Spectrum reshaping of amplified pulse in silicon nanowaveguide

A device consisting of a cascaded semiconductor optical amplifier (SOA) and silicon on insulator (SOI) optical waveguide is presented to amplify and reshape the frequency spectrum of optical pulses in the picoseconds time duration. Numerical simulations show that the output spectrum of the amplified pulse by SOA can be effectively reshaped by utilizing the SOI waveguide. The length of the SOI waveguide may be judiciously adjusted to significantly reduce the frequency chirp of the output pulse from the SOA resulting in reshaping of the output spectrum. We find that the property of pulse spectrum is sensitive to the input pulse power and its temporal width.     

Intraband effects on ultrafast pulse propagation in semiconductor optical amplifier

High bit-rate (>10 Gb/s) signals are composed of very short pulses and propagation of such pulses through a semiconductor optical amplifier (SOA) requires consideration of intraband phenomena. Due to the intraband effects, the propagating pulse sees a fast recovering nonlinear gain which introduces less distortion in the pulse shape and spectrum of the output pulse but introduces a positive chirping at the trailing edge of the pulse.     

半导体光放大器亚皮秒量级超快动态特性的研究

通过考虑脉冲频谱的增益色散，提出了一个分析亚皮秒光脉冲在半导体光放大器(SOA)中传输的新思路.分别对超快光脉冲在时域和频域内进行离散化，建立了一个更为合理的传输数值模型.基于该模型，观察到了脉冲频谱的明显漂移，并探讨了漂移量与脉冲载波波长、脉冲能量、脉冲宽度、SOA长度和注入电流等外部条件之间的关系，从理论上解释了文献没有证实的实验结果.同时，基于交叉增益调制原理，利用双光束结构，描述了载流子加热、光谱烧孔效应和双光子吸收等非线性效应引起的增益压缩.理论分析的结果为改善SOA的动态特性提供了指导. 关键词： 半导体光放大器 超快非线性效应 增益色散性     

Generation of subpicosecond electrical pulses by optical rectification

We describe the generation of subpicosecond electrical pulses by optical rectification of ultrashort optical pulses. The electrical pulses are generated by the second-order nonlinear response of a LiTaO(3) crystal bonded to a coplanar transmission line. A bipolar temporal waveform with a width of 875 fs was measured after a propagation distance of 175mum . This pulse width was limited by the response time of the photoconductive sampler. We observed both broadening and amplitude reduction in the temporal waveform owing to propagation.     

Patterning characteristics and its alleviation in high bit-rate amplification of bulk semiconductor optical amplifier

An investigation is carried out on high bit-rate pulse propagation through bulk semiconductor optical amplifier (SOA) and a tunable optical bandpass filter (OBPF) placed at the output of the SOA for the alleviation of patterning of the amplified signal due to gain saturation in SOA. The effect of the OBPF in terms of its central wavelength detuning and bandwidth on the quality of peak power equalization of the output signal is investigated for signal pulsewidth smaller and larger than the critical pulsewidth of the amplifier at bit-rates of 20 and 40 Gb/s, respectively. It is observed that the scheme is highly useful for the signal pulses with width greater than the critical width.     

Optical ultrawideband monocycle pulse generation based on cross-gain modulation in a semiconductor optical amplifier

A novel method for generating ultrawideband (UWB) monocycle pulses based on cross-gain modulation (XGM) in a semiconductor optical amplifier (SOA) is proposed and experimentally demonstrated. Thanks to the XGM in the SOA, a pair of polarity-reversed optical Gaussian pulses is generated at the output of the SOA, to which a Gaussian pulse pump and a continuous-wave probe are applied. The two polarity-reversed optical pulses are then time delayed by two cascaded fiber Bragg gratings to introduce a time delay difference. A UWB monocycle pulse with a full width at half-maximum of 48 ps and a fractional bandwidth of 188% is generated at the output of a high-speed photodetector.     

Analysis of ultrafast nonlinear phenomena��s influences on output optical pulses and four-wave mixing characteristics in semiconductor optical amplifiers

We have analysed the output pulse characteristics of semiconductor optical amplifier (SOA). It is shown that they can be modified due to the variation of input parameters, such as, gain, input pulsewidth, input pulse energy and effects imposed by the medium. Therefore, the influence of these parameters are analysed on the output pulse shape, spectrum, chirp and pulsewidth. We have used the nonlinear propagation equation taking into account the gain spectrum dynamics, gain saturation which depends on carrier depletion, carrier heating, spectral hole-burning, group velocity dispersion, self-phase modulation and two photon absorption. We have used the finite-difference beam propagation method to simulate the wave evolution both in time and spectral domain in the SOA. We have also simulated the four-wave mixing characteristics between pulses for various input pulses. An accurate output pulse shape can be achieved by controlling the mentioned parameters. To the authors knowledge, pulse shaping in co-propagation regime due to medium effect and input pulse shapes in presence of all nonlinear effects relevant to picosecond regime have been studied comprehensively, for the first time in this work.     

