The present invention relates to an optical device having a structure that
can make the diameter of the emitted Gaussian beam smaller. The optical
device is arranged between a first and second positions, and comprises a
first and second optical systems separated with a focus on an
intermediate position between the first and second positions. The first
optical system, arranged between the first and intermediate positions,
comprises a first light entrance surface on which a Gaussian beam having
a beam waist, whose radius becomes w.sub.0 at the first position, is
incident, and a first light emission surface from which a Gaussian beam
having a beam waist, whose radius becomes w.sub.1 at the intermediate
position, is emitted. The second optical system, arranged between the
intermediate and second positions, comprises a second light entrance
surface on which the Gaussian beam emitted from the first optical system
is incident, and a second light emission surface from which a Gaussian
beam having a beam waist, whose radius becomes w.sub.2 at the second
position, is emitted. The radial ratio of the beam waists
(w.sub.2/w.sub.0) is set so as to become smaller than a
geometrical-optical lateral magnification of the optical device.