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In this work we preset a new technique for measurement of harmonic distortion in microwave photodiodes. Our technique combines optical combs and high-resolution optical filtering to provide a pure optical two-tone excitation of the photo diode. Our technique removes the challenges presented by laser frequency drift and modulator bias control by utilizing a single laser and phase-modulation to synthesize the optical combs. We demonstrate our technique through measurement of the harmonic distortion generated in a commercial InGaAs photodiode and compare our results with those obtained via phase-locked laser measurements. We show that this technique provides essentially equivalent measurement fidelity to phase-locked laser techniques while offering decreased system complexity. We additionally provide a general discussion of the limits of nonlinearity measurements performed using optical heterodyne techniques.