Detection, Localization, and Handling of High-Attenuation Points in Optical Cables

In optical cable construction, attenuation performance is a critical evaluation metric. Based on conventional line conditions, this article analyzes the locations and handling methods of high-attenuation points in optical cable lines.

I. Common Locations of High-Attenuation Points

After completing cable splicing, OTDR testing is typically performed on the entire relay section. This test verifies whether the optical characteristics of the spliced cable meet construction specifications and acceptance standards. The evaluation mainly includes:

• Whether the total attenuation of the relay section is lower than the design specification (i.e., whether the average attenuation coefficient meets requirements)

• Whether the bidirectional average splice loss of joints meets acceptance standards and design requirements

• Whether the backscatter curve slope is uniform and smooth, with no large attenuation steps except for small steps caused by normal splice losses

When using an OTDR to test the relay section and locate high-attenuation points, it is essential to correctly set parameters such as range, wavelength, pulse width, refractive index, and averaging time:

• Test range: Set according to the relay section length so that the curve occupies about 2/3 of the display screen

• Wavelength: Typically 1310 nm and 1550 nm for long-distance trunk cables

• Refractive index: Set based on the fiber manufacturer’s specifications

• Pulse width: A key parameter—too small results in insufficient dynamic range and noisy curves; too large reduces measurement accuracy. It should be selected based on both distance and precision requirements

• Averaging time: Adjust until the curve tail is smooth without noticeable noise

To accurately locate fault points, OTDR analysis software can be used. Faults generally fall into two categories: splice-box faults and cable-body faults.

II. Handling of High-Attenuation Points

First, determine whether the high-attenuation point is at a splice location. At splice points, all fibers typically show attenuation steps of varying degrees. By analyzing multiple fiber curves simultaneously, if all curves show steps at the same location, it is likely a splice issue.

• Measure and calculate the bidirectional splice loss at that point

• Record any values exceeding standards

• Open the splice closure and perform corrective actions

If only some fibers show attenuation at a location while others do not, the issue is likely not a splice but a fault within the cable body.

Fault Location Methods

• Near-end faults: Use OTDR from the terminal to measure the distance from the nearest splice point

• Far-end faults: Due to reduced accuracy over long distances, test from a nearby splice closure instead

• Combine OTDR data with construction records and slack information

• Conduct field measurements to locate the fault within a range of about 10 meters

This approach reduces excavation scope, lowers costs, and shortens repair time.

Repair Methods

1. Splice-Point Faults

• Open the splice closure and re-splice fibers

• Monitor in real time using OTDR until acceptable loss levels are achieved

If repeated splicing fails:

• Check for deformation of fiber buffer tubes

• Ensure proper bending radius during fiber coiling

• Check for fiber compression

If issues persist, inspect the cable sections before and after the splice. If damage is suspected, cut back and re-splice all fibers.

2. Cable-Body Faults

Common causes include:

• Cable kinking or twisting

• External mechanical damage (e.g., pressure from rocks causing deformation)

• Buffer tube deformation leading to fiber compression

Solution:

• Cut out the damaged section and re-splice

• For severe damage, install a splice closure and repair affected fibers

• If necessary, strip the outer sheath and repair or replace damaged buffer tubes

Preventive Measures

• Inspect spare cable length before splicing

• Cut additional length from suspicious cable ends to avoid hidden damage

• Leave sufficient slack at known weak points for easier future repairs

Testing Requirements

Testing personnel should coordinate with field technicians and perform OTDR tests at the following stages:

1. After splicing is completed

2. After fiber coiling is finished

3. After splice closure sealing

Only after confirming that attenuation issues are resolved should the team leave the site.