Loss theory and solution of optical fiber in transmission

The transmission loss characteristic of optical fiber is one of the most important factors determining the transmission distance, transmission stability and reliability of optical network. The causes of optical fiber transmission loss are various. In the construction and maintenance of optical fiber communication network, the most noteworthy is the causes of transmission loss in the use of optical fiber and how to reduce these losses. The transmission loss caused by the use of optical fiber mainly includes connection loss (inherent loss, fusion loss and movable joint loss of optical fiber) and non connection loss (bending loss and loss caused by other construction factors and application environment).
1. Connection loss and its solution

1.1 connection loss
The connection loss of optical fiber mainly includes: inherent loss caused by intrinsic factors of optical fiber, fusion loss and movable joint loss caused by extrinsic factors of optical fiber.
(1) The inherent loss of optical fiber mainly comes from the inconsistency of mode field diameter of optical fiber; Fiber core diameter mismatch; The cross section of the fiber core is not round; The concentricity of fiber core and cladding is poor at four points; The most important factor is the inconsistency of mode field diameter.
(2) The welding loss of non intrinsic factors is mainly caused by axial dislocation; Inclination of axis (angle); End face separation (clearance); The end face of the optical fiber is incomplete; Refractive index difference; The unclean end face of optical fiber is caused by other factors, such as the operation level of splicing personnel, operation steps, the cleanliness of welding electromechanical electrode, the setting of welding parameters, the cleanliness of working environment and so on.
(3) The movable joint loss of non intrinsic factors is mainly caused by the poor quality, poor contact, uncleanness of the movable connector and some of the same factors as the welding loss (such as axial dislocation, end face clearance, folding angle, refractive index difference, etc.).
1.2 solution to connection loss
(1) High quality optical fibers with consistent characteristics shall be selected in engineering design, construction and maintenance. The same batch of high-quality famous brand bare fibers shall be used as much as possible in one line, so as to match the characteristics of optical fibers as much as possible, so as to minimize the impact of mode field diameter on optical fiber fusion loss.
(2) Optical cable construction shall be carried out in strict accordance with regulations and requirements
Try to configure the whole panel (single panel ≥ 500m) to minimize the number of joints. During laying, the cable reel number and end order shall be strictly followed to minimize the loss.
(3) Select experienced and well-trained successors for connection and testing
The level of the connecting personnel directly affects the size of the connecting loss. The connecting personnel shall connect in strict accordance with the optical fiber welding process flow and strictly control the joint loss. During the welding process, the optical domain reflectometer (OTDR) shall be used for monitoring at all times (the connecting loss ≤ 0.08db / piece). If it does not meet the requirements, it shall be welded again. When using optical time domain reflectometer (OTDR), the loss of the joint shall be measured from two directions, and the average of the two results shall be obtained to eliminate the human factor error of one-way OTDR measurement.
(4) Ensure that the connection environment meets the requirements
It is strictly prohibited to operate in the open air in dusty and humid environment. The optical cable connection parts, tools and materials shall be kept clean and shall not damp the optical fiber connector. The optical fiber to be cut must be clean and free of dirt. After cutting, the optical fiber shall not be exposed to the air for too long, especially in dusty and humid environment. When the connecting ambient temperature is too low, necessary heating measures shall be taken.
(5) Preparation of perfect optical fiber end face
The preparation of optical fiber end face is the most critical process of optical fiber connection. The perfection of optical fiber end face is one of the important reasons to determine the optical fiber connection loss. The high-quality end face shall be flat, free of burrs and defects, and perpendicular to the axis. The axis inclination of the optical fiber end face shall be less than 0.3 degrees, showing a smooth and flat mirror, and kept clean to avoid dust pollution. High quality cutting knife shall be selected and used correctly to cut optical fiber. The cleaning, cutting and welding of bare fiber shall be closely connected, and the interval shall not be too long. When moving the optical fiber, handle it gently to prevent damage to the end face of the optical fiber due to collision with other objects.
(6) Correct use of welding machine
Correct use of welding machine is an important guarantee and key link to reduce optical fiber connection loss.
① The welding machine shall be operated correctly in strict accordance with the operation instructions and procedures of the welding machine.
② Place the optical fiber reasonably. When placing the optical fiber in the V-groove of the welding machine, the action should be light. This is because for a single-mode fiber with a core diameter of 10 nm, if the fusion loss is less than 0.1dB, the radial offset of the fiber axis is less than 0.8nm.
③ Correctly and reasonably set the welding parameters (pre discharge current, time, main discharge current, main discharge time, etc.) according to the type of optical fiber.
④ The dust in the welding machine (especially the dust in the fixture, each mirror and V-groove and fiber debris) shall be removed in time during and after use.
