Ultracompact all-optical XOR logic gate in a slow-light silicon photonic crystal waveguide

High-Performance All-Optical Logic Operations Using Ψ-Shaped Silicon Waveguides at 1.55 μm

We simulate with FDTD solutions a complete family of basic Boolean logic operations, which includes XOR, AND, OR, NOT, NOR, NAND, and XNOR, by using compact Ψ-shaped silicon-on-silica optical waveguides that are operated at a 1.55 μm telecommunications wavelength. Four identical slots and one microring resonator, all made of silicon deposited on silica, compose the adopted waveguide. The operating principle of these logic gates is based on the constructive and destructive interferences that result from the phase differences incurred by the launched input optical beams. The performance of these logic operations is evaluated against the contrast ratio (CR) metric. The obtained results suggest that the considered functions designed with the employed waveguide can be realized all-optically with higher CRs and faster speeds than other reported designs.

Tunable topological slow-light in gyromagnetic photonic crystal waveguides with unified magnetic field

We have proposed a tunable topological slow-light in a photonic crystal (PC) waveguide with unified magnetic field. The waveguide is constructed by bringing close two gyromagnetic photonic crystals (GPCs) with different structural parameters and introducing a row of Al2O3 rods as the coupling layer. The two GPCs are applied with a unified external static magnetic field (ESMF) instead of two opposite ESMFs. Such waveguide supports a slow-light state originated from the coupling effect of two one-way edge states on both sides of the waveguide. By simply changing the strength of ESMF, one can achieve a tunable slow-light state with large normalized delay-bandwidth product (NDBP) (0.36< NDBP <0.84). Based on these excellent properties, we further design an optical delayer with a compact structure and expansibility simultaneously. This unique topological slow-light state with simple unified magnetic condition, high maneuverability and strong immunity to defects holds promise for many fields such as signal processing, optical modulation, and the design of various slow-light devices.

Optical Logic Gates Based on Z-Shaped Silicon Waveguides at 1.55 μm

In the last ten years, silicon photonics has made considerable strides in terms of device functionality, performance, and circuit integration for a variety of practical uses, including communication, sensing, and information processing. In this work, we theoretically demonstrate a complete family of all-optical logic gates (AOLGs), including XOR, AND, OR, NOT, NOR, NAND, and XNOR, through finite-difference-time-domain simulations using compact silicon-on-silica optical waveguides that operate at 1.55 μm. Three slots, grouped in the shape of the letter Z, make up the suggested waveguide. The function of the target logic gates is based on constructive and destructive interferences that result from the phase difference experienced by the launched input optical beams. These gates are evaluated against the contrast ratio (CR) by investigating the impact of key operating parameters on this metric. The obtained results indicate that the proposed waveguide can realize AOLGs at a higher speed of 120 Gb/s with better CRs compared to other reported designs. This suggests that AOLGs could be realized in an affordable manner and with improved outcomes to enable the satisfaction of the current and future requirements of lightwave circuits and systems that critically rely on AOLGs as core building elements.

Recent advances in integrated optical directed logic operations for high performance optical computing: a review

Optical directed logic (DL) is a novel logic operation scheme that employs electrical signals as operands to control the working states of optical switches to perform the logic functions. This review first provides an overview of the concept and working principle of DL. The developing trends of DL computing are then discussed in detail, including the fundamental optical DL gates, combinational optical DL operations, reconfigurable logic computing, low power optical logic computing, and programmable photonic network. The concluding remarks provide an outlook on the DL future development and its impacts in optical computing.

Photonic crystal integrated logic gates and circuits

This paper presents and demonstrates the three logic processing levels based on complementary photonic crystal logic devices through photonic integrated circuit modeling. We accomplished a set of logic circuits including AND, OR, NAND, NOR, XOR, FAN-OUT, HALF ADDER, and FULL ADDER based on photonic crystal slab platforms. Furthermore, we achieved efficient all-optical logic circuits with contrast ratios as high as 5.5 dB, demonstrated in our simulation results, guaranteeing well-defined output power values for logic representations; a clock-rate up to 2 GHz; and an operating wavelength at λ ≈ 1550 nm. Thus, we can now switch up for high computing abstraction levels to build photonic integrated circuits rather than isolated gates or devices.

Functional all-optical logic gates for true time-domain signal processing in nonlinear photonic crystal waveguides

We present a conceptual study on the realization of functional and easily scalable all-optical NOT, AND and NAND logic gates using bandgap solitons in coupled photonic crystal waveguides. The underlying structure consists of a planar air-hole type photonic crystal with a hexagonal lattice of air holes in crystalline silicon (c-Si) as the nonlinear background material. The remaining logical operations can be performed using combinations of these three logic gates. A unique feature of the proposed working scheme is that it operates in the true time-domain, enabling temporal solitons to maintain a stable pulse envelope during each logical operation. Hence, multiple concatenated all-optical logic gates can be easily realized, paving the way to multiple-input all-optical logic gates for ultrafast full-optical digital signal processing. In the suggested setup, there is no need to amplify the output signal after each operation, which can be directly used as a new input signal for another logical operation. The feasibility and efficiency of the proposed logic gates as well as their scalability is demonstrated using our original rigorous theoretical formalism together with full-wave computational electromagnetics.

