Ultra-small, low-power, all-optical switching and memory elements, such as all-optical flip-flops, as well as photonic integrated circuits of many such elements, are in great demand for all-optical signal buffering, switching and processing. Silicon-on-insulator is considered to be a promising platform to accommodate such photonic circuits in large-scale configurations. Through heterogeneous integration of InP membranes onto silicon-on-insulator, a single microdisk laser with a diameter of 7.5 µm, coupled to a silicon-on-insulator wire waveguide, is demonstrated here as an all-optical flip-flop working in a continuous-wave regime with an electrical power consumption of a few milliwatts, allowing switching in 60 ps with 1.8 fJ optical energy. The total power consumption and the device size are, to the best of our knowledge, the smallest reported to date at telecom wavelengths. This is also the only electrically pumped, all-optical flip-flop on silicon built upon complementary metal-oxide semiconductor technology. Scientists demonstrate that a single 7.5-μm-diameter microdisk laser coupled to a silicon-on-insulator wire waveguide can work as an all-optical flip-flop memory. Under a continuous bias of 3.5 mA, flip-flop operation is demonstrated using optical triggering pulses of 1.8 fJ and with a switching time of 60 ps. This device is attractive for on-chip all-optical signal buffering, switching, and processing.

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An ultra-small, low power all-optical flip-flop memory on a silicon chip

The remarkable achievements in the area of integrated optical memories and optical random access memories (RAMs) together with the rapid adoption of optical interconnects in the Datacom and Computercom industries introduce a new perspective for information storage directly in the optical domain, enabling fast access times, increased bandwidth and transparent cooperation with optical interconnect lines. This article reviews state-of-the-art integrated optical memory technologies and optical RAM cell demonstrations describing the physical mechanisms of several key devices along with their performance metrics in terms of their energy, speed and footprint. Novel applications are outlined, concluding with the scaling challenges to be addressed toward allowing light to serve as both a data-carrying and data-storage medium.

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Optical RAM and integrated optical memories: a survey

We demonstrate analytical frequency-domain transfer function expressions for an optical random access memory (RAM) cell that employs two SOA-based ON/OFF switches and two coupled SOA-MZI gates forming an optical flip-flop. Our theoretical model relies on first-order perturbation theory approximations applied for the first time to coupled optical switching structures, resulting to an optical RAM cell frequency response that allows for a qualitative and quantitative analysis of optical RAM memory speed and performance characteristics and their dependence on certain RAM cell device parameters. We show that the transfer function of an optical RAM cell and its incorporated flip-flop device exhibits periodic resonance frequencies resembling the behavior of optical ring resonator configurations. Its free spectral range is mainly dictated by the length of the waveguide that enables the coupling of the two SOA-MZI gates, yielding this coupling length as the dominant memory speed determining factor. The obtained results are in close agreement with experimental observations, demonstrating that optimized RAM cell designs with waveguide coupling lengths lower than 5 mm can enable RAM operation at memory speeds well beyond 40 GHz.

Memory Speed Analysis of Optical RAM and Optical Flip-Flop Circuits Based on Coupled SOA-MZI Gates

In this paper, the bifurcation behavior of light in the PANDA ring resonator is investigated using the signal flow graph (SFG) method, where the optical transfer function for the through and drop ports of the PANDA Vernier system are derived. The optical nonlinear phenomena, such as bistability, Ikeda instability, and dynamics of light in the silicon-on-insulator (SOI) PANDA ring resonator with four couplers are studied. The transmission curves for bistability and instability as a function of the resonant mode numbers and coupling coefficients for the coupler are derived by the SFG method and simulated. The proposed system has an advantage as no optical pumping component is required. Simulated results show that closed-loop bistable switching can be generated and achieved by varying mode resonant numbers in the SOI-PANDA Vernier resonator, where a smooth and closed-loop bistable switching with low relative output/input power can be obtained and realized. The minimum through-port switching time of 1.1 ps for resonant mode numbers of 5;4;4 and minimum drop port switching time of 1.96 ps for resonant mode numbers of 9;7;7 of the PANDA Vernier resonator are achieved, which makes the PANDA Vernier resonator an operative component for optical applications, such as optical signal processing and a fast switching key in photonics integrated circuits.

Ultrafast all-optical switching using signal flow graph for PANDA resonator.

