Waveform shaping of chirp-controlled signal by semiconductoroptical amplifier using Mach-Zehnder frequency discriminator

The optical output waveform of a semiconductor optical amplifier (SOA) is shaped using a Mach-Zehnder interferometer (MZI) as a frequency discriminator in order to suppress its distortion at high SOA input powers. This technique is applied to chirp control of an optical signal using an SOA, and the chirp control with waveform shaping greatly improves the transmission performance of a positively chirped optical signal in standard single-mode fiber over a wide range of SOA input powers. We also demonstrate that the chirp control with waveform shaping can be achieved without changing the drive settings of the SOA and MZI, even when the optical frequency is switched to another channel located on the wavelength-division-multiplexing frequency grid     

基于半导体光放大器交叉增益饱和的波长转换的理论分析

建立了基于半导体光放大器交叉增益饱和的波长转换的理论模型。分别讨论了小信号下波长转换特性和大信号下转换波形的畸变情况。结果表明,半导体光放大器的载流子寿命是导致输出波形畸变的主要因素。     

Self-phase modulation and spectral broadening of optical pulses insemiconductor laser amplifiers

Amplification of ultrashort optical pulses in semiconductor laser amplifiers is shown to result in considerable spectral broadening and distortion as a result of the nonlinear phenomenon of self-phase modulation (SPM). The physical mechanism behind SPM is gain saturation, which leads to intensity-dependent changes in the refractive index in response to variations in the carrier density. The effect of the shape and the initial frequency chirp of input pulses on the shape and the spectrum of amplified pulses is discussed in detail. Particular attention is paid to the case in which the input pulsewidth is comparable to the carrier lifetime so that the saturated gain has time to recover partially before the trailing edge of the pulse arrives. The experimental results, performed by using picosecond input pulses from a 1.52-μm mode-locked semiconductor laser, are in agreement with the theory. When the amplified pulse is passed through a fiber, it is initially compressed because of the frequency chirp imposed on it by the amplifier. This feature can be used to compensate for fiber dispersion in optical communication systems     

Power Equalization for SOA-Based Dual-Loop Optical Buffer by Optical Control Pulse Optimization

In this paper, a theoretical study and experimental demonstration are applied to achieve power equalization for semiconductor optical amplifier (SOA)-based dual-loop optical buffers (DLOBs). It is found that, due to the gain saturation and limited linewidth-enhancement factor of the SOA, the peak power of a packet pulse with an optically controlled delay of 9.9 mus is 4.83 dB lower than that of a packet pulse without storage. In order to eliminate the 4.83-dB output power fluctuation of the DLOB, a simple power-equalization method based on the optimization of an optical control pulse is proposed. By injecting a negative optical control pulse, the output power fluctuation of a packet pulse can be effectively reduced to zero. We have also investigated the peak power level of the optical control pulse required to fulfil the buffer function. It is found that the SOA with larger linewidth-enhancement factor and larger small-signal gain should be used to reduce the peak power of the optical control pulse. It is also theoretically found that, due to the negative optical-control pulse injection, the packet signal with Gaussian profile has some distortion after storage. However, the distortion effect is mitigated when the shape of the input pulse is more similar to the square profile. Finally, the proposed method for achieving power equalization in an SOA-based optical buffer has been justified by carrying out a 2.5-Gb/s 2times2 exchange-bypass optical switch experiment. We believe that this power-equalization method can be also applied to other SOA cross-phase modulation-based applications     

High-output-power polarization-insensitive semiconductor optical amplifier

A high-output-power 1550 nm polarization-insensitive semiconductor optical amplifier (SOA) was developed for use as a compact in-line optical amplifier. A very thin tensile-strained bulk structure was used for the active layer and active width-tapered spot-size converters (SSCs) were integrated on both input and output sides. The SOA module exhibited a high saturation output power of +17 dBm together with a low noise figure of 7 dB, large gain of 19 dB, and low polarization sensitivity of 0.2 dB for optical signals of 1550 nm wavelength. For the amplification of optical signals modulated at 10 Gb/s in the nonreturn-to-zero (NRZ) format, a good eye pattern without waveform distortion due to the pattern effect was obtained at an average output power of up to +12 dBm. Additionally, good amplification characteristics were demonstrated for the signal wavelength range corresponding to the C-band.     

