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Typical Fringe Pattern    [Back]

The zero order fringe is the point of contact between the fiber and the glass reference flat.  When counting fringes you start at the contact point, which is usually a dark fringe, and count the number of similar ( light or dark ) fringes.  For example, on the spherical surface above left, the contact point is the dark fringe in the center.  Counting from the center out you would find three complete fringes, or six transitions from dark to light.  On the flat surface above right, the contact point would be on the left.  Counting from left to right you would find four fringes.  Counting transitions is helpful when there are not an even number of light or dark fringes.  The surface on the right is flat but angled.  The zero order fringe is on the left side of the fiber surface, the contact point with the reference flat.  Starting from the contact point you would count four complete fringes.  For an angle of 1 degree you would have approximately 10 fringes, therefore four fringes would be equivalent to a 0.4 degree angle.

    When measuring the end face of a spherically polished connector, we are most concerned with the area that is occupied by the fiber itself.   By knowing that the fiber diameter is precisely 125mm. we can extrapolate the end face radius by counting the number of fringes that fall within the fiber diameter.  Properly polished PC type connectors should have a radius that is greater than 10mm and less that 25mm. 

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       Last modified: 02/27/08