Generation of 10-GHz duty-cycle tunable square optical pulse in an SOA-based mode-locked fiber laser

We demonstrate the generation of 10-GHz optical square pulses by injecting a picosecond pulse train into an SOA-based mode-locked fiber laser. The novel scheme exploits nonlinear effects and gain saturation phenomenon in the semiconductor optical amplifier (SOA). This technique uses gain-compression dynamics between the input pulses and the generated ones in gain-saturated SOA to form square pulses. The center wavelength of the generated optical square pulse can be tuned from 1530 to 1570 nm by adjusting the center wavelength of the optical band pass filter (OBPF) in the SOA-based mode-locked fiber ring laser. The duty cycle of the output pulse can be tuned from 12.7 to 88.4%, which strongly depends on the input power and intra-cavity power.     

光脉冲在标准单模光纤中演化形成孤子的实验研究

利用二次谐波频率分辨光学门脉冲分析仪从实验上分析研究了具有啁啾的10 GHz光脉冲在不同输入功率下演化形成孤子的规律和特点。实验发现:输入脉冲在光纤中传输3.5个色散长度时,其时间宽度、频率啁啾和时间带宽积都随着输入功率的增加而减小。当输入功率大于一阶孤子功率的理论值时,光脉冲能够演化形成孤子;脉冲在随后传输过程中其宽度基本保持不变,波形、频率啁啾和时间带宽积仍随着传输距离的变化而变化;输入功率越高,形成的一阶孤子脉冲的宽度越窄。当输入功率小于一阶孤子功率的理论值时,输出脉冲的时间宽度随着传输距离的增加而增加,频率啁啾随着传输距离的增加而减小,光脉冲不能演化形成孤子。     

Nonlinear pulse propagation: a time-transformation approach

We present a time-transformation approach for studying the propagation of optical pulses inside a nonlinear medium. Unlike the conventional way of solving for the slowly varying amplitude of an optical pulse, our new approach maps directly the input electric field to the output one, without making the slowly varying envelope approximation. Conceptually, the time-transformation approach shows that the effect of propagation through a nonlinear medium is to change the relative spacing and duration of various temporal slices of the pulse. These temporal changes manifest as self-phase modulation in the spectral domain and self-steepening in the temporal domain. Our approach agrees with the generalized nonlinear Schr?dinger equation for 100 fs pulses and the finite-difference time-domain solution of Maxwell's equations for two-cycle pulses, while producing results 20 and 50 times faster, respectively.     

Femtosecond time-resolved optical polarigraphy: imaging of the propagation dynamics of intense light in a medium

A time-resolved imaging technique for visualizing ultrafast propagation dynamics of intense light pulses in a medium has been demonstrated. The method probes the instantaneous birefringence induced by a pulse in the medium. Through consecutive femtosecond snapshot images of intense femtosecond laser pulses propagating in air, ultrafast temporal changes in the two-dimensional spatial distribution of the optical pulse intensity were clearly seen.     

All-optical switchable UWB pulses generation,modulation and transmission

We proposed and demonstrated all-optical polarity- and shape-switchable ultrawideband (UWB) Gaussian pulses generation schemes using a single polarization interferometer (PI) or a semiconductor optical amplifier (SOA) cascaded by a PI. When an optical signal with dark return-to-zero (RZ) format propagates directly through a single PI, a pair of polarity-reversed UWB monocycle pulses and a positive UWB doublet pulse is acquired respectively only by adjusting the polarization controllers, without changing the bias of the intensity modulator. If a SOA is connected at the input of the PI to firstly implement the first-order differential function to the input dark RZ pulse utilizing gain saturation effect of the SOA, both polarity- and shape-switchable UWB Gaussian monocycle and doublet pulses are all acquired from the cascaded system. Moreover, if a nonreturn-to-zero control signal is introduced into the SOA, utilizing the cross-gain modulation (XGM) effect of the SOA and the differential characteristic of PI, all-optical shape modulation to UWB signals can be realized, and the polarity of the modulated UWB pulses can also be switched. Also, UWB signal transmission characteristics in fiber link were investigated.     

