Photon point process for traveling-wave laser amplifiers

The authors determine the evolution of the photon point process of a light beam as it passes through a traveling-wave laser amplifier (TWA). In particular, when coherent light is presented to the input of the amplifier, the output photon statistics are characterized by a marked-Poisson (MP) point process, which has a noncentral-negative-binomial (NNB) output photon-number distribution (PND). Using this distribution we calculate the probability of error (PE) for an ON-OFF keying (OOK) direct-detection photon-counting communication system, and show that the results differ somewhat from those obtained when the Gaussian-PND approximation is used. It is shown that receiver performance is optimized by filtering the amplifier output. Analysis of the point process is of interest because it permits the time response of the amplifier to be determined; this, in turn, allows the effects of intersymbol interference to be calculated     

Pulsed laser sampling photon amplifier

The performance of semiconductor laser amplifiers can be significantly improved by injecting carriers with pulsed electric currents of subnanosecond duration. Pulsed operation is illustrated in a Fabry-Perot amplifier and in a traveling-wave amplifier. The resonant amplifier is most sensitive to an input light wave at the instant the carrier density is crossing over the critical region, giving a sharply pulsed sampling effect on the input light wave signal. Compared to a resonant amplifier operating at subcritical electron density, the pulsed amplifier gives much higher gain and peak power. In fact, pulsed operation of a resonant amplifier is also expected to give significantly higher gain than and about the same peak output power as a traveling-wave amplifier. Pulsed operation also improves the performance of a traveling-wave amplifier by attenuating its internally reflected waves.<>     

Computer model of an injection laser amplifier

Laser amplifiers can be used in two ways: as preamplifiers to enhance the sensitivity and improve the noise performance of detectors and, in a pulsed mode of operation, as modulators to boost and stabilize the output of an injection laser oscillator. Most mathematical models of injection lasers are based on time-dependent rate equations that ignore the spatial dependence of the electron and photon densities. The model discussed here is based on numerical solutions of traveling-wave equations. The noise output of the laser amplifier is treated by traveling-wave power equations, but the light signal is described by traveling-wave equations for its amplitude. The parameters responsible for spontaneous and stimulated emission are being related to each other by the requirement that the amplifier achieve optimal noise performance in the absence of internal losses and without gain saturation. The most important results obtained from this computer model of a laser amplifier are as follows. 1) The theory contains a heuristic electron injection efficiency parameter. To agree with experimental observations this parameter must be kept small and its value must decrease with increasing current. 2) Cavity amplifiers saturate more readily than amplifiers without feedback. 3) Because of internal loss mechanisms the amplifier supplies more noise than is required by quantum theory, but its noise performance is still surprisingly good. In particular, the optical signal-to-noise ratio prior to detection is insensitive to gain saturation by strong signals. It remains approximately 4 dB below the theoretical maximum value for weak to moderately strong input signals and drops dramatically only when the amplifier is almost completely saturated.     

Transmission-line amplifiers

A wideband thermionic vacuum-tube power amplifier in which an extended and continuous grid and cathode serve as an input transmission line that excites a corresponding traveling-wave signal on a similar transmission line between the screen grid and anode is analyzed. The usual gain-bandwidth product limitation is avoided by making the input and output capacitances part of the transmission lines. The gain mechanism is the same as that in the usual power tetrode in that bunched charges are delivered to the anode at the proper time and location along the anode transmission line to enhance the traveling-wave signal. If line attenuation losses are small, the power gain increases as the square of line length. Part of the backward wave can be made a forward wave by tapering the impedance of the output line. This type of amplifier could potentially use a field-emission cathode and thus avoid the high heater power and slow transit time of the thermionic emitter. Its disadvantages are low gain and low input impedance     

2-Gbit/s signal amplification at λ=1.53 μm in anerbium-doped single-mode fiber amplifier

The gain, saturation power, and noise of an erbium-doped single-mode traveling-wave fiber amplifier operating at a wavelength λ=1.53 μm are characterized. In continuous-wave (CW) measurements amplification at 2 Gbit/s was demonstrated with up to 17-dB gain for 1×10^-9 bit error rate at 1.531 μm and a 3-dB full bandwidth of 14 nm. From the determination of the fiber-amplifier's output signal-to-noise ratio versus input signal power during data transmission, it was concluded that, with signal levels used here, signal-spontaneous beat noise limited the receiver sensitivity improvement. With the fiber amplifier acting as an optical preamplifier of the receiver, the best sensitivity was -30 dBm, obtained after installing a polarizer at the fiber amplifier output to reject half of the applied spontaneous emission power. This sensitivity was 6 dB better than without the fiber amplifier, proving that the fiber amplifier can be used as a preamplifier     

Optical carrier linewidth broadening in a traveling wavesemiconductor laser amplifier

