Diffractive optical elements (DOEs) are specialised optical components that manipulate light using micro-scale patterns. They have become increasingly popular in various applications, including laser beam shaping, holography, and displays. In this article, we will delve into five leading principles of DOEs that are crucial to their functioning:
- Diffraction: The basic principle behind DOEs is diffraction, which is the scattering of light to multiple orders as it passes through a medium with a periodic structure. When light enters a DOE, the diffraction gratings on its surface cause the light to split into multiple beams, which can then be redirected, shaped, or combined in various ways.
- Interference: Interference occurs when two or more waves overlap and reinforce or cancel each other. In DOEs, the diffracted beams interfere with each other to produce a specific intensity distribution. This allows DOEs to be designed to produce specific patterns, such as beam shaping or holographic displays.
- Phase control: The phase of the diffracted beams is an important factor in determining the intensity distribution of the light. By carefully controlling the phase of the diffracted beams, DOEs can produce specific patterns and shapes. This can be achieved by designing the grating structure of the DOE or by adding additional optical elements to the system.
- Surface relief structures: DOEs typically have a surface relief structure, which is a pattern of ridges and valleys on the surface of the component. This pattern is responsible for diffracting the light and determining the intensity distribution of the diffracted beams. The surface relief structure can be designed to produce specific patterns and shapes, and can be fabricated using a variety of techniques, such as lithography or etching.
- Wavelength sensitivity: DOEs are designed for a specific wavelength of light, as only the correct wavelength creates the correct delay when the light goes through the surface relief. This means that for a certain relief depth, only the correct wavelength will diffract properly, while for other wavelengths the efficiency of diffraction falls and the diffraction angles change. This property can be useful for applications such as spectral filtering or colour separation, where the strong chromatic behaviour of DOEs is harnessed to separate colours.
In conclusion, DOEs have become an essential tool for manipulating light in various applications, and their design and fabrication have advanced significantly in recent years. These five principles provide a basic understanding of the underlying mechanisms behind DOEs and how they can be designed and utilised to produce specific light patterns.