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.     

Q-factor assessment of SOA-based ultrafast nonlinear interferometer

The performance of the semiconductor optical amplifier (SOA)-based ultrafast nonlinear interferometer (UNI) gated by an ultra high speed pseudorandom binary sequence is theoretically analyzed. For this purpose, a comprehensive model formed by a set of equations that describe the gain and phase evolution inside a SOA deployed as the nonlinear element in an interferometric switch is appropriately applied to this particular configuration. By undertaking a detailed numerical simulation, the impact of the SOA and input data key parameters on the Q-factor is thoroughly investigated and assessed enabling to extract useful design rules for their proper selection so as to optimize this metric. The calculations confirm the experimental evidence that the main technical limitation is imposed by the SOA carrier lifetime, which must be reduced below the bit period in order to avoid the deleterious consequences of the pattern effect on the switched-out pulses. Provided that this condition is satisfied and the rest of the parameters fulfill their specified requirements, the output amplitude fluctuations can be effectively eliminated resulting in a high quality eye diagram and error-free operation. The adopted model can be exploited for studying more sophisticated all-optical circuits and subsystems of enhanced functionality that rely critically on the SOA-based UNI as switching module.     

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.     

Parameter design and performance analysis of a ultrafast all-optical XOR gate based on quantum dot semiconductor optical amplifiers in nonlinear mach-zehnder interferometer

This paper presents the parameter design and performance analysis of a 160 Gb/s all-optical XOR gate based on cross-gain modulation (XGM) in a nonlinear Mach-Zehnder interferometer (MZI) with quantum dot semiconductor optical amplifiers (QD-SOAs). Detailed numerical simulations of the QD-SOA parameters and optical signal parameters are performed to elevate the gate performance. With the optimized parameters, a Q factor over 8 dB is obtained. The possibility of operating at higher speed of the XOR gate is demonstrated as well. The results will be helpful for the design and performance analysis of practical quantum dot devices.     

Effect of amplified spontaneous emission on semiconductor optical amplifier based all-optical logic

The performances of all-optical logic gates XOR, AND, OR, NOR and NAND based on semiconductor optical amplifier (SOA) have been simulated including the effects of amplified spontaneous emission (ASE). For the parameters used, all-optical logic gates using SOA are capable of operating at speed of 80 Gb/s.     

All-optical logic AND-NOR gate with three inputs based on cross-gain modulation in a semiconductor optical amplifier

We report the realization of a novel all-optical logic AND-NOR gate based on cross-gain modulation (XGM). The used scheme requires only one SOA to perform the logic gate with three input signals. A 8.5 dB dynamic extinction ratio with a switching time of about 650 ps for the rise time and 100 ps for the fall time.     

Circuit designs of ultra-fast all-optical modified signed-digit adders using semiconductor optical amplifier and Mach-Zehnder interferometer

The need for increasingly high-speed digital optical systems and optical processors demands ultra-fast all-optical logic and arithmetic units. In this paper, we combine the attractive and powerful parallelism property of the modified signed-digit (MSD) number representation with the ultra-fast all-optical switching property of the semiconductor optical amplifier and Mach-Zehnder interferometer (SOA-MZI) to design and implement all-optical MSD adder/subtracter circuits. Non-minimized and minimized techniques are presented to design and realize efficient circuits to perform arithmetic operations. Several all-optical circuits’ designs are proposed with the objective to minimize the number of the SOA-MZI switches, the time delay units in the adders, and other optical elements. To use the switching property of the SOA-MZI structure, two bits per digit binary encoding for each of the trinary MSD digits are used. The proposed optical circuits will be very helpful in developing hardware modules for optical digital computing processors.     

200-Gb/s wavelength conversion using a delayed-interference all-optical semiconductor gate assisted by nonlinear polarization rotation

The pattern-induced intensity fluctuation (PIF) of output signals with a bit-rate above 160 Gb/s has been one of the major issues regarding all-optical semiconductor gates. We have demonstrated that the nonlinear polarization rotation (NPR) in the semiconductor optical amplifier (SOA) plays a significant role in the high-frequency operation of a delayed-interference signal-wavelength converter (DISC). We did this using a cross-correlation system whose temporal resolution is 1.5 ps which was developed to monitor our 200-Gb/s, 4992-bit-long binary-patterned waveforms. When we experimentally optimized the NPR effect inside our DISC specially for our 200-Gb/s wavelength conversion, the PIF was significantly improved (from 5.0 to 1.5, for example). Our systematically measured dependence of the PIF on the polarization settings was qualitatively explained with the new gate model that we developed earlier in this work.     

40 Gb/s all-optical logic NOR and OR gates using a semiconductor optical amplifier: Experimental demonstration and theoretical analysis

We experimentally and theoretically demonstrate 40 Gb/s all-optical logic NOR and OR gates based on a semiconductor optical amplifier (SOA) and a blue shifted optical bandpass filter (OBF). Two kinds of data formats are discussed, namely return-to-zero (RZ) format and nonreturn-to-zero (NRZ) format. The logic NOR and OR functions of RZ format are realized at the OBF detuning of −0.22 nm and −0.44 nm, respectively. The logic NOR function of NRZ format is realized at the OBF detuning of −0.24 nm. The simulation is in good agreement with the experimental results when the linewidth enhancement factor is 5.5. The simulation also shows that the SOA with large linewidth enhancement factor is preferred to achieve NOR and OR functions with good performance. The input data signal is of good pulsewidth-tolerance for NOR function, whereas not for OR function. The high Q factor could be obtained at narrow pulses injection.     

