How to reduce the loss of fiber in the application?

The transmission loss characteristics of optical fibers are one of the most important factors that determine the transmission distance, transmission stability and reliability of optical networks. There are many reasons for optical fiber transmission loss. In the construction and maintenance of optical fiber communication network, the most important concern is the cause of transmission loss caused by the use of optical fiber and how to reduce these losses.

The transmission loss caused by the use of optical fiber mainly includes splice loss (inherent loss of optical fiber, splice loss and movable joint loss) and non-splicing loss (bending loss and losses caused by other construction factors and application environment).

1. Continuity loss and its solution

1.1 Continuity loss

The splicing loss of optical fiber mainly includes: the intrinsic loss caused by the intrinsic factor of the fiber, the splice loss and the movable joint loss caused by the extrinsic factor.

(1) The inherent loss of fiber is mainly due to the inconsistency of the fiber mode field diameter; the mismatch of the fiber core diameter; the non-circular core section;

(2) The splice loss of the splice loss extrinsic factor is mainly caused by the axial dislocation; the axis (folding angle) is inclined; the end face is separated (gap); the fiber end face is incomplete; the refractive index difference; Operation steps, cleanliness of welding machine electrodes, welding parameter settings, cleanliness of the working environment and other factors.

(3) The movable joint loss of the extrinsic factor of the movable joint loss is mainly caused by the poor quality of the movable connector, poor contact, uncleanness and some factors that are the same as the welding loss (such as axial dislocation, end face gap, bending angle, refractive index difference, etc. )cause.

1.2 The solution to the connection loss

(1) In engineering design, construction and maintenance, high-quality optical fibers with the same characteristics should be selected. One line should use the same batch of high-quality brand-name bare fibers as much as possible, so as to match the characteristics of the optical fibers as much as possible, so as to make the mode field diameter affect the fiber fusion loss. to a minimum.

(2) The construction of the optical cable should be carried out in strict accordance with the regulations and requirements

When matching disks, try to make the whole disk configuration (single disk ≥ 500 meters) to minimize the number of joints. When laying, lay out the cable tray number and terminal sequence strictly to minimize the loss value.

(3) Select experienced and well-trained connection personnel for connection and testing

The level of the splicing personnel directly affects the size of the splicing loss. The splicing personnel should strictly follow the optical fiber splicing process for splicing, strictly control the splice loss, and use an optical domain reflectometer (OTDR) to monitor the splicing process at all times (splicing loss ≤ 0.08dB/piece ), those that do not meet the requirements should be re-welded. When using an optical time domain reflectometer (OTDR), the loss of the joint should be measured from two directions and the average of these two results should be calculated to eliminate the human factor error of a one-way OTDR measurement.

(4) Ensure that the connection environment meets the requirements

It is strictly forbidden to operate in the open air in a dusty and humid environment. The connecting parts of the optical cable, tools and materials should be kept clean, and the optical fiber connector should not be damp. The optical fiber to be cut must be clean and free of dirt. After cutting, the fiber must not be exposed to air for too long, especially in dusty and humid environments. When the continuous ambient temperature is too low, the necessary heating measures should be taken.

(5) Prepare a perfect fiber end face

The preparation of the fiber end face is the most critical process for fiber splicing. Whether the fiber end face is perfect or not is one of the important reasons for determining the fiber splice loss. The high-quality end face should be flat, free of burrs and defects, and perpendicular to the axis. The axis inclination of the fiber end face should be less than 0.3 degrees, showing a smooth and flat mirror surface, and keep it clean to avoid dust pollution. A high-quality cleaver should be used, and the cleaver should be used correctly to cleave the fiber. The cleaning, cutting and splicing of bare fibers should be closely connected, and the interval should not be too long. When moving the fiber, handle it with care to prevent the fiber end face from being damaged by rubbing against other objects.

(6) Correct use of fusion splicer

Correct use of fusion splicer is an important guarantee and key link to reduce optical fiber splice loss.

① Operate the fusion splicer correctly in strict accordance with the operation instructions and operation procedures of the fusion splicer.

②Place the optical fiber reasonably. When placing the optical fiber in the V-groove of the fusion splicer, the action should be light. This is because for a single-mode fiber with a core diameter of 10 nm, if the splice loss is to be less than 0.1 dB, the radial offset of the fiber axis must be less than 0.8 nm.

③According to the fiber type, set the splicing parameters correctly and reasonably (pre-discharge current, time, main discharge current, main discharge time, etc.).

