A high resolution, wide spectral range, optical apparatus that includes an optical resonator cavity and a wavelength demultiplexer, arrangeable in multiple configurations. A method for increasing the resolution of a wavelength demultiplexer involves inputting light into an optical resonant cavity; inputting a plurality of different resonant output wavelengths to a wavelength demultiplexer; and routing each different resonant wavelength to a different output waveguide of the demultiplexer to generate a demultiplexer output spectrum. The method further involves performing either a time serialization or a space serialization procedure to increase the channel density and fully cover the spectrum of interest.