Coherent Artifact Reduction
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APPLICATION NOTE Coherent Artifact Reduction System (CARS) Introduction Artifacts in interferometric data can take many forms. A common artifact is a “bulls-eye” which is usually caused by dust on an optical surface in the coherent beam path of an interferometer. Noise from various sources, such as laser speckle, electronics, or turbulence can also appear in the data. These artifacts can print through on the height map affecting the magnitude and repeatability of the surface result. A New Approach to Artifact Reduction Zygo’s Ring of Fire™ extended source is a patented method for reducing noise and other artifacts, and can be selected in place of a standard point source to reduce noise in the interferometer. Building on this expertise, Zygo has developed a coherent artifact reduction system that is ideally suited to the DynaFiz™ dynamic laser interferometer. Similar to the Ring of Fire system, CARS switches from a point source to a ring source enabling a reduced noise floor. Unlike the Ring of Fire system, CARS synthesizes the ring by means of a sequential acquisition compatible with dynamic data capture. CARS reduces noise from wavefront shearing, speckle, or mottle by a factor of up to 10 allowing for more surfaces features to be resolved. For example, scratches that would be buried by noise in spot mode are visible using CARS as shown in the following images. CARS Functionality On systems with CARS, Ring Mode can be enabled by simply selecting it from the drop-down menu or using the remote. Internally, switching from Spot to Ring Mode allows the CARS arm to move the point source position (see Figure 2). Instead of a fixed spot, it operates in a circular motion taking frames at Figure 1a. Spot Mode Measurement. Visible bulls-eye and speckle noise makes data look rougher and increases PV. Figure 1b. CARS Ring Mode Measurement. Bulls-eye disappears, lower noise gives smoother data, and small scratches bec

Coherent Artifact Reduction System—CARS various positions around a ring. This moving point source simulates an extended ring source similar to the Ring of Fire system. The high energy rotating point source allows for fast shutter and carrier fringe acquisition with excellent contrast. With the CARS option, there are two distinctive Ring acquisition types: Fixed Mode and Convergence Mode. Fixed Mode When a time limit is set, the software will average as frequently as possible during the specified time limit. A convergence threshold defines the maximum acceptable RMS of the difference map...

Coherent Artifact Reduction System—CARS A spatial deviation map and its RMS value (plotted in the control chart) provide the appropriate metrics for monitoring any spatial variances in the averaged data over an extended period of time. This allows the effects of cavity air turbulence and/or other thermally induced phase variations to be minimized with confidence. Conclusion Artifact reduction is a key part of obtaining the highest quality interferometric data. Artifacts such as noise and bulls-eyes caused by dust or secondary reflections in the optical path can significantly affect final...