The specification describes a wavelength monitoring system for multiple wavelength
communications systems, such as WDM systems, based on the recognition that the
mechanism for spatially separating the individual wavelength bands can be achieved
within the optical fiber itself. Individual wavelength bands are separated using
a series of discrete gratings spaced longitudinally along the fiber core. The wavelength
bands are extracted from the fiber core by converting the energy in the selected
band from a core-guided mode to a radiation mode. By using a tilted grating, the
light in the radiation mode is directed through the cladding and out of the fiber.
Spatial resolution of the selected bands can be any desired physical length. An
important implication of this is that detection can be made in the near field using
inexpensive detecting apparatus. Near field is defined for convenience in this
case as the optical field of the radiated energy without any optically modifying
elements (collimating devices, diffraction elements, mirrors, etc.). This eliminates
nearly all of the optics, and the attendant expense, in prior art monitoring systems.
