Wavelength multiplexing by spatial beam shifting in multilayer thin-film structures

Wavelength Division Multiplexing (WDM) systems allow the transmission of multiple channels over a single fiber by encoding each channel with a different optical wavelength. Compact and cost-effective wavelength multiplexing and demultiplexing devices are needed for combining the different WDM channels on the transmitter side and splitting them at the receiver. This dissertation investigates the use of a single multilayer thin-film stack with high spatial dispersion for multiplexing or demultiplexing multiple WDM channels by spatial beam shifting. The thin-film stack is designed such that multiplexed light incident at an angle experiences a wavelength-dependent effective group propagation angle in the stack. This translates to a wavelength-dependent spatial beam shift and demultiplexing at the output surface.; We introduce four different types of thin-film stacks with high spatial dispersion: Periodic stacks using the “superprism effect” in one-dimensional photonic crystals, chirped stacks exploiting a wavelength-dependent penetration depth, resonator stacks with dispersion due to stored energy, and numerically optimized non-periodic stacks utilizing a mixture of the two previous dispersion effects. The experimental results of a 200-layer periodic stack and a 66-layer non-periodic stack are discussed and compared. Because of its greater design freedom, the non-periodic stack gives both a linear shift with wavelength, and a larger usable shift than the thicker periodic stack.; Multiple bounces off the stack can be performed to increase the spatial beam shift. Using eight bounces off the 66-layer stack, a nearly linear 100-μm shift is achieved between 827 and 841 nm. This shift is sufficient to separate four channels by their Gaussian beam widths. We discuss that the number of separable channels is proportional to the total beam shift. Investigating over 600 different stacks, we develop a heuristic model predicting the maximum shift for a given stack thickness, material system, and incidence angle. From this model we find that the multiplexing of eight to sixteen WDM channels using a single thin-film stack with high spatial dispersion seems well possible.