852 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 11, JUNE 1, 2007 Two-User 150-km Field Fiber Security Enhanced SPECTS O-CDMA Transmission Chunxin Yang, Nicolas K. Fontaine, Ryan P. Scott, Vincent J. Hernandez, Wei Cong, D. L. Harris, Katsunari Okamoto, Fellow, IEEE, Brian H. Kolner, Senior Member, IEEE, Zhi Ding, Fellow, IEEE, Jonathan P. Heritage, Fellow, IEEE, and S. J. B. Yoo, Fellow, IEEE Abstract—We demonstrate two-user error-free performance of security enhanced spectral phase-encoded time-spreading optical code-division multiple-access across a 150-km field fiber link. The testbed incorporates a 64-mode optical frequency comb source, a fully integrated silica arrayed waveguide grating spectral phase en- coder/decoder, and a tunable dispersion slope compensator. Phys- ical layer security is enhanced by using a bright code/dark code modulation format. Index Terms—Multiaccess communication, optical code-division multiple-access (O-CDMA), optical fiber communications. I. INTRODUCTION I N RECENT years, the realization of fiber-to-the-premises, along with increasing penetration of broadband access, has renewed interest in the optical code-division multiple-access (O-CDMA) technology that provides flexibility and enhanced physical layer security to optical access networks [1]. Spectral phase-encoded time-spreading (SPECTS) O-CDMA typically applies zero or phase shifts based on O-CDMA coding schemes to ultrashort pulses in the spectral domain, causing encoded pulses to spread in the time domain. The receiver applies the conjugate phase code and subsequent nonlinear optical signal processing [2] to recover the intended user’s signal. Recent SPECTS O-CDMA work has demonstrated network throughput up to 320 Gb/s [3], [4]. Long-distance transmission of spectral phase coding O-CDMA has been considered difficult because the ultra- short pulse source is extremely susceptible to dispersion effect and other fiber link impairments, e.g., polarization-mode dis- persion (PMD). Previous demonstrations have been limited to fiber transmission lengths of 50 km [5]. This letter presents multiuser SPECTS O-CDMA transmission over 150 km of Manuscript received December 23, 2006; revised March 4, 2007. This work was supported in part by the Defense Advanced Research Projects Agency (DARPA) and SPAWAR under Agreement N66001-02-1-8937 and in part by the Air Force Office of Scientific Research (AFOSR) through the University of California, Davis Center for Digital Security. C. Yang, W. Cong, and B. H. Kolner are with the Department of Applied Science, University of California, Davis, Davis, CA 95616 USA (e-mail: cxyang@ucdavis.edu). N. K. Fontaine, R. P. Scott, V. J. Hernandez, K. Okamoto, Z. Ding, J. P. Heritage, and S. J. B. Yoo are with the Department of Electrical and Computer Engineering, University of California, Davis, Davis, CA 95616 USA (e-mail: yoo@ece. ucdavis.edu). D. L. Harris is with Advanced Technology Labs, Sprint Nextel, Burlingame, CA 94010 USA. Digital Object Identifier 10.1109/LPT.2007.897489 field fiber, which is subject to environmental effects not easily replicated in the laboratory. A tunable dispersion slope com- pensator (TDSC) based on a flexible, cubic-function shaped mirror, along with traditional dispersion compensation, enables subpicosecond pulse transmission over the field fiber link. Instead of the previously demonstrated O-CDMA encoder based on polarization-dependent bulk optics [3], [6], which are not practical in telecommunication environments, this demonstration exploits a monolithic, fully integrated encoder based on high-density silica arrayed waveguide grating (AWG) technology. The AWG-based encoder is very compact, and polarization-independent, so that polarization tracking is no longer necessary in fiber transmission. This demonstration also includes the code switching or bright code/dark code (BCDC) modulation, a security enhancement against eavesdropping by simple energy detection on the up-link [7], [8]. With the advantages of several technologies including dispersion slope compensation, AWG encoder, and forward-error-correction (FEC), we demonstrate the viability of the security enhanced SPECTS O-CDMA system in the realistic environment. II. EXPERIMENTAL SETUP In this experiment, we use an optical frequency comb gen- erator (OFCG) [9] as the source. It generates over 120 optical comb lines spaced at 20 GHz and centered at 1550.4 nm, and produces 700-fs optical pulses. The OFCG is more stable than the optical source used in our previous demonstration. The AWG-based encoder consists of a 20-GHz passband spacing AWG pair, one demultiplexing an input onto 64 wave- length channels and the other multiplexing all channels to a single output. The zero- or -phase shifts applied to each wavelength channel are based on a 63-chip -sequence. The AWG pair has four input–output pairs forming four bidirec- tional encoders, each with a respective 20-GHz frequency shift. Therefore, each encoder applies a chip-shifted version of the 63-chip 20-GHz spacing -sequence to 63 modes of the OFCG (i.e., truly mode-by-mode coding). Fig. 1 shows the experimental arrangement. The FEC encoder adds 7% redundancy by a Reed–Solomon code [RS(255, 238)] to the pseudorandom bit sequence data source and generates a 2.48832-Gb/s (OC-48) data stream. This data stream drives two complimentary-output Mach–Zehnder modulators (COMZM), each modulating a 20-Gb/s pulse train split from the OFCG (i.e., each bit contains eight pulses). Each COMZM output goes through an encoder to be encoded with the BC or DC depending on whether the data bit is a “1” or a “0”, respectively. The BC and DC of a particular user are aligned in time and power 1041-1135/$25.00 © 2007 IEEE