Study Of Optical CDMA Technology Based On Bipolar Chirped Coding

Optical code division multiple access (OCDMA) is getting more and more attention for its good performances such as high security, soft capacity, asynchronous access, convenient network management. OCDMA is regarded as one of the most potential techniques in the future optical networks, especially in the optical access networks. OCDMA system based on intensity modulating/direct detecting (IM/DD) is easy to realized, but its performance is not good due to the non-ideal correlation of one-dimensional code. The bipolar code based on phase modulating has a good performance, but difficult to realized due to its high optics parameter. Combining the two scheme's benefit, the OCDMA based on IM/DD and bipolar coding is proposed in this paper.The positive and negative dispersion characteristic of gauss pulse which enters into linear chirp fiber Bragg grating from different ends is analyzed. A bipolar coding scheme for pulse is achieved using the positive and negative dispersion characteristic. A bipolar chirped OCDMA scheme, based on a LCFBG and fiber delay line, is proposed in this paper. The mathematical model of system is established, and the system bit error rate (BER) formula is presented. The system performance, such as BER, normalized throughput, largest single-user data rate and impact of LCFBG dispersion, is analyzed.The results show that the system BER in our schemes is much better than that in one-dimensional prime code OCDMA systems. With the same code length and error rate, the number of online users can be up to twice of that in binary phase shift keying (BPSK) CDMA system in the electronic domain. The peak normalized throughput appears when load equals to quarter the code length. When the load is moderate or heavy, it can get a larger normalized throughput by increasing the code length.In order to reduce the impact caused by fiber dispersion to system performance, the LCFBG dispersion in the scheme should be as larger as possible. But simulations in this paper find that the before-class LCFBG dispersion greater impact on the system performance. The before-class LCFBG ceiling dispersion is to ensure address chip do not interfere with each other. The after-class LCFBG dispersion little impact on the system performance.