  A high contrast imaging technique based on an optical vortex coronagraph
(OVC) is used to measure the spatial phase profile induced by an air plasma
generated by a femtosecond laser pulse. The sensitivity of the OVC method
significantly surpassed both in-line holographic and direct imaging methods
based on air plasma fluorescence. The estimated phase sensitivity of 0.046
waves provides opportunities for OVC applications in areas such as bioimaging,
material characterization, as well as plasma diagnostics.
