Abstract
In this article we review the application of all-optical signal processing using an ultra-fast all-optical nonlinear fiber wavelength converters to optical packet switching and ultra-fast network applications. The wavelength converter can be used as a basic building block for these networks and has been demonstrated at bit rates in excess of 40 Gbps. Experimental optical packet switching and OTDM add/drop multiplexer experimental results at 80 Gbps are reviewed.

Introduction
Within today’s Internet, packets (the basic unit of Internet data) are directed to their final destination using electronic routers. These packets are moved from router to router using optical fiber transmission and wavelength division multiplexing (WDM) systems where data is transported over different wavelength (colors) of light that are combined onto the same fiber. Today’s fiber systems carry a typical 32-80 wavelengths modulated at 2.5 Gbps (1 Gbps = 10⁹ bits per second) to 10 Gbps per wavelength while routers are required to handle almost 1 Terabits (10¹²) per second. Things become interesting when we consider that the data carrying potential of optical fibers continues to double every 8-12 months with state-of-the-art single fiber capacity exceeding 10 Tbps. Comparing this increase with that of electronic processor speeds that doubles every 18 months (Moore’s Law) and comes at the expense of increased chip power dissipation we see that there is a potential bandwidth mismatch in handling capability between fiber transmission systems and electronic routers and switching systems.

The story is more complex when we consider that future routers and switches will potentially terminate hundreds or thousands of optical wavelengths and the increase in bit-rate per wavelength will head out to 40 Gbps and beyond to 160 Gbps. Additionally, electronic memory access speeds only increase at the rate of approximately 5% per year, an important data point since memory plays a key role in how packets are buffered and directed through the router. It is not difficult to see that the process of moving a massive number of packets per second (100 million packets/second and beyond the 1 Billion packets/second mark) through the multiple layers of electronics in a router, can lead to router congestion and exceed the performance of electronics and the ability to efficiently handle the dissipated power.

In this article we review research at the University of California, Santa Barbara in fast optical signal processing as it applies to transmission, time division multiplexed and packet switched networks. We will also describe how the use of optical signal processing techniques can be used to alleviate the bottlenecks in transmission and routing as described above.

Figure 1. Two methods to transport packets on a network (a) using asynchronous multiplexing and (b) using synchronous multiplexing.

Synchronous and Asynchronous Networks
Two basic approaches exist to carry packet data over a network and are shown in Figure 1. The asynchronous approach in Figure 1a allows a router to take packets arriving at random times at its multiple inputs and redirect them to various outputs without overlapping. This function requires memory (buffers) inside the router to temporarily hold packets so they can be delayed with respect to other packets appropriately and merged at the outputs. This is analogous to the on-ramp of a highway where or a merge lane where cars must adjust their speed and wait time in order to merge without colliding. Packets that arrive on different wavelengths will most likely have to be merged onto a common output. The synchronous type network shown in Figure 1b is more analogous to loading up a railroad car and transporting the cars on a track. Before packets can be transported on the synchronous network, they are loaded up into “frames.” The network then switches and routes based on these frames and not the packets inside them.

Ultra-Fast Optical Wavelength Converter for Signal Processing and Network Functions

Figure 2. All-optical fiber optic cross-phase modulation (XPM) wavelength converter.

