Broadening the angular tolerance in two-dimensional grating resonance structures at oblique incidence.

Broadening of the angular response of two-dimensional (2D) guided mode resonant spectral filters at oblique incidence is investigated. Coupling into multiple fundamental guided resonant modes having the same propagation constant but propagating in different planes (inherent multiple-plane diffraction by 2D gratings) is shown to significantly broaden the angular tolerance while maintaining narrow linewidth. Resonances have symmetric and broad angular responses when the incident wave is coupled to four resonant modes in a structure with a hexagonal grating pattern. Further broadening is implemented by enhancing the second Bragg diffraction of the 2D grating structure. Resonance with a narrow spectral linewidth (Dlambda(FWHM) approximately 1.6 x 10(-4)lambda(0)) and angularly tolerant to an ~6 mum beam diameter is obtained. A second approach utilizing a dual 2D grating configuration with a second grating on the substrate side is shown to increase the lateral confinement, causing the merging of two successive resonant bands. This results in further improvement of the angular/spectral linewidth ratio by ~80%.

Control of resonance separation over a wide spectral range in multiwavelength resonant grating filters.

The spectral separation of resonances within multiwavelength resonant grating filters is studied through the use of a grating layer with constant structural parameters in combination with single-layer, multimode waveguides and multilayer, multimode waveguides. The use of multilayer, multimode waveguides are shown to provide the ability to control resonance separations for multiwavelength filters from hundreds of nanometers to only a few nanometers.

All-dielectric unidirectional dual-grating output coupler.

A novel concept for an all-dielectric unidirectional output double-grating coupler is proposed and rigorously analyzed. In addition to a superstrate side grating, a second grating is placed on the substrate side. The periodicities of the gratings are chosen such that no propagating diffracted orders are present outside the structure in the superstrate region and only a single order is present outside the structure in the substrate region. The concept provides a robust output coupler requiring neither phase-matching between gratings nor any resonances in the structure, and is very tolerant to potential fabrication errors. Up to 96% coupling efficiency from the substrate-side grating is obtained over a wide range of grating properties.

Multiple wavelength resonant grating filters at oblique incidence with broad angular acceptance.

Multilayer, multimode waveguides are utilized in resonant grating filters having a broadened angular acceptance bandwidth for multiple wavelengths at a single oblique angle of incidence. It is shown that the waveguide grating structure should support a few leaky modes in order to support a multiwavelength resonant filter at oblique incidence with broadened angle acceptance at each wavelength.

Multiline two-dimensional guided-mode resonant filters.

A novel multiline filter using a two-dimensional guided-mode resonant (GMR) filter is proposed. The filter concept utilizes the multiple planes of diffraction produced by the two-dimensional grating. Multiple resonances are obtained by matching the guided modes in the different planes of diffraction to different wavelengths. It is shown that the location and the separation between resonances can be specifically controlled by modifying the periodicity of the grating and the other physical dimensions of the structure. This is in contrast to the one-dimensional GMR filters where the location of the resonances is material dependent. Two-line reflection filter designs with spectral linewidths less than 1 nm and a controllable spectral separation of up to 23% of the short resonance wavelength are presented using rectangular-grid grating GMR structures. Three-line filters are designed in hexagonal-grid grating GMR structures with two independently controllable resonance locations.