Classification of Techniques Used for Optical Fiber Connection/Splicing
Optical fibers are joined either by fusion/mechanical splice, which is a permanent joint, or by connectors, which can be disengaged repeatedly. Optical connectors are used mostly at joints that need to be switched for optical service operation and maintenance reasons, while permanent joints are in use mostly in other applications.
Mechanisms of Light Loss at Optical Fiber Joint
When joining optical fibers, the opposed cores must be properly aligned.
Optical fiber connector/splice loss occurs mostly in the following manner.
(1) Poor concentricity
Poor concentricity of joined optical fibers causes a connector/splice loss.
In the case of general purpose single-mode fibers, the value of connector/splice loss is calculated roughly as the square of the amount of misalignment multiplied by 0.2. (For example, if the light source wavelength is 1310nm, misalignment by 1 µm results in approximately 0.2 dB of loss.)
(2) Axial run-out
A connector/splice loss occurs due to an axial run-out between the light axes of optical fibers to be joined. For example, it is necessary to avoid an increased angle at fiber cut end when using an optical fiber cleaver before fusion splicing, since such an angle can result in splicing of optical fibers with run-out.
(3) Gap
An end gap between optical fibers causes a connector/splice loss.
For example, if optical fiber end faces are not correctly butt-joined in mechanical splicing, a splice loss.
(4) Reflection
An end gap between optical fibers results in 0.6 dB of return loss at the maximum due to the change in refractive index from the optical fiber to the air. Cleaning optical fiber ends is important for optical connectors. In addition, the whole optical connector ends should be cleaned because loss can also occur due to dirt between optical connector ends
Classification and Principles of Fusion Splices
Fusion splicing involves the melting and joining of optical fibers using heat generated by an electric arc between electrodes. Fusion splicing is classified into the two methods, as follows.
(1) Core alignment method(core alignment)
Optical fiber cores observed with a microscope are positioned with the help of image processing so that they are concentrically aligned.
Then, an electric arc is applied to the fiber cores. The fusion splicer used has cameras for observation and positioning in two directions.
(2) Stationary V-groove alignment method (cladding alignment)
This fusion splicing method uses V-grooves produced with high precision to position and orient optical fibers and utilizes the surface tension of melted optical fibers for alignment effects (cladding alignment).
Splices made by this method achieve low loss thanks to the recent advancement of optical fiber production technology, which has improved the dimensional accuracy regarding the placement of core. This method is primarily used for splicing a multi-fiber cable in a single action.
Notes on Fusion Splicing
Fusion splicing procedures comprise
(1) the fitting of a fiber protection sleeve, (2) removal of cover layers, (3) fiber cleaning, (4) fiber cleaving, (5) fusion splicing, and (6) reinforcing the splice.
(1) Fitting of Fiber Protection Sleeve
The fiber protection sleeve is used to protect optical fibers exposed at the splice. Make sure that one of the optical fibers is passed through the protection sleeve before fusion splicing.
(2) Removal of Cover Layers
Using a jacket remover, remove the cover layers to expose the fiber glass.
Notes:
- After cover layer removal, off-cuts are present in the jacket remover. Remove off-cuts from the jacket remover and clean the blade.
- To remove cover layers from a fiber ribbon, use a heated jacket remover. For successful removal, warm the cover layers for about 5 seconds before removal.
(3) Fiber Cleaning
After cover layer removal, clean the fiber glass with alcohol.
Notes:
- Debris of cover layers if remaining on the fiber glass can cause poor concentricity in fusion splicing or increased splice loss.
Clean the glass fiber thoroughly. - In the case of a multi-fiber cable, fiber ends may stick together due to alcohol, causing defective cleaving of fibers.
Flip lightly with a finger to spread out the fibers.
(4) Fiber Cleaving
Follow the optical fiber cleaver operating procedure to cut the fiber.
Notes:
- The loss characteristic of a fusion splice depends on the cleaving.
To reduce cleaving defects, clean the fiber holder and blade of optical fiber cleaver on a regular basis. - Keep the cleaved end of an optical fiber away from an object including your fingers to eliminate the causes of defective splices.
- Avoid scattering fiber off-cuts.
(5) Fusion Splicing
Fusion-splice optical fibers following the operation manual of the fusion splicer.
Notes:
- Dirt in the V-grooves or clamp of a fusion splicer can cause an unusual light loss due to poor concentricity.
Clean the fusion splicer thoroughly - It is possible to detect faulty conditions of cleaved end if pre-splicing inspection capability with dual-axis observation is available.
- If the fiber has a curl, lightly squeeze the fiber with fingers to remove the curl. The placed fiber should bend downward
(6) Splice Reinforcing
Cover the optical fiber splice with the fiber protection sleeve.
Reinforce the fiber with the sleeve on the heater.
Notes:
- Avoid bending or twisting the fiber when moving it so as not to break the fiber.
- Position the fiber protection sleeve so that its midpoint is close to the center of the splice.
- When placing the reinforcement, make sure that the glass fiber is straight.
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