Wavefront Phase Map Interferograms

Interferograms offer another perspective on our uniques imaging systems. Specifically, interferograms are images of phase distributions. These images can be used to view the phase of the optical masks required for focus-invariant and aberration-invariant imaging. Even though our current phase masks are continuos phase structures, direct phase measurements through interferograms measure phase modulo one wavelength.

The above image is a simulated interferogram of a 2-D rectangularly separable cubic phase mask. Each set of light and dark bands on the interferogram denote a phase change of one wavelength. At optical frequencies a wavelength is around 0.5 microns. The maximum phase deviation of the mask above is measured along either the horizontal or vertical axes. Counting the light and dark bands along these axes shows that the maximum phase deviation of the displayed optical mask is about 20 wavelengths. Other masks can have more or less phase deviation depending on the application.

Fabricating cubic phase optical masks is challenging since optical precision is required over a large number of wavelengths of a non-spherical surface. Measurement of such surfaces is also challenging for the same reasons. One way to reduce the maximum amount of phase deviation is to add a linear phase or tilt component to the optical mask. By moving your mouse over the above image, you can view an interferogram of the tilted mask.

A tilted cubic phase mask has approximately 3/4 of the maximum phase deviation of the untilted mask. The effect on the image from this tilt component of the mask is to slightly displace the image from the optical axis by a few pixels.