An apparatus for monitoring an input optical signal at a plurality of distinct optical frequencies is disclosed wherein a demultiplexing arrayed waveguide grating (AWG) having a plurality of M>1 Vernier input ports is disposed between an optical switch and a photodiode array coupled to the output ports of the AWG. In operation, the optical switch sequentially provides the input optical signal into each of the Vernier ports, and signals detected by photodiodes are stored in a memory unit. The apparatus is capable of monitoring the input optical signal with a frequency step which is M times smaller than a frequency spacing between the AWG transmission bands, and obtain M frequency-resolved readings from each photodiode.