Combining self-phase modulation and optimum modulation conditionsto improve the performance of 10-Gb/s transmission systems using MQWMach-Zehnder modulators

For 10-Gb/s transmission over nondispersion shifted fiber, the combined use of self-phase modulation (SPM) and joint optimization of the bias and modulation voltages to increase the dispersion limited transmission distance is considered for multiple quantum well Mach-Zehnder modulators. For the dual drive (push-pull) modulation format, the dependence of the receiver sensitivity on fiber length and average transmitted optical power is determined for both conventional and π phase-shift modulators with either symmetric or asymmetric Y-branch waveguides. When SPM is negligible and the optical extinction ratio is maximized, the modulator design must he considered carefully in order to increase the transmission distance. By combining SPM and optimum modulation conditions, the dependence of the system performance on the modulator design is reduced substantially. For an average transmitted optical power of 12.5 dBm, the receiver sensitivity for transmission over 140 km of fiber varies by only 0.3 dB for the different modulator designs. This compares with a variation of 3.1 dB for maximum extinction ratio modulation     

Optimum operating points for electroabsorption modulators in 10Gb/s transmission systems using nondispersion shifted fiber

The chirp, optical extinction ratio and insertion loss of an electroabsorption modulator (EAM) depend on the properties of the bulk or multiple-quantum-well absorption layer of the device and on the bias voltage and modulating voltage waveform. For 10 Gb/s transmission over nondispersion shifted fiber, joint optimization of the bias and modulation voltages is considered for five different EAM's. To comprehensively explore this issue, the measured dependence of the absorption and α-parameter on applied voltage is used to accurately model an EAM in a system simulator. The effects of group velocity dispersion and self-phase modulation arising from the Kerr nonlinearity are included to permit assessment of the dependence of the optimum bias and modulation voltages on the average transmitted optical power for a given fiber length. The improvement in receiver sensitivity, relative to that obtained with maximum optical extinction ratio, depends quite significantly on the transmitted optical power and the specific properties of the modulator. This makes it difficult to determine optimum operating conditions which apply generally     

Nonsymmetric Mach-Zehnder interferometers used as low-drive-voltagemodulators

Optical communications systems require fast optical modulators which are compatible with existing microwave-frequency drive circuits and which have low voltage requirements. The authors have designed and demonstrated a novel type of integrated optical modulator: a nonsymmetric Mach-Zehnder interferometer, which has both low switching voltage and efficient coupling to microwave-frequency drive circuits. In 2-cm-long devices, made in z-cut LiNbO₃, switching voltages of 3.2 V and better than 20-dB extinction ratio were measured at λ=1.3 μm. Device performance in the nonsymmetric devices was independent of the asymmetry, which indicates that devices of this kind are tolerant of fabrication variations     

基于调制边带法的高次倍频光毫米波产生

提出了利用外调制技术的调制边带法的六倍频和八倍频光毫米波产生方案。方案仅采用一个马赫曾德尔调制器,并利用其非线性传输特性,通过调节MZM 的偏置电压和调制电压,控制边带的强度,仅保留三阶边带或四阶边带,从而实现六倍频及八倍频光毫米波的产生。采用将基带数据信号仅调制在一个三阶边带或四阶边带分量的方式,有效防止走离。数值分析结果表明,提出的方案仅需10 GHz 及7.5 GHz 的调制信号频率就能得到60 GHz 毫米波,大大减小了调制信号频率,增加了上变频系数,传输距离可达160 km,而功率代价变化不大。与提出的其他高倍频技术相比,由于系统仅采用一个马赫曾德尔调制器,提出的方案系统结构更为简单,且此方案色散影响较小,传输距离更长。     

Polymeric digital optical modulator based on asymmetric branch

A digital optical modulator based on an asymmetric Y-branch waveguide is proposed and fabricated by using an electro-optic polymer. The operating point is initially shifted to the off-state utilising the asymmetry in the branch to provide an initial zero-state with no electrical bias. It has been confirmed that the high extinction ratio can be obtained with a low drive voltage. An extinction ratio of 25 dB is demonstrated for a drive voltage of 20V using a polymer PMMA-DR1 with r_ss of 5 pm/V at 1.3 μm     

一种改进的载波抑制调制光毫米波信号的产生方案

为了克服光纤无线(ROF)系统中色散对光载波抑制(OCS)调制光毫米波信号传输的影响,提出一种改进的OCS调制方案。使用双驱动马赫-曾德尔调制器(MZM),通过调整两路输入射频信号相位、基带信号增益和直流偏置电压将2.5Gbit/s数据信号仅调制到(OCS)信号的一个边带上传输。理论分析表明,与传统OCS调制光毫米波信号产生方案相比,本文方案解决了色度色散引起的码元走离问题,大大增加了传输距离。仿真实验结果表明,经过110km光纤传输后信号的眼图仍然十分清晰,在BER=10-10条件下,信号经过20、40和60km光纤传输后的功率代价分别为0.78、1.7和1.9dB。     

