  We present the experimental evidence for the existence of two exciting
wavelengths, termed critical and cross-over wavelength in the transmission
spectrum of fiber-optic directional coupler, whose properties are conjugate to
each other. The spectral shift associated with the transmission maxima/minima
suddenly flip around these wavelengths and the spectrum shows largest (nil)
spectral shift for the transmission maxima/minima closest to the critical ( at
the cross-over) wavelength, corresponding to the same perturbation parameter. A
theoretical explanation of the observed experimental behavior has also been
presented, highlighting that the underlying mechanisms for the existence of
these wavelengths are entirely different. The knowledge of the precise spectral
location of these wavelengths is necessary to avoid false alarms.
