  We examined the existence of critical wavelength in transmission-spectrum of
a Directional Coupler, employing GeO 2 - doped SMF and showed that it differs
significantly from that observed in SMS structures. The origin of critical
wavelength is explained. The sensitivity has been observed to be maximum for
the transmission maximum or minimum nearest to the critical wavelength. The
existence of a wavelength below the critical wavelength called the cross-over
wavelength where the spectrum again changes its nature of spectral shift has
also been observed. The effect of increase of core radius and of seperations
between cores of the two fibers; and the change in GeO 2 doping concentration
inside core on critical wavelength has also been studied. It was observed that
there exist two critical wavelengths for a Directional Coupler consisting of a
fiber having B 2 O 3 doping along with GeO 2 while the other has only GeO 2
doping. The effect of temperature and strain on the transmission spectrum of
the Directional Coupler has been studied.

Optics