一种基于并级联MZM的20倍频光生毫米波方案

为进一步提高光载无线通信(radio over fiber,ROF)系统中光生毫米波的倍频系数,提出了一种基于4个马赫-曾德尔调制器(Mach-Zehnder modulator,MZM)共同作用的20倍频毫米波信号产生方案。推导了理想情况下倍频方案的产生机理,在仿真实验中,分别分析了非理想因素下调制器直流偏置电压漂移、调制指数、消光比等对系统性能的影响,结果显示,当合理设置各参数的取值范围,其光边带抑制比(optical sideband suppression ratio,OSSR)和射频杂散抑制比(radio frequency spurious sideband suppression ratio,RFSSR)的饱和值高达33.20 dB和27.21 dB;针对基于此倍频方案的ROF系统,对比分析了2.5 Gbit/s数据信号的单双边带调制两种不同传输方式,仿真结果表明,当光纤距离为40 km时,单边带依旧可达无差错传输,降低了传输过程中码间走离效应的影响,增加了系统的传输距离,更适合远距离传输,为微波光子学的发展提供了一种理论依据。     

Reduced bandwidth optical digital intensity modulation withimproved chromatic dispersion tolerance

A simple optical modulation scheme using a lithium niobate Mach-Zehnder modulator driven by a three level drive waveform is proposed. The two-level intensity modulated (IM) optical signal obtained possesses a smaller optical bandwidth and thus greater chromatic dispersion tolerance compared with existing two-level IM methods used for high data rate transmission (e.g. 10 Gbit/s)     

一种克服色度色散影响的四倍频光毫米波信号产生方法

提出一种克服色度色散影响的四倍频光毫米波信号产生方法。该方法使用一个双驱动马赫曾德尔调制器,通过调整上、下两路射频信号的相位差、直流偏置点、调制系数以及基带信号增益,将数据信号仅调制到四倍频光毫米波信号的一个2阶边带上传输,解决了色度色散引起的码元走离问题,有效增加了传输距离。理论分析和仿真实验结果表明,信号在光纤中传输120 km后眼图仍然十分清晰,经过60 km传输后的功率代价约为0.45 dB。另外,基于频率再用技术,没有调制数据的另一个2阶边带信号还可以作为全双工光纤无线通信(RoF)系统的上行链路光载波,简化了基站配置。仿真实验结果表明,双向2.5 Gbit/s数据信号在光纤中传输40 km后,功率代价小于0.6 dB。     

Chromatic dispersion tolerance of new duobinary transmitters based on two intensity modulators without using electrical low-pass filters

This paper describes an investigation of the effect of the extinction ratio and the chirp parameter of the first intensity modulator on the transmission performance of a proposed duobinary transmitter based on two intensity modulators without using electrical low-pass filters. This modulation scheme can generate duobinary signals using only two-level electrical signals without the conversion process to three-level electrical signals using electrical low-pass filters. The simulation results suggest that the appropriate chirp and extinction ratio of the first intensity modulator provide larger dispersion tolerance, compared with the conventional duobinary transmitter. The duobinary transmitter based on two intensity modulators can increase the dispersion tolerance without using electrical low-pass filters.     

一种载边比可调单边带全双工光载无线系统

理论分析了一种基于双平行马赫曾德尔调制器(MZM)的2倍频光单边带调制方案,仿真研究了基于该方案的载波重用全双工光载无线(RoF)通信系统性能.合理设置射频驱动信号相移及双平行MZM主调制器偏置电压产生单边带信号,再调整调制指数以实现光载边比ROCS的连续可调谐.讨论了射频信号相移、主调制器偏置电压、双平行MZM消光比...     

A simplified optical millimeter-wave generation scheme based on frequency-quadrupling

This paper analyzes and demonstrates a simplified frequency quadrupling configuration for optical millimeter-wave(mm-wave)generation,in which the electrical phase shifter and optical filter are omitted.Theoretical analysis is given to reach the optimum operating conditions including direct current(DC)bias voltage,optical transmission point of the dual-parallel Mach-Zehnder modulator(MZM)bias voltage,optical transmission point of the dual-parallel Mach-Zehnder modulator(DP-MZM),amplitude of the radio frequency(RF)driving signal and the impact of the extinction ratio(EF)on the optical sideband suppression ratio(OSSR)and radio frequency spurious suppression ratio(RFSSR).Experiments prove an OSSR of 15 dB and an RFSSR of 26 dB for the new frequency quadrupling scheme at 6 GHz,8 GHz and 10 GHz of RF driving signal without any electrical phase shifter or optical filter.This system exhibits the advantage of low wavelength dependence and large frequency tunable range.     

