The bending degree of a fiber optic coil is tested by measuring its minimum bend radius and evaluating optical performance under controlled bending conditions using standardized methods and specialized equipment.

Understanding Bend Radius

The bend radius is the minimum radius a fiber optic cable can be bent without causing signal loss or mechanical damage. It is measured from the center of the curve to the inside edge of the cable. There are two key specifications:

• Unloaded (static) bend radius: Minimum radius when the cable is at rest, without tension.
• Loaded (dynamic) bend radius: Minimum radius while the cable is under tension during deployment, typically 1.5–2 times the static radius . Exceeding these limits can cause microbending (small-scale pressure points causing scattering and attenuation) or macrobending (large curvature forcing light to escape the core), both of which degrade optical performance .

Standardized Testing Methods

Fiber optic bending tests are often conducted according to IEC 60794-1-2 standards:

• Method E10 (Kink Test): Determines the smallest loop a cable can handle before permanent deformation or fiber breakage occurs .
• Method E11 (Bend Test): Evaluates the cable's ability to withstand bending around a mandrel, often using a diameter 20 times the cable diameter . These tests simulate real-world stresses and ensure compliance with international standards.

Practical Testing Procedure

1. Select the appropriate mandrel or fixture: Use a mandrel with a known diameter corresponding to the desired bend radius.
2. Form the fiber into a loop: Carefully bend the fiber around the mandrel without twisting.
3. Monitor optical performance: Measure insertion loss or attenuation using an optical power meter or OTDR (Optical Time-Domain Reflectometer). Any significant increase indicates the fiber is approaching its bending limit .
4. Gradually reduce the radius: For kink tests, progressively decrease the loop diameter until permanent deformation or signal degradation occurs .
5. Record the minimum safe bend radius: This value defines the maximum bending degree the fiber can safely tolerate.

Equipment for Accurate Measurement

• Automated bend testers: Devices like the TT-OFCK tester provide precise, repeatable measurements and eliminate human error .
• OTDR or optical power meters: Used to detect signal loss during bending.
• Mandrels of varying diameters: To simulate different bending conditions.

Key Considerations

• Fiber type matters: Standard single-mode fibers (G.652) have higher minimum bend radii than bend-optimized fibers (G.657), .
• Cable design and load conditions affect bending tolerance. Always consult manufacturer datasheets for specific limits.
• Testing should differentiate between short-term installation bends and long-term operational bends, as the tolerances differ . By following these procedures and standards, you can accurately determine the bending degree of a fiber optic coil while ensuring both mechanical integrity and optical performance.