Fiber optic link failure

Causes of Fiber Link Failures

1. The optical cable is too long

Due to the defects of the fiber itself and the non-uniformity of the doping composition, the optical signal propagating in it is scattered and absorbed all the time. With the improvement of manufacturing materials and manufacturing process, today's optical fiber has reduced the attenuation of 20dB per kilometer in 1970 to 1dB per kilometer. At the same time, ISO11801, ANSI/TIA/EIA568B and other standardization organizations also expressly stipulate the unit distance attenuation of optical fiber links.

However, even so, the attenuation of the fiber itself still exists. Therefore, when the optical fiber link is too long, the overall attenuation of the entire link will exceed the threshold of the network design, resulting in a decrease in communication quality. In actual work, because there are many cables in the optical link, the length of the optical link is often greater than the physical distance of the actual communication nodes, and a little carelessness will cause the optical link to be too long. Therefore, in the wiring design, it is necessary to specify the length design of each section of the line to prevent the optical cable from being too long. At the same time, after the wiring construction is completed, measure the actual length of the optical link through the instrument, as shown in Figure 1 (the OptifiberTM of Flukenetworks company can measure the length of each section of wiring, so as to facilitate the correction of the link where necessary), so as to ensure the construction and design consistency.

2. Excessive bending

The essence of cable bending loss and pressure loss is that light does not meet the conditions of total internal reflection.

The optical fiber has a certain degree of flexibility. Although it can be bent, when the optical fiber is bent to a certain extent, it will cause a change in the propagation path of the light, so that a part of the light energy can penetrate into the cladding or pass through the cladding to become a radiation mode and leak outward. Loss, resulting in bending loss. When light travels in a curved section, the closer to the outside of the fiber the greater the speed of transmission. When it is transmitted to a certain position, its speed will exceed the speed of light, and the conduction mode becomes the radiation mode to cause loss. When the bend radius is too small, the losses due to bending become very noticeable. Therefore, it is generally recommended that the dynamic bending radius shall not be less than 20 times the outer diameter of the optical cable, and the static bending radius shall not be less than 15 times the outer diameter of the optical cable.

In actual use, the data in the optical fiber propagates along a straight line. If the optical fiber remains unbent, there will be no data problems; if it is bent a little, the data will start to overflow; if the optical fiber is tightly wound into a circle, the signal will be completely lost. Therefore, during the wiring construction, special attention should be paid to reserve sufficient angles for the wiring, such as slightly bending along the corners, corridors, and desktops, the transmission may fail.

On the other hand, bending can also be used to filter out high-order modes in the fiber, thereby improving the stability of light attenuation measurement. Figure 2 shows the principle of the radiation mode attenuation of the optical signal in the optical fiber, and the process of modulating the high-order mode through the reel.

3. The optical cable is compressed or broken

When the optical fiber is subjected to inhomogeneous stress, for example, when it is subjected to pressure or the sheathed optical fiber is subjected to temperature changes, the fiber axis will be slightly irregularly bent or even broken. As a result, the conduction mode is transformed into a radiation mode, resulting in loss of optical energy. In particular, when the break occurs inside the cable, the fault cannot be found from the outside, but the sudden change in the refractive index at the break of the fiber will even cause reflection loss, which will greatly reduce the signal quality of the fiber. At this point, you can use the OTDR tester to detect the inner bend or break point of the optical fiber. It should be pointed out that the distance in the LAN wiring is short, so the accuracy requirements for the OTDR tester are relatively high, and it is generally recommended to use a tester with an event death (ie, resolution accuracy) not greater than 1m.

4. The optical cable is poorly welded

In fiber optic cabling, fusion splicing technology is often used to fuse two lengths of fiber into one. Since the glass fiber of the core layer is fused, it is necessary to strip off the skin and filler of the fused optical fiber during the fusion splicing process, and then splice again. During on-site operation, due to improper operation and harsh construction environment, it is easy to cause pollution of glass fibers, which will lead to impurities mixed in during the welding process, density changes, and even bubbles, and ultimately the communication quality of the entire link will decline.

Therefore, whether it is hot-melt or cold-melt technology, in order to ensure that the attenuation of the splice point can reach the 0.3dB jointly stipulated by TIA and ISO, there are strict requirements and regulations on the fused optical fiber and the operation process. For example, it is necessary to ensure the cleanliness of the electrodes of the welding machine, the cleanliness of the glass fiber before welding, and the temperature and humidity of the construction environment on site. When attenuation is caused by fiber splicing problems, the position and loss of each splicing point can be accurately judged through OptifiberTM.

