High performance 70Gbit=s SPECTS optical-CDMA network testbed

Introduction: Remarkable increases in the internet traffic have recently propelled upgrades in the wide-area and metropolitan-area networks employing advanced wavelength-division-multiplexing (WDM) and time-division-multiplexing (TDM) communication systems. Modern local-area networks (LANs) on the other hand, require less processing at the centre nodes but more flexible high-speed access at the edges. Optical code division multiple access(O-CDMA) has promising advantages in LAN applications because of the high bandwidth capacity and the flexibility inherited from code-based reconfigurability without involving WDM or TDM switching components. While various O-CDMA schemes have been proposed and demonstrated in the past two decades [1, 2], they indicate that multi-user interference (MUI) is the primary hurdle in achieving a high capacity O-CDMA network. In this Letter we discuss an O-CDMA networking testbed adopting the spectral phase encoded time spreading (SPECTS) technique. In this scheme, short optical pulses are phase modulated with a unique code in the spectral domain which spreads them in time [1]. The receiver reconstructs the original short pulses by using the conjugate code. Codes are chosen so that the signal pulses from other users remain spread in time. It is important to note that the integrated pulse energy of a time-spread pulse is the same as that of a compressed pulse making it difficult for traditional bandwidth-limited detectors to differentiate between them. In SPECTS O-CDMA, discrimination is achieved through nonlinear thresholding and detection. The thresholder is based on optical fibre nonlinearity, which was first proposed by Sardesai and Weiner [3]. Utilising highly-nonlinear fibre (HNLF) instead of dispersion-shifted fibre (DSF) in the thresholder significantly improved the sensitivity and suppressed MUI. Recently, we reported bit error rate (BER) measurement results of a smaller SPECTS O-CDMA testbed using the HNLF thresholder for one and two users [4, 5]. In this Letter, we demonstrate a time slotted O-CDMA network operating at 10 Gbit=s=user with a total of seven simultaneous users.