  Silicon photonic wavefront phase-tilt sensors for wavefront monitoring using
surface coupling grating arrays are demonstrated. The first design employs the
intrinsic angle dependence of the grating coupling efficiency to determine
local wavefront tilt, with a measured sensitivity of 7 dB/degree. A second
design connects four gratings in an interferometric waveguide circuit to
determine incident wavefront phase variation across the sensor area. In this
device, one fringe spacing corresponds to approximately 2 degree wavefront tilt
change. These sensor elements can sample a wavefront incident on the chip
surface without the use of bulk optic elements, fiber arrays, or imaging
arrays. Both sensor elements are less than 60 um across, and can be combined
into larger arrays to monitor wavefront tilt and distortion across an image or
pupil plane in adaptive optics systems for free space optical communications,
astronomy and beam pointing applications.
