Fifth order versus third order intermodulation distortion

The third order intermodulation distortion (IMD₃) rather than the fifth order intermodulation distortion (IMD₅) is often considered when examining the performance of an analogue fibre optic system. This paper aims at looking into the possibility of the IMD₅ being comparable to the IMD₃. The IMD₅ generated when the laser is intensity-modulated by the multiple carriers signal is simulated and compared to the IMD₃. Individual IMD₅ level is found to be lower than the IMD₃ when the RF powers of the individual carriers are low or the modulation index per channel is low. However the composite IMD₅ level at channel position could exceed the composite IMD₃ especially when modulation index increases. This is contributed by the number of IMD₅ being greater than that of the IMD₃ based on counting. Hence indicating the IMD₅ should not be neglected in examining the performance of the analogue fibre optic system driven by multiple carriers.