Lightwave Signals Research Articles

MZM–SOA based RoF system for 30-tuple millimeter-wave generation

Abstract In this paper, we have reported radio over fiber system with 30-tuple millimeter-wave generation using dual parallel Mach–Zehnder modulator and semiconductor optical amplifier. The optimum choice of Mach–Zehnder modulator parameters has resulted into generation of signal lightwave and pump signal with frequency separation of 10f RF. Four wave mixing effect in semiconductor optical amplifier results into 30-tuple millimeter-wave signal. The proposed scheme is applicable to radio over fiber system with multiple base stations. The multiple number of frequencies such as 50 GHZ, 100 GHz, and 150 GHz have been generated using 5 GHz radio frequency only. NRZ data at 2.5 Gbps was modulated and successfully transmitted via optical fiber. The impact of injection current on the sideband ratio and BER is investigated. The proposed scheme confirms quality reception of demodulated signal at a transmission distance of 20 km.

High-density integrated delay line using extreme skin-depth subwavelength grating waveguides.

Optical delay lines control the flow of light in time, introducing phase and group delays for engineering interferences and ultrashort pulses. Photonic integration of such optical delay lines is essential for chip-scale lightwave signal processing and pulse control. However, typical photonic delay lines based on long spiral waveguides require extensively large chip footprints, ranging from mm2 to cm2 scales. Here we present a scalable, high-density integrated delay line using a skin-depth engineered subwavelength grating waveguide, i.e., an extreme skin-depth (eskid) waveguide. The eskid waveguide suppresses the crosstalk between closely spaced waveguides, significantly saving the chip footprint area. Our eskid-based photonic delay line is easily scalable by increasing the number of turns and should improve the photonic chip integration density.

6G-oriented ultra-wideband fiber-THz-fiber seamless converged communication system: architecture, key techniques and verification

Terahertz (THz) communication is widely regarded as a key component of the future 6G mobile communication system. This paper proposes a novel ultra-wideband fiber-THz-fiber seamless converged real-time architecture that fully exploits the commercially mature digital coherent optical module (DCO) to realize ultra-high capacity THz real-time wireless communication by conducting a comparative analysis for some of the main existing technical routes of THz up- and down-conversion in THz wireless communication systems. (1) The proposed architecture employs the techniques of dual-polarization photonic up-conversion for THz generation and hybrid optoelectronic down-conversion for THz reception, respectively, to enable seamless integration of optical fiber and THz communications. (2) Due to the limited bandwidth of the optoelectronic devices, multi-dimensional modulation techniques are adopted for ultra-wideband terahertz signals to improve the spectral efficiency and transmission capacity. (3) An intelligent nonlinear joint compensation technique based on deep neural network (DNN) is proposed, which can effectively improve the signal-to-noise ratio of the time-varying hybrid fiber-THz-fiber channel. Based on the above-mentioned investigations, we for the first time realize the photonics-aided record-high 100/200 GbE real-time THz wireless transmission at 360 (∼) 430 GHz band, with a capacity of 10 to 20 times that of 5G. The proposed fiber-THz-fiber architecture can realize the smooth conversion between high-speed THz signal and lightwave signal. Furthermore, the architecture can significantly reduce research complexity and development costs and thus greatly accelerate the commercialization of 6G THz technology by highly reusing commercial DCO modules, which is compatible with the physical layer transmission protocols such as IEEE 802.3 and ITU-T G.798. Finally, this paper discusses some potential research and development directions for higher-capacity, longer-distance, and more-integrated fiber-THz-fiber seamless communication.

Deep-Learning Computational Holography: A Review

Deep learning has been developing rapidly, and many holographic applications have been investigated using deep learning. They have shown that deep learning can outperform previous physically-based calculations using lightwave simulation and signal processing. This review focuses on computational holography, including computer-generated holograms, holographic displays, and digital holography, using deep learning. We also discuss our personal views on the promise, limitations and future potential of deep learning in computational holography.

