Optical Cable Splicing and Testing in CATV Networks

"Optical fiber in, copper out" is the development trend of broadband and bidirectional cable TV network. Cable TV network transformation should push optical fiber further to the user end to achieve the goals of high reliability, high bandwidth, high carrying capacity, manageability and operability. In the current bidirectional network transformation, optical fiber to the community, optical fiber to the building and even optical fiber to the home have become a reality. The application of optical fiber is unprecedentedly extensive. How to better play the advantages of optical fiber transmission bandwidth, large communication capacity, low loss, no electromagnetic interference, small optical cable diameter, light weight, and abundant raw material sources is worthy of our attention. Light will cause loss when it is transmitted in the optical fiber. This loss is mainly composed of the transmission loss of the optical fiber itself and the fusion loss at the optical fiber joint. Once the optical cable is ordered, the transmission loss of the optical fiber itself is basically determined, and the fusion loss at the optical fiber joint is related to the optical fiber itself and the on-site construction. Now introduce the steps and methods of the optical cable splicing project in our county.

1 Design and drawing of optical fiber splicing drawings

(1) Selection of the number of optical fiber cores. Our county has newly set up an optical node and arranged a 4-core optical cable, using 1 optical fiber to transmit the downlink signal, 1 to transmit the uplink signal, 1 to transmit the digital signal, plus a reserved 1. At the same time, when considering that there are several optical nodes in the same direction, for the convenience of installation, the optical fiber used by the optical node at the farthest point is included in the optical cable that is closer to the front end in sequence according to the distance from the end. For example, if there are 3 optical nodes in the same direction, each point is set with 4-core optical fibers. When installing the optical cable, a 12-core optical cable is used between the front end and the first optical node, an 8-core optical cable is used between the first optical node and the second optical node, and a 4-core optical cable is used between the second optical node and the third optical node. In this way, multiple fibers are used together to save investment.

(2) Drawing of optical cable splicing drawings. After designing the number of optical fiber cores, start drawing the optical cable splicing drawings. A complete optical cable splicing route diagram should consist of an optical link schematic diagram, an optical cable splicing diagram, an optical cable route diagram, an optical cable splicing route diagram, an optical link loss table, etc. Before the optical cable splicing, the planning and design department provides the optical cable splicing diagram to the construction department. The optical cable splicing diagram describes the various elements of the optical cable splicing, including the location of the optical packet, the optical cable route, the number of optical cables, the number of cores, the optical cable cross-section diagram, the optical cable docking diagram, etc. The optical fiber chromatogram in the loose tube of the optical cable splicing diagram is arranged in the order of the cable chromatogram blue, orange, green, brown, gray, white, red, black, yellow, and purple. Common optical fiber chromatograms also include white and light red, which can be arranged before the corresponding color. The cable chromatogram sorting method can form a more unified specification in the design and construction of optical cable projects, which is easy to remember and grasp and organize data.

2 Optical cable splicing steps

(1) The heat shrink tubing should be inserted before stripping and must not be inserted after the end face is prepared.

