  Strong turbulence conditions create amplitude aberrations through the effects
of near-field diffraction. When integrated over long optical path lengths,
amplitude aberrations (seen as scintillation) can nullify local areas in the
recorded image of a coherent beam, complicating the wavefront reconstruction
process. To estimate phase aberrations experienced by a telescope beam control
system in the presence of strong turbulence, the wavefront sensor (WFS) of an
adaptive optics must be robust to scintillation. We have designed and built a
WFS, which we refer to as a "Fresnel sensor," that uses near-field diffraction
to measure phase errors under moderate to strong turbulent conditions.
Systematic studies of its sensitivity were performed with laboratory
experiments using a point source beacon. The results were then compared to a
Shack-Hartmann WFS (SHWFS). When the SHWFS experiences irradiance fade in the
presence of moderate turbulence, the Fresnel WFS continues to routinely extract
phase information. For a scintillation index of $S = 0.55$, we show that the
Fresnel WFS offers a factor of $9\times$ gain in sensitivity over the SHWFS. We
find that the Fresnel WFS is capable of operating with extremely low light
levels, corresponding to a signal-to-noise ratio of only $\mbox{SNR}\approx
2-3$ per pixel. Such a device is well-suited for coherent beam propagation,
laser communications, remote sensing, and applications involving long optical
path-lengths, site-lines along the horizon, and faint signals.