⑤ The service life of welding electromechanical electrode is generally about 2000 times. After a long service time, the electrode will be oxidized, resulting in large discharge current and increased welding loss. At this time, the electrode can be removed, gently wiped with medical absorbent cotton dipped in alcohol, then installed on the welding machine, and discharged for cleaning once. If the discharge current is still too large after multiple cleaning, the electrode must be replaced again.
(7) High quality and qualified movable connectors shall be selected as far as possible to ensure that the performance indicators of connectors meet relevant regulations. The insertion loss of movable connectors shall be controlled below 0.3 dB / piece (or even lower), and the additional loss shall not be greater than 0.2 dB / piece
(8) The movable joint shall be well connected and tightly coupled to prevent light leakage
(9) Ensure that the movable connector is clean
During construction and maintenance, pay attention to cleaning the plug and adapter (flange) and ensure the cleanness of the machine room and equipment environment, strictly prevent the plug and adapter (flange) from dirt and dust and minimize scattering loss.
2. Non continuous loss and its solution
2.1 non continuous loss
The non connection loss caused by the use of optical fiber mainly includes bending loss and loss caused by other construction factors and application environment.
(1) Radiation loss caused by bending when the optical fiber is greatly bent and the bending radius is comparable to its core diameter, its transmission characteristics will change. A large number of conduction modes are transformed into radiation modes, which do not continue to transmit, but enter the cladding and are absorbed by the coating or cladding, resulting in additional loss of optical fiber. The bending loss of optical fiber has two types: macro bending loss and micro bending loss.
① Macro bending loss additional loss caused by bending (macro bending) of optical fiber with a curvature radius much larger than the diameter of optical fiber. The main reasons are: routing turning and bending in laying; Bending caused by various reservation of optical fiber and cable (reservation ring, various bending and natural bending); The coiling of optical fiber in the joint box, the coiling of tail fiber in the machine room and equipment, etc.
② Micro bending loss optical fiber axis generation μ The main reasons for the additional loss caused by M-class bending (micro bending) are: random micro bending caused by uneven stress of each part caused by small irregularity of the supporting surface during optical fiber cabling; Micro bending formed by unsmooth interface between core and cladding; Micro bending caused by uneven tension during optical cable laying; Micro bending formed by uneven lateral pressure on the optical fiber; When the optical fiber encounters temperature change, it forms micro bending due to thermal expansion and cold contraction.
(2) Losses caused by other construction factors and application environment
① Loss caused by nonstandard optical cable on the shelf. The layer twisted loose sleeve structure optical cable is prone to such loss because, first, multiple loose sleeves at the upper shelf of the optical cable are twisted with each other; Second, when the loose tube is bound to the fiber holding disc bayonet of the joint box with a tie, the loose tube is sharply bent; Third, when the optical cable is put on the shelf, the metal reinforcing member and the optical fiber loose sleeve are misaligned up and down. These factors will increase the loss.
② Loss caused by poor heat shrinkage and hot melt protection. The main reasons are: one is the quality problem of the hot-melt protective tube itself, which distorts and produces bubbles after hot-melt; Secondly, when the heater of the welding machine is heated, the heating parameters are improperly set, resulting in the deformation of the hot-melt protection tube or the generation of bubbles; The third is that the heat shrinkable pipe is not clean, there is dust or gravel, and the connection point is damaged during hot melting, resulting in increased loss.
③ Loss caused by nonstandard construction of directly buried optical cable. The reasons are as follows: first, the optical cable is not buried deep enough and is damaged after being rolled by heavy objects; The second is the improper routing of optical cable, which makes the optical cable subject to external forces beyond its allowable load range due to changes in environment and terrain; Third, the bottom of the optical cable trench is uneven, the optical cable is arched and hung, and there is residual stress after backfilling; Fourth, the outer protective layer of the optical cable is damaged due to other reasons, resulting in water inflow and hydrogen loss.
④ Loss caused by nonstandard construction of overhead optical cable. The main reasons are as follows: first, during the optical cable laying construction, the optical cable makes small circles, bends, twists and back buckles, jerks and surges during traction, and the instantaneous maximum traction force is too large; The second is the improper use of the optical cable hook, the clamping direction is inconsistent, there are snake bends, the spacing is too sparse, and the optical cable is stressed due to excessive sag; Third, the optical cable coiled on the pole is not fixed firmly, and the optical cable is damaged by long-term external force and short-term impact force; Fourth, the optical cable is arranged too tightly, and the natural elongation of the optical cable is not considered; Fifthly, the outer sheath of the optical cable is damaged due to other reasons, resulting in water inflow and hydrogen loss.
