Research On The Performance Of Grating-based Encoder/decoder Of OCDMA System

Recently, an all-fiber fast frequency hop optical code division multiple access (FFH-OCDMA) scheme is proposed that encodes/decodes optical signal both in time and frequency domain and achieves encoding/decoding passively employing tunable fiber Bragg grating (FBG) arrays. However, there are two key problems exist in designing the tunable FBG array based encoder/decoder. The first one is the need to control intervals (or distances) related to bit rate between adjacent FBGs precisely during fabrication since the decoding FBG array have to be the exact inversion of the encoding FBG array. The second one is how to tune the Bragg wavelength of each FBG employing state-of-the-art FBG tuning technique with high accuracy and reduce the effect on system performance which may have resulted from excursion of FBGs' Bragg wavelengths. These two problems will result in differences in properties such as Bragg wavelengths and relative positions between the encoder and decoder which we refer to as the parametrical asymmetry problem.In this paper, the performance of FFH-OCDMA system is studied and simulated on condition that there is the aforementioned parametrical asymmetry. The impact to the system performance is also simulated in case of the system data rate beyond the theoretic limit. The encoding/decoding FBG array consists of n FBGs respectively.Simulation result shows that bit error ratio (BER) drops down sharply with a larger wavelength or relative position inconsistency as anticipated, especially when n is smaller, and the precise of wavelength and position parametrical can be lower requested in considering of BER rise slowly when parametrical asymmetry is small, e.g. n=3, wavelength offset <20% or relative position offset <30%. It also can be concluded that the impact to the system performance is very small when the system data rate is below than three times to the theoretic limit in case there is few users. However, as a larger n will almost certainly increase the system complexity and manufacture difficulty that may result in a higher probability of asymmetric Bragg wavelengths, trade-off has to be made, though it could reduce above performance deterioration.