Accurate qualification of optical components is a vital source of feedback to an optical manufacturing operation. From plano to spherical to aspheric shapes, common measurands include surface form error, waviness and roughness. Depending on the type of part and its application, material and geometric properties such as transmitted wavefront, homogeneity, wedge, parallelism, thickness variation, and radius of curvature may also require careful control. Optical components are the building blocks of optical systems, making it critical to verify that each optic meets its requirements prior to integration into an assembly. Backed by over 50 years of experience supplying the highest quality optics and instrumentation, ZYGO’s laser interferometers and 3D optical profilers provide users with confidence in metrology for a wide range of optical components.
Lenses are made up of two surfaces which can be flat (plano), spherical, aspheric, or cylindrical. These surface shapes can all be measured for surface form errors, while the transmitted wavefront of a single lens or system of lenses can also be measured. These measurements can then be analyzed to provide information about the Point Spread Function, Modulation Transfer Function, Encircled Energy, and Strehl Ratio of an optical system or lens.
Windows consist of two flat polished surfaces. Primarily, the surface flatness of a window will determine its performance and needs to be measured. These surfaces may need to be parallel or have a defined wedge between the two. Parallel parts present unique measurement challenges and are best handled using ZYGO Verifire™ MST interferometers. Parallel parts also present a unique opportunity to simultaneously measure all the surfaces of a window.
Homogeneity of refractive index is another important metric used to define the quality of optical materials. Commonly measured by glass suppliers, homogeneity is the variation of refractive index throughout bulk optical materials. It is possible to measure homogeneity using any ZYGO interferometer, but ZYGO’s Verifire™ MST interferometers can provide a fast and reliable measurement of nonlinear and linear homogeneity.
Mirrors are reflective surfaces used to steer light. Mirrors have the advantage of providing the same performance regardless of wavelength. However, surface figure requirements for reflective components are much more demanding than their refractive counterparts. These surfaces can be plano, spherical, aspherical, cylindrical, or freeform in shape and can be measured using ZYGO interferometers and accessories.
Prisms and Cornercubes
Like mirrors, prisms are used to direct light where it is needed. Each face of a prism is built to a very specific angle relative to the others in a prism. This has the advantage of always providing the same output angle relative to the input. It is very important to measure these angles for accuracy since they cannot be changed after finishing.
Common prism measurements include right angle prism error, corner cube dihedral angle, surface flatness of prism faces, beam deviation, and transmitted wavefront error.
Beamsplitters are unique because they are intended to both transmit and reflect light. That makes the specifications on transmitted wavefront and surface form error critically important. Some beamsplitters are made up component prisms where it is important to measure each component before assembling.
Flat or Plano Surfaces
Flat surfaces are used in nearly all optical systems, and it is critical to ensure they meet specification. These surfaces are easily measured using interferometry and the results are extremely repeatable and reliable. Using a ZYGO Ultraflat and best practices, plano surfaces can be measured with an uncertainty of as low as λ/100. Large aperture beam expanders can be added to any of ZYGO’s interferometer systems to accommodate flat parts up to 32” in diameter.
Spherical surfaces used for lenses and mirrors are defined by their radius of curvature. Along with surface form error, these two quantities will drive the performance of the lens or mirror and it is essential to validate both to ensure the performance of an optical component. Spherical surface form is tested using a transmission sphere to generate an ideal reference wavefront. The same transmission sphere along with radius scale hardware is used to measure radius of curvature.
Cylindrical optics present a more difficult problem for optical testing than do flat or spherical optics. Testing cylindrical optics using a conventional interferometer requires generating a cylindrical wavefront to match the curvature of the surface. Because cylindrical optics are inherently more difficult to fabricate, it is difficult to manufacture good interferometric references that would allow cylindrical surfaces to be fabricated.
Conventional interferometry can provide useful information about a cylindrical optic's surface, but a Computer Generated Hologram (CGH) can be used to gather a complete description of the surface. A CGH can generate a cylindrical wavefront to match the curvature of the test surface. Similar to testing a sphere with a Transmission Sphere, a single cylinder CGH allows for testing a wide range of cylinder optics.
Aspheric and Freeform Surfaces
While some conic shapes have been used for a long time like parabolic and hyperbolic mirrors in telescope systems, more general aspheres have been difficult to manufacture and test. Advances in both are allowing optical designers to reliably use aspheric surfaces to improve optical performance and reduce size and weight of optical systems. Aspheric surfaces can be measured using the VeriFire Asphere system or Computer Generated Holograms (CGH). The wavefront error of optical elements or systems using aspheric surfaces can also be measured directly, avoiding the need to measure the aspheric surface.