552 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 5, MAY 2000 Reconfigurable Optical Packet Header Recognition and Routing Using Time-To-Wavelength Mapping and Tunable Fiber Bragg Gratings for Correlation Decoding M. C. Cardakli, S. Lee, A. E. Willner, Senior Member, IEEE, V. Grubsky, D. Starodubov, and J. Feinberg Abstract—Weachieve reconfigurable optical header recognition and penalty-free routing of a 2.5 Gb/s packet stream with a 1.6-ns guard time. Our method uses cross-gain compression in a semi- conductor optical amplifier for time-to-wavelength mapping, and two fiber Bragg grating arrays for tunable correlation decoding. This technique may be of value in future high-speed optical packet- switching nodes. Index Terms—Fiber Bragg gratings, header recognition, optical fiber communications, packet switching, wavelength shifting. I N FUTURE high-performance optical networks, optical data packets may be rapidly routed by all-optical switches [1]. For efficient and high-throughput switching, header bits in each packet must be decoded and acted upon quickly [2]. One brute- force method is to tap off a small portion of the signal and elec- tronically detect the bits, but this approach is not suitable for high-bit-rate packets for which electronic techniques are too dif- ficult. A potentially faster approach is to decode the header bits optically so that a given routing decision can be made on-the-fly. Additionally, any optical decoding should be sufficiently flex- ible and tunable to allow for reconfigurable networks. Previous device technologies used to achieve optical header recognition have included: 1) fiber-optic matched filters [3], which are not tunable; 2) spectroholographic filters [4], which require free- space optics; 3) optical AND gates [5], which have a low effi- ciency for small headers; and 4) nonlinear loop mirror config- urations with semiconductor optical amplifiers [6], which are appropriate for ultra-high-bit-rate packets, but tend to be envi- ronmentally sensitive. In this letter, we demonstrate optical recognition of the header information in a data packet, as well as routing of packets through an optical switch, based on the decoded infor- mation. Our technique is tunable and can recognize different packet headers in a reconfigurable network. The operation is accomplished by shifting each bit of a header packet onto a Manuscript received February 3, 1999; revised November 1, 1999. M. C. Cardakli, S. Lee, and A. E. Willner are with the Department of Elec- trical Engineering Systems, University of Southern California, Los Angeles, CA 90089-2565 USA. V. Grubsky and J. Feinberg are with the Department of Physics, University of Southern California, Los Angeles, CA 90089-0484 USA, and with D-STAR Technologies, Inc., Manhattan Beach, CA 90266 USA. D. Starodubov is with the D-STAR Technologies, Inc., Manhattan Beach, CA 90266 USA. Publisher Item Identifier S 1041-1135(00)03601-6. Fig. 1. Conceptual diagram of the optical header recognition module and routing experiment. different wavelength, introducing different time delays for each wavelength, and then using an optical decoder to determine if the series of header bits match the header code of a tunable optically-encoded look-up table. The wavelength shifting is accomplished by time-gated cross-gain compression in a semiconductor optical amplifier (SOA) [7], [8]. Tunable fiber Bragg gratings (FBG’s) provide the wavelength-dependent time delays. By tuning the FBG’s to intermediate wavelengths, we change the reflectivity of the FBG series. For example, for recognition of header “101,” we tune away the second grating of the first grating series. In this way, we form the grating series structure, i.e., present (“1” bit), not present (“0” bit), and present (“1” bit) which matches the header “101.” This corresponds to a change of the header look-up table in a reconfigurable optical network. When the header bits match the local code, a recognition pulse is produced that controls a 2 2 optical switch (see Fig. 1). We use this optical technique to recognize the header and switch a series of incoming 2.5-Gb/s data packets to one of two outputs for a reconfigurable network. We achieve penalty-free routing with a 1.6-ns guard time, limited only by the speed of the 2 2 switch. This technique is reconfigurable, stable, has the potential to recognize headers at very high speeds, and scales with both bit rate and routing header size. Since the wavelength-shifted control signals do not propagate beyond the look-up table and are only used for autocorrelation purposes, our method avoids the problems of chirp and extinction-ratio degradation which are usually associated with cross-gain compression wavelength shifting. Fig. 2 shows the header recognition module. Each 2.5-Gb/s packet in a packet stream is composed of a 3-bit header, 57-bit 1041–1135/00$10.00 © 2000 IEEE