Design of multi-channel optical code-division multiple-access encoders and decoders based on sampled fiber Bragg gratings

An encoding/decoding scheme of multi-channel optical code-division multiple-access (OCDMA) system using sampled fiber Bragg gratings (SFBGs) is proposed and numerically investigated. A multiple-phase-shift (MPS) technique based SFBG is first developed as a dense wavelength-division multiplexing (DWDM) device to accommodated multiple channels. The OCDMA scheme is achieved through phase coding/encoding on each individual channel of the SFBG and the auto correlative ratio for each user is larger than 14.3 dB. The whole system exhibits enhanced performance in terms of the capacity and security since the OCDMA technique is simultaneously implemented for all channels using the single SFBG.     

Photon Generation Scheme of 32-Fold Millimeter-Wave Signal Based on Mach-Zehnder Modulator

This paper proposes a 32-tupling frequency millimeter-wave (MMW) filter-free system based on four Mach-Zehnder Modulators (MZM) connected in parallel and cascaded with a simple radio-fiber (RoF) link structure. The four MZMs are all at the maximum transmission point (MATP), and the radio frequency (RF) driving voltage phase difference between MZMs is π /2. The center carrier is suppressed by using an optical attenuator (OATT) and an optical phase shifter (OPS). Two parallel MZMs can generate ±8th order and ±12th order optical sidebands, and the ±4th order optical sidebands can be suppressed by adjusting the modulation index m of the MZM, using cascaded two dual-parallel MZMS(DPMZM) and the phase difference of the RF signal source is π/4 to generate ±16th order optical sidebands. The theoretical analysis and simulation experiments are performed for the scheme proposed in this paper. The results show that the simulated and theoretical values of the optical sideband suppression ratio (OSSR) for ±16th order optical sideband signals are 60.02 and 59.96 dB, respectively, and the simulated and theoretical values of the RF sideband suppression ratio (RFSSR) for the 32-tupling MMW signal are 56.34 and 53.94 dB, respectively.     

Channel spacing-tunable sampled fiber Bragg grating by linear chirp and its application to multiwavelength fiber laser

We theoretically investigated the two methods for densifying the channel spacing of sampled fiber Bragg gratings (SFBGs), the multiple-phase-shift (MPS) method and the linear chirp method, and found that the two methods have similar principle and ability, although the chirp method has an advantage that the tuning of channel spacing is easier. Then we realized channel spacing-tunable SFBGs by use of deformation of ring and S-bend structures by 2- and 3-folds. Finally, we applied the chirp method to realize a wavelength spacing-tunable multiwavelength fiber laser.     

Operation of low driving voltage cascaded modulator over non-cascaded modulator for 4 dB extinction ratio at 100 giga bit per second (Gbps)

This paper describes improved results when comparing cascaded traveling wave electro absorption modulator (TWEAM) to non-cascaded TWEAM by simulation. Large signal modeling is used for both types of modulators to achieve 4 and 10 dB extinction ratios (ERs) with flat frequency response for applications in short distance as well as long distance optical fiber communication. To obtain 4 and 10 dB ERs with 110 GHz 3 dB bandwidth, a cascaded TWEAM requires 0.4 V peak to peak (V_P-P) and 1 V_P-P input driving voltages respectively. A non-cascaded TWEAM requires about two times the input driving voltage compared to the cascaded modulator to achieve the same values of ER and 3 dB bandwidth. Both modulators have been simulated with the same bias and also use the same circuit parameters except for the total active segment lengths (1 and 0.5 mm for cascaded and non-cascaded modulator respectively) and microstrip lengths to obtain the same ERs and 3 dB bandwidths.     

基于取样光纤布拉格光栅的全光纤拉曼测温分光系统设计及优化

为实现大气温度全天时和高精度主动遥感探测, 转动拉曼测温激光雷达的分光系统需要滤除强烈的背景光噪声, 以及对Mie-Rayleigh散射提供70 dB以上的带外抑制率. 本文提出了以可见光波段取样光纤布拉格光栅为核心的多级级联的特征光谱提取光路, 构建高抑制率的全光纤拉曼测温分光系统, 以实现大气温度的全天时和高精度探测. 根据分光系统光路的传输特性, 采用传输矩阵模型, 优化设计了影响取样光纤布拉格光栅带外抑制率的主要因素(折射率调制深度、栅区总长度、取样周期和占空比), 得到了优化的光谱分光系统参数. 利用该分光系统可实现太阳背景光强度和Mie-Rayleigh散射信号强度分别比转动拉曼散射信号强度弱40 dB和50 dB, 信噪比高于100时, 白天探测高度可达1.6 km. 该全光纤分光系统具有小型化、抗干扰和稳定性高的优点, 可为陆基及星载拉曼测温激光雷达提供一种全新的解决方案.     

全光纤瓦级纳秒脉冲掺Yb放大器

采用单模放大和包层抽运放大的级联方式,在主振荡功率放大系统中,将平均功率0.5 mW、脉宽20 ns以及重复频率为50 kHz的光脉冲安全放大到平均功率0.6 W、峰值功率600 W的脉冲输出.相应增益为30.8 dB.放大输出的脉冲信号信噪比为30 dB,脉冲形状基本没有发生畸变.在前置放大结构中抽运光通过级联熔融拉锥型波分复用器耦合,有效确保了抽运源(976 nm)的正常工作.     