We have utilized an all-optical fiber wavelength converter as the basic building block for both asynchronous and synchronous packet switched networks [1]. This wavelength converter can be used to imprint data from one optical wavelength onto a new optical wavelength without passing the data through electronics. In addition to wavelength conversion, it can also be used to regenerate the bits in a digital signal and implement higher-level functions for asynchronous and synchronous packet networks. This approach is especially useful when the data rate exceeds 40 Gbps where electronics is not readily available. The wavelength conversion process is based on cross-phase modulation (XPM) in dispersion shifted optical fiber and is shown in Figure 2 along with conversion of return-to-zero (RZ) 80 Gbps data stream. The converter operation is based on the principle of XPM in a non-linear fiber, such as a dispersion-shifted fiber (DSF). A CW signal or a pulse train at a new wavelength l_j is combined with an intensity modulated pulse train at wavelength l_j. The incoming data imposes a phase modulation of the CW signal or the pulse train due to XPM. This phase modulation causes a spectral broadening of the CW signal or the pulse train thereby generating sidebands. One of these sidebands is filtered to convert phase modulation to amplitude modulation. The filter arrangement consists of a fiber Bragg grating (FBG) and a tunable band-pass filter (BPF). The FBG notches out the original data signal and the non spectrally broadened part of the new signal and lets only the desired sideband through. This improves the extinction ratio of the converted signal. This method of wavelength conversion is in principle very fast since non-linear processes are almost instantaneous and thus can be used to wavelength convert very high bit-rate data. The wavelength converter also acts as a 2R regenerator as seen by the smoothing out of bit amplitude fluctuations at the output.

Optical Packet Switching and Label Swapping for Asynchronous Networks
All-Optical Label Swapping (AOLS) is a type of optical packet switching that is intended to solve the potential mismatch between fiber capacity and router packet forwarding capacity. AOLS imparts the functionality to direct packets through an optical network without the need to pass these packets through electronics whenever a routing decision is necessary [2-6]. Inherent to this approach is the ability to route packets independently of bit-rate, packet or coding format and packet length.

Figure 3. An optical label-swapping network and example of AOLS with 80 Gbps packets.

An example AOLS network is illustrated in Figure 3. Internet Protocol (IP) packets enter the network through an “ingress” node and are encapsulated with an optical label and then re-transmitted on a new wavelength. Once inside the network, only the optical label is used to make routing decisions and the wavelength is used to dynamically redirect (forward) packets. At the internal nodes, labels are read and optically erased, then a new label is attached to the packet and the optically labeled packet is converted to a new wavelength using all-optical wavelength conversion. Throughout this process, the contents (e.g., the IP packet header and payload) are not passed through electronics and are kept intact until the packet exits the optical network through the “egress” node where the optical label is removed and the original packet is handed back to the electronic routing hardware. For packet networks where the bit rate can exceed 40 Gbps, ultra-fast signal processing techniques have been used to perform the functions of (i) optical label removal, (ii) optical wavelength conversion, (iii) optical signal regeneration and (iv) optical label replacement. We have demonstrated that the XPM fiber optic wavelength converter can be used to perform these functions for packet bit-rates as high as 80 Gbps with the potential to scale to data rates in excess of 160 Gbps [7]. Figure 3 also shows results of AOLS with 80 Gbps packets and labels running at 10 Gbps.

Figure 4. WDM/OTDM network based on ultra-fast all-optical wavelength converters.

Synchronous Ultra-Fast WDM/OTDM Networks
We have demonstrated that the XPM fiber wavelength converter can be used to integrate WDM and OTDM networks all-optically. Various functions required by a hybrid WDM-OTDM network including the ability to a) multiplex several low bit-rate DWDM channels into a single high bit-rate OTDM channels, b) demultiplex a single high bit-rate OTDM channel into several low bit-rate DWDM channels c) add and/or drop a time-slot from an OTDM channel d) wavelength route OTDM signals [8-11]. In addition to these basic functionalities the ability to multicast high bit-rate signals can be a very useful feature. The advantages of performing these functions all-optically are scalability and potential lower costs by minimizing the number of O-E-O conversions. An example of a WDM-OTDM is presented in Figure 4. One example is the all-optical OTDM add/drop mutiplexer shown in Figure 5. This building block has the capability of removing bits in one slot of the OTDM time frame and replacing new bits without passing the high-speed bus through electronics.

Figure 5. Example of an all-optical OTDM add/drop multiplexer using the XPM fiber wavelength converter.

Acknowledgements
This work was supported by the DARPA supported multidisciplinary optical switching technology center (MOST), a DARPA sponsored NGI award, a DARPA DURIP award, and funding from Cisco Systems.

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