Causes and treatment methods of large attenuation points of optical cable lines, line maintenance and testing methods, and standardized operation procedures for construction and connection of optical cable lines. The article also briefly introduces the composition and structure of the optical cable, the naming method and the standard chromatographic arrangement of the optical fiber. very powerful!
ZR Cable fiber optic cable
Section 1 Causes and Treatment Methods of Large Attenuation Points in Optical Cable Lines
In the construction of the optical cable line, the attenuation index of the optical cable line is an important evaluation index. Not only the average fiber loss coefficient of the optical cable line after construction, but also the optical fiber scattering curve of the optical cable line and the average loss coefficient of the optical cable line should be evaluated. The total loss and total loss must not only meet the design requirements, but also meet the index requirements of construction specifications and acceptance standards, and the optical fiber scattering curve should be relatively uniform, and there should be no large attenuation steps on the curve to ensure the optical characteristics and technical indicators of the optical cable line. Meet the requirements of construction specifications and acceptance criteria.
1. Several phenomena and causes of large attenuation points of optical cables
1.1 Large attenuation point generated during laying
In the construction of optical cables, because the laying length of optical cables is generally between 2 and 3KM when directly buried, there are many obstacles to pass through. When there are many obstacles, such as: passing through protective steel pipes, turning, going up and down slopes, etc., the so-called back-buckling (dead bend) phenomenon of the optical cable will occur, which will cause serious damage to the optical cable. Once the dead bend occurs, it will inevitably appear here. At a large attenuation point, some or all of the optical fibers will be broken seriously, which is a fault phenomenon that is prone to occur in the construction of optical cables. In addition, when laying the optical cable, the optical cable at the end of the optical cable is the most vulnerable to damage. During the connection, it often shows a large loss value at the connection point. At this time, even if the fusion is repeated many times, the connection loss value cannot be reduced. Thus forming a larger decay point.
1.2 The large attenuation point generated in the connection process
In the process of optical cable splicing, large attenuation points often occur. We generally use OTDR (Optical Time Domain Reflectometer) to monitor, that is, for each optical fiber spliced, use OTDR to test the attenuation value of the splice point. During the test, the two-way monitoring method is adopted. Due to the differences in the optical fiber manufacturing process, the two fibers cannot be completely consistent, and there is always an inconsistency in the mode field diameter. As a result, the loss value measured by the OTDR is not the connection point. The actual loss value has positive and negative values. Generally, the arithmetic mean of the two-way test values is used as the actual loss value. During splicing, the real-time monitoring method is generally used, which can basically ensure that the splicing loss reaches the control target, but the reason for the frequent occurrence of large loss points is that when the optical fiber is contained after the splicing, some optical fibers are compressed or the bending radius is too small, which forms a big loss point.
Because the optical fiber with wavelength of 1550nm is very sensitive to micro-bending loss, once the optical fiber is compressed, a micro-bending point is generated, or when the fiber is coiled, the bending radius is too small, and the optical fiber signal also produces a large attenuation here, which is manifested in the optical fiber. On the backscattering curve, a large attenuation step is formed; in addition, a relatively easy to ignore reason is that after the optical cable splice box is assembled, when fixing the splice box and fixing the optical cable, because the optical cable is not fixed in the splice box. It is very firm, causing the optical cable to twist and deform the optical fiber bundle. Due to the compression of the optical fiber, the optical fiber attenuation value increases sharply, forming an attenuation step.
1.3 Large attenuation points caused by transportation and handling
When the optical cable is transported to the construction site, due to the harsh site environment, especially when laying the railway communication optical cable, the crane is often unable to reach the construction site. At this time, the optical cable is often loaded and unloaded manually. It is often damaged because the diameter of the optical cable reel is too small, which causes the outer optical cable to be too close to the ground. Due to the uneven soft and hard soil on the site, the optical cable reel sinks into the ground during the process of rolling the optical cable reel, causing the outer layer Optical cables are damaged by hard objects on the ground. The main reason is that some manufacturers use smaller optical cable reels to reduce production costs.
In addition, the optical cable reel is not covered with wooden boards (some are iron-frame optical cable reels, which cannot be packaged with wooden boards), but only wrapped with plastic sheeting on the outer layer of the optical cable, or after the single-reel test, the optical cable reel is not packaged. After the outer layer of the optical cable is damaged by hard objects such as stones, the optical fiber is compressed in the bundle tube, that is, an attenuation step is generated, which is reflected in the backscattering curve of the optical fiber. a larger decay point.
1.4 The large attenuation point generated in the process of end formation
In the process of terminating the optical cable, a large loss point is often generated. When the end is formed, since the splice loss monitoring is generally not carried out, and the operation is only based on experience, the probability of generating a large loss point is also greatly increased. In addition, when the accommodating tray is installed after optical fiber fusion, the bending radius of the optical fiber bundle tube near the accommodating tray is often too small or the optical fiber bundle tube is twisted and deformed, so that the optical fiber has a large attenuation point here. The attenuation point is generally relatively hidden, unlike the large attenuation point in the middle of the line, which can be directly measured with an OTDR.
