Abstract:We demonstrate for the first time a 10-user, truly-asynchronous, gigabit OCDMA experiment over 50 km transmission using 511-chip SSFBG encoder/decoder and supercontinuum -based optical thresholder. Suppression of both beat noise and MAI are keys to the success. Introduction The passive optical network (PON) is promising for FTTH systems. Optical code division multiple access (OCDMA) is one promising candidate for next-generation broadband multiple access technique attributing to full asynchronous transmission, low latency access as well as soft capacity on demand [1][2][3][4][5][6][7]. There are several different OCDMA implementations that could be roughly classified according to operation principle as incoherent and coherent OCDMA [1]. Recently, coherent OCDMA using ultra-short optical pulse is receiving increasing attention with the progress of reliable and compact encoder/decoder devices, such as spatial light phase modulator (SLPM), planar lightwave circuit (PLC) and superstructured fiber Bragg grating (SSFBG). In coherent OCDMA, encoding and decoding are based on optical field amplitude instead of power intensity. The coding can be either direct time-spreading the ultra-short optical pulse using PLC [2] and SSFBG [7][8] or spectral phase-encoded timespreading using SLPM [3][4][5][6].In a common multi-user OCDMA network, multiple access interference (MAI) noise is the main issue [1][2][3][4][5][6][7]. The MAI could be suppressed effectively by employing time gating [2] or optical thresholding techniques [3][4][5][6]. Recently, multi-user coherent OCDMA experiments have been demonstrated by utilizing optical thresholding based on second harmonic generation (SHG) in periodically-poled lithium niobate (PPLN) [3,4] and nonlinear effect in high nonlinear fiber (HNLF) [5,6] to significantly suppress the MAI noise.However, coherent OCDMA could suffer from severe signal-interference (SI) beat noise if the signal and interferences overlap each other. The SI beat noise, which dominates over the MAI noise in such system, eventually limits the maximum number of active users that can be supported in the network [1]. Unfortunately, the SI beat noise could not be suppressed effectively by optical thresholding as it accompanies with the recovered signal pulse. Therefore, either slot-level [3,[6][7] or chip-level [4-6] timing coordination has been applied in previous experiments to enable multi-user transmission. Slot-level coordination is a rough synchronous approach that the signal and interferences are intentionally separated in time, therefore no beat noise will arise. Chip-level coordination is a precise synchronous approach that the signal and each interference have to be precisely aligned on a chip-level with zero interference to mitigate the beat noise. Both of them are synchronous approaches that sacrifice the most desired characteristic of OCDMA: "asynchronism". Besides, the former one significantly lowers the frequency efficiency of the system, while the latter one requires very strict network synchroniza...

An optical thresholding technique based on super-continuum generation in dispersion flattened fiber is proposed and experimentally demonstrated to enable data-rate detection in optical code division multiple access networks. The proposed scheme exhibits an excellent discrimination between a desired signal and interference signals with features of pulse reshaping, low insertion loss, polarization independency as well as reasonable operation power.

A ten-user truly asynchronous gigabit coherentoptical-code-division-multiple-access (OCDMA) transmission was experimentally demonstrated without using any timing coordination. The enabling technologies are a record-length 511-chip superstructured-fiber-Bragg-grating (SSFBG) en/decoder and a supercontinuum (SC)-based optical-thresholding technique to significantly suppress the signal interference beat noise as well as the multiple-access-interference (MAI) noise.

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