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Cause Analysis of Optical Cable Link Failure

2022-08-15
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In the past two years, with the decline in the cost of optical fiber and the wide application and upgrade of 1000Base and 10G Ethernet, optical fiber communication has gradually become an important part of local area network wiring and FTTx network construction. So, what are the reasons for the failure of the optical cable link?

 

Optical fiber link failure reasons

1. The optical cable is too long

Due to the defects of the fiber itself and the non-uniformity of the doping components, the optical signal propagating in it is scattered and absorbed all the time. With the improvement of manufacturing materials and manufacturing processes, today's optical fiber has reduced the attenuation of 20dB per kilometer in 1970 to 1dB per kilometer. At the same time, standardization organizations such as ISO11801 and ANSI/TIA/EIA568B also explicitly stipulate the attenuation per unit distance 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 the degradation of communication quality. In actual work, due to the existence of many coils in the optical link, the length of the optical link is often larger than the physical distance of the actual communication node, and a little carelessness will cause the optical link to be too long. Therefore, when designing the wiring, it is necessary to clarify 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, the actual length of the optical link is measured by the instrument, as shown in Figure 1 (OptifiberTM of Flukenetworks can measure the length of each wiring section to facilitate the correction of the link where necessary) to ensure construction and design. consistency.

fiber optical cable

2. Excessive bending

In essence, the bending loss and compression loss of the optical cable are caused by the fact that the light does not meet the conditions of total internal reflection.

 

The optical fiber has a certain flexibility. Although it can be bent, when the optical fiber is bent to a certain degree, it will cause a change in the propagation path of the light, so that a part of the light energy penetrates into the cladding or passes through the cladding to become a radiation mode and leak out. loss, resulting in bending loss. When light travels in a curved portion, the closer to the outside of the fiber, the greater the speed of travel.

 

When transmitted to a certain position, its speed will exceed the speed of light, and the conduction mode will become a radiating mode, resulting in loss. When the bending radius is too small, the loss caused by bending becomes very noticeable. Therefore, it is generally recommended that the dynamic bending radius should not be less than 20 times the outer diameter of the optical cable, and the static bending radius should not be less than 15 times the outer diameter of the optical cable.

 

In actual use, the data in the optical fiber propagates in a straight line. If the optical fiber is kept unbent, there will be no problem with the data; if it is bent a little, the data will begin 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 reserving sufficient angles for the wiring. For example, along the corners, corridors, and desktops, the transmission may fail.

 

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

 

3. The optical cable is under pressure or broken

When the fiber is subjected to uneven stress, such as pressure or temperature change of the sheathed fiber, the fiber axis is slightly irregularly bent or even broken. In particular, when the break occurs inside the optical cable, the fault cannot be found from the outside, but at the break of the fiber, due to the sudden change of the refractive index, reflection loss will even be formed, so that the signal quality of the fiber is believed to be greatly reduced. At this time, the OTDR tester can detect and find the bend or break point inside the fiber. It should be pointed out that the distance in the local area network wiring is short, so the accuracy requirements of the OTDR tester are relatively high, and it is generally recommended to use a test instrument with an event death (ie, resolution accuracy) not greater than 1m.

 

4. Poor fusion of optical cable

In optical fiber cabling, fusion splicing technology is often used to fuse two optical fibers into one. Since the glass fiber in the core layer is spliced, the skin and filler of the fused fiber need to be stripped off during the splicing process, and then spliced. During on-site operation, due to improper operation and harsh construction environment, it is easy to cause contamination of glass fibers, which leads to the mixing of impurities, density changes, and even bubbles during the welding process, and ultimately the communication quality of the entire link is degraded.

 

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

 

5. Core diameter mismatch

Active connections are also frequently used wiring means in fiber optic wiring, 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 (as shown in Figure 4), the joint loss will be greatly increased. The core diameter mismatch refers not only to the mixed use of single-mode and multi-mode fibers, but also to the mixed use of 62.5- and 50-diameter multi-mode fibers.

 

Whether it is mode mixing or line diameter mixing, it is conceivable that the light path and attenuation produced by the incident light from a small diameter to a large diameter and the light incident from a large diameter to a 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 a "negative attenuation" phenomenon will occur. Core diameter mismatches can be easily found through double-ended power testing or OTDR testing (as shown in Figure 6).

 

It is worth mentioning that, in addition to different core diameters, single-mode fibers and multi-mode fibers cannot be mixed because of their completely different optical modes, dominant wavelengths and attenuation mechanisms.

 

6. The filler diameter does not match

Similar to the causes of core diameter mismatches, fiber filler diameter mismatches also occur during cable splicing. The mismatch of fillers will mainly cause misalignment of optical fiber splices, resulting in optical signal leakage and attenuation.

 

7. Connector pollution

The pollution of optical fiber joints and the dampness of pigtails are one of the main reasons for the failure of optical cable communication. MartinTechnicalResearch's independent survey found that 80% of users and 98% of suppliers have experienced failures caused by unclean 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 a large number of switching devices, and the insertion, replacement and transfer of optical fibers are very frequent. During such an operation, the falling of dust, the touch of fingers, and the loss of plugging and unplugging can easily contaminate the optical fiber connector. These pollutions will affect the transmission of light. Through a fiber microscope (such as the FiberInspector of Flukenetworks), we can clearly see the actual situation of the fiber end face of tens of nanometers online, so as to clean the contaminated end face.

 

8. Poor polishing at the joint

In addition to splice contamination, poor street polish is one of the main failures of optical links. In an ideal optical link, the end faces of the optical connectors are flat and flat. 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 (as shown in FIG. 7 ).

 

These flaws cannot be detected 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 it also produces diffusion and scattering, causing the 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

Poor connector contact mainly occurs at the end of the optical path, such as optical distribution boxes and optical switches. It may be due to operator negligence, equipment quality problems, or aging of 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.

 

In summary, although the optical fiber cabling 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 long optical cable, bending transition, optical fiber compression or breakage, poor fusion, Core diameter mismatch, mode mix, filler diameter mismatch, splice contamination, poor splice polish, poor splice contact, etc. As far as its principle is concerned, the essence of optical fiber failure is caused by the influence of total light reflection and transmission conditions. External force extrusion and excessive bending will cause deformation of the optical fiber; impurities and air bubbles mixed in during fusion will cause changes in optical path density; mismatched wire diameters, end-face pollution and poor polishing will cause sudden changes in the refractive index.

 

Compared with the electrical faults in traditional cable communication, the optical fiber communication faults caused by these physical property changes are essentially different in terms of causes, manifestations, and effects. What's more, the precision of optical fiber communication makes it difficult to detect the fault of optical fiber with the naked eye.

 

For example, the pollution on the end face and the fault caused by the internal break of the link cannot be found from the outside. This requires us to pay special attention to fiber cabling to prevent unnecessary fiber failures caused by human factors as much as possible. At the same time, when the wiring is completed, and during routine maintenance, the optical fiber maintenance instruments (such as the SimpliFiber optical power meter of Flukenetworks, the OptiFiber optical time domain reflectometer, the FiberInspetor end-face microscope, etc.) are used for acceptance and maintenance of the optical cable. This can not only detect the existing fiber fault problem in time, but also quickly locate and solve the problem when the problem occurs, so as to ensure the safety of network operation.

 

In order to ensure long-distance and low-loss transmission of optical signals, the entire optical fiber link must meet very harsh and sensitive physical conditions. Any slight geometric deformation or slight pollution can cause a huge attenuation of the signal, or even interrupt the communication.

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