The principle of fusion splicing is that two bare fiber ends (with the coating removed) are fused together under the influence of heat. More precisely, the fiber ends are initially brought into close contact with a small gap between the two. While the surfaces melt, they are pressed together so that the ends fuse together after they are heated for a short while.
Heating is often achieved with a high voltage discharge, but there are other methods: electrically heated Nichrome wire, a CO 2 laser or a gas flame.
Surface tension helps to achieve good orientation if the fiber core is on the fiber axis. Alternatively, it is possible to align the fibers precisely before splicing so that the cores (even when they are slightly off-centre) are well matched, but there is a danger that surface tension will pull the fibers towards a location where the total area ( rather than core) is a match.
Splicing multimode fiber is pretty uncritical. Here, large transition losses are to be expected only if the fiber parameters are not well matched, eg if the core regions are quite different. For single-mode fiber, the method is more critical. Here, the ideal conditions for reliable low-loss splices are:
The optical fiber is a silica optical fiber. Not all other glass materials are suitable for welding.
The parameters of the fusion splicer (in particular, the current and duration of the arc) are well optimized for a given fiber type (material and diameter).
The fibers have equal cladding diameters.
The fiber coating is completely removed, possibly using solvents.
Careful preparation at the end face of the fiber, with a perfectly vertical cut, no surface irregularities, and no dust. A well-done fiber cleave (some do with a fiber cleaver) should usually be good enough. Polishing results in the highest surface quality and angular accuracy, but is more time consuming.
The fiber core is perfectly on the fiber axis and is precisely aligned. (It is usually viewed under a microscope).
The active mode area is the same and not too small.
Under ideal conditions, splices are fairly reliable with extremely low conversion losses of around 0.02 dB. Little to no light will be reflected in the stitching. The joint position can then be observed with little under the microscope. However, the mechanical strength of the splice and its surroundings can be much lower than normal bare fiber if the fiber surface receives some damage during handling; very small scratches are sufficient for the effect. Note that the protective coating has to be removed for splicing, and this removal process carries the risk of damaging the fiber surface. After joining, it is common to apply a new coating or attach some other protective material (eg heat shrink protector or mechanical crimp protector) in order to obtain a sufficiently high mechanical strength.
Low-loss splices can also be achieved under non-ideal conditions, for example, for fibers with different diameters. When the fiber core is not centered, it may be necessary to do orientation while monitoring light throughput. In this case, however, the bonding process may be unreliable and require more care. A considerable portion of the joint may have to be redone until a satisfactory result is achieved.
After splicing, people often use a splice protection sleeve to protect the splice area. Note that stripping fibers is less reliable, so some extra protection is often required.
-Characteristics of fusion splicer
Devices suitable for high-quality fusion joints usually have the following features:
Carefully designed fiber clamps allow for precise fixation of the fiber optics. At least one clamp is precisely adjusted with micrometer screws.
For splicing polarization maintaining fibers, it is also necessary to wrap one of the fibers around its axis.
The microscope enables inspection of the quality and alignment of the fiber ends. Typically, there is a knob for switching between two orthogonal directions of view. The fiber core is also typically visible.
A “prefuse” allows a clean surface to be applied without touching the fiber.
Some splicers do automatic alignment based on camera images and/or monitor optical power throughput. For the latter, there must be a light source connected to one fiber end, and a light detector to the other.
Some devices can also measure the resulting joint quality.
– Test splices
The first test of splicing is inspection with a microscope of the splicing device. Normally, one should barely be able to see the splicing. However, problems may arise eg from a deflection of a fiber core.
When visible light is launched into the fiber, a large amount of stray light may result in a faulty splice, which is thus easy to identify but does not provide a quantitative measure of splice quality.
An accurate measurement of splice loss is helpful, but not easy. Start with the challenge of launching a precisely known amount of optical power into the fiber, and proceed with measuring the output power to an accuracy of 0.1 dB, for example.
An Optical Time Domain Reflectometer (OTDR) is commonly used to inspect fiber optic cables, including connectors. At least one serious flaw is easy to identify and locate with the technology. However, this requires expensive OTDR equipment.
-Advantages and limitations of welding
In comparison with other techniques for preparing fiber optic splices, fusion splicing has some significant advantages:
There is no other way to reduce the promise loss transformation and weaker thinking.
The resulting joint is very stable, that is, misalignment needs to be maintained and there is no dust to affect the joint.
No parts or materials are required, other than to protect the fibers after splicing.
Some non-ideal aspects of fusion splicing are:
The equipment used for splicing is quite expensive and its use requires adequate training.
Compared to a mechanical joint for equipment, for example, it is more difficult to use in the field because it is less compact and requires electrical power.
Connections are not easily cleared. Essentially, a new cut is required, possibly followed by surface preparation.
Also note that high power fiber lasers and amplifiers have optical power losses at the joint that can be sufficient for burning materials, especially fiber coatings. This means that their high-quality splicing is essential not only for power efficiency, but also for reliable operation.
The use of fusion splices is common for outdoor cables; long cables are usually made by splicing cables together, each with a length of several kilometers. When transmitting cables indoors, mechanical splices or fiber optic connectors are usually used, avoiding the need for expensive fusion splicers. Fusion splicing is also used in factories to manufacture stable fiber optic equipment, such as fiber optic fusion splicers and amplifiers.