The powerful spectral densities of the phase, intensity, and electron density noise derived in a previous paper are used to calculate the linewidth broadening of an optical carrier as it propagates through a traveling-wave semiconductor laser amplifier. It is found that the amount of broadening is dependent upon the input linewidth as well as amplifier parameters. The effect of saturation due to high input power is considered, as well as the operational regime where intensity, rather than phase, noise determines the amplifier output line shape. An example system calculation is used to demonstrate the impact of amplifier line broadening on system design     

1.5 µm GaInAsP traveling-wave semiconductor laser amplifier

This paper presents a theoretical and experimental study in terms of small-signal gain, signal gain saturation, and noise characteristics of a 1.5 μm GaInAsP traveling-wave amplifier (TWA), realized through the application of SiOxfilm antireflection coatings. This TWA, having a residual facet reflectivity of 0.04 percent, exhibits a wide, flat signal gain spectrum and a saturation output power of +7 dBm at a 20 dB signal gain. The TWA also has a noise figure of 5.2 dB, which is the smallest value reported for semiconductor laser amplifiers. The experimental results are confirmed to be in good agreement with the theoretical predictions based on the multimode traveling-wave rate equations in conjunction with the photon statistic master equation analysis, which takes into account the amplifier material and device structural parameters. Signal gain undulation, saturation output power, and noise figure are also theoretically evaluated as functions of the facet reflectivity. The superior performance of the TWA demonstrates that the device is favorable for use in linear optical repeaters in fiber transmission systems.     

20-nm optical wavelength conversion using nondegenerate four-wavemixing

Wavelength conversion of optical signals over 20 nm is demonstrated using highly nondegenerate four-wave mixing in a semiconductor traveling-wave optical amplifier. This technique has the potential for extremely-high-speed operation and allows continuous tuning of both input and output wavelengths over the amplifier gain bandwidth. It is demonstrated that, even for such a large wavelength conversion range, it is possible to obtain conversion efficiencies in excess of -10 dB and high extinction ratios. The feasibility of the technique is demonstrated by system measurements at 622 Mb/s, showing a 1.1-dB power penalty at 10^-9 bit error rate (BER)     

Traveling-wave amplifiers with prescribed frequency response

Microwave transistor amplifier combinations that have controlled frequency response over a specified bandwidth were designed. Theoretical analysis of such an amplifier with an arbitrary number of sections is presented. The response of the amplifier is controlled by tapering the frequency selectivity or Q of each section of the amplifier. To verify the theory, a three-section amplifier with maximally flat time delay response was designed, constructed, and evaluated. Existing traveling-wave amplifiers were modeled as lossy transmission lines. Although the amplifiers were relatively broadband, a prescribed frequency response was not achieved, and each transistor did not receive an equal portion of the signal power. Resistive elements were required for impedance matching at the input and output. The design seeks to improve on previous techniques by trading bandwidth for controlled gain. By making the transmission line that connects the amplifier sections nonuniform, the frequency response was controlled over the design bandwidth. The designs can easily be implemented using familiar components     

Influence of the confinement factor on the wavelength-dependentoutput properties of a tapered traveling-wave semiconductor amplifier

We analyze the influence of the confinement factor on the output properties of a tapered traveling-wave semiconductor amplifier as a function of the wavelength of the input field and current injected into the amplifier. The interaction of the optical field with the amplifying medium is described by means of a susceptibility function taking into account both the gain and the carrier-induced antiguiding effect, while the beam propagation method is used to describe the evolution of the field along the amplifier. We find that, for the same input power, devices with lower optical confinement factors allow for the same output optical power with better beam quality and higher saturation power than devices with higher optical confinement factor, in agreement with the available experimental results     

线性光放大器的宽带自发辐射噪声分析

建立含有垂直光场(VCL)的线性光放大器(LOAs)稳态仿真模型,考虑了宽带噪声特性、端面反射和纵向空间烧孔效应。计算了在不同输入信号功率和垂直腔分布布喇格反射器(DBR)反射率下总的输出ASE噪声功率、噪声系数(NF)、前向和后向传输的ASE噪声光子速率和噪声谱分布的变化。结果表明,与传统光放大器(SOAs)相比,在增益非饱和区,LOA有很好的噪声钳制作用,ASE噪声几乎不随信号功率的改变而变化。在增益饱和区,破坏了VCL的钳制作用,NF有所上升,但上升的速率小于SOA。提高DBR反射率,总的输出ASE噪声功率减小,但NF有所增大。     

Noise and error rate performance of semiconductor laser amplifiers in PCM-IM optical transmission systems

Applications of semiconductor laser amplifiers in intensity modulated digital optical transmission systems were studied theoretically. An optical linear amplifier repeater between electronically regenerating terminal repeaters and an optical linear preamplifier in front of a photodetector in an electronically regenerating repeater are discussed. Both traveling-wave type and Fabry-Perot cavity type laser amplifiers are considered. The noise and error rate performance in these systems are evaluated using formulations for semiconductor laser amplifiers. The mean and variance in the optical amplifier output photons calculated by the photon master equation [1] is used to obtain the worst case variance in the equalized output voltage [2] for these systems. The required receiving power reduction from direct detection scheme by a preamplifier system and the repeater spacing expansion between two electronically regenerating terminals by an optical linear amplifier repeater system are delineated.     

