Millimeter Wave Attenuation Due to Wind and Heavy Rain in a Tropical Region.

Millimeter wave fixed wireless systems in future backhaul and access network applications can be affected by weather conditions. The losses caused by rain attenuation and antenna misalignment due to wind-induced vibrations have greater impacts on the link budget reduction at E-band frequencies and higher. The current International Telecommunications Union Radiocommunication Sector (ITU-R) recommendation has been widely used to estimate rain attenuation, and the recent Asia Pacific Telecommunity (APT) report provides the model to estimate the wind-induced attenuation. This article provides the first experimental study of the combined rain and wind effects in a tropical location using both models at a frequency in the E band (74.625 GHz) and a short distance of 150 m. In addition to using wind speeds for attenuation estimation, the setup also provides direct antenna inclination angle measurements using the accelerometer data. This solves the limitation of relying on the wind speed since the wind-induced loss is dependent on the inclination direction. The results show that the current ITU-R model can be used to estimate the attenuation of a short fixed wireless link under heavy rain, and the addition of wind attenuation via the APT model can estimate the worst-case link budget during high wind speeds.

High-speed fiber-wireless-fiber system in the 100-GHz band using a photonics-enabled receiver and optical phase modulator.

This Letter describes a high-speed yet simple fiber-wireless-fiber system in the 100-GHz band using a photonics-enabled receiver and optical phase modulator. At the antenna site, a millimeter-wave signal is down-converted to the microwave band using an electronic mixer with a local oscillator signal generated remotely using photonic technology. The down-converted signal is converted to an optical signal using an optical phase modulator. At the receiver, a simple direct detection of the phase-modulated signal is performed using optical filtering technology. To demonstrate the proof-of-concept, we successfully transmitted a 64-quadrature amplitude modulation orthogonal frequency-division multiplexing signal with a record line rate of 80 Gb/s (net data rate of 55 Gb/s) over a system consisting of two radio-over-fiber links and a 5 m wireless link in the 100-GHz band. The proposed system with simple antenna sites and optical transceivers can facilitate the deployment of ultra-dense small cells in high-frequency bands in beyond-5G networks.

Pump-phase-noise-free optical wavelength data exchange between QAM signals with 50-GHz channel-spacing using coherent DFB pump.

An important challenge for implementing optical signal processing functions such as wavelength conversion or wavelength data exchange (WDE) is to avoid the introduction of linear and nonlinear phase noise in the subsystem. This is particularly important for phase noise sensitive, high-order quadrature-amplitude modulation (QAM) signals. In this paper, we propose and experimentally demonstrate an optical data exchange scheme through cascaded 2nd-order nonlinearities in periodically-poled lithium niobate (PPLN) waveguides using coherent pumping. The proposed coherent pumping scheme enables noise from the coherent pumps to be cancelled out in the swapped data after WDE, even with broad linewidth distributed feedback (DFB) pump lasers. Hence, this scheme allows phase noise tolerant processing functions, enabling the low-cost implementation of WDE for high-order QAM signals. We experimentally demonstrate WDEs between 10-Gbaud 4QAM (4QAM) signal and 12.5-Gbaud 4QAM (16QAM) signal with 3.5-MHz linewidth DFB pump lasers and 50-GHz channel spacing. Error-free operation is observed for the swapped QAM signals with coherent DFB pumping whilst use of free-running DFB pumps leads to visible error floors and unrecoverable phase errors. The phase noise cancellation in the coherent pump scheme is further confirmed by study of the recovered carrier phase of the converted signals. In addition to pump phase noise, the influence of crosstalk caused by the finite extinction ratio in WDE is also experimentally investigated for the swapped QAM signals.

Optical 8QAM and 8PSK synthesis by cascading arbitrary 2QAM with squared QPSK.