In this paper, we demonstrate a novel RAM cell based only on three traveling waveguide semiconductor optical amplifier-cross gain modulation (SOA-XGM) switches. The RAM cell features wavelength diversity in the incoming bit signals and provides Read/Write operation capability with true random access exclusively in the optical domain. Two of the SOA-XGM switches are coupled together through an 70/30 coupler to form an asynchronous flip-flop, which serves as the memory unit. Random access to the memory unit is granted by a third SOA-ON/OFF switch and all three SOAs together form the proposed RAM cell. Proof-of-principle operation is experimentally demonstrated at 8 Mb/s using commercial fiber-pigtailed components. The distinctive simplicity of the proposed RAM cell architecture suggests reduced footprint. The proposed flip-flop layout holds all the credentials for reaching multi-Gb/s operational speeds, if photonic integration technologies are employed to obtain wavelength-scale waveguides and ultrashort coupling lengths. This is numerically confirmed for 10 Gb/s using a simulation model based on the transfer matrix method and a wideband steady-state material gain coefficient.

Optical RAM and Flip-Flops Using Bit-Input Wavelength Diversity and SOA-XGM Switches

All-optical pulse reamplification, reshaping (2R), and retiming (3R) using a monolithic bistable semiconductor ring laser (SRL) is demonstrated for the first time. The regeneration performance of the SRL is characterized with an all-optical settable switching threshold, achieving significant increases in the extinction ratio (ER) of the output pulse for input ER as low as 1 dB. For retiming, a rectangular retiming window generated by a clean clock signal is used to eliminate the timing jitter in the input pulse. For input pulse with peak-to-peak timing jitter as high as ~ 12% of the bit period, the timing jitter in the retimed output pulse is reduced to < 2% of the bit period. The pulsewidth of the final regenerated data can be controlled by changing the width of the retiming window. The SRL is, therefore, shown to have a "hard" digital performance in both amplitude and time domain suitable for all-optical 3R.

Characterization of All-Optical Regeneration Potentials of a Bistable Semiconductor Ring Laser

This paper proposes and demonstrates an all-optical static random access memory (SRAM) cell using a bistable semiconductor ring laser set-reset flip-flop integrated with four SOAs at each output. Read and write operation at 1Gb/s are achieved with extinction ration ≫10dB.

Optical static random access memory cell using an integrated semiconductor ring laser

An optical static random access memory cell is experimentally demonstrated using a semiconductor ring laser set-reset flip-flop integrated with four SOAs at each output. Read and write operations at 1Gb/s are achieved.

Millimeter waves (MMWs), operating at 30–300 GHz band, are very promising to the next-generation 5G wireless communication systems, enabling data rates of multi Gbps per user. Photonic technology is increasingly considered to play a key role in a wide range of MMW devices, modules, and subsystems that are essential to successfully build next generation MMW-based 5G networks. This work considers the switching function of MMWs exploiting nonlinearity in photonic devices. In this paper, we perform a systematic investigation of the optimum operating conditions that enable an optical single sideband wavelength conversion, by exploiting the nonlinear effects in a semiconductor optical amplifier (SOA). This principle of switching carefully exploits SOA’s four-wave mixing, cross-gain modulation in addition to self-phase modulation effects. The key parameters under investigation include the injection current, the wavelength spacing between the probe and the pump signals in addition to their respective powers. We experimentally determine the optimal operating conditions that maximize the sideband suppression ratio and simultaneously reduce the useless left sideband signal intensity, leading to a dispersion free transmission in optical fiber. Further, we experimentally demonstrate a photonic-based MMW switch of MMW signals having 30 GHz frequency and carrying 3 Gbaud/QPSK modulated signals. A 14-dB sideband suppression ratio of modulated signal is reported.

Optimizing OSSB Generation Using Semiconductor Optical Amplifier (SOA) for 5G Millimeter Wave Switching

A monolithic semiconductor ring laser (SRL) is demonstrated to operate as an optical monostable and bistable that can be triggered by an optical pulse, showing potential for all-optical 2R regeneration and all-optical Flip-Flop applications.

M.I. Memon

Papers

Characterization of All-Optical Regeneration Potentials of a Bistable Semiconductor Ring Laser

Optical static random access memory cell using an integrated semiconductor ring laser

Optical static random access memory cell using an integrated semiconductor ring laser

Optimizing OSSB Generation Using Semiconductor Optical Amplifier (SOA) for 5G Millimeter Wave Switching

Optically Triggered Monostable and Bistable Flip-Flop Operation of a Monolithic Semiconductor Ring Laser