Fifth order versus third order intermodulation distortion

The third order intermodulation distortion (IMD
3
) rather than the fifth order intermodulation distortion (IMD
5
) is often considered when examining the performance of an analogue fibre optic system. This paper aims at looking into the possibility of the IMD
5
being comparable to the IMD
3
. The IMD
5
generated when the laser is intensity-modulated by the multiple carriers signal is simulated and compared to the IMD
3
. Individual IMD
5
level is found to be lower than the IMD
3
when the RF powers of the individual carriers are low or the modulation index per channel is low. However the composite IMD
5
level at channel position could exceed the composite IMD
3
especially when modulation index increases. This is contributed by the number of IMD
5
being greater than that of the IMD
3
based on counting. Hence indicating the IMD
5
should not be neglected in examining the performance of the analogue fibre optic system driven by multiple carriers.