Fiber Geometry

End users perform fiber geometry measurements to reduce system attenuation and coupling loss resulting from poor fiber fabrication. Fiber attenuation and intrinsic coupling loss result from mismatches in the inherent fiber characteristics of two connecting fibers. Fiber mismatches occur when manufacturers fail to maintain optical or structural (geometrical) tolerances during the fiber fabrication process. Fiber geometry measurements performed in the laboratory identify fiber mismatches before the optical fiber is installed.

The procedure for measuring multimode and single mode fiber geometry is detailed in EIA/TIA-455-176. The fiber-geometrical parameters measured include cladding diameter, cladding noncircularity, core-cladding concentricity error, and core noncircularity. Figure 4-8 (chapter 4) illustrates core noncircularity (ellipticity) and core-cladding concentricity error. The core noncircularity measurement is for multimode fibers only.

Other test methods are available for measuring other multimode and single mode fiber core parameters. Additional test methods exist for measuring multimode fiber core diameter and NA. For single mode fibers, the mode field diameter measurement replaces core diameter and NA measurements. Core diameter, numerical aperture, and mode field diameter measurements are identified and explained later in this chapter.

To make fiber geometry measurements, the input end of the fiber is overfilled and any cladding power stripped out. The output end of the fiber is prepared and viewed with a video camera. Generally the fiber is less than 10 m in length. An objective lens magnifies the output image (typically 20X) going to a video camera. The image from the video camera is displayed on a video monitor and is also sent to the computer for digital analysis.

The computer analyzes the image to identify the edges of the core and cladding. The centers r_c and r_g of the core and cladding, respectively, are found. The cladding diameter is defined as the average diameter of the cladding. The cladding diameter is twice the average radius (R_g). The core diameter is defined as the average diameter of the core. The core diameter is twice the average core radius (R _c).

Cladding noncircularity, or ellipticity, is the difference between the smallest radius of the fiber (R_gmin) and the largest radius (R_gmax) divided by the average cladding radius (R_g). The value of the cladding noncircularity is expressed as a percentage.

The core-cladding concentricity error for multimode fibers is the distance between the core and cladding centers divided by the core diameter. Multimode core-cladding concentricity error is expressed as a percentage of core diameter. The core-cladding concentricity error for single mode fibers is defined as the distance between the core and cladding centers.

Core noncircularity is the difference between the smallest core radius (R_cmin) and the largest core radius (R_cmax) divided by the core radius (R _c). The value of core noncircularity is expressed as a percentage. Core noncircularity is measured on multimode fibers only.

Q.14 Why do end users perform fiber geometry measurements in the laboratory?
Q.15 Define cladding diameter.
Q.16 Explain the difference between multimode and single mode core-cladding concentricity errors.