Control of residual amplitude modulation in Lithium Niobate phase modulators

This work provides a simple model for Residual Amplitude Modulation observed in Lithium Niobate phase modulators. It operates under two key assumptions: the optical field incident on the modulator is not perfectly aligned to the preferred axis, and the two linear polarizations become spatially separated while travelling down the waveguide. These assumptions allow for a straight forward transfer matrix based approach. The effects of chromatic dispersion present in the optical fiber following the modulator are included, as they become important for modulation frequencies over 20 GHz. The result is a closed form expression for the intensity modulated signal seen by the photodetector in a phase modulated system. The model describes a near-instantaneous control mechanism, which is useful in minimizing the residual amplitude modulation in fielded systems by offering over 40 dB of suppression. The model is compared to direct measurements, validating the polarization effects and control mechanism proposed. Furthermore, etalon effects are ruled out by doing a course temperature dependence measurement.