The performance of the phase-shifted superstructured fiber Bragg grating (SSFBG) for optical code (OC) recognition was investigated with different reflectivity as well as input pulse width. The auto-correlation peak (PA) and the ratios of PA to the maximum wing level (P/W) and cross-correlation level (P/C) were used to quantitatively evaluate the OC recognition performance. There is a conflict between obtaining high PA and high P/W and P/C ratios in high reflectivity regime. The approach of applying apodization technique to improve the performance in high reflectivity regime is proposed. The comparative experimental investigations with 127-chip 160-Gchip/s SSFBG are carried out to confirm the effectiveness of the proposed approach. Error-free transmission with multiplexing of two active users has been successfully achieved by the apodized SSFBG at a data rate of 1.25 Gbit/s.

The generation and recognition of a record-length 511-chip optical code is experimentally demonstrated by use of a superstructured fiber Bragg grating (SSFBG) with a chip rate of 640 Gchips/s. Very high reflectivity (92%) is achieved with high-quality correlation properties. The temperature deviation tolerance is approximately +/- 0.3 degrees C, which is within the package's temperature stability range (+/- 0.1 degrees C). Experimental results show good agreement with the theory. They indicate the SSFBG's potential for processing a long optical code with an ultrahigh chip rate, which could significantly improve the system's performance.

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...

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.

width are seen (varied from 126°to 63°, a variation of 63°). This behavior indicates that, by varying the diameter D, the 3-dB beamwidth of the monopole's radiation pattern can be effectively controlled. Figure 6 presents the measured antenna gain against frequency for the cases with h ϭ 0 (reference antenna) and 25 mm. The reference antenna shows an antenna gain level of about 1.5 dBi higher than that of the case with h ϭ 25 mm. This is because the reference antenna has a smaller 3-dB beamwidth, thus having a higher directiveity and a higher antenna gain. Measured antenna gain at 2442 MHz for other cases studied is also listed in Tables 1 and 2 for comparison. In general, a larger 3-dB beamwidth obtained will lead to a smaller antenna gain for the monopole antenna. CONCLUSIONSThe omnidirectional radiation pattern of a cylindrical monopole antenna with a truncated-conical ground plane has been experimentally studied for operation in the 2.4-GHz WLAN band. The results have indicated that, by varying the height h and opening diameter D of the truncated-conical ground plane, the maximum radiation direction and 3-dB beamwidth of the monopole's omnidirectional radiation pattern can be effectively controlled. With this pattern-control technique, the cylindrical monopole antenna is very suitable for WLAN access-point applications and can provide an optimal or near-optimal omnidirectional radiation pattern in order to cover the desired service area for enhanced WLAN communications. IMPROVEMENT OF THE CODING PERFORMANCE OF SSFBG EN/DECODER BY APODIZATION TECHNIQUE