News - Location, Identification, and Handling of High-Attenuation Points in Optical Cables

In the construction of optical cable lines, attenuation performance is a critical evaluation indicator. This article analyzes the locations and handling methods of high-attenuation points in optical cable lines based on typical line conditions.

1. Common Locations of High-Attenuation Points

After completing optical cable splicing, the entire repeater section is usually tested using an OTDR (Optical Time Domain Reflectometer). This test verifies whether the optical performance of the completed cable section meets construction specifications and acceptance standards. The evaluation mainly includes the following aspects:

Whether the total attenuation of the entire repeater section is lower than the design specification (i.e., whether the average attenuation coefficient is within the required limit);
Whether the bidirectional average splice loss of the joints meets acceptance standards and design requirements;
Whether the backscattering curve of the repeater section has a uniform slope and is smooth. Except for small steps caused by normal splice losses, there should be no significant attenuation steps on the curve.

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

Test range: Select according to the repeater section length so that the curve occupies about two-thirds of the display screen;
Wavelength: Determined by the system, typically 1310 nm and 1550 nm for long-distance trunk cables;
Refractive index: Set according to the fiber manufacturer’s specifications;
Pulse width: A critical parameter. If too small, the dynamic range is insufficient, resulting in noisy signals at the end of the trace; if too large, the test range increases but accuracy decreases. A suitable pulse width should be selected based on the cable length through trial testing;
Averaging time: Adjusted to ensure a smooth curve without noticeable noise at the tail.

To accurately locate faults, OTDR analysis software can be used to analyze test curves. Faults generally fall into two categories: splice point faults and cable body faults.

2. Handling of High-Attenuation Points

First, determine whether the high-attenuation point is at a splice location. At splice points, all fibers usually show attenuation steps of varying magnitudes. By analyzing multiple fiber traces simultaneously, you can observe steps at the same position across all curves. Measure and calculate the bidirectional splice loss at this point, record any values exceeding the standard, and arrange to reopen the splice closure for repair.

If the attenuation point is not at a splice location, simultaneous analysis of multiple curves will show that some fibers have attenuation steps while others do not. This indicates a fault within the cable itself rather than at a splice.

Fault Localization

Near-end faults: Can be located from the terminal station using OTDR to measure the distance from the nearest splice point;
Far-end faults: More difficult to locate due to reduced accuracy over long distances. In such cases, testing can be performed from a nearby splice closure. Combine OTDR measurements with construction records and field measurements to estimate the fault location, typically within a range of about ten meters, reducing excavation scope and cost.

Handling Methods

Splice faults: Open the splice closure and re-fuse the fibers while monitoring with OTDR until acceptable splice loss is achieved.
If repeated splicing does not meet requirements, check for:
- Deformation of the fiber tube causing compression;
- Excessive bending radius during fiber coiling;
- Fiber stress .
If issues persist, inspect the cable sections before and after the splice. Damaged cable ends may need to be cut back and all fibers re-spliced.
Preventive measures: Before splicing, carefully inspect reserved cable lengths. If suspicious, cut back additional cable length to avoid hidden defects.
Cable body faults: Often caused by:
- Sharp bends or kinks;
- Mechanical damage (e.g., pressure from stones causing deformation);
- External forces leading to fiber tube deformation and fiber.
Treatment involves cutting out the damaged section and re-splicing. In most cases, this eliminates the attenuation issue.

For severe damage, install a splice closure, strip the outer sheath, and repair or replace deformed tubes. If necessary, re-splice fibers in the affected tubes.