Design, fabrication, and characterization of a full complex-amplitude modulation diffractive optical element

The use of diffractive optical elements (DOEs) is increasing for several industrial applications. Most elements modulate the phase of incoming light or its amplitude, but not both. The phase modulation DOE is the most popular because it has a high diffraction efficiency. However, the phase-only limitation may reduce the freedom in the element design, increasing the design complexity for a desired optimal solution. To overcome this limitation, a novel, full complex-amplitude modulation DOE is presented. This element allows full control over both phase and amplitude modulation of any optical wave front. This flexibility introduces more freedom in the element design and improves the element's optical performance, even in a near-field operation regime. The phase grating of the element was fabricated in an amorphous hydrogenated carbon film. The amplitude modulation was obtained by patterning a reflective aluminum thin film, which was deposited on top of the phase grating. The apertures in the metal film determine the quantity of transmitted light. The use of a reflective layer in the fabrication decreases the risk of laser-induced damage since no absorption is involved in the process. With this device it is possible to obtain extremely efficient spatial filtering and reconstruct low noise images.