In dynamic optical networking scenarios, it is desirable that the optical transmitter chooses the most suitable modulation format in order to achieve optimal transmission performance. Owing to the ability of switching among different modulation formats, flexible optical transmitters based on reconfigurable optical devices are becoming a key component for the implementation of future flexible optical networks. In this paper, we experimentally demonstrate a flexible 8-ary transmitter to achieve adaptive switching between 8-ary phase-shift keying (8PSK) and circular 8-ary quadrature-amplitude modulation (8QAM) through reconfiguration of two cascaded in-phase/quadrature (IQ) modulators with different driving signals and biasing conditions. An arbitrary binary quadrature-amplitude modulation (2QAM) with constant or non-constant amplitude is proposed and experimentally demonstrated using an IQ modulator. Then, optical 8PSK or 8QAM modulation formats are successfully synthesized when a standard squared QPSK modulator is cascaded with a constant-amplitude or non-constant-amplitude 2QAM, respectively.

Laser-phase-fluctuation-insensitive offset-frequency-spaced two-tone optical coherent detection scheme and its transmission for radio-over-fiber systems.

We propose a new optical coherent detection scheme with a "two-tone" local light, which is also in principle insensitive to the laser phase fluctuation and is assisted by the digital signal processing technique for radio-over-fiber (RoF) systems. The main feature of our proposal is that the frequency separation of the two-tone local light is different from that of the RoF signal, which is called "offset-frequency-spaced" in this paper. First, we explain the principle of our new proposal and experimentally demonstrate the data recovery. Then, the influence of the frequency detuning between photo-detected modulated and unmodulated signals is discussed. Moreover, the transmission performance with an error vector magnitude (EVM) is evaluated for the optical coherent detection of a 10-Gbaud quadrature-phase-shift-keying RoF signal after a 20-km-long standard single-mode fiber transmission.

Lithium niobate ridged waveguides with smooth vertical sidewalls fabricated by an ultra-precision cutting method.

This paper demonstrates the application of ultra-precision cutting to the fabrication of ridged LiNbO3 waveguides for use in low-loss photonic integrated circuits. Ridged waveguides with sidewall verticality of 88° and ultra-smooth sidewalls were obtained in LiNbO3 crystals using this technique. In addition, the possibility of fabricating bent ridged waveguides via this mechanical micromachining method was examined. The root mean square surface roughness of the machined sidewall was 4.5 nm over an area of 2.5 × 10 µm, which is sufficiently low so as to minimize scattering losses of guided light. The propagation loss of the ridged waveguide produced during this work was well below 1 dB/cm at a wavelength of 1550 nm. The present technique should have significant applicability to the micromachining of ferroelectric materials and the fabrication of highly confined optical waveguides such as ridged waveguides and photonic wires.

High-capacity self-homodyne PDM-WDM-SDM transmission in a 19-core fiber.

We investigate a high-capacity, space-division-multiplexed (SDM) transmission system using self-homodyne detection (SHD) in multi-core fiber (MCF). We first investigate SHD phase noise cancellation with both kHz and MHz range linewidths for both quadrature-phase shift-keyed (QPSK) and 16 quadrature-amplitude modulation (16QAM) signals, finding that phase noise cancellation in SHD enabled transmission with MHz linewidth lasers that resulted in error floors when using intradyne detection. We then demonstrate a high throughput SHD transmission system using low-cost, MHz linewidth distributed feedback lasers. We transmit a CW pilot-tone on a single core of a 10.1 km MCF span with the remaining 18 cores used to transmit 125 wavelength-division multiplexed (WDM) QPSK and polarization-division-multiplexed (PDM)-QPSK signals with 50 GHz channel spacing at 25 GBd. For PDM transmission and assuming a 7% forward-error correction overhead this is equivalent 210 Tb/s transmission with a SE of 33.4 b/s/Hz. High-capacity transmission is achieved despite high inter-core crosstalk, broad transmitter linewidth and narrow channel spacing, showing that combining SHD with MCF enables high throughput, low-cost transmission in next-generation optical networks.

Pump-linewidth-tolerant optical wavelength conversion for high-order QAM signals using coherent pumps.

