High speed semiconductor optical amplifiers and their performance in pseudo-random bit-stream generation

Pseudo random bit sequence (PRBS) generation is important for a wide range of applications including communication systems, pattern generation and encryption. All optical pseudo random bit sequence (PRBS) generation using both regular and quantum dot (QD) semiconductor optical amplifier (SOA) based Mach-Zehnder interferometer have been studied. The speed of operation is limited by the gain and phase recovery times in the SOA. A key element in the fabrication of a PRBS generator is an all optical XOR. All optical XOR functionality has been demonstrated experimentally using regular semiconductor optical amplifier (SOA) based Mach-Zehnder interferometer at 80 Gb/s. The performance of the XOR operation has been analyzed by solving the rate equation of the SOA for SOAs with both regular and QD active region. QD-SOAs should result in significantly higher speed operation. The paper shows that QD-SOA based devices can be used to produce PRBS generators that operate near 250 Gb/s.

Linear feedback shift registers (LFSRs) are essential components of communications systems such as pseudo random bit sequence (PRBS) generators, convolutional encoders, scramblers and randomizers [1, 2]. Fig.1 (left) shows a LFSR scheme in which the XOR gate is operated among two tapped bits i and i+k within the shift register. The result bit is fed to the shift register at the last position (n) and the sequence in the register is 1-bit shifted as well. The experimental setup for demonstrating an all-optical LFSR is shown in Fig.1 (middle). The main components of the scheme are an optical buffer [3], a bit selecting circuit, and a XOR gate. The data vector (A) is generated by modulating the continuous wave (CW) λ s with a 10Gbps non-return-to-zero (NRZ) electrical pattern whose period is T P =33.3168µs. Only 5 bits are used as the initial sequence for the shift register. The travelling time of the packet in the buffer is T L =1.85 µs, meaning that up to 18 round trips can be accommodated by the pattern's period T P but one of those should be preserved for erasing the buffer before it is fetched a new sequence. The initial sequence enters the buffer as well as propagates directly to the bit selecting circuit. Every round trip, a new sequence as a replica of the previous one plus the feedback bit comes out. At the last round trip, a pump signal inserted into the buffer through the circulator will strongly reduce the gain of the semiconductor optical amplifier (SOA). Both the circulating packet and the accumulating in-band amplified spontaneous emission (ASE) noise will be suppressed. The buffer is now completely cleaned for a new cycle. The first and the last bits of the 1-bit shifted sequence are selected for the XOR operation. Referring to Fig.1 (left), i=1 and k=4, n=5. With this setup, the bit selecting circuit requires 2 clock signals (C) for tap the proper bits from the incoming sequence. The sampling process is done by exploiting four-wave-mixing (FWM) in a 250m-long highly nonlinear fiber (HNLF). Since at every round trip the sequence must be 1-bit shifted, the sampling clocks must be accordingly delayed for 1 bit, resulting the clock period of T C =T L +T B where T B =100ps is the bit time. The outputs of the bit selecting circuit are fed as pump signals to port a and d of a SOA Mach Zehnder interferometer (MZI). They are synchronized with a clock at λ s used as the probe signal (F) at port b. The output of the XOR gate is then fed back into the buffer. The feedback bit should be precisely allocated at the end of the packet circulating inside the buffer, meaning that the feedback time should be T F =T L +T B +T seq in which T seq is the original sequence duration and equal to 500ps in our setup. Fig.1 (right) depicts the results at the 10th round trip for two different input initial sequences (10001) and (10011) taken at point (B). The equalization between bits in the sequence and the quality of the pulses is strongly limited by the accumulating ASE noise inside the buffer, the long fiber-based structure and the bit's polarization. The fiber-based setup also causes misalignment between the clocks, spoils the essential synchronism and degrades the performance of the FWM and the XOR operation. One possible solution for solving all of these problems and hence improving the performance is the integration approach. The bit selection could be implemented exploiting nonlinear effects into an SOA or a periodic-poled lithium niobate (PPLN) instead of using a long HLNF. Moreover the extinction ratio of the pulses at the SOA-MZI XOR output (<13.5dB) could be improved, e.g. with a saturable absorber at its output, thus increasing the number of allowed round trips into the shift register and reducing the noise on the final bit sequence.

