How to generate a chaotic sequence in an optical domain and it is used as an address code in an optical code division multiple access (OCDMA) system is a new research direction in recent years. A generator producing simulated bipolar chaotic sequences completely in optical domains is designed by a Mach-zehender?interferometer. The amplitude and phase of an incident?light field are modulated by controlling the phase shift of two arms on the Mach-zehender?interferometer. So the vector of output optical field is equal to the product of input optical field vector and the cosine item. A chaotic sequence generator is made up of several interferometers, optical?fiber?delay?lines?and?phase?shifters. And a bipolar chaotic sequence is generated in an optical domain according to logistic?mapping. The chaotic sequence generator can be used as the encoder and decoder of the OCDMA system to improve security and increase numbers of users accommodated by the system.