On the compensation of chirp induced from semiconductor optical amplifier on RZ data using optical delay interferometer

The capability of an optical delay interferometer (ODI) to compensate the chirp induced on return-to-zero pulses amplified by a semiconductor optical amplifier (SOA) when operated under stressful conditions for its gain dynamics is investigated and demonstrated through extensive numerical simulation. The phase response of the ODI, which through its variation per time increment determines the chirp, is calculated at its crossed output port using an explicit expression. The theoretical analysis reveals that cascading the ODI after the SOA can reduce both the magnitude of the chirp and the variations of its peaks as well as those of the amplified pulses while ensuring error-free performance even for a tight combination of the critical parameters. In order for this goal to be successfully accomplished while not distorting the pulses acted on by the ODI the offset introduced by this passive element is computationally found that it must not exceed 10% of their repetition interval. Therefore the scheme can constitute a promising technological option for efficiently exploiting the chirp of an SOA and simultaneously using the SOA as gain block for direct amplification purposes.     

On the data rate extension of semiconductor optical amplifier-based ultrafast nonlinear interferometer in dual rail switching mode using a cascaded optical delay interferometer

The feasibility of increasing by a factor of two the data speed of the semiconductor optical amplifier (SOA)-based ultrafast nonlinear interferometer in dual rail switching mode by means of a cascaded optical delay interferometer (ODI) is explored and shown through numerical simulation. From the theoretical analysis it has been found that such extension cannot be done without employing this passive element for any selection of the critical parameters but the SOA carrier lifetime, for which the requirements are yet very demanding. If, however, the time delay introduced by the ODI is adjusted to almost 1/3rd of the bit period, then the result of Boolean XOR operation can be improved for a specified range of parameter values, which can be further selected to be more relaxed than is possible when the ODI is not being used. The use of the ODI allows both error-free and pattern-free performance at the output of the interferometric structure configured as ultrafast XOR gate. In this manner the scheme can offer a practical alternative solution for extending the operating rate of this logical module and enabling its exploitation as a basic building unit in more sophisticated all-optical circuits and subsystems.     

Simulation and evaluation of phase noise for optical amplification using semiconductor optical amplifiers in DPSK applications

A thorough simulation and evaluation of phase noise for optical amplification using semiconductor optical amplifier (SOA) is very important for predicting its performance in differential phase-shift keyed (DPSK) applications. In this paper, standard deviation and probability distribution of differential phase noise at the SOA output are obtained from the statistics of simulated differential phase noise. By using a full-wave model of SOA, the noise performance in the entire operation range can be investigated. It is shown that nonlinear phase noise substantially contributes to the total phase noise in case of a noisy signal amplified by a saturated SOA and the nonlinear contribution is larger with shorter SOA carrier lifetime. It is also shown that Gaussian distribution can be useful as a good approximation of the total differential phase noise statistics in the whole operation range. Power penalty due to differential phase noise is evaluated using a semi-analytical probability density function (PDF) of receiver noise. Obvious increase of power penalty at high signal input powers can be found for low input OSNR, which is due to both the large nonlinear differential phase noise and the dependence of BER vs. receiving power curvature on differential phase noise standard deviation.     

Small-signal analysis of two cascaded semiconductor optical amplifiers in a counterpropagating configuration

In this work, we present a small-signal analysis which reveals its usefulness to explain the evolution of the optical frequency responses obtained at the outputs of two-cascaded semiconductor optical amplifiers (SOAs) used in a counterpropagating configuration (TC-SOA-CC). To obtained these responses, the TC-SOA-CC configuration has been implemented on an electric simulator and we have used for each SOA a multisection rate equation based model. This model permits us to study both the static and dynamic behavior of the TC-SOA-CC optical outputs in the context of bidirectional transmission. Thanks to this model, we can also obtain the evolution of the carrier lifetimes in each SOA in order to determinate their influence on the optical frequency responses. The small-signal responses derived from our small-signal analysis are given as a function of the intrinsic parameters and for each specific situation for the value of the average injected optical power. This small-signal analysis is used to comment the behavior of the frequency responses obtained by simulation at the TC-SOA-CC optical outputs. Thanks to our small-signal analysis, we explain why for such a configuration, the features of the frequency responses obtained at the SOAs optical outputs can change from a low-pass, to a high-pass, and/or to a band-pass form by varying the average injected optical powers in the SOAs.     

