Resonators liberated from dispersion: Broad periodic waveguides operating at the Littrow regime

We show that broad periodic waveguides operating at the Littrow regime can form series of modes with a negative dispersion in their spacing (dFSR/dω > 0, FSR = Free Spectral Range), thanks to samples with extended range. We provide an analysis of this experimental FSR based on a nearly analytical consideration. For frequency combs, a uniform FSR (dispersion free) can be attained at any desirable frequency in the range λ = 1.5-4.3 μm for a silicon resonator.