Multilayer thin-film stacks with steplike spatial beam shifting

This paper demonstrates the use of a single multilayer thin-film stack for generating custom-engineered spatial dispersion, i.e., spatial shift as a function of wavelength. Based on group-velocity effects similar to the superprism effect observed in photonic crystals, this thin-film grating (TFG) device allows for multiplexing or demultiplexing multiple wavelength-division-multiplexed (WDM) channels using a single nonperiodic thin-film stack. We designed a four-channel TFG for coarse WDM applications with 20-nm channel spacing and flat-top passbands. This paper shows that flat-top passbands correspond to a steplike spatial beam shifting with wavelength. The influence of a finite beamwidth on the passband shape is discussed using Fourier decomposition of the beam into plane-wave components. The paper concludes with experimental results for a three-channel TFG demonstrating steplike beam shifting.