An adaptation procedure on envelope statistics for predistorter designs based on statistical modeling methods

There have long been some experiment findings that the behavior of a power amplifier (PA) to digitally modulated stimuli depends on its input statistics. Correspondingly, there have been efforts in characterizing PA behaviors and, more recently, in designing amplifier linearization devices by including the information of envelope statistics or using statistical methods. Superior performance have been reported for resultant linearizer designs compared with similar designs based on conventional approaches. In this brief, an adaptation procedure on envelope statistics will be introduced that can further improve the performance of any predistorter design derived from a statistical amplifier model, or more generally, derived from the sample data sets of the input and the output of the PA.     

脉冲形状对半导体激光放大器用于啁啾补偿的影响

光脉冲经过增益饱和的行波半导体激光放大器后,由于放大器的自相位调制,使放大脉冲附加上频率啁啾。合适的附加啁啾不但能抵销入射脉冲的初始啁啾,而且还有可能借助简单的群速度延迟线对脉冲进行压缩。本文计算了不同形状的入射脉冲经光放大器放大后的输出脉冲形状及其附加的频率啁啾,分析了它们对光放大器用于啁啾补偿的影响。     

可调增益均衡性宽带MMIC低噪声放大器设计

文章提出了一种新颖的具有可调增益均衡特性的宽带全单片低噪声放大器电路设计方法,它将用于微波功率模块(MPM)的固态功率放大器(SSPA)链前端中的两项功能独立的电路(多级单片宽带低噪声放大器LNA和单片宽带频率均衡放大器FEA)组合设计在一块单片电路芯片中。其中LNA部分采用高效率高增益宽带级联型分布放大器取代常规的行波式分布放大器,使得放大器级数显著减少;频率均衡放大器提出用一种利用FET作可调元件的嵌入式低损无源网络来取代,使得放大器的增益特性在频带中部下凹可调,补偿了行波管“山丘状”功率增益特性。在此基础上完成设计的 MMIC LNA,仅使用3个0．25μm×120μm pHEMT,在6GHz～18GHz频带内,小信号增益14．3 ±0．8 dB,输入、输出反射损耗<-10 dB,NF<5 dB;嵌入可调增益均衡网络后,在其他性能参数基本保持不变的前提下,频带中部引入的电调衰减范围超过6．5 dB,完全满足MPM要求。     

返波激励的级联折叠波导行波放大器

利用折叠波导返波振荡器(FW-BWO)作为激励源,用于激励工作频率为216 GHz的折叠波导行波放大器.利用3D-Magic进行仿真实验,通过仿真优化,最终得到96 W的输出功率,整个电路的长度被设计为只有1 cm左右.通过该方法,显著地缩短了高频结构长度,有利于实现小型化的真空电子学太赫兹源,对集成化的太赫兹源设计具有重要的参考价值.     

Strict calculation of the light statistics at the output of a travelling wave optical amplifier

A new method for calculating the probability density function of the photon number propagating through a travelling wave optical amplifier with no restriction on its working regime (linear and nonlinear) is reported. The authors show that the widely used Gaussian approximation of the probability density function does not match the real statistics if the incident optical power is small.<>     

垂直腔半导体光放大器增益的理论分析

针对研究增益饱和时,现有的垂直腔半导体光放大器(VCSOA)速率方程模型在确定输入信号的功率注入因子方面存在难题,根据法布里-珀罗腔边界条件,从行波方程和与位置相关的载流子方程出发,引入随轴向位置发生变化的增益增长因子刻画微腔内的驻波效应,构建出VCSOA的增益模型。利用该模型通过求方程的自洽得到了腔内载流子、光子的分布,并分析了反射增益,其结果与已报道的理论及实验基本一致。     

An Automatically Controlled Predistorter for Multilevel Quadrature Amplitude Modulation

In digital microwave transmission, the nonlinear characteristics in a high power amplifier, such as a TWT (traveling-wave tube) inhibit efficient output use. This note introduces a new predistorter control technique, and assesses the nonlinear compensation capability of a third-order predistorter incorporating this technique. Concerning 16-QAM (quadrature amplitude modulation) a 10 dB reduction in out-of-band emission and larger than 8 dBC/Nimprovement with respect to symbol error rate can be achieved at 3 dB TWT average output power backoff.