In post-silicon testing and validation of circuit functionality, an effective IO stress pattern can identify bugs quickly and provide adequate test coverage. A lot of work has been done to identify the right stress patterns specific to each IO interface. While some patterns can be generic enough to apply to all IOs, other patterns are interface topology specific. In addition to identifying the worst-case pattern, tradeoffs between test-time and test coverage must be made depending on the test goals. Pseudo Random Bit Stream (PRBS) generators are commonly used to generate test patterns because of the adequate frequency content in the PRBS patterns, the ease of implementation, and minimal gate count. This paper introduces an Advanced Pattern Generator and Checker (APGC) based on PRBS that retains all the aforementioned advantages. The APGC was implemented for a DDR memory interface where different LFSRs beat against each other spatially on neighboring IO lanes while rotating this form of aggressor-victim pattern in time. The results of the APGC stress patterns are compared to a form of advanced software-based learning algorithm based patterns that exhaustively search this complete parameter space. The comparison of APGC to software showed that the measured bit error rate (BER) plotted on a Q-scale of both methods is similar for the Receiver side. On the Transmitter side, APGC showed less eye opening than the software. In addition to the margin comparison, on the test execution side, APGC can speed up the test and validation execution time compared to the software by 32 to 2048 times depending on aggressor victim lane width of 8 to 64 lanes.

In this presentation I will discuss photonic crystal lasers with particular emphasis on devices capable of room temperature CW operation, devices capable of achieving approximately 100 mW of output power under pulsed operation, and devices with quantum dot active regions. The CW lasers have 3 dB bandwidths of just under 10 GHz with approximately 30 dB of side mode suppression. The photonic crystal lasers with quantum dot active regions have absorbed powers at threshold of under 10 microwatts. I will also discuss efforts to model the electromagnetic properties of these devices in the near and far field using both finite-difference time-domain and finite element methods and compare these predictions to the experimental data. The presentation will also address device issues associated with passive photonic crystal components such as optical loss, waveguide dispersion, and the design of waveguide junctions. Demonstrations of Mach-Zehnder interferometers and directional couplers will be presented and again results from experiments will be compared to numerical predictions.

Photon is established as a strong and promising candidate in all-optical signal processing and superfast computing. Again, it is found that photon has a successful approach to be used as a quantum mechanical particle. For this reason, it is used as a carrier of information in optical systems instead of electrons in electronic systems. All-optical quantum systems can provide very high speed, secured and noise free communication and computation. Over the last few decades, several all-optical logic processors, algebraic processors, logic gates and their integrated systems are developed by many researchers. All-optical quantum logic gates with qubit data are the basic building block of an optical quantum computer. In quantum computation, qubits or quantum bits are the quantum analogue of classical boolean bits and are used to store information. Several popular encoding techniques (like phase encoding, frequency encoding, intensity encoding, polarization encoding etc.) are used to develop the qubits. Here, in this paper, the gate matrix for quantum optical tristate CNOT logic gate is developed for the first time with a frequency encoding technique. Quantum optical CNOT gate is one of the most important logic gates in the logic family that is extensively used for performing information processing task in optical systems. Tristate-based optical logic operations are proved to increase the data handling capacity. Also, the speed of different types of optical logic operations can be increased with tristate-based optical logic systems. Introduction of contradiction state is another advantage of tristate logic systems. Frequency encoding principle is the best way to encode tristate, quaternary state and multivalued logic states. In the article, the authors have developed a proper truth table and gate matrix of frequency encoded tristate CNOT gate. The all-optical scheme for tristate CNOT gate can be implemented with semiconductor optical amplifier (SOA)-based optical switches. At the same time advantages of quantum optical operations are exploited.KeywordsSemiconductor optical amplifierTristate logicFrequency encoding principle

