  A lens performs an approximately one-to-one mapping from the object to the
image planes. This mapping in the image plane is maintained within a depth of
field (or referred to as depth of focus, if the object is at infinity). This
necessitates refocusing of the lens when the images are separated by distances
larger than the depth of field. Such refocusing mechanisms can increase the
cost, complexity and weight of imaging systems. Here, we show that by judicious
design of a multi-level diffractive lens (MDL) it is possible to drastically
enhance the depth of focus, by over 4 orders of magnitude. Using such a lens,
we are able to maintain focus for objects that are separated by as large as ~6m
in our experiments. Specifically, when illuminated by collimated light at
lambda=0.85mm, the MDL produced a beam that remained in focus from 5mm to
~1500mm from the MDL. The measured full-width at half-maximum of the focused
beam varied from 6.6um (5mm away from MDL) to 524um (1500mm away from MDL).
Since the sidelobes were well suppressed and the main-lobe was close to the
diffraction-limit, imaging with a horizontal X vertical field of view of 20deg
x 15deg over the entire focal range was demonstrated. This demonstration opens
up a new direction for lens design, where by treating the phase in the focal
plane as a free parameter, extreme depth-of-focus imaging becomes possible.
