  Gaussian beams are often used in optical systems. The fundamental Gaussian
TEM00 mode is the most common of the Gaussian modes present in various optical
devices, systems and equipment. Within an optical system, it is common that
this Gaussian TEM00 beam passes through a circular aperture of a finite
diameter. Such circular apertures include irises, spatial filters, circular
Photo-Detectors (PDs) and optical mounts with circular rims. The magnitude of
optical power passing through a finite-sized circular aperture is
well-documented for cases where the Gaussian beam passes through the center of
the clear circular aperture, and is chopped off symmetrically in all radial
directions on a given plane. More often than not, a non-axial incident Gaussian
Beam is not blocked in a radially uniform manner by a circular aperture. Such
situations arise due to a lateral displacement of the beam from tilted glass
blocks, manufacturing errors and imperfect surface flatness or parallelness of
surfaces. The fraction of optical power of a laterally-shifted Gaussian Beam
passing through a circular aperture is calculated in this paper through
conventional integration techniques.
