Diffractive Optical Elements: How they are Designed and Fabricated

Diffractive optical elements or DOEs have many uses including to generate optical patterns, particularly for applications that require the creation of the laser spots or pattern elements are defined positions. Examples of these patterns include those that represent rulers or scales and viewfinder patterns. To produce these complex light patterns, the design and fabrication of the diffractive elements must be done sophisticatedly.

How to Design a DOE

Placing a diffractive element behind a laser module’s lens will make a lot of spots visible. These spots can form a complex light pattern. To design the diffractive microstructure, an inverted light propagation from the plane must be simulated. Also, the interaction between the incident light field and the microstructure must be described properly. The majority of diffractive elements come with a spatially period transmission function that results in clearly defined spot positions in a given plane of interest. The transmission function’s design only influences how much of the incident light is diffracted into a certain diffraction order. But, the mere geometrical size of its special periodicity affects the orders’ propagation direction.

How to Fabricate a DOE

Diffractive optics and micro-optics provide options that are superior to traditional optics. DOEs are fabricated using high-resolution lithographic techniques. The easiest type to fabricate is binary elements that have two-phase levels. Fabricating these elements requires the creation of a chromium amplitude mask through a direct-writing process using an electron beam. Silica can be infused into the mask’s substrate, allowing the conversion of the amplitude element into a phase element through reactive-ion etching.

Because diffractive optical elements made from fused silica tend to have a high damage threshold, they are suitable for high-power laser applications. Also, these elements can be used to generate a replication tool. During fabrication, multilevel diffractive elements require greater effort. Fabricators can use multi-mask or direct-wire approaches for fabricating multilevel structures in a photoresist.

A lot of DOE fabricators use computer-controlled laser direct writing that works in polar coordinates. Direct laser writing has been used because of its ability to allow for the flexible, fast, and cost-efficient fabrication of DOEs. With this system, fabricating rotationally symmetric optics is significantly made easy. Generally, the system is composed of a writing beam focused onto a substrate coated with a photoresist. To fabricate grayscale structures, the intensity of the writing beam can be controlled. Through this method, the substrate is placed on a turntable that rotates with a certain angular velocity.

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