We studied the output optical power of microdisk lasers with InGaAs/GaAs quantum dots active region. An increase in the number of layers in the active region in the waveguide from 2 to 6 leads to increase in the peak output optical power due probably to increase of the gain. We also observe a corresponding increase of the threshold current due to the increase on the transparence current. The maximal optical power is achieved for structure with 6 layers at approximately 60 mA injection current. Further increase of the number of the QD layers to 10 results in increase of the threshold current and sudden drop of the output power.

We demonstrate two dimensional photonic crystal nanocavities embedding spatially-isolated active regions containing InAs/GaAs self-assembled quantum dots. We fabricated the GaAs-based structures by a combination of electron-beam lithography, dry/wet etching and crystal regrowth using molecular beam epitaxy. The fabricated nanocavity based on a photonic crystal double heterostructure supports a cavity mode with a high experimental quality factor over 10,000, despite the existence of a mound structure on the nanocavity formed during the regrowth. Using electromagnetic simulations, we discuss the origin of the cavity mode and a direction for further increasing the quality factor. The two dimensional photonic crystal nanocavities with buried quantum dot active regions will provide a useful platform for developing high performance nanolasers.

Summary form only given. Vertical cavity surface emitting lasers (VCSELs) emitting at 1.3 /spl mu/m and integrated with GaAs-based microelectronic circuits have many potential applications in optical communication systems. Several approaches exist for the realization of 1.3 /spl mu/m VCSELs. A promising approach for the realization of longer wavelength VCSELs is the use of self-organized InAs-InGaAs quantum dot (QD) active gain regions. In 1997 we demonstrated QD VCSELs on GaAs with peak emission wavelengths near 1.0 /spl mu/m and sub-milliampere threshold currents. We have now extended the QD peak emission wavelength to 1.3 /spl mu/m by selectively varying the physical size and composition of the InAs-InGaAs dots and herein report room temperature continuous wave (CW) 1.3 /spl mu/m QD VCSELs. While striving to maximize the uniformity of the QDs to increase device efficiency at a single emission wavelength, we have also experimented with sheets of QDs wherein the dot size is purposely nonuniform. Though less efficient, these nonuniform QD active regions have potential applications in tunable VCSELs where a broad gain bandwidth is required.

Sub-picosecond all-optical switch using a multi-quantum-well semiconductor optical amplifier

In this article, the acceleration attained in gain recovery dynamics of travelling-wave-type semiconductor optical amplifier (SOA) at the expense of structural optimization is illustrated via numerical simulations. A pump–probe scheme has been utilized in order to study the outcomes of optimization of SOA operational and structural parameters on its effective gain recovery time ( $${\tau _\mathrm{e}}$$ ). A set of optimized SOA parameters are formulated from gain recovery dynamics studies after keeping practical implementation considerations in vision. Further, the impacts of altering SOA structural and operational parameters such as injection current (I), amplifier length (L), active region width (w), active region thickness (t) and optical confinement factor ( $${\varGamma } $$ ) on gain recovery time improvement achieved are further investigated on the performance of a cross-gain modulation (XGM) in SOA-based all-optical half-subtracter in terms of two designated performance metrics: quality factor (Q-factor) and extinction ratio (ER). It has been revealed that reduced gain recovery time-optimized SOAs-based all-optical half-subtracter arranged in a co-propagating manner exhibits improved Q-factor and ER (dB) performance at high bit rates of operation ( $$\le $$ 80 Gbps).

The effect of p-type doping of the barriers of multiple quantum well semiconductor optical amplifiers is shown to significantly reduce the gain and carrier recovery times with phase recovery times as short as 11ps.