An IoT-Based Water Level Detection System Enabling Fuzzy Logic Control and Optical Fiber Sensor

The usage of wireless sensors has become widespread for the collection of data for various Internet of Things (IoT) products. Specific wireless sensors use optical fiber technology as transmission media and lightwave signals as carriers, showing the advantages of antielectromagnetic interference, high sensitivity, and strong reliability. Hence, their application in IoT systems becomes a research hotspot. In this article, multiple optical fiber sensors are constructed as an IoT detection system, and a Transmission Control Protocol (TCP)/Internet Protocol (IP) communication stack is used for the sensor module. Furthermore, design of gateway module, data server, and monitoring module is established in order to run the data server in the Windows system and communicate across the network segments. Furthermore, the optical fiber sensor is connected to the development board with WiFi, meanwhile considering the optical fiber wireless network’s congestion problem. The fuzzy logic concept is introduced from the perspective of cache occupancy, and a fiber sensor’s network congestion control algorithm is proposed. In the experiment, the IoT detection system with multiple optical fiber sensors is used for water level detection, and the sensor’s real-time data detected by the User Interface (UI) are consistent with the feedback results. The proposed method is also compared with the SenTCP algorithm and the CODA algorithm, and it was observed that the proposed network congestion control algorithm based on the fuzzy logic can improve network throughput and reduce the network data packet loss.

Research on Image Super-resolution Internet Transmission Technology Based on Virtual Optics

Image resolution is a set of performance parameters used to evaluate the richness of detailed information contained in an image, including temporal resolution, spatial resolution and color scale resolution, etc., which reflects the ability of the imaging system to reflect the detailed information of objects. In this paper, a super-resolution image transmission technology based on virtual optics is proposed. The original image is placed on the object plane of the virtual light path. Firstly, the light wave signal of the original image on the virtual diffraction plane is obtained by inverse diffraction calculation. Then, the light wave of the virtual diffraction plane is illuminated by spherical wave and then the forward diffraction calculation is performed. By changing the position of the observation plane, the original images with different magnification can be reconstructed. The simulation test results show that compared with the general interpolation amplification method, the amplified image has a good visual perception effect, especially in the significant area. The results obtained by the algorithm in this paper are better than the interpolation model and convolutional neural network algorithm as a whole; The above test results objectively show that the super-resolution effect of this algorithm is better.

Optimization and visualization of phase modulation with filtered and amplified maximal-length sequence for SBS suppression in a short fiber system: a theoretical treatment.

We use a model to investigate both the temporal and spectral characteristics of a signal lightwave which has been spectrally broadened through phase modulation with a maximal-length sequence (MLS), which is a common type of pseudo-random bit sequence. The enhancement of the stimulated Brillouin scattering (SBS) threshold of the modulated lightwave in a fiber system is evaluated by numerically simulating the coupled three-wave SBS interaction equations. We find that SBS can build up on a nanosecond-level time scale in a short fiber, which can reduce the SBS suppressing capability of MLS modulation waveforms with GHz-level clock rate, if the sub-sequence ("run") lengths with the same symbol (zero or one) of the MLS extend over several nanoseconds. To ensure the SBS buildup is perturbed and thus suppressed also during these long sub-sequences, we introduce a low-pass filter to average the signal over several bits so that the modulation waveform changes gradually even during long runs and amplify the RF modulation waveforms to the level required for sufficient spectral broadening and carrier suppression of the optical signal. We find that the SBS suppression depends non-monotonically on the parameters of the filtered and amplified MLS waveform such as pattern length, modulation depth, and the ratio of low-pass filter cutoff frequency to clock rate for maximum SBS mitigation. We optimize the SBS suppression through numerical simulations and discuss it in terms of the temporal and spectral characteristics of the lightwave and modulation waveform using derived analytical expressions and numerical simulations. The simulations indicate that the normalized SBS threshold reaches a maximum for a RMS modulation depth of 0.56π and a ratio of filter cutoff frequency to clock rate of 0.54 and that MLS9 is superior to other investigated patterns.

Detection Method and System of the Human Body Characteristic Index Based on TCM.

