Energy-efficient regeneration routing in satellite optical networks under translucent optical payload architectures.

The translucent optical payload architecture is most economical and feasible for optical switching in the satellite optical network (SON) using laser inter-satellite links (LISLs), where the wavelength division multiplexing (WDM) technology enables lightpaths to transparently pass through relay satellites, minimizing on-board processing. For the long-distance lightpath in SONs, lightpath regeneration is necessary to ensure the acceptable quality of transmission (QoT), where optical-electrical-optical (OEO) conversion causes non-negligible energy consumption. The rechargeable battery is an important component for low-earth-orbit (LEO) satellites, and unrestrained use batteries at a deep depth of discharge (DOD) will accelerate battery aging and shorten satellite lifetime, causing extremely high expenditure costs. How to improve energy efficiency in lightpath regeneration is a key problem in SONs, which has no related studies. This paper proposes energy-efficient regeneration routing (EE-RR) in SONs under translucent optical payload architectures, which is to reduce the battery life consumption of lightpath regeneration. We define the maximum bypass hops (MBH) to ensure the bit error rate (BER) requirement of lightpaths in SONs, considering the noise accumulation of amplifier spontaneous emission (ASE) and Doppler shift (DS) crosstalk. Two greedy baselines are proposed for EE-RR, which are the regeneration routing scheme minimizing the number of regeneration nodes (RRS-MRN) and the regeneration routing scheme minimizing single satellite's battery life consumption (RRS-MBL), respectively. Based on two greedy baselines, the regeneration routing scheme using genetic algorithms (RRSGA) is developed to improve the optimization ability of EE-RR. To our knowledge, this is the first study to propose taking battery aging into account in lightpath regeneration in SONs. Through numerical simulation, we find that blindly reducing the number of regeneration nodes in lightpaths may not reduce overall battery life consumption, and RRSGA can effectively reduce battery life consumption of lightpath regeneration in SONs.

Free-space optical (FSO) feeder link planning in space-ground integrated optical networks (SGIONs): trade off throughput and dynamics.

The low-orbit satellite communication can provide users with low-delay and ultra-wideband communication services worldwide. By the wide available bandwidth and immunity to interference, free-space optical (FSO) feeder links are presented as an ideal alternative to radio frequency (RF) feeder links for satellite-to-ground backhaul, and the space-ground integrated optical network (SGION) is gradually formed by using FSO feeder links to integrate the low-orbit satellites and the terrestrial optical network (TON) for backhaul services. The propagation channel characteristics of FSO feeder links vary significantly during low-orbit satellite passes, and atmospheric turbulence causes serious scintillation in FSO feeder links at low elevations, increasing link budget and restricting link capacity. Limiting the observation range of optical ground stations (OGSs) to the high elevation area can establish high-capacity feeder links to improve SGION's throughput, but inevitably increase the network dynamics and reduce the satellite visibility. This paper trades off SGION's throughput and dynamics by planning FSO feeder link handover and capacity adjustment. Two baseline schemes and the feeder link handover and capacity adjustment scheme based on non-dominated sorting genetic algorithm (NSGA-FLHCA) are proposed. By finding the Pareto edge of the multi-objective optimization problem, NSGA-FLPCA is more effective in improving network throughput and reducing network dynamics compared with two baselines.

Low-loss microwave photonic switching for satellite communication.

For traditional switching architecture, packet switching performs fine granularity data packet forwarding, but its digital signal processing (DSP) has high power consumption (PC). All-optical switching provides rapid exchange of wavelength resources, which has coarse granularity. In scenarios where the PC is limited, such as broadband satcom, a switching architecture with lower PC and finer granularity than optical switching would be useful. In this paper, we propose a novel, to the best of our knowledge, low-loss microwave photonic switching architecture that can exchange subband signals across beams and frequency bands. The switching process is realized by exchanging optical carriers instead of payload signals, which does not degrade the signal power, guaranteeing the signal-to-noise ratio (SNR). We conducted a proof-of-concept experiment of 2 × 2 switching with two 1.2-GBaud quadrature phase-shift keying (QPSK) signals; an error vector magnitude (EVM) of or less than 13.87% is realized after forwarding. The proposed system has the advantages of low PC, high SNR, and fine granularity, and is very promising for flexible forwarding in future satcom systems.