全光纤瓦级纳秒脉冲掺Yb放大器

采用单模放大和包层抽运放大的级联方式,在主振荡功率放大系统中,将平均功率0.5 mW、脉宽20 ns以及重复频率为50 kHz的光脉冲安全放大到平均功率0.6 W、峰值功率600 W的脉冲输出.相应增益为30.8 dB.放大输出的脉冲信号信噪比为30 dB,脉冲形状基本没有发生畸变.在前置放大结构中抽运光通过级联熔融拉锥型波分复用器耦合,有效确保了抽运源(976 nm)的正常工作.     

Linearization and dispersion tolerable technique for remote access unit in uplink RoF system based on gain-saturated RSOA cascaded EAM

A novel scheme of a remote access unit which is based on an electroabsorption modulator cascaded by a gain-saturated reflective semiconductor optical amplifier is proposed to improve a transmission performance of uplink signal. The cascaded gain-saturated reflective semiconductor optical amplifier plays a role in enhancing linearity of the electroabsorption modulator as well as suppressing dispersion-induced carrier suppression in the uplink transmission using its nonlinear gain property. In the proposed scheme, carrier-to-interference ratio of transmitted 10-GHz band uplink signal was improved by 10.7 dB and the dispersion-induced carrier suppression of 5.65-GHz RF carrier was greatly mitigated by 33.4 dB through the 75.6-km optical link.     

Power penalty of cascaded Bragg gratings in the presence of PMD and PDL

Considering the presence of birefringence in Bragg gratings, the spectral polarization mode dispersion (PMD) and polarization dependent loss (PDL) induced power penalty of eye opening is examined for a network with cascaded Bragg gratings. It is shown that when the birefringence in the grating is less than 10⁻⁵, the induced power penalty is less than 0.1 dB for a network with up to 30 cascaded Bragg gratings. However, when the grating birefringence is on the order of 10⁻⁴, the induced power penalty can be as high as 5.6 dB for a network with 14 cascaded Bragg gratings.     

Photonic generation of high quality frequency-tunable millimeter wave and terahertz wave

A scheme for the photonic generation of frequency-tunable millimeter wave and terahertz wave signals based on a highly flat optical frequency comb is proposed and demonstrated experimentally.The frequency comb is generated using two cascaded phase modulators(PMs)and an electro-absorption modulator(EAM).The frequency comb covers a 440-GHz frequency range,with 40-GHz comb spacing and less than 2-dB amplitude variation.By filtering out two of the comb lines with 50 dB out of the band suppression ratio,high frequency-purity and low phase noise millimeter wave or terahertz wave signals are successfully generated,with frequencies ranging from 40 to 440 GHz.     

A phase stable short pulse generator using a DPMZM and phase modulators for application in 160 GBaud DQPSK systems

A method of 40 GHz phase stable short pulses generation is experimentally demonstrated. It is based on a dual parallel Mach–Zehnder modulator (DPMZM) driven by only one electrical sinusoidal clock and two cascaded phase modulators. The generated pulses are characterized with full-width-at-half-maximum pulse width of 1.9 ps, extinction ratio of 27 dB, timing jitter of 36 fs and signal to noise ratio over 30 dB. The high quality and phase stability of the pulses are further experimentally verified in a 4 × 40 GBaud differential quadrature phase shift keying (DQPSK) optical-time-division-multiplexing (OTDM) system.     

Carrier suppression in quadruple frequency modulation by cascaded optical external modulators for millimeter-wave generation

The optical carrier suppression in optical quadruple frequency modulation by cascaded external modulators is investigated theoretically and experimentally. Theoretical analysis demonstrates that the optical carrier suppression ratio is related with not only the initial phase difference of electrical signals applied on the two modulators, but also the optical phase shift between the two modulators. The maximum suppression ratio can be achieved when the total phase difference is equal to nπ＋π/2 （n = 1, 2,…）, which is verified by experiments. By properly controlling the total phase shift, 40-GHz millimeter-wave is generated by using a 10-GHz radio frequency （RF） source and the modulators.     

Ultrafast optical switching by use of fully phase-matched cascaded second-order nonlinearities in a polarization-gate geometry

We show that cascaded second-order nonlinear-optical processes can occur in a convenient polarization-gate beam geometry. Our arrangement uses type II phase matching, and both individual second-order processes (upconversion and downconversion) are simultaneously phase matched. This geometry can be applied to efficient ultrafast optical switching. With a beta-barium borate crystal and lightly focused 250-fs, 7.3-microJ pulses, we achieve a switching efficiency of 15% and an on-off ratio of 3 x 10(4) on a pulse-length-limited time scale.     