As traditional Chinese medicine (TCM) has gained more and more recognition in the world, Chinese medicine has also played its important role. However, traditional Chinese medicine equipment is relatively deficient, with insufficient functions and low degree of digitalization. For example, existing auscultation equipment can obtain few human characteristic indicators, which is difficult to meet the needs of reference in traditional Chinese medicine diagnosis. Based on this, this paper designed a human body characteristic index detection system based on the principle of traditional Chinese medicine, which includes respiratory and heartbeat signal acquisition device, meridian and acupoint signal acquisition device, temperature signal acquisition device, pulse and blood pressure acquisition device, processing module, keyword module, and output module. The respiratory and heartbeat signal acquisition device is used to collect the respiratory and heartbeat signal of human body. Meridian acupoint signal acquisition device is used to collect human meridian acupoint radio signals. The temperature signal acquisition device is used to collect the infrared temperature light wave signal of human body. Pulse and blood pressure acquisition devices are used to collect pulse and blood pressure signals. The processing module is used to obtain one or more human body characteristic indicators according to one or more of the respiration and heartbeat signals, meridians and acupoints signals, temperature signals, pulse, and blood pressure, including Qi and blood characteristic indicators, viscera and six meridian characteristic indicators, and temperature characteristic indicators. The keyword corresponding module is used to obtain the corresponding keyword representing the physiological state information of human body according to the one or more human body characteristic indicators. The output module is used to output the human body characteristic index and the key words. It includes the key words of Qi and blood state information, the key words of viscera state information, the key words of Qi and blood state information, etc. The system can be used for serious disease screening, chronic disease management, and risk early warning.

Gesture Recognition Using Reflected Visible and Infrared Lightwave Signals

In this article, we demonstrate the ability to recognize hand gestures in a noncontact wireless fashion using only incoherent light signals reflected from a human subject. Fundamentally distinguished from radar, lidar, and camera-based sensing systems, this sensing modality uses only a low-cost light source (e.g., LED) and a sensor (e.g., photodetector). The lightwave-based gesture recognition system identifies different gestures from the variations in light intensity reflected from the subject's hand within a short (20-35 cm) range. As users perform different gestures, scattered light forms unique, statistically repeatable, time-domain signatures. These signatures can be learned by repeated sampling to obtain the training model against which unknown gesture signals are tested and categorized. These time-domain variations of the lightwave signals reflected from hand are denoised, standardized, and then classified by using machine learning classification tools such as $K$-nearest neighbors and support vector machine. Performance evaluations have been conducted with eight gestures, five subjects, different distances and lighting conditions, and visible and infrared light sources. The results demonstrate the best hand gesture recognition performance of infrared sensing at 20 cm with an average of 96% accuracy. The developed gesture recognition system is low-cost, effective, and noncontact technology for numerous human-computer interaction applications.

Optical Phase Conjugation Using Nonlinear SOA for Nonlinearity and Dispersion Compensation of Coherent Multi-Carrier Lightwave Systems

We study the use of nonlinear semiconductor optical amplifier (SOA) for generating optical phase conjugate towards compensation of distortions in short distance optical fiber transmission due to Kerr nonlinearity and chromatic dispersion in coherent multi-carrier lightwave signals. We experimentally demonstrate the effectiveness of the SOA-based phase conjugator to improve the link budget with a 100 km standard single mode fiber link for 20 GHz coherent OFDM signals, with QPSK and 16QAM modulations and a corresponding net bit-rate of 40 Gbps and 80 Gbps respectively. Mid-span spectral inversion scheme is employed where the optical phase conjugate is generated through a partially degenerate four-wave mixing process in a nonlinear SOA. We demonstrate a bit error rate performance within 2×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> for an average launched power of up to 12 dBm (9 dBm) for QPSK (16QAM) coherent OFDM signals, in a 100 km fiber link. We also investigate the possible improvement in link budget using numerical simulation for 16QAM and 64QAM CO-OFDM signals with the proposed scheme.

A ROF system based on 18-tuple frequency millimeter wave generation using external modulator and SOA

In this paper, we have reported a radio-over-fiber (ROF) system with 18-tuple frequency optical millimeter-wave using external modulator and four-wave mixing (FWM) in semiconductor optical amplifier (SOA). Signal lightwave and pump with frequency interval of 6fRF are generated by adjusting the parameters of the Mach-Zehnder modulator (MZM). After FWM in SOA and filtering, 18-tuple frequency millimeter-wave signal is generated. Based on the proposed scheme, a 90 GHz optical millimeter-wave is generated only using 5 GHz RF by numerical simulation. The simulation results show that our proposed scheme can provide 20 km single-mode fiber (SMF) transmission. In addition, the impact of SOA’s current on the sideband ratio (SR) and the system performance are also studied.

Nonlinearity-Assisted All-Optical Clock Recovery for Phase Modulated Lightwave Systems

We report all-optical clock recovery from 10 GBaud NRZ shaped lightwave signals in QPSK and 16QAM formats through injection mode-locking of an Erbium-doped fiber laser. Since these optical NRZ phase modulated coherent signals lack in the strength of the clock-tones, a pre-processing stage for the enhancement of these clock-tones based on a nonlinear semiconductor optical amplifier (SOA) is employed. Optical clock recovery is demonstrated experimentally for both modulation formats, along with wavelength tunability of the scheme over the entire C-band. Resilience of the clock recovery process to input OSNR conditions is also demonstrated. The RMS timing jitter of the recovered clock is measured to be ≤ 2 ps. We also report the performance analysis of the scheme for coherent modulation formats with respect to the dispersion-induced degradation in practical links through simulations and experiments.

