Characteristics of wavelength conversion of short optical pulses in quantum dot semiconductor optical amplifiers

The characteristics of short optical pulse four-wave mixing (FWM) and amplification in quantum dot semiconductor optical amplifiers (QD-SOAs) are investigated taken into account the effect of the multi-discrete QD energy levels. Different saturation and recovery response for the electron and hole states are observed, which is attributed to different energy spacing between the energy states. We found that the 3 dB saturation energy of QD-SOA depends on the pulse width for short input pulses. Also, the optimum time delay between the probe and pump pulses in QD-SOAs, which provides maximum FWM efficiency in QD-SOAs, is smaller than the optimum delay in quantum well SOA.     

基于光纤四波混频波长转换和色散的慢光实验研究

对基于光纤四波混频（FWM）波长转换和色散的慢光实现进行了详细和系统的实验研究.首先,实验测定了高非线性光纤中FWM带宽约为40 nm,从而确定了慢光的可调谐带宽;接着,在普通单模光纤和色散补偿光纤（DCF）中针对500 MHz正弦信号和100 ps短脉冲信号分别实现了34和198 ns的脉冲延迟,在DCF中还实现了209 ns的脉冲提前.讨论了增大延迟量的方法,指出随着宽带FWM波长转换的实现和大色散光纤的应用有望获得微秒量级的大延迟量,从而为高性能光纤延迟线和全光缓存器等应用提供支持. 关键词： 慢光 四波混频 色散     

Determination of Zero-Dispersion Wavelength by Four-Wave Mixing

We present an improved approach to determine the zero-dispersion wavelength by measurement of the four-wave mixing （FWM） effect employing the two-tunable-laser scanning method. The FWM behaviour of combined fibres with two different zero-dispersion wavelengths is investigated theoretically and experimentally. The results are compared with those by regular zero-dispersion wavelength test instrument using phase shift technique. The theoretical and experimental results confirm the feasibility of determination of zero-dispersion wavelength by FWM.     

Compensation of group-velocity mismatch in the frequency-doubling modelocker

We propose a method for reducing the deteriorating effect of group-velocity mismatch on the frequency-doubling modelocker time response. A birefringent plate is used to introduce a time delay between the fundamental and the second-harmonic pulses which is opposite in sign to that accumulated in the frequency-doubling crystal. In the new arrangement the modelocker may provide pulse compression at a sub-picosecond time scale     

Enhanced four-wave mixing in a hollow-core photonic-crystal fiber

Hollow-core photonic-crystal fibers are shown to substantially enhance four-wave mixing (FWM) of laser pulses in a gas filling the fiber core. Picosecond pulses of Nd:YAG fundamental radiation and its second harmonic are used to generate a signal at the frequency of the third harmonic by the FWM process 3omega = 2omega + 2omega - omega. The efficiency achieved for this process in a 9-cm-long, 13-microm-hollow-core-diameter photonic-crystal fiber, designed to simultaneously transmit a two-color pump and the FWM signal, is shown to be approximately 800 times higher than the maximum FWM efficiency attainable with the same laser pulses in the tight-focusing regime.     

Optimized all-optical amplitude reshaping by exploiting nonlinear phase shift in fiber for degenerated FWM

We describe a novel performance optimization method for all-optical amplitude reshaping via degenerated four-wave mixing(FWM) in highly nonlinear optical fiber.The proposed optimization method is achieved by judiciously configuring the FWM operational condition and exploiting the nonlinear phase shift induced by self-and cross-phase modulations to properly influence the FWM conversion efficiency.Through the proposed scheme,fully functional and prevailing reshaping performance,including significant amplitude jitter suppression and extinction ratio improvement,is obtained within a single FWM stage.Results of the present theoretical calculation and numerical simulation verify the feasibility and advantages of the proposed scheme.     

Slow and fast light in quantum-well and quantum-dot semiconductor optical amplifiersInvited Paper

Slow and fast light in quantum-well (QW) and quantum-dot (QD) semiconductor optical amplifiers (SOAs) using nonlinear quantum optical effects are presented. We demonstrate electrical and optical controls of fast light using the coherent population oscillation (CPO) and four wave mixing (FWM) in the gain regime of QW SOAs. We then consider the dependence on the wavelength and modal gain of the pump in QW SOAs. To enhance the tunable photonic delay of a single QW SOA, we explore a serial cascade of multiple amplifiers. A model for the number of QW SOAs in series with variable optical attenuation is developed and matched to the experimental data. We demonstrate the scaling law and the bandwidth control by using the serial cascade of multiple QW SOAs. Experimentally, we achieve a phase change of 160° and a scaling factor of four at 1 GHz using the cascade of four QW SOAs. Finally, we investigate CPO and FWM slow and fast light of QD SOAs. The experiment shows that the bandwidth of the time delay as a function of the modulation frequency changes in the absorption and gain regimes due to the carrier-lifetime variation. The tunable phase shift in QD SOA is compared between the ground- and first excited-state transitions with different modal gains.     