2. Find, locate and process the large attenuation point of the optical cable
2.1 Generally, the location of the large attenuation point
After the optical cable connection is completed, we generally use OTDR to test the entire hop. Through the test, we can check whether the optical characteristics of the connected hops meet the requirements of construction specifications and acceptance standards, mainly from the following aspects. Assessment: Whether the total attenuation of the hop is less than the design specification (that is, whether the average attenuation coefficient is less than the design specified value); whether the two-way average attenuation value of the hop joint is less than the acceptance criteria and design requirements; hop backscattering Whether the slope of the curve is uniform and the curve is smooth, there should be no large attenuation step on the curve except for the small step at the normal joint attenuation point. When using OTDR for optical hop test and human attenuation point positioning, the test parameters of the instrument should be correctly set first, such as test range, test wavelength, pulse width, refractive index and average processing time.
For the setting of the test range, generally select the appropriate range according to the length of the hop, so that the entire hop curve occupies 2/3 of the entire display screen; the test wavelength is determined according to the wavelength used by the system, and is generally used for long-distance trunk cables. The refractive index for 1310nm and 1550nm is set according to the refractive index of the fiber used by the manufacturer; the pulse width is an important setting parameter. If the pulse width is too small, the dynamic range of the test is too small, and the entire curve cannot be tested completely, and it appears as a noise signal at the end of the curve. If the pulse width is too large, the quality of the obtained curve will be poor; if the pulse width is too large, the test range will be larger, but the test accuracy will be worse. Generally, an appropriate test pulse width should be selected according to the length of the hop under test. , and also consider the test accuracy, select a suitable pulse width through the test; the setting of the averaging time is determined according to the averaged curve quality test, so that there is no obvious burr on the end of the averaged curve. In order to accurately determine the position of the optical fiber fault point on the line, the OTDR analysis software can be used to analyze the curve tested by the instrument. Generally, there are two cases of fault in the splice box and fault on the cable body.
2.2 Processing of large attenuation points
First, determine whether the large attenuation point is the joint position. Generally, at the joint position, all fibers have large or small attenuation steps. The curves of multiple fibers can be analyzed at the same time, and it can be seen that all the curves have sizes at the joint point. For different steps, we can test and calculate the bidirectional attenuation value of the joints at the same position of each optical fiber, make a record of the ones that are larger than the index requirements, and arrange to open the joint box for the large attenuation point of the joint position for processing. For the large attenuation points of some optical fibers that are not at the joint position, we analyze multiple curves at the same time, and we can see that some curves have attenuation steps at this point, and some have no attenuation steps. Based on this, it can be judged that this is not a joint. The fault of the position, but the fault of the optical cable in the middle of the optical cable line.
For the fault at the joint, its location is relatively easy to locate, and for the fault at the non-joint position, it is difficult to locate. The general principle is that the fault point close to the test end can be tested at the end station, and the OTDR can be used to measure the nearest fault point. The distance of the joint point, for the fault point far from the test point, due to the long distance, the accuracy of the test is relatively reduced, and it is difficult to locate accurately, you can open it at the nearest joint box, connect to the OTDR for testing, and measure the fault point. After the distance, combined with the various residuals recorded in the original construction data, according to the conditions of the direct buried path, the approximate location of the fault point can be measured on the spot, which can generally be located within a range of more than ten meters, so that the excavation range is relatively small, saving energy The construction cost is reduced and the time for troubleshooting is shortened. For the large loss point at the joint, we open the joint box for re-splicing processing, and use the OTDR to monitor in real time until the splice loss meets the requirements.
Sometimes after multiple fusion splices, the splicing loss fails to meet the requirements. At this time, it is necessary to check whether the optical fiber is compressed due to the deformation of the optical fiber bundle, whether the bending radius of the optical fiber is too small when the fiber is coiled, and whether the optical fiber is compressed. After these inspections, if the requirements cannot be met, it is necessary to consider whether there is a problem with the optical cables before and after the splice box. Because the optical cable at the end is more likely to be damaged during construction, it is necessary to cut off another section of the optical cable and re-splicing all the optical fibers. In order to avoid such problems, we can carefully check the remaining optical cables at the joints before connecting, and cut off more parts of the optical cables at the suspected end to avoid such problems.
For the treatment of the large attenuation point of the optical cable in the middle of the line, after finding the fault point, it can be found that such a fault or the optical cable has been buckled, or the optical cable has been damaged, such as being damaged by hard objects such as stones, which makes the optical cable appear. Deformation phenomena such as indentation and flattening, the deformation of the optical fiber bundle tube causes the optical fiber to be compressed, resulting in a large attenuation point, or other external force factors cause the optical cable to be damaged. When processing, this section of optical cable can be cut off and spliced again. Generally, after this processing, the large attenuation point basically disappears.
For the fault point of the back buckle found during construction, the fault point should be properly reserved for processing. For serious damage, when adding a splice box, the outer sheath of the optical cable can be peeled off, the deformed bundle tube can be processed, and if necessary, the optical fiber in the damaged bundle tube can be connected. The test point should contact the on-site welding personnel to conduct a test after the welding is completed, a test after the optical fiber is left, and a test after the splice box is tightened and sealed. After the test point test confirms that the decay point fault disappears, the site personnel evacuate.
