Dielectric stack separates wavelengths for WDM

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Valerie C. Coffey

Demultiplexing devices used for long-distance transmission, such as arrayed waveguide gratings, are complex to fabricate and relatively costly compared to diffraction gratings and prisms. Although diffraction gratings, traditional dispersive devices, are simpler and less expensive than arrayed waveguide gratings, they typically have an angular dispersion less than 1°/nm, preventing them from being sufficiently compact.

A recent experiment by scientists at the Ginzton Laboratory at Stanford University (Stanford, CA) demonstrated a simple one-dimensional (1-D) dielectric stack structure as an inexpensive, simple way to disperse a beam with a compact device. By deliberately directing light through a dielectric stack (commonly used as mirrors) at wavelengths just outside the main reflection band, the team found that strong group-velocity dispersion occurred and waves still propagated through the structure.

For a beam incident at an angle, this dispersion gives rise to a wavelength-dependent shift of the beam. The shift is due to the group velocity dispersion, not the phase velocity dispersion, which can be dramatically different in a periodic structure. The average direction of energy propagation can be shown to be the same as the direction of group velocity, and is given by the normal to the constant-frequency dispersion diagram.

The constant-frequency dispersion relation near the photonic band edge is parametric (see figure). Light with a given incident angle, qph, has different propagation angles within the structure for different wavelengths. Near the photonic band edge, the propagation angle changes rapidly with wavelength. This property enables a large beam-steering effect in the structure.

The structure used in the experiment is a 30-period stack of alternating gallium arsenide (GaAs; n = 3.6) and aluminum gallium arsenide (AlGaAs; n = 3.0) layers 80 nm thick, grown upon a 500-µm-thick GaAs substrate by molecular beam epitaxy. For wavelengths 3 nm away from the band edge, angular dispersion is greater than 5°/nm. Inherent scalability allows a dielectric stack structure to be designed at any wavelength of interest. For more information, contact Bianca E. Nelson at bnelson@leland.stanford.edu.Wdm93859 7

Constant-frequency dispersion relation is shown for two different wavelengths near the band edge. Real values of the wave vector, Re {K} (versus the wave vector in the direction parallel to the layers, b) are used to derive the phase and group velocity within the structure. Smaller arrows correlate to phase velocity. Larger, thicker arrows correlate to group velocity.

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