A novel scheme to implement duplex 60-GHz radio-over-fiber link with 20-GHz double-sideband optical millimeter-wave transmitted along the fiber

A duplex radio-over-fiber (RoF) link with a novel scheme to generate 60-GHz millimeter (mm)-wave using 20 GHz double sideband (DSB) optical mm-wave with signal carried only by its optical carrier is investigated. In the RoF downlink, the modulation frequency to generate the DSB optical mm-wave is reduced greatly. The theoretical analysis and numerical simulation show that this scheme cannot only eliminate the code form distortion caused by the time shifting of the sidebands, but also reduce the influence of the fading effect as the DSB optical mm-wave signal is transmitted along the fiber. Based on the scheme, the duplex RoF link with the frequency up- (down-) conversion of the down- (up-) link mm-wave signal is built and the uplink transmission performance is analyzed theoretically.     

采用单个相位调制器产生毫米波

实验研究了采用单个相位调制器(PM)产生毫米波的方案。该方案采用电混频器将射频(RF)信号与基带信号混频后再利用相位调制器产生双边带调制(DSB)信号,经光纤传输到基站后用一个光交叉复用器(IL)分离一阶边带和中心载波,一阶边带经过光电(O/E)检测器拍频产生两倍频于射频频率的毫米波,而中心载波可以作为上行链路载波重新利用。理论分析了该毫米波的传输性能,研究发现由于色散导致两个一阶边带时延不同,码元的占空比会随着传输距离的增加而减小,将限制毫米波的最大传输距离;实验中采用频率为20 GHz射频信号产生频率为40 GHz的毫米波,速率为2.5 Gbit/s的非归零(NRZ)码作为下行链路数据,经过20 km色散光纤传输后下行链路的功率代价为0.2 dBm。     

Simultaneous Cancellation of Fiber Loss, Dispersion, and Kerr Effect in Ultralong-Haul Optical Fiber Transmission by Midway Optical Phase Conjugation Incorporated With Distributed Raman Amplification

An alternative application of distributed Raman amplification (DRA) for ultralong-haul optical fiber transmission is proposed. In our study, the DRA is employed in a transmission system using midway optical phase conjugation (OPC) for amplifying an optical signal and, at the same time, for constructing signal power evolution, which is symmetrical with respect to the midpoint of the system where the OPC is performed. Then, the nonlinear signal waveform distortions that are caused by the Kerr effect, as well as fiber dispersion, are almost completely compensated by the OPC, whereas the fiber loss is compensated by the DRA. Three possible symmetrical signal power maps - a power map that has a reverse sign of the power map that is caused by lump amplification, a flat signal power map, and an arbitrary symmetrical signal power map - are numerically designed by using appropriate Raman pump powers. We show that the flat power map exhibits smaller difference from the target and a higher optical signal-to-noise ratio and requires lower pump power than the other two power maps. Numerical simulation results demonstrate that, by employing the flat power maps with a span of 40 km, a single-wavelength signal whose data rate is 160 Gb/s can be successfully transmitted over 5000 km, and the Kerr effect is sufficiently suppressed near limitation due to the nonlinear accumulation of noise. Finally, we study the feasibility of expanding our method to wavelength-division-multiplexed signal transmission by designing a DRA gain with multiple-wavelength pumping to simultaneously obtain a flat power map and a wide-and-flat gain bandwidth. By using four-wavelength Raman pumps while carefully choosing pump wavelengths and their powers, we achieve the DRA gain that simultaneously gives a fluctuation of the signal power of only 3.5%, a gain ripple of only 5.3%, and, at the same time, a gain bandwidth of as wide as 46 nm.     

光载毫米波在光纤中传输色散性能研究

提出了并理论分析了一种采用单电极Mach–Zehnder调制器(SD-MZM)产生光载毫米波的方案。采用电混频器将射频信号与基带数字信号混频后再利用单电极调制器产生双边带调制信号并发送至光纤,在基站使用一个交叉复用器(IL)将双边带信号的中心载波与一阶边带信号进行分离,中心载波可用于上行链路的光载波,而一阶边带产生光载毫米波;理论分析了该毫米波的色散特性并在仿真平台上验证了其正确性,研究发现由于光纤色散引起两个一阶边带的延时不同, 从而导致毫米波能量的损失和解调信号的码间干扰,限制了毫米波的最大传输距离。     

Mitigation of Signal Distortions Using Reference Signal Distribution With Colorless Remote Antenna Units for Radio-Over-Fiber Applications

