Christos Vagionas

Bringing WDM Into Optical Static RAM Architectures

Optical RAM appears to be the alternative approach towards overcoming the “Memory Wall” of electronics, suggesting use of light in RAM architectures to enable ps-regime memory access times. In this communication we take advantage of the wavelength properties of optical signals to present new architectural perspectives in optical RAM structures by introducing the WDM principles in the storage area. To this end, we report on a 4 × 4 WDM optical RAM bank architecture that exploits a novel SOA-based multi-wavelength Access Gate (WDM-AG) and a dual wavelength SOA-based SET-RESET All-Optical Flip Flop (AOFF) as fundamental building blocks. The WDM-AG enables simultaneous random access to a 4-bit optical word encoded in 8 different wavelengths, allowing for the four AOFFs of each RAM row to effectively share the same Access Gate. The scheme is shown to support a 10 Gbit/s operation for the incoming 4-bit data streams, with a power consumption of 15 mW/Gbit/s for the WDM-AG and 120 mW/Gbit/s for the AOFFs. The proposed optical RAM architecture reveals that exploiting the WDM capabilities of optical components can lead to RAM bank implementations with smarter column/row encoders/decoders, increased circuit simplicity, reduced number of active elements and associated power consumption, while enabling for re-configurability in optical cache mapping.

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

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.

XPM- and XGM-Based Optical RAM Memories: Frequency and Time Domain Theoretical Analysis

We demonstrate a frequency and time domain analysis for optical random access memory (RAM) cells that rely on semiconductor optical amplifier (SOA)-based switches but employ different switching mechanisms. The first RAM cell utilizes SOA cross gain modulation (XGM) switches both for the access gate as well as latching mechanism, whereas the second RAM cell configuration utilizes SOA-Mach-Zehnder interferometer cross phase modulation (XPM) switches. The frequency domain analysis exploits first-order perturbation theory approximations towards deriving the RAM cell frequency response, which is shown to exhibit in both RAM cell layouts a comb like resonant behavior. The free spectral range is dictated by the coupling length between the coupled switches that form the latching element, whereas the finesse depends on the temporal response of the switching mechanism employed. The qualitative speed and signal quality results obtained in the frequency domain are confirmed by a respective time-domain analysis carried out for both RAM cell layouts, using an experimentally validated time-domain SOA simulation model that relies on the transfer matrix method. Performance analysis in the time domain reveals in addition important quantitative RAM output signal measures like the extinction ratio and its dependence on the coupling length and the operational speed, as well as the input power dynamic range for successful RAM operation. Our holistic frequency- and time-domain analysis framework provides an in-depth understanding of performance-critical design parameters and their relationship to expected RAM cell performance characteristics. This is then utilized for a one-by-one system level comparison between the two RAM cell layouts in terms of readout extinction ratio, maximum speed, footprint, and power consumption concluding that the SOA-XGM-based RAM cell offers certain advantages when operational speeds not higher than 10 Gb/s are targeted, and the SOA-XPM-based RAM cell setup dominating when higher RAM serial speeds even up to 40 GHz are targeted.

Dual-Wavelength Bit Input Optical RAM With Three SOA-XGM Switches

We demonstrate a novel all-optical static RAM cell that exploits wavelength diversity in the incoming optical streams towards reducing the number of active elements. The circuit requires only three semiconductor optical amplifiers-cross gain modulation gates for successful read/write operation, yielding a 25% reduction in power consumption compared to state-of-the-art configurations. Proof-of-concept experimental verification is presented at 8 Mb/s using fiber-interconnected off-the-shelf bulk components.

Column Address Selection in Optical RAMs With Positive and Negative Logic Row Access

An optical RAM row access gate followed by a column address selector for wavelength-division-multiplexing (WDM)-formatted words employing a single semiconductor optical amplifier-Mach-Zehnder interferometer (SOA-MZI) is presented. RAM row access is performed by the SOA-MZI that grants random access to a 4-bit WDM-formatted optical word employing multiwavelength cross-phase-modulation (XPM) phenomena, whereas column decoding is carried out in a completely passive way using arrayed waveguide grating. Proof-of-concept experimental verification for both positive and negative logic access is demonstrated for 4 × 10 Gb/s optical words, showing error-free operation with only 0.4-dB-peak-power penalty and requiring a power value of 25 mW/Gb/s.