Security and privacy problems in communication systems and social media platforms where digital image/video are shared almost always, have attracted researchers interest on information security. Starting from this point of view, a novel one-dimensional chaotic maps based pseudo random bit generator is proposed to make a significant contribution to the literature about protection of personal data in any network. Electronic circuit realizations of Logistic and Tent maps as entropy source are designed on Orcad-Pspice environment and state variables are inputted to novel post-processor algorithm. The rest of main blocks of proposed pseudo random bit generator design are built up fixed-point binary conversion algorithm, XOR processor and H function post-processor. The generated pseudo random bit series are tested by using NIST 800.22 statistical test suite and applied to color image encryption in order to show the effectiveness of proposed design. Cryptanalysis processes such as histogram, NPCR-UACI and correlation analysis are demonstrated. Analysis results show that the proposed design can be used successfully in many secure communication and media transmission applications.

The use of a tensile-strained multiquantum-well (MQW) active region in 630-nm-band InGaAlP MQW laser diodes is investigated through photoluminescence (PL) measurements. The critical conditions for lattice relaxation of strained MQW layers are also discussed. Emissions due to electron–heavy-hole recombination and electron–light-hole recombination are observed in the PL spectrum. A large energy difference (about 38 meV) is found between the two peaks, and radiative recombination is dominant in the MQW structure. These are thought to decrease the threshold current of tensile-strained MQW laser diodes. The MQW active region is thought to be extremely close to the critical conditions for lattice relaxation, but no adverse effects on optical properties are observed. It is concluded that such a tensile-strained MQW active region has advantages for use as the active region of 630-nm-band laser diodes.

Development of modern optical fiber networks puts an increasing demand on the optical hardware. All-optical signal processing components enable the highest switching rates and allow all-optical regeneration of pulse streams without converting the optical signal into electrical current. The subject of our research is self-switching in a Mach-Zehnder interferometer (MZI) and its applications in optical telecommunication networks. In this device, light injected in one of the input ports is unequally distributed over the two interferometer arms. Due to the intensity dependent refractive index in the interferometer arms, there can be a nonlinear phase shift induced between the optical signals of unequal intensities. The two signals therefore interfere destructively or constructively depending on the input power: in this way we obtain nonlinear self-switching. Two mechanisms of nonlinear phase shifting were considered: active, based on semiconductor optical amplifiers (SOAs), and passive, based on quantum dots (QDs). Interferometers of two types were developed: 2-to-2 (two input ports and two output ports) and 2-to-1 (two input ports and one output port). The 2-to-2 SOA-MZIs based on self-switching have been investigated for two applications. One of them is the pattern effect compensator. If SOAs are employed for all-optical signal amplification, e.g. in optical access networks, an unwanted pulse distortion (known also as the pattern effect) takes place, as a result of the SOA gain saturation. Our component allows pattern-free amplification of the optical signals at bitrates up to 20 Gb/s. At 10 Gb/s it shows an extended input power range (up to 7 dB improvement) and comparable gain, which makes it suitable to be used as an optical amplifier. Another application of the 2-to-2 SOA-MZI is a 2R-regenerator. Optical amplifiers used in long distance optical links add noise to the optical signal, causing signal degradation. The signal can be regenerated by passing it through an optical gate with a nonlinear transfer function. The 2-to-2 SOA-MZI has such a nonlinear transfer function. The regeneration capabilities were demonstrated at 2.5 Gb/s by an improvement of the receiver sensitivity of about 2.5 dB. The dynamic characterization of the chips was carried out in a close cooperation with the research group COM at the Technical University of Denmark within the ePIXnet "network of excellence". The 2-to-1 MZI based on self-switching can be used as a low-loss optical combiner. An essential function in optical fiber networks is the combining of optical signals. A serious disadvantage of the conventional type of combiners used in the networks is that they let only half of the power (3 dB) through. In order to compensate for this loss, passive optical combiners are often used in combination with an in-line SOA. The first realization of the low-loss optical combiner uses SOAs as active phase shifters. Such an active low-loss combiner shows an improvement of transmission of over 2 dB compared to a conventional combiner with an in-line SOA. It is therefore expected that the output optical signal-to-noise ratio of the self-switching SOA-MZI is more than 2 dB better than that of a conventional combiner with an in-line SOA. While for the pattern effect compensator and the 2R-regenerator SOAs are used not only for inducing the nonlinear phase shift, but also for sufficient amplification, for the low-loss optical combiner the preferred nonlinear effects should be passive: combination of the signal is a passive function. Therefore, the second realization is based on a novel material, quantum dots. QDs provide improved all-optical nonlinearities resulting in a very small switching energy and large refractive index changes. Such a passive 2-to-1 QD-MZI based on self-switching showed an improvement up to 1.7 dB with respect to a conventional combiner. These improvements have a huge effect on e.g. the power budget in passive optical networks, where a large number of splitting stages are required. The Mach-Zehnder interferometers were realized in the InP/InGaAsP semiconductor material system, which is perfectly suitable for the integration of the photonic integrated circuits for the telecommunication applications. In order to realize both active components (such as e.g. SOAs) and passive components (such as e.g. waveguides, couplers), an active-passive integration technique was applied. This integration was realized in a close cooperation between JDS Uniphase Eindhoven and the COBRA Research Institute. It employs a three-step metal-organic vapor-phase epitaxy regrowth process. The quantum dot material was grown within the COBRA Research Institute. Our MZIs use a ridge waveguide design, for which a reactive ion etching process was developed in the COBRA cleanroom. As a result of photonic integration our integrated Mach-Zehnder interferometers have very small dimensions: less than a square millimeter.

