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.

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.

Integrated remotely tunable optical delay line for millimeter-wave beam steering fabricated in an InP generic foundry.

A compact and fabrication-tolerant integrated remotely tunable optical delay line is proposed for millimeter-wave beam steering and is fabricated in an InP generic foundry. The proposed delay line is based on a spectrally cyclic-arrayed waveguide grating feedback loop. Its major features include the tolerant architecture with reduced chip size, and bi-directional operation with simplified remote tuning. Moreover, its cyclic feature guarantees further cascaded operations either for 2D radio beam steering or for high-resolution delay generation. The experimental results show less than 6.5-dB insertion loss of the integrated delay line. Five different delays from 0 to 71.6 ps are generated with less than 0.67-ps delay errors.

Mode-multiplexed transmission over conventional graded-index multimode fibers.

We present experimental results for combined mode-multiplexed and wavelength multiplexed transmission over conventional graded-index multimode fibers. We use mode-selective photonic lanterns as mode couplers to precisely excite a subset of the modes of the multimode fiber and additionally to compensate for the differential group delay between the excited modes. Spatial mode filters are added to suppress undesired higher order modes. We transmit 30-Gbaud QPSK signals over 60 WDM channels, 3 spatial modes, and 2 polarizations, reaching a distance of 310 km based on a 44.3 km long span. We also report about transmission experiments over 6 spatial modes for a 17-km single-span experiment. The results indicate that multimode fibers support scalable mode-division multiplexing approaches, where modes can be added over time if desired. Also the results indicate that mode-multiplexed transmission distance over 300 km are possible in conventional multimode fibers.

Cyclic additional optical true time delay for microwave beam steering with spectral filtering.

Optical true time delay (OTTD) is an attractive way to realize microwave beam steering (MBS) due to its inherent features of broadband, low-loss, and compactness. In this Letter, we propose a novel OTTD approach named cyclic additional optical true time delay (CAO-TTD). It applies additional integer delays of the microwave carrier frequency to achieve spectral filtering but without disturbing the spatial filtering (beam steering). Based on such concept, a broadband MBS scheme for high-capacity wireless communication is proposed, which allows the tuning of both spectral filtering and spatial filtering. The experimental results match well with the theoretical analysis.

Toward multi-Gbps indoor optical wireless multicasting system employing passive diffractive optics.

This Letter presents the evaluation and demonstration of an optical free-space (FS) multicasting system for multi-Gigabits-per-second (multi-Gbps) indoor transmission. These simultaneous line-of-sight links are formed by infrared beams and are beam-steered using a passive diffraction grating. The experiment has resulted in error-free links (bit error rate <10(-9) at 2.5 Gbps on-off keying) and is scalable to support higher data rates. This system is proposed for short-range optical wireless communication and can be seamlessly integrated in in-building fiber networks.

Phase modulation parallel optical delay detector for microwave angle-of-arrival measurement with accuracy monitored.

A novel phase modulation parallel optical delay detector is proposed for microwave angle-of-arrival measurement with accuracy monitored by using only one dual-electrode Mach-Zehnder modulator. A theoretical model is built up to analyze the proposed system including measurement accuracy monitoring. The spatial delay measurement is translated into the phase shift between two replicas of a microwave signal. Thanks to the accuracy monitoring, the phase shifts from 5° to 165° are measured with less than 3.1° measurement error.

Steerable pencil beams for multi-Gbps indoor optical wireless communication.

We report a novel optical wireless communication (OWC) system solution that supports multi-Gbps (Gigabit-per-second) capacity for indoors. Narrow beams, termed as pencil beams, are directed to wireless users using a tunable laser and a passive diffractive optical element. This enables a wide coverage of ultra-high-capacity communication links to serve multiple network users simultaneously. Experimental results demonstrating data rates of up to 10 Gbps, with on-off keying modulation format, over a distance of more than 2.5 m, are reported. Error-free links beam-steered over a total wavelength range of 130 nm, with steering angle of 17.16°, have been achieved. This system is proposed for short-range OWC and is promising for seamless integration in in-building optical networks.

Synchronized signaling delivery for broadband 60 GHz in-building optical wireless network based on digital frequency division multiplexing and digital Nyquist shaping.

A simple and low-cost synchronized signaling delivery scheme has been proposed for a 60 GHz in-building optical wireless network with 12.7Gbps throughput based on digital frequency division multiplexing and digital Nyquist shaping.

Field demonstration of mode-division multiplexing upgrade scenarios on commercial networks.

We demonstrate three possible scenarios for upgrading current single-mode transmission networks with high capacity few-mode fiber technology using mode-division multiplexing (MDM). The results were obtained from measurements over a number of field-deployed single-mode fiber links with an additional experimental in-line amplified few-mode fiber link. The results confirm the viability of employing MDM using few-mode fiber technology to gradually replace legacy optical systems.

73.7 Tb/s (96 x 3 x 256-Gb/s) mode-division-multiplexed DP-16QAM transmission with inline MM-EDFA.

Transmission of a 73.7 Tb/s (96 x 3 x 256-Gb/s) DP-16QAM mode-division-multiplexed signal over 119 km of few-mode fiber transmission line incorporating an inline multi mode EDFA and a phase plate based mode (de-)multiplexer is demonstrated. Data-aided 6 x 6 MIMO digital signal processing was used to demodulate the signal. The total demonstrated net capacity, taking into account 20% of FEC-overhead and 7.5% additional overhead (Ethernet and training sequences), is 57.6 Tb/s, corresponding to a spectral efficiency of 12 bits/s/Hz.

Power-efficient impulse radio ultrawideband pulse generator based on the linear sum of modified doublet pulses.

