Optical Fiber Communication

1. Optical Fiber Communication Week 5 CIIT Wah

2. Fiber Fabrication • Fiber fabrication is very important step for the whole communication, as we have already discussed attenuation that is being occurred due to contamination and non-uniform fibers. • Fibers have been fabricated by a number of techniques, We will discuss two methods for directly producing fibers several methods for producing preforms. CIIT Wah

3. Double Crucible CIIT Wah

4. Conti…. • In the double crucible method molten core glass is placed in the inner vessel and the molten cladding glass occupies the outer vessel. • The two glasses come closer to each other at the base of the outer container forming a glass cladded core, this molten mixture is pulled into a fiber. • At the first glance it would appear that the double crucible technique could only produce step index fibers. CIIT Wah

5. Conti…. • That is not true graded index fiber may be produced by allowing the core cladding glasses to inter-diffuse after they come together. • Diffusion results in the gradual change of the refractive index between that of the core and the cladding glasses. CIIT Wah

6. Rod in Tube CIIT Wah

7. Conti… • In Rod-in-Tube procedure a rod of the core glass is placed inside the tube of the cladding glass. • The diameter of the core rod may be a few centimeters depending upon the requirement, and the diameter of the cladding rod is just larger. • The end of this combination is heated, softening the glass so that the thin fiber can be pulled from it. CIIT Wah

8. Fiber Preform production methods • There are several methods in doped deposited silica for producing preform of the fiber. • External deposition • Axial deposition • Internal deposition Bahria University

9. Doped deposited silica • The most extensively used fabrication processes involves building up a fiber preform by vapor deposition of the glass constituents. • Here pure silica is used as the base and the small amount of the dopants are added to produce the slight changes in the refractive index that are required. • The resultant cylindrical preform has the required refractive index variations but its cross sectional area is 160 times more than that a fiber having (like 125 micron) Bahria University

10. External Deposition

11. Conti….. • External decomposition is also referred as external chemical vapor decomposition, outside vapor phase oxidation etc. • The material vapors are oxidized in a flame. • The torch moves laterally depositing the glass particles onto a rotating bait. • The deposition forms a soot or powder on the bait, after the deposition is completed material is sintered, and the bait is removed. CIIT Wah

12. Axial Deposition

13. Conti…. • Axial deposition is also known as axial vapor deposition. • In this case the deposition occurs on the end of the rotating bait, which is with drawn as the preform builds up. • A very long core preform can be constructed in this manner, the cladding can be deposited on the core by flame hydrolysis. • Alternatively, a cladded fiber can be constructed by inserting the core preform inside the lower index glass tube. CIIT Wah

14. Internal Deposition CIIT Wah

15. Conti…. • In this process the chemical vapors are deposited inside of the glass tube that is rotating in the glass lathe, a burner is also moves along the tube. • Layer upon layer is deposited as the burner is moved repeatedly traverses the length of the tube, typically 30 to 100 layer can be deposited. • By changing the concentration of the dopants, the refractive index can be changed from layer to layer, creating a graded index fiber. CIIT Wah

16. Optical fiber cables • The amount of the protection a fiber needs varies from application to application. • In lab a fiber protected by a thin buffer coating might be good for experimental setting but for deploying such fiber requires more protection. • Cables are designed to minimize the Microbends during the construction and limit their occurrence later. CIIT Wah

17. Cables variations • Several general structural forms that produce adequate cables have evolved, among the variations are the followings ones: • Single fiber cables, multifiber cables • Tightly held cables, Loose held cables • Circular geometries and ribbon geometries • Centralized strengthening members CIIT Wah

18. Single and Multifiber cables • If only one fiber is required then a single fiber cable is certainly the best choice. • In some instance future needs might be economically accommodated by installing a multifiber cable. • Simple two-fiber cables are designed for duplex communication systems, one fiber handles transmission in one direction, the other carries signals in the opposite directions. CIIT Wah

19. Tightly and Loosely held Cables • As we discussed earlier, that fibers are coated with a buffer immediately after being drawn. • The buffered fiber may be completely enclosed in a cushioning material as the next step in the cabling process, this is tight buffer construction. • The cushioning helps minimize the micro-bending , provides the crush resistance and vibration isolation but adds very little to the fiber tensile strength. CIIT Wah

20. Loose Construction cable On white board CIIT Wah

21. Conti…. • An alternative to the tight buffer cushioning is shown in the previous slide, where the fiber lies loosely inside a surrounding plastic tube, this is known as loose-tube construction. • The fiber itself can adjust within the tube when the cable is distorted. • Microbending is almost completely eliminated by this technique. • Moisture protection can be added inside by placing a foam or jelly inside the tube. CIIT Wah

22. Strengthening Members • Strengthening members are added to the fiber cables for bending purpose and external support. • Steel wires and the textile fibers are the most popular materials for this purpose. • Steel is strong but heavier than textile fibers, but is found in some commercial cables. • The textile fiber Kevlar, a very strong polymer is one of the most frequently used strengthening material. CIIT Wah

23. Light Duty Cable • A light duty cable can be completed by the surrounding Kevlar with an outer jacket. • This cable weights 12.5 Kg/Km and can safely bent to radius of 5 cm. • It contains a tightly packed fiber with external strengthening materials. • The cable can be operate with the applied load to 50 N. CIIT Wah