I. Introduction
Optical phase modulators are fundamental elements for electrical-to-optical signal conversion, and have a large number of applications including microwave photonic signal processing [1], optical frequency comb generation [2], single sideband modulation [3], optical pulse generation [4], and heterodyne transmission systems. The dynamic response of the phase modulator, which shows the variation of modulation efficiency over a range of frequencies, is a key parameter that requires critical evaluation. However, because optical phase modulators inherently only modulate the phase of the light, the modulator response cannot be measured directly using conventional network analyzers.