Simultaneous temperature and pressure sensing based on a single optical resonator.

We propose a dual-parameter sensor for the simultaneous detection of temperature and pressure based on a single packaged microbubble resonator (PMBR). The ultrahigh-quality (∼107) PMBR sensor exhibits long-term stability with the maximum wavelength shift about 0.2056 pm. Here, two resonant modes with different sensing performance are selected to implement the parallel detection of temperature and pressure. The temperature and pressure sensitivities of resonant Mode-1 are -10.59 pm/°C and 0.1059 pm/kPa, while the sensitivities of Mode-2 are -7.69 pm/°C and 0.1250 pm/kPa, respectively. By adopting a sensing matrix, the two parameters are precisely decoupled and the root mean square error of measurement are ∼ 0.12 °C and ∼ 6.48 kPa, respectively. This work promises the potential for the multi-parameters sensing in a single optical device.

Flexibility and fragmentation aware routing, core and spectrum allocation for hybrid AoD nodes in SDM-EONs.

Space division multiplexing elastic optical network (SDM-EON) enables high-capacity transmission, in which the network nodes should provide high switching flexibility while limiting the complexity and costs of nodes. Architecture on demand (AoD) nodes can meet these requirements but the slow configuration time of optical backplane in AoD nodes makes it difficult to serve latency-sensitive requests. In this paper, we propose a hybrid optical backplane based on micro-electromechanical systems (MEMS) and semiconductor optical amplifier (SOA) switches to provide fast configuration time for AoD nodes. Moreover, we propose quantitative measures of node switching flexibility in SDM-EONs and of link configuration speed in AoD nodes. Based on the hybrid backplane architecture and the measurement approaches, we propose a flexibility and fragmentation aware routing, spectrum and core allocation algorithm and an AoD synthesis algorithm. Simulation results show that the hybrid AoD nodes with support of spatial lane change can reduce network blocking probability. The AoD based on this hybrid backplane structure can improve the network performance by 32.8% compared to the AoD based on the traditional MEMS. Compared with traditional reconfigurable optical add/drop multiplexers (ROADMs), the hybrid AoD nodes can control the number of wavelength selective switch (WSS) ports.

RGAIA: a reconfigurable AWGR based optical data center network.

Benefitting from the cost-effective and flexible interconnection between computing nodes and storing infrastructures, various applications and services are deployed in data centers (DCs). These traffic-boosting applications put tremendous pressures on current electrically switched DC networks (DCNs) which suffer the bandwidth bottleneck. Benefitting from the data-rate and format transparency, the optically switched DCN with intrinsic high-bandwidth characteristics is a promising solution to update the hierarchical electrical DCNs with bandwidth limitations. Moreover, the applications deployed in DCNs with mixed traffic characteristics require dynamic quality of service (QoS) provisioning. Optical DCNs thus need to be designed in a flexible topology with the capability of bandwidth reconfigurability to adapt the variety of the traffic. In this paper, we propose and experimentally investigate a reconfigurable optical packet switching DCN named RGAIA, based on flexible top of racks (ToRs) and fast optical switch, where the optical switch is implemented by tunable transceiver combing with arrayed waveguide grating router (AWGR). Under the management of the designed software defined network (SDN) control plane, RGAIA can dynamically distribute the wavelength resource and then reconfigure the bandwidth in real-time based on the monitored traffic characteristics. Experimental assessments validate RGAIA improving performance of 37% and 66% in latency and packet loss, respectively, compared with the network with rigid interconnections at the traffic load of 0.8.

Broadband dual-channel channelizer based on a microwave photonic power tunable image rejection down-conversion.

