Impact of Nonlinearity on RF-Modulated Frequency Combs with Different Modulation Depths in an MUTC Photodetector

We calculate the impact of nonlinearity in a modified uni-traveling carrier (MUTC) photodetector on an RF-modulated frequency comb that is generated using short optical pulses. We take into account bleaching (nonlinear saturation), which plays an important role due to the large peak-to-average power ratio. Nonlinear impairment of an RF-modulated continuous wave is characterized by the second- and third-order intermodulation distortion (IMD2 and IMD3). By contrast, an RF-modulated frequency comb is characterized by a distinct IMD 2
_n
and IMD >3
_n
for each comb line n. We study the effect of modulation depth on nonlinearity in frequency combs, comparing modulation depths of 4% and 8%. When we include bleaching, we find that the OIP3
_n
, third-order intercept point for the n-th comb line, increases when the modulation depth increases for comb lines at frequencies above 2 GHz (n ≥40), so that distortion decreases. We show that this effect occurs because bleaching attenuates lower frequencies more than higher frequencies and this preferential attenuation increases as the modulation depth increases.