双折射光纤环形镜滤波器特性研究

双折射光纤环形镜具有滤波特性,将两个串联的双折射环形镜作为光学滤波器可以改善滤波特性,使其更为灵活。提出了用两个双折射光纤环形镜相串联的简单方案改善环形镜滤波特性。分析了由双折射光纤和萨尼亚克环构成的环形滤波器的透射特性,并讨论了用两个双折射光纤环形镜串联的滤波特性。从理论上模拟了双折射光纤长度比为1：2时的透射光谱形式,给出了透射光的表达式。串联后光谱中主峰的3dB带宽约为7nm,与单个环形镜相比,压缩了环形镜的3dB带宽,品质因数Q约为4．5。实验结果与理论模拟相一致,因此证实了该理论的正确性。     

Compact polarization rotator on silicon for polarization-diversified circuits

We demonstrate a polarization rotator based on adiabatic mode evolution on silicon for polarization-diversified circuits. The rotator has a device length of 420?μm, a polarization-conversion efficiency of more than 90%, and an insertion loss less than 1?dB for a wavelength range of 80?nm. Combining the rotator with a compact, broadband polarization beam splitter based on cascaded directional couplers enhances the polarization conversion extinction ratio to over 30?dB with less than 1.5?dB total insertion loss over a 60?nm spectral range.     

X波段高功率微波一维相扫漏波天线阵

研究了一种可一维相扫的X波段高功率微波漏波天线阵,该天线阵由高功率微波功分网络、移相器和漏波平面阵组成。漏波平面阵由8个矩波导漏波线阵组成,增益29.6dB,口面效率70%,设计功率容量0.91GW;功分器网络采用圆波导TM01-双矩波导TE01模式变换和串列式矩波导功分器形式,输出端口间的不平衡度小于1.6dB,设计功率容量1.1GW;移相器采用侧壁簧片弯进改变矩波导宽度,实现0~360°移相,单路功率容量150MW。整阵相扫性能的全波仿真分析结果表明,在主瓣增益下降3dB的情况下,扫描角度可达到±40°。     

NRZ码全光帧头识别器研究

提出了一种基于太赫兹非对称光解复用器结构的NRZ码全光帧头识别方案.通过注入辅助的连续光,解决了信号在SOA中的竞争,实现三态到两态的转换.对40 Gb/s信号的数值分析表明,这种方案可获得消光比为13.29 dB的判决识别信号输出.对非线性相移差偏离的影响进行了分析,结果表明如果限制非线性相移差的偏离小于0.1 rad,仍然可获得消光比13 dB以上的判决识别信号输出.对2.5 Gb/s帧头信号的判决实验表明,理论分析结果与实验相符.     

Bandwidth scaling and spectral flatness enhancement of optical frequency combs from phase-modulated continuous-wave lasers using cascaded four-wave mixing

We introduce a new cascaded four-wave mixing technique that scales up the bandwidth of frequency combs generated by phase modulation of a continuous-wave (CW) laser while simultaneously enhancing the spectral flatness. As a result, we demonstrate a 10?GHz frequency comb with over 100?lines in a 10?dB bandwidth in which a record 75?lines are within a flatness of 1?dB. The cascaded four-wave mixing process increases the bandwidth of the initial comb generated by the modulation of a CW laser by a factor of five. The broadband comb has approximately quadratic spectral phase, which is compensated upon propagation in single-mode fiber, resulting in a 10?GHz train of 940?fs pulses.     

间隔叠合式双层亚波长光栅

提出了一种应用价值较高的防伪光栅结构-间隔叠合式双层亚波长光栅,可以展宽基本光栅的光谱峰值带宽、改善光谱线形和光变效果.分析了亚波长光栅光谱峰值带宽展宽和线形改善的原理,用矢量衍射理论验证了理论分析的合理性,并对特定防伪光变设计要求的双层光栅进行了参量优化和光变特性分析.结果表明,满足同相位条件的光谱线形与基本光栅相同,带宽为基本光栅的两倍;满足反相位条件的光谱线形为准矩形,带宽大于基本光栅;亚波长光栅在传统和非传统照明条件下倾斜均能产生彩色光变效果,且传统照明情况下的光变速率较大,利用这种光变速率差可以制作各向异性光变效果.     

Effect of variable particle size reinforcement on mechanical and wear properties of 6061Al–SiCp composite

Abstract

The influence of reinforcement particle size variation plays a major role on the properties of Al–SiC_p composite. Therefore, this study aim to investigate the mechanical and wear performance of single particle size (SPS) and multiple particle size (MPS) Al–SiC_p composite prepared by stir casting process. The SPS comprises three categories; fine (15 μm), intermediate (40 μm) and coarse (80 μm) particle sizes and combination of the three sizes accounts for the MPS in the ratio 1:1:2, respectively. Oxidation of the SiC_p and addition of 1 wt% Mg during composite processing resulted to interface reaction products such as MgO (magnesium oxide) and MgAl₂O₄ (magnesium aluminate) which suppresses the potential formation of undesired Al₃C₄ (aluminium carbide). The study reveals that MPS composite improved the hardness and impact properties with enhanced wear performance compared to SPS composite. Characterization of the composite morphology and phases was performed using scanning electron microscope and X-ray diffraction analysis. This study provides an effective method of optimizing the properties of Al–SiC_p composite by integrating MPS with low volume fraction of reinforcement phase.