Optical wavelength conversion (OWC) is expected to be a desirable function in future optical transparent networks. Since high-order quadrature amplitude modulation (QAM) is more sensitive to the phase noise, in the OWC of high-order QAM signals, it is crucial to suppress the extra noise introduced in the OWC subsystem, especially for the scenario with multiple cascaded OWCs. Here, we propose and experimentally demonstrate a pump-linewidth-tolerant OWC scheme suitable for high-order QAM signals using coherent two-tone pumps. Using 3.5-MHz-linewidth distributed feedback (DFB) lasers as pump sources, our scheme enables wavelength conversion of both 16QAM and 64QAM signals with negligible power penalty, in a periodically-poled Lithium Niobate (PPLN) waveguide based OWC. We also demonstrate the performance of pump phase noise cancellation, showing that such coherent two-tone pump schemes can eliminate the need for ultra-narrow linewidth pump lasers and enable practical implementation of low-cost OWC in future dynamic optical networks.

Wavelength conversion of optical 64QAM through FWM in HNLF and its performance optimization by constellation monitoring.

All-optical wavelength conversion (AOWC) plays an important role in the future transparent optical networks, in order to enhance the re-configurability and non-blocking capacity. On the other hand, high-order quadrature amplitude modulations (QAMs) have been extensively studied for achieving the high-speed and high-spectral-efficiency optical transmission. Since high-order QAMs are more sensitive to phase and amplitude noise, to implement an AOWC sub-system suitable for high-order QAM signals with minimized power penalty, it is important to optimize the operation conditions in order to avoid extra nonlinear distortions co-existed in the AOWC process. Our experimental results show that, constellation monitoring provides a more intuitive and accurate approach to monitor the converted high-order QAM signals, especially in presence of nonlinear phase noise such as self-phase modulation (SPM). We experimentally demonstrate an AOWC of 64QAM signal through four-wave mixing (FWM) in highly-nonlinear (HNLF). The performance of the AOWC is optimized through the constellation monitoring of the converted signal, achieving a negligible power penalty (<0.3 dB at BER of 10(-3)) for 60-Gbps 64QAM after conversion.

Simultaneous multichannel wavelength multicasting and XOR logic gate multicasting for three DPSK signals based on four-wave mixing in quantum-dot semiconductor optical amplifier.

In this paper, we experimentally demonstrate simultaneous multichannel wavelength multicasting (MWM) and exclusive-OR logic gate multicasting (XOR-LGM) for three 10Gbps non-return-to-zero differential phase-shift-keying (NRZ-DPSK) signals in quantum-dot semiconductor optical amplifier (QD-SOA) by exploiting the four-wave mixing (FWM) process. No additional pump is needed in the scheme. Through the interaction of the input three 10Gbps DPSK signal lights in QD-SOA, each channel is successfully multicasted to three wavelengths (1-to-3 for each), totally 3-to-9 MWM, and at the same time, three-output XOR-LGM is obtained at three different wavelengths. All the new generated channels are with a power penalty less than 1.2dB at a BER of 10(-9). Degenerate and non-degenerate FWM components are fully used in the experiment for data and logic multicasting.

Multi-tone parallel coherent matched detection for demultiplexing of superchannels.

This paper presents multi-tone parallel coherent matched detection that orthogonally detects superchannels without crosstalk between neighboring channels. The receiver consists of multiple sets of multi-tone coherent matched detector employed in parallel. In each detector, the received superchannels signal is homodyne mixed in multi frequency with locally generated multi-tone optical frequency comb; detected is a signal set that has the amplitude and phase exactly matched with the local comb. By launching orthogonal sets of local comb to the multiple parallel coherent matched detectors, the received superchannels are orthogonally downconverted to the baseband frequencies keeping the amplitude and phase information of all channels included. With an aid of n × n transform matrix, all channels are separately recovered from the downconverted signal sets. The system does not rely on any optical filters for channel demultiplexing and separation, with increased flexibility in wavelength arrangement. In addition, the parallel configuration equivalently enhance the bandwidth of the coherent matched detector keeping the speed in each tributary channel as high as possible. In this paper, it is experimentally demonstrated that even-odd interleaved 23 × 20-Gb/s QPSK superchannels are orthogonally demultiplexed and detected by two-tone coherent matched detection.

