Advanced Logic Gates for Ultrafast Network Interchanges

By overcoming speed bottlenecks from electronic switching as well as optical/electronic conversions, all-optical logic gates can permit further exploitation of the nearly 40THz of bandwidth available from optical fibers. We focus on the use of optical solitons and alloptical logic gates to implement ultrafast "interchanges" or switching nodes on packet networks with speeds of 100Gbit/s or greater. For example, all-optical logic gates have been demonstrated with speeds up to 200Gbit/s, and they may be used to decide whether to add or drop a data packet. The overall goal of our effort is to demonstrate the key enabling technologies and their combination for header processing in 100 Gbit/s, time-division-multiplexed, packed switched networks. Soliton-based fiber logic gates are studied with the goal of combining attractive features of soliton-dragging logic gates, nonlinear loop mirrors and erbium-doped fiber amplifiers to design logic gates with optimum switching energy, contrast ratio and timing sensitivity. First, the experimental and numerical work studies low-latency soliton logic gates based on frequency shifts associated with cross-phase modulation. In preliminary experiments, switching in 15m long low-birefringent fibers has been demonstrated with a contrast ratio of 2.73:1. Using dispersion-shifted fiber in the gate should lower the switching energy and improve the contrast ratio. Next, the low-birefringent fiber can be cross-spliced and wrapped into a nonlinear optical loop mirror to take advantage of mechanisms from both soliton dragging and loop mirrors. The resulting device can have low switching energy and a timing window that results from a combination of soliton dragging and the loop mirror mechanisms. I N T R O D U C T I O N Implementing an ultrafast information superhighway requires wide roadways and high-speed interchanges. Optical fibers, with their 40THz of potential bandwidth, provide the wide roadways. However, we can only tap a tiny fraction (a few gigabits-per-second) of this bandwidth because we use electronics at the 9 1995 American Institute of Physics 605 606 Advanced Logic Gates interchanges (switching nodes) where information enter and exit the network. We propose to overcome the electronic switching bottlenecks by using light-controllinglight (all-optical) switching to complement optical transport of information. Our research falls in the general category of ultrafast, time-division-multiplexed (TDM), serial optical processing in networks. Serial optical processing is particularly attractive when information enters and exits in optical format on fibers. Also, TDM is advantageous because it permits a trully all-digital system with dynamic allocation of bandwidth; i.e., by time-interleaving various users, idle channels in the network can be avoided. High-speed TDM is important for alloptical networks for several reasons. First, TDM is more natural for self-routing packet switching, and it permits us to take advantage of the over 25 years of experience that we have from the electronics arena. Second, this work complements other wavelength-division-multiplexed (WDM) research, since highspeed TDM can eventually be used to upgrade the speeds of individual WDM channels. Finally, high-speed TDM may simplify the system architecture and protocol by avoiding the challenges of parallel programming required for a multichannel WDM systems. All-optical switching means that one light beam controls the passage or modulates another light beam by interacting in a nonlinear medium. Light-light manipulations can be very fast if nonresonant, virtual optical interactions are used: i.e., the interaction is through deformation of electron clouds or wave functions rather than through the generation of electron-hole pairs, which must then recombine before the switch is reused. There are several compelling reasons for turning toward alloptical switching for serial, terabit-rate processing. First, speeds exceeding 50 Gbit/s can be achieved, which is beyond where electronic systems might be expected to operate. Second, by processing optically we can remove bottlenecks from converting between optics and electronics, which is particularly important if the information enters and exits in optical format. Third, electronic gates or devices that absorb light tend to generate heat, which becomes a major problem at very high bit rates. On the other hand, if nonresonant interactions are exploited, then at least in principle heat dissipation is not a limiting factor. Finally, and perhaps most importantly, all-optical gates enable us to distribute intelligence throughout the network and implement "optical control" in addition to optical transport. For example, since the data and routing information can be in the same optical format, the destination can be encoded with the data. This enables what is commonly called "self-routing packet switching," which means a packet can route itself through the network. To implement such a network electronically requires that the optical information be converted to electrical signals, which runs into the optical/electronic conversion bottlenecks. Considerable research is focused on "solitons" in optical fibers and their applications to long-distance telecommunications and ultra-high-speed information processing and networks. Solitons are robust pulses that propagate nearly distortion-free for long distance in fibers [1]. For example, Mollenauer [2] has transmitted solitons virtually error-free over 13,000 kilometers at 20 Gbit/sec. In addition, Islam [1] has demonstrated the world's fastest logic gates based on solitons in special types of fibers. With the considerable experimental and theoretical studies on soliton transmission and switching in the United States as well as Europe and Japan, there is a growing need for integration of soliton generation, propagation and switching for network applications. Solitons arise in optical fibers in the anomalous group velocity dispersion regime (wavelengths longer than 1.3 ktm), and solitons represent a balance between the