10 Gb/s全光3R再生的研究

报道了对10Gb／s恶化数据信号的全光3R(再放大、再定时、再整形)再生的实验研究。针对利用基于半导体光放大器的注入锁模环形激光器进行时钟恢复时的码型效应问题,提出了利用梳状滤波器对信号进行预处理。利用这个技术极大的减小了由于码型效应造成的时钟信号的幅度和时间抖动,从恶化的数据信号中恢复出无码型效应的高质量光时钟脉冲。信号整形的关键是光判决门,性能良好的判决门可以进一步提高信号的消光比、减小抖动。研究了基于电吸收调制器的信号再整形。利用上述器件构成的3R再生器,实现了10Gb／s恶化信号的全光3R再生。     

Simultaneous chromatic dispersion monitoring,extinction ratio enhancement and wavelength conversion using four-wave mixing

This paper presents a multi-functional all-optical device performs simultaneous chromatic dispersion (CD) monitoring, extinction ratio (ER) improvement and tunable wavelength conversion using four-wave mixing (FWM) in a single piece of highly nonlinear optical fiber (HNLF). The multi-functional ability arises from the nonlinear power transfer function (PTF) between the input signal and the output idler wave, which not only maps the CD information of the signal onto the average power of idler wave, but also transfers the data information to the idler wave with ER enhancement, making the device compact and cost effective. Furthermore within the wavelength tuning range the monitoring sensitivity and output ER can be much higher compared to all optical monitors and regenerators proposed before. Numerical simulations are then used to demonstrate the efficiency of this method.     

Ultrafast all-optical signal processing with Symmetric Mach–Zehnder type all-optical switches

We present Symmetric Mach–Zehnder (SMZ) type all-optical switches: an SMZ all-optical switch, a polarization-discriminating SMZ (PD-SMZ) all-optical switch, and a delayed interference signal wavelength converter (DISC). These switches are capable of ultrafast, low control power, and low chirp switching, which is not restricted by slow relaxation of highly efficient nonlinearities. High repetition operation unrestricted by slow relaxation is also possible for these switches. This is because of a push–pull modulation scheme or sometimes called a differential phase modulation scheme. These three devices are similar, but different in some important aspects, thus a comparison is made among the three. Then semiconductor optical amplifiers (SOAs) are discussed as a nonlinear phase shifter for these devices. Then, ultrafast all-optical signal processing using SOA based SMZs is demonstrated. Error-free demultiplexing from 168 to 10.5 Gbit/s is presented, in which a hybrid-integrated SMZ (HI-SMZ) is used as a demultiplexer. In pulse regeneration experiment, the signal pulses at 84 Gbit/s are regenerated by the PD-SMZ and the regenerated pulses are demultiplexed to 10.5 Gbit/s by the HI-SMZ to verify error-free operation. The retiming capability of this scheme is quantitatively demonstrated. Also presented is error-free all-optical wavelength conversion at 168 Gbit/s using the DISC. These results represent the fastest error-free operations reported to date in each category.     

Picosecond cascadable inverter gate using second harmonic pumping

The possibility to realize one all-optical picosecond inverter or one NOR gate has been demonstrated several times. However, the difficult problem of cascading this device (i.e. to use one gate output for driving another gate) is still open. We report in this communication a first step towards simply cascadable logical NOR gates by demonstrating the possible operation of an all-optical inverter which uses an intermediate frequency doubling of the input signal. This implies that a cascadable all-optical NOR gate operating in an “effective” one-wavelength mode (i.e. with an intermediate frequency doubling) is possible. Experiments are carried out on a II–VI compound, namely CdSe, at liquid nitrogen temperature. The inverter operation is investigated around the wavelength 673 nm. The switching energy of the non-linear optical material in this work was 30 μJ/cm² at 337 nm (0.3 pJ/μm²).     

High speed logic gate using two-photon absorption in silicon waveguides

The switching speed of conventional silicon-based optical switching devices based on plasma dispersion effect is limited by the lifetime of free carriers which introduce either phase or absorption changes. Here we report an all-optical logic NOR gate which does not rely on free carriers but instead uses two-photon absorption. High speed operation was achieved using pump induced non-degenerate two-photon absorption inside the submicron size silicon wire waveguides. The device required low pulse energy (few pJ) for logic gate operation.     

