All-Optical Encryption and Decryption Using Quantum Dot Semiconductor Optical Amplifiers with Two Photon Absorption

We have developed a theory of high-speed operation of quantum dot (QD) semiconductor optical amplifiers (SOAs), and showed that pattern-effect-free amplification of pulse trains, cross gain modulation (XGM) and cross phase modulation (XPM) can take place in QD SOA in the regime with maximum gain. Formulas, which relate the maximum bit-rate for the pattern-effect-free operation and the average SOA output power to the SOA pump current density, were derived. XGM without pattern effect can be realized in the regime with maximum gain due to spectral hole burning effects. Possibility of ultrafast frequency conversion and demultiplexing of data pulse streams through this nonlinearity is illustrated. Expression for the nonlinear refractive index η nl due to spectral hole burning in QD structure was obtained. The value of η nl in QD SOAs can be by 4-5 orders larger than η nl in silica; and efficient ultrafast XPM without pattern effects can be carried out in QD SOA through this nonlinearity. In whole, usage of the regime with maximum gain in QD SOAs can lead to development of new generation of high-speed devices for ultrafast optical processing and communications.

We study the interplay between coherent light-matter interactions and non-resonant pulse propagation effects when ultra-short pulses propagate in room-temperature quantum-dot (QD) semiconductor optical amplifiers (SOAs). The signatures observed on a pulse envelope after propagating in a transparent SOA, when coherent Rabi-oscillations are absent, highlight the contribution of two-photon absorption (TPA), and its accompanying Kerr-like effect, as well as of linear dispersion, to the modification of the pulse complex electric field profile. These effects are incorporated into our previously developed finite-difference time-domain comprehensive model that describes the interaction between the pulses and the QD SOA. The present, generalized, model is used to investigate the combined effect of coherent and non-resonant phenomena in the gain and absorption regimes of the QD SOA. It confirms that in the QD SOA we examined, linear dispersion in the presence of the Kerr-like effect causes pulse compression, which counteracts the pulse peak suppression due to TPA, and also modifies the patterns which the coherent Rabi-oscillations imprint on the pulse envelope under both gain and absorption conditions. The inclusion of these effects leads to a better fit with experiments and to a better understanding of the interplay among the various mechanisms so as to be able to better analyze more complex future experiments of coherent light-matter interaction induced by short pulses propagating along an SOA.

The small-signal cross-gain modulation (XGM) characteristics of a p-doped quantum-dot (QD) semiconductor optical amplifier (SOA) are measured and compared with a theoretical model. The measured small-signal XGM responses show an average 3-dB bandwidth of 3.5 GHz and have no significant dependence on the pump-probe detuning wavelength. This indicates that the main XGM mechanism of the p-doped QD SOA is the total carrier density change rather than spectral hole burning. The measured small-signal conversion efficiency is compared with a theoretical model, which is derived from rate equations for QD SOAs. Based on our theoretical results, we find that the reduced intersubband (from the QD excited to ground state) carrier relaxation time due to p-type doping will decrease the small-signal 3-dB XGM bandwidth of QD SOAs

P-doped and undoped quantum dot (QD) semiconductor optical amplifiers (SOAs) having a similar chip gain of 22-24 dB are compared with regard to their static and dynamic characteristics. Amplified spontaneous emission (ASE) spectra reveal the influence of p-doping on the gain characteristics and the temperature stability. In contrast to QD lasers, p-doping does not significantly increase the thermal stability of QD SOAs. The static four-wave mixing efficiency is larger and more temperature stable in undoped devices, leading to a maximum chip conversion efficiency of -2 dB. Small-signal cross-gain modulation (XGM) experiments show an increase in the small-signal bandwidth from 25 GHz for the p-doped SOAs to 40 GHz for the undoped QD SOAs at the same current density. P-doped QD SOAs also achieve small-signal bandwidths beyond 40 GHz but at a larger bias. The XGM is found to be temperature stable in the range of 20°C to 40°C.

High speed optical communications systems are evolving rapidly. Commercial systems achieve high aggregate data rates utilizing wavelength division multiplexing, where multiple wavelength channels carry information at electronic rates, typically 2.5 Gb/s. Data encryption in these systems will most likely be implemented electronically. However, future system may also utilize time division multiple access (TDMA) schemes and technologies for 100 Gb/s, single stream TDMA networks are currently being developed. These high speed TDMA networks will rely on all-optical switches and processors to interface the high-speed electronics in the users nodes to the ultra-high-speed optical data bus. Data encryption in these networks may need to be implemented using optical logic gates. Straightforward duplication of electronic encryption circuits using optical logic gates is not feasible because optical logic gates have low fan-out, require high optical powers, are difficult to synchronize and have high latency. In this paper, we propose a high- speed electro-optic scheme for reconfigurable feedback shift registers (RFSRs) that relies upon electronic encryption circuits to reconfigure a sequence of optical logic gates and which makes use of the latency in the optical gates as memory. We show that, for linear RFSRs, the low number of optical gates is not a drawback and that the period of the sequences is generally very large. Non-linear feedforward functions, such as all-optical bit swapping, many also be introduced to improve the pseudo-random properties of the sequences.

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.

We investigate all-optical logic operation in quantum-dot semiconductor optical amplifier (QD-SOA) based Mach–Zehnder interferometer considering the effects of two-photon absorption (TPA). TPA occurs during the propagation of sub-picosecond pulses in QD-SOA, which leads to a change in carrier recovery dynamics in quantum-dots. We utilize a rate equation model to take into account carrier refill through TPA and nonlinear dynamics including carrier heating and spectral hole burning in the QD-SOA. The simulation results show the TPA-induced pumping in the QD-SOA can reduce the pattern effect and increase the output quality of the all-optical logic operation. With TPA, this scheme is suitable for high-speed Boolean logic operation at 320 Gb/s.

