To eliminate multiple access interference(MAI) noise in optical time division/code division multiple access igital communication systems, a novel all-optical threshold decision technique based on cross gain modulation(XGM) effect in distributed feedback(DFB) lasers cavity is proposed. By injecting optical digital pulse signal into DFB laser, XGM effect appears inside the laser. Using optical bandpass filter(OBPF) to filter out the new spectrum produced by XGM effect can eliminate MAI noise in the signal, thus improving the reception performance of digital signal.