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.