Non-line-of-sight optical wireless communication system enabled by wavefront shaping for multi-user indoor access.

In this Letter, we experimentally investigate a non-line-of-sight (NLOS) optical wireless communication (OWC) system that utilizes wavefront shaping techniques to realize simultaneous data transmission for multiple users. Wavefront shaping techniques are employed to address the issue of low intensity of diffusely reflected light at the receiver in NLOS scenarios for indoor high-speed access. To achieve communication path planning and tracing for two different users in free-space optical communication, the pixels of the spatial light modulator (SLM) are divided into two halves to separately manipulate the wavefront of two independent data carriers centered at different wavelengths. The maximum received optical power can be effectively improved by more than 15 dB with the wavefront shaping technique. To avoid power enhancement of non-target wavelength, the wavelength difference of two different users is experimentally studied. The difference in power enhancement ratio (DPER) is increased with the wavelength difference, and 14.95 dB DPER is obtained with a 10 nm wavelength difference. Under the aforementioned wavelength planning strategy, successful transmission and reception of 2 × 160 Gbit/s 16-QAM signals for two users with coherent detection is achieved using wavelengths of 1550 and 1560 nm in an indoor access scenario.

Digital ray: enhancing cataractous fundus images using style transfer generative adversarial networks to improve retinopathy detection.

BACKGROUND/AIMS: The aim of this study was to develop and evaluate digital ray, based on preoperative and postoperative image pairs using style transfer generative adversarial networks (GANs), to enhance cataractous fundus images for improved retinopathy detection. METHODS: For eligible cataract patients, preoperative and postoperative colour fundus photographs (CFP) and ultra-wide field (UWF) images were captured. Then, both the original CycleGAN and a modified CycleGAN (C2ycleGAN) framework were adopted for image generation and quantitatively compared using Frechet Inception Distance (FID) and Kernel Inception Distance (KID). Additionally, CFP and UWF images from another cataract cohort were used to test model performances. Different panels of ophthalmologists evaluated the quality, authenticity and diagnostic efficacy of the generated images. RESULTS: A total of 959 CFP and 1009 UWF image pairs were included in model development. FID and KID indicated that images generated by C2ycleGAN presented significantly improved quality. Based on ophthalmologists' average ratings, the percentages of inadequate-quality images decreased from 32% to 18.8% for CFP, and from 18.7% to 14.7% for UWF. Only 24.8% and 13.8% of generated CFP and UWF images could be recognised as synthetic. The accuracy of retinopathy detection significantly increased from 78% to 91% for CFP and from 91% to 93% for UWF. For retinopathy subtype diagnosis, the accuracies also increased from 87%-94% to 91%-100% for CFP and from 87%-95% to 93%-97% for UWF. CONCLUSION: Digital ray could generate realistic postoperative CFP and UWF images with enhanced quality and accuracy for overall detection and subtype diagnosis of retinopathies, especially for CFP.\ TRIAL REGISTRATION NUMBER: This study was registered with ClinicalTrials.gov (NCT05491798).

Cost-effectiveness and cost-utility of a digital technology-driven hierarchical healthcare screening pattern in China.

Utilization of digital technologies for cataract screening in primary care is a potential solution for addressing the dilemma between the growing aging population and unequally distributed resources. Here, we propose a digital technology-driven hierarchical screening (DH screening) pattern implemented in China to promote the equity and accessibility of healthcare. It consists of home-based mobile artificial intelligence (AI) screening, community-based AI diagnosis, and referral to hospitals. We utilize decision-analytic Markov models to evaluate the cost-effectiveness and cost-utility of different cataract screening strategies (no screening, telescreening, AI screening and DH screening). A simulated cohort of 100,000 individuals from age 50 is built through a total of 30 1-year Markov cycles. The primary outcomes are incremental cost-effectiveness ratio and incremental cost-utility ratio. The results show that DH screening dominates no screening, telescreening and AI screening in urban and rural China. Annual DH screening emerges as the most economically effective strategy with 341 (338 to 344) and 1326 (1312 to 1340) years of blindness avoided compared with telescreening, and 37 (35 to 39) and 140 (131 to 148) years compared with AI screening in urban and rural settings, respectively. The findings remain robust across all sensitivity analyses conducted. Here, we report that DH screening is cost-effective in urban and rural China, and the annual screening proves to be the most cost-effective option, providing an economic rationale for policymakers promoting public eye health in low- and middle-income countries.

