An optical module for optically coupling an input side and an output side
with each other through an optically functional portion inserted between
the input side and the output side. At least one of the input side and the
output side includes a plurality of collimators. At least one of the
collimators is made different from the other collimators in at least one
of the distance between the focal point of a corresponding lens and a
corresponding light exit or incident surface, the numerical aperture of
the light exit or incident surface, the effective focal length of the
lens, the wavelength used and the distance between optical axes of
adjacent ones of the collimators so that the size and position of a beam
waist on an input side are made substantially coincident with those on an
output side. In the case of a fiber collimator 14 constituted by a
combination of a lens 12 and an optical fiber 10, an end surface of the
optical fiber serves as a light exit or incident surface and the mode
field diameter of the optical fiber also serves as one of variable
characteristic parameters.
