The visibility of interference fringes related to optical coherence in interferometric microscopes provides a powerful non-contact sensing mechanism for 3D measurement and surface characterization. Coherence Scanning Interferometry (CSI) extends interferometric measurement techniques to complex surfaces in terms of roughness, steps, discontinuities, and structures (such as transparent films). Other benefits include automatic focusing at every point in the field of view and suppression of stray interference from scattered light.
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Overview
In the simplest conceptualization, surface height is inferred by noting where the interference effects are strongest. Thus, a defining characteristic of Coherence Scanning Interferometry CSI is that, by design, interference fringes are only strongly observed within a narrow range of surface heights.
The figure below shows how the appearance of interference fringes changes as the interferometric objective is scanned vertically along its optical axis. The conclusion drawn from the visual appearance of the fringes is that the outer edge of the sample is lower than the center, which is evident from the scanning position, where the contrast of the fringes at the edge is lower compared to the center. Despite modern instruments using various optical configurations and data processing methods to extract surface data, the highly correlated variations in fringe contrast are a common method across all CSI instruments.
In commercial equipment, CSI is an automated system with electronic data acquisition that provides signals as a function of the scanning position for each image pixel. The resulting signal is shown below, displaying the signal itself and the overall modulation envelope with peak positions that conceptually provide non-contact optical measurements of surface height.
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Typical Configuration of CSI
The figure below illustrates a generic CSI. The object has height features h that vary with the object’s surface. A mechanical scanner provides smooth, continuous scanning of the interferometric objective in the z-direction. During scanning, the computer records intensity data I for each image point or pixel in continuous camera frames.
The light source used for CSI is an incoherent light source. For example, incandescent lamps, halogen lamps, and currently the most common light source is white LED. The coherence scanning interferometer using white light is also known as a white light interferometer.
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Signal Formation
White light in interferometers is typically defined as a light source composed of larger bandwidth components, which visually appears white. It has the characteristic of poor temporal coherence, making the interference phenomenon less easily found, and the range of interference is usually only a few microns. As shown in the figure below, several beams of electromagnetic waves with different frequencies produce interference (left), resulting in an interference wave packet.
Since interference phenomena only occur at positions with the same optical path difference, controlling the relative position of the reference surface and the surface to be measured will only form interference at specific optical path difference positions. Therefore, by continuously changing the distance from the measured object to the reference surface and recording the positions of the corresponding interference fringe peaks generated at different height points on the measured surface, we can obtain the surface height profile of the measured surface.
As shown in the figure below, pixels at different horizontal positions on the CCD will obtain corresponding brightness changes of the interference wave packet at different vertical positions. As long as we can obtain the peak positions of the interference wave packets, we can obtain the surface height at the corresponding horizontal position and thus restore the surface profile of the measured surface. Of course, all of this is relative positioning.
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Application Case – MPO Fiber End Interferometer
Originally, FC, SC, LC, and other fiber connectors were single fiber. In the last 20 years, the rapid development of fiber communication, especially the explosion of chatGPT, has rapidly increased the demand for high-density multi-core fiber connections. The figure below shows the multi-core fiber connector MPO.
The measurement of the physical end faces of fiber connectors was primarily based on PSI. However, the fiber protrusions of high-density multi-core connectors MPO are about 1500nm, which cannot be measured due to the phase wrapping issue of PSI. CSI can just solve this problem.
