Method and apparatus for monitoring multi-wavelength optical systems

Apparatus for monitoring multi-wavelength optical systems includes an optical fiber system (44) and apparatus (42) for separating the different wavelengths of transmitted light transmitted therethrough and for monitoring the respective optical power in the separated spectral components. More specifically, embodiments of the invention scan or modify the physical parameters of in-fiber gratings (46) that couple light between spatially different modes of light within a wavelength-division-multiplexed optical fiber system, separate the spatial modes using a mode-discriminating device (47) and monitor or detect the separated spectral components using a conventional or other suitable detector (48). By scanning the in-fiber gratings, the peak wavelength of coupling between two dissimilar modes is modified, thus allowing control of the coupling within the fiber optic system. Scanning the grating is performed, for example, by changing the temperature or modifying the physical dimensions of the grating. In one embodiment, the system employs a long-period grating that couples light between two co-propagating, spatially different modes. In an alternative embodiment, the system uses a short-period grating that couples light between a forward propagating mode and a spatially dissimilar, backward propagating mode.