半导体光放大器引起的光控器件中的信号损伤分析

在双环耦合全光缓存器的基础上分析了全光分组交换网络中以半导体光放大器（SOA）为相移器件的反馈型全光缓存器输出的信号损伤,包括SOA的非线性及载流子恢复时间限制引起的脉冲畸变与连续码流中的图样失真和SOA的自发辐射噪声累积引起的信噪比恶化及缓存器结构引起的“漏光”问题.理论分析及实验结果表明,在采用反相控制并注入高功率控制光的情况下,脉冲畸变与图样失真被抑制,由信噪比恶化及漏光决定的光分组的缓存圈数被限制在20—30圈.得到的结果对基于SOA的光缓存器及逻辑器件同样具有借鉴作用. 关键词： 全光分组交换 全光缓存器 信号损伤 半导体光放大器的噪声分析     

All optical SR and D flip-flop employing XGM effect in semiconductor optical amplifiers

This paper presents a simple and novel scheme for all-optical SR and D flip-flop employing cross gain modulation (XGM) effect in two wideband semiconductor optical amplifiers. The proposed flip-flop has a fast response, with less than 20 ps transition times for both rising and falling edges. The FF speed-limit is mainly determined by the SOA recovery time and the intra-FF coupling length. The simulation results exhibit a contrast ratio of 13 dB between two states with an AM of less than 2.5 dB.The distinctive simplicity of the flip-flop implies reduced footprint and low power consumption which makes it ideal for photonic integration.     

Demultiplexing of OTDM-DPSK signals based on a single semiconductor optical amplifier and optical filtering

We propose and demonstrate the use of a single semiconductor optical amplifier (SOA) and optical filtering to time demultiplex tributaries from an optical time division multiplexing-differential phase shift keying (OTDM-DPSK) signal. The scheme takes advantage of the fact that phase variations added to the target channel by cross-phase modulation from the control signal are effectively subtracted in the differential demodulation scheme employed for DPSK signals. Demultiplexing from 80 to 40 Gbit/s is demonstrated with moderate power penalty using an SOA with recovery time twice as long as the bit period at 80 Gbit/s. Large dynamic ranges for the input power and SOA current are experimentally demonstrated. The scheme is expected to be scalable toward higher bit rates.     

Controlled shift of optical bistability hysteresis curve and storage of optical signals in a four-level atomic system

The dependence of the shift of an optical bistability hysteresis curve on the nonlinear phase shift induced by a controlling light is observed in a four-level atomic system of 87 Rb inside an optical ring cavity. In the process the intensity of the coupling beam keeps constant and the atomic system is operated at near conditions of coherent population trapping due to atomic coherence. The refractive and absorptive chi3 nonlinearities enhanced by atomic coherence provide the physical mechanism of the phenomena. Based on the effects, all-optical flip-flop and storage of optical pulse signals with a low peak power of several tens of microwatts are implemented.     

Improved optical single-sideband generation using the self-modulation birefringence difference in semiconductor optical amplifiers

By employing the self-modulation birefringence difference in a semiconductor optical amplifier (SOA), an improved method is proposed to generate a complete optical single-sideband (OSSB) signal. Over 30 dB sideband suppression ratios (SSRs) of lower OSSB signals are obtained over a 12 dB input power range and a 36 nm wavelength span, with a maximum of over 35 dB. Upper OSSB signals with an SSR of over 15 dB are observed using a SOA for what is believed to be the first time. This method is effective even for the carrier-suppressed signal. The theory for OSSB generation in an SOA is extended and verified by experiment.     

Power penalty in multitone radio-over-fibre system employing direct and external modulation with optical amplifiers

This paper presents a multitone RoF system employing a central station and a remote antenna station to transmit modulated microwave or mm-wave signals. Two configurations of the system employing direct and external modulation are presented. First, at the central station, the modulating signal is directly and externally modulated at 1549.5 and 1550.5 nm, respectively, and the combined signal is then transmitted using wireless mode. In the second configuration, the combined signal is transmitted over 20 km to the remote antenna station using optical fibre connectivity. At the remote antenna station, both channel wavelengths are separated and amplified with an EDFA and SOA and are detected. The demonstration results show that there exists a small power penalty between the direct and the external modulation. But a large power penalty is reported depending upon the choice of optical amplifier viz. EDFA or SOA. Better performance is recorded in later case. The results also show that power is being distributed among the operating frequency and sidebands adjacent to it after transmission over a fibre link.     

