Losses and Solutions in Fiber Optic Applications

1.1 Connection loss

Optical fiber splicing loss mainly includes three types: inherent loss caused by intrinsic factors of optical fiber, fusion loss caused by non-intrinsic factors, and active joint loss.

(1) The intrinsic loss of optical fiber mainly comes from four factors: inconsistent mode field diameter; mismatch of optical fiber core diameter; non-circular core cross section; and poor concentricity between the core and cladding. Among them, the most influential factor is inconsistent mode field diameter.

(2) Non-intrinsic factors of splicing loss The splicing loss is mainly caused by axial misalignment, axis (angle) inclination, end face separation (gap), incomplete fiber end face, refractive index difference, unclean fiber end face, and other factors such as the operating level of the splicing personnel, operating steps, cleanliness of the welding machine electrodes, welding parameter settings, and cleanliness of the working environment.

(3) Non-intrinsic factors of active joint loss Active joint loss is mainly caused by poor quality of active connectors, poor contact, uncleanness and some factors similar to welding loss (such as axial misalignment, end face gap, angle, refractive index difference, etc.).

1.2 Solutions to Solve Connection Loss

(1) In engineering design, construction and maintenance, high-quality optical fibers with consistent characteristics should be selected. High-quality brand-name bare fibers from the same batch should be used as much as possible on a line to ensure that the characteristics of the optical fibers are matched as much as possible and to minimize the impact of the mode field diameter on the optical fiber fusion loss.

(2) Optical cable construction should be carried out strictly in accordance with regulations and requirements

When arranging cables, try to configure the entire cable reel (single reel ≥ 500 meters) to minimize the number of joints. When laying cables, strictly follow the cable reel number and end sequence to minimize the loss value.

(3) Select experienced and well-trained personnel to carry out the connection and testing

The level of the splicing personnel directly affects the size of the splicing loss. The splicing personnel should strictly follow the fiber fusion splicing process to perform splicing, strictly control the splicing loss, and use the optical time 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-splice. When using the optical time domain reflectometer (OTDR), the loss of the splice should be measured from two directions, and the average of the two results should be calculated to eliminate the human error of the unidirectional OTDR measurement.

(4) Ensure that the connection environment meets the requirements

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

(5) Prepare a perfect optical fiber end face

The preparation of the fiber end face is the most critical process for fiber splicing. The perfection of the fiber end face is one of the important factors that determine the fiber splicing loss. A 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, presenting a smooth and flat mirror surface, and kept clean to avoid dust pollution. A high-quality cleaver should be selected and used correctly to cut the fiber. The cleaning, cutting and welding of the bare fiber should be closely connected and the interval should not be too long. When moving the optical fiber, handle it with care to prevent it from rubbing against other objects and damaging the fiber end face.

(6) Use the welding machine correctly

Correct use of the fusion splicer is an important guarantee and key link in reducing optical fiber splicing losses.

① Strictly follow the operating instructions and operating procedures of the welding machine and operate the welding machine correctly.

② Place the optical fiber properly and place it in the V-groove of the fusion splicer with gentle movements. This is because for a single-mode optical fiber with a core diameter of 10 nm, if the fusion loss is to be less than 0.1 dB, the radial offset of the optical fiber axis must be less than 0.8 nm.

③ Correctly and reasonably set the welding parameters (pre-discharge current, time, main discharge current, main discharge time, etc.) according to the optical fiber type.

④ The dust in the fusion splicer should be removed promptly during and after use (especially the dust and fiber fragments in the clamps, mirrors and V-grooves).

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

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

(8) The movable joints should be well connected and tightly coupled to prevent light leakage.

(9) Ensure that the active connector is clean

During construction and maintenance, care should be taken to clean the plug and adapter (flange) and ensure the cleanliness of the machine room and equipment environment to prevent dirt and dust from forming on the plug and adapter (flange) and to minimize scattering losses.