④ The dust in the fusion splicer should be removed in time during and after use (especially the dust and fiber debris in the fixture, each mirror surface and the v-shaped groove).

⑤ The service life of the welding machine electrode is generally about 2000 times. After a long time of use, the electrode will be oxidized, resulting in a large discharge current and an increase in the welding loss value. At this time, the electrode can be removed, wiped gently with medical absorbent cotton dipped in alcohol, and then installed on the fusion splicer, and cleaned by discharge once. If the discharge current is still too large after multiple cleanings, the electrode must be replaced.

(7) Try to choose high-quality and qualified movable connectors to ensure that the performance indicators of the connectors meet the relevant regulations. The insertion loss of the movable connectors should be controlled below 0.3 dB/piece (or even lower), and the additional loss should not be greater than 0.2 dB/piece

(8) The movable joint should be well plugged and tightly coupled to prevent light leakage

(9) Ensure that the movable connector is clean

During construction and maintenance, care should be taken to clean the plugs and adapters (flanges) and ensure the cleanliness of the equipment room and equipment environment. Strictly prevent the plugs and adapters (flanges) from being contaminated with dirt and dust, and minimize scattering losses.

2. Discontinuous loss and its solution

2.1 Discontinuous loss

The non-splicing losses caused by the use of optical fibers mainly include bending losses and losses caused by other construction factors and application environments.

(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 conducted modes are converted into radiation modes, which do not continue to transmit, but enter the cladding and are absorbed by the coating or cladding, thereby causing additional losses in the fiber. There are two types of bending loss in optical fiber: macrobending loss and microbending loss.

①The additional loss caused by the curvature radius of the optical fiber is much larger than the diameter of the optical fiber (macrobending), the main reasons are: routing turning and bending in laying; bending caused by various reservations of optical fibers and cables (reserved coils, various bends, natural bending); the coiling of optical fibers in the splice box, the coiling of pigtails in the equipment room and equipment, etc.

②Microbending loss The optical fiber shaft produces additional losses caused by μm-level bending (microbending). The main reasons are: when the optical fiber is cabling, the small irregularities on the supporting surface cause the random microbending of various parts caused by uneven stress; The microbend formed by the uneven interface between the core and the cladding; the microbend formed by uneven tension everywhere when the optical cable is laid; the microbend formed by the uneven lateral pressure on the optical fiber; the optical fiber encounters temperature changes, due to heat Microbends formed by expansion and contraction.

(2) Losses caused by other construction factors and application environment

①The loss caused by the installation of irregular optical cables. The layer-twisted loose-tube structure optical cable is prone to such losses. The reason is that one is that multiple loose tubes are twisted with each other at the upper shelf of the optical cable; When the port is opened, the loose tube will be bent sharply; the third is that the metal reinforcing member and the optical fiber loose tube will be dislocated up and down when the optical cable is put on the shelf. These factors cause increased losses.

② Loss caused by poor thermal shrinkage and thermal melt protection. The main reasons are: one is the quality problem of the hot-melt protection tube itself, distortion occurs after hot-melting, and bubbles are generated; the other is that 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 tube 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 non-standard construction of direct buried optical cable. The reason is that one is that the optical cable is not buried deep enough and is damaged after being crushed by a load-bearing object; the other is that the optical cable is not properly routed, and the optical cable is subjected to external forces beyond its allowable load range due to changes in the environment and terrain; the third is the bottom of the optical cable trench. Uneven, the optical cable appears arched and hung, and there is residual stress after backfilling; the fourth is that the outer sheath of the optical cable is damaged by other reasons and water enters, resulting in hydrogen loss.

④The loss caused by the irregular construction of the overhead optical cable. The main reasons are as follows: one is that in the construction of optical cable laying, the optical cable is lapped, bent, twisted and buckled. When pulling, there is a surge, and the instantaneous maximum pulling force is too large; The direction of the hook is inconsistent, and there is a meandering bend, the interval is too sparse, and the optical cable is stressed due to excessive sag; the third is that the optical cable left on the rod is not fixed firmly, and the optical cable is damaged by long-term external force and short-term impact force; The fourth is that the fiber optic cable is too tight, and the natural elongation of the fiber optic cable is not considered; the fifth is that the outer sheath of the fiber optic cable is damaged by other reasons and water enters, resulting in hydrogen loss.

⑤The loss caused by the irregular construction of the pipeline optical cable. The reason is that, firstly, when the optical cable is deployed by the net sleeve method, the traction speed is not well controlled, and the optical cable is buckled and surged. It is due to other reasons that the outer sheath of the optical cable is damaged and water enters, resulting in hydrogen loss.

