Jon R. Sauer

Photonic packet switches: architectures and experimental implementations

Photonic packet switches offer high speed, data rate and format transparency, and flexibility required by future computer communications and cell-based telecommunications networks. In this paper, we review experimental progress in state-of-the-art photonic packet switches with an emphasis on all-optical guided-wave systems. The term all-optical implies that the data portion of a packet remains in optical format from the source to the destination. While the data remain all-optical, both optical and optoelectronic techniques have been used to process packet routing functions based on extremely simple routing protocols. An overview of the design issues for all-optical photonic packet switching is given and contrasted with electronic packet switch implementations. Low-level functions that have been experimentally implemented include routing, contention resolution, synchronization, and header regeneration. System level demonstrations, including centralized photonic switching and distributed all-optical multihop networks, will be reviewed. >

Four-wave mixing in wavelength-division-multiplexed soliton systems: damping and amplification

Four-wave mixing in wavelength-division-multiplexed soliton systems with damping and amplification is studied. An analytical model is introduced that explains the dramatic growth of the four-wave terms. The model yields a resonance condition relating the soliton frequency and the amplifier distance. It correctly predicts all essential features regarding the resonant growth of the four-wave contributions.

Multisoliton interactions and wavelength-division multiplexing

Multisoliton interactions are studied with an asymptotic expansion of the N-soliton solution in the limit of large frequency separation between the channels. In this limit the spectral distortion is small and the peak frequency shift of a channel is the sum of pairwise shifts as a result of interaction with other channels. These results, derived for collisions among an arbitrary number of channels, will be useful in estimating the limits on the minimum channel spacings and packet sizes for a wavelength-multiplexed optical communication system.

A soliton ring network

The recent demonstration of ultrafast, cascadable, all-optical soliton gates, although with long latency and at an early stage of research, opens the possibility of niche exploitation in architectures whose performance is primarily limited by the absence of a few such logic elements. A candidate system is a widely distributed, self-routing short packet, slotted ring system running at peak rates well beyond that of the conventional electronic hosts at each access node. The authors describe an architecture for a system with a 1.25 GHz packet rate, 32-bit payload, and 100 Gb/s peak bit rate serving a few hundred user nodes. An optical format is retained by through-going node traffic, so that the overhead of conversion to/from electronics is incurred only at the source and destination. This design effort has served to sharpen their understanding of the strengths and weaknesses of using such gates in carefully chosen applications. >

A new time domain, multistage permutation algorithm (switching systems)

It is shown that a frame of N time slots can be arbitrarily permuted with 2log/sub 2/N-1 controlled exchange switches with associated delay elements. This is an improvement over previously known interconnection networks that require O(N) exchange elements. The proof utilizes the recursive algorithm of V.E. Benes (1965) and the time interchange properties of a particular configuration of a single exchange element. The architecture is especially applicable in optical systems, since optical exchange switches are among the simplest optical logic devices to build, are inherently very fast, and are the best developed, although expensive. >

Design and implementation of a prototype optical deflection network

We describe the design and implementation of a packet-switched fiber optic interconnect prototype with a ShuffleNet topology, intended for use in shared-memory multiprocessors. Coupled with existing latency-hiding mechanisms, it can reduce latency to remote memory locations. Nodes use deflection routing to resolve contention. Each node contains a processor, memory, photonic switch, and packet routing processor. Payload remains in optical form from source to final destination. Each host processor is a commercial workstation with FIFO interfaces between its bus and the photonic switch. A global clock is distributed optically to each node to minimize skew. Component costs and network performance figures are presented for various node configurations including bit-per-wavelength and fiber-parallel packet formats. Our efforts to implement and test a practical interconnect including real host computers distinguishes our work from previous theoretical and experimental work. We summarize obstacles we encountered and discuss future work.

Data-dependent timing jitter in wavelength-division-multiplexing soliton systems

Soliton timing jitter that is due to amplifier noise has been studied extensively. In wavelength-multiplexed soliton systems jitter also results from collisions between solitons on different channels. We derive an expression for the variance in pulse arrival times because of such collisions and predict system performance using this result. Jitter depends not only on perturbations to the fiber but also on the encoding of information. When the expression is applied to a system with loss and amplification, it is shown that extensive wavelength multiplexing may significantly enhance soliton communication systems.

Multi-Gb/s optical interconnect

The National Science Foundation has granted funding to the Centerfor Optoelectronic Computing Systems located in Boulder, Colorado. A new initiative has been undertaken there in collaboration with the Center for Telecommunications Research at Columbia University. This project is to design and build a multi-GHz optoelectronic data transport network using self-routing packets in a multi-hop network. The single electronic word packet payloads are highly compressed using optical techniques, and remain optical from source to target while traversing the switching nodes. Optical packet switching is performed with custom LiNbO3 directional coupler switches. The routing is done with a lean, self-routing hot potato protocol in order to avoid the need for data storage at the switching nodes and to provide a fixed node latency equivalent to a few meters of fiber. Sustainable throughput both in to and out of Ihe electronic host at each node should exceed 1 Gb/s, with bursts close to the 10-100 Gb/s peak link bandwidth. Some technical details of the optical compression and decompression schemes, the hot potato switching protocol, and the wrap-around shuffleexchange interconnection network will be given. The project timetable anticipates a lower speed, proof-of-principle four node network in three years, and a higher speed, larger, engineering demonstration in five years. The project received NSF funding in September 1989.

Four-wave mixing in wavelength-division- multiplexed soliton systems: ideal fibers

Analytic expressions for four-wave-mixing terms in an ideal, lossless wavelength–division-multiplexed soliton system are derived with an asymptotic expansion of the N-soliton solution of the nonlinear Schrodinger equation. The four-wave contributions are shown to grow from a vanishing background and then to decay. Their importance becomes evident in real, nonideal fibers, where they grow by an order of magnitude and equilibrate to a stable value as an effect of periodic amplification.

Wavelength-tolerant WDM networks

We describe a novel approach for implementing WDM networks that does not rely on fixed wavelength channels. Rather than tuning to a preassigned fixed wavelength channel any two nodes that require a link select a currently unused portion of the spectrum. Rather than tying to make trnamitters and receivers tune to fixed wavelength channels, the medium access control unit determines the approximate channel wavelength to be used for a given link. The receiver then dynamically locks onto this channel.