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.     

Reconfigurable all-optical logic gate using four-wave mixing (FWM) in HNLF for NRZ-PolSK signal

We demonstrate a reconfigurable all-optical logic gate for NRZ-PolSK signal based on FWM in a highly nonlinear fiber at 10 Gb/s. Half subtracter, XOR, AB?, āB or XNOR, AND, and NOR logic gates can be implemented simultaneously. The input power for the HNLF is optimized to be as low as about 15.2 dBm and the high Q factors above 8 dB for eye diagrams are achieved. Experimental results show Q factors of AB?, āB, AND, and NOR were higher than those of XOR, and XNOR. Error-free operation is achieved experimentally for 10 Gb/s 2⁷-1 pseudorandom bit sequence (PRBS) data. Power penalties for the logic gate are less than 3 dB. Simulation analysis about the wavelength characteristic for all logic gates is given and it predicts that the reconfigurable logic gate can realize error-free operation when the wavelength separation is less than 5 nm.     

Static and dynamic analysis of an all-optical inverter based on a Vertical Cavity Semiconductor Optical Amplifier (VCSOA)

We report the static and dynamic properties of an all-optical inverter based on an 850 nm Vertical Cavity Semiconductor Optical Amplifier (VCSOA). The inverter exhibits low switching power requirements (~ 15 μW), large on/off contrast ratio (> 11 dB), and high speed operation (~ 1.4 GHz). Large and small signal measurements show that the speed of operation and the on/off contrast ratio improve with increased bias current. This holds important prospects for the development of VCSOA-inverters for high-speed, low-power optical logic applications. Finally, a theoretical model of the VCSOA-inverter has been employed giving good agreement with experiments.     

Realization of XNOR logic function with all-optical high contrast XOR and NOT gates

In this article, we propose the realization of XNOR logic function by using all-optical XOR and NOT logic gates. Initially, both XOR and NOT gates are designed, simulated and optimized for high contrast outputs. T-shaped waveguides are created on the photonic crystal platform to realize these logic gates. An extra input is used to perform the inversion operation in the NOT gate. Inputs in both the gates are applied with out of phase so as to have a destructive interference between them and produce negligible intensity for logic ‘0'. The XOR and NOT gates are simulated using Finite Difference Time Domain method which results with a high contrast ratio of 55.23?dB and 54.83?dB, respectively at a response time of 0.136?ps and 0.1256?ps. Later, both the gates are cascaded by superimposing the output branch of the waveguide of XOR gate with the input branch of the waveguide of NOT gate so that it can be resulted with compact size for XNOR logic function. The resultant structure of XNOR logic came out with the contrast ratio of 12.27?dB at a response time of 0.1588?ps. Finally, it can be concluded that the proposed structures with fair output performance can suitably be applied in the design of photonic integrated circuits for high speed computing and telecommunication systems.     

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.     

Design of ultra compact all-optical XOR and AND logic gates with low power consumption

We propose a novel ultra-compact all-optical XOR and AND logic gates without using nonlinear optics. In order to realize these devices, we adopt photonic crystal waveguides (PCWs) based on multi-mode interference devices. Numerical results show that the operating bandwidth of the ON to OFF logic-level contrast ratio of not less than 6.79 dB is 35 nm for XOR logic gate and 9 nm for AND logic gate. Proposed logic gates have the potential to be key components for an optical packet switching system due to their small feature sizes and low power consumption.     

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.     

All optical NOR and NAND gate based on nonlinear photonic crystal ring resonators

In this paper we proposed optical NOR and NAND gates. By combining nonlinear Kerr effect with photonic crystal ring resonators first we designed a structure, whose optical behavior can be controlled via input power intensity. The switching power threshold obtained for this structure equal to 2 kW/μm². For designing the proposed optical logic gates we employed two resonant rings with the same structures, both rings at the logic gates were designed such that their resonant wavelength be at λ = 1550 nm. Every proposed logic gate has one bias and two logic input ports. We used plane wave expansion and finite difference time domain methods for analyzing the proposed structures.     

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.     