(2) Preparation of the end face. The preparation of the optical fiber end face includes stripping, cleaning and cutting. A qualified optical fiber end face is a necessary condition for fusion splicing, and the quality of the end face directly affects the quality of fusion splicing. The stripping of the optical fiber coating layer should master the three-character fiber stripping method of flat, steady and fast. "Flat", that is, hold the fiber flat. Pinch the optical fiber with the thumb and index finger of the left hand to make it horizontal. The exposed length is about 5 cm. The remaining fiber is naturally bent between the ring finger and the little finger to increase the strength and prevent slipping. "Steady", that is, the fiber stripping pliers should be held steadily. "Fast", that is, the fiber stripping should be fast. The fiber stripping pliers should be perpendicular to the optical fiber, tilted inward at a certain angle, and then the pliers should be used to gently clamp the optical fiber. Then, the right hand should be used to push it out along the axis of the optical fiber. The whole process should be natural and smooth. To clean the bare fiber, first observe whether the coating layer of the stripped part of the optical fiber has been completely stripped. If there is any residue, it should be re-stripped. If there is a very small amount of coating layer that is difficult to strip, you can use a cotton ball dipped in an appropriate amount of alcohol and gradually wipe it while dipping. Tear the cotton into small fan-shaped pieces with flat layers, dip a little alcohol (it is best to pinch it with two fingers without overflow), fold it into a "V" shape, clamp the stripped optical fiber, wipe it along the axis of the optical fiber, and strive to succeed once. A piece of cotton should be replaced in time after 2 to 3 times of use. Use different parts and layers of cotton each time, which can not only improve the utilization rate of cotton, but also prevent the double contamination of the probe. Cutting is the most critical part of the preparation of the optical fiber end face. Precision and excellent cutters are the foundation, and strict and scientific operating specifications are the guarantee. First, check the drawings and materials to understand the type of fiber to be connected. The placement of the optical fiber should be "front push and back lift, front in and back out", that is, hold the optical fiber, slightly ahead of the scale requirement and place it flat in the guide groove, lift the back slightly upward, so that the front half of the optical fiber is close to the bottom of the guide groove, and then withdraw it to the required scale to ensure that the optical fiber fits the "V" guide groove and is perpendicular to the blade. When cutting, the action should be natural, smooth, not heavy, and not hasty, to avoid the generation of bad end faces such as broken fibers, bevels, burrs, and cracks.

(3) Fiber fusion splicing. Fiber fusion splicing is the central link of the splicing work, so high-performance fusion splicers and scientific operation during the fusion splicing process are very necessary. Before fusion splicing, set the optimal pre-melting main melting current and time and fiber feeding amount and other key parameters according to the material and type of the optical fiber. During the fusion splicing process, the fusion "V" groove, electrodes, objective lens, fusion splicing chamber, etc. should be cleaned in time, and observe whether there are bubbles, too thin, too thick, virtual melting, separation and other undesirable phenomena in the fusion splicing at any time. Pay attention to the OTDR tracking monitoring results, analyze the causes of the above undesirable phenomena in time, and take corresponding improvement measures. If the virtual melting phenomenon occurs multiple times, check whether the materials and models of the two optical fibers to be fused match, whether the cutter and fusion splicer are contaminated by dust, and check the oxidation status of the electrodes. If there are no problems, the fusion splicing current should be appropriately increased. [page]

(4) Fiber coiling. Fiber coiling is a technology as well as an art. Scientific fiber coiling methods can make the fiber layout reasonable, reduce additional loss, withstand the test of time and harsh environment, and avoid fiber breakage caused by extrusion. Fiber coiling methods: 1. First the middle and then the two sides, that is, first place the heat-shrinkable sleeves in the fixed grooves one by one, and then deal with the remaining fibers on both sides. This is conducive to protecting the fiber joints and avoiding damage caused by fiber coiling. 2. Start coiling from one end, that is, start from the fiber on one side, fix the heat shrink tube, and then deal with the remaining fibers on the other side. This is convenient and quick, and can avoid sharp bends and small loops. 3. Handling of special cases, such as individual optical fibers that are too long or too short, they can be placed at the end and coiled separately. When there are special optical devices, they can be coiled separately. If they are coiled together with ordinary optical fibers, they should be gently placed on the ordinary optical fibers, and a buffer pad should be added between the two to prevent fiber breakage caused by extrusion. The tail fiber of the special optical device should not be too long. 4. Use a variety of fiber coiling patterns based on actual conditions. Coil the fiber naturally according to the length of the remaining fiber and the size of the reserved coil space. Do not pull it hard. Use a variety of fiber coiling patterns such as circle, ellipse, CC, etc. flexibly (note R > 4 cm) to maximize the use of the reserved coil space and effectively reduce the additional loss caused by fiber coiling.