In this paper, we propose and demonstrate a bidirectional radio-over-fiber (ROF) system using a reference signal distribution. In order to minimize the radio-frequency (RF) signal attenuation as well as limit the transmit power to moderate levels, small cell size, or picocell is used in the ROF system. This will result in the need of many base stations (BSs) to provide sufficient network coverage. Because of this, colorless remote antenna unit (RAU) with optical carrier generated and distributed from the head end (HE) is employed. Experimental results show that error-free signal transmission and remodulation using downstream differential phase shift keying (DPSK) and upstream on-off keying (OOK) is achieved in a 20-km reach ROF system. Experimental characterization of the reflective semiconductor optical amplifier (RSOA)-based remodulation unit is also performed. Numerical simulation is performed to compare the proposed scheme with several conventional optical millimeter-wave (mm-wave) schemes, including double sideband (DSB), optical carrier suppression (OCS), and single sideband (SSB). Results show that the proposed scheme could be a potential candidate to mitigate signal distortions due to signal fading and code time shifting.     

Impact of Nonlinear Transfer Function and Imperfect Splitting Ratio of MZM on Optical Up-Conversion Employing Double Sideband With Carrier Suppression Modulation

Generation of optical millimeter-wave (mm-wave) signal using a Mach–Zehnder modulator (MZM) based on double-sideband (DSB), single-sideband (SSB), and double-sideband with carrier suppression (DSBCS) modulation schemes have been demonstrated for various applications, such as broadband wireless signals or optical up-conversion for wavelength-division-multiplexing (WDM) radio-over-fiber (RoF) network, wideband surveillance, spread spectrum, and software-defined radio. Among these schemes, DSBCS modulation offers the best receiver sensitivity, lowest spectral occupancy, the least stringent requirement of electrical bandwidth, and the smallest receiving power penalty after long transmission distance. Nonetheless, the inherent nonlinear E/O (electrical/optical) conversion response of a MZM is such that the signal quality of the optical mm-wave suffers. Fabrication tolerances make a balanced 50/50 splitting ratio of the MZM's y-splitter particularly difficult to achieve. As a result, imbalanced MZMs have a finite extinction ratio (ER) and degrade the optical carrier suppression ratio (OCSR) using DSBCS modulation. In this paper, the effect of the MZM nonlinearity and imbalanced y-splitter on optical mm-wave generation by DSBCS modulation is theoretically and experimentally investigated. A novel approach with better performance and greater cost-effectiveness than dual-electrode MZM (DD-MZM) is presented to realize a DSBCS modulation scheme based on a single-electrode MZM (SD-MZM).      

Transmission Performance of the Optical Millimeter-wave Up-conversion Link Using a Phase Modulator

Numerically analyzed is the transmission performance of the optical millimeter(mm)-wave generated by frequency up-conversion via a phase modulator along the dispersive fiber.60 GHz mm-wave sub-carrier(SC)signals can be obtained after fiber transmission,simultaneously,the phase-modulated signals can be converted to the intensity-modulated ones.The numerical results show that the optical mm-wave at fading loops has better performance,and the eye diagram still keeps open when optical mm-wave signal is transmit...     

A millimeter-wave radio-over-fiber system for overcoming fiber dispersion-induced signal cancellation effect

Millimeter-wave(mm-wave) radio over fiber(ROF) using dispersive single-mode fiber is susceptible to signal cancellation effect at the output of the uni-travelling carrier photodiode at the base station(BS).The fiber dispersion effect produces different phase shifts of the sidebands of the intensity-modulated lightwave which can produce a cancellation of the output signal when mixed with the optical carrier.In this paper,we propose and analyze a novel scheme of mm-wave ROF which uses microwave modulation at the central station(CS) and frequency upconversion before the BSs.This scheme can overcome fiber dispersion-induced signal cancellation effect.     

基于光载无线技术的毫米波轨道角动量多维联合调制/解调研究

提出了一种基于光载无线(ROF)技术的W波段涡旋毫 米波生成及轨道角动量(OAM)多维联合调制 /解调方法。采用ROF技术在光域对微波源进行倍频生成W 波段光学毫米波,并通 过集成光调制 器对光学毫米波的相位和幅度进行调控。通过合理调控圆环天线阵列(CAAs)的辐射毫米波 相移,从而实 现涡旋毫米波的生成以及状态切换。同时,设计了64阶OAM、幅度和 相位的三维联合调制格式, 并通过模拟仿真,成功实现了75Gbit/s高阶联合调制信号的调制/解 调。研究结果表明,OAM 作为一个新 的物理维度不仅可以通过复用来实现传输容量密度的提升,同时也可以作为调制手段实现 传输容量的提 升,在提高无线频谱效率以及保密通信中具有重要的潜在应用价值。     

