A vernier configuration in a 2.5-GHz-spaced 128-channel arrayed-waveguide grating (AWG) for use as a secondary demultiplexer in a planar optical spectrum analyzer was incorporated with a tandem configuration. The frequency changes were 3.0 and 2.5 GHz for adjacent input and output ports, respectively. By selecting any of the eight input ports of the AWG, new passbands could be generated in the gaps between adjacent passbands and in the gaps produced when the diffraction order of the AWG was changed. Enough new passbands were generated to achieve frequency sampling at 1-GHz intervals without dead frequency points in the C and L bands of optical fiber amplifiers.
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