Radiofrequency (RF) channelization has potential high frequency and wideband advantages in frequency-domain channel segmentation and down-conversion reception. In this paper, we propose a compact dual-channel channelizer that can process high-frequency wideband signals. It uses double-polarization double-sideband electro-optic modulation and Hartley structure photoelectric conversion to realize down-conversion channelization of the high-frequency wideband signal. The power matching between two polarization signals can be realized by controlling the modulator bias, so the crosstalk between the two output signals can be suppressed. The proposed channelizer has a compact structure since the electro-optic modulation is based on one single laser and one single integrated modulator. No filters are used in the structure, contributing to a very wide RF operation bandwidth and low constraints of laser wavelength. In the experiment, the single frequency signal pairs from 9 GHz to 15 GHz can achieve an inter-channel image rejection ratio of 53 dB. Furthermore, the channelizer slices multi-octave bandwidth quadrature phase shift keying (QPSK) signals up to 16 GHz with the wideband isolation higher than 10 dB and outputs them to two channels in parallel. The error vector magnitudes (EVM) of 9-17 GHz and 18-26 GHz band QPSK signals are guaranteed to be under 23.58% after channelized separation. To the best of our knowledge, the proposed channelizer provides high inter-channel interference suppression at dual-band adjacent signals with 8 GHz bandwidth for the first time. Therefore, the proposed channelizer has great application value for the reception and processing of millimeter signals in the future.

Generation of rotational orbital angular momentum beams in the radio frequency based on an optical-controlled circular antenna array.

The orbital angular momentum (OAM) has been widely used in the wireless short-range communication system, but for long-distance communication, the huge difficulty of beam receiving is a great challenge. In this paper, to overcome this challenge, a generation system of radio-frequency rotational orbital angular momentum (RF-ROAM) beams based on an optical-controlled circular antenna array (CAA) is proposed. The ROAM beam is an OAM beam rotating at a certain speed around the beam axis. According to the rotational Doppler effect, the rotation of the OAM beam will induce a frequency shift proportional to the OAM mode and the rotation speed. Thereby, by rotating an OAM beam at a fixed speed scheduled in advance in the transmitting end, the beam can be mode-distinguished by just detecting the frequency shift without receiving the whole wavefront vertical to the beam axis in the receiving end. This provides a partial reception scheme for the OAM-based wireless communication system. The generation system of RF-ROAM beams is proposed and constructed, and the proof-of-concept experiment is performed. In the system, the optical-controlled CAA is constructed to generate the general RF-OAM beam, the optical signal processor (OSP) is employed to control the phase shifts to further control the OAM mode, and the signal with time-varying phase is generated as the rotation factor to control the rotation speed. In the experiment, the RF-ROAM beams with different mode and mode combination are generated and successfully measured by detecting the frequency shift of the signal received in a fixed point.

Data-aided channel equalization scheme for FAST radio over fiber transmission system.

The Five-hundred-meter Aperture Spherical radio Telescope (FAST) located in Guizhou, China, is a very sensitive single dish telescope. Due to the large size of the telescope, optical fiber is used for the transmission of the 3-km astronomical signal from the telescope to the signal processing center. The optical fibers are suspended in the air above the telescope reflector, very easy to slide when the telescope feed cabin moves, resulting in phase drifts for the transmission signal. This phase drift has a negative impact on the observation mode of very long baseline interferometry, and can be compensated by the frequency transfer system in the FAST. In this manuscript, we propose a new phase drift compensation scheme, which is denoted as data-aided channel equalization scheme. The proposed scheme is based on a hypothesis of linear phase relationship between different wavelengths in the same optical fiber, and uses the channel response information of the data-aided channel to conduct signal recovery for the astronomical signal channel. Not only the phase drift, but also the frequency-dependent distortion of the broadband transmission link can be compensated. The proposed scheme has simple transmission structure, and the function part is well modularized, so that the Astronomer users can easily turn it on or off. In the proof-of-concept experiments, the estimation deviation can be significantly reduced by estimated channel responses averaging over training sequence repetitions, showing very high accuracy of the astronomical signal channel estimation.

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.

XT-considered multiple backup paths and resources shared protection scheme based on ring covers.

With the rapid development of space division multiplexing (SDM) and flexible grid technology, the problem of resource allocation in optical network becomes much more complicated. Although there emerge a substantial number of works about link protection or restoration in SDM-EONs mesh networks, the topic of survivability is dug deeper in this work. It is acknowledged that protection schemes based on ring covers bring the advantages of shorter restoration time and lower costs. However, the RSCA problem in SDM-EON for link protection based on ring covers has rarely been investigated. To enhance the survivability of SDM-EON and make the best use of ring covers, we initially select a set of rings for the protection of all the links in a network topology according to constrains, aiming at taking full advantages of network resources. After that, we propose an algorithm to recover the traffic which will break off under link failures, basing on the set of rings. At the end of this protection scheme, we arrange the resources fulfilling the constrains of spectrum contiguity and core continuity constrains by using several different algorithms considering physical impairment. Based on the allocation in both spacial and spectral dimensions, our algorithms achieve better results of survivability. According to the simulations conducted for the evaluation of the proposed algorithms, our algorithms manage to recover at least 72.8% of traffic when the traffic request number is set to be 1000.