Demonstration of nanophotonic NOT gate using near-field optically coupled quantum dots

We propose and demonstrate the operation of a nanometric optical NOT gate using CuCl quantum dots coupled via an optical near-field interaction. The device was smaller than 20 nm and its repeated operation was verified. The operating energy of this device was much lower than that of a conventional photonic device. We also introduce all-optical NAND and NOR gates using coupled quantum dots. Toward an actual nanophotonic device, we discuss the possibility of coupled InAlAs quantum dots. A double layer of InAlAs quantum dots for nanophotonic device operation was prepared using molecular beam epitaxial growth. We obtained a near-field spectroscopy signal, indicating that the InAlAs quantum dots coupled with the optical near field acted as a NOT gate. The experimental results show that the sample has great potential as an actual nanophotonic device. PACS 78.67.Hc; 07.79.Fc; 42.79.Ta     

Highly efficient second-harmonic generation in buried waveguides formed by annealed and reverse proton exchange in periodically poled lithium niobate

Efficient three-wave mixing devices have numerous applications, including wavelength conversion, dispersion compensation, and all-optical switching. Second-harmonic generation (SHG) is a useful diagnostic for near-degenerate operation of these devices. With buried waveguides formed in periodically poled lithium niobate by annealed and reverse proton exchange, we demonstrate what is believed to be the highest normalized conversion efficiency (150%/W cm(2)) for SHG in the 1550-nm communications band reported to date.     

Theoretical analysis and performance investigation of ultrafast all-optical Boolean XOR gate with semiconductor optical amplifier-assisted Sagnac interferometer

A comprehensive theoretical model of an ultrafast all-optical Boolean XOR gate implemented with a semiconductor optical amplifier (SOA)-assisted Sagnac interferometer is presented. The model accounts for the SOA small signal gain, linewidth enhancement factor and carrier lifetime, the switching pulses energy and width and the Sagnac loop asymmetry. By undertaking a detailed numerical simulation, the influence of these key parameters on the metrics that determine the quality of switching is thoroughly investigated and simple design rules are extracted for their proper selection so as to ensure optimum operation. The obtained results are in good agreement with the published experimental measurements and confirm the feasibility of realizing the gate at 10 Gb/s with fairly high performance. The model can be extended for studying more complex all-optical circuits of enhanced functionality in which the XOR gate is the basic building block.     

Flip-flop operation in opto-thermal bistable devices with localized absorption

We consider nonlinear interferometric devices based on opto-thermal bistability with localized absorption and show how the introduction of composite structures within the interferometer makes feasible its setting and resetting between stable outputs by applying positive light pulses on different parts of the bistable device. Experimental demonstration of all-optical flip-flop operation is reported for two interferometric configurations, one of them being extremely simple and well suited for parallel processing.     

多波长飞秒激光激发下GaAs纳米线SHG特性研究

本文基于有限元法研究了直立生长于GaAs衬底的GaAs纳米线的光场响应和光场增强性质. 实验使用多个波长的飞秒激光脉冲激发GaAs纳米线, 测得了较高效率的二次谐波信号, 并首次使用宽带超连续飞秒脉冲 (1000–1300 nm) 在纳米线上获取了宽带、无杂散荧光噪声的二次谐波信号. 这种高效的二次谐波产生过程主要归因于纳米结构引起的局域场增强效应. 本文阐明了GaAs纳米线的二次谐波倍频特性, 这些结果对于其在纳米光学中的光器件、 光集成等领域的进一步研究和实际应用具有很好的参考价值. 关键词： GaAs纳米线 二次谐波 飞秒激光     

Ultra-high on/off ratio optical trigger based on SOI–SOA waveguide

We present a device able to perform an all-optical trigger by utilizing the cascaded silicon-on-insulator (SOI) and semiconductor-optical-amplifier (SOA) optical waveguides, where both the controlling pulse and the signal pulse are co-propagating along the device. The results of numerical simulations show that the all-optical trigger behavior with ultra-high on/off ratio can be easily achieved by adjusting the device parameters. To improve and optimize the optical trigger operation, the negative initial delay (the signal pulse before the controlling pulse) and the positive initial delay (the signal pulse after the controlling pulse) are considered for performing the optical trigger operation.     

