Cladding layer thickness-controlled directional coupler opticalswitch

The AlGaAs/GaAs cladding layer thickness-controlled directional coupler optical switch (CTC-SW) is discussed. To increase the efficiency of coupling between two waveguides, the CTC-SW was designed with a thin i-AlGaAs cladding layer h between the two waveguides. The coupling length L_c can be reduced, without reduction of the extinction ratio and increase of the switching voltage. For instance, L_c of CTC-SW (h=0.07) can be reduced 40% more than that of a conventional directional coupler switch (h=0). The extinction ratio is near 20 dB, both for the crossover (applied voltage V=0 V) and straight-through (V =19 V) states. A technique for adjusting L_c to the device length was developed     

Add-drop multiplexer with UV-written Bragg gratings and directionalcoupler in SiO2-Si integrated waveguides

The needs of wavelength multiplexers is growing and therefore optical add-drop multiplexers (OADMs) become key components for the advanced high bit rate telecommunication networks. Currently, one of the solutions to make OADMs use fiber Bragg gratings and circulators. In order to take advantage of integrated optics and to avoid the use of expensive circulators, we perform this kind of OADM in ridge silica waveguide on silicon. This technology makes possible short based directional coupler (DC) device. In previous work, we firstly realized an OADM which involves a symmetrical directional coupler combined with two Bragg gratings inside the coupling zone. The results meet difficult tradeoff between a high extinction ratio and a narrow spectral width of the rejection band. This paper reports some fabrication improvements for DC-OADM that lead to more efficient components, progress results from a lot of works concerning both the photo-writing process of Bragg gratings and the lowering of the waveguide internal stresses. Thus, we show that apodization of gratings and low birefringence in the waveguides enables us to achieve DC-OADM with an extinction ratio of 30 dB and a spectral bandwidth less than 1.5 nm     

High performance fused-type mode-selective coupler using ellipticalcore two-mode fiber at 1550 nm

In this letter, we demonstrate a high performance fused-type mode selective coupler that couples the LP₁₁ mode in one fiber and the LP₀₁ mode in another using highly elliptical core two-mode fibers. The phase-matching condition was achieved by etching and prepulling portions of two-mode fibers. The coupling efficiency and the mode extinction ratio of 56% to 80% and 22-32 dB, respectively, were achieved with high temperature stability over 1515- to 1595-nm wavelength range     

Nonlinear optical limiter and digital optical switch by cascadednonlinear couplers: analysis

In this paper, an all-optical digital switch and a limiter are proposed and analyzed, both of which consist of several nonlinear directional couplers in series. These two devices utilize the nonlinear restraining effect of the nonlinear coherent coupler on its input power. In the devices, the OFF-ON extinction ratio, the output power fluctuation and the dynamic switching range all decrease with the number of the couplers in series increasing, and the output power level can be adjusted by varying the coupling length. The characteristics of these devices with different structures are discussed, and the numerical results show that the OFF-ON extinction ratio and the dynamic switching range of the optical switches can be less than -90 dB and 0.03P_c /100, respectively, and that the fluctuation of the limiting output power of the optical limiter can be less than 10^-7P _c     

Channel management in microcell/macrocell cellular radio systems

In this paper, we study spectrum management in a two-tier microcell/macrocell cellular system. Two issues are studied: micro-macro cell selection and frequency spectrum partitioning between microcells and macrocells. To keep the handoff rate in a two-tier cellular system at an acceptable level, low mobility users (with speed υ0 ) should undergo handoffs at microcell boundaries, and high mobility users (with speed υ>V₀) should undergo handoffs at macrocell boundaries. The mobile determines user mobility from microcell sojourn times and uses it for channel assignment at call origination and handoff. The probability of erroneous assignment of a mobile to a microcell or macrocell is shown to be significantly lower than previous approaches. We investigate the optimal velocity threshold, V₀, and propose that it may be dynamically adjusted according to traffic load. Finally, we propose a systematic way for finding an optimal partition of frequency spectrum between microcells and macrocells. This partitioning is based on the traffic load and velocity distribution of mobiles in the system     

