Accurate optoelectronic third-order nonlinearity characterization using improved carrier-shifted phase modulation

We propose and demonstrate an optoelectronic method for accurately characterizing the third-order nonlinearity of photodetectors (PDs) using improved carrier-shifted phase modulation. A carrier-shifted phase-modulated probe signal is injected into the PD under test, and the third-order nonlinear characteristics are extracted by measuring the powers of the fundamental and third-order intermodulation distortion (IMD3) components as the probe signal power is swept. To enhance measurement accuracy, a continuous-wave laser is employed for power compensation, maintaining a constant operating condition for the PD throughout the sweep. Moreover, an additional optical frequency shifter is inserted before the phase modulator (PM) to reduce the frequency spacing between the fundamental components, and a de-embedding procedure is employed to remove the contribution of the PM's nonlinearity. Experimentally, the spur-free dynamic ranges (SFDRs) of a commercial PD are measured over 10-67 GHz, and the effects of frequency spacing and input power on measurement accuracy are investigated.