An optical attenuator has first and second lenses provided individually on
the respective end faces of first and second optical fibers that are
opposed to each other across a space, and a shutter plate interposed
between the lenses. The distance between the shutter plate and the second
lens and a mode-field radius of propagating light on the second-lens-side
surface of the shutter plate are set so that the respective signs of the
real and imaginary parts of an evaluation function, representing a
diffraction pattern of the propagating light that propagates from the
first optical fiber to the second optical fiber, are inverted 60 times or
more when a variable of the evaluation function is changed within an
evaluation range.