Gain saturation characteristics of traveling-wave semiconductorlaser amplifiers in short optical pulse amplification

The gain saturation characteristics of traveling-wave semiconductor laser amplifiers (TWAs) are theoretically and experimentally investigated. In the amplification of an isolated pulse whose repetition period is short compared to the carrier lifetime, the gain saturation is related through the carrier lifetime to the gain saturation in CW amplification. When the output pulse energy is smaller than the saturation energy, short optical pulses can be amplified without pulse shape distortion, whereas high-energy pulses suffer from pulse shape distortion due to the temporal gain variation during the pulse radiation. FWHM pulse duration variation in amplification by TWAs depends on the input pulse shape. The pulse energy gain saturation was experimentally confirmed to be independent of pulse durations and to be determined only by the pulse energy. In extremely-high-repetition-rate pulse amplification, the saturation of the pulse energy gain is determined by the average signal power     

Demonstration of CSRZ Signal Generator Using Single‐Stage Mach‐Zehnder Modulator and Wideband CMOS Signal Mixer

In this paper, we demonstrate an electrically band‐limited carrier‐suppressed return‐to‐zero (EB‐CSRZ) signal generator operating up to a 10 Gbps data rate comprising a single‐stage Mach‐Zehnder modulator and a wideband signal mixer. The wideband signal mixer comprises inverter stages, a mixing stage, and a gain amplifier. It is implemented by using a 0.13 μm CMOS technology. Its transmission response shows a frequency range from DC to 6.4 GHz, and the isolation response between data and clock signals is about 21 dB at 6.4 GHz. Experimental results show optical spectral narrowing due to incorporating an electrical band‐limiting filter and some waveform distortion due to bandwidth limitation by the filter. At 10 Gbps transmission, the chromatic dispersion tolerance of the EB‐CSRZ signal is better than that of NRZ‐modulated signal in single‐mode fiber.     

Analysis of signal degree of polarization degradation used ascontrol signal for optical polarization mode dispersioncompensation

The basic property of degree of polarization (DOP) degradation of optical signal induced by polarization mode dispersion (PMD) in high-speed optical fiber transmission is investigated in detail. The DOP of the optical signal reflects the degree of waveform degradation caused by PMD, therefore, it is proposed to be used as the control signal judging the best compensation point for the optical adaptive PMD compensation techniques. However, the signal DOP is not only affected by PMD, but also by various factors, such as the modulation format, modulator chirp, fiber nonlinearity, amplified spontaneous emission (ASE), and so on. We use numerical simulations and experiments to explore the basic DOP property to detect PMD with these factors. We also show that using the signal DOP as control signal is especially useful for the optical duo-binary modulation because of its high sensitivity and wide PMD detection range     

An Enhanced SOA-Based Double-Loop Optical Buffer for Storage of Variable-Length Packet

To extend the buffer depth of a fiber loop optical buffer, we have experimentally demonstrated an enhanced semiconductor optical amplifier (SOA)-based dual-loop optical buffer (DLOB) for storing variable-length optical packets. We have theoretically derived constraints governing the buffer depth of the DLOB, in which the SOA not only provides a nonlinear phase shift in the loop to implement the buffer function but also compensates for the fiber loop attenuation during long-time storage. It is found that the maximum allowable length of a stored packet to avoid the counter-propagation packet collision inside the SOA depends on the SOA bias position as well as the length of the fiber loop. To demonstrate the effectiveness of the proposed enhanced configuration, we have successfully demonstrated the storage of 2.5-Gbps variable-length packets even when the length of the input packet exceeds the corresponding length of the fiber loop. Another unique advantage of the proposed enhanced DLOB configuration is that it can also overcome the problem of power leakage of the stored packet due to a directional gain difference of single SOA and gain saturation. Index     

