Image Number 5 for United States Patent #6002822.
A dispersive optical waveguide tap comprises a blazed refractive index grating in the core of the waveguide, coupling means, focusing means and utilization means. The grating is selected such that guided mode light of predetermined wavelength will, in the absence of the coupling means, be directed into one or more cladding modes of the waveguide. The presence of the coupling means, in optical co-operation with the waveguide, changes the guiding conditions such that the cladding modes are substantially eliminated from a portion of the waveguide that includes the cladding, whereby the grating directs the guided mode light into one or more radiation modes. The blaze angle typically is .ltoreq.15.degree.. The focusing means serve to bring the radiation mode light substantially to a focus in at least one dimension, the focal point (or line) depending on the wavelength of the light. The utilization means exemplarily comprise an array of photodetectors, and the coupling means exemplarily comprise an appropriately shaped glass member and index matching means. Dispersive waveguide taps are advantageously used in WDM optical communication systems, e.g., to provide status information (e.g., channel wavelength, channel power, including presence or absence of a channel) to, e.g., a system maintenance unit. The status information facilitates maintenance of operating conditions by conventional feedback control. Any optical element that can bring the tapped radiation to a focus on the utilization means can serve as a focusing element. Exemplary focusing elements are optical lenses (cylindrical or non-cylindrical), diffraction gratings, volume gratings (holograms), and combinations thereof.