Attosecond-fast internal photoemission

The photoelectric effect has a sister process relevant in optoelectronics called internal photoemission. Here an electron is photoemitted from a metal into a semiconductor. While the photoelectric effect takes place within less than 100 attoseconds, the attosecond time scale has so far not been measured for internal photoemission. Based on the new method CHArge transfer time MEasurement via Laser pulse duration-dependent saturation fluEnce determinatiON, CHAMELEON, we show that the atomically thin semi-metal graphene coupled to bulk silicon carbide, forming a Schottky junction, allows charge transfer times as fast as (300 $\pm$ 200) attoseconds. These results are supported by a simple quantum mechanical model simulation. With the obtained cut-off bandwidth of 3.3 PHz for the charge transfer rate, this semimetal-semiconductor interface represents the first functional solid-state interface offering the speed and design space required for future light-wave signal processing.

Cost-efficient, polarization-insensitive and widely-tunable AOWC of OFDM signal based on FWM in SOA.

In this paper, we propose a cost-efficient, polarization-insensitive and widely-tunable wavelength conversion scheme for orthogonal frequency division multiplexing (OFDM) signal based on four-wave mixing (FWM) in semiconductor optical amplifier (SOA). In this scheme, electrical OFDM signal directly drives directly modulated laser (DML) to achieve intensity modulation, then the modulated signal lightwave and pump are coupled and injected into a polarization-diversity structure for performing FWM in SOA. The analytical results show that the proposed scheme is polarization-insensitive and has a wider conversion range compared with the parallel pump scheme and orthogonal pump scheme. We have also numerically demonstrated wavelength conversion of 16QAM-OFDM signal by employing the proposed scheme. The results can be agreed with the theoretical analyses. In addition, the impact of input optical power, line-width of external cavity laser (ECL) and injection current of SOA on the system performance are also studied.

Proposal of InGaAs/InAlAs multiple quantum well Mach–Zehnder modulator integrated with array of planar antennas

We propose an InGaAs/InAlAs multiple quantum well (MQW) Mach–Zehnder modulator integrated with an array of gap-embedded planar antennas for 60 GHz band radio-over-fiber system. The modulator enables us to directly convert from wireless millimeter-wave (MMW) signals to light-wave signals without any connection cables. A newly designed MQW for the modulator exhibits a large refractive index change at low electric field owing to its unique quantum confined Stark effect, which can achieve push-pull driving in modulation without any external electrical power supply. In addition, by introducing a PIN structure to waveguides, an electric field induced in the antena gap can be applied to a core layer of the waveguide efficiently under the irradiation of MMW signals. The modulator with a 2-element array planar antenna can be driven under an MMW power density of 98 W m−2. We theoretically discuss designs and characteristics of the MQW, planar antenna, and modulator.

Efficient telecom-to-visible spectral translation through ultra-low power nonlinear nanophotonics.

The ability to spectrally translate lightwave signals in a compact, low-power platform is at the heart of the promise of nonlinear nanophotonic technologies. For example, a device to link the telecommunications band with visible and short near-infrared wavelengths can enable a connection between high-performance chip-integrated lasers based on scalable nanofabrication technology with atomic systems used for time and frequency metrology. While second-order nonlinear () systems are the natural approach for bridging such large spectral gaps, here we show that third-order nonlinear () systems, despite their typically much weaker nonlinear response, can realize spectral translation with unprecedented performance. By combining resonant enhancement with nanophotonic mode engineering in a silicon nitride microring resonator, we demonstrate efficient spectral translation of a continuous-wave signal from the telecom band (≈ 1550 nm) to the visible band (≈ 650 nm) through cavity-enhanced four-wave mixing. We achieve such translation over a wide spectral range >250 THz with a translation efficiency of (30.1 ± 2.8) % and using an ultra-low pump power of (329 ± 13) μW. The translation efficiency projects to (274 ± 28) % at 1 mW and is more than an order of magnitude larger than what has been achieved in current nanophotonic devices.

