In the construction of Fiber Cable link, the attenuation index is an important assessment parameter, not only the average loss coefficient of the completed fiber cable link but also the fiber back-scattering signal curve. The average loss coefficient and total loss of the fiber cable link must meet the design requirements, but also meet the construction specifications and acceptance standards of the index requirements, and ensure a relatively uniform fiber back-scattering curve. There should not be a large attenuation step on the curve to ensure that the optical characteristics of the cable link conform to construction specifications and acceptance standards.

During Cable laying

During optical cable construction, since the cable installation length is typically 2-3 km, when using direct burial installation, there are often many obstacles to cross. During construction, with multiple installation workers and long installation distances, it’s difficult to ensure coordinated action among all installation personnel. Especially when encountering numerous obstacles such as crossing protective steel pipes, making turns, and navigating slopes, this can lead to what’s commonly known as “cable kinking” (at the end of the article to get more details about it).

For pipeline/duct and aerial fiber cables, inconsistencies frequently occur between the cable drum release speed and the cable installation speed during the laying process. If the cable drum release speed exceeds the cable installation speed, excess cable will accumulate on the ground. As fiber cable is a type of wire material, it naturally contains stress when coiled; without external force, coiling is an inevitable phenomenon, so the excess cable on the ground will unavoidably form coils automatically. If these coils aren’t eliminated promptly, due to limitations in manual organization or equipment deficiencies during construction, combined with the inertial force of forward cable pulling, these coils will form sharp bends, resulting in what’s known as the “kinking” situation.

During Splicing Process

Large attenuation points frequently happened during fiber cable splicing. We typically monitor this using an OTDR (Optical Time Domain Reflectometer), testing the attenuation value of each splice point after each fiber fusion. Specifically, we use a bi-directional monitoring method. Due to differences in the fiber manufacturing process, no two fibers are exactly identical, and there are always inconsistencies in mode field diameter. This leads to OTDR measurements that don’t reflect the actual loss at the splice point, with values that can be either positive or negative. Generally, the arithmetic mean of bi-directional test values is used as the actual attenuation value.

During splicing, real-time monitoring methods are typically used, which can generally ensure that splice losses meet control targets. However, large loss points often occur when storing fibers after splicing is complete. This happens when some fibers are under pressure or have too small a bending radius. Some fibers are particularly sensitive to microbending loss – when a fiber is under pressure, it creates a microbending point, or when coiling the fiber, if the bending radius is too small, the fiber signal experiences significant attenuation at these points. This appears as a large attenuation step in the fiber backscatter curve.

Additionally, a commonly overlooked cause occurs after assembling the cable splice closure. When fixing the splice closure and securing the cable, if the cable isn’t firmly secured inside the splice closure, it can cause cable twisting. This deforms the fiber buffer tubes, and the resulting pressure on the fibers causes a sharp increase in fiber attenuation, forming an attenuation step.

During Transportation and handling

When fiber cables are transported to construction sites, the site conditions are often harsh, especially when installing railway communication fiber cables. The Cranes often cannot reach the construction site, so manual loading and unloading of fiber cables is common. During the unloading process, the outer layer of the cable frequently gets damaged because the cable full coiled onto the drum, so the outer layer of cable to be too close to the ground. Due to uneven and varying soil conditions at the site, when rolling the cable reel, it can sink into the ground, causing the outer layer of cable to be damaged by hard objects on the ground. This primarily occurs because some manufacturers use smaller cable drum to reduce production costs.

When the outer layer of the cable is damaged by rocks or other hard objects, the fiber becomes compressed within the buffer tube, creating an attenuation step. This appears as a large attenuation point on the fiber backscatter curve.

During Cable Termination

High attenuation points frequently occur during fiber cable termination. During termination, since the loss monitoring is generally not performed and operations according on experience of the engineer, the probability of large attenuation points increases significantly. Furthermore, when installing storage trays after fiber splicing, the fiber buffer tubes near the storage tray often end up with too small a bending radius or become twisted and deformed, causing a significant attenuation point in the fiber at this location. These types of large attenuation points are generally more concealed and cannot be directly measured with an OTDR like the large attenuation points in fiber cable link.

More details of “Cable Kinking”

Specific Manifestations of Cable Kinking

Cable Twisting: The fiber cable twists during installation, causing uneven stress on the optical fibers.

Excessive Bending: The cable’s bending radius becomes less than 15 times cable diameter (national standard requirement), sometimes be get sharp bends.

Sharp Bends: Irreversible bends form during cable installation, leading to fiber damage.

Industry Terminology

In the telecommunications industry, “cable kinking” is typically referred to as fiber cable twisting or fiber cable sharp bending. This phenomenon must be strictly avoided during construction, as it leads to cable performance degradation and may require re-installation or repair.

Harmful Effects of Cable Kinking

Increased Fiber Attenuation: Kinking causes micro or macro bending in fibers, increasing fiber signal attenuation.

Fiber Breakage: Severe kinking can lead to fiber breakage, affecting communication link continuity.

Construction Rework: Kinked cables fail construction acceptance standards, potentially requiring cable re-installation, increasing both cost and time.

How to Prevent Cable Kinking

Control Bending Radius: Ensure the cable’s bending radius exceeds 15 times cable diameter; during construction, it’s recommended to maintain it above 20 times.

Standardize Installation Procedures: When using mechanical pulling, control pulling speed and tension to prevent cable twisting or excessive stress.the pulling force must be less than 80% of the fiber’s maximum allowable tension, and the force should be applied to the strength members of the fiber cable (not directly on the optical fibers).

Use Protective Devices: During cable installation, use anti-kinking devices to reduce the risk of cable damage.