Analytical approach of developing wavelength encoded AND, NAND and X-OR logic operations and implementation of the theory using semiconductor optical amplifiers

Frequency encoding technique is a very promising and faithful technology for very fast long-haul optical communication and super fast computation. Implementation of different logic gates based on the principle of frequency conversion is the key mechanism of frequency encoded data processing and networking. It is established that semiconductor optical amplifiers (SOA) have been used successfully for the purpose of frequency conversion. One of the important techniques of frequency conversion is the conjugate beam generation by four wave mixing (FWM) in SOA and ultimately conversion of it into desired frequency by means of reflecting semiconductor optical amplifier (RSOA). However the efficiency of conjugate beam generation is restricted by polarization dependent gain saturation of SOA. This dependency can be successfully removed using polarization diversity scheme. Another technique of the frequency conversion is based on nonlinear rotation of the state of polarization of the linearly polarized probe beam. An important advantage of using polarization rotation in SOA is that a small change in rotation of the state of polarization will lead to a large difference in output power. Here in our present communication we propose a method of developing wavelength encoded AND, NAND and X-OR logic operations exploiting the above mentioned functions of SOA. For this purpose we have developed an analytical treatment based on which above mentioned three logic gates are conducted. The satisfactory simulation result proves also the validity of the developed theory.     

High-speed all-optical differentiator based on a semiconductor optical amplifier and an optical filter

We demonstrate a novel all-optical differentiator that carries out the first-order temporal derivation of optical intensity variation at high speed. It consists of a semiconductor optical amplifier (SOA) and an optical filter (OF) serving as an optical phase modulator and a frequency discriminator, respectively. A polarity-reversed derivative pair with a certain bias can be obtained by locating the probe wavelength at the positive or negative slope of the OF. Differentiations of super-Gaussian and Gaussian signals are obtained at various data rates. Defined as the mean absolute deviation of the measured result from the ideal result, total average errors of less than 0.12 are observed in all cases. Input power dynamics as well as control wavelength dependence are investigated and show that the cross-gain modulation in the SOA is detrimental to the differentiation performance.     

Nonlinear distortion generated by semiconductor optical amplifier boosters in analog optical systems

Closed-form theoretical expressions for the second- and third-order harmonic distortions generated by a semi-conductor optical amplifier (SOA) operating under slight saturation conditions are derived for what is believed to be the first time. The distortion results provided by these expressions agree very well with those obtained by means of simulation and experiments, showing that one should carefully adjust the optical power launched into the SOA to obtain both the good linearity and high dynamic range required by analog optical systems.     

Modeling of semiconductor optical amplifier RIN and phase noise for optical PSK systems

Phase modulation schemes are attracting much interest for use in ultra-fast optical communication systems because they are much less sensitive to fibre nonlinearities compared to conventional intensity modulation formats. Semiconductor optical amplifiers (SOAs) can be used to amplify and process phase modulated signals, but with a consequent addition of nonlinear phase noise (NLPN). Existing SOA NLPN models are simplistic. In this paper we show that a more accurate model can be used, which results in simple expressions for SOA nonlinear noise, in particular when used to amplify differential phase shift keyed modulated data. The model is used to calculate the optical signal to noise ratio introduced by a power booster SOA and the first inline amplifier of a 40 Gb/s NRZ-DQPSK single channel link.

