Simulated SA-SOA-TPA-Based Receiver

Figure 4.22: (a) BER curves as a function of received average power for the SA-SOA-TPA- based receiver simulation model (b) Eye diagrams measured for each transmitting scenario.

Figure 4.22 shows the simulated results for the SA-SOA-TPA-based receiver. Again, the simulated BER curves as a function of received average power are shown in Figure 4.22 (a) with the simulated eye diagrams recorded at bit error rates of 1×10−9 shown in Fig- ure 4.22 (b). From Figure 4.22 (a) it can be seen that the power penalty associated with the addition of the interfering channel to the system is 7 dB. This value is the same as

that for the SA-TPA receiver, resulting in no improvement between the SA-TPA and SA- SOA-TPA receivers when adding a single interferer. However, the addition of 3 dB to the interfering channels power results in a power penalty of 9 dB. In comparison to the same result for the SA-TPA receiver, there is an improvement of 6 dB in this power penalty. As a result, the overall power penalty of this system using the SA-SOA-TPA-based receiver is 16 dB. This is an overall reduction of 15 dB when compared to the TPA-based receiver.

The simulation results presented above reinforce the experimental data discussed in sec- tion 4.3 that shows that the addition of each nonlinear optical device can significantly im- prove the performance of an OCDMA system in the presence of both MAI and optical beat noise. These results also demonstrate that a nonlinear receiver consisting of a saturable absorber, an SOA and a TPA-based detector can successfully recover a data pattern in the presence of both MAI and beat noise while operating at the data rate of the incoming signal rather than requiring a higher-speed photodetector in the receiver.

4.5

Summary

Multiple access interference and optical beat noise generated by the presence of interfering channels in an OCDMA system place strict limits on the performance that can be achieved for such a system. Optical beat noise becomes a significant problem when there is some level of coherence between the optical signals used in each channel resulting in a large amount of noise at the receiver. In this chapter it was shown that a saturable absorber can be used directly before a TPA-based detector in order to suppress MAI and improve the ex- tinction ratio of the incoming signal. However, the nonlinear responses of both the saturable absorber and the TPA-based detector, while successful in the suppression of MAI, can in- crease the level of optical beating present on the optical signal. Therefore some form of beat noise suppression is also required. In this chapter it was shown that a gain-saturated SOA can be used in conjunction with a saturable absorber and a TPA-based detector to success- ful reject both MAI and optical beat noise while operating at the data rate of the incoming optical signal, resulting in improved system performance. Such a receiver structure has the advantage of the possible integration of each device into a single, higher-speed, small area device that could be used in future OCDMA applications due to the semiconductor nature of the nonlinear devices.

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