Ultrafast all-optical XOR logic gate based on a symmetrical Mach-Zehnder interferometer employing SOI waveguides

We present an integrated Silicon-on-Insulator (SOI) based Mach-Zehnder interferometer (MZI) in order to perform ultrafast all-optical XOR logic gate operation with a bit rate of ∼ 0.33 Tb/s. A numerical simulation is carried out in order to study various parameters such as extinction ratio and eye-opening parameters, characterizing the performance of the XOR logic gate. The output XOR logic gate signal can have improved extinction and eye margin if the initial powers of primary signals and the probe continuous-wave (CW), and SOI waveguide length are judiciously adjusted.     

Spin transport and relaxation in graphene

We review our recent work on spin injection, transport and relaxation in graphene. The spin injection and transport in single layer graphene (SLG) were investigated using nonlocal magnetoresistance (MR) measurements. Spin injection was performed using either transparent contacts (Co/SLG) or tunneling contacts (Co/MgO/SLG). With tunneling contacts, the nonlocal MR was increased by a factor of ∼1000 and the spin injection/detection efficiency was greatly enhanced from ∼1% (transparent contacts) to ∼30%. Spin relaxation was investigated on graphene spin valves using nonlocal Hanle measurements. For transparent contacts, the spin lifetime was in the range of 50-100 ps. The effects of surface chemical doping showed that for spin lifetimes in the order of 100 ps, charged impurity scattering (Au) was not the dominant mechanism for spin relaxation. While using tunneling contacts to suppress the contact-induced spin relaxation, we observed the spin lifetimes as long as 771 ps at room temperature, 1.2 ns at 4 K in SLG, and 6.2 ns at 20 K in bilayer graphene (BLG). Furthermore, contrasting spin relaxation behaviors were observed in SLG and BLG. We found that Elliot-Yafet spin relaxation dominated in SLG at low temperatures whereas Dyakonov-Perel spin relaxation dominated in BLG at low temperatures. Gate tunable spin transport was studied using the SLG property of gate tunable conductivity and incorporating different types of contacts (transparent and tunneling contacts). Consistent with theoretical predictions, the nonlocal MR was proportional to the SLG conductivity for transparent contacts and varied inversely with the SLG conductivity for tunneling contacts. Finally, bipolar spin transport in SLG was studied and an electron-hole asymmetry was observed for SLG spin valves with transparent contacts, in which nonlocal MR was roughly independent of DC bias current for electrons, but varied significantly with DC bias current for holes. These results are very important for the use of graphene for spin-based logic and information storage applications.     

Narrow gated Raman and luminescence of explosives

Narrow gated Raman spectroscopy is used to detect Raman signals of explosives, which are usually screened by their intrinsic or background luminescence. It was found that the Raman/luminescence ratio is improved by 2-10 times with gate width of 500 ps compared to the 10 ns gate. It enables in certain cases to combine the luminescence suppression by gating with higher identification ability of Raman signals achievable with green excitation.     

80 Gb/s All-optical logic AND operation using Mach-Zehnder interferometer with differential scheme

An all-optical logic AND gate is demonstrated by using a semiconductor optical amplifier (SOA) based Mach-Zehnder interferometer (MZI). The AND results are numerically analyzed by solving the rate equation of SOA. Q-factor values have been calculated. The operation of the AND logic gate is experimentally demonstrated at 80 Gb/s. Operation at higher data rates is feasible using SOAs with shorter phase recovery time.     

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.     

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.     

Tunable slow and fast light with large bandwidth in the band-edge of gain spectrum of the wide band fiber optical parametric amplifier

We propose a novel method theoretically to generate the slow and fast light with large bandwidth and low gains, which is based on the parametric process in fiber. In our scheme, the wide band fiber optical parametric amplifier is employed and the whole signal bandwidth should be located at a certain frequency range of the band-edge of gain spectrum, and then signal waves will be delayed or advanced with low signal gains because of the peculiar feature of signal gain and phase shift. By changing the pump power, the delay time is continuously-tunable optically. The ultimate delay bandwidth and the delay bandwidth product are constrained by the shape of time delay spectrum. Our simulation verifies that 22.4 ps delay or advanced time for the bandwidth of 10 GHz with little distortion can be obtained at certain wavelengths in the optical communication waveband, and their gains are nearly zero. The tunable range is from 0 ps to 22.4 ps for the signal bandwidth of 10 GHz, and it is from 0 ps to 15.6 ps for the bandwidth of 15 GHz. This type of slow and fast light in wide band FOPA has the potential capability to produce the tunable slow and fast light for large bandwidth with low signal gains in future.     

