BACKGROUND

Integrated photonics has grown over the past several decades to be valuable to a wide range of applications. One of the more recent of these applications is in the generation of a magneto-optical trap, or MOT. A necessary component of a MOT is an array of three optical beams, travelling orthogonally to each other, which intersect in free space. Typically, complex sets of discrete optical components are required to generate the three optical beams, but it has been recently shown that a single integrated photonics chip can achieve the same task.

Grating couplers are structures that are essential to integrated photonics, as these couplers allow for light to be diffracted out of the plane of a photonics chip and into free space. However, in order to emit an optical field with a desired intensity distribution, it is necessary to make the grating stronger or weaker as a function of position. The typical way to achieve this is by way of a term known as the “duty cycle.” Each period of a grating is composed of a high refractive index region and a low refractive index region, and in the absence of apodization, each of these regions occupies an equal percentage of each grating period, meaning the duty cycle is equal to 0.5. If this ratio is either increased or decreased, the grating strength will be reduced. However, this state-of-the-art approach cannot be used to drive the grating strength all the way to zero because of lithographic limitations, and this approach additionally generates higher-order diffraction events, which waste power by emitting light in incorrect directions.

To achieve the needed performance from a photonics chip, a method is needed that provides for the emission of optical beams with uniform intensity distributions.

SUMMARY

An optical coupler device comprises a substrate having a substantially planar upper surface, and a grating structure on the upper surface of the substrate. In one embodiment, the grating structure comprises a copropagating array of waveguides that are substantially parallel to each other and extend along at least a portion of the upper surface of the substrate. Each of the waveguides has opposing sidewalls, wherein a width of each waveguide is defined by a distance between the opposing sidewalls. The opposing sidewalls each have a periodic structure that produces a sidewall modulation for each of the waveguides. An input port is in optical communication with the grating structure. The input port is configured to direct an input light beam in plane into the grating structure such that the input light beam propagates along the array of waveguides. The grating structure is configured to diffract the input light beam out of plane and into free space.