     

半导体光放大器的光-光互作用及其应用

半导体光放大器（SOA）中的非线性系数约为普通光纤的10^9，为光子晶体光纤的10^7，而且有4种光-光互作用，即交叉增益调制（XGM）、交叉相位调制（XPM）、交叉偏振调制（XSM）及四波混频（FWM），可以灵活地组成各种光信号处理器件，如波长变换器、全光触发器、全光逻辑、全光时钟恢复、全光缓存器……等，正成为整个光信号处理的基础。文章介绍了它们的原理和简单应用。     

All-optical 40 Gbits/s packet regeneration by means of cross-gain compression in a semiconductor optical amplifier

We experimentally demonstrate all-optical reshaping of 40 Gbits/s packets by using cross-gain compression in a semiconductor optical amplifier (SOA). This scheme, which is based on cross-saturation effects between two conjugate signals copropagating in a single SOA, is wavelength preserving and polarization independent and does not suffer from any transient effect at packet edges. We report evidence of noise redistribution and packet reshaping by means of the bit-error rate versus threshold measurements for different input optical signal-to-noise ratios.     

A method of developing wavelength encoded all-optical S–R flip-flop by the uses of semiconductor optical amplifier based Mach–Zenhder interferometer and phase conjugation system

A scheme for a high-speed wavelength encoded all-optical S–R flip-flop (or a digital memory cell for storing of optical bits) based on wavelength conversion (MZI) in semiconductor optical amplifier (SOA) and phase conjugation system (PCS) is proposed. The switching action of semiconductor optical amplifier (SOA) does not give too high operational speed because of electrical pumping power. But optical phase conjugation mechanism gives us ultrahigh operational speed. So, joint use of them gives rise to a more high speed system comparatively to only SOA based switches. Here two logic states of the whole system is encoded by two wavelengths as well as frequencies, since the information in the bit is unaffected throughout the communication not having regard to the loss of light energy due to reflection, refraction polarization, etc.     

Design of ultrafast all-optical PolSK DMUX based on semiconductor optical amplifiers

A novel ultrahigh-speed all-optical demultiplexer (DMUX) with polarization-shift-keying (PolSK) modulation input signals is proposed. This design is based on four-wave mixing (FWM) in a semiconductor optical amplifier (SOA). For analyzing each amplifier, we use finite-difference method (FDM) based on solution of the traveling wave coupled equations. Using numerical simulation, the all-optical DMUX is theoretically realized at 40 Gb/s. We also study the relation between optical confinement factor and thickness of active layer of the SOA section successfully, and investigate the increasing effect of confinement factor on the DMUX optical output power. With this work, the confinement factor is increased from 0.3 to 0.48, and as a result, the output power approximately twice of its initial value is achieved. Moreover, the effects of polarization dependence of SOA on the output performance of all-optical DMUX for PolSK signal are theoretically investigated in detail.     

Comparative analysis of the effects of internal lasing oscillation and external light injection on semiconductor optical amplifier performance

One of the key differences of a semiconductor optical amplifier (SOA) with internal lasing oscillation (ILO) from a SOA with external light injection (ELI) lies in a carrier-sharing mechanism. Since the internal lasing mode shares the same pool of carriers with the signals, the carriers (or photons) withdrawn from the circulating laser mode speed up the gain recovery. On the other hand, the external light injected into the SOA shortens the carrier recovery time through optical pumping without any carrier sharing involved. To find out a better scheme, we have made a comparative investigation on the effects of the ILO and ELI on the SOA performance. It turns out by way of simulation that the ELI scheme provides faster gain recovery, shorter carrier lifetime, and higher saturation power when the external injection power is higher than the internal lasing power. The performance enhancement is not so pronounced with the carrier-sharing mechanism, as the internal lasing mode itself gives rise to severe longitudinal spatial hole burning (LSHB). Nevertheless, the ILO scheme is preferable for linear-amplification applications. We also examine the use of the ELI for low-crosstalk optical amplifiers. It is found that the ELI scheme does not bring in a very strong resonance peak in the crosstalk, which appears in a SOA with ILO due to relaxation oscillations of the lasing mode. In comparison to the ILO in SOAs, the ELI into SOAs is likely to leave more optical gain for multi-channel amplification without any sacrifice on the crosstalk.     