(5) Optical cable splicing quality inspection. Strengthening OTDR monitoring is of great significance to ensure the quality of optical fiber splicing and reduce the additional loss caused by fiber coiling and the damage that may be caused to the optical fiber by the sealing box. During the entire splicing process, the OTDR 4-channel monitoring procedure must be strictly implemented:

1. During the splicing process, each fiber core is tracked and monitored in real time to check the quality of each melting point; 2. After each fiber coiling, the coiled fiber is inspected to determine the additional loss caused by the fiber coiling; 3. Before sealing the splicing box, all optical fibers are tested to find out whether there is any missed test and whether the fiber and connector are squeezed between the fiber reserved coils; 4. After the box is sealed, all optical fibers are tested in the end to check whether the box is damaged. After 4 tests, the optical cable splicing loss test table is recorded in detail.

(6) Fixing of optical cable splicing box and handling of excess cable. The horizontal splicing box of underground manhole should be placed on the ground first, with the box body as the reference point, and the optical cable should be coiled into a circle, then dragged into the well, and tied with rust-proof wires in four directions: up, down, left and right. Do not hang it with thin wire. The vertical splicing box on the pole head should pay attention to the horizontal rotation of the box body, so that the incoming and outgoing optical cables are naturally free of twists and the bending arc of the optical cable is controlled. Specific operation method: 1. Multiple people operate to form a party cooperation of one sending and one pulling of the excess cable. In principle, one person is at the box body and each excess cable rack. It is strictly forbidden to sing a double reed by one person. If necessary, a walkie-talkie is equipped to enhance communication. 2. First the middle and then the two sides, that is, drag the box body flat, fix it on the steel wire, and then handle the excess cables on both sides. If the excess cable itself twists, it should be pulled out along the steel wire to the opposite end in advance. When there is no need to deal with the remaining cables temporarily, the box body should be fixed first, and then all the remaining cables should be hung on the pole head at the opposite end to prevent damage to the junction box and the optical fiber inside during the secondary construction.

(7) Optical cable splicing records. After the optical cable splicing is completed, various records should be made in a timely manner, including sequence, number of cores, color spectrum, splicing loss, etc.

3 Other issues that should be paid attention to in optical fiber splicing

(1) When stripping the optical cable before fusion splicing, the starting end of the optical cable pulled during laying must be cut off for more than 2 m, because the pulling relay will always have an adverse effect on this section of optical fiber. For parallel steel wire optical cables, due to the tension, the loose tube will shrink slowly, and it will take at least half a year or even a year to shrink into place. This shrinkage phenomenon sometimes breaks the optical fiber. Therefore, after stripping the optical cable, the steel wire should be pulled while pulling the outer protective layer of the optical cable to minimize its shrinkage coefficient.

(2) The metal reinforcement core of the optical cable and the joint box must be clamped tightly, and the reinforcement core must be bent. In the case of multiple optical cable bundles, the reinforcement core should be reasonably separated according to the arrangement of the fusion tray and bundles to prevent the optical cable bundle from twisting, which may cause the optical fiber to be tightened or even broken.

(3) The quality of the selected shrink tube must be qualified. The metal wire in the tube should be straight and hard. The tube should not be bent or deformed. It should match the slot in the joint box. When placing multiple fusion joints, the slot should be free of pressure and have space. This is because the tightness of the slot may cause the optical fiber in the shrink tube to break. In addition, when placing the fusion joint with both hands, pinch the two ends with the thumb and index finger to prevent the middle from being subjected to external force (because the fusion joint is in the middle) to prevent the fusion joint from breaking.

(4) When there are many optical fibers, the lengths are often different when they are coiled. They must be fixed in sequence with insulating tape. Otherwise, the excess length of the optical fiber after fusion splicing will easily pop out. It is easily compressed by the waterproof glue of the joint box, resulting in excessive loss. Therefore, try to make the excess length of the optical fiber fusion splicing the same when stripping the fiber. At the same time, check whether there is any optical fiber popping out of the tray when packaging.