MANUFACTURE OF DEPRESSED INDEX OPTICAL FIBERS

A method for producing depressed index optical fibers is provided to obtain both advantages of a soot deposition process and those of a rod-in-tube process while needing no long processing time and preventing contamination of a preform. A method for producing depressed index optical fibers comprises the steps of: (a) causing at least one free precursor gas flow and fuel gas flow to flow together with to form a first soot gas mixture in a first vapor axis deposition(VAD) torch, and (ii) forming a first soot flame to ignite the first soot glass mixture and to produce a first glass soot in a first vapor axis deposition(VAD) torch; (b) causing at least one free precursor gas flow and fuel gas flow to flow together with to form a second soot gas mixture in a second vapor axis deposition torch, and (ii) forming a second soot flame to ignite the second soot glass mixture and to produce a second glass soot in a second vapor axis deposition torch; (c) depositing the first glass soot to allow the first VAD torch to form a first soot, and allowing the first VAD torch to precede the second VAD torch so that the second glass soot is deposited on the first glass soot, while optionally orienting a support bar to the first and second VAD torches; (d) moving the support bar to the torches from the starting point to the end point so as to form a double layer of soot; (g) heating the double layer of soot to cause the soot to be closely inserted into a glass bar; (h) inserting the glass bar into a glass pipe containing fluorine-doped glass; and (i) heating the glass pipe to collapse the glass pipe around the glass bar.