Abstract
We systematically investigate the interaction effects during time-of-flight(TOF) expansion of a pure condensate released from an optical lattice potential. By simulating the expansion process both in the spatial and momentum space, we find that the condensate fraction keeps decreasing due to the inter-particle interaction, showing a universal power-law dependent of the expansion time and interaction. Our results imply that the so-called ”condensate fraction” measured in the standard TOF experiment can be very different from the true condensate fraction inside the optical lattice due to such dynamical depletion. We further propose a theoretical scheme to estimate the condensate fraction based on the existing TOF experimental results, including both the finite time and finite interaction effects.