Flexible high-order QAM transmitter using tandem IQ modulators for generating 16/32/36/64-QAM with balanced complexity in electronics and optics.

In order to adapt to the dynamics in the future optical networks, we propose a flexible high-order QAM transmitter using a tandem in-phase/quadrature (IQ) modulators to synthesize different high-order quadrature amplitude modulation (QAM) formats, such as 16QAM, 32 or 36QAM and 64QAM. To generate high-order QAMs, an offset-QAM is firstly generated using an IQ modulator driven by electronics with reduced modulation-level, and then mapped to other quadrants through another following IQ modulator configured as a standard quadrature phase-shift keying (QPSK) modulator. All of the embedded sub-Mach-Zehnder modulators are operated in push-pull configurations to avoid introducing excess phase chirp. In contrast with the schemes based on a single IQ modulator driven by multilevel electronics or a highly-integrated parallel modulator, by deploying commercially-available optical modulators and driving electronics with reduced modulation-level, the transmitter complexity in optics and electronics is well-balanced. In the case of generating optical 64QAM, different from another tandem scheme deploying dual-drive IQ modulator driven by independent four binary streams, less phase chirp is observed in our proposed scheme, and comparable implementation penalty is obtained even without applying additional specific compensation algorithm in the coherent receiver. Moreover, thanks to the tandem structure and the deployment of QPSK modulator, the obtained high-order QAM is naturally differentially coded, which is helpful to solve the phase ambiguity at coherent receiver. We experimentally demonstrate the generations of these high-order QAMs including 16QAM, 32/36QAM and 64QAM, and confirm the error-free operations with comparable BER performance to the "electrical" approach based on a single IQ modulator.

Investigating self-homodyne coherent detection in a 19 channel space-division-multiplexed transmission link.

We investigate the performance of a self-homodyne coherent detection (SHCD) system using a 19 core multi-core fiber (MCF) and 16 wavelength-division-multiplexed channels. We show that SHCD, with the pilot-tone transmitted on a single MCF core and information carrying signals on the remaining cores, is compatible with space-division-multiplexed transmission, potentially relaxing laser linewidth and digital signal processing requirements due to phase noise cancellation. However, inter-core crosstalk can have an impact on performance and core selection.

Polarization division multiplexed 4 × 10 Gbps simultaneous transmissions in 1.0-µm waveband and C-waveband over a 14.4-km-long holey fiber using an ultra-broadband photonic transport system.

Polarization division multiplexing (PDM) and wavelength division multiplexing (WDM) are essential techniques for enhancing the capacity of photonic networks and facilitating the efficient use of optical frequency resources. 2 PDM × 2 WDM × 10 Gbps error-free simultaneous transmissions in the 1.0-µm waveband and C-waveband are successfully demonstrated for the first time using an ultra-broadband photonic transport system over a 14.4-km-long holey fiber transmission line.

Optical and millimeter-wave radio seamless MIMO transmission based on a radio over fiber technology.

Multi-input multi-output (MIMO) transmission of two millimeter-wave radio signals seamlessly converted from polarization-division-multiplexed quadrature-phase-shift-keying optical signals is successfully demonstrated, where a radio access unit basically consisting of only optical-to-electrical converters and a radio receiver performs total signal equalization of both the optical and the radio paths and demodulation with digital signal processing (DSP). Orthogonally polarized optical components that are directly converted to two-channel radio components can be demultiplexed and demodulated with high-speed DSP as in optical digital coherent detection. 20-Gbaud optical and radio seamless MIMO transmission provides a total capacity of 74.4 Gb/s with a forward error correction overhead of 7%.

Air-gap structure between integrated LiNbO3 optical modulators and micromachined Si substrates.

