In recent years, a holey fiber becomes popular as an optical fiber suitable for a long transmission distance. The holey fiber is an optical fiber in which a refractive index of a clad is reduced by holes. It is known that the holey fiber obtains an optical characteristic that cannot be obtained by a conventional optical fiber in which a refractive index of a clad is reduced by an impurity.
As described in Patent Literatures 1 through 4, the holey fiber is manufactured by the steps of (1) preparing a columnar base material (hereinafter, referred to as a “preform”) made from silica glass, (2) forming, in the preform, through holes which are to be holes, and (3) drawing the preform in which the through holes have been formed.
The optical characteristic of the holey fiber is influenced by positions of the holes. Accordingly, it is important to form the through holes in predetermined appropriate positions in the preform in order to manufacture an optical fiber having a desired optical characteristic.
In the step (3), through holes extending in a direction vertical to end surfaces of the preform are formed by a drilling process. If a machine tool for forming the through holes has low machining accuracy, perforating positions of the through holes are gradually shifted as the perforating is proceeded even if the perforating is started from appropriate positions of one end surface. Accordingly, positions of the through holes are largely shifted at the other end surface, or a through hole is connected to another through hole in the middle of the preform. For this reason, after the through holes are formed, it is necessary to whether or not through holes are formed in respective appropriate positions.
The inspection as to whether or not through holes are formed in respective appropriate positions has been normally carried out by a method in which a preform is observed from an end surface by use of an optical microscope.
However, the conventional inspecting method by use of an optical microscope can inspect only forming positions of holes in the vicinity of an end surface of a preform. That is, the forming positions of the holes in the middle of end surfaces (at an arbitrary cross-section between both end surfaces) cannot be inspected. There is another method in which forming positions of through holes in the middle of a preform are estimated from (i) forming positions of holes in one end surface and (ii) forming positions of the holes in the other end surface. However, such estimation cannot accurately specify the forming positions of the holes in the middle of the end surfaces. In view of the circumstances, it is necessary to inspect which portion of a preform is suitable for manufacturing an optical fiber by cutting the preform in round slices in a case where forming positions of through holes in an end surface are largely shifted from respective appropriate positions. This inspection increases a manufacturing cost.
On the contrary, Patent Literature 5 discloses a method for inspecting whether or not through holes are formed in respective appropriate positions on the basis of an intensity distribution of forward scattered light which is generated by parallel light entering from a side surface of a preform.