On the output characteristics of a semiconductor optical amplifier driven by an ultrafast optical time division multiplexing pulse train

A comprehensive theoretical analysis of a semiconductor optical amplifier (SOA) that is subject to an ultrafast optical time division multiplexing pulse stream is presented with the help of a simple but efficient model developed for this purpose. The model combines the necessary set of mathematical equations with the appropriate simplifying assumptions to describe in the time domain gain saturation and recovery for the case of multiple incoming pulses. In this manner, analytical expressions can be obtained for the power and chirp profile of the amplified pulses, essentially extending the work that has been performed for a single pulse only. This allows to identify the critical operational parameters and to investigate and evaluate their effect on these two output characteristics. The derived simulation curves are thoroughly studied to specify the limitations imposed on the SOA small signal gain and carrier lifetime as well as on the full-width at half-maximum (FWHM) and energy of the input pulses and, based on a series of logical arguments, to extract useful rules concerning their selection so as to achieve improved performance with respect to the practical applications of all-optical switching and pulse compression. The obtained results indicate that due to the continuous insertion of pulses, the requirements for the SOA small signal gain and the input pulse energy are stringent than those for the case of isolated pulse amplification. The combination of these two parameters determines also the regime in which the amplifier must be biased to operate in order to ensure distortionless pulse amplification and enhanced chirp for efficient pulse compression and it has been found that low saturation is necessary for the former case whilst heavy saturation for the latter. The scopes of the corresponding requirements for the carrier lifetime and the FWHM are also tight but to a less extent and can be simply satisfied with the available photonics technology. These results are in good agreement with the available experimental data essentially proving the validity and robustness of the model. The model can be thus applied to predict the behavior of more complex all-optical circuits of enhanced functionality in which the SOA is the basic functional device.     

All optical frequency encoding method for converting a decimal number to its equivalent binary number using tree architecture

In any kind of computing and data processing system the use of binary numbers are found very much suitable and reliable. On the other hand several natural representations have been realized using decimal numbers. So conversion of a decimal number to its binary equivalent and vise-versa are of great importance in the field of computation technology. There lie already a number of established methods regarding such conversion processes. Again optical tree architecture is one of the most promising systems for realizing the optical conversion of any decimal number to its equivalent binary. Here in this communication the authors propose a new method for optical conversion of a decimal number to its binary equivalent using tree architecture based system and frequency encoding principle. In frequency encoding system, frequency of light is used for encoding of decimal digits or binary bits instead of intensity variation. For example 0 and 1 bits of binary number are coded by two different frequencies of light signal, instead of representing the presence of light as 1 and absence by 0. The proposed conversion process has multifaceted advantages in communication, as well as in data processing. To implement the above conversion some characteristic features of semiconductor optical amplifier (SOA) have been used massively. The wavelength conversion property, cross gain modulation and some nonlinear properties of SOA are exploited to get the frequency encoded response. The proposed system carries all the basic advantages of optical processing as well as those of frequency encoding also.     

All optical logic NAND gate based on two-photon absorption in semiconductor optical amplifiers

We propose a new scheme to realize all optical logic NAND operating at high speeds up to 250 Gb/s utilizing the ultrafast phase response during two-photon absorption (TPA) process in semiconductor optical amplifiers (SOA). NAND gate is important because other Boolean logic elements and circuits can be realized using NAND gates as basic building blocks. Rate equations for semiconductor optical amplifiers (for input data signals with high intensity) configured in the form of a Mach-Zehnder interferometer have been solved. The input intensities are high enough so that the two-photon induced phase change is larger than the regular gain induced phase change. The performance of this scheme is analyzed by calculating the quality factor of the resulting data streams. The results show that both AND and NAND operations at 250 Gb/s with good signal to noise ratio are feasible.     

基于半导体光放大器的非归零信号时钟恢复

提出了一种基于半导体光放大器加窄带光纤光栅滤波器，将非归零信号转换为伪归零信号，再把伪归零信号注入到主动锁模环行腔激光器进行时钟提取的非归零信号时钟恢复方案.利用该方案实现了10 Gb/s伪随机非归零信号的全光时钟恢复，对工作原理和结果进行了分析和讨论.实验证明该方案具有结构简单，调整容易，输出波形好的特点.     

