Dynamically Reconfigurable All-Optical Correlators to Support Ultra-fast Internet Routing

A new all-optical networking function is demonstrated using a bank of novel thermallytuned FBG-based optical correlators to construct an “optical bypass” to vastly accelerate conventional electronic internet routers. Based on a simple set of rules, a software algorithm configures the correlators as a routing table cache that can quickly determine the destination port for a large percentage of the incoming traffic by only examining a small subset of the address bits. 2001 Optical Society of America OCIS codes: (060.2330) Fiber optics communications; (060.4250) Networks Introduction As transmission speeds in the network core approach 40Gb/s and beyond, the need to make faster routing decisions at each networking node becomes increasingly evident. It is therefore important for optical designers to examine how current electronic routers function and determine where it may be feasible to employ optical techniques to aid the electronics in making ultra-fast routing decisions. In conventional Internet routers, packets are steered towards their destinations by interrogating their 24-bit destination IP addresses and matching them to entries in a large routing table using complicated “longest prefix” matching algorithms. This can be time consuming given that core routing tables contain upwards of 100,000 entries. As a result, a true all-optical router would need to be capable of 24-bit lookups into 100,000-entry tables at ≥40 Gb/s. Such capabilities are beyond current optical technologies, but some recent developments hint at the feasibility of a partial solution. Given that most core routers have only four to eight outgoing ports, it may be possible to determine a packet’s outgoing port by looking at only a small subset of the 24 bits in the destination address. So although building a true all-optical router is beyond current optical technologies, it is feasible to build an “optical bypass” to vastly accelerate a conventional router. A subset of the traffic would be routed by the optical bypass without any O/E conversion, at increased throughput and decreased latency. The remainder of the traffic, which requires more complicated processing, is handled by a conventional electronic router. The optical bypass can utilize a subset of the routing table with as few as 100 entries and still successfully optically route as much as 90% of the incoming traffic [1]. The remaining challenge is to determine how to design a 24-bit input, 100-entry optical index using a manageable number of optical correlators. The optical correlators required for this application must be tunable and designed to easily scale to 40Gb/s and beyond. Optical correlators operate by sequentially splitting the optical bit pattern and recombining it after one branch has experienced a one to few bits time delay. The tiny distances corresponding to these delays would present a serious challenge for previously reported correlators which, for example, are implemented with either optical splitters and fiber mirrors [2] , or an array of discrete fiber Bragg gratings (FBG) tuned with separate piezoelectric stretchers [3]. In these cases, the inter-mirror or inter-grating spacings would have to be 2.5 mm for a one-bit round trip time delay at 40Gb/s – an impractical length for a device using discrete fiber components. Thus, we propose a new correlator design in which an FBG array is constructed from a single uniform fiber grating that is divided into separate, electrically-tunable sections using thin film micro-heaters. Millimeter, and even micron sized spacings are easily achieved with this technology. An additional advance over previously reported correlators is that our approach uses two grating arrays per correlator, one to correlate with the “1-bits” in the desired code and the other for the “0-bits,” allowing all possible input codes to produce unique correlation outputs. Concept To implement an effective optical bypass for an electronic router, the key design decision is to combine a software algorithm with a small set of dynamically configurable fiber-Bragg-grating based optical correlators. A conceptual diagram showing how the optical bypass is implemented in an IP router is shown in Fig. 1. A small portion of the incoming optical packet stream is tapped off and sent to the correlator module. The optical signal is amplified, split and sent into the multiple correlators, each of which can be configured to produce a “match” signal for any number, X ,of 24 bits. For example, if the algorithm has predetermined that any incoming packets with bit positions 1, 4, and 5 equal to 1, 0, and 1 should go to port 1, then the correlator is configured to provide a “match” signal for any input pattern with “1xx01” for its first five bits and anything for the rest (where the “x”s indicate a “don’t care” bit). There will be a group of correlators for each output port since there will be a group of bit patterns that the software has determined should all be routed to a particular port. The goal of the algorithm is to reduce the number of correlators required. Threshold detectors are used at the outputs of the optical correlators to provide an electrical match/no match condition to the optical switch. The switch uses these signals to determine which output port each packet should be routed to. For any packet that the correlators fail to find a match for, the switch sends it to an auxiliary port for electrical processing by a conventional electronic router.