Preform manufacturing method
A preform manufacturing method of the present invention has a hole forming step of forming a plurality of holes in a glass body to produce a glass pipe, and a heating integration step of heating the glass pipe with core rods including core portions being inserted in the respective holes, thereby to implement integration of the core rods and the glass pipe. In the hole forming step, a peripheral hole out of the holes to be formed in the glass body is formed at a position determined in consideration of positional variation of the core portion before and after the integration.
1. A preform manufacturing method for manufacturing a multicore optical fiber preform comprising a plurality of core portions each extending in a predetermined axial direction, and a common cladding portion covering each of the plurality of core portions, the preform manufacturing method comprising:
a hole forming step of preparing a glass body to constitute a part of the common cladding portion, and perforating the glass body to form a plurality of holes each extending in the axial direction of the glass body, thereby producing a glass pipe; and
a heating integration step of preparing a plurality of core rods each including a respective core portion of the plurality of core portions, inserting each of the plurality of core rods into a respective hole of the plurality of holes of the glass pipe, and heating the glass pipe with the plurality of core rods being inserted in the respective holes, thereby to implement integration of the plurality of core rods and the glass pipe,
wherein, when as a structure parameter after the integration, d is defined as a distance between a center position of a peripheral core portion located off a central axis of the multicore optical fiber preform and the central axis of the multicore optical fiber preform, and
as structure parameters before the integration, r is defined as a radius of a peripheral core rod corresponding to the peripheral core portion, R as a radius of a peripheral hole into which the peripheral core rod is to be inserted, out of the plurality of holes to be formed in the glass body, and D as a distance between a center position of the peripheral hole and the central axis of the glass body,
the hole forming step is configured to perforate the glass body to form a peripheral hole at a position satisfying the following relation on a straight line connecting the center position of the peripheral core portion and the central axis of the glass body:
d<D≦d+R−r.
2. The preform manufacturing method according to claim 1 , wherein, when φ is defined as a diameter reduction ratio of an outer diameter of a multicore optical fiber to be manufactured by drawing the multicore optical fiber preform, with respect to an outer diameter of the multicore optical fiber preform,
the hole forming step is configured to set the radius R of the peripheral hole so as to satisfy the following condition:
R−r≦ 0.5 um/φ.
3. The preform manufacturing method according to claim 1 , wherein the hole forming step is configured to make a difference between the radius R of the peripheral hole and the radius r of the peripheral core rod not less than 0.15 mm.
4. A preform manufacturing method for manufacturing a multicore optical fiber preform comprising a plurality of core portions each extending in a predetermined axial direction, and a common cladding portion covering each of the plurality of core portions, the preform manufacturing method comprising:
a hole forming step of perforating a glass body constituting a part of the common cladding portion to form a plurality of holes each extending in the axial direction of the glass body, thereby producing a glass pipe; and
a heating integration step of preparing a plurality of core rods each including a respective core portion of the plurality of core portions, inserting each of the plurality of core rods into respective hole of the plurality of holes of the glass pipe, and heating the glass pipe with the plurality of core rods being inserted in the respective holes, thereby to implement integration of the plurality of core rods and the glass pipe,
wherein, when as a structure parameter after the integration, d is defined as a distance between a center position of a peripheral core portion located off a central axis of the multicore optical fiber preform and the central axis of the multicore optical fiber preform, and
as structure parameters before the integration, r is defined as a radius of a peripheral core rod corresponding to the peripheral core portion, R as a radius of a peripheral hole into which the peripheral core rod is to be inserted, out of the plurality of holes to be formed in the glass body, D as a distance between a center position of the peripheral hole and the central axis of the glass body, and S a sectional area of a clearance in another hole existing between the peripheral hole and a central axis of the glass pipe,
the hole forming step is configured to perforate the glass body to form a peripheral hole at a position satisfying the following relation on a straight line connecting the center position of the peripheral core portion and the central axis of the glass body:
d<D≦d+ 2 R−r −√{square root over ( R 2 −S /π)}.
5. The preform manufacturing method according to claim 4 , wherein, when φ is defined as a diameter reduction ratio of an outer diameter of a multicore optical fiber to be manufactured by drawing the multicore optical fiber preform, with respect to an outer diameter of the multicore optical fiber preform,
the hole forming step is configured to set the radius R of the peripheral hole and the clearance so as to satisfy the following condition:
R−r≦ 0.5 μm/φ.
6. The preform manufacturing method according to claim 4 , wherein the hole forming step is configured to make a difference between the radius R of the peripheral hole and the radius r of the peripheral core rod not less than 0.15 mm.