An exemplary optical apparatus comprises: an optical element having multiple sets of diffractive elements; and a photodetector. The diffractive elements of each set are collectively arranged so as to comprise corresponding spectral and spatial transformation information for each set. At least two of the sets differ with respect to their corresponding spectral and spatial transformation information. The diffractive elements of each of the sets are collectively arranged so as to transform a portion of an input optical signal into a corresponding output optical signal according to the corresponding spectral and spatial transformation information. At least one photodetector is positioned for receiving at least one of the corresponding output optical signals.