Digital-filter-aided crosstalk-mitigation for a high spatial resolution AWGR-based 2D IR beam-steered indoor optical wireless communication system.

The growing demand for wireless connectivity is attracting interest in optical wireless communication (OWC) technique. In this paper, a filter-aided crosstalk mitigation scheme, employing digital Nyquist filters, is proposed to eliminate the trade-off between the spatial resolution and the channel capacity for the AWGR-based 2D infrared beam-steered indoor OWC system. By shaping the transmitted signal for narrow spectral occupancy, the inter-channel crosstalk resulting from the imperfect AWGR filtering can be avoided, which enables a denser AWGR grid. In addition, the spectral-efficient signal reduces the bandwidth requirement of the AWGR, which allows a low-complexity AWGR design. Thirdly, the proposed method is not sensitive to the wavelength misalignment between AWGRs and lasers, which relaxes the design of high wavelength stability lasers. Moreover, the proposed method is cost-efficient as we can make use of the mature DSP technique without additional optical components. The 20-Gbit/s data rate OWC capacity using PAM4 format has been experimentally demonstrated over a 6-GHz bandwidth-limited AWGR-based 1.1-m free-space link. The experimental results show the feasibility and effectiveness of the proposed method. By combining our proposed method with the polarization orthogonality technique, a promising capacity per beam of 40 Gbit/s is potentially attainable.

Monolithic integrated two-stage cascaded SOA-PIN receiver for high-speed optical wireless communication.

Monolithic integrated receivers are highly desired due to the potential of mass production and the reduction of device size and cost. In this Letter, a monolithic integrated optical wireless communication (OWC) receiver with optical preamplifiers is designed, fabricated, and investigated to achieve high sensitivity based on photonic integration technology. The proposed receiver consists of one waveguide PIN photodetector integrated with two semiconductor optical amplifiers (SOAs). Compared with using a one-stage optical amplifier, using two independent SOAs as a two-stage amplifier offers the advantage of optimizing the noise figure of each amplifier independently by tuning their injection currents, which leads to the reduction of the total noise and an improvement of the receiver sensitivity. The achieved sensitivity for a 10-Gb/s OOK signal with 10-dBm launch power at 1550-nm wavelength by using the designed receiver is up to -27.5 dBm at a bit-error-ratio (BER) level of 3.1×10-3 over a 0.9-m indoor free-space link. The experimental results show the potential to achieve a high-speed OWC link with high sensitivity by using a cascaded SOA/PIN monolithic integrated receiver.

Efficient light coupling between conventional silicon photonic waveguides and quantum valley-Hall topological interfaces.

Robust and efficient light coupling into and out of quantum valley-Hall (QVH) topological interfaces within near-infrared frequencies is demanded in order to be integrated into practical two-dimensional (2D) optical chips. Here, we numerically demonstrate efficient light coupling between a QVH interface and a pair of input/output silicon photonic waveguides in the presence of photonic crystal line defects. When the topological QVH interface is directly end-butt coupled to the silicon waveguides, the input-to-output transmission efficiency is lower than 50% and the exterior boundaries associated with a QVH interface also cause inevitable back-reflections and high-order scatterings, further reducing the transmission efficiency. The transmission efficiency is substantially increased to 95.8% (94.3%) when photonic crystal line defects are introduced between the bridge (zigzag) QVH interface and the waveguides. The buffering line defect mode, with an effective group refractive index between the interface state and the waveguide mode will ease their mode profile conversion. The design we present here brings no fabrication complexity and may be used as a guide for future implementation of on-chip 2D topological photonics.

Optical polarization division multiplexing fiber-wireless integration system at Ka-band based on a low-cost dual-drive MZM.