The air-gap structure between integrated LiNbO(3) optical modulators and micromachined Si substrates is reported for high-speed optoelectronic systems. The calculated and experimental results show that the high permittivity of the Si substrate decreases the resonant modulation frequency to 10 GHz LiNbO(3) resonant-type optical modulator chips on the Si substrate. To prevent this substrate effect, an air-gap was formed between the LiNbO(3) modulator and the Si substrate. The ability to fabricate the air-gap structure was demonstrated using low-temperature flip-chip bonding (100 °C) and a Si micromachining process, and its performance was experimentally verified.

Rectangular QPSK for generation of optical eight-ary phase-shift keying.

Quadrature phase-shift keying (QPSK) is usually generated using an in-phase/quadrature (IQ) modulator in a balanced driving-condition, showing a square-shape constellation in complex plane. This conventional QPSK is referred to as square QPSK (S-QPSK) in this paper. On the other hand, when an IQ modulator is driven in an un-balanced manner with different amplitudes in in-phase (I) and quadrature (Q) branches, a rectangular QPSK (R-QPSK) could be synthesized. The concept of R-QPSK is proposed for the first time and applied to optical eight-ary phase-shift keying (8PSK) transmitter. By cascading an S-QPSK and an R-QPSK, an optical 8PSK could be synthesized. The transmitter configuration is based on two cascaded IQ modulators, which also could be used to generate other advanced multi-level formats like quadrature amplitude modulation (QAM) when different driving and bias conditions are applied. Therefore, the proposed transmitter structure has potential to be deployed as a versatile transmitter for synthesis of several different multi-level modulation formats for the future dynamic optical networks. A 30-Gb/s optical 8PSK is experimentally demonstrated using the proposed solution.

Reconfigurable multilevel transmitter using monolithically integrated quad Mach-Zehnder IQ modulator for optical 16-QAM and 8-PSK generation.

We propose and demonstrate a reconfigurable multilevel transmitter using a monolithically-integrated quad Mach-Zehnder in-phase/quadrature (QMZ-IQ) modulator with binary driving electronics. Different from previous parallel-integrated quadrature amplitude modulation (QAM) transmitter solutions, only one electrode is required to adjust the relative phase offset among embedded sub-Mach-Zehnder modulators in the proposed IQ superstructure. By feeding different RF driving electronics and operating the integrated modulator as different bias conditions, different advanced multilevel modulation formats, such as QAM and 8-ary phase-shift keying (8-PSK), could be synthesized. In this paper, a 40-Gb/s 16-QAM and a 30-Gb/s 8-PSK are generated using the proposed multilevel transmitter, respectively. Offline digital processing is employed for bit-error rates estimation and constellation reconstruction.

40 Gb/s W-band (75-110 GHz) 16-QAM radio-over-fiber signal generation and its wireless transmission.

The generation of a 40-Gb/s 16-QAM radio-over-fiber (RoF) signal and its demodulation of the wireless signal transmitted over free space of 30 mm in W-band (75-110 GHz) is demonstrated. The 16-QAM signal is generated by a coherent polarization synthesis method using a dual-polarization QPSK modulator. A combination of the simple RoF generation and the versatile digital receiver technique is suitable for the proposed coherent optical/wireless seamless network.

Narrow-line-width 1.31-µm wavelength tunable quantum dot laser using sandwiched sub-nano separator growth technique.

A wide wavelength tunable quantum dot (QD) external cavity laser operating in the 1.31-µm waveband with a narrow line-width is successfully demonstrated. A high-density, high-quality InAs/InGaAs QD optical gain medium for the 1.31-µm waveband was obtained using a sandwiched sub-nano separator growth technique. A wide wavelength tunability of 1.265-1.321 µm and a narrow line-width of 210 kHz were successfully achieved using a compact and robust external cavity system constructed with multiple optical band-pass and etalon filters for active optical mode selection. The laser also achieved an error-free 10-Gb/s photonic data transmission over an 11.4-km-long holey fiber.