2. Non-connection loss and its solution

2.1 Non-connection loss

The non-splicing losses caused during 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 an 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 converted into radiation modes and no longer continue to be transmitted. Instead, they enter the cladding and are absorbed by the coating or cladding, causing additional losses in the optical fiber. There are two types of bending losses in optical fibers: macrobending losses and microbending losses.

① Macrobend loss The additional loss caused by the bend (macrobend) in which the curvature radius of the optical fiber is much larger than the diameter of the optical fiber is mainly due to: routing turns and bends during laying; bends caused by various reservations of optical fiber cables (reserved loops, various bends, natural bends); the coiling of optical fibers in the joint box, the coiling of pigtails in the machine room and equipment, etc.

② Microbend loss Additional loss caused by μm-level bending (microbend) of the optical fiber axis. The main reasons are: when the optical fiber is cabled, the tiny irregularities of the supporting surface cause uneven stress in various parts and form random microbends; the interface between the core and the cladding is not smooth and forms microbends; when the optical cable is laid, the tension in various places is uneven and forms microbends; the lateral pressure on the optical fiber is uneven and forms microbends; the optical fiber encounters temperature changes and forms microbends due to thermal expansion and contraction.

1. Connection loss and its solution

1.1 Connection loss

The splicing loss of optical fiber mainly includes three types: inherent loss caused by intrinsic factors of optical fiber, fusion loss caused by non-intrinsic factors, and active joint loss.

(1) The intrinsic loss of optical fiber mainly comes from four factors: inconsistent mode field diameter; mismatch of optical fiber core diameter; non-circular core cross section; and poor concentricity between the core and cladding. Among them, the most influential factor is inconsistent mode field diameter.

(2) Non-intrinsic factors of splicing loss The splicing loss is mainly caused by axial misalignment, axis (angle) inclination, end face separation (gap), incomplete fiber end face, refractive index difference, unclean fiber end face, and other factors such as the operating level of the splicing personnel, operating steps, cleanliness of the welding machine electrodes, welding parameter settings, and cleanliness of the working environment.

(3) Non-intrinsic factors of active joint loss Active joint loss is mainly caused by poor quality of active connectors, poor contact, uncleanness and some factors similar to welding loss (such as axial misalignment, end face gap, angle, refractive index difference, etc.).

1.2 Solutions to Solve Connection Loss

(1) In engineering design, construction and maintenance, high-quality optical fibers with consistent characteristics should be selected. High-quality brand-name bare fibers from the same batch should be used as much as possible on a line to ensure that the characteristics of the optical fibers are matched as much as possible and to minimize the impact of the mode field diameter on the optical fiber fusion loss.

(2) Optical cable construction should be carried out strictly in accordance with regulations and requirements

When arranging cables, try to configure the entire cable reel (single reel ≥ 500 meters) to minimize the number of joints. When laying cables, strictly follow the cable reel number and end sequence to minimize the loss value.

(3) Select experienced and well-trained personnel to carry out the connection and testing

The level of the splicing personnel directly affects the size of the splicing loss. The splicing personnel should strictly follow the fiber fusion splicing process to perform splicing, strictly control the splicing loss, and use the optical time 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-splice. When using the optical time domain reflectometer (OTDR), the loss of the splice should be measured from two directions, and the average of the two results should be calculated to eliminate the human error of the unidirectional OTDR measurement.

(4) Ensure that the connection environment meets the requirements

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

(5) Prepare a perfect optical fiber end face

The preparation of the fiber end face is the most critical process for fiber splicing. The perfection of the fiber end face is one of the important factors that determine the fiber splicing loss. A 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, presenting a smooth and flat mirror surface, and kept clean to avoid dust pollution. A high-quality cleaver should be selected and used correctly to cut the fiber. The cleaning, cutting and welding of the bare fiber should be closely connected and the interval should not be too long. When moving the optical fiber, handle it with care to prevent it from rubbing against other objects and damaging the fiber end face.

(6) Use the welding machine correctly

Correct use of the fusion splicer is an important guarantee and key link in reducing optical fiber splicing losses.

① Strictly follow the operating instructions and operating procedures of the welding machine and operate the welding machine correctly.