Performance analysis and improvement of semiconductor optical amplifier direct modulation with assistance of microring resonator notch filter

We demonstrate through numerical simulations that a passive single-bus microring resonator (MRR) employed as notch filter enables to directly modulate an ordinary semiconductor optical amplifier (SOA) at 10 Gb/s. This data rate constitutes a fourfold increase in the speed that it is possible to modulate the SOA due to its finite differential carrier lifetime. The theoretical analysis unveils how the MRR radius and detuning must be selected so that the MRR-based filtering scheme efficiently overcomes the pattern-dependent impairments incurred by the SOA narrow electrical bandwidth. Moreover, the small-signal analysis conducted to correlate the perturbations in power of the modulated optical signal inserted in the MRR with those at the MRR output confirms the enhancement in the electrical bandwidth of the SOA–MRR combination. Provided that the MRR response is tailored as suggested, which is technologically feasible, the MRR action permits the encoded signal to exhibit at extended data rate a pulse and eye diagram profile of acceptable quality, tolerable amplitude difference indices, an error probability below the forward error correction limit, and a sufficient system net gain for medium-haul transmission in standard single-mode fiber with dispersion compensation. These performance improvements can be beneficial for the direct modulation of standard SOAs. Together with SOAs inherent amplification capability, they can open new perspectives in using these active elements as external modulators in various target applications.     

Analysis of format conversion from optical 16QAM to QPSK based on cascaded four-wave mixing in semiconductor optical amplifier

A scheme of format conversion from optical 16-ary quadrature amplitude modulation (16QAM) to quadrature phase shift keying (QPSK) signal based on cascaded four-wave mixing (FWM) in semiconductor optical amplifiers (SOAs) is proposed. Theoretical analysis and simulations of the format conversion scheme are conducted to validate the feasibility of the proposal. In this proposal, the phase conjugated of 16QAM signal is generated after the first FWM process in an SOA, and then the QPSK signal is converted due to the second non-degenerate FWM (ND-FWM) process in another SOA. The performance and the optimal design of the 10 Gbit/s format conversion system under various key parameters of SOAs are evaluated and discussed. Simulation results present useful to enable interconnection between backbone network and access network.     

A method of developing frequency encoded multi-bit optical data comparator using semiconductor optical amplifier

Optical data comparator is the part and parcel of arithmetic and logical unit of any optical data processor and it is working as a building block in a larger optical circuit, as an optical switch in all optical header processing and optical packet switching based all optical telecommunications system. In this article the author proposes a method of developing an all optical single bit comparator unit and subsequently extending the proposal to develop a n-bit comparator exploiting the nonlinear rotation of the state of polarization of the probe beam in semiconductor optical amplifier (SOA). Here the dataset to be compared are taken in frequency encoded/decoded form throughout the communication. The major advantages of frequency encoding over all other conventional techniques are that as the frequency of any signal is fundamental one so it can preserve its identity throughout the communication of optical signal and minimizes the probability of bit error problem. For frequency routing purpose optical add/drop multiplexer (ADM) is used which not only route the pump beams properly but also to amplify the pump beams efficiently. Switching speed of ‘MZI-SOA switch’ as well as SOA based switches are very fast with good on–off contrast ratio and as a result it is possible to obtain very fast action of optical data comparator.     

DSB optical mm-wave with signal only on optical carrier generated by embedded LiNbO3 Mach-Zehnder modulator

We have investigated the generation of the 40-GHz double-sideband optical millimeter (mm)-wave with signal carried only by its optical carrier via an embedded LiNbO₃ Mach-Zehnder modulator (LN-MZM). Since the optical carrier and its two first-order sidebands are dominant and their powers are well balanced, the first-order harmonic in the photocurrent gets maximal. As the optical mm-wave signal is transmitted along the fiber, there is no code outline distortion because the signal is only modulated on the optical carrier. Although the first-order harmonic shows the periodical fading effect when the optical mm-wave signal is transmitted along the fiber, its degradation on the radio-over-fiber link can be avoided by adjusting the position of the fading nodes via varying the main MZM bias voltage, and the signal still keeps much good eye diagram even after 50-km fiber transmission. The experimental results prove our theory.     

A self-consistent model for a SOA-based fiber ring laser including the mode-locked pulse properties

In this paper, we present a self-consistent model of an optically mode-locked semiconductor fiber ring laser. The fiber laser uses a semiconductor optical amplifier (SOA) as the gain medium, while mode-locking is achieved by its gain modulation, via an external optical pulsed signal. We solved the model analytically developing a novel technique, where we have assumed double saturation of the SOA by both the mode-locked and the externally introduced pulsed signal. The study revealed the locus of the laser parameters to achieve mode-locking. In particular, it was found that SOA gain and energy of the externally introduced signal are two critical parameters that must simultaneously set properly for exact mode-locking. Another outcome of our analysis is that the study of the chirp parameter should be carried out keeping the nonlinear terms of the SOA gain. We have also investigated a slightly detuning regime of operation that revealed a fast change of the mode-locking process.     