In this paper, a low-cost dual-drive Mach-Zehnder modulator (DDMZM) based 40 Gbit/s polarization division multiplexing (PDM) fiber-wireless-integration system at Ka-band is experimentally demonstrated. Since the DDMZM is biased at the quadrature point for electro-to-optical (E/O) conversion, a high-power direct current (DC)/radio frequency (RF) component will appear within the received signal bandwidth. This high-power component becomes a narrowband interference due to the phase noise of lasers, which will lead to the incorrect convergence of the constant-modulus algorithm (CMA) during equalization at the receiver side. In order to deal with the broadened DC component, twin-single-sideband (twin-SSB) signal with bandwidth interleave and RF-pilot based phase noise compensation scheme are adopted. Enabled by the combination of optical PDM technique and heterodyne coherent detection, a 40 Gbit/s PDM twin-SSB Nyquist-shaped quadrature-phase-shift-keying (QPSK) signal transmitting over 20-km single mode fiber (SMF) and 1-m 2×2 multiple-input multiple-output (MIMO) wireless distance is achieved with the bit error rate (BER) below the hard-decision forward-error-correction (HD-FEC) threshold of 3.8×10-3.

Extended Risley scanning system with 30 × 360 coverage.

We propose a beam scanning system based on an extended Risley prism structure. This system uses a rotating mirror structure and expands the original construction of the balanced and stable Risley prism. There is much research on extended Risley prism structures, but the scanned angle of their extended systems is less than 180°. The proposed system in this paper can expand scanned angle to 360°, showing practical significance and application value in the field of beam scanning. This is achieved by using reflectors and multiple transmitter and receiver structures with optimized positions and transmission directions. Simulation results show that the scanning range is 360° and ±14∘ in the horizontal and vertical directions, respectively, at 110 m while the angular resolution is 0.2∘×2∘.

Analog radio of fiber link of 2-Gbaud OOK/BPSK radio frequency-orbital angular momentum beam transmission over 19.4 km.

The 5G mobile communication system provides ultrareliable, low-latency communications at up to 10 Gbps. However, the scale and power consumption of 5G is tremendous owing to a large number of antenna drivers required by the massive multiple-input multiple-output technique. The 6G system will require an architectural paradigm shift to resolve this problem. In this study, we propose an analog RoF downlink scheme for 6G wireless communications. The upcoming oversized base station problem is solved using photonics techniques. The antennas are driven together within the optical domain at a centralized station. The proposed system uses orbital angular momentum (OAM) beams as the generated space-division-multiplexing beams. An RF-OAM beam has a weak coupling effect between different modes, which will dramatically decrease the complexity of the signal processing. In our proof-of-concept experiment, the generated RF-OAM beam was shown to carry a 2-Gbaud OOK/BPSK signal in the Ku-band. Signals were transmitted over a 19.4-km RoF link without dispersion-induced power fading. In addition, by switching the OAM beams, a two-dimensional direction scanning was achieved.

Active mode-locking optoelectronic oscillator.

The optoelectronic oscillator (OEO) has been widely investigated to generate ultra-pure single-frequency microwave signals. In this study, we propose and experimentally demonstrate an active mode-locking optoelectronic oscillator (AML-OEO), which can generate broadband microwave frequency comb (MFC) signals. An additional intensity modulator is inserted into the OEO as an active mode-locking device for loss modulation to realize phase-locking between adjacent oscillation modes. Through the active mode-locking technique, steady multi-mode oscillation is achieved, which is difficult to realize in a conventional OEO due to the mode-competition effect. By tuning the frequency of the active modulation signal (AMS), both fundamental and harmonic AML-OEOs can be established. In the experiments, MFC signals with a frequency spacing of 195 kHz and 50.115/100.035 MHz are generated with fundamental and harmonic AML-OEOs.

Photonic compressive sensing of sparse radio frequency signals with a single dual-electrode Mach-Zehnder modulator.

A novel approach to realizing compressive sensing (CS) of sparse radio frequency (RF) signals based on photonic random demodulation (RD) is proposed. The key function of mixing the RF signal under test and the bipolar pseudo-random binary sequence (PRBS) in photonic RD is implemented with a single dual-electrode Mach-Zehnder modulator (DEMZM). By properly setting the DC bias of the DEMZM at Vπ and the voltages of the PRBS at ±Vπ/2, a pure desired multiplication term between the signal and the bipolar PRBS is obtained after an AC-coupled photodetector (PD), which not only simplifies the modeling of the CS link but also improves the recovery performance. A proof-of-concept experiment is demonstrated where a sparse signal with spectral components of 500 MHz and 950 MHz is successfully identified with a compression ratio of 20. Simulation results are also given to show the advantage of the given photonic CS scheme with bipolar random mixing.

