108. NEXT GENERATION PLANAR IMAGER USING ACTIVE ILLUMINATION
Name: Jason H. Karp
Grad Year: 2010
Optical rays contain both amplitude and phase components, however detection systems are only sensitive to light intensity. Phase contains the optical-path-difference of the light providing important three-dimensional relations. Phase measurements are needed to map surface profiles in manufacturing metrology, atmospheric aberrations for astronomy and corneas of the eye for corrective vision surgery. Lensless imaging is achieved by sensing the entire complex data field.
A wavefont sensor maps phase information into intensity so it can be recorded and processed. Most implementations are application specific and force a tradeoff between resolution and dynamic range. We propose a simple and compact system using a phase transmission grating attached directly to a CMOS image sensor providing flexibility and adaptation based on the input profile.
With coherent illumination, the phase grating produces overlapping diffraction orders to form an interference pattern with lateral shear. Using Fourier analysis and integration techniques, the gradient data as well as the wavefront profile can be reconstructed. A Matlab simulation platform was created to develop and test the algorithm for arbitrary wavefront shapes. Experimental results using lenses as phase objects are in agreement with commercial profilometer measurements.
The sensitivity of the wavefront sensor is largely dependent on the input phase profile and can be maximized at a specific shearing distance. The amount of lateral shear is controlled by the frequency of the grating and the propagation distance to the sensor. If the fixed grating were replaced by a phase modulator, the system could adapt to achieve maximum sensitivity for any wavefront profile. This would enable wavefront sensors to be used in a variety of new applications where the general profile is not known a priori as well as surpass the limitations inherent to other sensing modalities.