Terahertz Direct Modulation Techniques for High-speed Communication Systems

Terahertz communication technology can provide abundant frequency resources,strong confidentiality,antijamming capability,communication tracking capability and the ability to achieve highspeed data transmissions and can serve as an important technical method for high-speed communications in the future.Among these terahertz communication technologies,terahertz direct modulation technology is a key means to achieve low system complexity and power consumption.In this paper,a review and outlook of terahertz direct modulation technology are proposed from the aspects of high-electron-mobilitytransistor-based terahertz direct modulation,parallelswitch terahertz direct modulation,diode-based terahertz direct modulation,quantum cascade laser-based terahertz direct modulation and new-material-based terahertz direct modulation.We hope through this paper that more readers can gain knowledge about the development and challenges of terahertz direct modulation technology for high-speed communication systems,thus promoting the development of high-speed terahertz communication technology based on direct modulation.

Security-Enhanced Directional Modulation Based on Two-Dimensional M-WFRFT

Directional modulation(DM)is one of the most promising secure communication techniques.However,when the eavesdropper is co-located with the legitimate receiver,the conventional DM has the disadvantages of weak anti-scanning capability,anti-deciphering capability,and low secrecy rate.In response to these problems,we propose a twodimensional multi-term weighted fractional Fourier transform aided DM scheme,in which the legitimate receiver and the transmitter use different transform terms and transform orders to encrypt and decrypt the confidential information.In order to further lower the probability of being deciphered by an eavesdropper,we use the subblock partition method to convert the one-dimensional modulated signal vector into a twodimensional signal matrix,increasing the confusion of the useful information.Numerical results demonstrate that the proposed DM scheme not only provides stronger anti-deciphering and anti-scanning capabilities but also improves the secrecy rate performance of the system.

Direct Modulation of the Guard Cell Outward- Rectifying Potassium Channel （CORK） by Abscisic Acid

Dear Editor,Abscisic acid （ABA） induces turgot loss and hence stomatal closure by promoting rapid net K^＋ efflux from guard cells （GCs） through outward-rectifying K^＋ （K^＋out） channels （Schroeder et al., 1987; Blatt, 1990）. The mechanisms of ABA signaling in GCs are detailed elsewhere （see Munemasa et al., 2015; Weiner et al., 2010; Pandey et al., 2007）. Briefly, ABA binds to the PYR/ PYL/RCARs, a family of soluble steroidogenic acute regulatory- related lipid transfer （START） proteins, and, in turn, inactivates the downstream PP2C （type 2C protein phosphatase）, leading to the activation of SnRK2.6 （SNF1 [sucrose non-fermenting-1- related protein kinase]/OST1 [open stomata 1]） protein kinases.

SOA Direct Modulation in Wavelength-Selectable Laser Array Light Sources

We demonstrated, for the first time, 2.5-Gb/s SOA direct modulation in the wavelength-selectable DFB laser array. The SOA direct modulation was achieved in the wide tuning range of 22.7nm by selecting arrayed laser and tuning the temperature. 40-km transmission was also confirmed..

Stochastic resonance in a gain-noise model of a single-mode laser driven by pump noise and quantum noise with cross-correlation between real and imaginary parts under direct signal modulation

Stochastic resonance （SR） is studied in a gain-noise model of a single-mode laser driven by a coloured pump noise and a quantum noise with cross-correlation between real and imaginary parts under a direct signal modulation. By using a linear approximation method, we find that the SR appears during the variation of signal-to-noise ratio （SNR） separately with the pump noise self-correlation time τ, the noise correlation coefficient between the real part and the imaginary part of the quantum noise λq, the attenuation coefficient γ＇ and the deterministic steady-state intensity I0. In addition, it is found that the SR can be characterized not only by the dependence of SNR on the noise variables of and λq, but also by the dependence of SNR on the laser system variables of γ and I0. Thus our investigation extends the characteristic quantity of SR proposed before.

