A low cost compact integrated 4-port single-channel OADM with improved optical
isolation and low insertion loss is provided in which a 4-fiber pigtail includes
I/O fibers and add/drop fibers, which are arranged on the same side of the multiplexer.
A lens, filter and reflector are arranged in a double-pass configuration to route
the optical signals to and from the appropriate fibers. The fibers are suitably
arranged with the input and drop fibers at the corners of one diagonal and the
output and add fibers at the corners of the crossing diagonal with the midpoints
of the diagonals intersecting. Furthermore, a point halfway between the input and
output fibers suitably lies at or near the central axis of the collimating lens.