⑤ Loss caused by nonstandard construction of pipeline and optical cable. The reasons are as follows: firstly, when the optical cable is deployed by the net method, the traction speed is not well controlled, and the optical cable has back buckle and surge; Second, when the optical cable is put through, there is no protective plastic sub tube, and the optical cable is scratched; Third, the outer sheath of the optical cable is damaged due to other reasons, resulting in water inflow and hydrogen loss.
⑥ The binding and winding of pigtail and optical fiber jumper in the machine room and equipment are not standard, and cross winding occurs, resulting in loss.
⑦ The quality of the optical cable connector box is poor, the packaging and installation of the connector box are not standardized, and the connector box is damaged due to external effects, resulting in water and hydrogen loss.
⑧ The loss caused by the tensile deformation of the optical cable during erection, too much pressure for clamping the optical cable in the splice box, too tight clamping of the hot-melt tube in the fiber tray, and non-standard winding of the optical fiber in the fiber tray.
2.2 solution to non connection loss
(1) During engineering survey, design and construction, the best route and route laying mode shall be selected.
(2) It is very important to establish and select a high-quality construction team to ensure the construction quality. Any negligence in construction may increase the optical fiber loss.
(3) During design, construction and maintenance, practical and effective "four prevention" measures (lightning protection, electricity protection, corrosion protection and mechanical damage protection) shall be actively taken to strengthen protection.
(4) Use the bracket to hold up the cable reel to lay the optical cable. Do not lay the optical cable by the method similar to that from the spool after the cable reel is placed down, and do not let the optical cable be twisted. During the laying of optical cables, unified command and communication shall be strengthened, and scientific and reasonable traction methods shall be adopted. The deployment speed shall not be too fast; The continuous deployment length should not be too long. If necessary, the inverted "8" word should be used to deploy from the middle to both ends. Be careful and take necessary protective measures at corners and other places that may damage the optical cable. In case of laying optical cables in downtown areas and other situations requiring temporary laying of optical cables, 8-shaped disc shall be used to keep the optical cables from torsion.
(5) During the laying of optical cables, attention must be paid to the allowable rated tension and the limitation of bending radius. During the laying of optical cables, it is strictly prohibited to make small circles, bend and twist the optical cables to prevent back buckle and surge. The traction force shall not exceed 80% of the allowable of the optical cable, and the instantaneous maximum traction force shall not exceed 100%. The traction force shall be added to the reinforcement of the optical cable, and special attention shall be paid not to jerk and kink. When the optical cable turns, the bending radius shall not be less than 15 ~ 20 times of the outer diameter of the optical cable.
(6) Do not use inferior heat shrinkable sleeves, especially those that have been bent and deformed. Such sleeves will generate internal stress during heat shrinkable, which will be applied to the optical fiber to increase the loss. When carrying and storing the casing, pay attention to cleaning and prevent foreign matters from entering the casing.
(7) During the connection operation, the stripping length shall be determined according to the size of the receiving disc, and the stripping length shall be longer as far as possible, so that the optical fiber can be wound in the closing calmly (the reserved length of the disc is 60 ~ 100cm). Attention should be paid to the storage of optical fiber after fusion (fiber winding and fixation of optical fiber). During fiber winding, the larger the radius of the coil, the greater the radian, and the smaller the loss of the whole line. Therefore, it is necessary to maintain a certain radius (R ≥ 40mm) to avoid unnecessary loss. The key to the connection of large core optical cable is storage. During the connection operation, the depth of the cable cutter cutting into the optical cable shall be well controlled, and the loose sleeve shall not be flattened to force the optical fiber. Use qualified joint materials and correctly package and install the joint box according to the specifications and operation requirements.
(8) The machine room shall be as clean as possible. The tail fiber shall be protected by a winding belt, or a wire shall be used for the tail fiber separately to avoid cross winding between the tail fiber or with other connecting lines, and the tail fiber (even for temporary use) shall not be placed where the foot can step on. When terminating the optical cable, pay attention to avoid the right angle of the jumper in the routing, especially do not tie the jumper into a right angle with plastic tape, otherwise the loss of the optical fiber will increase due to long-term stress. The jumper shall follow the curve when turning, and the bending radius shall not be less than 40mm. During laying, ensure that the jumper is free from stress and pressure, so as to avoid long-term stress fatigue of the jumper. Do not bundle the pigtails too tightly during ODF.
(9) Strengthen the daily maintenance and technical maintenance of optical cable lines.
Optical fiber is the inevitable development of the information age, and optical network interconnection is the tomorrow of the digital earth. With the construction and operation of various optical fiber communication networks at all levels, facing and solving the transmission loss caused by the use of optical fiber will greatly improve and optimize the transmission performance of optical fiber communication network in the design, construction and maintenance of optical fiber communication engineering.