Femtosecond laser ablation of silver foil with single and double pulses

The average ablation depth per pulse of silver foil by 130 fs laser pulses has been measured in vacuum over a range of three orders of magnitude of pulse fluence up to 900 J cm⁻². In addition, double pulses with separations up to 3.4 ns have been used to probe time scales of relevance for femtosecond ablation. The double pulse ablation depth, when each pulse fluence is 0.7 J cm⁻², falls to that of a single pulse as the pulse separation is increased from 0 ps to 700 ps. This time scale decreases to only 4 ps as the fluence is increased to 11 J cm⁻². It then jumps to 500 ps across a transition fluence where the slope of the ablation depth versus logarithmic fluence characteristic changes abruptly to a higher value. In addition, for pulse separations near 1000 ps, the second pulse can cause re-deposition of ejecta from the first pulse resulting in a double pulse ablation depth only 40% that of the first pulse alone. This has important implications for the interpretation of double pulse femto-LIBS intensities. Our results suggest that the optical properties of nano or mesoparticles play a significant role in double pulse ablation with large pulse separations.     

Experimental demonstration on 40 Gbit/s all-optical multicasting logic XOR gate for NRZ-DPSK signals using four-wave mixing in highly nonlinear fiber

We propose and demonstrate all-optical multicasting logic XOR gate for non-return-to-zero differential phase-shift keying (NRZ-DPSK) signals by using non-degenerate four-wave mixing (FWM) in a highly nonlinear fiber (HNLF). Theoretical analysis regarding the operation principle of NRZ-DPSK logic XOR gate is clearly described by deriving an analytical solution under the non-depletion approximation. The NRZ-DPSK logic XOR operation is attributed to the linear relationship of complex amplitudes between converted idlers and input NRZ-DPSK signals. By using three non-degenerate FWM processes in an HNLF, 40 Gbit/s all-optical multicasting logic XOR gate for NRZ-DPSK signals are successfully demonstrated in the experiment.     

Protein dynamics on different timescales

Protein dynamics is studied on metmyoglobin by Mössbauer investigations with synchrotron radiation, conventional Mössbauer spectroscopy and incoherent neutron scattering. In the center of interest is the time sensitivity of mean square displacements, 〈x²〉 of special atoms in the protein molecule. Phonon assisted Mössbauer effect labels internal vibrations at the heme iron on a time scale from 6.5 fs to 0.65 ps. The incoherent neutron scattering yields quasi diffusive motions of side chain hydrogens on a time scale faster 100 ps. The quasi diffusive broad lines in the Mössbauer spectrum indicate slow motions of larger segments of the molecule between about 100 ns and 100 ps.     

Design and demonstration of a novel optical CDMA platform for use in avionics applications

We designed, built, and demonstrated a highly scalable incoherent optical CDMA platform under DARPA contract which was delivered to Lockheed Martin for additional testing in avionic applications. The platform enables users to communicate with each other at ∼1.25 Gbit/s per user with raw BER of less than 10⁻¹². The system architecture uses (3, 11) fast wavelength-hopping, time-spreading prime codes with a chip size of 73 ps utilizing picosecond optical pulses allocated in the time and wavelength domains. A novel design of a “dual code” optical encoder and decoder realized a novel optical layer implementation of an XOR gate and enabled secure network connectivity using a “One-time pad” encryption approach. The testbed is also designed to conduct eavesdropping studies on testbed users. The incoherent OCDMA approach is compatible with existing DWDM optical networks and uses off-the-shelf components.