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Micro-Interferometry Theory    [Back]

The End-View Interferometric microscope utilizes the principal of a Fabry Perot type of interferometer.  Constructive and destructive interference is created when the monochromatic (single wavelength or color) light reflects back and forth between the end of the fiber face and a glass reference flat.  When the light passes through the reference, some of the light (about 4%) is reflected back toward the microscope objective, by the process known as Fresnell Reflection.  Of the remaining light that continues on toward the fiber, an additional 4% is then reflected from the surface of the fiber.  As shown below, depending on the distance between the fiber surface and the surface of the reference flat, the light can add up either constructively

or destructively.  When this occurs, the resulting light forms a series of lines, known as Fringes.  The distance between each fringe corresponds to a vertical change in distance that is equal one half of the wavelength of the light.  Only The End-View Micro-Interferometer uses Blue light with a wavelength of 472 nanometers or 0.472 micron.  That means there is a vertical distance of 0.236 micron from one fringe to another.  This may be from one dark fringe to the next dark fringe or from one light fringe to the next light fringe.  Therefore the distance from one light fringe to the next dark fringe is a mere 0.118 micron.  This is called a Transition, and is equal to one half fringe.  When counting fringes, every time you transition from light to dark or dark to light, you have a vertical change in height equal to 0.118 micron. Depending on the shape of the fiber end face you may see different patterns formed by the fringes.  Basically the fringe patterns are just like a contour map of the fiber and ferrule surface.   

A perfect spherical surface will produce a series of concentric circular fringes.  By counting the number of fringes in a given area (such as the end of the fiber) you can estimate the radius of curvature.  The more fringes that are present the smaller the radius of curvature.  As you approach a flat surface the circle become so large that they appear as straight lines.  By counting the number of straight line fringes that appear across the face of a fiber, you can calculate the angle of a cleaved or polished fiber end. 

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       Last modified: 02/27/08