Fully reconfigurable optical add-drop multiplexer based on parallel configuration using Mach-Zehnder module

Reconfigurable optical add-drop multiplexer (ROADM) with the ability of dynamic configuration will be one of the core equipment for the future optical transport networks. This paper proposes an effective scheme for fully ROADM with parallel modularized structure and ability of upgrading. Each module consists of a Mach-Zehnder interferometer based on fiber Bragg grating (FBG), a 1 × 2 optical switch and a Y-model combiner. Optical signal with single wavelength or multiwavelength could be dropped or added at one drop port or add port by configuring the optical switch in each module. The proposed ROADM with four-stage parallel connection is designed and experimentally investigated to prove its performance and advantage in dynamic selection of the wavelengths and upgrading. It is also showed that this ROADM can also reduce the ununiformity of power at output port and drop port to improve the optical signal to noise ration (OSNR) of the whole system.     

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.     

On the feasibility of full pattern-operated all-optical XOR gate with single semiconductor optical amplifier-based ultrafast nonlinear interferometer

The possibility of implementing an ultrafast all-optical XOR gate using a single semiconductor optical amplifier (SOA)-based ultrafast nonlinear interferometer (UNI) is theoretically investigated and demonstrated. For this purpose a comprehensive model that characterizes the performance of a SOA when it is successively driven by two strong pseudorandom binary sequences is applied to simulate the specific module under dual rail switching mode of operation. In this manner an extensive set of curves is obtained allowing to analyze and evaluate the impact of the input data, SOA and interferometer critical parameters on the fully loaded Q-factor. Their thorough study and interpretation reveals that the satisfaction of their requirements in order to render acceptable this metric is feasible from a technological perspective and thus if their selection is made according to the extracted guidelines then pattern-free and error-free modulo-2 arithmetic can be straightforwardly realized at 20 Gb/s. This prediction can be of practical interest in simplifying and assisting the design of more sophisticated interconnections of enhanced combinatorial and sequential functionality in which the XOR gate is the core logical unit.     

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.     

Transmission performance of optical OFDM signals with low peak-to-average power ratio by a phase modulator

We have experimentally generated optical orthogonal frequency-division multiplexing (OOFDM) signals by a phase modulator (PM). The generated OOFDM signal can tolerant higher nonlinear effects in fiber than that generated by an intensity modulator because of its lower peak-to-average power ratio (PAPR). It is shown that, by using a PM, the PAPR of the OOFDM signal has a 2-3 dB reduction and the input power in fiber can be improved over 3 dB.     

Theoretical analysis and performance investigation of ultrafast all-optical Boolean XOR gate with semiconductor optical amplifier-assisted Sagnac interferometer

A comprehensive theoretical model of an ultrafast all-optical Boolean XOR gate implemented with a semiconductor optical amplifier (SOA)-assisted Sagnac interferometer is presented. The model accounts for the SOA small signal gain, linewidth enhancement factor and carrier lifetime, the switching pulses energy and width and the Sagnac loop asymmetry. By undertaking a detailed numerical simulation, the influence of these key parameters on the metrics that determine the quality of switching is thoroughly investigated and simple design rules are extracted for their proper selection so as to ensure optimum operation. The obtained results are in good agreement with the published experimental measurements and confirm the feasibility of realizing the gate at 10 Gb/s with fairly high performance. The model can be extended for studying more complex all-optical circuits of enhanced functionality in which the XOR gate is the basic building block.     

一种全光归零码到非归零码变换的新技术方案

报道了一种利用单只半导体光放大器和光滤波器实现全光归零码到非归零码变换的新技术方案.当探测（Probe）光和数据信号光同时输入到SOA时,基于SOA中的交叉增益调制和交叉相位调制现象,探测光光谱的后沿和前沿将分别产生红移和蓝移.通过调节光滤波器和探测光的中心波长之间的失调量,滤出光谱的特定部分,可以得到转换后的NRZ码光信号.这种新型的全光码型变换器具有结构简单、偏振不敏感、控制参量少和稳定性高的特点.分别采用仿真和实验的方式实现了20 Gbit/s光数据信号从RZ码到NRZ码的全光码型变换,并且仿真结果和实验结果相吻合.