Future optical networks may require both wavelength conversion and bit shape regeneration in an all-optical domain. The possibility of pulse reshaping while providing wavelength conversion may support new demands over medium and large distances links (Kelly, 2001). Indeed, during propagation the optical data signal suffers deterioration due to the amplified spontaneous emission (ASE) from optical amplifiers, pulse distortion from intrinsic dispersion, crosstalk, and attenuation. All-optical regenerators may be important components for the restoration of these signals, providing complexity and cost reductions with the avoidance of optoelectronic conversions. The regeneration could be 2R (reamplification and reshaping) or 3R, which also provide retiming to solve jitter (Simon et al., 2008). Several 3R regenerators using the semiconductor optical amplifier (SOA) have been proposed, such as cascaded SOAs setups (Funabashi et al., 2006) or SOA based MachZehnder interferometers (MZI) (Fischer et al., 1999). However for small and medium distances systems, where the signal amplitude noise and distortions form the main problem and where jitter has fewer magnitudes, the simpler 2R processes can be adequate to keep signal quality (Simon et al., 1998). In addition, the SOA is a helpful device for both 2R-regeneration (Ohman et al., 2003) and wavelength conversion (Durhuus et al., 1996). Several techniques for 2R-regeneration based on SOAs have been proposed and tested, for example by using four-wave mixing (FWM) (Simos et al., 2004), cross-gain modulation (XGM) (Contestabile et al., 2005), integration within MZI (Wang et al., 2007), multimode interferometric SOA (Merlier et al., 2001), cross-phase modulation (XPM) with filtering (Chayet et al., 2004), and feed forward technique (Conforti et al., 1999). However, these techniques require complex designs and involve critical operation points, even the simplest ones based on XGM features. In addition, most of these techniques are not capable of wavelength conversion and regeneration simultaneously. Recently a regenerator based on cross-gain modulation was proposed using one SOA for wavelength conversion (in a counter-propagating mode) and another deeply saturated SOA (synchronized by an optical delay line) to achieve cross-gain compression (Contestabile 2005). This efficient approach has similarities with the all-optical feed-forward techniques. In addition, this regenerator could not done wavelength conversion if the wavelength of the