Silicon photonic crystal all-optical logic gates

All-optical logic gates, including OR, XOR, NOT, XNOR, and NAND gates, are realized theoretically in a two-dimensional silicon photonic crystal using the light beam interference effect. The ingenious photonic crystal waveguide component design, the precisely controlled optical path difference, and the elaborate device configuration ensure the simultaneous realization of five types of logic gate with low-power and a contrast ratio between the logic states of “1” and “0” as high as 20 dB. High power is not necessary for operation of these logic gate devices. This offers a simple and effective approach for the realization of integrated all-optical logic devices.     

Development of coherent tunable source in 2–16 μm region using nonlinear frequency mixing processes

A very convenient way to obtain widely tunable source of coherent radiation in the infrared region is through nonlinear frequency mixing processes like second harmonic generation (SHG), difference-frequency mixing (DFM) or optical parametric oscillation (OPO). Using commonly available Nd:YAG laser and its harmonic pumped dye laser radiation as parent beams, we have been able to generate coherent tunable infrared radiation (IR) in 2–16 μm region using different nonlinear crystals by DFM and OPO. We have also generated such IR source in the 4–5 μm region through SHG of CO₂ laser in different infrared crystals. In the process we have characterized a large number of nonlinear crystals like different borate group of crystals, KTP, KTA, LiIO₃, MgO:LiNbO₃, GaSe, AgGaSe₂, ZnGeP₂, AgGa_1?x In _x Se₂, HgGa₂S₄ etc. To improve the conversion efficiencies of such frequency conversion processes, we have developed some novel schemes, like multipass configuration (MC) and positive optical feedback (POF). The significance of the obtained results lies in the fact that to get the same conversion in SHG or DFM, one now requires fundamental input radiation with much lower intensity.     

On the feasibility of ultrafast all-optical NAND gate using single quantum-dot semiconductor optical amplifier-based Mach–Zehnder interferometer

The feasibility of implementing an all-optical NAND gate for 160 Gb/s return-to-zero data pulses using a single quantum-dot semiconductor optical amplifier (QD-SOA)-based Mach–Zehnder interferometer is theoretically investigated and demonstrated. The proposed scheme exploits a modified Fredkin gate simultaneously driven by the pair of data streams between which the Boolean NAND function is to be executed, a sequence of continuous pulses and the complement of the first data input. The impact of the peak data power as well as of the QD-SOAs current density, small signal gain and electron relaxation time from the excited state to the ground state on the amplitude modulation of the switching outcome is explored and assessed by means of numerical simulation. The interpretation of the obtained results allows to specify the conditions under which the QD-SOAs must be biased to operate so that the defined performance metric becomes acceptable. By following the extracted guidelines whose satisfaction is technologically feasible and making a suitable choice for the critical parameters the NAND gate can be realized both with logical correctness and high quality at the target ultrafast data rate while being cascadable and scaleable for constructing more complex all-optical circuits.     

Effects of pattern dependence on high-power polarization-division-multiplexing applications

High-power polarization-division-multiplexing (PDM) systems or functional modules, such as self-phase-modulation (SPM)-based all-optical regenerators, cross-phase-modulation (XPM)-based wavelength convertors or format convertors, all-optical logical gate, and so on, may suffer from the effects of pattern dependence. Such effects are experimentally investigated using relative time delay variation between bit sequences with orthogonal polarization states in a 2 × 10.65 Gb/s high-power on-off keying (OOK) PDM system. Eye-diagram-based signal-to-noise ratio (SNR) and bandwidth of broadened spectrum are measured and compared. An eye-diagram-based SNR fluctuation of up to 4 dB may occur as the delay changes.     

