How to reduce fiber joint splice loss

1 The concept of fiber optic connector loss is to use a fully automatic special equipment - Fusion Splitter to connect the two fibers in the two cables, and use a short arc to fuse two fibers during welding. The end faces are integrated into one body. This connection method has a small size, high mechanical strength, and stable performance after the fiber is connected, and thus is widely used. After the fiber is connected, the light is transmitted to the joint and a certain amount of loss is called the splice loss or the splice loss. Because the quality of the fiber connection affects the transmission loss of the fiber line, and the transmission line distance of the fiber line is not relayed, the welding loss at the fiber connector is required to be as small as possible to ensure the transmission quality of the fiber CATV signal. At present, most welding methods can achieve a splice loss of less than 0.1 dB, and can even reach a level of less than 0.05 dB. For a specific fiber-optic CATV project, it can be based on specific conditions such as the length of the fiber-optic line hop and the optical device. The allowable splice loss value at each fiber joint is determined by the power and receiving sensitivity and the system quantity, and is specified as the splice loss index in the relevant technical documents. The line transmission loss of each hop on the fiber-optic CATV transmission line should also be clearly defined. Because the measurement of the transmission quality of the fiber-optic line after the completion of all the fiber-optic joints is the transmission loss of the fiber-optic line, the current requirement is 0.25 dB/km. The following (including welding loss). Since the development direction of the fiber-optic CATV transmission network is the broadband data service network, there should be high requirements for the splice loss of the fiber-optic connector and the transmission loss of the fiber-optic line, especially some fiber-optic CATV trunk networks, such as the full participation of Lightcom. It is more than 18 00km long and connects 13 provinces under the jurisdiction of the province with a double-ring structure to transmit radio and television programs as the main business of Jiangsu Broadcasting and Television Cable Transmission Provincial Trunk Network. It is required that the transmission loss of optical fiber lines in the 1550nm window should not exceed 0.23dB/km. The splice loss value of the fiber connector is currently not to exceed 0.06 dB.

2 Measurement of splice loss of fiber splices Measuring the splice loss of fiber splices requires the use of an Optical Time Domain Reflectometer (OTDR), which uses a backscatter method to measure the splice loss at the fiber splice. Although the welding loss value is also shown on the fusion splicer, it is measured by local monitoring using the optical fiber mandrel direct view method, and the actual welding loss is reflected only in a very ideal state, so it is generally for reference only. Due to the difference in refractive index, core diameter, mode field diameter and Rayleigh scattering coefficient of the optical fiber, the measured values ​​of the splice loss in the two directions obtained by measuring the splice loss from the two ends of the optical fiber joint are different and the difference is large, so GB /T15972-1995 "Analytical Specifications for Optical Fibers" Appendix A "Analysis of Fiber Backscatter Power Curves" stipulates that the measurement of splice loss should be measured from both ends of the fiber optic connector, that is, bidirectional measurement, taking two directions of measured value algebra and The average value is taken as the value of the splice loss at the joint; due to the difference in the scattering properties of the two fibers that are connected, the OTDR measures that the splice loss value of the fiber splice may be positive or negative, and the fiber with a negative splice loss The joint can be considered as a fusion joint, and generally does not re-weld; the OTDR measurement value of the weld loss of each joint during welding should generally be less than 1/2-2/3 of the index value required for the welding loss, if the index requirement is less than 0.1 dB, then Unidirectional measurements should generally be less than 0.05-0.06 dB. The method for measuring the splice loss generally has a remote monitoring method, that is, the OTDR placed in the equipment room is connected to the optical fiber cable to be tested through the pigtail with the connector, and the optical fiber connection point is continuously moved forward, and the OTDR always performs the connection point in the equipment room. The quality monitoring and welding loss measurement has the advantages of small measurement deviation, and the disadvantage is that it can only measure in one direction, and is suitable for optical fibers with good mode field diameter consistency. The near-end monitoring method, that is, the OTDR always has a cable length at the front of the connection point. The disadvantage is that the OTDR needs to move forward continuously, which affects the use of the instrument. The advantage is that the measurement range of the OTDR is not too large. The values ​​of the splice loss measured by the above two methods are one-way measured values. After all the fiber joints are welded, the splice loss values ​​of the respective fiber splices are sequentially measured from the other end of the optical fiber line, and then the two directions of each joint are taken. The measured values ​​are summed to average the splice loss of the joint. The remote loopback bidirectional monitoring method is to temporarily loop the optical fibers in the light to form a loop, so that the optical fiber joint can be bidirectionally measured, thereby avoiding the shortcoming that the unidirectional measurement cannot obtain the welding loss value in time, and the measuring method requires the OTDR. The instrument measures a wide range of distances, but because the measurement method is too complicated, it is only suitable for optical cables below 12 cores. For fiber optic CATV projects, remote monitoring is generally used, provided that the mode field diameters of the two fibers in the splicing must be the same. The following is an example of remote loopback two-way monitoring method using the 8-core layer twisted Yongding cable used in the transmission of the provincial trunk cable. There are 4 PBT beam tubes in red, green and white in the cable. Each beam has one blue and white fiber. The length of each cable is 2km. The OTDR is placed in the machine room and measured in the first and second wiring packages. There is a group of welding construction personnel and are called Group 1 and Group 2 respectively. Firstly, the second group is temporarily welded to the blue fiber and the white fiber in the second cable red tube at the second wiring package, and then In the first group, the blue fiber and the white fiber in the first and second cable red tubes are respectively welded together. When the OTDR in the equipment room is connected with the white fiber of the first cable, the first wiring package is measured at 2 km. The splice loss value a of the joint of the white fiber in the red tube from the A end to the B end is 11, the splice loss value 612 of the joint B to the A direction of the blue fiber is measured at 6 km, and the OTDR is connected to the blue fiber and the blue is measured at 2 km. The splice loss value a12 of the fiber joint from A to B direction, and the splice loss value b11 of the white fiber joint from B to A in 6 km, the splice loss value of the white fiber joint is S white = (a11+ B11)/2, the splice loss value of the blue fiber is S blue=(a12+b12)/2. If it meets the requirements, the two fibers in the green tube are fused in the above method until the fibers in the four bundle tubes are all After the connection is completed, the first group is moved to the third junction package for temporary welding after the wiring package is sealed. The welding method is the same as the welding method of the second group at the second wiring package, and the second group is officially welded to the second wiring package. After the welding, the fiber is temporarily transferred to the 4th wiring package for temporary welding, the second group is officially welded to the 3rd wiring package, and so on, until the fiber connector is completely welded. This method avoids the fiber connection disorder and promptly presses the two-way. The measurement requires measuring the splice loss of the fiber optic connector and judging whether the loss value exceeds the standard, and avoiding the joint in which the one-way measurement cannot measure the splice loss in time and cause the rework value to exceed the standard in the future.