Real-time discrete Fourier transformer with complex-valued outputs based on the inverse temporal Talbot effect.

Discrete Fourier transform (DFT) plays an important role in digital signal processing. In this paper, we present a novel optical real-time discrete Fourier transformer with complex-valued outputs, which is enabled by the inverse temporal Talbot effect. In the system, an input pulse train is first quadratically phase-modulated as in an inverse temporal Talbot system and then split into two channels. In the first channel, the pulse train is further amplitude-modulated pulse-by-pulse by a discrete data sequence to be transformed. In the second channel, a reference signal modulates the pulse train, which is for removing the residual quadratic phase profile in the output pulse train. The pulse trains in the two channels propagate through a shared dispersion medium with a proper dispersion value determined by the inverse temporal Talbot effect. A 90-degree optical hybrid and two balanced photodetectors are employed to retrieve the real and imaginary parts of the DFT results. In this scheme, the pulse repetition rate of the output pulse train is equal to the input one. In addition, we present a full theoretical framework to explain exactly the DFT relationship. We also demonstrate that the input data sequence can be complex-valued with the help of an I/Q modulator.

Ultra-high-capacity wireless communication by means of steered narrow optical beams.

The optical spectrum offers great opportunities to resolve the congestion in radio-based communication, aggravated by the booming demand for wireless connectivity. High-speed infrared optical components in the 1500 nm window have reached high levels of sophistication and are extensively used already in fibre-optic networks. Moreover, infrared light beyond 1400 nm is eye-safe and is not noticeable by the users. Deploying steerable narrow infrared beams, wireless links with huge capacity can be established to users individually, at minimum power consumption levels and at very high levels of privacy. Fully passive diffractive optical modules can handle many beams individually and accurately steer narrow beams two-dimensionally by just remotely tuning the wavelength of each beam. The system design aspects are discussed, encompassing the beam-steering transmitter, wide field-of-view optical receiver and the localization of the user's wireless devices. Prototype system demonstrators are reported, capable of supporting up to 128 beams carrying up to 112 Gbit s-1 per beam. Hybrid bidirectional systems which use a high-speed downstream optical link and an upstream radio link at a lower speed can provide powerful asymmetric wireless connections. All-optical bidirectional beam-steered wireless communication will be able to offer the ultimate in wireless capacity to the user while minimizing power consumption. This article is part of the theme issue 'Optical wireless communication'.

Reconfigurable beam system for non-line-of-sight free-space optical communication.

In this paper, we propose a reconfigurable beam-shaping system to permit energy-efficient non-line-of-sight (NLOS) free-space optical communication. Light is steered around obstacles blocking the direct communication pathway and reaches a receiver after reflecting off of a diffuse surface. A coherent array optical transmitter (CAO-Tx) is used to spatially shape the wavefront of the light incident on a diffuse surface. Wavefront shaping is used to enhance the amount of diffusely reflected light reaching the optical receiver. Synthetic NLOS experiments for a signal reflected over an angular range of 20° are presented. A record-breaking 30-Gbit/s orthogonal frequency-division multiplexing signal is transmitted over a diffused optical wireless link with a >17-dB gain.

Optical I/Q modulation utilizing dual-drive MZM for fiber-wireless integration system at Ka-band.

In this Letter, a fiber-wireless integration system at Ka-band adopting heterodyne coherent detection is proposed and experimentally demonstrated. Optical I/Q up-conversion is realized with a low-cost dual-drive Mach-Zehnder modulator (DD-MZM) instead of the traditional I/Q nested MZM. To avoid non-convergence during constant-modulus-algorithm-based equalization, DC elimination is applied to suppress the ultra-high peak in the frequency domain after frequency down-conversion and symbol-phase-average-processing-based coarse phase noise estimation. Using DD-MZM for optical I/Q modulation, transmission, and reception of single polarization, 20-Gbit/s quadrature phase shift keying (QPSK) over 40-km single-mode fiber (SMF) and 5-m free space at Ka-band is demonstrated.

Cost-efficient half-duplex 10 Gbit/s all-optical indoor optical wireless communication enabled by a low-cost Fabry-Perot laser/photodetector.