A Novel Demodulation Scheme of Fiber-Optic Interferometric Sensor Based on FM SSB Signal

A novel vibration signal demodulation scheme of fiber-optic interferometric sensor is proposed and demonstrated. A frequency-modulated (FM) single sideband (SSB) light-wave signal is generated by applying an FM microwave signal to a Mach–Zehnder modulator (MZM) for the demodulation of fiber-optic vibration interferometric sensor. A method of 12-sample orthogonal demodulation algorithm (ODA) based on setting the system sampling rate to 12 times of the FM frequency is adopted for the generation of two orthogonal signals for arctangent transformation. The experimental results show that the harmonic distortion is well suppressed compared with the traditional phase generation carrier (PGC) arctangent (PGC-Arctan) algorithm, and a minimum detectable phase shift is achieved about $6.3\times 10^{-5}$ rad/ $\surd $ Hz at 5 kHz.

Millimeter-wave band optical single-sideband modulator using array-antenna-electrode and polarization-reversed structures with asymmetric Mach–Zehnder waveguide

The radio-over-fiber (RoF) technology is effective in millimeter-wave (MMW) wireless systems for future broadband massive mobile communication. In constructing MMW RoF systems, a device for converting MMW signals into lightwave signals is important. In particular, a conversion device for optical single-sideband (SSB) modulation is rather attractive since the signal fading caused by the chromatic dispersion effect of an optical fiber does not occur. We have proposed a new optical SSB modulator using array-antenna-electrode and polarization-reversed structures with an asymmetric Mach–Zehnder waveguide for MMW band RoF systems. By introducing polarization-reversed structures in electro-optic modulators, it is possible to control the optical modulation phase arbitrarily. Therefore, by combining with an asymmetric Mach–Zehnder waveguide, we can obtain the SSB optical modulation in the MMW band without an external power supply. In the experiment, SSB modulation characteristics and the switching of enhanced sidebands were successfully demonstrated in the 60 GHz band.

An optical-frequency synthesizer using integrated photonics.

Optical-frequency synthesizers, which generate frequency-stable light from a single microwave-frequency reference, are revolutionizing ultrafast science and metrology, but their size, power requirement and cost need to be reduced if they are to be more widely used. Integrated-photonics microchips can be used in high-coherence applications, such as data transmission 1 , highly optimized physical sensors 2 and harnessing quantum states 3 , to lower cost and increase efficiency and portability. Here we describe a method for synthesizing the absolute frequency of a lightwave signal, using integrated photonics to create a phase-coherent microwave-to-optical link. We use a heterogeneously integrated III-V/silicon tunable laser, which is guided by nonlinear frequency combs fabricated on separate silicon chips and pumped by off-chip lasers. The laser frequency output of our optical-frequency synthesizer can be programmed by a microwave clock across 4 terahertz near 1,550 nanometres (the telecommunications C-band) with 1 hertz resolution. Our measurements verify that the output of the synthesizer is exceptionally stable across this region (synthesis error of 7.7 × 10-15 or below). Any application of an optical-frequency source could benefit from the high-precision optical synthesis presented here. Leveraging high-volume semiconductor processing built around advanced materials could allow such low-cost, low-power and compact integrated-photonics devices to be widely used.

Graphene-based plasmonic waveguide devices for electronic-photonic integrated circuit

Graphene is a one-atom thick carbon film packed in a honeycomb structure, and because it has extraordinary optoelectronic properties, it has attracted a great deal of attention in the field of photonics. Graphene-based plasmonic photonic devices have been developed which are capable of emitting, transmitting, modulating, and detecting light wave signals using a single material. In this paper, we propose the concept of on-chip graphene electronic-photonic integrated circuits (EPICs) architecture, and present developments and perspectives on the essential graphene plasmonic-based photonic components, including the plasmonic waveguide, modulator, and photodetector, which are based on the graphene EPICs. The optical characteristics and design considerations of the graphene-based photonic devices are discussed based on experimental and theoretical investigations at telecommunications wavelengths. In parallel, we provide new device design concepts and a potential solution for further improving their operating characteristics. We also performed a numerical investigation of the characteristics of the photonic devices to explain their operating principles and to predict their operating performances. The proposed photonic components were fabricated on an organic photonic chip using an optimized fabrication process, and their optical characteristics were experimentally demonstrated. Based on these results and demonstrations, we hope to introduce new perspectives on the future direction of graphene-EPICs in graphene plasmonics and the optical telecom field.