Fiber Dispersion Influence on Transmission of the Optical Millimeter-Waves Generated Using LN-MZM Intensity Modulation

We have theoretically investigated the transmission performance of the optical millimeter-waves (mm-waves) generated using intensity modulation via a Mach-Zehnder modulator, with the double and single sidebands (DSB and SSB) and optical carrier suppression (OCS) schemes. According to our theoretical analysis, fiber chromatic dispersion leads to fading effect and time shifting of the codes; therefore, signals are greatly degraded. Of all signals, DSB optical mm-wave suffers from both the fading effect and the time shifting of the codes. However, the optical mm-waves generated by SSB and OCS schemes are immune to the fading effect, while the time shifting of the codes limits their transmission distance. Experimental and simulation results confirm the theoretical analysis.     

Compensation for the self-steepening effects in optical fiber communication system using midway optical phase conjugation

Effects of self-steepening (SS) of chirped Gaussian pulses on optical fiber communication system using midway optical phase conjugation (OPC) are analyzed. Dynamic evolution of the ultrashort pulses is simulated numerically. It is found that OPC cannot compensate for pulse waveform distortion due to SS.The initial chirp of pulses and dispersion can counteract SS and improve the compensation performance for the distortion.     

Simultaneous suppression of third-order dispersion and sideband instability in single-channel optical fiber transmission by midway optical phase conjugation employing higher order dispersion management

In optical phase conjugation (OPC) systems, the third-order dispersion (TOD) of optical fibers and the nonlinear resonance at well-defined signal sideband frequencies called sideband instability (SI) mainly limit the transmission performance. We propose, for the first time, a scheme for simultaneous suppression of both TOD and SI in OPC systems using a periodic higher order dispersion-managed link consisting of standard single-mode fibers (SMFs) and reverse dispersion fibers (RDFs). Computer simulation results demonstrate the possibility of 200-Gb/s data transmission over 10 000 km in the higher order dispersion-managed OPC system, where the dispersion map is optimized by our system design strategies.     

Fiber-optic millimeter-wave downlink system using 60 GHz-bandexternal modulation

In this paper, a fiber-optic millimeter-wave (mm-wave) downlink system using 60 GHz-band external modulation is investigated. We prepare the fiber-optic 60 GHz-band mm-wave downlink testbed. It consists of an optical modulation section with a mm-wave signal generator, an optical single sideband (SSB) filter, a standard single-mode fiber (SMF), an optical detection section with a 60 GHz-band radio transmitter and a 60 GHz-band radio receiver. To modulate the laser output with 60 GHz-band mm-wave signals directly, a specially designed electro-absorption modulator with high-efficiency at around 60 GHz is used. The use of this modulator makes the simpler system configuration possible. Using the downlink testbed, the 5 m-long free-space propagation of subcarrier multiplexed 156 Mb/s-DPSK 60 GHz-band mm-wave signals recovered by optical direct detection is successfully demonstrated. The transmission of the mm-wave signals over 85 km-long standard SMF is also successfully demonstrated, using an optical SSB filtering technique to overcome the fiber dispersion. The BER of 10^-9 is achievable at the optical received power of -7.0 dBm     

基于级联马赫-曾德电光调制器的60GHz光载毫米波传输系统的研究

提出并实现了一种新的产生毫米波的六倍频技术。采用两个相同的串联马赫一曾德电光调制器（MZM）,调节第1个MZM调制器的偏置电压使其工作在最小传输模式,只输出奇次光谐波分量;调节第2个MZM调制器的偏置电压使其工作在最大传输模式,抑制所有的奇次光谐波分量。实验显示,采用10GHz的射频（RF）信号驱动这两台MZM调制器后...     

自陡峭效应对相位共轭系统脉冲传输的影响

光学相位共轭(OPC)技术能够同时且高效地补偿光纤传输过程中色散及非线性效应所导致的信号失真，且该技术同脉冲调制方式无关。从理论上分析了在自陡峭效应(SS)作用下高斯脉冲信号在中距相位共轭系统中的传输演化特性，数值模拟了在其作用下超短飞秒高斯脉冲的动态传输过程，讨论了自陡峭效应对中距相位共轭系统复原性能的影响。结果表明自陡峭效应将导致高斯脉冲信号发生峰值漂移和脉冲后沿变陡，相位共轭系统不能补偿由此导致的脉冲失真和畸变。引入合适的色散可以减小这种信号失真，并使得相位共轭系统能够同时补偿由于色散、自相位调制和自陡峭效应而引起的信号失真。     