Design of an ultrafast all-optical NOR logic gate based on Mach-Zehnder interferometer using quantum-dot SOA

This paper proposes a design for all-optical NOR logic gate, based on Mach-Zehnder interferometer (MZI) using quantum-dot semiconductor optical amplifier (QD-SOA). In this regard, a theoretical model for an ultrafast all-optical signal processor is developed using QD-SOA to achieve high bit rate operation. We have demonstrated the NOR gate operation in two cases of with and without an optical control pulse. Simulations have been carried out at data bit rates 160 Gb/s, 200 Gb/s, and 250 Gb/s for the case that control pulse is not applied, and also at data bit rates 1 Tb/s and 2 Tb/s in presence of control pulse which leads to improvement of gain recovery time and ultrafast NOR logic operation. In addition, quality factors of the output signals in presence and without the control pulse at different bit rates with different bias currents have been investigated for pseudo-random binary sequence (PRBS) of word length 2⁸–1.     

Characterization and optimization of SOA-based UNI recirculating shift register switching window

The operation of an all-optical recirculating shift register implemented with the semiconductor optical amplifier (SOA)-based ultrafast nonlinear interferometer (UNI) in a counter-propagating, inverting configuration is methodically studied and analyzed. This is achieved by applying an appropriate model for a SOA deployed as the nonlinear element in an interferometric switch driven by ultrafast and strong feedback optical pulses. By means of numerical simulation the impact of the critical functional parameters on the formation of the switching window is thoroughly investigated and assessed enabling to provide useful design rules for their proper selection and combination so as to optimize the specific metric and ensure high performance. The undertaken theoretical treatment can be extended for characterizing other more complex all-optical circuits and subsystems of enhanced functionality that employ this type of shift register as the key building module.     

Simultaneous implementation of all-optical OR and AND logic gates for NRZ/RZ/CSRZ ON-OFF-keying signals

An all-optical scheme for simultaneously realizing OR and AND logic gates based on three-input four-wave mixing (FWM) arising in a single semiconductor optical amplifier (SOA) is proposed and demonstrated. It has the ability to process not only conventional non-return-to-zero-ON-OFF-keying (NRZ-OOK) and return-to-zero-OOK (RZ-OOK) formats but also carrier-suppressed return-to-zero-OOK (CSRZ-OOK) format signals. Firstly, the performance of 40 Gb/s logic operation is numerically evaluated by a comprehensive dynamic SOA model considering three input signal induced FWM effect. Then, 10 Gb/s experimental demonstrations with clear waveforms and high extinction ratios (ERs) further verify the logic integrity of this scheme. Thus, the OR and AND logic gates simultaneously achieved within a single logic unit is compact and cost-effective for future optical signal processing applications.     

Theoretical analysis of pattern effect suppression in semiconductor optical amplifier utilizing optical delay interferometer

The ability of an optical delay interferometer (ODI) to suppress the pattern effect that is inherently present in a straightforward, solitary semiconductor optical amplifier (SOA) whose dynamic response is slower than the period of its driving high-speed return-to-zero (RZ) data signal is theoretically investigated. For this purpose an existing comprehensive model that simulates and links the operation of these two elements is methodically applied to their concatenated configuration. In this manner an extensive set of curves is numerically obtained, which allow to analyze and assess the impact of the input pulse energy and width as well as of the SOA carrier lifetime, linewidth enhancement factor and small signal gain on the amplitude modulation of the transmitted sequence at the output of each one of these block units. Their thorough study and interpretation reveals that the employment of the ODI can significantly reduce the value of this quality metric resulting from a single SOA only. The main offered benefit, however, is that any technical restrictions regarding the involved critical parameters can be considerably relaxed while at the same time their useful operational range can be extended. These important findings highlight the necessity of placing this passive device after the SOA and exploiting it in order to effectively alleviate the detrimental pattern-dependent degradation. This fact in conjunction with its overall practicality renders it a promising candidate for enhancing, within the frame of the proposed scheme, the performance of SOAs that are employed as pure amplification elements in fiber-optic communication systems and networking applications.     

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 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 NRZ-OOK-to-RZ-OOK format conversions with tunable duty cycles using nonlinear polarization rotation of a semiconductor optical amplifier

We experimentally demonstrate an all-optical 10 Gb/s format conversion from non-return-to-zero (NRZ) on-off-keying (OOK) to return-to-zero (RZ)-OOK with tunable duty cycle in the whole C-band using nonlinear polarization rotation (NPR) arising in an semiconductor optical amplifier (SOA). The experimental results show that, by tuning the polarizer at the SOA output, an RZ signal with tunable duty cycle from 33% to 66% could be obtained with an extinction ratio(ER) over 10 dB. In addition, we show that the NRZ-to-RZ conversion with duty cycle of 33-66% can be obtained with less than 1 dB power penalty at the bit error ratio (BER) of 10⁻⁹. The device can facilitate the cross-connection between optical transmission networks employing different modulation formats.