Method of implementing frequency encoded multiplexer and demultiplexer systems using nonlinear semiconductor optical amplifiers

Multiplexing and demultiplexing are the essential parts of any communication network. In case of optical multiplexing and demultiplexing the coding of the data as well as the coding of control signals are most important issues. Many encoding/decoding mechanisms have already been developed in optical communication technology. Recently frequency encoding technique has drawn some special interest to the scientific communities. The advantage of frequency encoding technique over any other techniques is that as the frequency is fundamental character of any signal so it remains unaltered in reflection, refraction, absorption, etc. during transmission of the signal and therefore the system will execute the operation with reliability. On the other hand, the switching speed of semiconductor optical amplifiers (SOA) is sufficiently high with property of best on/off contrast ratio. In our present communication we propose a method of implementing a ‘4-to-1’ multiplexer (MUX) and a ‘1-to-4’ demultiplexer (DEMUX) exploiting the switching character of nonlinear SOA with the use of frequency encoded control signals. To implement the ‘4-to-1’ MUX and ‘1-to-4’ DEMUX system, the frequency selection by multiquantum well (MQW)-grating filter-based SOA has been used for frequency routing purpose. At the same time, the polarization rotation character of SOA has also been exploited to get the desired purpose. Here the fast switching action of SOA with reliable frequency encoded control input signals, it is possible to achieve a faithful MUX/DEMUX service at tera-Hz operational speed.     

Filter-free ultrawideband generation based on semiconductor optical amplifier nonlinearities

A filter-free scheme for ultrawideband (UWB) generation with single semiconductor optical amplifier (SOA) is proposed and demonstrated. A pair of polarity-reversed optical pulses is generated due to cross gain modulation (XGM) in the pump-probe scheme, whereas the amplified pump pulse becomes sharp at its leading edge such that the power peak is shifted forward. Hence, the combination of the pump and probe signals at the SOA output is quasi-monocycle shape. Our scheme is an improved scheme to avoid exploiting optical filters and time-delay devices. The generated UWB radio frequency spectrum shows good stability when the input probe power varies from −10 dBm to 6 dBm, the probe wavelength varies in the whole C-band, and the bias current varies from 100 mA to 240 mA. Although the generated quasi-monocycle deviates from a standard Gauss monocycle to some extent, the frequency spectrum conforms to the UWB regulation. Two SOAs with different XGM dynamics are compared in generating UWB signals.     

All-optical flip-flop based on vertical cavity semiconductor optical amplifiers

We report the operation of an all-optical set-reset (SR) flip-flop based on vertical cavity semiconductor optical amplifiers (VCSOAs). This flip-flop is cascadable, has low optical switching power (~10 microW), and has the potential to be integrated on a small footprint (~100 microm(2)). The flip-flop is composed of two cross-coupled electrically pumped VCSOA inverters and uses the principles of cross-gain modulation, polarization gain anisotropy, and highly nonlinear gain characteristics to achieve flip-flop functionality. We believe that, when integrated on chip, this type of all-optical flip-flop opens new prospects for implementing all-optical fast memories and timing regeneration circuits.     

双波长数据包并行缓存

以半导体光放大器为非线性元件的双环耦合全光缓存器(DLOB)为基础,实现了速率为2.5 Gb/s双波长数据的并行缓存.不同波长光信号合成后的功率随机波动,导致由半导体光放大器交叉相位调制产生的相位差随机波动.在考虑吸收损耗的情况下,对常用的半导体光放大器增益特性曲线进行了修正并与实测值拟合,得到了更为准确的增益特性曲线.在此基础上进行了理论分析,提出了通过调节控制光功率到最佳点以便减小相位差波动的方法.     

Analysis of multitone microwave signals on optical fiber system using external laser modulation

In this paper, we Analyze multitone microwave signals on optical fiber with different tone signals and utilizing various possibilities of regeneration and amplification by different means. We compare out the performance analysis of Erbium doped fiber amplifiers (EDFAs) and Semi-conductor Optical Amplifiers (SOA). One tone's frequency varies from 1 to 20 GHz through parametric runs and other has fixed frequency of 5 GHz and third harmonic tones may be monitored.