Provided are a small-sized, low-cost, and easy-to-use laser optical path
length difference detecting device, a laser optical path length
difference detecting device, and a coherent optical coupling device. The
laser optical path length difference detecting device detects an optical
path length difference between propagation paths of a first laser beam
(1) and a second laser beam (2), which are mutually coherent when the
beams are propagated through two arbitrary optical paths of a plurality
of laser beam optical paths. The laser optical path length difference
detecting device is provided with an optical path length difference
variable means (4) for changing the optical path length difference
between the first laser beam (1) and the second laser beam (2), a
wavefront inclination generating means (5) for inclining a wavefront of
at least one of the first laser beam (1) and the second laser beam (2),
and a two-dimensional detector (6) for detecting the interference light
intensity distribution of the first laser beam (1) and the second laser
beam (2) which have passed through the wavefront inclination generating
means.