Modulation bandwidth enhancement in monolithic integrated two-section DFB lasers based on the detuned loading effect

Modulation bandwidth enhancement in a directly modulated two-section distributed feedback(TS-DFB)laser based on a detuned loading effect is investigated and experimentally demonstrated.The results show that the 3-dB bandwidth of the TS-DFB laser is increased to 17.6 GHz and that chirp parameter can be reduced to 2.24.Compared to the absence of a detuned loading effect,there is a 4.6 GHz increase and a 2.45 reduction,respectively.After transmitting a 10 Gb/s non-return-to-zero(NRZ)signal through a 5-km fiber,the modulation eye diagram still achieves a large opening.Eight-channel laser arrays with precise wavelength spacing are fabricated.Each TS-DFB laser in the array has side mode suppression ratios(SMSR)>49.093 dB and the maximum wavelength residual<0.316 nm.

35 km amplifier-less four-level pulse amplitude modulation signals enabled by a 23 GHz ultrabroadband directly modulated laser

The 4-level pulse amplitude modulation(PAM4)based on an 23 GHz ultrabroadband directly modulated laser(DML)was proposed.We have experimentally demonstrated that based on intensity modulation and direct detection(IMDD)56 Gbps per wavelength PAM4 signals transferred over 35 km standard single mode fiber(SSMF)without any optical amplification and we have achieved the bit error rate(BER)of the PAM4 transmission was under 2.9×10–4 by using feed forward equalization(FFE).

Directional Modulation Based on a Quantum Genetic Algorithm for a Multiple-Reflection Model

Directional modulation is one of the hot topics in data security researches.To fulfill the requirements of communication security in wireless environment with multiple paths,this study takes into account the factors of reflections and antenna radiation pattern for directional modulation.Unlike other previous works,a novel multiple-reflection model,which is more realistic and complex than simplified two-ray reflection models,is proposed based on two reflectors.Another focus is a quantum genetic algorithm applied to optimize antenna excitation in a phased directional modulation antenna array.The quantum approach has strengths in convergence speed and the globe searching ability for the complicated model with the large-size antenna array and multiple paths.From this,a phased directional modulation transmission system can be optimized as regards communication safety and improve performance based on the constraint of the pattern of the antenna array.Our work can spur applications of the quantum evolutionary algorithm in directional modulation technology,which is also studied.

Two-dimensional directional modulation with dual-mode vortex beam for security transmission

A two-dimensional directional modulation(DM)technology with dual-mode orbital angular momentum(OAM)beam is proposed for physical-layer security of the relay unmanned aerial vehicle(UAV)tracking transmission.The elevation and azimuth of the vortex beam are modulated into the constellation.which can form the digital waveform with the encoding modulation.Since the signal is direction-dependent,the modulated waveform is purposely distorted in other directions to offer a security technology.Two concentric uniform circular arrays(UCAs)with different radii are excited to generate dual vortex beams with orthogonality for the composite signal,which can increase the demodulation difficulty.Due to the phase propagation characteristics of vortex beam,the constellation at the desired azimuth angle will change continuously within a wavelength.A desired single antenna receiver can use the propagation phase compensation and an opposite helical phase factor for the signal demodulation in the desired direction.Simulations show that the proposed OAM-DM scheme offers a security approach with direction sensitivity transmission.

High-speed directly modulated distributed feedback laser based on detuned loading and photon-photon resonance effect

A monolithic integrated two-section distributed feedback(TS-DFB)semiconductor laser for high-speed direct modulation is proposed and analyzed theoretically.The grating structure of the TS-DFB laser is designed by the reconstructionequivalent-chirp(REC)technique,which can reduce the manufacturing cost and difficulty,and achieve high wavelength controlling accuracy.The detuned loading effect and the photon-photon resonance(PPR)effect are utilized to enhance the modulation bandwidth of the TS-DFB laser,exceeding 37 GHz,while that of the conventional one-section DFB laser is only 16 GHz.When the bit rate of the non-return-to-zero(NRZ)signal reaches 55 Gb/s,a clear eye diagram with large opening can still be obtained.These results show that the proposed method can enhance the modulation bandwidth of DFB laser significantly.