② Place the optical fiber properly and place it in the V-groove of the fusion splicer with gentle movements. This is because for a single-mode optical fiber with a core diameter of 10 nm, if the fusion loss is to be less than 0.1 dB, the radial offset of the optical fiber axis must be less than 0.8 nm.

③ Correctly and reasonably set the welding parameters (pre-discharge current, time, main discharge current, main discharge time, etc.) according to the optical fiber type.

④ The dust in the fusion splicer should be removed promptly during and after use (especially the dust and fiber fragments in the clamps, mirrors and V-grooves).

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

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

(8) The movable joints should be well connected and tightly coupled to prevent light leakage.

(9) Ensure that the active connector is clean

During construction and maintenance, care should be taken to clean the plug and adapter (flange) and ensure the cleanliness of the machine room and equipment environment to prevent dirt and dust from forming on the plug and adapter (flange) and to minimize scattering losses.

2. Non-connection loss and its solution

2.1 Non-connection loss

The non-splicing losses caused during 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 an 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 converted into radiation modes and no longer continue to be transmitted. Instead, they enter the cladding and are absorbed by the coating or cladding, causing additional losses in the optical fiber. There are two types of bending losses in optical fibers: macrobending losses and microbending losses.

① Macrobend loss The additional loss caused by the bend (macrobend) in which the curvature radius of the optical fiber is much larger than the diameter of the optical fiber is mainly due to: routing turns and bends during laying; bends caused by various reservations of optical fiber cables (reserved loops, various bends, natural bends); the coiling of optical fibers in the joint box, the coiling of pigtails in the machine room and equipment, etc.

② Microbend loss Additional loss caused by μm-level bending (microbend) of the optical fiber axis. The main reasons are: when the optical fiber is cabled, the tiny irregularities of the supporting surface cause uneven stress in various parts and form random microbends; the interface between the core and the cladding is not smooth and forms microbends; when the optical cable is laid, the tension in various places is uneven and forms microbends; the lateral pressure on the optical fiber is uneven and forms microbends; the optical fiber encounters temperature changes and forms microbends due to thermal expansion and contraction.

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

① Losses caused by non-standard optical cable racking. Layered loose tube structure optical cables are prone to this type of loss. The reasons are: first, multiple loose tubes are twisted together at the optical cable rack; second, when the loose tube is tied to the fiber tray of the joint box with a cable tie, the loose tube is bent sharply; third, when the optical cable is racked, the metal reinforcement member and the optical fiber loose tube are misaligned up and down. These factors will cause increased losses.

② Losses caused by poor heat shrinkage of hot melt protection. The main reasons are: first, the quality of the hot melt protection tube itself, which will be distorted and produce bubbles after hot melting; second, when the heater of the welding machine is heated, the heating parameters are improperly set, causing the hot melt protection tube to deform or produce bubbles; third, the heat shrink tube is not clean, has dust or gravel, and the connection point is damaged during hot melting, causing increased losses.

③ Losses caused by non-standard construction of direct buried optical cables. The reasons are: first, the optical cable is not buried deep enough and is damaged by being crushed by a load; second, the optical cable route is improperly selected, and the optical cable is subjected to external forces beyond its allowable load range due to changes in the 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, other reasons cause the outer sheath of the optical cable to be damaged and water to enter, causing hydrogen loss.

④ Losses caused by non-standard construction of overhead optical cables. The main reasons are: first, during the installation of optical cables, the optical cables are looped, bent, twisted, and buckled, and they are pulled violently and surges occur during traction, resulting in excessive instantaneous maximum traction; second, the optical cable hooks are improperly used, the directions of the hooks are inconsistent, serpentine bends occur, the spacing is too sparse, and the optical cables are stressed due to excessive verticality; third, the optical cables coiled on the poles are not firmly fixed, and the optical cables are damaged by long-term external forces and short-term impact forces; fourth, the optical cables are laid too tightly, and the natural elongation of the optical cables is not considered; fifth, other reasons cause the outer sheath of the optical cable to be damaged and water to enter, causing hydrogen loss.

