A method includes taking backscattering light of a first wavelength in a
first direction of a first transmission line and coupling the same so
that the light may travel in a second direction of a second transmission
line; taking a second wavelength in the first direction of the first
transmission line and coupling the same so that the second wavelength may
travel in the first direction of the second transmission line; taking
backscattering light of the first wavelength in the second direction of
the second transmission line and coupling the same so that the
backscattering light may travel in the first direction of the first
transmission line; taking the second wavelength in the second direction
of the second transmission line and coupling the same so that the second
wavelength may travel in the second direction of the first transmission
line.