FDTD simulation of wavelength conversion of ultrashort pulses utilizing intersubband absorption in AlGaN/GaN quantum wells

The four-wave mixing (FWM) characteristics of 500-fs pulses in nonlinear optical waveguides utilizing the saturation of the intersubband absorption at 1.55 μm in nitride multiple quantum wells are calculated by a one-dimensional finite-difference time-domain (FDTD) method combined with three-level rate equations describing the intersubband carrier dynamics. Efficiency for a 100-nm wavelength conversion in a 160-μm waveguide is predicted to be higher than 3% both for up- and down-conversions. The extinction ratio is higher than 20 dB. As for the short-pulse application, however, the conversion efficiency is strongly dependent on the phase difference between the pump and signal pulses, especially when the efficiency is high. This causes ambiguity in the output power. Therefore, the cross-loss-modulation (XLM) converter is preferable to the FWM converter for OTDM application. The FWM converters are considered to be applicable to simultaneous conversion of WDM signals.     

DWDM系统非简并四波混频串扰的分析

考虑到DWDM系统中的比特序列的随机性、信道间脉冲走离效应等多种因素的影响,完善了归零调制格式下的四波混频效率的计算方法,给出了相应的非简并四波混频噪声标准差的计算公式,并利用该公式进行实例分析,计算结果表明,当占空比和信道间隔变小时,不同信道内的随机比特序列的相对初始时延及光纤色散值的取值组合对四波混频效率的影响变得更大,为保证系统的低色散和低四波混频串扰特性,必须对这些量取值组合进行优化. 关键词： 非简并四波混频 脉冲走离 密集波分复用     

Investigation of plumes produced by material ablation with two time-delayed femtosecond laser pulses

We experimentally investigated and herewith reported the results of laser ablation of copper and gold with two time-delayed femtosecond laser pulses at 800 nm in vacuum. The ablation plume dynamic was monitored by fast plume imaging and time- and space-resolved optical emission spectroscopy. Optical microscopy was used to follow the ablation depth as a function of the delay between the two laser pulses. Nanoparticles deposition on mica substrates was analysed by atomic force microscopy.We estimate roughly the plume's atomization degree - that is the mass fraction of atomized material over the total ablated mass - from the relative intensities of radiation emitted from atoms and nanoparticles. It is shown that the atomization degree depends critically on the time delay between both laser pulses and on the characteristic time of electron-lattice relaxation. The increase of the atomization degree is accompanied by the decrease of the ablation depth. Atomic force microscopy measurements confirm the partial atomization of nanoparticles, as the analyses of particle deposition on mica substrates show a large decrease of the number of nanoparticles for large delay between the two pulses.     

Ultra-short optical pulse shaping using semiconductor optical amplifier

Optical ultra-short pulse compression and amplification using semiconductor optical amplifier (SOA) is presented. Using pump-probe pulse configuration, we present an SOA model which includes the nonlinear effects such as, spectral hole burning (SHB), carrier heating (CH), two-photon absorption (TPA) and group velocity dispersion (GVD) taking into account gain spectrum effect. Then by adjusting time delay between the pump pulse and probe pulse we use the model for simultaneous compression and amplification of probe pulse. We also analyze the four wave mixing (FWM) signal during pulse compression process. It is shown that dispersive effect of GVD on output probe pulse becomes more important for larger cavity length and probe-pump pulses relative time delays.     

Efficient generation of maximally entangled states via four-wave mixing in a semiconductor quantum-dot nanostructure

We investigate the generation of the maximally entangled state of two weak-light pulses (the probe and generated pulses) via four-wave mixing (FWM) in a semiconductor quantum dot (SQD) with a biexciton-exciton cascade configuration. The results show that this maximally entangled state can propagate with an ultraslow group velocity under suitable parameter conditions. For application, our proposed scheme is probably achievable with the present technology by applying the standard GaAs/InGaAs self-assemble quantum dots (QDs). Furthermore, our calculations provide a guideline for the realization of the maximally entangled state in the SQD solid-state system, which can be much more practical than that in an atomic system because of its flexible design and the wide tunable parameters.     