Beyond the 100 Gbaud directly modulated laser for short reach applications

It is very attractive to apply a directly modulated laser(DML)-based intensity-modulation and direct-detection(IM/DD)system in future data centers and 5G fronthaul networks due to the advantages of low cost,low system complexity,and high energy efficiency,which perfectly match the application scenarios of the data centers and 5G fronthaul networks,in which a large number of high-speed optical interconnections are needed.However,as the data traffic in the data centers and 5G fronthaul networks continues to grow exponentially,the future requirements for data rates beyond 100 Gbaud are challenging the existing DML-based IM/DD system,and the main bottleneck is the modulation bandwidth of the DML.In this paper,the data rate demands and technical standards of the data centers and 5G fronthaul networks are reviewed in detail.With the modulation bandwidth requirements,the technical routes and achievements of recent DMLs are reviewed and discussed.In this way,the prospects,challenges,and future development of DMLs in the applications of future data centers and 5G fronthaul networks are comprehensively explored.

Widely Tunable Two-Section Directly Modulated DBR Lasers for TWDM-PON System

Wavelength tunable and directly modulated distributed Bragg reflector （DBR） lasers with butt-joint technology are designed, fabricated and characterized. The DBR laser consists of a gain section and a DBR section. To increase the electrical isolation between the gain section and the DBR section, parts of a p-doped material in the isolation region are etched off selectively. Over 2kΩ isolation resistance is realized ultimately without the need of ion implantation, which simplifies the fabrication process. The laser exhibits high speed modulation with a large tunable range. The 3dB direct modulation bandwidth of the device is over 8GHz in a 12nm tunable range. This widely tunable DBR laser with the simple structure is promising as a colorless light source for the next-generation time and wavelength division multiplexed passive optical network （TWDM-PON） systems.

A novel modulation and direct detection scheme of optical phase shift keying

This paper introduces a new modulation and direct detection scheme of optical phase shift keying (PSK) which is simple and practical in fiber optical communication. A phase modulator is used to modulate a continuous wave (CW) laser source and return-to-zero (RZ) signal that is changed from the initial transmitting information is used to control a phase modulator to form a optical PSK signal. In the receiver terminal, just add a signal delayed a half of one bit to itself so that the initial information can be restored.

Directly modulated 25 Gbaud/s tunable in-series DFB laser array for WDM systems

In this Letter,we proposed and experimentally demonstrated a directly modulated tunable laser based on the multi-wavelength distributed feedback(DFB)laser array.The lasers are placed in series to avoid the usage of an optical combiner and additional power loss.A three-section design is utilized to reduce the interference from other lasers and improve the electro-optic response bandwidth.Besides,the reconstruction-equivalent-chirp technique is used to simplify the grating fabrication and precisely control the grating phase.We realized 12 channels with 100 GHz spacing with high side mode suppression ratios of above 50 dB.The output power of all the channels is above 14 mW.The 3 dB electro-optic bandwidth is above 20 GHz at a bias current of 100 mA for all four lasers.A 25 Gb/s data transmission over a standard single-mode fiber of up to 10 km is demonstrated for all 12 channels,and 50 Gb/s data per wavelength is obtained through the four-level pulse amplitude modulation.The proposed directly modulated tunable in-series DFB laser array shows the potential for a compact and low-cost light source for wavelength division multiplexing(WDM)systems,such as next-generation front-haul networks and passive optical networks.

Physical Layer Security for MmWave Communications:Challenges and Solutions

The mmWave communication is a promising technique to enable human commutation and a large number of machine-type commu⁃nications of massive data from various non-cellphone devices like Internet of Things(IoT)devices,autonomous vehicles and remotely con⁃trolled robots.For this reason,information security,in terms of the confidentiality,integrity and availability(CIA),becomes more important in the mmWave communication than ever since.The physical layer security(PLS),which is based on the information theory and focuses on the secrecy capacity of the wiretap channel model,is a cost effective and scalable technique to protect the CIA,compared with the traditional cryptographic techniques.In this paper,the theory foundation of PLS is briefly introduced together with the typical PLS performance metrics secrecy rate and outage probability.Then,the most typical PLS techniques for mmWave are introduced,analyzed and compared,which are classified into three major categories of directional modulation(DM),artificial noise(AN),and directional precoding(DPC).Finally,several mmWave PLS research problems are briefly discussed,including the low-complexity DM weight vector codebook construction,impact of phase shifter(PS)with finite precision on PLS,and DM-based communications for multiple target receivers.