Deep neural network based OSNR and availability predictions for multicast light-trees in optical WDM networks.

The quality of transmission (QoT) of a light-tree is influenced by a variety of physical impairments including attenuation, dispersion, amplified spontaneous emission (ASE), nonlinear effect, light-splitting, etc. Moreover, a light-tree has multiple destinations that have different distances away from the source node so that the QoT of the received optical signal at each destination is different from each other. Since the optical network is a living network, the real-time network state is difficult to obtain. Therefore, it is difficult to accurately and rapidly determine the QoT or availability of a light-tree. However, the QoT or availability of a light-tree obtained in advance not only guarantees the quality of service (QoS) but also helps to network optimization design. This paper studies the problems of the optical signal-to-noise ratio (OSNR) and availability predictions for multicast light-trees while leveraging deep neural network (DNN) in optical WDM networks. The DNN based OSNR and availability prediction methods are developed and implemented. Numerical results show that the DNN based OSNR prediction method reaches an accuracy of about 95%. And the DNN based availability prediction method reaches a high accuracy greater than 98%. These two methods provide a fast decision approach for light-tree construction.

Chromatic dispersion immune microwave photonic phase shifter based on double-sideband modulation.

A chromatic dispersion (CD) immune microwave photonic phase shifter (MPPS) based on double-sideband (DSB) modulation is proposed and demonstrated. An optical spectrum processor introduces the phase shift to the MPPS. The DSB signals along two orthogonal polarizations are demodulated to two RF signals with both quadrature amplitude and phase items, transferring the CD-induced power fading to the phase item of the synthetic RF signals. Experimental results show that the RF signals over 14-25 GHz obtain random phase shift in 360° range without a power fading point (PFP) after passing through a dispersion compensation fiber with CD of -331 ps/nm. The phase variation and power variation of the phase-shifted signal are <±5.7° and <±0.9 dB, respectively, at the original PFP at 16 GHz.

Photonic generation of background-free binary and quaternary phase-coded microwave pulses based on vector sum.

A photonic microwave phase-coded pulse generator is proposed and experimentally demonstrated based on the principle of vector sum. The key component of the proposed pulse generator is an integrated polarization-division multiplexing Mach-Zehnder modulator (PDM-MZM) and a 90° hybrid coupler. By properly setting the data sequences applied to the specially biased PDM-MZM, binary and quaternary phase-coded microwave pulses (PCMPs) that are free from the background signals can be generated. Since no filters and polarization adjustment are involved, the proposed pulse generator is characterized by a simple structure, low-loss, flexible frequency tunability and high long-term stability. The experimental results show that background-free 4 Gb/s Barker and Frank PCMPs at 18 GHz and 2 Gb/s Barker and Frank PCMPs at 24 GHz are successfully generated. The calculated pulse compression ratio and peak-to-side lobe ratio are in good agreement with the theoretical values.

All-optical generation of binary phase-coded microwave pulses without baseband components based on a dual-parallel Mach-Zehnder modulator.

In this paper, we propose an all-optical system for the generation of binary phase-coded microwave pulses without baseband components. The scheme is based on a dual-parallel Mach-Zehnder modulator (DPMZM). By properly applying the coding signals and the microwave signals to the precisely biased DPMZM, accurate π phase shift binary phase-coded microwave pulses without baseband components can be generated. The proposed system has an extremely simple and stable all-optical structure, leading to a large frequency tuning range and a high signal quality. The operation of the system is very easy. The generation of the 2-Gbit/s 14-GHz and 4-Gbit/s 16-GHz binary phase-coded microwave pulses under different coding signal amplitudes and microwave carrier powers are experimental verified. The results show that the proposed binary phase-coded microwave pulses generation system has high quality and performance.

Equivalent photonic switch for microwave frequency shift keying signal generation.