Polarization-selective mode coupling in two-mode Hi-Bi fibers

We propose and demonstrate a mode coupler which converts either of the LP₀₁ polarization states in a two-mode high-birefringence (Hi-Bi) fiber to the LP₁₁ mode with the same polarization. We use coupled-mode theory to develop design rules based on the polarization splitting of the beat length between the two lower-order modes. The device can be operated either as a narrow-band device in a region with large intermodal group delay difference or as a broadband device in a region with zero group delay difference. We use this novel device as a key component in a two-mode Hi-Bi fiber polarizer. In this configuration either of the polarization eigenstates can be selected and transmitted with an adjustable extinction ratio which can be as large as 30 dB. The coupling loss in the transmitted state of polarization can be less than 0.2 dB. In the broadband polarizer we demonstrate -20 dB extinction over 42 nm with a potential for considerable improvement. We also propose and experimentally investigate an increased differential group delay obtained by propagating one polarization state in the LP₁₁ mode instead of in the LP₀₁ mode. The largest differential group delay measured in this configuration is 14.5 ps/m which is seven times larger than the differential group delay between the polarization modes. We discuss several possible uses in fiber sensors and measure the transmission of a proposed two-coupler configuration     

A Compact Slow-Wave Microstrip Branch-Line Coupler With High Performance

One compact slow-wave microstrip branch-line coupler is presented. The new structure not only effectively reduces the occupied area to 28% of the conventional branch-line coupler at 2.0 GHz, but also has high second harmonic suppression performance. The measured results indicate a bandwidth of more than 200 MHz has been achieved while the phase difference between S₂₁ and S₃₁ is within 90deg plusmn 1deg. Furthermore, the measured insertion loss is comparable to that of a conventional branch-line coupler. The new coupler can be easily implemented by using the standard printed-circuit-board etching processes and is very useful for wireless communication systems.     

Theoretical and experimental study of 10 Gb/s transmissionperformance using 1.55 μm LiNbO3-based transmitters withadjustable extinction ratio and chirp

This paper has experimentally and theoretically investigated transmission performance depending on chirping and extinction ratio for a 10 Gb/s transmission system with the standard single-mode fiber. The transmission performance can be dramatically degraded or improved by adjusting chirp and extinction ratio in a 1.55 μm LiNbO₃ modulator-based transmitter and erbium-doped fiber amplifier (EDFA)-pin diode receiver configuration. To estimate the transmission performance, bit error rate (BER) characteristics rather than eye-opening penalty (EOP) have been calculated by solving the nonlinear Schrodinger equation with including the model of chirping and extinction ratio for the transmitter, and noise and intersymbol interference for the receiver. This simulation can predict the measured BER characteristics well enough to see interplaying between chirping and extinction ratio     

Adaptive H∞ channel equalization for wirelesspersonal communications

In this paper, a new adaptive H_∞ filtering algorithm is developed to recursively update the tap-coefficient vector of a decision feedback equalizer (DFE) in order to adaptively equalize the time-variant dispersive fading channel of a high-rate indoor wireless personal communication system. Different from conventional L ₂ (such as the recursive least squares (RLS)) filtering algorithms which minimize the squared equalization error, the adaptive H _∞ filtering algorithm is a worst case optimization. It minimizes the effect of the worst disturbances (including input noise and modeling error) on the equalization error. Hence, the DFE with the adaptive H_∞ filtering algorithm is more robust to the disturbances than that with the RLS algorithm. Computer simulation demonstrates that better transmission performance can be achieved using the adaptive H_∞ algorithm when the received signal-to-noise ratio (SNR) is larger than 20 dB     

Fluctuational crosstalk in Ti:LiNbO3 directional couplerswitches

It has been found that the fluctuations in the Ti concentration along waveguides of Ti:LiNbO₃ directional coupler switches degrade the switch extinction ratio and introduce an unwanted crosstalk between both channels. We discuss analytically the effect of effective optical index fluctuations on the transfer matrix of the optical switch. We demonstrate the sensitivity of the mismatch to the statistics of the fluctuations of the Ti concentration     

Transmission performance of 10-Gb/s optical duobinary transmissionsystems considering adjustable chirp of nonideal LiNbO3Mach-Zehnder modulators due to applied voltage ratio and filterbandwidth