Kerr non-linearity and four-wave mixing in a double-cascade type four-level system of multiple quantum wells

We study the non-linear optical response in multiple quantum wells structure with a double-cascade type four-level configuration based on excitons and biexcitons transitions. By analysing the Kerr non-linear effects, we obtain the slow, mutually matched group velocities and giant Kerr non-linearity of probe and signal fields. While when the signal (or probe) field is removed, the non-linear optical phenomenon four-wave mixing (FWM) originating from quantum interference is demonstrated. The FWM efficiency of the system study is about 50%. Such a semiconductor system is much more practical than its atomic counterpart because of its flexible design and the controllable interference strength.     

Power-independent technique to extract the dispersion parameters of an optical fiber using four wave mixing

Conventional methods which extract dispersion parameters of an optical fiber using four wave mixing rely on the measurement of optical power in the generated wavelengths, and hence are prone to measurement errors. We propose and demonstrate a power-independent method to extract the dispersion curve of a fiber using four wave mixing (FWM). The analytical equations that have been traditionally used to estimate the phase mismatch in FWM are modified to cater to low-dispersion fibers, and are verified. Experiments to obtain the dispersion curve of a low-dispersion and highly nonlinear fiber are discussed and the results are analyzed. Limitations of any FWM-based method to extract the dispersion curve of a fiber are presented for the first time.     

Enhancement and Suppression of Four-Wave Mixing in a Four-Level Atomic System

We report observations of the enhancement and suppression of four-wave mixing （FWM） in an electromagnetically induced transparency window in a Y-type ^85 Rb atomic system. The results show the evolution of the dressed effects （from pure enhancement to partial enhancement/suppression, and finally into pure suppression） in the degenerate-FWM processes. Moreover, we use the perturbation chain method to describe the FWM process. Finally, we observe the polarization dependence of the enhancement and suppression of the FWM signal.     

Theory of pulse propagation and four-wave mixing in a quantum dot semiconductor optical amplifier

A theory, combining the relations of pulse traveling into quantum dot (QD) semiconductor optical amplifier (SOA) with the four-wave mixing (FWM) theory in these SOAs, is developed. Carrier density pulsation (CDP), carrier heating (CH), and spectral hole burning (SHB) contributions on FWM efficiency are discussed. Effect of QD ground state and wetting layer are included. An additional parameter appears in the gain integral relation of QD SOAs. An equation formulating pulses in the QD SOAs is introduced. We have found that FWM in QD SOAs is detuning and is pulse width dependent. For short pulses, CH is dominant at high detunings (10–100 GHz) while at higher detunings (>100 GHz) the SHB is the dominant one. Undesired paunch behavior is shown in QD SOAs then, CDP must be reduced.     

Phase Matching Property of Cross Polarization Four Wave Mixing in BaTiO3 Crystal

Four wave mixing (FWM) is of considerable interest in generation of a phase conjugate beam with photorefractive crystals. The common methods of FWM are parallel polarization FWM and cross polarization FWM. In this paper, we analyze the phase matching property of the latter. First, we consider the phase matching condition, and derive equations representing the angler relationship of interacting beams. Then, we consider the phase mismatching which is caused by shift of the probe beam angle from the phase matching condition, and calculate the effect of the shift of this angle on the amount of phase mismatching. We also determine and closely examine the coupling constant.     

Excitation of the spin density and current by coherent light pulses in quantum wells

We study the orbital and spin dynamics of charge carriers induced by non-overlapping linearly polarized light pulses in semiconductor quantum wells. It is shown that such an optical excitation with coherent pulses leads to a spin orientation of photocarriers and an electric current. The effects are caused by the interference of optical transitions driven by individual pulses. The distribution of carriers in the spin and momentum spaces depends on the crystallographic orientation of quantum wells and can be efficiently controlled by the pulse polarizations, time delay and phase shift between the pulses, as well as an external magnetic field.     

A fiber-based differential phase shift quantum key distribution scheme with higher key creation efficiency

In this paper, a differential phase shift key distribution system with a selectable delay unit is proposed, in which higher key creation efficiency can be achieved. A photon from a single-photon source is split into four pulses with the same time interval T through two cascaded Mach-Zehnder interferometers. In Bob’s site, a 4T delay loop with an optical switch is employed to reuse the two end pulses discarded in the conventional differential phase shift scheme, and then entire superposition of the all single photon pulses is achieved. The utilization ratio of photons can reach 100% in perfect condition.     

超短光脉冲波形研究

本文讨论了光脉冲技术发展中的脉冲波形测量问题。介绍了一种通用的光脉冲波形测量新方法,该方法可以给出光脉冲的波形、脉宽以及光脉冲内含的频率啁啾信息。这种测量方法已应用于半导体激光器产生ps光脉冲的实验研究,其可行性得到了验证。