Realization of true all-optical AND logic gate based on nonlinear coupled air-hole type photonic crystal waveguides

In this manuscript we propose an easily scalable true all-optical AND logic gate for pulsed signal operation based on band-gap transmission within nonlinear realistic air-hole type coupled photonic crystal waveguides (C-PCW). We call it "true" all-optical AND logic gate, because all AND gate topologies operate with temporal solitons that maintain a stable pulse envelope during the optical signal processing along the different C-PCW modules yielding ultrafast full-optical digital signal processing. We directly use the registered (output) signal pulse as new input signal between multiple concatenated nonlinear C-PCW modules (i.e. AND gates) to setup a multiple-input true all-optical AND logic gate. Extensive full-wave computational electromagnetic analysis proves the correctness of our theoretical studies and the proposed operation principle of the multiple-input AND logic gate is vividly demonstrated for realistic C-PCWs.

Ultra-fast all-optical decoder based on nonlinear photonic crystal ring resonators

In this paper, a 2-to-4 all-optical decoder based on photonic crystal ring resonators is introduced. The photonic crystal structure has a 2D square chalcogenide rod lattice whose maximum response time is 2 ps. Three ring resonators including nonlinear rods with 9×10^-17  m²/W for a Kerr coefficient carry out a switching operation at 1550 nm wavelength. The switching speed of the device is 500 GHz, which is more than that in previously presented works. Also, the small size of the structure is sufficient for optical integrated circuits.

Phase-shift-controlled logic gates in Y-shaped nonlinearly coupled chains

We introduce a model system composed of two input discrete chains nonlinearly coupled to a single output chain which mimics the geometry of Y-shaped carbon nanotubes, photonic crystal wave guides, and DNA junctions. We explore the capability of the proposed system to perform logic gate operations based on the transmission of phase-shifted harmonic incoming waves. Within a tight-binding approach, we determine the exact transmission spectrum which exhibits a nonlinear induced bistability. Using a digitalization scheme of the output signal based on amplitude modulation, we show that AND, OR, and XOR logic operations can be achieved. Nonlinearity strongly favors the realization of logic operations in the regime of large wavelengths, while phase shifting is required for the OR logic gate to be realizable. A detailed analysis of the contrast ratio shows that optimal operation of the AND and OR logic gates takes place when the nonlinear response is the predominant physical property distinguishing the coupling and regular sites. These results point towards the possibility of Y-branched junctions to perform logic operations based on the transmission of traveling waves.

Reconfigurable dual-channel all-optical logic gate in a silicon waveguide using polarization encoding

A reconfigurable dual-channel all-optical logic gate is proposed and experimentally demonstrated using four-wave mixing in a silicon waveguide for polarization encoding signals. Six logic functions, XNOR, AND, NOR, XOR, AB¯, and A¯B are implemented at two different wavelength channels by adjusting the polarization states of two 10 Gb/s non-return-to-zero polarization-shift keying (NRZ-PolSK) signals modulated by 10-bit on-off keying (OOK) sequences. The eye diagrams of the logic signals are clearly observed, and the logic functions are well demonstrated as the two incident NRZ-PolSK signals are both modulated by the OOK sequences, which originate from 2(31)-1 pseudo-random binary sequences.

Designing photonic crystal waveguides for broadband four-wave mixing applications

We present photonic crystal waveguide designs which exhibit large four-wave mixing efficiencies over a wide wavelength region. These designs are identified using an optimization process taking into account sophisticated figure-of-merits that depend on the pump bandwidth and the signal/pump tunability. The obtained designs achieve up to -18.9  dB conversion efficiency, tunable over a 10 nm tunability range. We also present alternative designs that are less efficient but have smaller power requirements and are far more compact.

Route-asymmetrical optical transmission and logic gate based on optical gradient force

We put forward a route-asymmetrical optical transmission scheme employing optical gradient force, which means that forward and backward propagation of an optical device have different transmittance provided they are not present simultaneously. The device is based on optical gradient force between two single-mode waveguides followed by a Mach-Zehnder interferometer. Our numerical investigation shows that the forward transmittance is about -6 dB while the backward transmittance is suppressed below -20.5 dB in C + L bands. The proposed device is passive, wideband, and compatible with complementary metal-oxide semiconductor (CMOS) process. Furthermore, we demonstrate the applications of route-asymmetrical transmission such as an all-optical switch and all-optical AND gate for all-optical information processing.

