Freeform optics - systems of the future

Freeform optical lenses without rotational symmetry enable the implementation of completely new concepts for optical systems. The use of freeform optics can reduce the number of elements in an optical system, allowing smaller, lighter and more efficient systems. Optical design and production of freeform optics require a high degree of precision and manufacturing know-how.

Specification of freeform surfaces

Freeform optics are refractive and reflective surfaces that differ significantly from spherical and aspheric geometries. Freeform optics manufacturing begins with the definition of the surface. In addition to a clear description of the optics, an equation, point cloud or 3D model is required for production.

One possible kind of freeform optics is the decentered use of an actually rotationally symmetrical lens, which is thereby adapted to the off-axis range. The optically relevant part is cut out (see Fig. 1), also to save space in systems. In terms of production, a symmetrical lens is thus cheaper and easier to manufacture. Regarding the optical design, its surface is already a freeform due to the optical effect achieved. However, a side effect of such a decentered lens is the increase of aberrations with oblique incident light, which ultimately leads to a rapid reduction in image quality. The great advantage of "real" freeforms is, they enable a very good imaging quality thanks to their special geometry and high number of degrees of freedom. However, the demands on the optical design and manufacturing process are much more complex and demanding compared to conventional symmetrical lenses.

Advantages and challenges of freeform optics

Based on their special surface shape, freeform optics can provide functions that cannot be achieved with classic optics. Especially in optical systems that require a folding of the beam path, e.g. monoliths, freeform optical lenses are extremely advantageous and can replace existing, larger arrangements such as mirror systems. To illustrate this, the following figure shows the optical design of a monolithic system consisting of three aspheric and one freeform surface with simulated beam path folding. The shown monolith was created within the research and development platform fo+.

Compared to spheres and aspheres, improvements in field of view (FOV) and numerical aperture (NA) as well as in size and weight can be achieved using freeforms (see Fig. 3). However, a high quality of the freeform optics is an absolute prerequisite for a constant high quality of the optical system.

The advantages of freeform optics mainly are

  • their compact shape,
  • the potential reduction in size of optical systems, and
  • the enabling of new optical functions.

Due to their optical shapes/surfaces, which have few or no symmetry, freeform optics have many degrees of freedom. Their manufacturing is similar to that of highly complex aspheres. The surface shape and local changes in inclination influence the complexity of the geometry, the manufacturing process and the measurement. Thanks to modern manufacturing and measuring equipment, asphericon also produces unconventional freeforms and systems. Furthermore, we offer optical design consulting, optical coating, mounting and CGH-free measurement (complete component, including all position tolerances).

Freeform optics made by asphericon

Discover the advantages of freeform optics and systems that can be realized with asphericon's innovative technologies.

  • Unconventional shapes in all materials (also ceramics)
  • Lenses, mirrors, monolithical elements with diameters up to 300 mm
  • Multi-focal, compact systems
  • Excellent surface quality (up to at least RMSi 50 nm)
  • CGH-free measurement including all position tolerances (complete component)
  • Individual coating and mounting concepts
  • Optical design consulting for production-optimized system designs

Growth core fo+

Together with regional partners, companies and research institutions, we have been working on the research project "Freefrom Optics Plus (fo+)" research project since 2014 on the development of innovative freeform optical systems. Highlight of this cooperation so far is the awarding of the science prize "Research in the Group" by the Stifterverband der Deutschen Wissenschaft for innovative approaches to the manufacturing freeforms in 2018.

Details about the production of freeform optics at asphericon can also be seen in the following video of fo+, which was shot during Photonics West 2020.

Asphericon Asphericon
FREE your mind, FORM your optics
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Alvarez System

Discover also our freeform systems based on Alvarez lenses. Such systems are particularly convincing due to their compact freeform optical design. Thanks to the surface shape it is possible to vary an optical function infinitely. Examples are the dynamic change of the focal length of a system, the compensation of divergence of laser beams, as well as the creation of square Top-Hat profiles. We will be happy to answer your questions about our Alvarez lenses in personal contact.

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Comparison

 

Standard Quality

Precision Quality

Diamond-Turning

Diameter8 – 300 mm4 – 300 mm1 - 150 mm
Diameter Tolerance± 0.1 mm± 0.01 mm 
Center Thickness2 - 60 mm2 - 60 mmfrom 0,5 mm
Center Thickness Tolerance± 0.1 mm± 0.05 mm 
RMS Irregularities (RMSi)0.75 – 0.3 µm0.09 µm0.1 µm
Surface Roughness2.0 – 3.0 nm1.5 – 2.0 nmup to 1 nm
Full-surface Interferometric  
Measurement
optionaloptionaloptional
Coatingcustomer-specificcustomer-specificcustomer-specific
Materialscustomer-specific
(including ceramics)
customer-specific
(including ceramics)
customer-specific
(including ceramics)
Mountingcustomer-specificcustomer-specificcustomer-specific
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Angela Eckstein

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