Modulator-spurious-free electrooptic method for analyzing harmonic distortion of photodetectors with frequency-shifted heterodyne

A modulator-spurious-free electro-optic method is proposed for analyzing the harmonic distortion of photodetectors (PDs) based on frequency-shifted optical heterodyne. The proposed method utilizes the heterodyne between phase-modulated optical signals and a frequency-shifted optical carrier as an actual overall electro-optical stimulus. The harmonic distortion of PDs under an ideal single-tone electro-optical stimulus can be de-embedded from the spurious distortion in the actual overall electro-optical stimulus by tracking the modulation depth of the phase modulation. Therefore, the proposed method equivalently provides an ideal single-tone stimulus for implementing modulator-spurious-free measurement of harmonic distortion of PDs. Moreover, our method enables bias-independent and frequency-swept analysis for PDs, thanks to electro-optic phase modulation. For a proof-of-concept, second- and third-order harmonic output intercept points (HOIP2, HOIP3) of a commercial PD are measured at different frequencies, and the consistency between our method and conventional methods verifies that the modulator-spurious-free analysis provides a minimalist setup but accurate method for the harmonic distortion characterization of PDs.