⑤ Losses caused by non-standard construction of pipeline optical cables. The reasons are: first, when the optical cable is laid using the net sleeve method, the pulling speed is not well controlled, and the optical cable has back buckles and surges; second, when the optical cable is laid, no plastic sub-tube is laid, and the optical cable is scratched; third, other reasons cause the outer sheath of the optical cable to be damaged and water to enter, causing hydrogen loss.

⑥ The fiber pigtails and fiber jumpers in the machine room and equipment are not tied and coiled in a standardized manner, resulting in cross-entanglement and other phenomena that cause losses.

⑦ The quality of the optical cable junction box is poor, the junction box packaging and installation are not standardized, the junction box is damaged due to external factors, etc., causing water ingress and hydrogen loss.

⑧ The loss is caused by the tensile deformation of the optical cable during the installation process, too much pressure on the optical cable clamped in the splicing box, too tight clamping of the hot melt tube in the fiber tray, and irregular winding of the optical fiber in the fiber tray.

2.2 Solutions to non-connection losses

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

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

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

(4) Use a bracket to hold up the cable reel when laying the optical cable. Do not lay the cable reel down and then lay the optical cable in a similar way to laying it from a spool. Do not subject the optical cable to torque. When laying the optical cable, unified command should be maintained, communication should be strengthened, and a scientific and reasonable traction method should be adopted. The deployment speed should not be too fast; the continuous deployment length should not be too long. If necessary, an inverted "8" shape should be used to lay the cable from the middle to both ends. Be careful and take necessary protective measures at bends and other places where the optical cable may be damaged. When encountering situations where temporary cable reels are required, such as laying optical cables in downtown areas, use an 8-shaped reel to prevent the optical cable from being subjected to torque.

(5) When laying optical cables, attention must be paid to the limits of the allowable rated tension and bending radius. During the installation of optical cables, it is strictly forbidden to make small circles, bends, or twists in the optical cables to prevent back buckles and surges. The pulling force should not exceed 80% of the allowable force of the optical cable, and the instantaneous maximum pulling force should not exceed 100%. The pulling force should be applied to the reinforcement of the optical cable, and special attention should be paid to not pulling violently or causing kinks. When the optical cable turns, 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 shrink tubing, especially those that have been bent and deformed. Such tubing will generate stress inside when it shrinks, which will increase the loss of the optical fiber. When carrying or storing the tubing, pay attention to cleaning and do not allow foreign matter to enter the tubing.

(7) During the splicing operation, the stripping length should be determined according to the size of the receiving reel. Try to strip longer so that the optical fiber can be coiled more easily in the receiving reel (the coiling length is 60 to 100 cm). Attention should be paid to the receiving of the optical fiber after fusion splicing (fiber coiling and fixing). When coiling, the larger the radius of the coil and the larger the arc, the smaller the loss of the entire line. Therefore, a certain radius (R ≥ 40 mm) must be maintained to avoid unnecessary loss. The key to splicing large-core optical cables is receiving. During the splicing operation, the depth of the cable cutter cutting into the optical cable should be controlled well, and the loose tube should not be flattened to cause stress on the optical fiber. Use qualified joint materials and correctly package and install the joint box in accordance with specifications and operating requirements.

(8) Keep the room as clean as possible. The pigtails should be protected by wrapping tape, or a separate wire should be used for the pigtails. Do not cross-wrap the pigtails or other connecting wires. Try not to place the pigtails (even if they are temporarily used) where they can be stepped on. When terminating the optical cable, be careful to avoid right angles in the routing of the jumper. In particular, do not use plastic tape to tie the jumper into a right angle, otherwise the optical fiber will be affected by long-term stress and cause increased loss. The jumper should be curved when turning, and the bending radius should be no less than 40mm. During the routing, ensure that the jumper is not subjected to force or pressure to avoid long-term stress fatigue of the jumper. Do not bundle the pigtails too tightly during the optical fiber termination operation (ODF).

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

Fiber-to-the-home (FTTH) is an inevitable development in 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 transmission loss problem 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.