A photonic microwave frequency shift keying (FSK) signal generator is proposed and experimentally demonstrated based on an equivalent photonic switch (EPS). The EPS is constructed using a polarization-multiplexing dual-drive Mach-Zehnder modulator (PM-DMZM). By properly controlling the data sequences and RF signals applied to the PM-DMZM, microwave FSK signals with flexible frequency intervals can be obtained. The proposed FSK signal generator features the advantages of a simple structure, low loss, good stability, and great frequency tunability. In addition, the proposed setup can also be easily reconfigured to generate microwave amplitude shift keying and phase shift keying signals. The experimental results show that 2 Gb/s at 5/14 GHz and 1 Gb/s at 6/20 GHz microwave FSK signals are successfully generated, after transmission over 5 km single-mode fiber. The required received optical power at 7% forward error correction threshold is only -14.48 dBm.

Optical network solution to the synchronization of distributed coherent aperture radar.

Distributed coherent aperture radar (DCAR) is an important direction for next-generation radar due to its high sensitivity. The challenge to realize DCAR is the synchronization among geographically distributed radar units. We propose an optical network for DCAR synchronization. The proposed network achieves functions of phase-coded pulse generation, time synchronization, and phase synchronization with the help of microwave photonics techniques. Proof-of-concept experiments are conducted in fiber and space transmission scenarios. The combined radar beams have negligible energy loss when synchronization is achieved.

2D optically controlled radio frequency orbital angular momentum beam steering system based on a dual-parallel Mach-Zehnder modulator.

An optically controlled system for generating and continuously steering radio frequency (RF) signals with double orbital angular momentum (OAM) modes is proposed and experimentally demonstrated. The optical carrier's utilization efficiency can be doubled through the distinct electro-optical modulation, which is based on two single-sideband modulation operations on a single optical carrier through a customized dual-parallel Mach-Zehnder modulator. A constructive antenna phase feeding method of a circular antenna array for collectively forming and steering an OAM radio beam is proposed and illustrated. A proof-of-concept experiment is conducted to generate and steer a dual-mode RF-OAM beam to two different two-dimensional (2D) directions, independently and simultaneously. One 17 GHz OAM beam with mode L=1 is continuously steered to 2D directions (:, 0°, 0°), (:, 0°, 1.70°), (:, 0°, 3.87°), (:, 0°, 6.17°), and(:, 0°, 7.80°), with vortex properties, where ":" means "any value of." Meanwhile, the 19 GHz OAM beam with mode L=-1 carried is steered from (:, 0°, 0°) to (:, 0°, -6.72°), and the constellations are obtained successfully.

Broadband chromatic-dispersion-induced power-fading compensation for radio-over-fiber links based on Hilbert transform.

A Hilbert-transform-based broadband chromatic dispersion (CD) compensation scheme for radio-over-fiber links is proposed and experimentally demonstrated. By constructing a Hilbert transform path, CD-induced phase shifts, which initially lead to periodic power fading of the output RF signals, are transferred to the phases of the RF signals. As a result, the powers of the output RF signals are free from the effect of CD in a broadband frequency range. Experimental results show that a flat normalized amplitude-frequency response is actualized within 2-24 GHz, with only 3.02 dB/4.27 dB power fluctuation after transmission over an equivalent of a 38.6 km/43.6 km single-mode fiber. Besides, compared with a conventional dispersive path, the proposed CD compensation scheme significantly improves the third-order spurious-free dynamic range by 23.60 dB.

A Small-Divergence-Angle Orbital Angular Momentum Metasurface Antenna.

Electromagnetic waves carrying an orbital angular momentum (OAM) are of great interest. However, most OAM antennas present disadvantages such as a complicated structure, low efficiency, and large divergence angle, which prevents their practical applications. So far, there are few papers and research focuses on the problem of the divergence angle. Herein, a metasurface antenna is proposed to obtain the OAM beams with a small divergence angle. The circular arrangement and phase gradient were used to simplify the structure of the metasurface and obtain the small divergence angle, respectively. The proposed metasurface antenna presents a high transmission coefficient and effectively decreases the divergence angle of the OAM beam. All the theoretical analyses and derivation calculations were validated by both simulations and experiments. This compact structure paves the way to generate OAM beams with a small divergence angle.