A new code structure for spectral-amplitude-coding(SAC)optical CDMA system based on modified LSOOC(MLSOOC)was proposed.It has an ideal in-phase cross correlation(CC)and reaches the upper bound.The structure based on fiber Bragg gratings spectral encoder/decoder was proposed.The principle analysis of SAC based spectral encoder/decoder and the characteristic anlalysis of the new code have been done.We also analysed the SNR and BER by taking into account the effects of phase-induced intensity noise(PIIN),shot noise,thermal noise and dark current noise.Our results reveal that MAI can be canceled at an ideal source and channel and the new code can effectively suppress the intensity noise.For a stated BER,the total active synchronous users are increased greatly compared with that using Hadamard.