What are the Environment Effect on Optical Fiber? What is OFC Splicing?
- There are however always small defects at the surface of the fiber, called microcracks. These cracks grew when water vapour is present and the fiber simultaneously is under strain, hence shortening the life of the fiber.
- Another effect ingress of water, which may increase of concentration of water vapour around the fiber.
- Temperature variation may cause Expansion/ Contraction of fibers and affect the performance to some extent. By proper choice of materials and by adjusting the excess length of fiber in the loose tube, the temperature variation effect can be neglected.
CABLE DRUM LENGTH :
Cables come reeled in various lengths, typically 1 to 2 km, although lengths of 5 or 6 kms are available for single mode fibers. Long lengths are desirables for long-distance applications since cable must be spliced end to end over the run. Each splice introduces additional loss into the system. Long cable lengths mean fewer splices and less loss.
METALLIC OR NON-METALLIC CABLES :
Fiber optic cables sometimes also contain copper conductors, such as twisted pair. One use of these conductors is to allow installers to communicate with each other during installation of the fiber especially with long distance telephone installation. The other use is to power remote equipment such as repeaters. Sub-marine cables, cables for overhead mounting, highly, armoured cables of railways etc are also coming in category of metallic cables. In such cables strength member will typically be of steel wire and the cable will also contain one or two copper service pairs. It is also common to include an aluminium water barrier.
It is possible to construct completely metal free cables, used in areas suffering from high frequency of lightening. Strength member is made of fiber glass rod. Induction effect due to lightening or power line parallelism is not at all on such non-metallic cables.
Splices are permanent connection between two fibers. The splicing involves cutting of the edges of the two fibers to be spliced.
Single–Fiber Mechanical Splicing
- Single Fiber Capillary
- Aligns two fiber ends to a common centerline, thereby aligning cores.
- Clean, cleaved fibers are butted together and index matched.
- Permanently secured with epoxy or adhesive.
Examples : Siecor, See Splice GTE Elastomeric Splice.
The following three types are widely used :
- Adhesive bonding or Glue splicing.
- Mechanical splicing.
- Fusion splicing.
- Adhesive Bonding or Glue Splicing
1. Adhesive bonding or Glue splicing.
This is the oldest splicing technique used in fiber splicing. After fiber end preparation, it is axially aligned in a precision V–groove. Cylindrical rods or another kind of reference surfaces are used for alignment. During the alignment of fiber end, a small amount of adhesive or glue of same refractive index as the core material is set between and around the fiber ends. A two component epoxy or an UV curable adhesive is used as the bonding agent. The splice loss of this type of joint is same or less than fusion splices. But fusion splicing technique is more reliable, so at present this technique is very rarely used.
This technique is mainly used for temporary splicing in case of emergency repairing. This method is also convenient to connect measuring instruments to bare fibers for taking various measurements.
The mechanical splices consist of 4 basic components :
(i) An alignment surface for mating fiber ends.
(ii) A retainer
(iii) An index matching material.
(iv) A protective housing
A very good mechanical splice for M.M. fibers can have an optical performance as good as fusion spliced fiber or glue spliced. But in case of single mode fiber, this type of splice cannot have stability of loss.
The fusion splicing technique is the most popular technique used for achieving very low splice losses. The fusion can be achieved either through electrical arc or through gas flame.
The process involves cutting of the fibers and fixing them in micro–positioners on the fusion splicing machine. The fibers are then aligned either manually or automatically core aligning (in case of S.M. fiber) process. Afterward the operation that takes place involve withdrawal of the fibers to a specified distance, preheating of the fiber ends through electric arc and bringing together of the fiber ends in a position and splicing through high-temperature fusion.
If proper care taken and splicing is done strictly as per schedule, then the splicing loss can be minimized as low as 0.01 dB/joint. After fusion splicing, the splicing joint should be provided with a proper protector to have following protections:
- Mechanical protection
- Protection from moisture.
Sometimes the two types of protection are combined. Coating with Epoxy resins protects against moisture and also provides mechanical strength at the joint.
Now–a–days, the heat shrinkable tubes are most widely used, which are fixed on the joints by the fusion tools.
The fusion splicing technique is the most popular technique used for achieving very low splice losses. The introduction of single mode optical fiber for use in long haul network brought with it fiber construction and cable design different from those of multimode fibers.
The splicing machines imported by BSNL begins to the core profile alignment system, the main functions of which are :
- Auto active alignment of the core.
- Auto arc fusion.
- Video display of the entire process.
- Indication of the estimated splice loss.
The two fibers ends to be spliced are cleaved and then clamped in accurately machined vee–grooves. When the optimum alignment is achieved, the fibers are fused under the microprocessor contorl, the machine then measures the radial and angular off–sets of the fibers and uses these figures to calculate a splice loss. The operation of the machine observes the alignment and fusion processes on a video screens showing horizontal and vertical projection of the fibers and then decides the quality of the splice.
The splice loss indicated by the splicing machine should not be taken as a final value as it is only an estimated loss and so after every splicing is over, the splice loss measurement is to be taken by an OTDR (Optical Time Domain Reflectometer). The manual part of the splicing is cleaning and cleaving the fibers. For cleaning the fibers, Dichlorine Methyl or Acetone or Alcohol is used to remove primary coating.
With the special fiber cleaver or cutter, the cleaned fiber is cut. The cut has to be so precise that it produces an end angle of less than 0.5 degree on a prepared fiber. If the cut is bad, the splicing loss will increase or machine will not accept for splicing. The shape of the cut can be monitored on the video screen, some of the defect noted while cleaving are listed below :
(i) Broken ends.
(ii) Ripped ends.
(iii) Slanting cuts.
(iv) Unclean ends.
It is also desirable to limit the average splice loss to be less than 0.1 dB.