3 Main factors affecting the splice loss of the fiber splice The influencing factors of the splice loss of the fiber can be divided into intrinsic factors and extrinsic factors. The intrinsic factor refers to some factors of the fiber itself, such as the inconsistent mode field diameter of the two fibers, the fiber core mismatch, the core cross section is not round, the core and the cladding are not concentric, and the mode field diameter is inconsistent. The impact of the splice loss of the fiber optic connector is large. The G652 standard of the International Telegraph and Electric Advisory Committee (CCITT) stipulates that the nominal value of the mode field diameter of the 1310 nm window is within 9-10 pm, and the deviation must not exceed 10% of the nominal value. In the difference range, an optical fiber with a mode field diameter of 11 pm and another fiber with a mode field diameter of 9 pm can be welded under very good connection conditions, and the theoretical calculation value of the splice loss at the joint can reach 0.17 dB. It is even higher. The extrinsic factor refers to the influence of various human factors, instruments and other factors on the welding loss, such as the theory that the fiber is not aligned during welding, and the axis of the two fiber cores is radially offset by 2Pm. The value can reach 0.74dB; the theoretical value of the splice loss can reach O.46 dB when the tilt angle of the two fibers is 1 degree; the theoretical value of the fiber splice is 0.21 dB when the sum of the tilt angles of the fiber end face is 1 degree. The operation level of the continuator also affects the splice loss. It is reported that the same instrument and equipment are operated by different people. The total loss difference Z of the 10 splice points is up to 0.32 dB. In addition, the coil of the fiber in the wiring package is pre-wired. The coiling of the optical cable, the welding parameter setting of the fusion splicer and the cleaning condition of the discharge electrode, and whether the working environment is clean or not, have different effects on the splice loss of the optical fiber.

4 Methods for reducing the splice loss of the fiber splice The factors affecting the splice loss of the fiber splice are many. Only by eliminating the influence of various adverse factors can the splice loss of the fiber splice be fundamentally reduced, thereby reducing the transmission loss of the fiber CATV line. According to the author's practice and related information, it is recommended to take the following measures to reduce the splice loss of the fiber connector:

(1) The mode field diameter of the fiber at the break after disconnection at a certain point is the same, so the fusion at the breakage can minimize the influence of the fiber mode field diameter on the splice loss, so the cable manufacturer must select the same production batch. The second-class high-quality brand-name bare fiber is continuously produced according to the order length. According to the specified length of the cable, the optical cable is disconnected according to the length of the disk, and the winding cable is continuously numbered and divided into A and B ends (disconnected on the previous disk) For the B end, it is the A end on the next rear plate. It must not be hopped or garbled. When laying, according to the determined route, the B end of the previous cable should be arranged in order. The A-end of the last coil is connected to ensure that the fiber can be fused at the break, thereby avoiding the disadvantage that the splice loss of the fiber joint is too large due to the inconsistent diameter of the mode field of the fiber.

