IEEE PHOTOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 5, MAY 2000 573 Fast Wavelength-Hopping Time-Spreading Encoding/Decoding for Optical CDMA S. Yegnanarayanan, A. S. Bhushan, and B. Jalali, Senior Member, IEEE Abstract—We demonstrate a new technique for implementation of fast wavelength-hopping incoherent optical code-division mul- tiple-access (CDMA). The output pulse from a mode locked laser is spectrally broadened through supercontinuum generation. This pulse is then encoded into fast wavelength-hopped time-spread waveforms through a wavelength-selective time-delay device. At the receiver, matched-filter decoding is used to recover data. We present a 1-Gb/s digital transmission experiment through a 15-km dispersion-shifted (DS) single-mode fiber link. This technique avoids the need for a fast wavelength tunable optical source. Index Terms—Mode locked laser, optical CDMA, optical pro- cessing. I. INTRODUCTION O PTICAL code-division multiplexing (OCDM) is an attractive technique that takes advantage of the enormous bandwidth (almost 25 THz) in single-mode fiber to achieve random asynchronous communication access among many users, free of network control [1]–[3]. Every user is assigned a specific code sequence. Through a proper choice of the OCDM codes, the signals from all network nodes can be made mutually noninterfering. In incoherent OCDM, an encoder maps each bit “1” of source information into a high bit rate optical sequence of ultrashort light pulses, while the bit “0” is not encoded. Encoding results in time spreading of the signal. The encoded signal is then broadcast to all node receivers. At the receiver, the optical signal is correlated with the local code in a matched filter thereby reconstructing the signal. A threshold detector is used to detect the autocorrelation peak [4]. This approach to multiplexing allows transmission without delay and handles multiaccess interference (contention) as an integral part of the multiplexing scheme. In order to improve their cross-correlation property, conven- tional incoherent OCDM systems employ very sparse codes with a small number of codes in the family [5]. This results in a small number of operating nodes and simultaneous users and, more importantly, a very high chip (signaling) rate for a partic- ular data rate. Recently, in analogy to hybrid systems in elec- trical code-division multiple-access (CDMA), there has been a proposal to integrate time spreading with a wavelength-hopping pattern to achieve a perfect needle-shaped autocorrelation func- tion with zero sidelobes and a very low cross correlation [6]. Every pulse (chip) within the code sequence is transmitted at a Manuscript received December 16, 1999; revised February 9, 2000. This work was supported in part by DARPA. The authors are with the Optoelectronic Circuits and Systems Laboratory, Department of Electrical Engineering, University of California Los Angeles, Los Angeles, CA 90095-1594 USA (e-mail: jalali@ucla.edu). Publisher Item Identifier S 1041-1135(00)03595-3. different wavelength according to a hopping algorithm. At the receiver, the original signal is recovered by a matched-filter cor- relator which is the conjugate of the transmission filter. Since every wavelength occurs only once in every hopping pattern, for a prime-hop sequence of order the autocorrelation is a max- imum of at zero time-shift and sidelobes are completely ab- sent, while crosscorrelation gives individual coincidences of at most one. For the worst case cross correlation (when chips are synchronous), the average variance can be shown to be [6] (1) which is very low. Consequently, the number of stations in the network and the number of simultaneous users is expected to be greatly increased. The number of orthogonal codes in the family is also increased from in prime codes (time spread only) to for prime-hopped (time-spread plus wavelength- hopped) codes. However, wavelength-hopped OCDM requires an extremely high repetition rate modelocked tunable laser with very fast wavelength tunability to achieve rapid wavelength hop- ping. Hence, such wavelength-hopping time-spreading OCDM systems remained a concept in paper until recent simultaneous experimental demonstrations by two groups [7], [8]. In this letter, we propose a novel implementation of wave- length-hopped time-spread OCDM, where the ultrafast tuning requirement and extremely high repetition rate for the mod- elocked laser is eliminated. This is achieved in an optically incoherent system. A fixed-wavelength modelocked laser is used to obtain the entire optical wavelength spectrum through supercontinuum generation [9], [10]. This is followed by parallel optical processing through splitting and recombining on a wavelength basis using wavelength-division multiplexing (WDM) multiplexers and demultiplexers and encoding using optical delay lines [11]. To verify the concept, a 1-Gb/s digital transmission experiment over a 15-km dispersion-shifted (DS) single-mode fiber link using an all-optical three-chip code for wavelength hopping and time spreading is demonstrated. The experiment setup is shown in Fig. 1. An actively mod- elocked fiber laser generating 3 ps optical pulses at a 1-GHz repetition rate is used as the light source. To achieve a spectral width of 2 nm (for three chips in spectrum spaced by 0.8 nm) we amplify the pulses and use supercontinuum generation in 4 km of DS fiber followed by a tunable bandpass filter to select a 2-nm band. Fig. 2 shows the spectrum after the laser and after supercontinuum generation. We obtain a 10-nm spectral band- width after supercontinuum broadening. While this is sufficient for our experiment, it has been shown that more than 200-nm supercontinuum broadening with relatively flat spectral response 1041–1135/00$10.00 © 2000 IEEE