1. High attenuation points caused during laying
During optical cable installation, especially in direct-buried laying over lengths of 2–3 km, many obstacles are often encountered. The construction typically involves many workers and long distances, making it difficult to ensure coordinated actions among all personnel. This is particularly problematic when passing through obstacles such as protective steel pipes, bends, slopes, and elevation changes. As a result, a phenomenon commonly known as “back buckling” (dead bends) may occur, causing severe damage to the cable. Once a dead bend occurs, a significant attenuation point will inevitably appear at that location. In severe cases, partial or complete fiber breakage may occur. This is a common fault during optical cable construction.
In addition, during cable laying, the cable ends are most vulnerable to damage. During splicing, a relatively high attenuation value often appears at the splice point. Even after repeated fusion splicing, the loss cannot be reduced, resulting in a large attenuation point.
2. High attenuation points caused during splicing
High attenuation points frequently occur during the splicing process. Typically, an OTDR (Optical Time Domain Reflectometer) is used for monitoring. That is, after each fiber is spliced, the attenuation value at the splice point is tested. In practice, a bidirectional testing method is used. Due to variations in fiber manufacturing, no two fibers are exactly identical, and differences in mode field diameter always exist. As a result, the loss value measured by the OTDR is not the actual splice loss; it may be either positive or negative. Generally, the arithmetic average of bidirectional test values is taken as the actual attenuation value.
During splicing, real-time monitoring is usually applied to ensure that splice loss meets control targets. However, a common cause of large attenuation points occurs after splicing, during fiber storage. Some fibers may be subjected to الضغط or have too small a bending radius, forming a high attenuation point. This is because fibers operating at a wavelength of 1550 nm are highly sensitive to micro-bending loss. Once the fiber is compressed, micro-bending occurs; similarly, if the bending radius is too small during fiber coiling, significant signal loss occurs at that point. On the OTDR backscatter curve, this appears as a large attenuation step.
Another often overlooked cause occurs after the splice closure is assembled. When fixing the closure and securing the cable, if the cable is not firmly fixed inside the closure, twisting may occur, deforming the fiber buffer tubes. Compression of the fibers then leads to a sharp increase in attenuation, forming a step loss.
3. High attenuation points caused during transportation and handling
When optical cables are transported to the construction site, the environment is often harsh. In particular, when laying railway communication cables, cranes are often unable to reach the site. In such cases, cables are frequently loaded and unloaded manually. During unloading, the outer layer of the cable is easily damaged. One reason is that the cable drum diameter is too small, causing the outer cable layer to be too close to the ground. The ground conditions at construction sites are often uneven, with varying hardness. While rolling the cable drum, it may sink into the ground, causing the outer cable to be damaged by hard objects. The main reason is that some manufacturers use smaller drums to reduce production costs.
Additionally, if the cable drum is not properly protected with wooden boards (some drums use metal frames and cannot be fully enclosed with wood), and only plastic wrapping is used, or if the protective covering is not restored after single-drum testing, the cable is inadequately protected. When the outer sheath is damaged by hard objects such as stones, the fibers inside the buffer tubes are compressed, resulting in attenuation steps. On the OTDR backscatter curve, this appears as a large attenuation point.
4. High attenuation points caused during termination
High attenuation points also frequently occur during cable termination. During termination, splice loss monitoring is usually not performed, and operations rely largely on experience, increasing the likelihood of large attenuation points. Furthermore, after fiber splicing, when installing the fiber storage tray, the buffer tubes near the tray may be bent with too small a radius or twisted and deformed. This causes significant attenuation at those points.
Such attenuation points are often concealed and are not as easily detected as those in the middle of the cable using an OTDR.
Post time: Apr-23-2026