(2) When laying the optical cable, it is necessary to adopt the mechanical traction method of stepless speed regulation with the traction speed wood greater than 20m/min. The traction force shall not exceed 80% of the allowable tension of the optical cable, and the instantaneous maximum traction force shall not exceed 100%. The traction force must be applied in the optical cable. On the reinforced part, the elongation of the optical cable after the erection is subjected to the maximum load shall be less than 0.2%. In order to avoid the force and distortion of the optical fiber during the traction process, the optical cable pulling end shall be made when necessary, and the bending radius of the optical cable during construction shall be greater than 20 times the diameter of the cable, the cable must be released from the top of the system and kept loose and curved without twisting, and it is strictly prohibited to bend and twist the small circle, so as to reduce the chance of damage to the fiber in the cable as much as possible, avoiding the end of the cable. The fiber is damaged and the joint splice loss is increased.

(3) There should be well-trained construction workers to complete the connection of the optical fiber. It is necessary to strictly measure the welding process and measure the splice loss of the optical fiber connector. The joint with the unsatisfactory welding loss must be newly welded, and the number of times of repeated welding is 3- 4 times is appropriate, after continuous welding for 3 times, the improvement is still not good. After removing the cause of the fusion splicing machine, generally only need to reach the lowest value of the three times of welding, and do not repeatedly weld to avoid excessive consumption of the fiber to adversely affect the fiber. . The remaining length of the optical fiber coiled on the fiber storage tray of the wiring package should be not less than 60cm, and the radius of the coiled circle should be as large as possible. If the welding loss of the previous joint on the same optical fiber is negative during the connection, the next rear joint is a joint. The value of the splice loss can be larger. If the value of the splice loss of the front joint is large, the value of the splice loss of the next joint must be small or negative, which will affect the splice loss to avoid damage to the fiber at the end of the cable. In the preparation of the cable fusion, the cable head can be cut off.

(4) The connecting fiber must be carried out in a neat environment, such as in a construction vehicle or a small tent, which is not suitable for welding in dusty and humid environments. The fiber connection part and the connecting tool must be kept clean and dry. When preparing the fiber cross section, it must be wiped and cut. The prepared fiber section must be clean and free of dirt, and the wood should be exposed to the air for a long time. The cross-section of the fiber should be neatly cut, and the angle between the two sections should be less than 0.3 degrees. When the fiber is placed in the V-groove of the fusion splicer, the action is light, because for a single-mode fiber with a core diameter of 10 Pm, if the splice loss is less than 0.1 dB, the radial offset of the fiber axis is smaller than 0.8 Pm.

(5) Both ends of the cable entering the wiring package must be fixed firmly, so as to avoid the position of the fiber connector being displaced due to the twisting of the cable when the wiring package is mounted, resulting in a large loss measurement value at the joint. In the welding construction, when the welding is found, the welding loss value measured under the window of 1550 nm meets the requirements, but the value of the loss at the retest joint after sealing the wiring package is too large, which is usually caused by the positional displacement of the fiber joint. At this time, it can be re-measured in the window of 1310nm. If the measured value is too small, the position of the fiber connector is wrong, and the excess length of the fiber must be re-coiled. If it is too large, it is a fusion problem and must be re-fused. To avoid this phenomenon, Secure the fiber connector and the fiber length to the fiber storage tray with a dry tape. The coil diameter of the remaining length of the cable on both sides of the wiring package is controlled to be about 40cm, which should not be too small, so as to avoid damage to the optical fiber due to excessive distortion.

(6) The fusion splicer and the cutting knives have a great influence on the splice loss of the optical fiber. When welding, the welding parameters, such as the pre-melting current, the pre-melting time, the main melting current, and the main melting time, are correctly and reasonably set according to the type of the optical fiber. During the welding, the fiber waste and dust in the V-groove of the fusion splicer and the cutting blade ju should be removed in time. After the fusion splicer is used, the dust on the machine casing must be removed. If it is used in a humid environment, it must be protected from moisture. The service life of the welding machine electrode is generally about 2000 times. It is required to run the cleaning program to clean the electrode after 20 times of welding. However, after the fiber cleaning and connection conditions are good, it can be welded for about 60 times and then discharged once. The working conditions are better. When it is poor, it can be welded for 30-40 times and then discharged once, which not only prolongs the service life of the electrode, but also increases the welding loss. The electrode of the fusion splicer with a long time will have a layer of gray scale, which will cause the discharge current to be too large and the welding loss value will increase. At this time, the electrode can be removed, wiped gently with alcohol cotton, then loaded onto the fusion splicer and discharged. Once, if the discharge current is still too large after multiple cleanings, the electrode must be replaced. In addition, it is necessary to select a fusion splicer with strong dustproof capability for field work to splicing the fiber. 5 Conclusion Reducing the splice loss of the fiber optic connector can effectively reduce the transmission loss of the optical fiber CATV cable line, thereby improving its transmission quality, and thus has important practical significance. As long as the various measures mentioned in the paper can be used to reduce the splice loss to various factors that affect the splice loss of the fiber splice, so that the fiber optic CATV has a high-quality optical cable transmission line, and the data service will be carried out for the cable television station in the future. A good foundation for fiber to the home.