Navy

Micro-fabricated diffractive optical elements

Increased optical performance, increased mechanical and thermal precision tolerance, and reduction in size and weight relative to traditional optical designs

Photonics

Fabrication of precise DOEs that circumvents the need for multiple lithography steps and increases the number of surface steps to, for example, over 100.

Diffractive optical elements (DOEs) are used in many applications, such as optical storage devices, sensing, and communications. A DOE serves to wave-shape incoming light. Whereas standard refractive optical elements, such as mirrors and lenses, are often bulky, expensive and limited to a specific use, DOEs are generally light-weight, compact, easily replicated, and can modulate complicated wavefronts. Diffractive lenses can be used to reduce the number of elements in conventional lens systems and eliminate the need for exotic materials in correcting chromatic aberrations. The fabrication of a DOE is a rather complex process. The 3-D surface profile of the DOE determines how the element will shape an incoming wavefront. Hence, the key feature of any DOE is its complicated 3D surface topography. Some gratings can be blazed or cut, but most DOEs are made by micro-fabrication techniques. This usually involves many lithography and etching steps each of which can introduce errors degrading the optical performance.

Navy researchers have improved upon the gray-tone lithography technique to fabricate DOEs. A stack of bi-layers is deposited on a substrate. Each bi-layer includes an etch stop layer and a bulk layer with respective indices of refraction that approximate each other. A 3-D photoresist structure is formed by using gray-tone lithography as plasma is etched (reactive ion etching) into the bi-layer stack, thereby generating at least one plasma-etched bi-layer stack. The first chemical etchant stops at the etch stop layer. Optionally, a second chemical etch is used for the bulk layer, such as a standard phosphoric acid-based solution used to etch alumina.

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