We investigate interplay between the residual and applied chirp of optical duobinary modulated signals in order to improve transmission performance. To find the best performance for 10-Gb/s optical duobinary transmission systems, we consider the residual chirp accompanying from the finite extinction ratio, the applied chirp adjusted by the applied voltage ratio (the chirp parameter) between two electrodes of LiNbO₃ modulators, and the bandwidth of electrical low-pass filters used in duobinary transmitters. The simulation results suggest that nearly zero chirp during the mark (`1') period and large peak chirp at the middle of the space (`0') provide the best transmission performance. This zero chirp around marks and high peak chirp at the middle of each space can be controlled by the applied voltage ratio between two electrodes of modulator and the filter bandwidth, respectively     

Further investigations of a 2-μm Tm:YVO4 laser

The performance of a Tm:YVO₄ laser has been optimized by varying the thickness of the laser crystal and the reflectivity of the output coupler. This has led to the measurement of a slope efficiency of 48% for the laser output. Formulas describing the saturation of the pump and the upper laser level in the presence of up-conversion loss have been developed. Fitting of the experimental data to the saturation curves calculated using these formulas has yielded an up-conversion rate constant from ³F₄ to ³ H₅ of 2·10^-18 cm³ s^-1      

A 1.3 μm directional coupler polarization splitter by ionexchange

A glass waveguide polarization splitter for operation in the 1.3 μm wavelength region is reported. The device, which has a symmetric directional coupler configuration, exploits the stress-induced birefringence in K⁺-Na⁺, ion-exchanged waveguides, giving rise to an adequate difference in the coupling lengths for the two polarizations. Starting from the measured potassium concentration (refractive index) profile of the structure and utilizing a combination of the multilayer stack theory and the effective-index method, the normal mode propagation constants and mode field profiles are calculated to determine the polarization splitting length and the extinction ratio, and the results are compared with the experimental data. It is shown that in a given coupler, the splitting occurs at several wavelengths in the 1.0-1.45 μm range. A 25 mm-long coupler, fabricated by thermal diffusion of K⁺ ions in BK7 glass, exhibits an extinction ratio of 18.2 dB at 1.32 μm, in excellent agreement with the simulation results     

分块压缩感知的全变差正则化重构算法

针对分块压缩感知(BCS)重建图像质量较差问题,该文提出一种最小化l₀范数的分块压缩感知全变差(TV)正则化迭代阈值图像重构算法(BCS-TVIT)。BCS-TVIT算法考虑图像的局部平滑、有界变差等性质,将最小化l₀范数与图像的全变差TV正则项结合,构建目标函数。针对目标函数中l₀范数项和分块测量约束项无法直接优化问题,采用迭代阈值法使重构图像l₀范数最小化,并通过凸集投影保证满足约束条件,完成了目标函数的优化求解。实验表明,与基于l₀范数最小化的分块压缩感知平滑投影算法(BCS-SPL)相比,BCS-TVIT算法重构图像峰值信噪比提高2 dB,能消除BCS-SPL的“亮斑”效应,且在视觉效果上明显优于BCS-SPL算法;与最小全变差算法相比,BCS-TVIT算法重构图像峰值信噪比提升1 dB,且能降低重构时间约2个数量级。     

High-Speed (60 GHz) and Low-Voltage-Driving Electroabsorption Modulator Using Two-Consecutive-Steps Selective-Undercut-Wet-Etching Waveguide

Based on a novel structure of waveguide, a broadband electroabsorption modulator (EAM) with low driving voltage and high extinction ratio has been demonstrated in this letter. The waveguide of InGaAsP-InP p-i-n layer structure is fabricated by two consecutive steps of selective undercut-wet-etching: 1)HCl : H₃PO₄ on p-InP (p- layer), and 2)H₃PO₄ : H₂O₂ : H₂O on InGaAsP (active region), showing a wide ridge with a narrow undercut active region. Low capacitance and low cladding impedance can thus be simultaneously attained in such waveguides, leading to low microwave loss and high-speed electrooptical (EO) response. A ridge as wide as 8 mum with a 3-mum- wide active region and a 450-nm gap height in the undercut portion has been fabricated. A 350- mum -long waveguide of EAM is designed, revealing a high extinction ratio of > 30 dB (D.C.) and a modulation efficiency of > 20 dB/V (D.C.) with polarization-insensitive operation at a wavelength of 1550 nm. As high as 60 GHz of a 3-dB bandwidth is measured in the high-speed EO conversion. Calculations by an equivalent circuit model are quite fitted with the measurement, revealing that broadband performance is mainly attributed to the low microwave propagation loss in such waveguides.     