啁啾光脉冲在高阶色散控制光纤链中的稳定传输

基于一种二阶和三阶色散补偿的光纤级联系统模型 ,用数值法对啁啾皮秒光脉冲作了传输模拟。结果表明 ,完全补偿的高阶色散控制系统消除了三阶色散所引起的脉冲边沿部的振荡 ,减弱了脉冲峰的时间移动 ;另外 ,在确定的配置下 ,给输入脉冲附加一最佳的频率啁啾 ,可使得色散控制孤子稳定传输。脉冲宽度和啁啾以及光强度都围绕在初始值附近波动 ,在每个补偿周期末端 ,基本恢复到初始值 ;最佳啁啾的选取与二阶色散的配置有关 ,与三阶色散的配置无关 ;文中画出了 10 0Gbit/s码率的 64位随机高斯光脉冲序列在完全补偿系统中传输 10 0 0 0km后的眼图。由清晰的眼图可知 ,这种完全补偿系统减弱了脉冲之间的相互作用     

Theory of signal degradation in semiconductor laser amplifiers indirect detection systems

Theoretical studies are carried out on long haul direct detection optical fiber communication systems, with inline optical semiconductor amplifier repeaters. Calculations are made of the noise, eye diagrams, and bit-error-rate characteristics of lightwave systems with optical amplifiers. Indications are given of the effect of amplifier characteristics such as spontaneous noise and signal distortion due to gain saturation on the system performance. The nonlinear process within semiconductor laser amplifiers leads generally to pulse amplitude-temporal distortions due to gain saturation. This theoretical study demonstrates that the system penalty caused by these nonlinear effects appears progressively as the optical input power at each amplifier is increased. For example, nonregenerated fiber transmission using traveling wave semiconductor laser amplifiers was simulated, and results obtained at 0.5 and 2.5 Gb/s are presented. In order to improve the system performance, the influence of structure and bulk dimensions of the amplifier cavity is also considered     

Dispersion of microwave-modulated optical signals

Experimental and analytical evaluation of dispersion effects that control intersymbol interference in gigabit, single-mode-fiber, digital systems is described. Both direct modulation of distributed-feedback (DFB) lasers and external modulation by Mach-Zehnder modulators are studied. The experiment uses sinusoidal modulation of the light at a microwave frequency, e.g. 5 GHz, and the subsequent measurement of signal gain or loss in a dispersive fiber. For DFB lasers, the transmission measurements are used to derive the transient and adiabatic components of chirp. It is shown that, for external modulation at 5 GHz, the 1.55- mu m optical signals suffer relatively little dispersion loss for transmission distances up to 60 km. Analytical solutions for waveform distortion are given for external modulation and show good agreement with measurements.<>     

dBm average optical output power operation of small-chirp 40-gbps electroabsorption Modulator with tensile-strained asymmetric quantum-well absorption Layer

A small-chirp 40-Gbps electroabsorption modulator (EAM) with high optical output power capability has been developed for the first time. An optimized tensile-strained asymmetric quantum-well structure is employed as the absorption layer of the EAM so that small chirp and reduction of the lifetime of the photogenerated holes for high optical output power tolerance is obtained. Deteriorations of frequency response and chirp due to carrier pileup under high optical output power conditions were prevented by enhancing carrier sweepout, which was experimentally confirmed as a hole lifetime as short as 35 ps under high optical output power conditions. As a result, good frequency response (bandwidth > 30 GHz) and small chirp (/spl alpha/<1) were obtained under the condition of the zero bias voltage and +4.5 dBm continuous-wave (CW) optical output power (P/sub out,CW/). Clear eye opening and high dynamic extinction ratio under 40-Gbps non-return-to-zero modulation persisted to a high average output power (P/sub out,ave/) condition of P/sub out,ave/=+1.0 dBm.     

Transmission Over 360 km Using a 2.5-Gb/s SOA-Modulator Integrated Full$C$-Band Wavelength-Tunable Transmitter

We present an external-cavity wavelength-tunable laser with a semiconductor optical amplifier-modulator monolithically integrated on the gain chip. The modulator chirp is analyzed, and it is explained how the chirp can be partially cancelled by self-phase modulation in the transmission fiber. Full$C$-band transmission of 2.5-Gb/s data over 360 km is demonstrated with small power penalty.     