5. The core diameter does not match

Active connections are also frequently used in fiber optic cabling, such as flange connections. This method is flexible, simple, convenient and reliable, and is mostly used in computer network wiring in buildings. The loss of the active connection is generally about 1dB, but if the fiber end face is not clean, the joint is not tight, and the core diameter does not match when making the active connection (as shown in Figure 4), the loss of the joint will increase greatly. The core diameter mismatch not only refers to the mixed use of single-mode and multi-mode fibers, but also includes the mixed use of 62.5 and 50 diameter multi-mode fibers.

Whether it is mode mixing or wire diameter mixing, it is conceivable that the light path and attenuation produced by light incident from small diameter to large diameter and light incident from large diameter to small diameter will be very different. Therefore, at this time, the attenuation test results of the same fiber in different directions will be very different, and sometimes even the phenomenon of "negative attenuation" will occur. Through the double-ended power test or OTDR test (as shown in Figure 6), the core diameter mismatch problem can be found more conveniently.

It is worth mentioning that in addition to the difference in core diameter, single-mode fiber and multimode fiber cannot be mixed because of their completely different transmission light modes, dominant wavelengths and attenuation mechanisms.

6. The filling diameter does not match

Similar to the cause of the core diameter mismatch, the fiber filler diameter mismatch will also occur during the splicing process of the fiber optic cable. The mismatch of fillers will mainly cause dislocation of optical fiber connection, resulting in optical signal leakage and attenuation.

7. Joint pollution

Pollution of optical fiber joints and dampness of pigtails are one of the most important causes of optical cable communication failures. MartinTechnicalResearch's independent survey found that 80% of users and 98% of suppliers have experienced failures caused by dirty optical fiber termination surfaces, and another 72% of users and 88% of suppliers have experienced problems caused by poor polishing. This indicator is much higher than the fiber failure caused by other reasons.

Especially in the local area network, there are a large number of short jumpers and numerous switching devices, and the insertion, removal, replacement, and transfer of optical fibers are very frequent. During such an operation, dust falling, finger touch, insertion loss, etc. can easily contaminate the optical fiber connector. And these pollutions will affect the transmission of light. Through fiber optic microscopes (such as FiberInspector from Flukenetworks), we can clearly see the actual situation of tens of nanometer fiber end faces online, so as to clean the contaminated end faces.

8. Poor polishing of joints

In addition to connector contamination, poor street polish is also one of the main failures of optical links. In an ideal optical link, the end faces of the optical connectors are flat and fitted. When the optical signal passes through the end face, a small amount of light is reflected, and most of the light continues to propagate through the end face. However, ideal optical connectors do not exist in reality, and they all have certain protrusions, depressions, or inclinations more or less.

These defects cannot be found by the naked eye, but when the optical signal in the link encounters such a joint, the reflection caused by the irregular light on the joint surface is much larger than the ideal state, and at the same time, it will also cause diffusion and scattering, resulting in optical signal attenuation. On the OTDR curve, the attenuation dead zone of the poorly polished end face is much larger than that of the normal end face.

9. Poor contact at the joint

The poor contact of the connector mainly occurs at the end of the optical path, such as optical distribution boxes and optical switches. It may be due to the negligence of the operator, or equipment quality problems, or the aging of the connectors, etc., resulting in loose optical fiber connectors, resulting in reflection loss and leakage attenuation of optical signals. In addition, if the tolerance of the connector installation accuracy exceeds the standard, it will also cause the loosening of the optical connector, resulting in the drift of the performance parameters of the entire optical link.

To sum up, although the optical fiber wiring system is completely immune to electromagnetic interference, due to its own physical characteristics, the optical fiber communication system also has many hidden troubles: such as too long optical cable, bending transition, optical fiber compression or fracture, poor fusion, Core diameter mismatch, pattern mix, filler diameter mismatch, joint contamination, poor joint polish, poor joint contact, etc. In terms of its principle, the essence of fiber optic faults is caused by the influence of total light reflection and transmission conditions. Extrusion by external force and excessive bending will cause the deformation of the optical fiber; impurities and air bubbles mixed in the fusion splicing will cause changes in the optical path density; mismatched wire diameters, contamination of the end face and poor polishing will cause sudden changes in the refractive index.