宝鸡高新区冬季大气消光系数及其组成特征

为了解宝鸡市高新区气溶胶及气体的消光特性,利用宝鸡市高新区2018年1月浊度仪和 黑碳仪的逐时观测数据,以及同时段的PM2.5、NO2质量浓度和气象数据,分析了冬季大气 消光系数Bext、气溶胶散射系数Bsp、吸收系数Bap与气体吸收系数Bag的变化特征,并估算了气溶胶消光系数和气体消光系数对大气消光的贡献率。结果表明,宝鸡高新区Bext范围为200.63~948.87 Mm^-1, 均值为483.12 Mm^-1; Bsp范围为170.83~890.06 Mm^-1, 均值为444.11 Mm^-1; Bap范围为4.89~20.44 Mm^-1,均值为9.69 Mm^-1。 PM2.5与Bext、Bsp和Bap的相关系 数分别为0.952、0.950、0.572,说明宝鸡市高新区PM2.5对大气消光效应的影响不容忽视。 Bsp与相对湿度(RH)呈显著正相关,与能见度呈显著负相关。Bsp和 Bap均具有显著的双峰双谷型变化特征,这与宝鸡高新区早晚高峰、机动车运行 时间有关。黑碳(Black carbon, BC)质量浓度、Bsp和Bap随污染 等级增加而升高。整体分析表明,气溶胶消光系数占大气消光系数93.93%,其中散射 系数Bsp和吸收系数Bap分别占91.92%和2.01%,而气体消光系数 占总大气消光系数的6.07%。     

Fabrication of LiTaO3 optical waveguides by Ni diffusionbelow the Curie temperature

Using Ni diffusion into LiTaO₃ below the Curie temperature for the fabrication of optical waveguides was presented for the first lime. Ordinary and extraordinary polarization waveguide modes were obtained. The index distribution profiles of both modes were measured by a prism coupler. Loss in planar waveguides at 0.6328 μm is 0.7±0.1 dB/cm for the ordinary mode and 1.3±0.2 dB/cm for the extraordinary mode. Moreover, a Mach-Zehnder interferometer was fabricated by this method for electrooptic characterization. The measured half-wave voltage is 5.5 V and the extinction ratio is greater than 29 dB     

Effect of intrinsic phase mismatch on linear modulator performanceof the 1×2 directional coupler and Mach-Zehnderinterferometer

The performance of 1×2 directional couplers and Mach-Zehnder interferometers as linear modulators is compared. Models describing the operation of each device, incorporating the intrinsic phase mismatch Δβ₀ which is present in nonideal devices, are developed. Distributions of Δβ₀ are obtained for each device and are shown to be independent of device type. Linear dynamic range and sensitivity measurements showing how Δβ₀ affects performance agree with theory. The additional effect of the coupling coefficient in the 1×2 directional coupler is demonstrated. This information can be used to predict device yield when combined with the measured Δβ₀ distributions     

Technique for velocity-matched traveling-wave electroopticmodulator in AlGaAs/GaAs

A design for a velocity-matched traveling-wave directional-coupler intensity modulator in AlGaAs/GaAs is proposed. The structure utilizes a thin coating of Ta₂O₅ on the top of the modulator/electrode structure in order to achieve velocity matching between the optical wave and microwave signal. The film does not significantly affect the optical properties or voltage requirements of the modulator. The optical and RF characteristics of the modulator are analyzed using the effective-index and finite-difference methods. The optical bandwidth is calculated numerically, taking into account both the anticipated velocity mismatches due to fabrication tolerances and the calculated frequency-dependent microwave losses. The predicted small-signal bandwidth of a 3 mm long direction coupler biased at a null point is greater than 45 GHz, and exceeds 100 GHz (~50 GHz electrical bandwidth) when the coupler is biased in the linear region. This device is designed to operate at 830 nm with a maximum modulation voltage of 5 V. The figure of merit of the proposed device is therefore at least 10 GHz/V when an electrical bandwidth of 50 GHz is used