We propose a new and power-efficient impulse radio ultawideband (IR-UWB) pulse design concept. The proposed concept is based on a linear sum of modified doublet pulses. The proposed concept is both simulated and experimentally demonstrated. The experimental demonstration employs a photonic scheme that generates the designed pulse using two main steps, mainly optical shaping and differential detection. The optical shaping is performed using a single electro-optic modulator biased in the nonlinear portion of its transfer function, and the differential detection is performed using a balanced photodetector. The generated IR-UWB pulse is fully Federal Communications Commission compliant, even in the highly power-restricted global positioning system band. The proposed optical scheme has potential to be integrated on a compact optical chip and thus suitable for reliable, low-cost, high-speed, short-range UWB wireless access, such as in-building networks.

Cost optimization of optical in-building networks.

Optical fiber-based in-building network solutions can outperform in the near future copper- and radio-based solutions both regarding performance and costs. POF solutions are maturing, and can already today be cheaper than Cat-5e solutions when ducts are shared with electricity power cabling. We compare the CapEx and OpEx of in-building networks for fiber and Cat-5E solutions. For residential homes, our analysis shows that total network costs during economic lifetime are lowest for a point-to-point duplex POF topology.

10-Gb/s transmission over 20-km single fiber link using 1-GHz RSOA by discrete multitone with multiple access.

We demonstrate a novel 10.5-Gbit/s transmission scheme over 20-km single fiber link by using a remotely fed 1-GHz reflective semiconductor optical amplifier (RSOA). Discrete multitone (DMT) modulation with adaptive bit-/power-loading is applied to overcome the bandwidth limitation of the RSOA. Transmission performance of the proposed scheme is analyzed in terms of various system parameters, such as the nonlinearity of the RSOA, optical signal-to-noise ratio of the optical seed carrier, the overhead size impact on dispersion, the number of DMT subcarriers, and the reflection noise from the single fiber link. We also report flexible-bandwidth-allocated multiple access operation based on the proposed scheme. The throughput for all cases is approximately 10 Gbit/s with BER < 10(-3).

Record high-speed short-range transmission over 1 mm core diameter POF employing DMT modulation.

We report multigigabit/second transmission capacity in 1 mm core diameter graded index plastic optical fiber (POF) exploiting off-the-shelf low-cost components and discrete multitone (DMT) modulation. Transmission capacities of 10.1 Gbits/s x 15 m and 12.7 Gbits/s x 3 m are achieved for average bit-error rates less than 10(-3).

Mode-selective spatial filtering for increased robustness in a mode group diversity multiplexing link.

We propose a very simple optical method to reduce the cross talk among the channels of a mode group diversity multiplexing (MGDM) link. MGDM is an intensity modulation, direct detection, multiple-input, multiple-output technique that creates independent communication channels over a multimode fiber (MMF). The cross talk among the channels is mitigated electronically. However, by properly employing a lens between the output of a graded-index MMF and the detectors, we achieve mode-selective spatial filtering (MSSF) and optically reduce the cross talk. The robustness of the link is then increased when compared with an implementation without MSSF. This allows for a larger number of channels.

All-optical demultiplexing of 640 to 40 Gbits/s using filtered chirp of a semiconductor optical amplifier.

We present a high-capacity ultrafast all-optical time demultiplexer that can be employed to retrieve 40 gigabits/second (Gb/s) base-rate channels from a 640 Gb/s single-polarized signal. The demultiplexer utilizes ultrafast effects of filtered chirp of a semiconductor optical amplifier. Excellent demultiplexing performance is shown at very low switching powers: +8 dBm (640 Gb/s data) and -14 dBm (40 GHz clock). The demultiplexer has a simple structure and, in principle, allows monolithic integration.

Near-field intensity pattern at the output of silica-based graded-index multimode fibers under selective excitation with a single-mode fiber.

Selective excitation of graded-index multimode fibers (GI-MMFs) with a single-mode fiber (SMF) has gained increased interest for telecommunication applications. It has been proposed as a way to enhance the transmission bandwidth of GI-MMF links and/or create parallel communication channels over the same GI-MMF. Although the effect of SMF excitation on the transmission bandwidth has been investigated, its impact on the near-field intensity pattern at the output face of the GI-MMF has not been systematically addressed. We have carried out an analysis of the near-field intensity pattern at the output face of silica-based GI-MMFs excited by a radially offset SMF. Simulation results exhibit all of the features displayed by experimental ones. It turns out that differential mode attenuation and delay, full intra-group mode mixing, and small deviations in the refractive index profile of the GI-MMF do not affect the overall shape of the near-field intensity, which is determined by the radial offset of the input SMF. This can be exploited in mode group diversity multiplexing links. The effect of defects in the refractive index profile, such as a central dip or peak, is also examined.

Time domain add-drop multiplexing for RZ-DPSK OTDM signals.

We show all-optical time domain add-drop multiplexing for a phase modulated OTDM signal for the first time, to our knowledge. The add-drop multiplexer is constructed of a Kerr shutter consisting of a 375 m long highly nonlinear fiber (HNLF), gamma=20 W(-1)km(-1). Successful time domain add-drop multiplexing is shown for 80 Gb/s RZ-DPSK OTDM signals with a 10 Gb/s base rate.

Optical node with time-space-and-wavelength domain contention resolution, deflection and dropping capability.

We experimentally demonstrate an optical node with time-space-and- wavelength domain contention resolution, deflection and dropping capability. The node is composed of an optical buffer based on an optical crossconnect and a wavelength converter. Although the experimental results are shown at 10 Gbit/s the bitrate can be increased substantially. Bit-error rate measurements are shown, sustaining only 3.5 dB power penalty after 10mus of optical buffering and agile wavelength conversion over 18nm span.