Polarization insensitive frequency converter with the capability ofchirp removal

We report on a new type of polarization insensitive frequency conversion technique using four-wave mixing (FWM) in a semiconductor optical amplifier (SOA). In this technique the input signal is spectrally duplicated by a Mach-Zehnder type phase modulator. Then, the resulting two waves generate the frequency converted output signal by FWM with use of an additional pump wave. The converted output wavelength is independent of the wavelength of the input signal. In addition to the frequency conversion, the chirp of the input signal is removed by the FWM process bit-error-rate (BER)-measurements after transmission of the converted signal over standard communication fiber are reported     

基于FTTH的CATV系统CNR的影响分析

为满足半导体光放大器SOA在FTTH接入网中的广泛应用,FTTH系统中SOA噪声特性对有线电视信号CNR的影响成为一个关键的问题。首先介绍了半导体光放大器的特点,通过SOA的噪声特性、输入光功率对光纤系统中视频信号CNR劣化的影响分析,计算FTTH网络用户端的CNR,结果显示,增大SOA的输入光功率是减小CNR劣化值的最佳方法,必须在系统CNR和SOA增益之间作出权衡,以满足FTTH的光纤CATV系统的设计要求。     

Reducing crosstalk and signal distortion in wavelength-division multiplexing by increasing carrier lifetimes in semiconductor optical amplifiers

A simple theory for the crosstalk in multichannel wavelength-division multiplexing (WDM) communication systems caused by the cross gain saturation in semiconductor optical amplifier (SOA) is developed. It is shown that increasing the carrier recombination lifetime while reducing the differential gain in the SOA greatly mitigates the crosstalk. An SOA using type-II quantum wells with reduced recombination probability is designed, and its characteristics are numerically modeled for a number of different WDM systems. It is shown that when used as a preamplifier or in local/metro networks with a small number of amplifiers, error-free transmission can be achieved with the type-II SOA.     

半导体光放大器的超快动态增益特性

提出了一种包括载流子密度脉动(CDP)、载流子加热(CH)和光谱烧孔(SHB)效应在内的半导体光放大器(SOA)的时域动态模型。利用该模型分析了半导体光放大器中的增益饱和、超快增益动态以及光脉冲在增益饱和半导体光放大器中的波形畸变,其中重点考虑了超短脉冲的情况。模拟计算表明,对于10ps量级以下的短脉冲,分析半导体光放大器的动态增益特性时,不能忽略载流子加热和光谱烧孔等带内超快非线性效应的影响。     

System outage probability due to first- and second-order PMD

A theoretical approach is proposed that allows one to quantify the impact of fiber polarization mode dispersion (PMD) on optical binary transmission taking into account not only first-order polarization mode dispersion, but also signal distortion induced by second-order PMD. Using this approach the impact of the spectral signal width on PMD-induced system outage probability could be studied for the first time. An analysis of 10-Gb/s transmission exhibits that, as long as the mean PMD remains below the commonly accepted limit (about 10 ps) for negligible outage, a linear chirp of up to 30 GHz does not lead to an additional increase of the system outage. This result confirms that low bandwidth modulation schemes (external modulator, low chirp laser) do not suffer from additional outage degradation due to second-order PMD     

双辅助光作用下半导体光放大器性能分析

半导体光放大器以其良好的非线性在全光网络中具有广泛应用,但较长的载流子恢复时间一直是制约其用于超快全光信号处理的速率瓶颈,基于包含自发辐射噪声的半导体光放大器模型,探讨了提高半导体光放大器增益恢复时间的有效途径,通过对制约透明波长移动,增益饱和与有效载流子寿命的相关因素进行数值分析,得出以下结论:与单辅助光相比,采用双辅助光可以在不牺牲信号增益的前提下进一步缩短载流子寿命,因而是提高半导体光放大器增益恢复时间的有效途径,这一点对工程设计和应用具有一定的指导意义.