For this reason, relatively sophisticated optical filters can be constructed using integrated Bragg gratings, as described in related reports. Despite the flexibility afforded by Bragg gratings, their application in integrated-optical devices has been limited to relatively simple components, largely because the required fabrication techniques have not been adequately developed. While the waveguides can be patterned using conventional optical photolithography, high-resolution nanolithography must be used to print the Bragg gratings. Moreover, these two lithography levels must be precisely aligned relative to one another. We use a combination of many different lithographies to generate the Bragg grating patterns for our devices.
Interference lithography is a cornerstone of our Bragg grating work.
In interference lithography, two coherent laser beams are crossed, generating a standing wave interference pattern. Which serves as a fiducial reference for subsequent steps. In order to allow precise alignment between the gratings and the waveguides, we have developed a technique of adding alignment marks to interferometrically-generated patterns. Before writing the alignment marks, the e-beam system samples the existing grating in order to ensure that the marks are precisely aligned to the submicron gratings. In most cases, the techniques mentioned above are not applied directly to a device, but instead to an x-ray mask.