Memory Speed Analysis of an Optical Flip-Flop Employing a SOA-MZI and a Feedback Loop

We present analytical expressions for the frequency-domain transfer function of an optical flip-flop (O-FF) cell that employs an semiconductor optical amplifier-Mach-Zehnder interferometer gate and a feedback loop. Our analysis relies on first-order perturbation theory approximations applied for the first time to optical switching structures employing feedback-loop, resulting to an O-FF frequency response that allows for a qualitative and quantitative analysis of memory speed and performance characteristics and their dependence on certain device parameters. We show that the transfer function of the O-FF exhibits periodic resonance frequencies resembling the behavior of optical linear cavity configurations and its free spectral range is mainly dictated by the length of the waveguide that forms the feedback loop, revealing this loop length as the main memory speed determining factor. Experimental verification is provided, achieving good agreement with theoretical observations. The presented design guidelines show the way for achieving memory speeds beyond 30 GHz if employing feedback loop lengths lower than 5 mm, while we provide a direct comparison between O-FFs employing feedback loops or coupled switches, giving insight for future optical memories.

All optical flip flop with two coupled travelling waveguide SOA-XGM switches

We demonstrate a novel optical SR-flip flop with simple architecture, employing only two SOA XGM switches. Proof-of-principle operation is experimentally demonstrated at 8 Mb/s and numerically evaluated at 10 Gb/s.

Digital Optical Physical-Layer Network Coding for mm-Wave Radio-Over-Fiber Signals in Fiber-Wireless Networks

We demonstrate a digital Optical Physical-layer Network Coding (OPNC) for mm-wave fiber-wireless signals modulated with up to 2.5 Gb/s On/OFF Keyed (OOK) data. The proposed OPNC concept uses an all-optical XOR gate comprising a Semiconductor Optical Amplifier-Mach Zehnder Interferometer (SOA-MZI) with SOAs being driven at low moderate electrical currents in order to perform the all-optical encoding between the OOK envelopes of the data, ignoring the high-frequency Sub-Carrier (SC) signal. In this scheme, network coding is performed on-the-fly at the central office and the resultant packet is broadcasted at the client nodes, where the decoding takes place. We demonstrate experimental results of OPNC using OOK data signals modulated on a 10 GHz SC with the aid of a second all-optical XOR gate for the decoding process at the clients site, reporting error-free performance for both synchronous/asynchronous data packets. The scenario of all optical encoding for 60 GHz SC frequencies followed by electrical decoding at the end-users is evaluated via PHY-layer simulations. Going a step further and considering the network level, we present a performance improvement on the network throughput by using the proposed network coding.

WDM-Enabled Optical RAM at 5 Gb/s Using a Monolithic InP Flip-Flop Chip

We experimentally demonstrate an all-optical static random access memory (RAM) cell using a novel monolithic InP set-reset flip-flop (FF) chip and a single hybridly integrated semiconductor optical amplifier-Mach-Zehnder interferometer (SOA-MZI)-based access gate employing wavelength division multiplexing (WDM) data encoding. The FF device is a 6×2 mm2 InP chip having a 97.8% reduced footprint compared with previous FF devices that were successfully employed in optical RAM setups. Successful and error-free RAM operation is demonstrated at 5 Gb/s for both read and write functionalities, having a power penalty of 4.6 dB for write and 0.5 dB for read operations. The theoretical potential of this memory architecture to allow RAM operation with memory speeds well beyond 40 GHz, in combination with continuously footprint-reducing techniques, could presumably lead to future high-speed all-optical RAM implementations that could potentially alleviate electronic memory bottlenecks and boost computer performance.

Optical RAM row access and column decoding for WDM-formatted optical words

We present a multi-wavelength SOA-MZI-based access gate and an AWG-based column decoder that control random access of 4×10Gb/s WDM-formatted words into a 4-bit optical RAM row. Error-free decoding with 0.4dB peak power penalty is presented.