Distorted grating simplifies wavelength sensor

A research team at the Defence Evaluation and Research Agency (Worcestershire, England) has developed a simple sensor for generating test wavefronts that combines a distorted diffraction grating with simple optics and a single camera. Although the measurement scheme is similar to that of wavefront curvature sensing, the new technique reportedly has several advantages, including noniterative reconstruction of the wavefront, robustness to scintillation, beaconless wavefront sensing on extended sources, and an ability to sense anisoplanatic, multiconjugate wavefront distortions.¹ According to research team member Alan Greenaway, one application that could benefit is wavefront sensing for adaptive optics.

A key component of the new sensor system configuration is a grating that has been distorted according to a quadratic function such that a different focal length can be associated with each diffraction order. This allows simultaneous imaging of multiple object planes with one camera. One benefit is that the quadratically distorted (qd) grating eliminates crosstalk between temporal and spatial wavefront structures. The system also can collect both the image of the object and the wavefront sensor data in the same focal plane.

An earlier multiplane imaging and wavefront sensing system developed at the agency had the qd grating placed close to a lens so that the diffraction orders imaged three unequally spaced real object planes onto one image plane. As a result, a different magnification was associated with the image in each diffraction order. The scientists report that the difference is small and addressable with post processing for many applications, including the imaging of multiple layers within a compact three-dimensional structure. Unfortunately, this is not the case with wavefront sensing applications in which the object planes to be imaged could be several meters apart. The resulting postprocessing demands would thus be impractical for a real-time adaptive optics system or a wavefront analyzer. The newer sensor system addresses this issue.

According to Greenaway, the current phase-diversity wavefront sensor generates the two pupil-plane images required for wavefront sensing in the -1 and +1 orders, with the zero order providing a direct image of the scene. With the zero order focused at infinity, the images in the other diffraction orders correspond to the object plane's equal distances on either side of the lens. Their magnifications are equal, but inverted.

To validate the system for different aberrations, including astigmatism, coma, trefoil, and spherical abberation, the scientists measured individual Zernike modes for 11 grating wavefront modulators with different magnitudes of distortion (see figure). They also were able to reconstruct a representative atmospheric phase map with good accuracy.

REFERENCE

• P. M. Blanchard et al, Applied Optics 39, 6649 (Dec. 10, 2000).