Computer Generated Holograms for Null Testing

Arizona Optical Metrology is well-known for our CGH null correctors that allow interferometric measurements of aspheric surfaces, lenses, and optical systems.

CGH null correctors are:

  • Well understood, mature technology
  • Easy and quick to specify and purchase
  • Easy to install, align, and perform measurements
  • incredibly accurate
  • Capable of measuring nearly anything: aspheres, freeforms, cylinders, systems
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Tech Notes coming soon!

How A CGH is used for measuring aspherical surfaces

CGH Nulls AOM

Custom Manufactured CGH Nulls for Measuring Aspheric Surfaces

AOM makes optical testing with CGH Nulls accessible to all levels of expertise. Below we have broken the process of ordering and using a CGH Null from AOM into 6 easy-to-follow steps.

Step 1 - Provide the specifications to AOM

Our CGH’s are custom designed to meet your needs. AOM holograms are designed with computer algorithms and manufactured using state of the art computer controlled e-beam and laser writing machines. The resulting holograms use diffraction to accurately generate practically any wavefront for measuring or aligning surfaces, lenses, or systems. When combined with a high-quality interferometer, the CGH corrects for the difference between the interferometer’s spherical wavefront and the desired light distribution for metrology. AOM CGHs include additional patterns that are used for alignment of the test. AOM offers several standard CGH sizes. The size needed for a specific test is determined from a combination of the part size and aspheric departure as well as added alignment features. Request a quote or contact AOM engineers directly. What are your needs?

Contact AOM engineers today:

  • By email: Send drawings and specifications to info@cghnulls.com
  • Call for consultation: 520-833-5761
  • Submit a request for information using the CGH null worksheet

Step 2 - Order CGH from AOM (delivery in 5-6 weeks)

Verify the specifications in the quotation and place an order for the CGH, mounting hardware, and alignment references. Expect delivery in 5-6 weeks  (2 week expedite available upon request).

Step 3 - Install and align CGH

It is easy to install the CGH and align it to your interferometer using the AOM EasyNull system. The CGH is precision mounted into a frame that has kinematic coupling to a 6-degree-of-freedom (6DoF) alignment stage. Simply place the stage within a few millimeters of the specified position and click the CGH into the magnetic seats. Use the reflected light from the built-in alignment CGH to guide adjustment of the 6DoF stage.

 

Step 4 - Align the optic under test

AOM can provide alignment references to put your optic in the right place for the measurement.  The details depend on your requirements of datum references and alignment procedures.  We can work with you to provide a wide range of reference features and techniques:

      • Projected bright spots that are picked up visually or with a camera

    • Catseye focus off the test optic

    • Projected spherical wavefronts that are reflected by tooling balls

    • Projected line focus references that are reflected by sphere mounted retroreflectors (SMRs)

    • Datum reference features attached to the CGH

    • Metrology platform with optical references for alignment to the CGH and mechanical datum features for repeatable placement of the test optic.

 

Step 5 - Collect and process the data

In many cases, the data that comes straight out of the interferometer has everything you need, and you’re already done. There are specialized operations that can be performed with AOM CGHplus software:

  • Correcting for mapping distortion
  • Subtracting alignment terms from the data
  • Correction for as-built CGH substrate distortion
  • Perform tolerance analysis

Step 6 - Report the results

AOM can help make it easy to provide the data in a complete, attractive format, including the following:

  • Measurement result after compensation
  • Tolerance analysis
  • Measurement uncertainty from as-built hologram and alignment tolerances
  • Traceable Quality Control

CGHs for measurement of optical systems

Computer Generated Holograms accurately create virtually any wavefront that we design. These wavefronts are used for measuring optical systems as well as optical surfaces. Some examples of systems measured by AOM holograms are:

  • Assemblies of lenses, such as the field lenses for the Rubin Telescope (Aka LSST)
  • Multiple field angle testing for wide-field telescopes

AOM Certified Accuracy

Arizona Optical Metrology applies rigorous quality control for the design, manufacture, and documentation of all product. Each CGH is provided with a certificate of accuracy, listing the QC measurements and verifications.