The process of lock acquisition in a high-finesse suspended Fabry–Perot cavity used in the LIGO 40-m interferometer is numerically simulated. The simulation, including a model of the cavity optical transient response as the mirrors swing through resonance, demonstrates that acquisition of lock by the controller depends on the relative velocity of the mirrors and establishes a threshold velocity below which acquisition may take place. The model results are used to implement a real-time controller that analyzes the transient response, extracts the mirror velocity, and then guides the mirrors into resonance with relative velocity under the threshold. The result is a factor-of-10 decrease in the experimentally observed acquisition time.
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