Modulation characteristics of semiconductor Mach-Zehnder opticalmodulators

The transmission and chirp characteristics are described for two types of semiconductor Mach-Zehnder modulators, distinguished by the differential phase shift between the two arms of the interferometer in the unbiased state. The conventional modulator has a differential phase shift of 0 radians, while the π-shift modulator has a differential phase shift of π radians. The nonlinear dependence on the applied voltage of the attenuation and phase constants of the optical signal propagating in the p-i-n waveguide leads to different characteristics for the two modulators. The influence of the splitting ratio of the Y-junctions is considered for single-arm and dual-arm (push-pull) modulation formats. The π-shift modulator is shown to yield better transmission performance for 10 Gb/s systems compared to the conventional modulator     

Phase-engineered III-V MQW Mach-Zehnder modulators

We have optimized the negative frequency chirp while maintaining a high extinction ratio in an InP-InGaAsP multiple-quantum-well (MQW) Mach-Zehnder modulator. Our procedure is to elongate one modulator arm such that a /spl pi/ phase shift is produced at zero bias relative to the opposing arm. The resulting de extinction ratio is improved to /spl ap/16 dB from 10 dB relative to a symmetric design. The frequency chirp of the modulator at the rising edge is about -3 GHz enabling excellent transmission performance over a 95 km of conventional fiber.     

Synthesis of photonic microwave filters based on external opticalmodulators and wide-band chirped fiber gratings

The synthesis of photonic microwave filters employing optical sources with modulation side-bands, as multimode optical sources, and chirped fiber gratings is addressed. A single wavelength laser externally modulated using electrooptical Mach-Zehnder modulators and electroabsorption modulators have been proved as optical sources with modulation sidebands. Microwave filter features such as wide range tunability, compactness, readily reconfigurable frequency responses and flexibility have been achieved and demonstrated for the proposed structure. Experimental results using both a dual-electrode Mach-Zehnder electrooptical modulator and an electroabsorption modulator as optical sources with modulation sidebands are provided to demonstrate the feasibility of this technique     

基于双平行相位调制器的OFDM–ROF系统仿真

建立一种基于双平行相位调制器的OFDM-ROF实验仿真系统.在中心站采用双平行相位调制器产生双边带光载毫米波,利用光交错复用器将信号进行上边带和下边带分离,将OFDM信号通过马赫曾德尔调制器调制到一个边带上进行传输.OFDM的调制和解调部分使用MATLAB软件编写,OFDM-ROF系统实验仿真平台基于软件Optisystem进行搭建,并给出了光载OFDM信号的产生模型和系统仿真结果.实验表明,OFDM-ROF系统经过色散补偿后大幅度提高了系统的传输距离.     

16-arrayed electrooptic polymer modulator

A 16-arrayed polymeric optical modulator is fabricated using an electrooptic (EO) polymer with a large EO coefficient and good thermal stability. The 16-arrayed modulator has lumped-type electrodes with a response time of less than 1 ns. The 16-arrayed modulator has good uniform modulation characteristics between the individual modulators. The deviation of half-wave voltages is 0.2 V and that of insertion losses about 1 dB. Crosstalks range from -28 to -36 dB and extinction ratios are more than 21 dB.     

A lithium niobate light modulator for fiber optical communications

An electrooptic intensity modulator using lithium niobate has been developed for applications in binary fiber optical digital communications at the wavelength of 1.06 µm. We have shown that many shortcomings generally associated with electro-optic modulators can be surmounted. The modulator was driven by a compact transistor amplifier, temperature dependence of the static birefringence was minimized, and the optical bias was made adjustable by a dc voltage superposed on the signal. The modulator has been operated at 70-Mb/s pulse rate and 100-percent modulation, its extinction ratio is better than 40 to 1 and the optical insertion loss is about 1 dB.     

High-speed bulk InGaAsP-InP electroabsorption modulators withbandwidth in excess of 20 GHz

Bulk InGaAsP-InP electroabsorption optical modulators with high extinction ratio, low drive voltage, and high modulation bandwidth at λ=1.3 μm are reported. The devices have a tapered-fiber-to-modulator-to-tapered-fiber extinction ratio greater than 20 dB at a drive voltage of <5 V. Very low capacitance modulators (<0.2 pF) were fabricated using SiO₂ bonding pad isolation, resulting in a measured electrical modulation bandwidth in excess of 20 GHz