Dispersion Limit for IM/DD Fiberoptic Transmission Systems

A general formula for the dispersion limit of single-mode-fiber IM/DD (Intensity Modulation/Direct Detection) systems is derived for arbitrary given normalized pulse width P, eye opening penalty X and source linewidth enhancement factor α. From the comparison with published theoretical results, computer simulation and experimental data, its validity and convenience in system design and evaluation are shown. Using this simple and general formula, one can easily obtain the dispersion limit of various fiber types under different working conditions in IM/DD systems.

Bidirectional intensity modulated/direct detection optical OFDM WDM-PON system

In this paper, we have evaluated a bidirectional wavelength division multiplexing passive optical network(WDM-PON) employing intensity modulated/direct detection optical orthogonal frequency division multiplexing(IM/DD-OFDM). The proposed system employs 100 Gbit/s 16 quadrature amplitude modulation(16-QAM) downstream and 5 Gbit/s on-off keying(OOK) upstream wavelengths, respectively. The proposed system is considered low-cost as non-coherent IM/DD OFDM technology and a simple reflective semiconductor optical amplifier(RSOA) colorless transmitter are employed and no dispersion compensating fiber(DCF) is needed. Based on the bit error rate(BER) results of WDM signals, the proposed WDM-PON system can achieve up to 1.6 Tbit/s(100 Gbit/s/λ × 16 wavelengths) downstream transmission over a 30 km single mode fiber(SMF).

25 Gb/s directly modulated ground-state operation of 1.3 μm InAs/GaAs quantum dot lasers up to 75℃

We report 25 Gb/s high-speed directly modulated ground-state operation of 1.3 μm In As/Ga As quantum dot(QD) lasers grown by molecular beam epitaxy. The active region of the lasers consists of eight layers of p-doped In As QDs with high uniformity and density. Ridge-waveguide lasers with a 3-μm-wide and 300-μm-long cavity show a low threshold current of 14.4 m A at 20°C and high temperature stability with a high characteristic temperature of 1208 K between 20°C and 70°C. Dynamic response measurements demonstrate that the laser has a 3 d B bandwidth of 7.7 GHz at 20°C and clearly opened eye diagrams even at high temperatures up to 75°C under a 25 Gb/s direct modulation rate.

112 Gbit/s transmitter optical subassembly based on hybrid integrated directly modulated lasers

Based on the hybrid integration technology, an ultra-compact and low cost transmitter optical subassembly module is proposed. Four directly modulated lasers are combined with a coarse wavelength division multiplexer operated at the O-band. The bandwidth for all channels is measured to be approximately 3 GHz. The 112 Gb∕s transmission is experimentally demonstrated for a 10 km standard single mode fiber（SSMF）, in which an optical isolator is used for avoiding the back-reflected and scattered light to improve the bit error rate（BER） performance. A low BER and clear eye opening are achieved for 10 km transmission.

Design of a single-mode directly modulated orbital angular momentum laser

We propose a design of single-mode orbital angular momentum(OAM) beam laser with high direct-modulation bandwidth. It is a microcylinder/microring cavity interacted with two types of second-order gratings: the complex top grating containing the real part and the imaginary part modulations and the side grating. The side grating etched on the periphery of the microcylinder/microring cavity can select a whispering gallery mode with a specific azimuthal mode number, while the complex top grating can scatter the lasing mode with travelling-wave pattern vertically. With the cooperation of the gratings, the laser works with a single mode and emits radially polarized OAM beams. With an asymmetrical pad metal on the top of the cavity, the OAM on-chip laser can firstly be directly modulated with electrical pumping. Due to the small active volume, the laser with low threshold current is predicted to have a high direct modulation bandwidth about 29 GHz with the bias current of ten times the threshold from the simulation. The semiconductor OAM laser can be rather easily realized at different wavelengths such as the O band, C band, and L band.