Dispersion-Tolerant Millimeter-Wave Photonic Link Using Polarization-Dependent Modulation

A novel technique to modulate one carrier in a dual-carrier millimeter-wave (mm-wave) photonic system is presented and experimentally evaluated. The technique utilizes the polarization dependence of an optical phase modulator and the properties of optical heterodyning and is demonstrated for both amplitude- and phase-modulated mm-wave signal outputs. Experimental verification of the technique is presented with transmission of up to 2.5-Gb/s nonreturn-to-zero data, multilevel quadratic-amplitude modulation, and IEEE 802.11a WLAN data signals on a 40-GHz carrier over optical fiber and wireless transmission. The tolerance for fiber dispersion is investigated, and penalty-free transmission over 44 km is verified. The linearity of the system is evaluated by measuring the spurious-free dynamic range. [All rights reserved Elsevier].     

Generation of frequency-doubling mm-wave signal using a Mach–Zehnder modulator with three arms to overcome fiber chromatic dispersion

We propose a novel method to generate optical frequency-doubling mm-wave signal by using a three-arm Mach–Zehnder modulator (MZM) for radio-over-fiber (RoF) systems. Since the optical carrier and its two 2nd-order sidebands are dominant, the 2nd-order harmonic in the photocurrent gets maximal. When the optical mm-wave signal is transmitted in standard single-mode fiber (SSMF), there is no time shift of code edges because the data signal is only carried by the optical carrier. The 2nd-order harmonic in the photocurrent consists of two beating products, so it experiences a periodical fading effect when the optical mm-wave signal is transmitted in SSMF. However, if a proper direct-current (DC) bias is applied to the data modulation arm in the MZM architecture to adjust the position of the fading nodes, the degradation on the photocurrent can be avoided. A RoF link is built based on the novel scheme by simulation, and the transmission performance of the proposed scheme is presented. The eye diagram still keeps open and clear even after 40 km transmission. Simulation results prove our theoretical analysis.     

Generation of optical phase-conjugate waves and compensation forpulse shape distortion in a single-mode fiber

The generation of optical phase-conjugate waves and the application of optical phase conjugation (OPC) to optical communication systems is described. The method of pulse shape distortion compensation by OPC is outlined including distortion due to both fiber dispersion and the optical Kerr effect. The generation of a forward-going phase-conjugate wave in a third-order nonlinear medium is discussed and that by a nondegenerate forward four-wave mixing in a zero-dispersion single-mode fiber (SMF) is investigated. Suppressing the stimulated Brillouin scattering (SBS) of a pump wave in the fiber prevents saturation of the generation efficiency of the phase-conjugate wave even when the pump power exceeds the SBS threshold. In transmission experiments through a 200-km standard SMF with a 16-Gb/s intensity-modulated signal and a 5-Gb/s continuous-phase FSK (CPFSK) modulated signal, it is shown the applicability of OPC is modulation independent and that OPC effectively compensates for both chromatic dispersion and the optical Kerr effect     

A Novel Scheme to Generate Single-Sideband Millimeter-Wave Signals by Using Low-Frequency Local Oscillator Signal

We have proposed and experimentally demonstrated a novel scheme to generate optical millimeter-wave (mm-wave) signals by using single-sideband modulation with low-frequency local oscillator (LO) signals. In this architecture, by incorporating the proper dc bias of the modulator in central office, the optical mm-wave carriers are generated with two times frequency of the LO signal while largely reducing the bandwidth requirement of the modulator. We quantify the optical carrier-to-sideband ratio (CSR) of downstream transmission in this radio-over-fiber (ROF) link and establish that the performance of the ROF system can be significantly improved when the optical signals are transmitted at CSR equal to 0 dB.     

An Optical Millimeter-Wave Generation Technique Based on Phase Modulation and Brillouin-Assisted Notch-Filtering

A novel approach to generate continuously tunable microwave and millimeter-wave (mm-wave) signals is proposed and experimentally demonstrated based on an optical phase modulator and a Brillouin-assisted notch-filtering technique. The strong optical carrier of the phase modulated signal is deeply suppressed by using stimulated Brillouin scattering (SBS) in a dispersion-shifted-fiber ring while the sidebands keep unchanged. Therefore, only even-order electrical harmonics exist at the output of a photodetector. As this SBS-assisted carrier-filter is narrow bandwidth and wavelength-independent, a wavelength-flexible way of optical generation mm-wave signals can be successfully realized.