⑥The pigtails and optical fiber jumpers in the equipment room and equipment are not bundled and coiled irregularly, and the phenomenon of cross-winding will cause loss.

⑦The quality of the optical cable splice box is poor, the encapsulation and installation of the splice box are not standardized, the splice box is damaged due to external effects, etc., resulting in water ingress and hydrogen loss.

⑧The cable is stretched and deformed during the erection process, the pressure of clamping the optical cable in the splice box is too high, the hot-melt tube clamp in the fiber accommodating disk is too tight, and the optical fiber coiling in the fiber accommodating disk is not standardized.

2.2 The solution to the discontinuous loss

(1) During the engineering survey, design and construction, the best route and line laying method should be selected.

(2) It is very important to form and select a high-quality construction team to ensure the construction quality. Any negligence in construction may cause the increase of optical fiber loss.

(3) During the design, construction and maintenance, effective "four defenses" measures (lightning protection, electricity protection, corrosion protection, and mechanical damage prevention) should be actively taken to strengthen the protection work.

(4) Use the bracket to hold up the cable reel to lay out the optical cable. Do not lay the optical cable in a similar way to laying the optical cable from the spool after laying the cable reel down, and do not allow the optical cable to be twisted. When laying the optical cable, the command should be unified, the liaison should be strengthened, and the scientific and reasonable traction method should be adopted. The arming speed should not be too fast; the continuous arming length should not be too long. If necessary, the inverted "8" should be used, and it should be deployed from the middle to the two ends. Be careful and take necessary protection measures in places where the optical cable may be damaged, such as corners. In the case of laying optical cables in downtown areas and other situations that need to be temporarily coiled, use a figure-8 coil to prevent the optical cables from being twisted.

(5) When laying the optical cable, attention must be paid to the allowable rated tensile force and the limitation of the bending radius. In the optical cable laying construction, it is strictly forbidden to make small circles, bending and twisting of the optical cable, so as to prevent the back buckle and the surge phenomenon. The traction force should not exceed 80% of the allowable optical cable, and the maximum instantaneous traction force should not exceed 100%. The traction force should be added to the reinforcement of the optical cable, and special attention should be paid not to yank and kink. When the optical cable is turned, the bending radius should not be less than 15 to 20 times 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-shrinking, which will increase the loss when applied to the fiber. When carrying and storing the casing, pay attention to cleaning and do not allow foreign objects to enter the casing.

(7) During the splicing operation, the stripping length should be determined according to the size of the receiving tray, and the stripping length should be as long as possible, so that the optical fiber can be coiled in the closing tray more calmly (the coil length is 60-100cm). Attention should be paid to the storage of the optical fiber after fusion (the coiling and fixing of the optical fiber). When the fiber is coiled, the larger the radius of the coil, the larger the arc, and the smaller the loss of the entire line, so a certain radius must be maintained (R≥40mm) , to avoid unnecessary losses, the key to the connection of large-core fiber optic cables is storage. During the connection operation, the depth of the cable opening knife cutting into the optical cable should be well controlled, and the loose tube should not be flattened to cause the optical fiber to be stressed. Use qualified joint materials and properly package and install the joint box in accordance with the specifications and operating requirements.

(8) The equipment room should be kept as clean as possible. The pigtails should be protected by a coiled tape, or use a separate wire for the pigtails, so as not to cross the pigtails or with other cables, and try not to wrap the pigtails (even temporary use) where the foot can step on it. When terminating the optical cable, pay attention to avoid the right angle of the jumper in the wiring, especially do not use the plastic tape to tie the jumper to a right angle, otherwise the loss of the optical fiber will increase due to the long-term stress. The jumper should follow a curve when turning, and the bending radius should not be less than 40mm. Make sure that the jumper is not under stress or pressure during placement to avoid long-term stress fatigue of the jumper. Do not bundle the pigtails too tightly during fiber end operation (ODF).

(9) Strengthen the daily maintenance and technical maintenance of optical cable lines.

Fiber to the home (FTTH) is the inevitable development of the information age, and optical network interconnection is the future of the digital earth. With the massive construction and operation of various types of optical fiber communication networks at all levels, facing and solving the problem of transmission loss caused by the use of optical fibers will greatly improve and optimize the transmission performance of optical fiber communication networks in the design, construction, and maintenance of optical fiber communication projects.