Traditional laser scanning systems, such as those used for microscopy,
typically image objects of finite thickness. If the depth-of-focus of such
systems is low, as is the case when a simple clear pupil is used, the
object must be very thin or the image will be distorted. Several methods
have been developed to deal with this problem. A microscope with a thin
annular pupil has a very high depth-of-focus and can image the entire
thickness of a sample, but most of the laser light is blocked, and the
image shows poor contrast and high noise. In confocal laser microscopy, the
depth-of-focus problem is eliminated by using a small aperture to discard
information from all but one thin plane of the sample. However, such a
system requires scanning passes at many different depths to yield an image
of the entire thickness of the sample, which is a time-consuming process
and is highly sensitive to registration errors.
In this thesis, a novel type of scanning system is considered. The sample
is simultaneously scanned with a combination of two Gaussian laser beams of
different widths and slightly different temporal frequencies. Information
from scanning with the two beams is recorded with a photodetector,
separated electronically, and processed to form an image. This image is
similar to one formed by a system using a difference-of-Gaussians pupil,
except no light has been blocked or wasted. Also, the entire sample can be
scanned in one pass. The depth-of-focus characteristics of this synthesized
difference-of-Gaussians pupil are examined and compared with those of
well-known functions such as the circular, annular, and conventional
difference-of-Gaussians pupils.
