Jump to content

Optical manufacturing and testing

From Wikipedia, the free encyclopedia
This is an old revision of this page, as edited by Holographer1 (talk | contribs) at 21:50, 9 February 2024 (→Testing techniques). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

Optical manufacturing and testing spans an enormous range of manufacturing procedures and optical test configurations.

The manufacture of a conventional spherical lens typically begins with the generation of the optic's rough shape by grinding a glass blank.[1] This can be done, for example, with ring tools. Next, the lens surface is polished to its final form. Typically this is done by lapping—rotating and rubbing the rough lens surface against a tool with the desired surface shape, with a mixture of abrasives and fluid in between. Typically a carved pitch tool is used to polish the surface of a lens. The mixture of abrasive is called slurry and it is typically made from cerium or zirconium oxide in water with lubricants added to facilitate pitch tool movement without sticking to the lens. The particle size in the slurry is adjusted to get the desired shape and finish.

Types of lapping include planetary lapping, double-sided lapping, and cylindrical lapping.[2]

During polishing, the lens may be tested to confirm that the desired shape is being produced, and to ensure that the final shape has the correct form to within the allowed precision. The deviation of an optical surface from the correct shape is typically expressed in fractions of a wavelength, for some convenient wavelength of light (perhaps the wavelength at which the lens is to be used, or a visible wavelength for which a source is available). Inexpensive lenses may have deviations of form as large as several wavelengths (λ, 2λ, etc.). More typical industrial lenses would have deviations no larger than a quarter wavelength (λ/4). Precision lenses for use in applications such as lasers, interferometers, and holography have surfaces with a tenth of a wavelength (λ/10) tolerance or better. In addition to surface profile, a lens must meet requirements for surface quality (scratches, pits, specks, etc.) and accuracy of dimensions.

Fabrication techniques

Unconventional techniques include single-point diamond turning (SPDT) and magnetorheological finishing (MRF).[3]

Free-abrasive grinding

Free-abrasive grinding is a technique to grind down the surface of a material before polishing. It involves the use of small particles of grit to grind away small chips of material from the surface of an optical workpiece. The grit particles are known as free abrasives. The particles are added to a liquid slurry, which goes between a grinding plate and the material. Sliding motions between the grinding plate and the material are used.[4]

After grinding, there is a small amount of surface roughness, which is based on the size of the grit. Additionally, a small amount of fracturing below the surface of the material is present, which is known as subsurface damage (SSD).[4]

Additional grinding at a smaller grit size can be used to reduce the amount of roughness and subsurface damage.[4] A set of two or three stages of grinding with decreasing grit size is typically used. A typical set of grit stages is 30 micrometer, then 15 micrometer, then 9 micrometer. An alternate set of grit stages that is used is 20 micrometer, then 12 micrometer, then 5 micrometer.[4]

Types of abrasives include aluminium oxide, industrial diamond, and silicon carbide. Diamond is typically only used for grinding down very hard materials, or for certain crystals.[5]

Polishing

Optics are polished in a slurry of abrasive particles, a fluid carrier, and optional additives.[6]

Types of abrasive particles that can be used include cerium(IV) oxide, diamond, aluminum oxide, and colloidal silica.[6]

Optional additives include suspension agents, lubricants, and detergents.[6]

Material Selection

There are various materials that can be used for optical components, including BK7 glass, fused silica, silicon, high-index glass, and crystal quartz.[7] Calcium fluoride (CaF2) can be used as an optical material, although it is easily fractured and scratched.[7]

Materials for infrared optical components include zinc selenide (ZnSe), zinc sulfide (ZnS), and gallium arsenide (GaAs).[7]

Component Specifications

The specifications for optical components vary based on their type:

Specifications for prisms include pyramidal error, beam path, beam displacement and deviation, base angle, roof edge chips, wavefront, and polarization.[8]

Specifications for aspheric lenses include base radius with tolerance, conic and polynomial coefficients, best-fit sphere reference, sag table reference, sag error tolerance, slope errors versus bandwidth, wavefront per specified test, tilt, and decenter.[8]

Optical coating specifications include apertures, reflection, transmission, absorption, phase shift, adhesion, abrasion resistance, and damage threshold.[8]

Testing techniques

The Fizeau interferometer is the standard type of interferometer that is used in optical fabrication.[9]

For testing aspheres, stitching interferometry can be used. It involves performing subaperture tests that are stitched together into a single-high resolution image.[10]

See also

Notes and references

  1. ^ Shorey, Aric B.; Golini, Don; Kordonski, William (October 2007). "Surface finishing of complex optics". Optics and Photonics News. 18 (10). Optical Society of America: 14–16.
  2. ^ Williamson, Ray (2011). Field guide to optical fabrication. SPIE field guides. Bellingham, Wash: SPIE Press. pp. 45–48. ISBN 978-0-8194-8676-9.
  3. ^ Williamson, Ray (2011). Field guide to optical fabrication. SPIE field guides. Bellingham, Wash: SPIE Press. ISBN 978-0-8194-8676-9.
  4. ^ a b c d Williamson, Ray (2011). Field guide to optical fabrication. SPIE field guides. Bellingham, Wash: SPIE Press. p. 26. ISBN 978-0-8194-8676-9.
  5. ^ Williamson, Ray (2011). Field guide to optical fabrication. SPIE field guides. Bellingham, Wash: SPIE Press. p. 27. ISBN 978-0-8194-8676-9.
  6. ^ a b c Williamson, Ray (2011). Field guide to optical fabrication. SPIE field guides. Bellingham, Wash: SPIE Press. p. 38. ISBN 978-0-8194-8676-9.
  7. ^ a b c Williamson, Ray (2011). Field guide to optical fabrication. SPIE field guides. Bellingham, Wash: SPIE Press. p. 8. ISBN 978-0-8194-8676-9.
  8. ^ a b c Williamson, Ray (2011). Field guide to optical fabrication. SPIE field guides. Bellingham, Wash: SPIE Press. p. 10. ISBN 978-0-8194-8676-9.
  9. ^ Williamson, Ray (2011). Field guide to optical fabrication. SPIE field guides. Bellingham, Wash: SPIE Press. p. 87. ISBN 978-0-8194-8676-9.
  10. ^ Williamson, Ray (2011). Field guide to optical fabrication. SPIE field guides. Bellingham, Wash: SPIE Press. p. 12. ISBN 978-0-8194-8676-9.
  • Malacara, D., Optical Shop Testing - 2nd Edition, John Wiley and Sons, 1992, ISBN 0-471-52232-5
Retrieved from "https://en.wikipedia.org/w/index.php?title=Optical_manufacturing_and_testing&oldid=1205532317"