All-optical XOR logic gate using intersubband transition in III-V quantum well materials

A monolithically integrated all-optical exclusive-OR (XOR) logic gate is experimentally demonstrated based on a Michelson interferometer (MI) gating device in InGaAs/AlAsSb coupled double quantum wells (CDQWs). The MI arms can convert the pump data with return-to-zero ON-OFF keying (RZ OOK) to binary phase-shift keying (BPSK) format, then two BPSK signals can interfere with each other for realizing a desired logical operation. All-optical format conversion from the RZ OOK to BPSK is based on the cross-phase modulation to the transverse electric (TE) probe wave, which is caused by the intersubband transition excited by the transverse magnetic (TM) pump light. Bit error rate measurements show that error free operation for both BPSK format conversion and XOR logical operation can be achieved.

XOR and XNOR operations at 12.5 Gb/s using cascaded carrier-depletion microring resonators

We report the implementation of the XOR and XNOR logical operations using an electro-optic circuit, which is fabricated by CMOS-compatible process in the silicon-on-insulator (SOI) platform. The circuit consists of two cascaded add-drop microring resonators (MRRs), which are modulated through electric-field-induced carrier depletion in reverse biased pn junctions embedded in the ring waveguides. The resonance wavelength mismatch between the two nominally identical MRRs caused by fabrication errors is compensated by thermal tuning. Simultaneous bitwise XOR and XNOR operations of the two electrical modulating signals at the speed of 12.5 Gb/s are demonstrated. And 20 Gb/s XOR operation at one output port of the circuit is achieved. We explain the phenomena that one half of the resonance regions of the device are much more sensitive to the round-trip phase shift in the ring waveguides than the other half resonance regions. Characteristic graphs with logarithmic phase coordinate are proposed to analyze the sensitivity of the demonstrated circuit, as well as several typical integrated optical structures. It is found that our circuit with arbitrary chosen parameters has similar sensitivity to MRRs under the critical coupling.

Three-input optical addition and subtraction of quaternary base numbers

We present an approach to implementing three-input addition and subtraction of quaternary base numbers in the optical domain using multiple non-degenerate four-wave mixing (FWM) processes in a single highly nonlinear fiber (HNLF) and differential quadrature phase-shift keying (DQPSK) signals. By employing 100-Gbit/s three-input return-to-zero DQPSK (RZ-DQPSK) signals (A, B, C), we demonstrate 50-Gbaud/s three-input quaternary hybrid addition and subtraction (A + B-C, A + C-B, B + C-A). Moreover, by adding a conversion stage from C to -C via conjugated degenerate FWM, we also demonstrate 50-Gbaud/s three-input quaternary addition (A + B + C). The power penalties of three-input quaternary addition and subtraction (A + B-C, A + C-B, B + C-A, A + B + C) are measured to be less than 6 dB at a bit-error rate (BER) of 10(-9). In addition, no significant degradations are observed for RZ-DQPSK signals (A, B, C or -C) after the operations of quaternary addition and subtraction.

Slow-light dispersion engineering of photonic crystal waveguides using selective microfluidic infiltration

We experimentally demonstrate dispersion engineering of slow light photonic crystal (PhC) waveguides using selective infiltration of the first two rows of air holes with high index ionic liquids. The infiltrated PhC waveguide exhibits a dispersion window of 3 nm with a nearly constant group velocity of ~c/80 that depends on the liquid physical properties. We investigate how the effective refractive index changes in time due to the dynamics of the liquids in the holes. This demonstration highlights the versatility, flexibility, and tunability offered by optofluidics in PhC circuits.

Four-wave mixing in slow light photonic crystal waveguides with very high group index

We report efficient four-wave mixing in dispersion engineered slow light silicon photonic crystal waveguides with a flat band group index of n(g) = 60. Using only 15 mW continuous wave coupled input power, we observe a conversion efficiency of -28 dB. This efficiency represents a 30 dB enhancement compared to a silicon nanowire of the same length. At higher powers, thermal redshifting due to linear absorption was found to detune the slow light regime preventing the expected improvement in efficiency. We then overcome this thermal limitation by using oxide-clad waveguides, which we demonstrate for group indices of n_g = 30. Higher group indices may be achieved with oxide clad-waveguides, and we predict conversion efficiencies approaching -10 dB, which is equivalent to that already achieved in silicon nanowires but for a 50x shorter length.

Energy efficient nonlinear optics in silicon: are slow-light structures more efficient than nanowires?

We compare the energy performance of four-wave mixing in nanowires and slow-light photonic crystals and outline the regimes where each platform exhibits salient advantages and limitations, including analysis of the impact of future fabrication improvement. These results suggest a route towards energy efficient silicon integrated photonics.