We propose schemes for binary adder, subtractor and parity checker using optical logic gates. These schemes could be useful for calculations using optical systems. Utilizing optical logic gates, we can achieve functions of binary adder, subtractor and parity checker of high-speed optical signals. Due to two-photon absorption in the wetting layer, quantum dot-semiconductor optical amplifier Mach-Zehnder interferometer (QD-SOA-MZI) can work as high data rate optical logic gates. The simulated result supports the idea that it is possible to realize all-optical binary adder, subtractor and parity checker at high optical signal rates.

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

This paper describes the application of a monolithic Si bipolar IC to serve as a single-chip measurement instrument for pseudo-random binary sequence (PRBS) generation, bit error detection, de-scrambling, and trigger derivation up to 12.5 Gb/s. The package interconnect problem of high I/O count multi-Gb/s IC's is addressed. The paper presents a packaging solution with improved ground contact and odd-mode controlled impedance differential microstrip transmission lines. A method for phase synchronization of multiple PRBS generators is proposed and its feasibility is demonstrated by measurement results. >

A 2 11 -1 pseudo-random binary sequence (PRBS) generator with trigger synchronization output (9.77-MHz rate) is implemented using synthetic transmission lines for the clock distribution. The full-rate data sequence is sourced from a 2:1 multiplex of dual shift register outputs synchronized to a half-rate clock. Quadrature half-rate clocks generated by a dual-mode (Dynastat) divide-by-2 are distributed via the synthetic lines to optimize power-speed tradeoffs in the design. The 790 × 620 μm 2 PRBS designed in 130-nm SiGe BiCMOS (200/280 GHz f T /fmax) consumes 250 mA at 2.5 V (i.e., 625 mW).

In this paper the experimental validation of a novel, modified double scroll chaotic attractor circuit, employed as a true random binary generator (TRBG) is presented. The double scroll attractor is modeled on Chua's circuit constituted as independent chaotic oscillator using passive only nonlinear device for nonlinear operation leading to chaotic behavior. The output from the chaotic circuit which is a correlated binary sequence is scrambled with a pseudo random binary sequence generator (PRBSG) topology to yield a true random binary source for key stream generation. The modified chaotic circuit has been first modeled in PSpice software. The randomness attributes of the modified generator, obtained from both the PSpice model and hardware implementation, using the PRBSG de-correlator were successfully tested by the well known NIST Test Suite and Diehard Test Set for statistical validation. Output binary streams from the proposed modified generator were examined for randomness using both Test Suites with all tests successfully passed for both PSpice model and experimental chaotic circuit generator. A physical TRBG has been constructed on the basis of the proposed PRBSG modification with all statistical tests successfully passed confirming theoretical expectations. (6 pages)

Objective: Design of quadriphase codes with superior merit factor in basically a nonlinear multivariable optimization problem. Quadriphase transformation is applied to low side lobes in bi phase generation of binary sequence. In order to provide real time hardware for identification and generation of quadriphase pulse compression sequences. Methods/ Statistical Analysis: An efficient VLSI architecture has been proposed. The objectives to achieve high main lobe and low side lobes by poly phase codes. Findings: Initially pulse compression techniques are applied followed by binary quadriphase transformation. By applying a transformation to the biphase code using of sub pulses consequently results in the generation of codes. Near constant envelopes are created. Improvements/Applications: The systematic quadriphase sequences are useful for application to radar and continuation. There is scope for further development of merit factor. Keywords: Merit Factor (MF), Pseudo Random Binary Sequence (PRBS) Generator, Pulse Compression, Quadriphase Sequence

On-chip high-frequency test for Rapid Single Flux Quantum (RSFQ) circuits without external high-frequency equipment is very attractive and promising. The test method based on Shift Register (SR) is widely used for long time. Recently, we have proposed two test methods based on Pseudo Random Binary Sequence (PRBS) generator. In this paper, a Full Adder (FA) is taken as an example of circuit under test and the design methodology of the above approaches is presented. For FA with the traditional and two proposed approaches, the high-frequency test circuits are fabricated and tested. We demonstrate the successful measurement of the three circuits with the maximum operating frequency of the full adder at 23.3 GHz, 26.2 GHz and 27.4 GHz respectively. Based on the experimental results, the clock frequency measurement in the proposed two PRBS based test systems is more accurate and time-saving, which is advantageous for high-frequency RSFQ circuits measurement.

Optical frequency encoding/decoding tech- niques are important in optical signal processing and communication because of its inherent advantages. There are several research works are already reported where this frequency encoding/decoding mechanism are successfully used to develop logic gates in optical do- main. The frequency encoded logic gates can show very low bit error rate and with a very small optical power the logic operation can be conducted too. Here in this paper the authors report an all optical NAND logic gate using frequency encoding/decoding techniques success- fully. At the same time the concept of frequency encoded controlled NAND logic is proposed here. The simulated results of the NAND logic are also reported in this paper. To implement the NAND operation non- linear activity of semiconductor optical amplifier (SOA) is exploited here for switching purpose. As a whole the whole logic gates perform its operation at a very high speed (greater than THz frequency). The simulated re- sult of the operation also supports the theoretical pre- diction of the operation.