optical beam using manipulations of multi-mode fibers (MMFs) in an all-fiber configuration. In addition, the sufficiently excited higher order modes (HOMs) of MMFs enable a homogenized distribution of received optical powers (ROPs) within the coverage area, which facilitates the mobility of end-users. Comparative analysis against back-to-back links without free-space transmission demonstrates the proposed scheme’s ability to achieve low power penalties. With the minimized link losses, experimental results demonstrate a 10 Gbps error-free (BER < 10−13) LOS OWC downlink transmission at 2.5 m over an angular range of 10° × 10° without using any optical pre-amplifications at a typical PIN receiver. The proposed scheme provides a simple and low-cost solution for high-speed and short-range indoor wireless applications.

Extending quantum key distribution through proxy re-encryption

Modern quantum key distribution (QKD) network designs are based on sending photons from one node to another and require free-space or dedicated fiber optic cables between nodes. The purpose of this is to co-generate secret key material on both sides of the quantum channel. In addition to this quantum link, there are several insecure classical channels that allow QKD algorithms to exchange book-keeping information and send symmetrically encrypted data. The attenuation of photons transmitted through fiber becomes too high to practically generate key material over fiber at distances of more than 100 km. Free-space transmission through the atmosphere or the vacuum of space can reduce attenuation, but at the cost of system complexity and sensitivity to other impairments, such as weather. To extend the effective range of QKD networks, we present a method that combines QKD algorithms with post-quantum, homomorphic key-switching to allow multiple parties to effectively share secret key material over longer distances through semi-trusted relay nodes. We define how such a system should work for arbitrary network topologies and provide proofs that our scheme is both correct and secure. We assess the feasibility of this solution by building and evaluating two implementations based on lattice-based cryptography: learning with errors.

Single-wavelength polarization- and mode-division multiplexing free-space optical communication at 689 Gbit/s in strong turbulent channels.

We experimentally demonstrated, for, it is believed, the first time, high-capacity polarization- and mode-division multiplexing free-space optical transmission with adequate strong turbulence resiliency. A compact spatial light modulator-based polarization multiplexing multi-plane light conversion module was employed to emulate strong turbulent links. By employing advanced successive interference cancellation multiple-input multiple-output decoder and redundant receive channels, the strong turbulence resiliency was significantly improved in a mode-division multiplexing system. As a result, we achieved a record-high line rate of 689.2 Gbit/s, channel number of 10, and net spectral efficiency of 13.9 bit/(s Hz) in a single-wavelength mode-division multiplexing system with strong turbulence.

Securing 2D information carriers over dynamic and turbulent media in a free-space optical channel.

In this Letter, a new, to the best of our knowledge, scheme is proposed to realize high-fidelity secured free-space optical information transmission through dynamic and turbulent media by encoding 2D information carriers. The data are transformed into a series of 2D patterns as information carriers. A novel differential method is developed to suppress noise, and a series of random keys are also generated. A different number of absorptive filters are arbitrarily combined to be placed in the optical channel to generate ciphertext with high randomness. It is experimentally demonstrated that the plaintext can be retrieved only when correct security keys are applied. Experimental results demonstrate that the proposed method is feasible and effective. The proposed method provides an avenue for securing high-fidelity optical information transmission over dynamic and turbulent media in a free-space optical channel.

Bi-Directional Free-Space Visible Light Communication Supporting Multiple Moveable Clients Using Light Diffusing Optical Fiber.

In this work, we put forward and demonstrate a bi-direction free-space visible light communication (VLC) system supporting multiple moveable receivers (Rxs) using a light-diffusing optical fiber (LDOF). The downlink (DL) signal is launched from a head-end or central office (CO) far away to the LDOF at the client side via a free-space transmission. When the DL signal is launched to the LDOF, which acts as an optical antenna to re-transmit the DL signal to different moveable Rxs. The uplink (UL) signal is sent via the LDOF towards the CO. In a proof-of-concept demonstration, the LDOF is 100 cm long, and the free space VLC transmission between the CO and the LDOF is 100 cm. 210 Mbit/s DL and 850 Mbit/s UL transmissions meet the pre-forward-error-correction bit error rate (pre-FEC BER = 3.8 × 10-3) threshold.

Optical analog-signal transmission system in a dynamic and complex scattering environment using binary encoding with a modified differential method.

High-fidelity optical transmission through dynamic scattering media is challenging, since transmission errors are induced due to dynamic scattering media. In this paper, a new scheme is proposed to realize high-fidelity free-space optical analog-signal transmission in dynamic and complex scattering environments using binary encoding with a modified differential method. Each pixel of an analog signal to be transmitted is first divided into two values, and each of them is encoded into a random matrix. Then, a modified error diffusion algorithm is utilized to transform the random matrix into a 2D binary array. Each pixel of the analog signal to be transmitted is eventually encoded into only two 2D binary arrays, and transmission errors and dynamic scaling factors induced by dynamic and complex scattering media can be temporally corrected. Dynamic smoke and non-line-of-sight (NLOS) are created as a dynamic and complex scattering environment to verify the proposed method. It is experimentally demonstrated that analog signals retrieved at the receiving end are always of high fidelity using the proposed method, when average path loss (APL) is less than 29.0 dB. Only the half number of measurements is used compared to that in conventional methods. The proposed method could open up a novel research perspective for high-fidelity free-space optical analog-signal transmission through dynamic and complex scattering media.

Ultrabright polarization-entangled photon pair source for frequency-multiplexed quantum communication in free-space.

The distribution of entanglement via satellite links will drastically extend the reach of quantum networks. Highly efficient entangled photon sources are an essential requirement towards overcoming high channel loss and achieving practical transmission rates in long-distance satellite downlinks. Here we report on an ultrabright entangled photon source that is optimized for long-distance free-space transmission. It operates in a wavelength range that is efficiently detected with space-ready single photon avalanche diodes (Si-SPADs), and readily provides pair emission rates that exceed the detector bandwidth (i.e., the temporal resolution). To overcome this limitation, we demultiplex the photon flux into wavelength channels that can be handled by current single photon detector technology. This is achieved efficiently by using the spectral correlations due to hyper-entanglement in polarization and frequency as an auxiliary resource. Combined with recent demonstrations of space-proof source prototypes, these results pave the way to a broadband long-distance entanglement distribution network based on satellites.

Aiming for high-capacity multi-modal free-space optical transmission leveraging complete modal basis sets

Free-space optical (FSO) communication has gained much interest due to its capability of providing not only a cost-efficient solution where the optical fiber is too expensive or too difficult to deploy but also a much higher system capacity than its radio-frequency competitor. In addition to conventional polarization-division multiplexing and coherent detection, combining with mode-division multiplexing can unlock the full potential of FSO communications and achieve ultra-high spectral efficiency. In this paper, we review the recent progress of multi-modal FSO communication systems leveraging complete modal basis sets. Some practical issues for system design are discussed. We also present two latest demonstrations that achieved the record-high single-wavelength data rate using a commercial transponder and a pair of commercial multiplexer/demultiplexer for multi-modal FSO transmission without turbulence and in atmospheric turbulence, respectively, showing the potential of such a technique.