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Manufacturing Issues



                                                                       Manufacturing Issues  337


                                   Burner moves back and forth

                                          Burner
                                          Burner
                        O 2 + metal halide vapors
                                                     Glass particles




                      Rotation          Soot preform   Bait rod (mandrel)
                      Figure 20.2. Illustration of the outside vapor-phase oxidation
                      (OVPO) process.































                    Figure 20.3. Illustration of the
                    vapor-phase axial deposition (VAD)
                    method.

                        When the deposition process is complete, the mandrel is removed and the
                      porous tube is vitrified to a clear glass preform. The central hole in the tube pre-
                      form collapses during the fiber drawing process.

          20.1.2. Vapor-phase axial deposition
                      In the vapor-phase axial deposition (VAD) method, shown in Fig. 20.3, the soot
                      particles are formed in the same way as in the OVPO process. However, in this
                      case the particles are deposited on the end of a silica glass rod, which acts as a
                      seed. A porous glass preform is grown in the axial direction by moving the rod


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