Modeling Phase Transients in Acousto-Optic Modulators

Abstract

In neutral-atom quantum computing, realizing robust, high-fidelity two-qubit gates requires precisely shaped optical control pulses. These time-varying amplitude and phase profiles are standardly engineered using an Acousto-Optic Modulator (AOM). However, during turn-on, turn-off, and fast-switching, the AOM introduces optical distortions to the aforementioned amplitude and phase profiles, due to the finite propagation speed of the acoustic wave across the aperture, and the ring-up dynamics of the transducer circuit. While these transients are currently mitigated using empirical feedback loops that treats the AOM as a black box, this thesis aims to develop a quantitative, first-principles theoretical model of AOM-induced phase transients during turn-on and turn-off. Our framework integrates a transducer circuit model to simulate the acoustic wave's amplitude and phase profiles during turn-on and turn-off, alongside a spatial coupling model that calculates the time-dependent overlap of a clipped Gaussian beam entering a single-mode fiber. We find that this model successfully reproduces the effects of angular fiber misalignment on the phase transient when driven at the resonant frequency of 110 MHz. However, it fails to predict the system's behavior under off-resonance driving conditions. Specifically, our circuit model predicts an odd-symmetric phase flip across the resonance, whereas experimental data exhibits an even-symmetric response. This fundamental discrepancy suggests that our transducer circuit model may be incorrect or incomplete, or that off-resonance phase transients are only marginally influenced by electrical ring-up dynamics. Future efforts may find it fruitful to desolder electrical components in the transducer circuit to procure a precise measurement of component values, and to experimentally obtain the electrical properties of the piezoelectric transducer. It may also be helpful to model the precise acousto-optic interaction during turn-on and turn-off within some non-infinitesimal interaction length.