To develop an indoor optical wireless communication (OWC) system, both the system complexity/cost and data rate need to be taken into consideration. In this Letter, a cost-efficient half-duplex OWC system for photonic home area network applications is proposed and experimentally demonstrated. A low-cost Fabry-Perot laser diode is proposed to be employed as both the downlink receiver (Rx) and uplink transmitter at the user side. Enabled by the Fabry-Perot transceiver, the indoor transmission of 10 Gbit/s four-level pulse-amplitude-modulation signal for both downlinks and uplinks is experimentally achieved over a 1.7 km single-mode fiber and 1.1 m free space. Moreover, the proposed scheme also enables us to operate an orthogonal frequency division multiplexing (OFDM) signal. The bit error rate levels of multi-gigabit OFDM data for both downlinks and uplinks over a 10 h measurements are all under a 7% forward error correction limit of 3.8×10-3, which indicates that the proposed system is robust and, thus, can provide a promising solution for high-speed low-cost home area OWC networks.

40 Gb/s indoor optical wireless system enabled by a cyclically arranged optical beamsteering receiver.

Indoor optical wireless communication with optical beamsteering capability is currently attracting a lot of attention. One major two-dimensional (2D) optical beamsteering scheme is realized by 2D grating or its active counterpart, which is usually based on a spatial light modulator (SLM). However, there is a fundamental trade-off between the field of view (FoV) and power efficiency due to the inherent feature of gratings. In this Letter, we propose a new class of 2D beamsteering, named cyclically arranged optical beamsteering (CAO-BS), which can break that trade-off. Traditional 2D gratings extend the optical beam in the Cartesian coordinates (1D grating in horizontal + 1D grating in vertical), while CAO-BS extends the optical beam in the polar coordinates (1D grating + angular rotation). Since only 1D grating is engaged, the power efficiency increases with the number of grating lobes reduced. In the polar coordinates, the angle rotation tuning in a SLM is quasi-continuous in a full 2π range. The CAO-BS is demonstrated at the receiving end in an indoor experimental system. The FoV is 18° by 360° in polar coordinates without any additional mechanical parts. Based on the CAO-BS, 40 Gbit/s on-off keying data is also successfully transmitted over 1 km single-mode fiber and 0.5 m free space.

Free-space transmission with passive 2D beam steering for multi-gigabit-per-second per-beam indoor optical wireless networks.

In order to circumvent radio spectrum congestion, we propose an innovative system which can provide multiple infrared optical wireless beams simultaneously where each beam supports multi-gigabit-per-second communication. Scalable two-dimensional beam steering by means of wavelength tuning is proposed. A passive beam-steering module constructed with cascaded reflection gratings is designed for simultaneous multi-user coverage. We experimentally characterized the beam-steered system and thoroughly evaluated the performance of steered channels using the spectrally efficient and robust discrete multitone modulation in a bandwidth-limited system deploying 10 GHz telecom transceivers. This study reports the achievement of at least 37 Gbps free-space transmission per beam over a distance of up to 2 m over 5.61° × 12.66° scanning angles.

Characterization of Rayleigh backscattering arising in various two-mode fibers.

We experimentally characterize the mode dependent characteristics of Rayleigh backscattering (RB) arising in various two-mode fibers (TMFs). With the help of an all-fiber photonic lantern, we are able to measure the RB power at individual modes. Consequently, mode dependent power distribution of RB light caused by arbitrary forward propagation mode superposition can be obtained. The total RB power of the TMFs under test is higher than that of single mode fiber by at least 2 dB over the C band. Meanwhile, the RB light occurs among all guided modes in the TMFs with specific power ratios. The experimental characterization agrees well with the theoretical calculations.

Mode-dependent characterization of photonic lanterns.

We propose and experimentally demonstrate a simple method for characterizing the power transfer matrix of photonic lanterns (PLs) used for mode division multiplexing (MDM) transmission. Due to the optical reflection arising at output facet of the few-mode fiber (FMF), we are able to detect the power at the individual single-mode fiber (SMF) input port and exploit a series of equations based on the theory of energy conservation to obtain mode-dependent characteristics of the PL, including the property of mode selectivity, insertion loss (IL), and channel-dependent loss (CDL). The proposed method is experimentally verified for both the mode selective and the nonmode selective photonic lanterns.