Analysis of an optical logic gate using a symmetric coupler operating with pulse position modulation (PPM)

In this paper we propose the operation of an all-optical logical gate based in a symmetric nonlinear directional coupler (NLDC) operating with a pulse position modulation (PPM). The performance of a symmetric NLDC realizing two-input AND/OR logical functions, which can be applied in transmission and processing of signals in all-optical form in TDM systems, is examined. This integrated symmetric NLDC logical gate operates with two ultrashort soliton light pulses (2 ps), which are modulated in agreement with the technique of pulse position modulation (PPM). Initially, we evaluate the effect resulting of an increment in the PPM coding parameter offset (ε), for the temporal position of the output pulse, considering the anomalous group-velocity dispersion (GVD), nonlinear self phase modulation (SPM) and without loss propagation regime of input pulses, in the cores 1 and 2 of the NLDC. In this situation, we analyze the four possible situations for the two-input logical gate, modulating the 1 and 2 input pulses through temporal displacement and allowing a variation in the coding parameter offset. We can conclude that is possible to get AND/OR logical operations for the cores 1 or 2, without to insert PPM error, since a phase control (Δ?) exists applied in agreement with the logical level of the input pulse in the core 1. Finally we define the truth table, considering the adequate phase difference and coding parameter offset for the stable operation of the AND/OR logical gate based in the symmetric NLDC.     

All-optical flip-flop based on an active Mach-Zehnder interferometer with a feedback loop

A novel architecture for an all-optical flip-flop is validated experimentally. The architecture comprises a single semiconductor optical amplifier-based Mach-Zehnder interferometer with an external feedback loop. The experimental results show optical bistable operation for a latching device with an on-off contrast ratio of 11 dB that employs set and reset pulses of less than 250 pJ, although the energy of these pulses could be greatly reduced by optical integration of the whole device.     

All-Optical Logic Device Using Bent Nonlinear Tapered Y-Junction Waveguide Structure

We propose an all-optical logic device made of a bent tapered Y-junction waveguide with a Kerr-type nonlinear interface. It could provide an AND gate, OR gate, and exclusive-OR (XOR) gate. We could obtain different transmission results by adjusting the bending angle. The numerical simulation results show that the device functions as AND, OR, and XOR gates.     

Differential group delay monitoring using an all-optical signal spectrum-analyser

We demonstrate an all-optical device that monitors differential group delay (DGD) degradation of picosecond optical pulses. This device is based on an ultrafast all-optical signal analyser that uses nonlinear effects (cross-phase modulation) to transfer rapid temporal fluctuations into frequency domain effects that can be measured on an conventional optical spectrum analyser (incorporating a slow-detector). This monitoring scheme will enable rapid dynamic monitoring and compensation of DGD in ultrafast optical networks, at 160 Gb/s data rates and beyond, where electronic monitoring techniques cannot operate. We discuss the required signal polarisation condition.     

Approximately analytical optimization of Cherenkov cascading second order nonlinear effects in a dissipative waveguide

The coupled-mode equation in dissipative waveguides in Cherenkov configuration has been detailedly discussed for the first time, to our knowledge. Based on this equation, Cherenkov second harmonic generation (SHG) and Cherenkov nonlinear phase shift (NPS) caused by Cherenkov cascading second order nonlinear (CSON) effects have been analyzed and optimized comprehensively and systematically by an analytical method, taking into account both the conversion depletion and the waveguide loss, and the analytical expressions for Cherenkov SHG and NPS has been presented. The dependence of the NPS and the conversion efficiency on varied parameters, such as the waveguide thickness, the fundamental wavelength, the input fundamental power and the temperature, has been discussed completely. The results show that a large enough NPS (>10π) can be attained for nondissipative waveguides with a very small depletion of the fundamental power, at the same time, this Cherenkov CSON configuration has some advantages over the quasi-phase matching CSON configuration, e.g., the much looser tolerances of the waveguide parameters and the noncollinear feature of the fundamental wave and the second harmonic wave, these characteristics make the Cherenkov CSON configuration a promising direction to realize all-optical devices.