FTI Presents at SPIE: Distinguishing Characteristics of Diffractive Optical Elements and Fresnel Lenses
An experiment to explore and explain the difference between often-confused Fresnel lenses and Diffractive Optical Elements (DOEs)
The distinction between Diffractive Optical Elements (DOEs) and Fresnel lenses is often confused due to their similar appearances and overlapping terminologies. However, understanding their unique characteristics is crucial for their effective application in various optical systems. This article dives into the distinguishing features of DOEs and Fresnel lenses, based on the research conducted by A. Guidry, M. Kyrish, L. Stone, M. Fraelich, O. Lechuga, and N. Claytor at Fresnel Technologies, Inc.
Background and Optical Principles
Diffractive optics, another name for meta lenses and metasurfaces, which are trending as a solution in AR/VR, are often confused with Fresnel lenses--much due to terminology issues. Especially for those with a background in lasers who may be familiar with Fresnel zone plates, it's an understandable confusion. Fresnel zone plates are a type of basic diffractive optical element used to create a pattern of alternating transparent and opaque concentric rings, or Fresnel zones, which diffract light in a controlled manner. Fresnel zone plates are not Fresnel lenses--though both are named after Augustin-Jean Fresnel, a prolific inventor.
The distinguishing characteristics between DOEs and Fresnel lenses include feature size, optical performance, and operational phenomena. DOEs’ optical properties stem from diffraction/interference, whereas Fresnel lenses operate primarily via refraction. Since both light propagation methods exist on a spectrum, many optical components utilize both to some degree. However, the dominant physical phenomenon determines how the optic is classified. If the element’s optical performance can be sufficiently modeled and explained via geometrical optics, it falls into the refractive domain. If physical optics (also known as wave optics) are necessary to describe the element’s optical performance adequately, it is a diffractive optic.
Design and Fabrication
The design and fabrication processes for DOEs and Fresnel lenses further underscore their differences. Fresnel lenses are created by reducing the material between the front and back interfaces, forming annular zones that focus light to a common point. This design minimizes weight and absorption while maintaining optical performance. On the other hand, DOEs are designed with microstructures that alter the phase of incident light, producing desired diffraction patterns. In this experiment, both elements were fabricated by precise diamond-turning techniques to achieve the required surface profiles.
Optical Performance and Metrology
The optical performance of DOEs and Fresnel lenses is evaluated through metrological techniques. In the study, samples of both types of acrylic optics were subjected to focal-length metrology. The results demonstrated that DOEs exhibit greater axial chromatic aberration compared to Fresnel lenses, with focal lengths varying significantly across different wavelengths. This variation is attributed to the distinct operational phenomena of diffraction and refraction. Additionally, surface roughness measurements indicated low levels of scattering, ensuring high optical performance.
Differences between DOEs and Fresnel lenses
| Fresnel Lenses | DOEs | |
| Optical Principle | Function primarily through refraction | Operate based on diffraction and interference |
| Design & Structure | Created by reducing material between the front and back interfaces, forming annular zones that focus light to a common point | Designed with microstructures that alter the phase of incident light to create diffraction patterns |
| Fabrication | Fabricated by molding or machining to create the stepped surface structure | Fabricated using precise diamond turning techniques to achieve required surface profiles |
| Optical Performance | Have more consistent focal lengths across wavelengths and lower chromatic aberration | Exhibit greater axial chromatic aberration with focal lengths varying significantly across different wavelengths |
| Applications | Used in applications requiring lightweight and thin lenses, such as imaging systems and illumination | Used in applications requiring complex light manipulation, such as beam shaping and splitting |
Side-by-side view of a Fresnel lens (left) and Diffractive Optical Element (right). Chromatic dispersion can be seen in the glossy reflection off the surface of the DOE.
Surface profile sections of the Fresnel lens sample (A-left) and the DOE sample (B-right). Only a small region of the Fresnel grooves near the vertical step is shown.
Why It Matters
Understanding the distinguishing characteristics of DOEs and Fresnel lenses is essential for their effective application in optical systems.
Considering the right approach to your optical system? FTI offers a comprehensive range of services, including design assistance, diamond machining, prototyping, tooling, and production solutions. The company excels in producing molded optics with submicron features, catering to applications in imaging, automatic lighting, high-end security, tailored illumination, head-up displays (HUD), biomedical systems, and laser beam shaping. We are happy to talk early in your optical design process to suggest approaches that will best meet your performance, cost and size/weight requirements.
Access the paper in the SPIE Digital Library.