Ultrawideband monocycle generation using cross-phase modulation in a semiconductor optical amplifier

We propose a novel scheme to generate ultrawideband (UWB) monocycle pulses based on cross-phase modulation (XPM) of a semiconductor optical amplifier (SOA). The proposed system consists of a SOA and an optical bandpass filter (OBF). Due to the XPM, a continuous wave (CW) probe signal is phase modulated by another optical Gauss pulse in the SOA. The OBF will convert the phase modulation to intensity modulation. A pair of polarity-reversed UWB monocycle pulses is achieved by locating the probe carrier at the positive and negative linear slopes of the OBF. Both cases conform to the UWB definition of the Federal Communications Commission.     

强超短脉冲抽运下半导体光放大器中的非线性过程

建立了包含载流子浓度脉动(CDP)、自由载流子吸收(FCA)、受激辐射(SE)、双光子吸收(TPA)、光谱烧孔(SHB)和超快非线性折射(UNR)过程的半导体光放大器(SOA)理论模型,通过与已报道的实验结果的比较对模型进行了验证,实现了对已有 SOA模型的修正,并对UNR,FCA和TPA效应对强超短光脉冲传输特性的影响进行了分析.当脉宽为几个皮秒的强光脉冲注入工作于透明电流下的SOA时,其强度特性主要受FCA和TPA效应的影响.由于加入了FCA效应,使模型对200fs脉冲强度传输特性的仿真结果与实验结果 关键词： 非线性过程 强超短光脉冲 SOA理论模型 增益透明     

Effects of initial frequency chirp on the linear propagation characteristics of the exponential optical pulse

In this paper, the linear propagation characteristics of the exponential optical pulse with initial linear and nonlinear frequency chirp are numerically studied in a single mode fibre for \be₂<0. It can be found that the temporal full width at half maximum and time-bandwidth product of exponential pulse monotonically increase with the increase of propagation distance and decrease with the increase of linear chirp C for C<0.5, go through an initial decreasing stage near \zeta=1, then increase with the increase of propagation distance and linear chirp C for C\geq0.5. The broadening of pulses with negative chirp is faster than that with positive chirp. The exponential pulse with linear chirp gradually evolves into a near-Gaussian pulse. The effect of nonlinear chirp on waveform of the pulse is much greater than that of linear chirp. The temporal waveform breaking of exponential pulse with nonlinear chirp is first observed in linear propagation. Furthermore, the expressions of the spectral width and time-bandwidth product of the exponential optical pulse with the frequency chirp are given by use of the numerical analysis method.     

An improved nonlinear optical pulse propagation equation

A new equation for self-focusing of extremely focused short-duration intense pulses is derived using a method that treats diffraction and dispersion to all orders with nonlinearity present, and self-consistently determines the nonlinear derivative terms present in the propagation equation. It generalizes both the previous formulation of linear optical pulse propagation to the nonlinear regime, and the cw nonlinear regime propagation to the pulsed regime by including temporal characteristics of the pulse. We apply the new equation and propagate a tightly focused picosecond pulse in silica and explicitly show the effects of spatial-derivative nonlinear coupling terms.     

Transform-limited spectral compression due to self-phase modulation in fibers

We demonstrate near-transform-limited pulse generation through spectral compression arising from nonlinear propagation of negatively chirped pulses in optical fiber. The output pulse intensity and phase were quantified by use of second-harmonic generation frequency-resolved optical gating. Spectral compression from 8.4 to 2.4 nm was obtained. Furthermore, the phase of the spectrally compressed pulse was found to be constant over the spectral and temporal envelopes, which is indicative of a transform-limited pulse. Good agreement was found between the experimental results and numerical pulse-propagation studies.     

Pulse propagation analysis based on the temporal Radon-Wigner transform

Based on the space-time duality and the temporal imaging theory, an optical implementation of the Radon-Wigner transform in the time domain is proposed for analyzing the propagation of ultrashort light pulses in guided dispersive media. As an application of this approach, we discuss the temporal selfimaging or Talbot effect originated by a periodic pulse train. The frequency chirp of the pulses is also considered in order to evaluate its effect on both the integer and the fractional selfimages.     

All-optical ultrawideband monocycle generation utilizing gain saturation of a dark return-to-zero signal in a semiconductor optical amplifier

We demonstrate a simple and compact scheme to generate ultrawideband (UWB) monocycle pulses utilizing gain saturation of a dark return-to-zero (RZ) signal in a semiconductor optical amplifier (SOA). When optical pulses with a dark RZ format propagate through the SOA, the output power at the rising edge will be overamplified compared with that at other durations due to the gain unsaturation. As a result, the output pulses are monocyclelike. The UWB frequency spectra at different injected currents, different input pulse widths, and different input wavelengths are analyzed. Our experiments show that the monocycle generation has good tolerance to the SOA bias current and the input signal wavelength.