Plane-parallel optics are transparent components with parallel faces, or systems with two or more parallel surfaces. Measuring plane-parallel optics with a laser interferometer can be challenging, as the parallel surfaces can all contribute interference fringes, making measurement extremely difficult or impossible.
4D's Surface Isolation Source is an optional, external laser source that excludes all but the surface of interest. Both surfaces of a transparent optic can then be measured, and transmitted wavefront error, optical thickness and homogeneity can be calculated. The adjustable path match mechanism provides flexibility, letting you dial in any surfaces that are within 88 through 112 millimeters from the aperture.
Accufiz Surface Isolation Source (SIS) with Adjustable Path Matching
Fringes on a plane-parallel glass disk without SIS (left) and with SIS (right).
In a single setup you can measure both the front and back surfaces of an optic without repositioning it. Homogeneity, transmitted wavefront error and optical thickness can be readily obtained from a combination of measurements.
An AccuFiz equipped with SIS can perform all of the functionality of a standard AccuFiz as well as measuring plane-parallel optics. For example, you can use the Surface Isolation Source to measure the flat surface of a plano-convex lens, then switch to the internal source and add a transmission sphere to measure the spherical surface.
Measurement of front and back surfaces of a plane-parallel sapphire window. The back surface was measured through the optic.
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Dynamic Interferometry
4D's patented wavefront sensors enable vibration insensitive Dynamic Interferometry. US Patents 6,304,330, 6552,808, 7,230,717, others. Other patents pending.
4D Technology dynamic laser interferometers use proprietary, high speed, high resolution wavefront sensors to measure the shape of optical surfaces with extreme accuracy despite vibration and environmental noise.
In a conventional phase-shifting laser interferometer, multiple frames of data are acquired over many milliseconds—enough time for vibration and turbulence to degrade the measurement results. In a dynamic interferometer, all phase data is acquired simultaneously, in as little as thirty microseconds. This speed enables 4D dynamic interferometers to measure in noisy environments, despite vibration solation or air turbulence.
Dynamic interferometry® enables complex tests between multiple unisolated tables, measurements in environmental chambers, and measurements with long stand-off distances. Odd shapes, pupil obscurations, poor signal contrast, moving parts, air turbulence and mechanical vibration are not problems. Special wavefront sensors have also been developed for wavelength insensitivity, higher spatial resolution, and applications within tight space restrictions.
