Sources of nonlinearity and mitigation of phase noise in MUTC photodetectors at comb-line frequencies

We calculate the phase noise for the first 100 comb line frequencies of two modified uni-traveling-carrier (MUTC) photodetectors. The frequency comb is generated by one-picosecond pulses with a 2-GHz repetition frequency. We observe a non-monotonic increase in the phase noise and investigate its origins. Our model identifies the interplay among the space charge effect, the heterostructure design, and the nonlinear relationship between the electric field and the electron drift current that leads to a complex variation of the phase noise as a function of the comb line frequency. Based on the findings, we present ways to reduce the phase noise of the photodetectors. While the optimal design depends on the desired frequency range of operation, we find a design that can reduce the phase noise over a wide range of comb-line frequencies.